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© 2012, American Railway Engineering and Maintenance-of-Way Association 8-28-1

Part 28

Temporary Structures for Construction

— 2015 — TABLE OF CONTENTS

Section/Article Description

28.1 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Page

. . . . . . . . . . . . . 8-28-2 28.1.1 Scope (2015) . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28-2

28.1.2 Criteria (2002) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28-2 28.1.3 Qualifications (2015). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28-2 28.1.4 Responsibility (2015) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

28.1.5 Types of Temporary Structures (2015) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28-3

28.2 Information Required . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28-4 28.2.1 Field Surveys and Records (2002). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28-4 28.2.2 Soil Investigation (2002) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28-4 28.2.3 Loads (2002) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28-4 28.2.4 Drainage (2002) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28-4 28.2.5 Soil Properties (2002) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28-4

28.3 Computation of Lateral Forces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28-5

28.4 Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28-5

28.5 Design of Shoring Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28-5 28.5.1 Design of Cantilever Sheet Pile Walls (2015) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28-5 28.5.2 Design of Anchored Sheet Pile Walls (2002) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28-7 28.5.3 Design of Cantilever Soldier Beam with Lagging Walls (2015). . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28-7 28.5.4 Design of Anchored Soldier Beam with Lagging Walls (2015) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28-9 28.5.5 Design of Braced Excavations (2002) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28-11 28.5.6 Design of Cofferdams (2015) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28-11

28.6 Design of Falsework Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28-15 28.6.1 Review and Approval of Falsework Drawings (2015) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28-15 28.6.2 Design Loads (2015) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28-16 28.6.3 Design Stresses, Loadings, and Deflections (2015). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28-17 28.6.4 Special Conditions (2015) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28-18 28.6.5 Falsework Construction (2015) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28-19 28.6.6 Removing Falsework (2015) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28-19 Commentary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28-20

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Concrete Structures and Foundations

© 2012, American Railway Engineering and Maintenance-of-Way Association

8-28-2 AREMA Manual for Railway Engineering

LIST OF FIGURES

Figure Description Page 8-28-1 Apparent Earth Pressure Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-28-9

SECTION 28.1 GENERAL

28.1.1 SCOPE (2015)

a. This Part provides a recommended practice for the design of the most commonly used temporary structures. Other types of temporary structures may be used with the approval of the Engineer. This Part is intended for SERVICE LOAD DESIGN only.

b. Temporary structures are defined as those structures used to facilitate the construction of a permanent structure. The temporary structures addressed by this Part are primarily shoring and falsework systems. This Part is intended for evaluating earth pressure loading, tieback anchor design, wall design, stability considerations and corrosion protection requirements.

c. All temporary structures anticipated to be in service for more than an 18-month period are not within the scope of this Part.

d. Temporary bridges to carry railroad traffic shall be designed as permanent structures and are not included in this Part.

28.1.2 CRITERIA (2015)

a. All temporary structures shall be designed and constructed to provide safe support and adequate rigidity for the loads imposed.

b. All temporary structures shall be constructed with minimal interference to the operating tracks.

28.1.3 QUALIFICATIONS (2015) The performance of temporary support structures is strongly influenced not only by the methods and materials used but also the experience of the constructor. The constructor should be able to show sufficient expertise, through past projects and experience and in addition, be able to demonstrate that proper design capabilities are available and will be used for the project, as required.

28.1.4 RESPONSIBILITY (2015)

a. The Contractor shall be solely responsible for the design, construction and performance of a temporary structure unless it is provided by others.

b. Designs completed by the Contractor shall be submitted to the Engineer, including working drawings and design calculations for the temporary structures. The drawings and calculations shall be signed and sealed by a Registered Professional Engineer. The temporary structure(s) shall follow the lines, grades and location as shown on the plans. The temporary structure(s) shall be designed to conform to the right-of-way and easement restrictions provided and shall protect facilities and utilities shown on the plans or known to exist.

c. Review by the Engineer of the Contractor’s designs and working drawings shall in no way relieve the Contractor of full responsibility for the temporary structure, or its effect upon other adjacent facilities.

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AREMA Manual for Railway Engineering 8-28-3

28.1.5 TYPES OF TEMPORARY STRUCTURES (2015)

28.1.5.1 Shoring Systems

a. A cantilever sheet pile wall is a structure designed to provide lateral support for a soil mass and derives stability from passive resistance of the soil in which the sheet pile is embedded.

b. An anchored sheet pile wall is a structure designed to provide lateral support for a soil mass and derives stability from passive resistance of the soil in which the sheet pile is embedded and the tensile resistance of the anchor. Anchors may be cement-grouted tiebacks or other types of anchors acceptable to the Engineer.

c. A cantilever soldier beam with lagging wall is a structure designed to provide lateral support for a soil mass and derives stability from passive resistance of the soil in which the soldier beam is embedded. Soldier beams include steel H-piles, wide -flange sections or other fabricated sections that are driven or set in concrete in drilled holes. Lagging refers to the members spanning between soldier beams.

d. An anchored soldier beam and lagging wall is a structure designed to provide lateral support for a soil mass and derives stability from passive resistance of the soil in which the soldier beam is embedded and the tensile resistance of the anchors. Anchored soldier beam with lagging walls are generally designed as flexible structures which have sufficient lateral movement to mobilize active earth pressures and a portion of the passive pressure.

e. A braced excavation is a structure designed to provide lateral support for a soil mass and derives stability from passive resistance of the soil in which the vertical members are embedded and from the structural capacity of the bracing members. The vertical members of the braced excavation system include steel sheet piling or soldier beams comprised of steel H-piles, wide -flange sections, or other fabricated sections that are driven or installed in drilled holes. Wales are horizontal structural members designed to transfer lateral loads from the vertical members to the struts. Struts are structural compression members that support the lateral loads from the wales.

f. A cofferdam is an enclosed temporary structure used to keep water and soil out of an excavation for a permanent structure such as a bridge pier or abutment or similar structure. Cofferdams may be constructed of timber, steel, concrete or a combination of these. This Part considers cofferdams primarily constructed with steel sheet piles.

