aashto committee on bridges and structures … · • a soil nail wall is an earth-retaining system...

AASHTO COMMITTEE ON BRIDGES AND STRUCTURES

ATTACHMENT B -2018 AGENDA ITEM 11

Technical Committee T-15 Substructures and Retaining Walls

ATTACHMENT B – 2018 AGENDA ITEM 11 - T-15

In Article 3.4.1, revise Table 3.4.1-2 as follows: Table 3.4.1-2—Load Factors for Permanent Loads, γp

Type of Load, Foundation Type, and Method Used to Calculate Downdrag

Load Factor Maximum Minimum

DC: Component and Attachments DC: Strength IV only

1.25 1.50

0.90 0.90

DD: Downdrag Piles, α Tomlinson Method Piles, λ Method Drilled shafts, O’Neill and Reese (2010) Method

1.40 1.05 1.25

0.25 0.30 0.35

DW: Wearing Surfaces and Utilities 1.50 0.65 EH: Horizontal Earth Pressure

• Active • At-Rest • AEP for anchored walls

1.50 1.35 1.35

0.90 0.90 N/A

EL: Locked-in Construction Stresses 1.00 1.00 EV: Vertical Earth Pressure

• Overall and Compound Stability • Retaining Walls and Abutments • Rigid Buried Structure • Rigid Frames

• Flexible Buried Structures o Metal Box Culverts, Structural Plate Culverts with Deep Corrugations,

and Fiberglass Culverts o Thermoplastic Culverts o All others

• Internal and Compound Stability for Soil Failure in Soil Nail Walls

1.00 1.35 1.30 1.35

1.50 1.30 1.95

1.00

N/A 1.00 0.90 0.90

0.90 0.90 0.90

N/A

ES: Earth Surcharge 1.50 0.75


ATTACHMENT C -2018 AGENDA ITEM 11


SECTION 11: WALLS, ABUTMENTS, AND PIERS 1

ATTACHMENT C – 2018 AGENDA ITEM 11 - T-15 Section 11—Walls, Abutments, and Piers Revise Article 11.1 as follows:

11.1 SCOPE

This Section provides requirements for design of abutments and walls. Conventional retaining walls, nongravity cantilevered walls, anchored walls, mechanically stabilized earth (MSE) walls, and prefabricated modular walls, and soil nail walls are considered.

Add the following to the end of Article 11.2:

11.2—DEFINITIONS • A soil nail wall is an earth-retaining system containing passive reinforcing elements that are drilled and

grouted sub-horizontally in the ground.

In Article 11.3, add and revise the following Notation: 11.3—NOTATION 11.3.1—General AH = cross-sectional area of the head of a stud (in.2) (11.12.6) ahm = cross-sectional area of horizontal reinforcement per unit width at midspan of facing (in2/ft) (11.12.6) ahn = cross-sectional area of horizontal reinforcement per unit width at nail head (in2/ft) (11.12.6) A'HN = equivalent cross-sectional area at the head in the vertical direction (in2) (C11.12.6) AS = peak seismic ground acceleration coefficient modified by short-period site factor (11.6.5) (C11.8.6)

(11.10.7.1); cross-sectional area of the shaft of a headed-stud (in2) (11.12.6) At = cross-sectional area of soil nail tendon (in2) (11.12.5) A'VN = equivalent cross-sectional area at the head in the horizontal direction (in2) (C11.12.6) avm = cross-sectional area of vertical reinforcement per unit width at midspan of facing (in2/ft) (11.12.6) avn = cross-sectional area of vertical reinforcement per unit width at nail head (in2/ft) (11.12.6) CF = factor to consider nonuniform soil pressures behind soil nail wall facing (dim.) (11.12.6) CP = correction factor to account for contribution of soil support in punching shear (dim.) (11.12.6) D'C = effective equivalent diameter of conical slip surface at soil nail head (ft) (11.12.6) DDH = drill hole diameter (in.) (11.12.4) df = distance from the outer edge of a final facing section in compression to the centroid of the reinforcement

(in.) (11.12.6) di = distance from the outer edge of an initial facing section in compression to the centroid of the

reinforcement (in.) (11.12.6) DSC = diameter of the shaft of a headed stud (in.) (11.12.6) DSH = diameter of the head of stud (in.) (11.12.6) f ′c = compressive strength of concrete (ksi) (11.12.6.2.3) fy = yield strength of steel (ksi) (11.12.5) fy-hs = yield strength of headed-stud (ksi) (11.12.6) h = vertical distance between ground surface and wall base at the back of wall heel (ft); thickness of facing

(in.) (11.6.3.2) (11.10.7.1) (11.12.6) hc = effective depth of conical surface (ft) (11.12.6)


hf = thickness of final facing (in.) (11.12.6) hi = height of reinforced soil zone contributing horizontal load to reinforcement at level i (ft); thickness of

initial facing (in.) (11.10.6.2.1) (11.12.6) LBP = length of a bearing plate (ft) (11.12.6) LP = pullout length behind slip surface (ft) (11.12.4) LS = length of headed-stud (ft) (11.12.6) NH = number of headed-studs (dim.) (11.12.6) qU = bond strength of soil nails (ksi) (11.12.4) RFF = nominal resistance of soil nail to flexure (i.e., bending) of facing (kip) (11.12.6) RFH = nominal tensile resistance of headed-studs located in final facing (kip) (11.12.6) RFP = nominal punching shear resistance of facing (kip) (11.12.6) RPO = nominal pullout resistance of soil nail (kip) (11.12.4) RT = nominal tensile resistance of tendon (kip) (11.12.5) Sh = horizontal reinforcement spacing (ft) (11.10.6.4.1); horizontal spacing of soil nails (ft) (11.12.1,

C11.12.1, C11.12.6; 11.12.6) SHS = spacing of headed-studs (ft) (11.12.6) Smax = maximum spacing of soil nails (ft) (C11.12.6) Sv = vertical spacing of reinforcements (ft); vertical spacing of soil nails (ft) (11.10.6.2.1) (11.12.1) Tmaxsn = maximum soil nail force (kip) (11.12.2) Tosn = tensile force at the nail head (kip) (11.12.6.2) tP = thickness of bearing plate (ft) (11.12.6) tSH = thickness of the head of stud (ft) (11.12.6) VF = punching shear force acting through facing (kip) (11.12.6) α = scale effect correction factor, or wall height acceleration reduction factor for wave scattering (dim.);

(11.10.6.3.2) (A11.5) ρij = reinforcement ratio (percent) (11.12.6) φFF = resistance factor for flexure (dim.) (11.12.6) φFH = resistance factor for headed stud in tension (dim.) (11.12.6) φFP = resistance factor for punching shear in facing (dim.) (11.12.6) φPO = resistance factor for pullout (dim.) (11.12.4) φT = resistance factor for tension (dim.) (11.12.5) Revise Article 11.5.5 as follows: 11.5.5—Resistance Requirement

Abutments, piers and retaining structures and their

foundations and other supporting elements shall be proportioned by the appropriate methods specified in Articles 11.6, 11.7, 11.8, 11.9, 11.10, or 11.11, or 11.12 so that their resistance satisfies Article 11.5.6.

The factored resistance, RR, calculated for each applicable limit state shall be the nominal resistance, Rn, multiplied by an appropriate resistance factor, φ, specified in Table 11.5.7-1.

C11.5.5 Procedures for calculating nominal resistance are

provided in Articles 11.6, 11.7, 11.8, 11.9, 11.10, and 11.11, and 11.12 for abutments and retaining walls, piers, nongravity cantilevered walls, anchored walls, mechanically stabilized earth walls, and prefabricated modular walls, and soil nail walls, respectively.

Revise Article 11.5.7 as follows: 11.5.7—Resistance Factors—Service and Strength

Resistance factors for the service limit states shall be taken as 1.0, except as provided for overall stability in Article 11.6.2.3.

C11.5.7

The resistance factors given in Table 11.5.7-1, other than those referenced back to Section 10, were calculated


For the strength limit state, the resistance factors provided in Table 11.5.7-1 shall be used for wall design, unless region specific values or substantial successful experience is available to justify higher values. Resistance factors for geotechnical design of foundations that may be needed for wall support, unless specifically identified in Table 11.5.7-1, are as specified in Tables 10.5.5.2.2-1, 10.5.5.2.3-1, and 10.5.5.2.4-1.

If methods other than those prescribed in these Specifications are used to estimate resistance, the resistance factors chosen shall provide the same reliability as those given in Tables 10.5.5.2.2-1, 10.5.5.2.3-1, 10.5.5.2.4-1, and Table 11.5.7-1.

Vertical elements, such as soldier piles, tangent-piles and slurry trench concrete walls shall be treated as either shallow or deep foundations, as appropriate, for purposes of estimating bearing resistance, using procedures described in Articles 10.6, 10.7, and 10.8.

by direct correlation to allowable stress design rather than reliability theory.

Since the resistance factors in Table 11.5.7-1 were based on direct correlation to allowable stress design, the differences between the resistance factors for tensile resistance of metallic versus geosynthetic reinforcement are based on historical differences in the level of safety applied to reinforcement designs for these two types of reinforcements. See Article C11.10.6.2.1 for additional comments regarding the differences between the resistance factors for metallic versus geosynthetic reinforcement.

Region-specific resistance factor values should be determined based on substantial statistical data combined with calibration or substantial successful experience to justify higher values. Smaller resistance factors should be used if site or material variability is anticipated to be unusually high or if design assumptions are required that increase design uncertainty that has not been mitigated through conservative selection of design parameters. See Allen et al. (2005) for additional guidance on calibration of resistance factors.

Some increase in the prescribed resistance factors may be appropriate for design of temporary walls consistent with increased allowable stresses for temporary structures in allowable stress design.

The evaluation of overall stability of walls or earth slopes with or without a foundation unit should be investigated at the service strength limit state based on the Service I Load Combination using γp for overall stability as specified in Article 3.4.1, and an appropriate resistance factor as specified in Article 11.6.3.7.