28.1.6 Falsework

a. Falsework is defined in general terms as a temporary construction work on which a main or permanent work is wholly or partially supported until it becomes self-supporting.

b. Falsework for roll-in/roll-out construction methods is not covered in this Part.

28.1.6.1 Types of Falsework Systems

a. Conventional falsework typically consists of timber posts and caps, timber bracing, and either timber or steel stringers and timber joists. Foundation support is usually provided by timber pads or sills set on the surface of the ground, although poor soil conditions may require the use of concrete footings, or by steel sills designed to distribute the loads to adequate timber pads or cribbing.

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b. Large-diameter, typically 20 inches (500 mm) or more, welded steel pipe columns are occasionally used to support steel caps and girders. When properly braced, pipe columns may provide an economical design when falsework is tall and spans are long.

c. Patented steel shoring typically consists of individual components that may be assembled into modular units and erected in place to make any desired falsework configuration. When erected, the shoring consists of a series of internally-braced steel towers which, either directly or through a cap system, support the load-carrying members.

d. Depending on load-carrying capacity, steel shoring systems are classified as pipe-frame shoring, heavy-duty shoring or intermediate strength shoring. For bridge falsework the use of pipe-frame shoring is limited to installations where tower leg loads do not exceed 11 kips (49 kN). In contrast, a properly designed heavy-duty shoring system will be capable of supporting loads of 100 kips (445 kN) per tower leg. Intermediate strength shoring will have a load carrying capacity of up to 25 kips (111 kN) per tower leg. Typically, timber caps and stringers are used with pipe-frame intermediate strength systems, whereas rolled-beams or welded plate girders will be more economical for the longer spans which are possible with heavy-duty shoring. Pipe-frame shoring is usually supported on timber pads; however, the larger leg loads associated with heavy-duty shoring will require, depending on soil conditions, solid timber cribbing or reinforced concrete footings.

SECTION 28.2 INFORMATION REQUIRED

28.2.1 FIELD SURVEYS AND RECORDS (2002)

Sufficient information shall be furnished in the form of profiles and cross sections, or topographical maps to determine general design and structural requirements. Existing and proposed grades and alignment of tracks and roads shall be indicated together with records of reference datum, maximum and minimum high water, minimum and mean low water, existing ground water level, location of utilities, construction history of the area, indication of any conditions which might hamper proper installation of the piling, soldier beams, ground anchors, depth of scour, allowance for over dredging, and wave heights.

28.2.2 SOIL INVESTIGATION (2002)

The characteristics of the foundation soils shall be investigated as indicated in Part 22, this Chapter with the investigation being done specifically for the temporary structure being designed.

28.2.3 LOADS (2002)

Loads shall be as indicated in Part 20, Article 20.2.3, this Chapter.

28.2.4 DRAINAGE (2002)

Drainage shall be as indicated in Part 20, Article 20.2.4, this Chapter.

28.2.5 SOIL PROPERTIES (2002)

Soil properties shall be determined and soils classified as indicated in Part 20, Article 20.2.5, this Chapter.


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SECTION 28.3 COMPUTATION OF LATERAL FORCES (2002)

Computation of lateral forces shall be as indicated in Part 20, Section 20.3, this Chapter.

SECTION 28.4 STABILITY (2015)

The stability of the system shall be investigated as indicated in Part 20, Section 20.4, this Chapter or as indicated in Article 28.5.6.5 for cellular cofferdams.

SECTION 28.5 DESIGN OF SHORING SYSTEMS

28.5.1 DESIGN OF CANTILEVER SHEET PILE WALLS (2015)

28.5.1.1 Restrictions on Use1

a. Cantilever sheet pile walls shall not exceed 12 feet (3.7 m) in height and shall be used only in granular soils or stiff clays.

b. If used for shoring adjacent to an operating track the wall should be at least 10 feet (3 m) away from the centerline of track, and its maximum height should not exceed 10 feet (3 m).

28.5.1.2 Depth of Embedment2

a. The total depth of embedment D shall be determined as indicated in Part 20, Section 20.3 of this Chapter. The coefficient of passive resistance Kp shall be multiplied by 0.66 to provide a factor of safety of 1.5.

b. Conditions such as unrealistically short penetration requirements into relatively strong layers, potential for overall instability, scour or erosion shall be taken into account, and the depth of embedment increased to not less than the height of the wall.

28.5.1.3 Maximum Moment1 (2002)

Determine the depth at which the shear in the wall is zero by starting from the top of the wall and finding the point at which the areas of the driving and resisting pressure diagrams are equivalent. Calculate the maximum bending moment at the point of zero shear.

28.5.1.4 Allowable Stresses (2015)

The allowable stresses shall be determined on the following basis:

(1) Sheet Pile Section: 2/3 tensile yield strength for new steel. Allowable stresses shall be reduced depending on the extent of usage for reused material.

(2) All other structural material shall comply with applicable parts of the AREMA Manual.

28.5.2 DESIGN OF ANCHORED SHEET PILE WALLS (2002)

The design of anchored sheet pile wall systems shall be as indicated in Part 20, this Chapter. Requirements of



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Article 28.5.4.1, 28.5.4.2, 28.5.4.3, 28.5.4.5, and 28.5.4.6 shall be satisfied as applicable to the anchored sheet pile walls.

1 See Commentary




28.5.3 DESIGN OF CANTILEVER SOLDIER BEAM WITH LAGGING WALLS (2015)

28.5.3.1 Restrictions of Use1

a. Cantilever soldier beam with lagging walls shall not exceed 12 feet (3.7 m) in height and shall be used only in granular soils or stiff clays.

b. If used for shoring adjacent to an operating track, the wall should be at least 13 feet (4.0 m) away from the centerline of track, and its maximum height shall not exceed 8 feet (2.4 m).

28.5.3.2 Depth of Embedment2

a. The total depth of embedment D shall be determined using the guidelines given in Article 28.5.1.2 except that the pressure distribution on the soldier piles below the excavation elevation shall be adjusted based on their equivalent width. The equivalent width for passive pressure shall be assumed to equal the width of the soldier pile multiplied by a factor of 3 for granular soils and a factor of 2 for cohesive soils. The width of the soldier piles shall be taken as the width of the flange or diameter for driven sections and the diameter of the concrete-filled hole for sections encased in concrete. Also, when determining the passive pressure distribution on the soldier piles, a depth of 1.5 times the width of the soldier pile in soil, and a depth of one foot in rock below the excavation elevation shall not be considered in providing passive lateral support.

b. For conditions such as unrealistically short penetration requirements into relatively strong layers, the potential for overall instability, scour or erosion shall be taken into account, and the depth of embedment increased to not less than the height of the wall.

28.5.3.3 Maximum Moment

Determine the depth at which the shear in the soldier piles is zero by starting from the top of the wall and finding the point at which the areas of the driving and resisting pressure diagrams are equivalent. Calculate the maximum bending moment at the point of zero shear.

28.5.3.4 Allowable Stresses

Allowable stresses shall comply with applicable sections of this AREMA Manual.

(1) 28.5.3.5 Lagging

a. The design load on the lagging is the theoretical pressure computed to act on it. When arch action can form in the soil behind the lagging (e.g., in granular or stiff cohesive soils where there is sufficient space to permit the in -place soil to arch and the back side of the soldier piles bear directly against the soil) the moment computed based on simple end supports may be reduced by one third.

b. Well -compacted fill shall be provided behind the lagging.

1 See Commentary 2 See Commentary


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28.5.4 DESIGN OF ANCHORED SOLDIER BEAM WITH LAGGING WALLS (2015)

28.5.4.1

Design Criteria

a. The lateral earth pressures shall be computed as indicated below:

(1) For single tier anchored walls, lateral earth pressures shall be computed using Part 20, this Chapter.

(2) For masses which do not have a history of sliding, the magnitude of lateral pressures on multi-tiered anchored walls shall be computed following the guidelines on Figure 8-28-1.

(3) Refer to Part 20, this Chapter, for the application of live load surcharge.

Figure 8-28-1. Apparent Earth Pressure Diagrams

Braced Cuts in Sand

Braced Cuts in Soft and Medium Clay If then the larger of or Braced Cuts in Stiff Clay If then Where γ = unit weight of soil (lb/ft3) H = depth of excavation (ft) Ka = active earth pressure coefficient c = undrained cohesion (lb/ft2)

b. The width of the soldier beam shall be assumed to be equal to the width of the flange for driven sections and the shaft diameter of the drilled sections. The resultant passive resistance of a soldier beam assumes that passive resistance is mobilized across an equivalent width described in Article 28.5.3.2, Paragraph a. The effects of backfill compaction and surcharge loads applied to the surface behind the wall shall be considered in the design earth pressure. The design stresses shall be in accordance with the current edition of Chapter 15 of the Manual.


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c. The unbonded tendon length shall extend beyond the critical failure surface and be a minimum of 15 feet (4.6 m) in length. The critical failure surface starts at the bottom of the excavation. The wall-anchor system shall be checked for adequate stability. The overall stability of the earth mass being retained shall be checked and shall have a minimum factor of safety of 1.3.

28.4.5.2 Submittals

The drawings shall include all details, dimensions, cross-sections, and sequence of construction necessary to construct the wall. The drawings and calculations shall include, but not be limited to:

(1) A description of the tieback installation including drilling, grouting and stressing information;

(2) Anchor capacity, type of tendon, anchorage hardware, minimum unbonded lengths, minimum anchor lengths, angle of installation and tieback locations and spacings;

(3) Testing schedule and procedures for tiebacks;

(4) A section view indicating the elevation at the top and bottom of the wall and the centerline of track including all horizontal and vertical dimensions;

(5) A plan view of the wall indicating the offset from the construction and track centerlines to the face of the wall at all changes in horizontal alignment;

(6) All details for construction of drainage facilities associated with the wall clearly indicated;

(7) The relationship between existing and proposed utilities; and

(8) A top of rail monitoring plan.