Table 11.5.7-1—Strength Limit State Resistance Factors for Permanent Retaining Walls

Wall-Type and Condition Resistance Factor Nongravity Cantilevered and Anchored Walls

Axial compressive resistance of vertical elements Article 10.5 applies Passive resistance of vertical elements 0.75 Pullout resistance of anchors (1) Cohesionless (granular) soils

Cohesive soils Rock

0.65 (1) 0.70 (1) 0.50 (1)

Pullout resistance of anchors (2) Where proof tests are conducted 1.0 (2) Tensile resistance of anchor tendon

Mild steel (e.g., ASTM A615 bars) High strength steel (e.g., ASTM A722 bars)

0.90 (3) 0.80 (3)

Overall stability, soil failure Article 11.6.3.7 applies Flexural capacity of vertical elements 0.90

Mechanically Stabilized Earth Walls, Gravity Walls, and Semigravity Walls Bearing resistance Gravity and semigravity walls

MSE walls 0.55 0.65

Sliding 1.0 Tensile resistance of metallic reinforcement and connectors

Strip reinforcements (4) Static loading Grid reinforcements (4) (5) Static loading

0.75

0.65

Tensile resistance of geosynthetic reinforcement and connectors

Static loading 0.90

Pullout resistance of tensile reinforcement

Static loading 0.90

Overall and compound stability, soil failure

Article 11.6.3.7 applies

Prefabricated Modular Walls Bearing Article 10.5 applies Sliding Article 10.5 applies Passive resistance Article 10.5 applies Overall stability, soil failure Article 11.6.3.7 applies

Soil Nail Walls (6) Lateral sliding 1.00 Overall and Compound stability, soil failure

Article 11.6.3.7 applies

Tensile resistance of nail tendon Mild steel bars (Grades 60 and 75) High resistance bars (Grades 95 and 150)

0.75 0.65

Pullout resistance of nail 0.65 Facing flexure Initial and final facing 0.90 Facing punching shear Initial and final facing 0.90 Tensile resistance of headed stud A307 steel bolt (7)

A325 steel bolt 0.70 0.80


(1) Apply to presumptive ultimate unit bond stresses for preliminary design only in Article C11.9.4.2. (2) Apply where proof test(s) are conducted on every production anchor to a load of 1.0 or greater times the factored load on the

anchor. (3) Apply to maximum proof test load for the anchor. For mild steel apply resistance factor to Fy. For high-strength steel apply

the resistance factor to guaranteed ultimate tensile strength. (4) Apply to gross cross-section less sacrificial area. For sections with holes, reduce gross area in accordance with Article 6.8.3

and apply to net section less sacrificial area. (5) Applies to grid reinforcements connected to a rigid facing element, e.g., a concrete panel or block. For grid reinforcements

connected to a flexible facing mat or which are continuous with the facing mat, use the resistance factor for strip reinforcements.

(6) Additional, special cases of limit states, as well as corresponding resistance factors, for soil nail walls are presented in FHWA-

NHI-14-007/FHWA GEC 7 (Lazarte et al. 2015). (7) Equivalent to AWS D1.1 Type B studs, with fy = 60 ksi.

Change Article 11.12 to 11.13 and add the following references:

11.1213—REFERENCES Byrne, R.J., D. Cotton, J. Porterfield, D. Wolschlag, and G. Ueblacker. (1998). “Manual for Design and Construction Monitoring of Soil Nail Walls,” Report FHWA-SA-96-69R, Federal Highway Administration, Washington, DC. Elias, V. and I. Juran. (1991). “Soil Nailing for Stabilization of Highway Slopes and Excavations,” Publication FHWA-RD-89-198, Federal Highway Administration, Washington, DC. Lazarte, C.A., H. Robinson, J. E. Gómez, A. Baxter, A. Cadden, and R. R. Berg. (2015). “Soil Nail Walls,” Geotechnical Engineering Circular No. 7, Publication FHWA-NHI-14-007, Federal Highway Administration, U.S. Department of Transportation, Washington, DC, 305p. Porterfield, J.A., Cotton, D.M., and Byrne, R.J. (1994). “Soil Nailing Field Inspectors Manual, Demonstration Project 103,” Report No. FHWA-SA-93-068, Federal Highway Administration, U.S. Department of Transportation, Washington, DC.


ATTACHMENT D -2018 AGENDA ITEM 12



NEW SECTION

ATTACHMENT D – 2018 AGENDA ITEM 12 - T-15

Note: The Following Pages Containing 11.12 comprise a New Subsection for Section 11. It is NOT Underlined as it is All New, Added Language.

11.12 SOIL NAIL WALLS

11.12.1 General Considerations C11.12.1

Soil nail walls, illustrated in Figure 11.12.1-1, most commonly consist of: (a) a soil nail (i.e., steel bar) that is placed in a pre-drilled hole, then grouted along its entire length in the hole; (b) connectors in the soil nail head; and (c) a structurally continuous reinforced shotcrete or concrete cover (facing) connecting all nail heads.

The feasibility of using a soil nail wall at a particular location should be based on the suitability of the soil and rock conditions at both the wall face and along the length of the nails. Where fill is to be placed above the soil nail wall, or if the wall supports a combination of cut and fill, the fill placed near the wall face should be limited regarding depth of fill placed below the wall top to no more than down to the first row of nails, and the placement and compaction of the fill shall not damage the nails or facing or otherwise compromise the ability of these elements to resist load.

Soil nail walls are top-down construction structures that are particularly well suited for ground conditions that require vertical or near-vertical cuts, and may be used for both permanent and temporary support. Favorable ground conditions make soil nailing technically feasible and cost effective, compared with other techniques, when:

• the soil being excavated is able to stand

unsupported, 3 to 6 ft high, vertically or near vertical, for 1 to 2 days;

• all soil nails are above the groundwater table; and

• the long-term integrity of the soil nails can be maintained through corrosion protection.

If the soil nail wall retains fill in the upper part of

the wall, provisions to protect against nail and facing damage resulting from backfill and subsoil settlement, or backfill and compaction activities above the anchors, should be included.

Soil nail walls may also be feasible to stabilize marginally unstable existing slopes in which a sufficient length of nails can penetrate behind the failure zone and in which the wall installation process will not exacerbate, temporarily or permanently, the stability problem.

The availability or ability to obtain underground easements and proximity of buried facilities to nail locations should also be considered in assessing feasibility.

Horizontal nail spacing, Sh, is typically the same as vertical nail spacing, Sv, and is typically between 4 ft and 6.5 ft, with a maximum influence area of Sh × Sv ≤ 40 ft2. Soil nail spacing may be modified to accommodate the presence of existing underground structures or utilities behind the wall.


NEW SECTION Subsurface conditions that are generally well-

suited for soil nails applications include dense to very dense granular soils with apparent cohesion; weathered rock without adverse planes of weakness; stiff to hard fine-grained soils; engineered fill; some residual soils; and some glacial till soils.

Examples of unfavorable soil types and ground conditions include dry, poorly-graded cohesionless soils; granular soils with high groundwater; soils with cobbles and boulders; soft to very soft fine-grained soils; collapsible soils; organic soils; highly-corrosive soils; weathered rock with unfavorable planes of weakness; karstic formations; loess; and expansive soils.

Corrosion protection is provided by grouting, epoxy coating, galvanized coating, and encapsulation, or a combination of these methods (not shown in Figure 11.12.1-1). See Section 11.12.8 and FHWA-NHI-14-007/FHWA GEC 7 (Lazarte et al. 2015) for corrosion protection design considerations.


NEW SECTION

Figure 11.12.1-1—Typical Cross-Section of a Soil Nail Wall: (a) overall cross-section, and (b) nail head detail. (Lazarte et al., 2015, after Porterfield et al. 1994)

(a)

(b)


NEW SECTION

11.12.2 Loading C11.12.2

The provisions of Article 11.6.1.2 shall apply. Limit equilibrium analysis methods shall be used to

assess wall loading and stability for final design. When additional surcharge loads, such as a structure footing load or live load, are applied to the top of the reinforced zone of the soil nail wall, they are factored as specified in Article 3.4.1 for the Strength I Limit state.

For soil nail walls, the limit-equilibrium slope stability analyses to assess overall stability (external to the nail reinforced section) and to determine nail loads (i.e., Tmaxsn, the maximum soil nail force in each soil nail row determined from the limit equilibrium stability analysis) for internal and compound stability (i.e., nail tensile design, nail pullout and facing design) are as illustrated in Figure C11.12.2-1. Determination of Tmaxsn for internal and compound stability for nail tensile resistance, pullout, and facing design is done at the strength limit state, and if extreme events such as seismic are being considered, Tmaxsn is estimated also considering the additional loads applicable to the extreme event being considered.

Available soil nail wall analysis computer programs analyze soil nail tension and pullout as a single design step to achieve a target minimum level of safety. The tension in the nails Tmaxsn is determined at the target soil failure resistance factor (see Article 11.5.7 and Article 11.6.3.7) for the wall (i.e., 1/FS output by the computer program, in which FS is typically 1.3 or 1.5). Reduction factors (also termed “safety factors”) are applied to soil nail tensile and pullout resistance to account for variability in the pullout and nail tensile resistance. See Lazarte et al. (2015) for additional guidance on the application of typical soil nail wall design programs to a LRFD framework.

Available computer programs used for soil nail wall stability analysis typically provide values of Tmaxsn in each soil nail row that corresponds to the target level of safety. The Tmaxsn values obtained may vary depending on the volume of soil between the wall face and the critical surface (which is a function of the slope stability FS), the type of surface analyzed (e.g., circular, log spiral, two part wedge, etc.), and the distribution of force along the length of the nails. How these factors affect the results may vary depending on the software used for the wall design. The designer should consider these factors when selecting values for Tmaxsn to be used in the limit state equations specified in Articles 11.12.5 and 11.12.6 for designing the nails for tensile and pullout resistance, and the strength of the facing needed.

The load and resistance factors specified for use for nail pullout (Article 11.12.5.2), nail tensile resistance (Article 11.12.6.1), and facing design (Article 11.12.6.2) have been calibrated by fitting to past Allowable Stress Design (ASD) practice. For example, for ASD pullout resistance, a slope stability FS of 1.3 was used, and the


NEW SECTION nominal pullout resistance was reduced by a FS of 2.0. For the LRFD limit state equations corresponding to this example, Tmaxsn values are obtained from the slope stability analysis using a soil failure resistance factor of 0.75 and load factor of 1.0, a load factor for vertical earth pressure applied to Tmaxsn of γEV = 1.35, and a pullout resistance factor of 0.65. The equations produce a similar result as used in past ASD practice. Additional background information regarding the development of load and resistance factors for soil nail wall design is provided in Lazarte et al. (2015).

Figure C11.12.2-1—Failure Modes in Soil Nail Walls

11.12.3 Movement at the Service Limit State

The provisions of Articles 10.6.2.2, 10.7.2.2 and 11.8.2.1 shall apply as applicable.

The effects of wall movements on adjacent facilities shall be considered in the development of the wall design.

A first order estimate of horizontal and vertical displacements at the top of the wall can be estimated with the procedure presented in FHWA-NHI-14-007/FHWA GEC 7 (Lazarte et al. 2015). Displacements are a function of the soil nail length to wall height ratio, soil nail spacing, wall batter, and soil conditions.


NEW SECTION 11.12.4 – Safety Against Soil Failure (External and

Overall Stability – Strength Limit State) 11.12.4.1 – Sliding

The provisions of Article 10.6.3.4 shall apply.