Soldier Beam Installation

(1) Soldier beams may be installed by driving with impact or vibration hammers or set in predrilled holes and encased with concrete below subgrade elevation and with lean concrete backfill above subgrade elevation. Encasement below subgrade level shall be concrete with a minimum 28-day compressive strength of 3,000 psi (20.7 MPa). Methods and equipment used for soldier beam installations shall be determined by the Contractor. The effect on existing structures should be considered.

(2) For driven soldier beams, leads or spuds shall be centered in such a manner as to afford freedom of movement to the hammer and shall be rigged to hold the soldier beam and hammer in alignment during driving. The soldier beam shall be driven with equipment which will ensure a properly distributed hammer impact on the soldier beam and prevent damage while driving.

(3) For drilled-in soldier beams, side wall stability shall be maintained during drilling. If required by soil and water conditions, provide casing for hole excavation. Provide casing of sufficient strength to withstand handling stresses, lean concrete backfill pressure and surrounding earth and/or water pressure. Drilling mud may also be used to maintain side wall stability of soldier beam holes subject to the approval of the Railroad. Pump water from drill holes. Contractors may use tremie methods in lieu of pumping water.

Above subgrade elevation, the soldier beam shall be fully encased in lean concrete backfill after completion of soldier beam hole excavation. The soldier beam may be set prior to, or after, concrete placement at the option of the Contractor. Free fall lean backfill may be used. Vibrating of lean backfill mix is not required.

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(4) Soldier beams may be furnished in full-length sections or may be spliced according to the method of splicing as shown on the plans. Field welding will be allowed only in accordance with the requirements for welding as specified in AWS D1.1, except as amended on the plans.




(5) Structural welding of steel, steel reinforcement and soldier beams shall be made by personnel qualified to perform the type of welding involved in accordance with the qualification procedure of AWS D1.1 and D1.4, except as amended on the plans.

28.5.4.4 Anchors (Tiebacks)

a. Unless otherwise directed, the Contractor shall select the tieback type and the installation method, and determine the bond length, anchor length and anchor diameter in accordance with Article 20.5.5 Anchorages, this Chapter.

28.5.4.5 Allowable Stresses

a. Allowable stresses shall be in accordance with Article 20.5.7 this Chapter.

28.5.5 DESIGN OF BRACED EXCAVATIONS (2015) Braced excavations shall be designed using the apparent earth pressure diagram, based on soil type, shown in

Figure 8-28-1.

28.5.6 DESIGN OF COFFERDAMS (2015)

28.5.6.1 General

a. This section deals primarily with cofferdams constructed with steel sheet piles. This section applies to the case where the water level lies above the soil or rock level such as in rivers, lakes and bays.

b. A single-wall cofferdam consists of a single wall of sheet piling driven in the form of an enclosure. Single-walled cofferdams shall be designed as flexible sheet pile bulkheads or braced excavations.

c. A double-walled cofferdam consists of two rows of steel sheet piling driven parallel to each other and tied to each other with anchors and wales. Double wall cofferdams shall be designed similar to single-wall cofferdams. The two rows of sheet piles shall not be assumed to share equally in resisting the outside pressure unless concrete fill or rigid bracing is used between them. The use of double-wall cofferdams over single-wall cofferdams is usually to provide increased water tightness.

d. A cellular cofferdam consists of soil-filled interconnected circular or diaphragm cells constructed of steel sheet piling. Cellular cofferdams are designed as gravity retaining structures.

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28.5.6.2 Required Data

The required information about the site includes the following:

High water elevation

Velocity of water flow

Wave height and period Distance to existing piers and foundation type of existing piers

Ice conditions

Scour potential

Ground line cross-sections and profiles

Existing soil types, layer thicknesses, and properties

Properties of backfill materials Ground water conditions

Navigation and lighting requirements

Vessel impact potential

28.5.6.3 Design Stresses and Factors

of Safety

a. The maximum stresses for cofferdam materials shall be in accordance with Article 20.5.7, this Chapter. The minimum factors of safety for stability of cofferdams shall be 1.25. The factors of safety shall be calculated as the sum of the resisting forces or moments divided by the sum of the applied forces or moments. The factors of safety may be calculated on a unit length of cofferdam. An analysis shall be conducted to determine the stability of the bottom of the excavation.

28.5.6.4 Applied Forces

In determining the stability of cofferdams, the applied forces shall include the following as applicable:

Hydrostatic water pressure

Seepage force

Stream flow pressure

Wave forces

Active earth pressure

Vessel impact

Ice forces


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28.5.6.5 Design of Cellular Cofferdams

28.5.6.5.1 Equivalent Width

The stability of cellular cofferdams may be determined using an equivalent width. The equivalent width of a cofferdam is defined as the width of an equivalent rectangular section having an area equal to that of the actual cofferdam.

28.5.6.5.2 Saturation Line

The location of the line of saturation or phreatic surface within a cofferdam cell may be taken as a straight line sloping downward from the water surface level on the outboard side to the inboard side. The slope of the saturation line may be assumed as shown below. A horizontal line representing the average level of saturation may be assumed for stability calculations.

Cell Fill Material Slope (Horizontalto Vertical) Free draining coarse -grained 1 to 1

Silty coarse -grained 2 to 1 Fine -grained 3 to 1

28.5.6.5.3 Sliding

Cofferdams shall be investigated for sliding at the base. The resisting forces shall consist of the frictional resistance of the soil along the bottom of the cofferdam, the passive resistance of soil on the inboard face, and the passive resistance of a berm, if any, on the inboard face. The unit weight of the soil below the saturation line shall be the submerged unit weight.

28.5.6.5.4 Overturning

Cofferdams shall be investigated for overturning about the inboard toe. The resultant of the applied forces and the cell weight shall lie within the middle one-third of the cofferdam.

28.5.6.5.5 Piling Uplift

Cofferdams shall be investigated for uplift of the outboard piling. The moments shall be summed about the inboard toe. The resisting moments shall be those due to the frictional forces on the inner and outer surfaces of the outboard sheeting plus the effective passive resistance of the soil and berm, if any, on the outboard face. The weight of the cell fill shall not be used for resisting moment.

28.5.6.5.6 Vertical Shear

a. Cofferdam cells shall be investigated for vertical shear failure on the centerline of the cells. The total shearing force, Q, on the neutral plane at the centerline of the cell shall be as follows:

Q = 3M/2E

Q = total shearing force per unit length of cofferdam

M = net overturning moment per unit length of cofferdam

E = equivalent width of cofferdam



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b. The shearing force, Q, shall be resisted by vertical shear within the cell fill and friction in the interlocks of the sheeting. In computing the vertical shear resistance of the fill material, the coefficient of earth pressure K shall be as follows:

𝐾𝐾 = cos φ

2− cos φ

φ = angle of internal friction of cell fill

c. The total centerline shear force resistance of the cell fill per unit length of cofferdam shall be the resultant lateral force due to soil fill material multiplied by tan φ The frictional resistance of the sheet pile interlocks per unit length of cofferdam shall be the interlock tension multiplied by the coefficient of friction of the interlocks.

28.5.6.5.7 Horizontal Shear

a. Cofferdam cells shall be investigated for tilting failure through horizontal shear in the cell fill material. The resisting moments shall be those due to the lateral resistance of the cell fill, the frictional resistance of the sheet pile interlocks, and the passive resistance of the berm if one is used.

b. The lateral resisting moment, M, of the cell fill about the base of the cofferdam shall be:

M = 𝛾𝛾

(𝐻𝐻 –𝐸𝐸 𝑡𝑡𝑡𝑡𝑡𝑡 ) (𝐸𝐸 𝑡𝑡𝑡𝑡𝑡𝑡 )2

2 +

(𝐸𝐸 𝑡𝑡𝑡𝑡𝑡𝑡 )3

3

M = resisting moment per unit length of cofferdam

H = height of cofferdam

E = equivalent width of cofferdam

𝛾𝛾 = submerged unit weight of fill material

𝜙𝜙 = angle of internal friction of fill material

c. The resisting moment due to frictional resistance of the interlocks shall be the interlock tension multiplied by the coefficient of friction of the interlocks multiplied by the equivalent width of the cofferdam.

28.5.6.5.8 Interlock Tension

a. The hoop or interlock forces for circular cells and

connecting arcs shall be calculated by the following equation:

T = PR

M =

H =

E =

T =

P =

R =

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T = hoop or interlock force

P = maximum lateral pressure from cell fill and water

R = radius of circle or arc

b. The lateral pressure may be taken as maximum at 1/4 the height from the mudline to the top of the cofferdam.

c. The interlock force at the connection of arc to circular cell shall

be calculated by the following

equation:

Tc = PLsec 𝜃𝜃

Tc = hoop or interlock force at connection

P = maximum lateral pressure from cell fill and water

L = 1/2 the center-to-center distance of full circular

cells

𝜃𝜃 = angle between centerline of cells and a line from center of cell to point on cell periphery where connecting arc is attached.

d. The interlock tension shall not exceed the manufacturer's recommended values.

e. The maximum coefficient of friction of steel on steel at the interlocks shall not exceed 0.3.

28.5.6.6 Construction Requirements

a. Cofferdams for foundation construction shall be carried well below the bottom of the excavation or as far as the bottom of the excavation as conditions will permit and shall be well braced and as watertight as practical. The interior dimensions of cofferdams shall provide sufficient clearance inside the wales for constructing forms, driving piles, pumping outside the forms, and inspection.

b. Cofferdams which are tilted or moved out of position by any cause during the process of construction shall be righted or enlarged as necessary.

c. No bracing which will induce stress, shock, or vibration in the permanent structure will be permitted in cofferdams.

d. Cellular cofferdams with diaphragm walls shall be filled equally on each side of the diaphragm walls to avoid distortion of the cells.

e. After completion of the construction, the cofferdams with all sheeting and bracing shall be removed as directed by the Engineer or as shown on the plans. Such removal shall be done in a manner that will not disturb or mar the permanent structure.