11.12.4.2 Overall Stability

The potential failure surfaces to be considered in overall stability should not intersect soil nails but instead go behind the ends of the nails. With regard to the minimum level of safety required, the provisions of Article 3.4.1 for load factors and Article 11.6.3.7 for soil resistance factors shall apply. For the case of failure surfaces intersecting soil nails, see Articles 11.12.2, 11.12.5, and 11.12.6.

Overall stability analyses should also be conducted for intermediate excavation conditions. For soil nail walls with complex geometry (e.g., multiple-tiered walls), the provisions of Article 11.10.4.3 shall apply. 11.12.5 – Safety against Soil Failure (Internal and

Compound Stability – Strength Limit State) 11.12.5.1 – Soil Shear Strength

As specified in Article 11.12.2, limit equilibrium analyses shall be conducted to assess internal and compound stability of soil nail walls in the Strength Limit State. If the wall supports a structural element, the load applied to the wall by the structural element should be factored as specified in Article 3.4.1 for the Strength I Limit state. With regard to the minimum level of safety required, the provisions of Article 11.6.3.7 for soil resistance factors shall apply. 11.12.5.2 – Soil Nail Pullout

Soil nails shall be designed to resist pullout of the length of nail extending beyond the critical failure surface. It shall be verified that the factored pullout resistance available is sufficient to obtain the desired minimum level of safety as specified in Articles 11.12.2 and 11.12.5.1, and as further described in Articles C11.2.2 and C11.12.5.1. The following shall be verified for each row of nails, considering both internal and compound stability:

C11.12.4.2

Overall stability of soil nail walls is commonly evaluated using two-dimensional limit-equilibrium-based methods. For this specific limit state, only failure surfaces that are behind the soil nails are considered. Failure surfaces that intersect one or more nail rows, including both surfaces that are defined as compound stability and those that intersect all the nail rows (Figure C11.12.2-1) are addressed in the Strength Limit State for internal wall stability. Detailed guidance for evaluating the overall stability of soil nail walls, including intermediate excavation conditions, and for defining nominal tensile and pullout forces in the soil nails is provided in FHWA-NHI-14-007/FHWA GEC 7 (Lazarte et al. 2015).

C11.12.5.1

See Article C11.12.2 for background on limit equilibrium analysis procedures for soil nail walls. As discussed in that article, the resistance of the soil (i.e., its shear strength) is handled separately from LRFD analysis of the nails.

While compound stability is in a sense both internal and external, because the failure surface involves the resistance of the structural elements (i.e., nails), this limit state is dealt with in the Strength Limit State. The resistance of the nails in pullout and as tensile members is assessed as specified in Articles 11.2.5.2 and 11.12.6.1. C11.12.5.2

A uniform distribution of bond stresses along the

pullout length behind the critical failure surface, LP, is assumed.

When using the pullout limit state Eqs. 11.12.5.1-1 and 11.12.5.1-2, Lp is usually determined using the critical failure surface from the final wall configuration.

Soil nail bond strength is influenced by soil or rock type, overburden stress, drilling method, drill hole cleaning, grouting procedure, and grout


NEW SECTION φ𝑝𝑝𝑝𝑝𝑅𝑅𝑝𝑝𝑝𝑝 ≥ 𝛾𝛾𝑝𝑝𝑇𝑇𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 (11.12.5.2-1)

where:

Rpo= nominal pullout resistance (kip) φpo= resistance factor for soil nail pullout (dim.) γp = the maximum load factor for vertical earth pressure EV from Table 3.4.1-2 (dim.). Tmaxsn = maximum soil nail force determined from the limit equilibrium stability analysis in Article 11.12.2 (kip)

Variables for pullout are illustrated in Figure 11.12.5.2-1.

The nominal pullout resistance, Rpo (kips), shall be computed as:

Pdhu LDqπR PO = (11.12.5.2-2)

where:

RPO = nominal pullout resistance (kip)

qu = bond strength per unit area (ksf)

Ddh = drill hole diameter (ft)

LP = the length of soil nail behind the critical failure surface (ft)

Figure 11.12.5.2-1—Definition of Pullout Variables

characteristics. As a guide, the presumptive values in Tables C11.12.5.2-1 and C11.12.5.2-2 may be used to estimate the bond strength for different drilling methods and gravity-grouted in coarse-grained and fine-grained soils, respectively. Presumptive bond strength values in weathered rock and in rock, for rotary drilling method, are provided in Table C11.12.5.2-3.

Table C11.12.5.2-1—Presumptive Bond Strength for Soil Nails in Coarse-Grained Soils (FHWA-NHI-14-007/FHWA GEC 7, Lazarte et al. 2015; after Elias and Juran 1991)

Drilling Method Soil Type Bond

Strength, qu (psi)

Rotary Drilled Sand/gravel 15 - 26

Rotary Drilled Silty sand 15 - 22

Rotary Drilled Silt 9 - 11

Rotary Drilled Piedmont residual 6 - 17

Rotary Drilled Fine Colluvium 11 - 22

Driven Casing Sand/gravel

w/low overburden

(1) 28 - 35

Driven Casing Sand/gravel

w/high overburden (1)

41 - 62

Driven Casing Dense Moraine 55 - 70

Driven Casing Colluvium 15 - 26

Augered Silty sand fill 3 - 6

Augered Silty fine sand 8 - 13

Augered Silty clayey sand 9 - 20 Note: (1) Low and high overburden are defined as effective overburden pressure being, respectively, less than and greater than 1.5 tsf.


NEW SECTION Table C11.12.5.2-2—Presumptive Bond Strength for Soil Nails in Fine-Grained Soils (FHWA-NHI-14-007/FHWA GEC 7, Lazarte et al. 2015; after Elias and Juran 1991)

Drilling Method Soil Type

Bond Strength, qu (psi)

Rotary Drilled Silty clay 5 - 7

Driven Casing Clayey silt 13 - 20

Augered Loess 4 - 11

Augered Soft clay 3 - 4

Augered Stiff clay 6 - 9

Augered Stiff clayey silt 6 - 15

Augered Calcareous sandy clay 13 - 20

Table C11.12.5.2-3—Presumptive Bond Strength for Soil Nails in Rock (FHWA-NHI-14-007/FHWA GEC 7, Lazarte et al. 2015; after Elias and Juran 1991)

Rock Type Bond Strength, qu (psi)

Marl/limestone 44 - 58

Phyllite 15 - 44

Chalk 73 - 87

Soft dolomite 58 - 87

Fissured dolomite 87 - 145

Weathered sandstone 29 - 44

Weathered shale 15 - 22

Weathered schist 15 - 25

Basalt 73 - 87

Slate/Hard shale 44 - 58

The use of the current value of EV in Table 3.4.1-2 for the load factor in this case should be considered an interim measure until research is completed to quantify load prediction bias and uncertainty.


NEW SECTION

11.12.6 – Safety against Structural Failure (Internal and Compound Stability – Strength Limit State)

11.12.6.1 Soil Nail in Tension

C11.12.6.1

It shall be verified that the factored soil nail tensile resistance available is sufficient to obtain the desired minimum level of safety as specified in Articles 11.12.2 and 11.12.5.1, and as further described in Articles C11.2.2 and C11.12.5.1. Therefore, the following limit state shall be verified for each row of nails, considering both internal and compound stability:

φ𝑇𝑇𝑅𝑅𝑇𝑇 ≥ 𝛾𝛾𝑝𝑝𝑇𝑇𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 (11.12.6.1-1)

where:

φT = resistance factor for bar in tension from Table 11.5.7-1 (dim.)

RT = nominal tensile resistance of bar (kip)

γP = the maximum load factor for vertical earth pressure EV from Table 3.4.1-2 (dim.)

Tmaxsn = maximum soil nail force determined from the limit equilibrium stability analysis in Article 11.12.2 (kip)

The nominal tensile resistance of a soil nail bar shall be computed as:

ytT fAR = (11.12.6.1-2)

where:

At = cross-sectional area of soil nail bar (in2)

fy = yield strength of steel (ksi)

The use of the current value of EV in Table 3.4.1-2 for the load factor in this case should be considered an interim measure until research is completed to quantify load prediction bias and uncertainty.

The contribution of the grout to the nominal

resistance in tension should be disregarded.

11.12.6.2 Soil Nail Wall Facing - – Strength Limit State

11.12.6.2.1 General C11.12.6.2.1

The failure modes of the facing of a soil nail wall that shall be considered include: (a) flexure (i.e., bending); (b) punching shear; and (c) headed-stud in tension. These failure modes are shown schematically in Figure

The failure modes for flexure and punching shear in the facing shall be considered separately for the temporary and the permanent facing. The failure mode


NEW SECTION 11.12.6.2.1-1. Nail loads used as part of the facing system analysis are determined as specified in Article 11.12.2 considering both internal and compound stability, assuming that Tosn, the load in the soil nail at the nail head, is equal to Tmaxsn.

for tension in the headed-stud shall be considered only in permanent facings.


NEW SECTION

Figure 11.12.6.2.1-1—Failure Modes to be Considered in Soil Nail Wall Facings: (a) Typical Section, (b) Flexure; (c) Punching Shear in Initial Facing; (d) Punching Shear in Final Facing; and (e) Headed-Stud in Tension


NEW SECTION

11.12.6.2.2 Facing Flexure C11.12.6.2.2

As part of the slope stability limit equilibrium analysis, as specified in Article 11.12.2 and as further described in Article C11.2.2, for flexure in the facing, the following equation shall be satisfied:

φ𝐹𝐹𝐹𝐹𝑅𝑅𝐹𝐹𝐹𝐹 ≥ 𝛾𝛾𝑝𝑝𝑇𝑇𝑝𝑝𝑚𝑚𝑚𝑚 (11.12.6.2.2-1)

in which:

φFF = resistance factor for flexure in the facing from Table 11.5.7-1 (dim.)

RFF = flexural resistance of facing (kip)

Tosn = tensile force at the nail head (kip)

RFF shall be estimated using the following equations:

RFF = 3.8CF fy F (11.12.6.2.2-2)

This equation is valid for shotcrete with fꞌc > 4,000

psi.