Tc =

P =

L =

Φ

=

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SECTION 28.6 DESIGN OF FALSEWORK SYSTEMS

28.6.1 REVIEW AND APPROVAL OF FALSEWORK DRAWINGS 2015)

a. Falsework design drawings and calculations prepared by, or for an outside agency covering falsework adjacent to or over Railroad's operating tracks shall be certified to be complete and satisfactory to the submitting agency prior to being submitted to the Railroad.

b. There shall be sufficient detail in the drawings to permit a complete stress analysis. In particular, the drawings shall show the size of all load-supporting members; all lateral and longitudinal bracing, including connections; the method of adjustment; and similar design features.

c. All design-controlling dimensions shall be shown, including, but not limited to, beam length; beam spacing; post location and spacing; vertical distance between connectors in diagonal bracing; overall height of falsework bents; and similar dimensions critical to the analysis.

d. Minimum horizontal and vertical clearances to the centerline of all tracks, tops of rails and adjacent facilities shall be shown on the plans.

e. Where cast-in-place concrete will be supported by falsework, a diagram showing the placing sequence and construction

joint locations shall be provided. When a schedule of placing concrete is shown on the contract plans, no deviation will be permitted without the approval of the design engineer.

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e. When footing type foundations are to be used, the Contractor shall determine the bearing value of the soil and shall show the values assumed in the design of the falsework on the falsework drawings.

f. Anticipated total settlements of the falsework and forms shall be shown on the falsework drawings.

g. Falsework footings shall be designed to carry the load imposed upon them without exceeding the estimated soil bearing values and anticipated settlements. Refer to Part 3 of this Chapter for allowable soil pressures of various material and settlements.

h. When falsework will be supported on pile bents, the required pile capacity and the maximum allowable driving tolerances shall be shown.

i. The support systems for form panels supporting concrete deck slabs and overhangs on girder bridges shall also be considered to be falsework and designed as such.

j. The falsework drawings shall show all openings which are required through the falsework. Horizontal and vertical clearances shall be adequate and be shown on the plans.

k. Temporary bracing shall be provided to all falsework bents adjacent to the operating tracks, and shall be designed to withstand all imposed loads during erection, construction and removal. Wind loads shall be included in the design of such bracing.

l. In addition to the falsework drawings, the design engineer shall submit a copy of design calculations. The design calculations shall show the stresses and deflections of all load-supporting members. Calculations furnished by the design engineer are for information only, rather than for review and acceptance. Accordingly, design and/or construction details, which may be shown in the form of sketches with the calculation sheets, shall be shown on the falsework drawings as well; otherwise the drawings will not be considered complete.

28.6.2 DESIGN LOADS (2015)

a. The design loads for falsework shall consist of the sum of dead and live vertical loads, and the assumed horizontal load. The minimum total design load for any falsework shall be not less than 100 pounds per square foot (4.8 kPa) for the combined live and dead load regardless of slab thickness.

b. Dead load shall include the weight of concrete, reinforcing steel, forms and falsework. The weight (mass density) of concrete, reinforcing steel and forms shall be assumed to be not less than 160 pounds per cubic foot (2600 kg/m3 ) for normal concrete.

c. Live loads shall consist of the actual weight of equipment to be supported by the falsework applied as concentrated loads at the points of contact and a uniform load of not less than 20 pounds per square foot (960 Pa) applied over the area supported, plus 75 pounds per linear foot (1100 N/m) applied at the outside edge of deck overhangs.

d. The assumed horizontal load to be resisted by the falsework bracing system shall be the sum of the actual horizontal loads due to equipment, construction sequence or other causes and an allowance for wind, but in no case shall the assumed horizontal load to be resisted in any direction be less than 2 percent of the total dead load.

e. The falsework shall be designed so that it will have sufficient rigidity to resist the assumed horizontal load without considering the weight of the supported structure.

f. The minimum horizontal load to be allowed for wind on each heavy-duty steel shore having a vertical load carrying capacity exceeding 30 kips (133 kN) per leg shall be the sum of the products of the wind impact area, shape factor, and the applicable wind pressure value for each height zone. The wind impact area is the total projected area of all the

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elements in the tower face normal to the applied wind. The shape factor for heavy-duty shoring shall be taken as 2.2. Wind pressure values shall be determined from the following table:

WIND PRESSURE

Height Zone Shores Adjacent to At Other Locations Distance above ground Traffic Openings

0 to 30 ft. (0 to 9 m) 20 psf (960 Pa) 15 psf (720 Pa)

30 to 50 ft. (9 to 16 m) 25 psf (1200 Pa) 20 psf (960 Pa) 50 to 100 ft. (16 to 30 m) 30 psf (1440Pa) 25 psf (1200 Pa)

Over 100 ft. (30 m) 35 psf (1680 Pa) 30 psf (1440 Pa)

g. The minimum horizontal load to be allowed for wind on all other types of falsework, including falsework supported on heavy-duty shoring, shall be the sum of the products of the wind impact area and the applicable wind pressure value for each height zone. The wind impact area is the gross projected area of the falsework and any unrestrained portion of the permanent structure, excluding the areas between falsework posts or towers where diagonal bracing is not used. Wind pressure values shall be determined from the following table:

WIND PRESSURE VALUE

Height Zone For Members over and Bents Adjacent to At Other Locations (Feet above ground) Traffic Openings

0 to 30 (0 to 9 m) 2.0 Q psf (Pa) 1.5 Q psf (Pa)

30 to 50 (9 to 16 m) 2.5 Q psf (Pa) 2.0 Q psf (Pa) 50 to 100 (16 to 30 m) 3.0 Q psf (Pa) 2.5 Q psf (Pa)

Over 100 (30 m) 3.5 Q psf (Pa) 3.0 Q psf (Pa)

The value of Q in the above tabulation shall be determined as follows:

Q = 1 + 0.2W ( Q = 48(1 + .656W) ); but shall not be more than 10 (480)

In the preceding formula, W is the width of the falsework system in feet (meters), measured in the direction of the wind force being considered.

h. The entire superstructure cross-section, except railing, shall be considered to be placed at one time. If the concrete is to be prestressed, the falsework shall be designed to support any increased or readjusted loads caused by the prestressing forces.

28.6.3 DESIGN STRESSES, LOADINGS, AND DEFLECTIONS (2015)

a. The maximum allowable design stresses and loadings listed are based on the use of undamaged, high-quality structural grade material. Stresses and loadings shall be reduced by the design engineer if lesser quality materials are to be used.

b. The maximum allowable stresses, loadings and deflections used in the design of the falsework shall be as follows:







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28.6.3.1 Timber

a. Allowable stresses shall be in accordance with Chapter 7 – Timber Structures, Part 2 – Design, or Chapter 7, Appendix 2 – Temporary Structures, of this Manual.

b. Deflection due to the weight of concrete shall not exceed L/240 of the span irrespective of the fact that the deflection may be compensated for by camber strips.

c. The maximum modulus of elasticity, E, for timber shall be 1.6 x 106 psi (11.0 x 103 MPa).

d. The maximum loading on timber piles shall be 45 tons (400 kN).

e. Timber connections shall be designed in accordance with the stress and loads allowed in the National Design Specification of Wood Construction, as published by the National Forest Products Association except that (1) reductions in allowable loads required therein for high moisture condition of the lumber and service conditions shall not apply, and (2) the design value of bolts in two member connections (single shear) when used for falsework bracing shall be 0.75 of the tabulated design value.

28.6.3.2 Steel

a. For identified grades of steel, design stresses, except stresses due to flexural compression, shall not exceed those specified in Chapter 15 of this Manual.

b. When the grade of steel cannot be positively identified, design stresses shall not exceed those specified for ASTM Designation A36. For compression members L/r shall not exceed 120.

c. For all grades of steel, deflections due to the weight of concrete shall not exceed L/240 irrespective of the fact that the deflection may be compensated for by camber strips.

Author 7/15/14 11:22 AMDeleted: <#>Compression perpendicular to the grain 450 psi (3,100 kPa).<#>In the foregoing formulas, L is the unsupported length, d is the least dimension of a square or rectangular column, or the width of a square of equivalent cross-sectional area for round columns.

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j. The modulus of elasticity, E, used for steel shall be 29x106 psi (2.0 x103 MPa).

28.6.3.3 Manufactured Assemblies

a. The maximum loadings and deflections used on jacks, brackets, columns, joists and other manufactured devices shall not exceed the manufacturer's recommendations except that the dead load deflection of such joists used at locations other than under deck slabs between girders shall not exceed L/240. If requested by the Engineer, the design engineer shall furnish engineering data from the manufacturer verifying the manufacturer's recommendations or shall perform tests as necessary to demonstrate the adequacy of any such device proposed for use.

28.6.4 SPECIAL CONDITIONS (2015)

a. In addition to the minimum requirements specified in Section 28.6.2 falsework over or adjacent to the railroad tracks which are open to traffic shall be protected from impact by motor vehicles and construction equipment. The falsework design shall include, but not be limited to, the following minimum provisions:

b. The vertical load used for design of falsework posts and towers, but not footings, which support the portion of the falsework over openings, shall be the greater of the following:

(1) 150 percent of the design load calculated in accordance with the provisions for the design load previously specified but not including any increased or readjusted loads caused by the prestressing forces, or

(2) The increased or readjusted loads caused by the prestressing forces.

c. Falsework posts adjacent to railroads shall consist of either steel with a minimum section modulus about each axis of 9.5 inches cubed (156,000 mm3) or sound timbers with a minimum section modulus about each axis of 250 inches cubed (4,100,000 mm3).

d. Each falsework post adjacent to railroad shall be mechanically connected to its supporting footing at its base, or otherwise laterally restrained, so as to withstand a force of not less than 2,000 pounds (8.90 kN) applied at the base of the post in any direction except toward the railroad track. Such posts also shall be mechanically connected to the falsework cap or stringer. Such mechanical connection shall be capable of resisting a load in any horizontal direction of not less than 1,000 pounds (4.45 kN).

e. For falsework spans over railroads all stringers shall be mechanically connected to falsework cap or framing. Such mechanical connections shall be capable of resisting a load in any direction, including uplift on the stringer, of not less than 500 pounds (2.22 kN).

f. When timber members are used to brace falsework bents which are located adjacent to railroads, all connections for such timber bracing shall be bolted type using 5/8 inch (16 mm) diameter or larger bolt.

g. Falsework bents adjacent to tracks shall have a minimum horizontal clearance of twelve feet (3.7 m) from centerline of track or as required by the Engineer. Falsework shall be sheathed solid on the side adjacent to track between 3 feet (0.9 m) and 17 feet (5.2 m) above the top of rail elevation. Sheathing shall consist of plywood not less than 5/8 inch (16 mm) thick or lumber not less than one inch thick (25 mm), nominal. Bracing on such bents shall be adequate so that the bent will resist the required assumed horizontal load or 5,000 pounds (22.2 kN) whichever is greater. Collision posts and sheathing shall not be required if horizontal clearances to falsework is 18 feet (5.5 m) or greater.