𝐹𝐹 = 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 𝑜𝑜𝑜𝑜

⎩⎨

⎧(𝑠𝑠𝑣𝑣𝑚𝑚 + 𝑠𝑠𝑣𝑣𝑚𝑚) × �𝑆𝑆𝐻𝐻𝑆𝑆𝑉𝑉ℎ�

(𝑠𝑠ℎ𝑚𝑚 + 𝑠𝑠ℎ𝑚𝑚) × �𝑆𝑆𝑉𝑉𝑆𝑆𝐻𝐻ℎ�

(11.12.6.2.2-3) where:

CF = factor to consider non-uniform soil pressures behind soil nail wall facing (dim) see Table 11.12.6.2.2-1

h = Nominal facing thickness, hi for initial facing or hf for final facing (ft)

avn = Cross-sectional area of vertical reinforcement per unit width direction, at nail head (in.2/ft) see Table 11.12.6.2.2-2

avm = Cross-sectional area vertical reinforcement per unit width at midspan (in2/ft) see Table 11.12.6.2.2-2

ahn = Cross-sectional area of horizontal reinforcement per unit width at nail head (in2/ft) see Table 11.12.6.2.2-2

The nominal resistance for flexure in the facing depends on the soil pressures mobilized behind the facing, horizontal and vertical soil nail spacing, soil conditions, and facing stiffness. The pressure distribution acting on the facing can be non-uniform, with load concentrating near the soil nail and decreasing toward the mid-point between the nails. See Lazarte et al. (2015) for a more in depth explanation of the pressure distribution behind the soil nail wall facing. CF is used to account for this non-uniformity.

Reinforcement can be welded wire mesh (WWM) or concrete reinforcement bars.

See FHWA-NHI-14-007/FHWA GEC 7 (Lazarte et al. 2015) for guidance on minimum and maximum amount of steel reinforcement ratio ρij to be placed at different locations of the facing. The reinforcement ratio is defined as:

100 h 0.5

a=ρ ij

ij (C11.12.6.2.2-1)

Where aij can take the values of avm, avn, ahm, or ahn; and h can take the values of hi or hf.

With regard to Eqs. 11.12.6.2.2-2 and 11.12.6.2.2-3, more general equations are provided in FHWA-NHI-14-007/FHWA GEC 7 (Lazarte et al. 2015).


NEW SECTION ahm = Cross-sectional area of horizontal

reinforcement per unit width at midspan (in2/ft) see Table 11.12.6.2.2-2

Table 11.12.6.2.2-1—Factor CF

Facing Layer

Nominal Facing Thickness, hi or hf

(in) Factor CF

initial

4 2.0

6 1.5

8 1.0

Final All 1.0

The cross-sectional areas of reinforcement per unit

width in the vertical or horizontal direction and around and in-between nails are shown schematically in Figure 11.12.6.2.2-1.

Reinforcement areas per unit width shall be determined as summarized in Table 11.12.5.2.2-2.

Table 11.12.6.2.2-2—Facing Reinforcement Area per Unit Width

Direction Location Cross-Sectional Area of Reinforcement per

Unit Width

vertical nail head (1) avn = avm +

mid-span avm

horizontal nail head (2) ahn = ahm +

V

HN

SA'

mid-span ahm

Notes: (1) At the nail head, the total cross-sectional area (per unit length) of reinforcement is the sum of the welded-wire mesh area (avm) and the area of additional vertical bars (A'VH) divided by the horizontal spacing (Sh).

(2) At the nail head, the total area is the sum of the area of the welded-wire mesh (ahm) and the area of additional horizontal bars (i.e., waler bars, A'HN) divided by Sv.

If (vertical) bars are used behind the nail heads, the total reinforcement area per unit length in the vertical direction shall be calculated as:


NEW SECTION

(11.12.6.2.2-4)

where:

A'VH = equivalent cross-sectional area at the head in the vertical direction (in2)

Similarly, this concept should be applied if

additional horizontal rebar (i.e., waler bars) is used. The total reinforcement area per unit length in the horizontal direction shall then be calculated as:

ahn = ahm + 𝐴𝐴𝐻𝐻𝐻𝐻′

𝑆𝑆𝑉𝑉 (11.12.6.2.2-5)

A'HN = equivalent cross-sectional area at the head in

horizontal direction (in2)

Figure 11.12.6.2.2-1—Geometry used for Flexural Failure Mode


NEW SECTION 11.12.6.2.3 Facing Punching Shear Resistance

As part of the slope stability limit equilibrium analysis, as specified in Article 11.12.2 and as further described in Article C11.2.2, for punching shear in the facing, it shall be verified that: φ𝐹𝐹𝐹𝐹𝑅𝑅𝐹𝐹𝐹𝐹 ≥ 𝛾𝛾𝑝𝑝𝑇𝑇𝑜𝑜𝑠𝑠𝑜𝑜 (11.12.6.2.3-1)

where: φFP = resistance factor for punching shear in

facing (dim)

RFP = nominal punching shear resistance of facing (kip)

in which:

(11.12.6.2.3-2)

where: VF = punching shear force acting through facing (kip)

CP = correction factor to account for contribution of soil support (dim). Cp should normally be assumed to be 1.0.

The nominal punching shear resistance shall be

calculated as:

𝑉𝑉𝐹𝐹 = 18.34�𝑜𝑜′𝑐𝑐𝜋𝜋𝐷𝐷′𝑐𝑐ℎ𝑐𝑐 (11.12.6.2.3-3)

where: 𝑜𝑜𝑐𝑐′ = compressive strength of concrete (ksi)

D'c = effective equivalent diameter of conical surface at soil nail head (ft)

hc = effective depth of conical surface (ft)

For the initial facing:

D'c = LBP + hc (11.12.6.2.3-4)

where:

LBP = length of the bearing plate (ft)

C11.12.6.2.3 The failure mode for punching shear may involve

the formation of a localized, conical failure surface around the nail head. The failure surface may extend behind the bearing plate or headed studs and may punch through the facing thickness at an inclination of about 45 degrees and form two punching failure surfaces (Figure 11.12.6.2.3-1).

The size of the conical failure surfaces depends on the facing thickness and the type of the nail-facing connection (i.e., bearing-plate or headed-studs).

Generally, the contribution from the soil support is

ignored and, CP = 1.0. If the soil reaction is considered, CP can have values up to 1.15.

Typical dimensions for bearing plates and headed

studs can be found in FHWA-NHI-14-007/FHWA GEC 7 (Lazarte et al. 2015).


NEW SECTION For the final facing:

𝐷𝐷𝑐𝑐′ = 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 𝑜𝑜𝑜𝑜 �𝑆𝑆𝐻𝐻𝐻𝐻 + ℎ𝑐𝑐2ℎ𝑐𝑐

(11.12.6.2.3-5)

where:

SHS = spacing of the headed studs

These equations shall be separately used to design initial and final facing. The maximum and average diameters of the failure surface (Dc and D'c on Figure 11.12.6.2.3-1), as well as the effective depth of the failure surface (hc) shall be selected separately for initial and final facings.

For initial facings, only the dimensions of the bearing plate and facing thickness should be considered. For final facings, the dimensions of headed-studs, bearing plate, and the facing thickness shall be considered.


NEW SECTION

Figure 11.12.6.2.3-1—Punching Shear Failure Modes


NEW SECTION

11.12.6.2.4 Headed-Stud in Tension C11.12.6.2.4

As part of the slope stability limit equilibrium analysis, as specified in Article 11.12.2 and as further described in Article C11.2.2, for design of the facing headed-stud in tension, it shall be verified that:

φ𝐹𝐹𝐻𝐻𝑅𝑅𝐹𝐹𝐻𝐻 ≥ 𝛾𝛾𝑝𝑝𝑇𝑇𝑝𝑝𝑚𝑚𝑚𝑚 (11.12.6.2.4-1)

where: φFH = resistance factor for headed-stud in tension

(dim)

RFH = nominal tensile resistance of headed-stud (kip)

in which:

(11.12.6.2.4-2)

where:

NH = number of headed-studs in the connection (dim)

AS = cross-sectional area of the shaft of a headed-stud (in2)

fy-hs = minimum yield stress of headed-stud (ksi)

In addition, it should be verified that:

AH ≥ 2.5 AS (11.12.6.2.4-3)

tSH ≥ 0.5 (DSH - DSC) (11.12.6.2.4-4)

where (see Figure 11.12.6.2.4-1):

AH = cross-sectional area of the stud head (in2)

tSH = head thickness (in)

DSH = diameter of the stud head (in)

DSC = diameter of the headed-stud shaft (in)

To provide sufficient anchorage, the length of the headed-studs shall extend beyond the mid-section of the facing, while maintaining 2 in. minimum cover.

When threaded bolts are used in lieu of headed-stud connectors, the effective cross-sectional area of the bolts must be employed in equations 11.12.6.2.4-2, 11.12.6.2.4-3 and 11.12.6.2.4-4. See FHWA-NHI-14-007/FHWA GEC 7 (Lazarte et al. 2015) for computation of the effective cross-sectional area of threaded anchors.


NEW SECTION

Figure 11.12.6.2.4-1—Geometry of a Headed-Stud

11.12.7 – Seismic Design of Soil Nail Walls

Soil nail walls shall be designed for seismic loads when applicable. Article 11.5.4.2 defines when earth-retaining structures require analysis for loads at the Extreme-Event I limit state. The criteria for MSE walls, another earth-supporting system regarded as flexible, is considered appropriate for soil nail walls.

C11.12.7

Soil nail seismic resistance factors, for use with MSE wall seismic design criteria, are presented in FHWA-NHI-14-007/FHWA GEC 7 (Lazarte et al. 2015).

11.12.7.1 – External and Global Stability External stability of soil nail walls for seismic

loading conditions shall be conducted as specified in Article 11.6.5 except as modified in Article 11.10.7.1 for MSE walls for sliding stability. For global stability, a horizontal seismic coefficient, per Article 11.6.5, shall be incorporated into the limit equilibrium analysis.

11.12.7.2 – Internal Stability Internal stability of soil nail walls for seismic

loading conditions shall be performed by incorporating a horizontal seismic coefficient, per Article 11.6.5, into the limit equilibrium analysis to quantify nominal soil nail tensile load under seismic loading. Resistance factors for internal stability for seismic conditions shall be as follows:

• Bar pullout – 0.65 • Bar rupture

C.11.12.7.2 Limit equilibrium slope stability analyses are used to

quantify the nominal tensile load under seismic loads in the soil nail bars.


NEW SECTION o Bar grades 60-75 (ASTM A615) –

0.75 o Bar grade 95/150 (ASTM A722) –

0.65 o Facing flexure resistance – 0.90 o Facing punching shear – 0.90 o Headed stud in tension

A307 steel bolt – 0.65 A325 bolt – 0.75

11.12.8 – Corrosion Protection

For all permanent soil nail walls and, in some cases, for temporary walls, the soil corrosion potential shall be evaluated and considered part of the design.

Soils shall typically be considered nonaggressive if the soils meet all of the following criteria:

• pH = 5 to 10 • Resistivity ≥ 3,000 ohm-cm • Chlorides ≤ 100 ppm • Sulfates ≤ 200 ppm • Organic Content ≤ 1 percent by weight

Soils not meeting one or more of these criteria shall be considered to be aggressive. If the resistivity is greater than or equal 5,000 ohm-cm, the chlorides and sulfates requirements may be waived.