Author 7/15/14 11:22 AMDeleted: <#>In the foregoing formulas, L is the unsupported length; d is the least dimension of rectangular columns, or the width of a square of equivalent cross-sectional area for round columns, or the depth of beams; b is the width and t is the thickness of the compression flange; and r is the radius of gyration of the member. All dimensions are expressed in inches (millimeters). Fy is specified minimum yield stress in psi (MPa), for the grade of steel used.

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h. A minimum vertical clearance of 22'-6" (6.9 m), or as established by the Railroad, above top of higher rail shall be maintained at all times.

28.6.5 FALSEWORK CONSTRUCTION (2002)

a. The falsework shall be constructed to conform to the falsework drawings. The materials used in the falsework construction shall be of quality necessary to sustain the stress required by the falsework design. The workmanship used in falsework construction shall be of such quality that the falsework will support the loads imposed on it without excessive settlement or take-up beyond that shown on the falsework drawings.

b. Falsework shall be founded on solid footings, safe against undermining, protected from softening, and capable of supporting the loads imposed on it. When requested by the Engineer, the Contractor shall demonstrate by suitable load tests that the soil bearing values assumed for the design of the falsework do not exceed the supporting capacity of the soil.

c. When falsework is to be supported on piles, the piles shall be driven until the required pile capacity is obtained as shown on the falsework drawings.

d. For falsework over or adjacent railroad tracks, all details of the falsework system which contribute to the horizontal stability and resistance to impact, except for bolts in bracing, shall be installed at the time each element of the falsework is erected and shall remain in place until the falsework is removed.

e. Falsework shall be designed to compensate for falsework deflection, vertical alignment and anticipated structure deflection.

f. Contractor shall provide tell-tales attached to the soffit forms and readable from the ground in enough systematically placed locations to determine the total settlement of the entire portion of the structure where concrete is being placed.

28.6.6 REMOVING FALSEWORK (2002)

a. Falsework supporting any span of a simple span concrete bridge shall not be released before 10 days after the last concrete, excluding concrete above the bridge deck, has been placed in that span and in the adjacent portions of each adjoining span of a length equal to at least ½ the length of the span where falsework is to be released.

b. Falsework for cast-in-place prestressed portions of structures shall not be removed until after the prestressing tendons have been tensioned and released.

c. Falsework supporting any span of a continuous or rigid frame bridge shall not be removed until all required prestressing has been completed in that span and in the adjacent portions of each adjoining span for a length equal to at least ½ the length of the span where falsework is to be removed.

d. Falsework supporting overhangs, deck slabs between girders and girder stems which slope 45 degrees or more off vertical shall not be removed before 7 days after the deck concrete has been placed.

e. In addition to the above requirements, no falsework for bridge spans shall be removed until the supported concrete has attained a compressive strength of 2,600 pounds per square inch (17.9 MPa) or 80 percent of the specified strength, whichever is higher.

f. When falsework piling are used to support falsework within the limits of the railroad right-of-way, such piling within this area shall be removed to at least 2 feet (0.6 m) below the finished grades or as required by the Engineer.

g. All debris and refuse resulting from the work shall be removed and the premises left in a neat and presentable condition.

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C - SECTION 28.5 DESIGN OF SHORING SYSTEMS

C - 28.5.1 DESIGN OF CANTILEVER SHEET PILE WALLS (2015)

C - 28.5.1.1 Restrictions of Use

A cantilever wall derives support from the passive resistance below the excavation line to support the active pressure from the soil above excavation elevation without an anchorage. Cantilever walls undergo large lateral deflections, and the member stresses increase rapidly with height. Therefore, it is important to restrict the maximum height of the wall and require good quality soil below the excavation line that can provide adequate passive resistance.

C - 28.5.1.2 Depth of Embedment

The large moment and deflections that need to be resisted in cantilever type walls may require quite large penetration depths.

Penetration depths of 2 or more times the height of the wall may be necessary.

. C - 28.5.1.3 Maximum MomentSee Steel Sheet Piling Design Manual, US Steel, 1984, , for charts that may be used to obtain preliminary values for the depth of penetration D and the maximum moment for the case of a cantilever sheet pile wall in homogeneous granular soil and in a cohesive soil with granular soil behind above the excavation elevation. The D values obtained from the charts should be increased by 20 percent.

C - 28.5.3 DESIGN OF CANTILEVER SOLDIER BEAM WITH LAGGING WALLS (2002)

C - 28.5.3.1 Restrictions of Use

A cantilever soldier pile wall behaves similarly to a cantilever sheet pile wall. The active soil pressure and surcharge loadings are transmitted through the lagging to the soldier piles above the excavation elevation. Below the excavation the soldier piles utilize the soils passive resistance to resist the driving pressures. Due to the rapid increase in deflections and moments with the wall height, maximum height restrictions needed to be imposed.

C - 28.5.3.2 Depth of Embedment

The depth of embedment of the soldier piles must be sufficient to mobilize the passive resistance. The arching capability of soils allows the use of an equivalent width for the soldier pile below the excavation.

C – 28.5.6.3 Design Stresses and Factors of Safety

Flow nets may be used for this analysis.

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