The primary corrosion protection depends on the aggressiveness of the in-situ soils. Two classes of corrosion protection shall be used.

• Class A – Bar Encapsulation • Class B – Epoxy coated or galvanized bars

Class A bar encapsulation shall consist of providing either a 40-mil thickness PVC or 60-mil thickness HDPE corrugated sheath. The sheath shall have a minimum diameter of the bar plus 0.4 inches. Place the sheath around the bar and grout the annular space between the bar and the sheath. The grout cover shall be a minimum of 0.2 inches on all sides of the bar. See Figure 11.12.8-1.

C11.12.8 An in depth discussion on corrosion of metallic

components used to construct soil nail walls is provided in FHWA-NHI-14-007/FHWA GEC 7 Soil Nail Walls (Lazarte et al., 2015).

The criteria provided for assessing corrosion potential are based on Elias et al., 2009.

Recommended test methods for soil chemical property determination include AASHTO T-289 for pH, AASHTO T-288 for resistivity, AASHTO T-291 for chlorides, AASHTO T-290 for sulfates, and AASHTO T-267 for organics.

Resistivity should be determined under the most adverse condition (i.e., a saturated state) in order to obtain a resistivity that is independent of seasonal and variations in soil-moisture content (Elias et al., 2009).

A nonaggressive site does not mean that no corrosion will occur. Rather, for a nonaggressive site, a tolerable level of corrosion over the life of the structure should be expected, provided that the electro-chemical characteristics of the site do not change over the life of the structure.

Class A corrosion protection is used for sites that are

considered aggressive or if the electro-chemical environment is unknown or not adequately defined. Class B corrosion protection is used for sites that are considered nonaggressive. However, Class A corrosion protection may be used for nonaggressive sites at the discretion of the Owner.


NEW SECTION

Figure 11.12.8-1—Bar Encapsulation (Lazarte et al., 2015)


NEW SECTION Class B corrosion protection consists of epoxy coating and/or galvanization. The epoxy coating on the bars, plates, washers and nuts shall have a required thickness ranging from 7 to 12 mils.

The other Class B corrosion protection is galvanization (i.e., zinc coating). The minimum galvanization that should be applied is 2.0 oz/ft2 or 3.4-mil thickness. The galvanization of the bar shall adhere to the requirements of AASHTO M111 (ASTM A123). The plates, nuts and washers shall be galvanized to the requirements of ASTM A153.

The minimum grout cover around the bar shall be 1 inch to provide a secondary level of corrosion protection. This minimum cover shall account for the sag of the bar between centralizers.

The epoxy coating is applied as required in ASTM A775 or A934.

The epoxy coatings used on the bars are color-coded; green, gray or purple. The green epoxy coating meets ASTM A775 and is used due to its flexibility. Green epoxy coating is suitable for rebar that will be bent into different shapes. The gray and purple epoxy coatings are less flexible, but have greater chemical resistance. The purple epoxy coating is better suited for marine or harsh environments.

ASTM A123 requires a minimum average galvanization thickness of 3.4-mil for all bars on the project and a minimum of 3.1-mil on any individual bar. The zinc coating provides a sacrificial anode that corrodes while protecting the base metal.

The corrosion protection indicated applies only to customary rebar used as bars. The use of high strength bars requires project specific corrosion projection to be developed.

The use of sacrificial steel is not recommended as a corrosion protection method. However, if the Owner wants to use sacrificial steel as corrosion protection method, see Lazarte et al., 2015 for the procedure.

11.12.9 - Soil Nail Testing Testing of soil nails shall be conducted to verify

the bond strength, qu, and nominal load transfer rate, rpo, between the soil and soil nail. This testing shall consist of, as a minimum, verification tests conducted on sacrificial nails installed and tested prior to production nail installation, and proof tests conducted during production nail installation. At least one verification test should be conducted in each soil unit encountered in the wall using the same installation technique and nail inclination as will be used for the production nails. Regarding proof tests, enough tests should be conducted in each nail row to account for the variability of the soil and the installation process.

C11.12.9 During design, the bond strength should be estimated

in accordance with Article 11.12.5.2, and as further described in Article C11.12.5.2 and used as input to conduct the soil nail wall design, from which the nail spacing and length are determined. Using that bond strength, the nominal load transfer rate, rpo, in units of load/unit length of nail can be determined for use in the construction contract documents.

The actual bond strength and load transfer rate are strongly dependent on the soil nail installation process. Hence, the verification and proof testing are used to confirm that qu and rpo are obtained. Typically, a minimum of 5 percent of the nails in a nail row are proof tested, and for short rows, at least one nail should be proof tested. When selecting nails to proof test, consideration should be given to making sure that proof tests are conducted in all the geologic units. If some of the nails will be installed within ground water, some of the test nails located within the ground water should also be proof tested.

While the nominal load transfer rate applies to the entire nail length, it is most important that this load


NEW SECTION transfer rate is obtained behind the critical failure surface obtained from the limit equilibrium slope stability analyses used to design the wall. Test nails are usually installed to test a minimum bond length (typically 10 ft or more) to simulate the pullout resistance of the nail behind the critical failure surface. The bonded length of the test nail is grouted, while the remainder of the nail between the wall face and the nail bonded zone is left open. The bonded length of test nail should begin far enough away from the wall face such that the load carried by the grout-soil interface in the bonded zone is not unduly influenced by the presence of the reaction frame placed against the wall face during nail loading. Typically, a minimum of 3 to 5 ft between the wall face and the front of the bonded zone is required to accomplish this. If the portion of the test nail that is left open has a tendency to cave, a weak PVC casing can be used to hold the hole open and removed once testing is complete, at which point the open portion of the test nail is filled with a non-structural filler and is abandoned if the nail is sacrificial or filled with the grout used for the bonded zone if the nail is a production (i.e., permanent) nail.

It is best if proof tests are conducted on non-production nails placed between production nails (either between nail rows or between nails within a row) to avoid complications that could arise due to:

• the need to have an unbonded (i.e., ungrouted)

length of nail in front of the bonded zone that must be grouted later, and

• the need for an increased bar size since the nails are tested to a load that is significantly greater than the nail design load.

If it is desired to proof test production nails, the above should be considered when setting up the construction contract.

The load increments used in the verification and proof testing schedules are based on the nail design load, DL, determined as follows: DL = φpo x rpo x Lb (C11.12.9-1) where: φpo = nail pullout resistance factor rpo = nominal load transfer rate, and Lb = length of test nail bonded zone


NEW SECTION For verification tests, the test nails should be loaded

to twice the nail design load (i.e., 2.0xDL), applied in increments of 25% of the test nail design load. This will take the verification test nail to near the nominal pullout resistance of the test nail bonded zone. Each load increment is held for 10 minutes, except for the load equal to 1.5xDL, which is held for 60 minutes. The purpose of the 60 minute load hold is to assess the likely long-term performance of the nail to insure that it will safely carry the design load throughout the wall design life with minimal creep displacements.

For proof tests, the test nails should be loaded to 1.5xDL the nail design load, applied in increments of 25% of the test nail design load. This test is primarily focused on the consistency of the nail performance as production nails are installed, including consistency with the verification nail performance located in the same geologic unit as the considered production nails. Each proof test load increment is held for 5 minutes, except for the load equal to 1.5DL, which is held for 10 minutes.

11.12.10 - Drainage

The provisions of Article 3.11.3 shall apply. Surface water runoff and groundwater shall be

controlled both during and after construction of the soil nail wall. Soil nail walls shall contain sufficient drainage elements such that:

• No build-up of hydrostatic forces occurs between the wall facing and excavation face.

• Pore water pressures from perched water or groundwater do not negatively affect nail performance.

If perched groundwater or the groundwater table is anticipated to be above the base of the wall, horizontal or sub-horizontal drains should be used to reduce the effect of pore pressure on the stability of the wall and any impacts on the long-term performance and durability of the nails. If horizontal drains are not practical to use, then the wall shall be designed for the long-term presence of ground water in the wall, considering both wall stability and the effect of long-term water exposure on the life of the soil nails and connection to the facing.

C11.12.10

See Article C3.11.3. Also see FHWA-NHI-14-007/FHWA GEC 7 (Lazarte et al. 2015) for further guidance about methods to control drainage behind soil nail walls.

A concrete-lined diversion ditch should be located at the top of the wall to control surface water runoff toward the wall face to limit infiltration into the retained soil. A diversion ditch may not be necessary if the area above the wall slopes away from the wall face.

Vertical geocomposite strip drains installed behind the initial facing and adjacent to the excavation face should be used to prevent the build-up of hydrostatic pressures behind the wall facing due to incidental water and minor surface infiltration. The strip drains should be fitted with drainage elements at the bottom of the wall that allow exit of the water from the strips to the outside of the wall. Vertical strip drains should not be solely relied upon to provide complete reduction of hydrostatic pressure if perched water or elevated groundwater is present. Vertical strip drains are typically spaced at one to two times the horizontal spacing of the nails, SH, with a spacing equal to SH being more common. Strip drain width and spacing are commonly selected based on judgment and experience; however, the designer should consider problems with sloughing of shotcrete if not enough soil face contact area is provided.


NEW SECTION To design a drainage system consisting of drilled

horizontal or sub-horizontal drains, a seepage analysis should be performed to determine the diameter, length and spacing of drilled horizontal drains. Typically, horizontal drains are installed horizontally or with mild upslope (5-10 degrees from horizontal).


ATTACHMENT D -2019 AGENDA ITEM 40


1

ATTACHMENT D – 2019 AGENDA ITEM 40 - T-15

LRFD Design Specifications – Section 11 (Walls, Abutments, and Piers)

2018 AGENDA ITEM as Approved by Bridge Committee 6-2018

(ARTICLES NOT RELEVANT TO THE PROPOSED CHANGES ARE NOT SHOWN)

11.12—SOIL NAIL WALLS

11.12.7.2—Internal Stability Internal stability of soil nail walls for seismic

loading conditions shall be performed by incorporating a horizontal seismic coefficient, per Article 11.6.5, into the limit equilibrium analysis to quantify nominal soil nail tensile load under seismic loading. Resistance factors for internal stability for seismic conditions shall be as follows:

• Bar pullout – 0.65 • Bar rupture

o Bar grades 75 60-80 (ASTM A615 AASHTO M31) – 0.75

o Bar grade 95/150 (ASTM A722 AASHTO M275) – 0.65

o Facing flexure resistance – 0.90 o Facing punching shear – 0.90 o Headed stud in tension

A307 steel bolt – 0.65 A325 bolt – 0.75

C.11.12.7.2 Limit equilibrium slope stability analyses are used

to quantify the nominal tensile load under seismic loads in the soil nail bars.

11.12.9—Soil Nail Testing Testing of soil nails shall be conducted to verify

the bond strength, qu, and nominal load transfer rate, rpo, between the soil and soil nail. This testing shall consist of, as a minimum, verification tests conducted on sacrificial nails installed and tested prior to production nail installation, and proof tests conducted during production nail installation. At least one verification test should be conducted in each soil unit encountered in the wall using the same installation technique and nail inclination as will be used for the production nails. Regarding proof tests, enough tests should be conducted in each nail row to account for the variability of the soil and the installation process.

C11.12.9 During design, the bond strength should be

estimated in accordance with Article 11.12.5.2, and as further described in Article C11.12.5.2 and used as input to conduct the soil nail wall design, from which the nail spacing and length are determined. Using that bond strength, the nominal load transfer rate, rpo, in units of load/unit length of nail can be determined for use in the construction contract documents.

The actual bond strength and load transfer rate are strongly dependent on the soil nail installation process. Hence, the verification and proof testing are used to confirm that qu and rpo are obtained. Typically, a minimum of 5 percent of the nails in a nail row are proof tested, and for short rows, at least one nail should be

2

proof tested. When selecting nails to proof test, consideration should be given to making sure that proof tests are conducted in all the geologic units. If some of the nails will be installed within ground water, some of the test nails located within the ground water should also be proof tested.

While the nominal load transfer rate applies to the entire nail length, it is most important that this load transfer rate is obtained behind the critical failure surface obtained from the limit equilibrium slope stability analyses used to design the wall. Test nails are usually installed to test a minimum bond length (typically 10 ft or more) to simulate the pullout resistance of the nail behind the critical failure surface. The bonded length of the test nail is grouted, while the remainder of the nail between the wall face and the nail bonded zone is left open. The bonded length of test nail should begin far enough away from the wall face such that the load carried by the grout-soil interface in the bonded zone is not unduly influenced by the presence of the reaction frame placed against the wall face during nail loading. Typically, a minimum of 3 to 5 ft between the wall face and the front of the bonded zone is required to accomplish this. If the portion of the test nail that is left open has a tendency to cave, a weak PVC casing can be used to hold the hole open and removed once testing is complete, at which point the open portion of the test nail is filled with a non-structural filler and is abandoned if the nail is sacrificial or filled with the grout used for the bonded zone if the nail is a production (i.e., permanent) nail.

It is best if proof tests are conducted on non-production nails placed between production nails (either between nail rows or between nails within a row) to avoid complications that could arise due to:

• the need to have an unbonded (i.e., ungrouted)

length of nail in front of the bonded zone that must be grouted later, and

• the need for an increased bar size since the nails are tested to a load that is significantly greater than the nail design load.

If it is desired to proof test production nails, the above should be considered when setting up the construction contract.

The load increments used in the verification and proof testing schedules are based on the nail design load, DL, determined as follows:

3

DL = φpo x rpo x Lb (C11.12.9-1) where: φpo = nail pullout resistance factor rpo = nominal load transfer rate, and Lb = length of test nail bonded zone

For verification tests, the test nails should be loaded

to twice the nail design load (i.e., 2.0xDL), applied in increments of 25% of the test nail design load. This will take the verification test nail to near the nominal pullout resistance of the test nail bonded zone. Each load increment is held for 10 minutes, except for the load equal to 1.5xDL, which is held for 60 minutes. The purpose of the 60 minute load hold is to assess the likely long-term performance of the nail to insure that it will safely carry the design load throughout the wall design life with minimal creep displacements.

For proof tests, the test nails should be loaded to 1.5xDL the nail design load, applied in increments of 25% of the test nail design load. This test is primarily focused on the consistency of the nail performance as production nails are installed, including consistency with the verification nail performance located in the same geologic unit as the considered production nails. Each proof test load increment is held for 5 minutes, except for the load equal to 1.5DL, which is held for 10 minutes.


ATTACHMENT E -2019 AGENDA ITEM 41


ATTACHMENT E – 2019 AGENDA ITEM 41 – T-15

NEW SECTION 34 of the LRFD Construction Specifications

34-1

Since all text is new, the text has not been underlined.

SECTION 34

SOIL NAILS 34.1—DESCRIPTION

This work shall consist of designing, furnishing,

installing, and testing permanent cement-grouted soil nails in accordance with these Specifications and the contract documents.

C34.1 Soil nails and ground anchors have many of the

same or similar materials, fabrication and installation requirements. Soil nails and ground anchors are both reinforcing elements typically grouted in drilled holes; however, ground anchors are stressed after installation (“post-tensioned”) and soil nails are passive elements that are tensioned when movement occurs at the nail heads.

Soil nails are generally unsuitable, or are more difficult and expensive to design and construct in caving and other unfavorable soil conditions. See See Lazarte, et al. (2015) for descriptions of unfavorable soil types and ground conditions.

Soil nails are most common in retaining walls but are also used for slope stabilization. This section only addresses soil nails; soil nail retaining wall construction is addressed in Article 7.6.5. For guidance on construction techniques such as use of hollow bars or pressure grouting see Lazarte, et al. (2015). Launched soil nails are not addressed in these Specifications.

34.2—WORKING DRAWINGS

At least four weeks before installation is to begin,

the Contractor shall submit to the Engineer for review and approval complete working drawings and design calculations describing the soil nail system or systems intended for use. The submittal shall include at least the following:

1) Soil nail tendon details with bar sizes and grades,

corrosion protection and encapsulation systems, centralizers and nail head assembly drawings.

2) Grout mix designs and nail material testing results and certifications.

3) Soil nail construction procedures including drilling and grouting methods and plans for controlling groundwater seepage or unstable holes.

4) Soil nail monitoring and testing methods with example construction records.

5) Test nail setup including bond lengths, recent calibration certificates and testing equipment information about frames and supports, jacks, load cells and gauges. The Engineer shall approve or reject the Contractor's

C34.2 The contract documents generally give the

Contractor considerable latitude in the selection of materials and method of installation that may be used; therefore, complete working drawings are required to control the work.

Soil nails are typically installed by a specialty

geotechnical contractor. Most agencies ensure that the contractor has sufficient soil nail experience by requiring prequalification based on a history of completed satisfactory soil nails, personnel qualifications and certifications, reference lists and equipment capabilities.

Working drawing preconstruction submittals for soil nail retaining walls should address soil nails and other wall components such as facing and drainage systems as well as wall construction procedures for excavating lifts, shooting shotcrete and forming facing.

working drawings within four weeks of receipt of a complete submittal. No work on soil nails shall begin until working drawings have been approved in writing by the Engineer. Such approval shall not relieve the Contractor of any responsibility under the contract documents for the successful completion of the work.

34.3—MATERIALS Soil nail materials shall be comprised of soil nail

tendons with corrosion protection elements and nail heads. Soil nail tendons shall be centered in drilled holes typically with centralizers and are grout-protected.

34.3.1—Soil Nail Components Soil nail tendons shall consist of deformed steel bars

conforming to the requirements of AASHTO M 275 (ASTM A722) or M 31 (ASTM A615). Nail head assemblies shall consist of hex nuts, washers, and bearing plates with shear studs welded to plates. Bearing plates shall be fabricated to meet the same requirements for ground anchors in Article 6.3.3, “Steel Elements”. Shear studs shall conform to the requirements of Article 11.3.3, “Welded Stud Shear Connectors”. Nuts and washers shall conform to the requirements of AASHTO M 291, Grade B.

C34.3.1 Typical maximum manufactured bar length is 60 ft

and soil nail tendons are generally continuous without splices or welds. If couplers are necessary to extend the length of bars, couplers should conform to the requirements of Section 10, “Prestressing”.

Typical bar grades used for soil nails include, for AASHTO M 31, grades 60 to 80, and for AASHTO M 275, grade 150.

34.3.2—Grout

Grout shall conform to the requirements for ground

anchors in Article 6.3.2, “Grout” and have a minimum compressive strength of 1,500 psi at 3 days and 4,000 psi at 28 days.

C34.3.2 Neat cement grout is more common than sand

cement grout for soil nails and admixtures are not normally used for most soil nail applications. If admixtures are used, they should not be mixed in quantities that exceed the Manufacturer's recommendations.

In addition to compressive strength, grout temperature, flow, water/cement ratio, and relative density/specific gravity may be specified for quality control of the grout. Typically, water/cement ratio for grout used in soil nails ranges from 0.4 to 0.5 in neat cement mixes, which is equivalent to a specific gravity of 1.8 to 1.9. See Lazarte, et al. (2015) for a discussion of grout parameters and the relationship between them as well as the effect these properties have on grout performance.

SECTION 34: SOIL NAILS

34-3

34.3.3—Corrosion Protection Elements The level of corrosion protection shall meet either

Class A or B selected in accordance with AASHTO LRFD Bridge Design Specifications, Article 11.12.8.

Bar encapsulation for Class A corrosion protection shall consist of one of the following sheathings:

• Corrugated high-density polyethylene (HDPE) tube having a minimum wall thickness of 60 mils and conforming to AASHTO M 252 requirements.

• Corrugated PVC tube having a minimum wall thickness of 40 mils.

Bar coatings for Class B corrosion protection shall consist of at least one of the following coatings:

• Fusion-bonded epoxy conforming to the requirements of AASHTO M 284 (ASTM A775) or ASTM A934.

• Galvanizing a minimum of 2 oz/ft2 conforming to the requirements of AASHTO M111.

C34.3.3 If the nail head assemblies will be exposed, all

components of the nail heads should be galvanized or epoxy coated. Otherwise, the need for corrosion protection on nail heads will depend on the environment and cover thickness from the facing. See Lazarte, et al. (2015) for further information on corrosion protection for nail heads.

34.3.4—Centralizers Centralizers shall conform to the requirements for

ground anchors in Article 6.3.5, “Miscellaneous Elements”.

34.4—FABRICATION

Tendons for soil nails may be either shop- or field-

fabricated from materials conforming to the requirements of Article 34.3, “Materials.” Tendons shall be fabricated as shown on the approved working drawings. Soil nails consist of production nails and test nails. Soil nail tendons for test nails shall be sized so that the maximum design or test load does not exceed 90 percent of minimum yield strength for Grade 60 through 80 bars and 80 percent of minimum ultimate tensile strength for Grade 150 bars.

C34.4 There is a potential for steel bars used as production

nails to be overstressed under higher test nail loads. Therefore, it may be necessary to use larger size or higher grade bars for test nails; shortening the bond lengths to less than the minimum required is not permitted in production nails.

34.4.1—Test Nails

Test nails consist of temporary unbonded and bond

lengths. Fully grouted soil nails shall not be tested. The Contractor shall determine the test nail bond length necessary to satisfy the load test requirements and the contract documents. The maximum test nail bond length shall be as calculated in Articles 34.5.5.2 and 34.5.5.3. The minimum test nail unbonded length shall be 3.0 ft or as indicated in the approved working drawings.

The minimum test nail bond length for nails with a total length 13.0 ft or more shall be 10.0 ft. The minimum test nail bond length for nails with a total length less than 13.0 ft shall not be less than the minimum length shown in the contract documents.

34.4.2—Production Nails and Nail Head Assemblies Production nails are fully grouted soil nails and shall

be the length shown in the approved working drawings based on the nail pullout resistance verified by load testing.

Soil nail tendons shall be encapsulated by a grout-filled corrugated plastic or deformed steel tube, or by a fusion-bonded epoxy coating. Encapsulation shall be fabricated and grouted as specified in Article 34.3.3. The soil nail tendon shall be centralized within the encapsulation and the tube sized to provide an average of 0.20-in. of grout cover for the steel bar.

Centralizers shall be placed along the soil nail tendon. They shall be located at 10.0 ft maximum centers and about 1.5 ft from each end of the nail.

Hex nuts, washers, and bearing plates with welded stud shear connectors shall be sized in accordance with the soil nail wall facing design described in AASHTO LRFD Bridge Design Specifications, Article 11.12.6.2. The size of nail head components shall not be less than that shown in the contract documents or on the approved working drawings.

34.4.3—Tendon Storage and Handling

Soil nail tendons shall be stored and handled in such

a manner as to avoid damage or corrosion. Damage to tendon steel bars as a result of abrasions, cuts, nicks, welds, and weld splatter will be cause for rejection by the Engineer. Grounding of welding leads to the steel bars is not permitted. A slight rusting, provided it is not sufficient to cause pits visible to the unaided eye, shall not be cause for rejection. Prior to inserting a soil nail tendon into the drilled hole, its corrosion protection elements shall be examined for damage. Any damage found shall be repaired in a manner approved by the Engineer.

Repairs to encapsulation shall be in accordance with the tendon Supplier's recommendations.

34.5—INSTALLATION

The Contractor shall select the drilling method and

grouting procedure to be used for the installation of the soil nails as necessary to satisfy the design and testing requirements.

34.5.1—Drilling

The drilling method used may be core drilling,

rotary drilling, percussion drilling, auger drilling, or driven casing. The method of drilling used shall prevent loss of ground above the drilled hole that may be detrimental to the soil nails or existing structures. Depending on the presence of and allowable movement for adjacent structures or nearby underground utilities, a monitoring plan may be required during soil nail installation.

Casing for nail holes, if used, shall be removed,

C34.5.1 The longitudinal axis of the drilled hole and that of

the soil nail tendon should be parallel. The bar should not be bent to accommodate connecting the bearing plate.

See Lazarte, et al. (2015) for additional information on the protection of structures and recommended monitoring methods.

Greater deviations from planned inclination and


34-5

unless permitted by the Engineer to be left in place. The location, inclination, and alignment of the drilled hole shall be as shown in the contract documents. Inclination and alignment shall be within ±3 degrees of the planned angle at the bearing plate, and within ±1.0 ft of the planned location at the ground surface (point of entry).

alignment may be needed in some cases due to unanticipated obstructions. In such cases, the impact of this greater deviation should be assessed by the wall designer.

34.5.2—Tendon Insertion

The soil nail tendon shall be inserted into the drilled

hole to the desired depth without difficulty. When the tendon cannot be completely inserted, it shall be removed and the drill hole cleaned or redrilled to permit insertion. Partially inserted bars shall not be driven or forced into the hole.

34.5.3—Grouting

The grouting equipment shall produce a grout free of lumps and undispersed cement. Grout shall be placed under gravity flow or with a nominal, low pressure for soil nails. The grout pump shall be equipped with a pressure gage to monitor grout pressures. The pressure gage shall be capable of measuring pressures of at least 0.150 ksi or twice the actual grout pressures used, whichever is greater. The grouting equipment shall be sized to enable the grout to be placed in one continuous operation. The mixer shall be capable of continuously agitating the grout.

Grout cubes in accordance with AASHTO T 106 are required for testing compressive strength. If required by the contract documents, grout temperature, density and flow shall be measured during grouting with at least the same frequency grout cubes are made. Density and flow field tests shall be performed in accordance with American National Standards Institute/American Petroleum Institute Recommended Practice 13B-1 (Section 4, Mud Balance) and ASTM C939 (Flow Cone), respectively.

Shortly after the soil nail tendon is placed, grout shall be injected by tremie methods through a grout pipe inserted to the end of the drill hole until grout fills the hole. Due to the fluidity of the grout and the inclination of the drill hole, the grout will not fill a space above the bottom elevation of the hole opening. This void is called the “bird’s beak” due to its shape and must be filled with either grout or shotcrete.

The bird’s beak will be larger for test nails to accommodate the temporary unbonded length. If the test nail is a production nail and the unbonded length cannot be satisfactorily grouted after testing, the test nail shall become sacrificial and be replaced with an additional production nail without additional cost to the Owner.

Upon completion of grouting, the grout tube may remain in the drill hole provided it is filled with grout. After grouting, test nails shall not be load tested for a minimum of three days.

C34.5.3 The bird’s beak is typically filled with additional

grout after placing a temporary cover over the drill hole opening or filled with shotcrete when shooting shotcrete. Since this area of the soil nail tendon is the most vulnerable to water infiltration and therefore corrosion, it is critical that it be completely filled to ensure full coverage and protection from moisture.

Typical testing frequency of grout cubes is one set

per day’s production for soil nails.

34.5.4—Nail Head Assembly As the shotcrete starts to cure when shooting

shotcrete, a bearing plate shall be placed over the soil nail tendon protruding from the drill hole and lightly pressed into the fresh shotcrete. Hex nuts and washers shall be installed to engage the nail head against the bearing plate. The hex nut shall be wrench-tightened within 24 hours of placing the bearing plate.

C34.5.4 The bearing plate requires suitable bearing against

the shotcrete face to transfer load. See Lazarte, et al. (2015) for additional information on suitable bearing.

34.5.5—Testing Designated soil nails shall be load-tested by the

Contractor using either the verification or proof test procedures specified herein. Testing shall consist of, as a minimum, verification tests conducted on sacrificial (non-production) nails installed and tested prior to production nail installation, and proof tests conducted during production nail installation.

C34.5.5

Considerations for selecting soil nail verification and proof test locations, number of tests to be conducted, and location of the test nail bonded zone with respect to soil units encountered, groundwater, and the anticipated location of the critical failure surface determined during soil nail wall design are provided in the LRFD Bridge Design Specifications, Article C11.12.9.

34.5.5.1—Testing Equipment Soil nail testing equipment consisting of dial gauges,

a jack, a pressure gauge and a load cell shall be used as specified in Article 6.5.5.1. The soil nail movement shall be measured by two dial gauges at each load increment and each time step. Each gauge shall be supported with respect to separate independent fixed reference points.

34.5.5.2—Verification Test The number of sacrificial nails verification-tested

shall be as indicated in the contract documents. At least one verification test should be conducted in each soil unit encountered using the same installation technique and nail inclination as will be used for the production nails. Verification testing shall be completed before installing production nails to confirm the appropriateness of the Contractor’s drilling and installation methods, and to verify the required bond strength.

The maximum bond length for verification testing, LBVTmax (ft), shall be taken as:

LBVTmax = (CRT × At × fs) / (rPO) (34.5.5.2-1) where: CRT = reduction coefficient, 0.9 for Grade 60 to 80

bar or 0.8 for Grade 150 bar At = cross-sectional area of the test tendon (in2) rpo = nominal load transfer rate (kips/ft) = (π × qu × Ddh) / 12 qu = bond strength (ksf) Ddh = drill hole diameter (in) fs = nominal resistance of the test tendon, fy or fu

(ksi), in which, fy = nominal yield resistance of the test tendon

(ksi) for Grade 60 to 80 bar

C34.5.5.2

See Article C11.12.5.2 in the LRFD Bridge Design Specifications for guidance in selecting qu.


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fu = minimum ultimate tensile strength of the test tendon (ksi) for Grade 150 bar If LBVTmax is less than 10.0 ft, then the size of the test

tendon shall be increased until LBVTmax is at least 10.0 ft. The test nail bond length for verification testing LBVT shall be between 10.0 ft and LBVTmax. The nominal bond strength qu shall be as determined in the LRFD Bridge Design Specifications, articles 11.12.5.2 and 11.12.9.

The verification test shall be made by incrementally loading the test nail in accordance with the following schedule unless a different maximum verification test load, VTL, or verification test schedule is indicated in the contract documents:

Table 34.5.5.2-1—Verification Test Schedule

Load Observation Period, min AL Apply AL

and set dial gauges to “zero” 0.13VTL 10 (record movement at 1, 2, 5, 10) 0.25VTL 10 (record movement at 1, 2, 5, 10) 0.38VTL 10 (record movement at 1, 2, 5, 10) 0.50VTL 10 (record movement at 1, 2, 5, 10) 0.63VTL 10 (record movement at 1, 2, 5, 10) 0.75VTL

(Creep Test) 60 (record movement at

1, 2, 5, 6, 10, 20, 30, 50, 60) 0.88VTL 10 (record movement at 10) 1.00VTL 10 (record movement at 10)

AL Reduce to AL and record permanent set

VTL shall be taken as: VTL = LBVT × rpo (kips/ft) × 0.75 (34.5.5.2-1) where: VTL = Verification Test Load (kips) LBVT = verification test nail bond length (ft) rpo is as defined in Article 34.5.5.2. AL shall be taken as: AL ≤ 0.025VTL (34.5.5.2-2) where, AL = Alignment Load (kips)

Each test load in a verification test shall be held for

10 min except the 0.75VTL load increment which shall be creep tested for 60 min. The jack shall be repumped as necessary in order to maintain a constant load. The load-hold period shall start as soon as each test load is applied, and the soil nail movement shall be recorded at each load increment and each time step in Table

34.5.5.2-1. A graph shall be constructed showing a plot of soil

nail movement versus load for each load increment and a plot of soil nail movement versus time load is held for each creep test. Graph format shall be approved by the Engineer prior to use.

34.5.5.3—Proof Test The number of soil nails proof-tested shall be equal

to at least five percent of production nails per nail row or a minimum of one test nail per production row, whichever is greater. The number and location of proof tests to be conducted in each nail row should be sufficient to account for the variability of the soil and installation techniques.

The maximum bond length for proof testing, LBPTmax (ft), shall be taken as: LBPTmax = (CRT × At × fs) / (rpo × 0.75) (34.5.5.3-1) where, CRT, At, fs, fy, fu and rpo are as defined in Article 34.5.5.2.

The test nail bond length for proof testing LBPT shall be 10.0 ft or LBPTmax, whichever is less. The value of rpo for the proof tests shall be equal to the value of rpo verified through the verification tests for the anticipated soil units to be encountered.

The proof test shall be made by incrementally loading the test nail in accordance with the following schedule unless a different maximum proof test load, PTL, or proof test schedule is indicated in the contract documents:

Table 34.5.5.3-1—Proof Test Schedule

Load Observation Period, min AL Apply AL

1 (and set dial gauges to “zero)

0.17PTL Record movement when it stabilizes 0.33PTL Record movement when it stabilizes 0.50PTL Record movement when it stabilizes 0.67PTL Record movement when it stabilizes 0.83PTL Record movement when it stabilizes 1.0PTL

(Creep Test) 10

(record movement at 1, 2, 5, 6, 10) AL Reduce to AL

and record permanent set

PTL shall be taken as: PTL = LBPT × rpo (kips/ft) × 0.75 (34.5.5.3-2) where: PTL = Proof Test Load (kips) LBPT = proof test nail bond length (ft)

C34.5.5.3 Proof tests may be conducted on production nails or

sacrificial (non-production) nails placed in between production nails (either between nail rows or between nails within a row) to avoid complications that could arise due to the following:

• Unbonded (i.e., ungrouted) length of nail in front of the bonded zone that must be grouted later

• Increased bar size since test nails are tested to a load that is significantly greater than the nail design load

If it is desired to proof test production nails, the above should be considered when preparing the contract documents.


34-9

rpo is as defined in Article 34.5.5.2. AL shall be taken as: AL ≤ 0.025PTL (34.5.5.3-3) where, AL = Alignment Load (kips) PTL = Proof Test Load (kips)

Each test load in a proof test shall be held until

movement stabilizes except the 1.0PTL load increment which shall be creep tested for 10 min or 60 min. If the soil nail movement measured for the creep test between 1 min and 10 min exceeds 0.04 in., the PTL shall be maintained for 50 more minutes and additional movement recorded at 20 min, 30, 50, and 60 min.

The jack shall be repumped as necessary in order to maintain a constant load. The load-hold period shall start as soon as each test load is applied, and the soil nail movement shall be recorded at each load increment and each time step in Table 34.5.5.3-1.

A graph shall be constructed showing a plot of soil nail movement versus load for each load increment and a plot of soil nail movement versus time load is held for each creep test. Graph format shall be approved by the Engineer prior to use.

If the soils that are nailed are relatively

susceptible to deformation or creep, each load increment should be held for 10 min and soil nail movement recorded at 1 min, 2, 5, and 10 min.

Note that the verification test nail bond length may not be the same as the proof test nail bond length.

34.5.5.4—Soil Nail Load Test Acceptance Criteria

Pullout shall be defined as the load at which attempts to

further increase the test load increment results in continued test nail movement. A verification-tested or proof-tested soil nail shall be deemed acceptable if:

• The soil nail resists the creep test load with less than

0.04 in. of movement between 1 min and 10 min; and • If the creep test load is maintained for 60 min, the soil

nail resists the creep test load with less than 0.08 in. of movement between the 6 min and 60 min; and

• The creep rate is linear and decreasing throughout the creep test observation period; and

• Total soil nail movement at the maximum test load exceeds 80 percent of the theoretical elastic elongation of the temporary unbonded length; and

• Pullout does not occur before achieving the maximum test load. When a verification-tested nail fails, the Contractor

shall modify the design and/or the installation procedures and install replacement sacrificial nails for verification testing.

When a proof-tested nail fails, the production nail shall be rejected. Additional proof testing, at the discretion of the Engineer, may be required to delineate the area of unsatisfactory production nails. The Contractor shall evaluate and modify the design and construction procedures as applicable.

Any modification which requires changes to the structure shall be approved by the Engineer. Any modifications of design or construction procedures shall be without additional cost to the Owner and without extension of the contract documents time.

C.34.5.5.4 Modifications may include, but are not limited to,

installing replacement soil nails, modifying the installation methods, and/or revising the soil nail design by increasing the bond length or reducing the design load with more soil nails spaced closer together.

34.6—MEASUREMENT AND PAYMENT

Soil nails will be measured and paid for by the

number of units installed and accepted as shown in the contract documents or ordered by the Engineer. No change in the number of soil nails to be paid for will be made because of the use by the Contractor of an alternative number of soil nails.

C34.6 Some agencies prefer to pay for verification and

proof tests separately to avoid the uncertainty of testing costs. Local experience will determine the desirability of such separate pay clauses.

The contract unit price paid for soil nails shall include full compensation for furnishing all labor, materials, tools, equipment, and incidentals, and for doing all the work involved in installing the soil nails (including testing), complete in place, as specified in these Specifications, the contract documents, and as directed by the Engineer.

Only changes to this article shown, i.e., edited references and new references only:

34.7—REFERENCES AASHTO. AASHTO LRFD Bridge Design Specifications, Ninth Edition, LRFD-89. American Association of State Highway and Transportation Officials, Washington, DC., 2020.


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Lazarte, C A., Robinson, H., G�́�𝑜mez, J. E., Baxter, A., Cadden, A., and Berg, R., 2015. GEC 7 Soil Nail Walls Reference Manual, Publication No. FHWA-NHI-14-007. US Department of Transportation Federal Highway Administration, Washington, DC, 425 pp.


ATTACHMENT F -2019 AGENDA ITEM 41


1

ATTACHMENT F – 2019 AGENDA ITEM 41 - T-15

LRFD Construction Specifications – Section 7 (Earth Retaining Systems)

(Articles not relevant to the proposed changes are not shown)

7.3—MATERIALS

7.5.4—Geocomposite Drainage Systems

Geocomposite drainage systems shall be installed at

the locations shown in the contract documents or on the approved working drawings. The geocomposite drainage material shall be placed and secured tightly against the excavated face, lagging, or back of wall as specified in the contract documents. When concrete is to be placed against geocomposite drainage material, the drainage material shall be protected against physical damage and grout leakage.

7.5.5—Horizontal Drains

Horizontal or sub-horizontal drains consisting of a

PolyVinyl Chloride (PVC) pipe with slotted perforations in section that will intercept water shall be constructed as specified in the contract documents or on the approved working drawings.

7.6—CONSTRUCTION The construction of earth-retaining systems shall

conform to the lines and grades indicated in the contract documents, on the working drawings, or as directed by the Engineer.

7.6.5—Soil Nail Walls

Soil nail walls shall consist of a combination of a

permanent facing and shotcrete facing, or only shotcrete facing, connected to soil nails.

7.6.5.1—Excavation The ground contour above the wall shall be

established to its final configuration and slope as shown in the Plans prior to beginning excavation of the soil for the first row of soil nails.

Soil nail walls shall be constructed by excavating from the top down in staged horizontal lifts. The excavation shall be limited to no more than 3.0 ft below the elevation at which the soil nails will be installed for the current lift. The reinforced shotcrete facing shall be placed prior to the vertical cut becoming unstable but no more than 24 hours. A lift shall not be excavated until the

C7.6.5

See Lazarte, et al. (2015) for additional information

on the components of soil nail walls. While cast–in-place concrete facing is commonly used for soil nail walls, other facings have been used.

2

nail installation and reinforced shotcrete placement for the preceding lift has been completed and accepted.

After a lift is excavated, the cut surface shall be cleaned of all loose materials, mud, rebound, and other foreign matter that could prevent or reduce shotcrete bond. The accuracy of the ground cut shall be such that the required thickness of shotcrete can be placed within a tolerance of plus or minus 2.0 in. from the defined face of the wall, and over excavation does not damage overlying shotcrete sections by undermining or other causes.

If an excavation becomes unstable at any time, soil nail wall construction shall be suspended and the excavation shall be temporarily stabilized by immediately placing an earth berm up against the unstable excavation face.

7.6.5.2—Soil Nails Soil nails shall be constructed in conformance with

the requirements of Section 34 “Soil Nails”. 7.6.5.3—Drainage Components Geocomposite drainage systems and horizontal or

subhorizontal drains shall conform to the requirements of Articles 7.5.4 and 7.5.5, respectively.

7.6.5.4—Shotcrete and Concrete Facing Shotcrete for stabilizing lifts and permanent shotcrete

facing shall be constructed in conformance with the requirements of Section 24, “Pneumatically Applied Mortar”. Cast-in-place concrete for permanent concrete facing shall be constructed in conformance with the requirements of Section 8, “Concrete Structures”.

The height and length of an excavation will depend

on how long the soils can vertically stand unsupported as well as the requirements shown on the approved working drawings or contract documents.

See Lazarte, et al. (2015) for further details about excavating for soil nail walls and construction methods for soils that do not have sufficient stand up time.

7.7—MEASUREMENT AND PAYMENT Unless otherwise designated in the contract

documents, earth-retaining systems shall be measured and paid for by the square foot. The square foot area for payment shall be based on the vertical height and length of each section built, except in the case when alternative earth-retaining systems are permitted in the contract documents. When alternative earth-retaining systems are permitted, the square foot area for payment will be based on the vertical height and length of each section of the system type designated as the basis of payment whether or not it is actually constructed. The vertical height of each section shall be taken as the difference in elevation on the outer face, from the bottom of the lowermost face element for systems without footings, and from the top of footing for systems with footings, to the top of the wall, excluding any barrier.

The contract price paid per square foot for earth-retaining systems shall include full compensation for furnishing all labor, materials, tools, equipment, and incidentals, and for doing all the work involved in constructing the earth-retaining systems including, but not limited to, earthwork, piles, footings, and drainage systems, complete in place, as specified in the contract

3

documents, in these Specifications and as directed by the Engineer.

Full compensation for revisions to drainage system or other facilities made necessary by the use of an alternative earth-retaining system shall be considered as included in the contract price paid per square foot for earth-retaining system and no adjustment in compensation will be made therefore.

7.8—REFERENCES AASHTO. 2008. AASHTO Guide Specifications for Highway Construction, Ninth Edition, GSH-9, American Association of State Highway and Transportation Officials, Washington, DC. AASHTO. 2009. Standard Specifications for Transportation Materials and Methods of Sampling and Testing, 29th Edition, HM-29, American Association of State Highway and Transportation Officials, Washington, DC. GSA. 1966. Adhesive, Bonding Vulcanized Rubber to Steel, Federal Specification MMM-A-121, U.S. General Services Administration. Lazarte, C. A., H. Robinson, J. E. G�́�𝑜mez, A. Baxter, A. Cadden, and R. Berg. 2015. GEC 7 Soil Nail Walls Reference Manual, Publication No. FHWA-NHI-14-007. US Department of Transportation Federal Highway Administration, Washington, DC, 425 pp.

aashto committee on bridges and structures … · • a soil nail wall is an earth-retaining system...

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