support of excavation

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209. Support of Excavation

209.1. Types of Excavation

There are two basic types of excavations: (1) open excavations where stability

is achieved by providing stable side slopes and (2) braced excavations where

vertical or sloped sides are stabilized by structural systems that can be

restrained laterally by internal or external structural elements. Some

examples are skeleton shoring (excavation in most soils up to 20 ft depth),

box shoring (depths up to 40 ft), telescopic shoring (very deep trenches).

In selecting and designing the excavation system, the primary controlling

factors are (1) soil-type and soil-strength parameters, (2) groundwater

conditions, (3) slope protection, (4) side and bottom stability, and (5) vertical

and lateral movements of adjacent areas and effects on existing structures.

A trench shieldis a rigid prefabricated steel unit which extends from the

bottom of the excavation to within a few feet of the top of the cut. Pipes are

laid within the shield, which is pulled ahead, as trenching proceeds. Typically,

this system is useful in loose granular or soft cohesive soils where the

excavation depth does not exceed 12 ft.

In trench timber shoring , braces and shoring of trench are carried along

with the excavation. Braces and diagonal shores of timber should not be

subjected to compressive stresses in excess of

(209.1)

Support of Excavation
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where L= unsupported length (in)

D= least side of the timber (in)

= allowable compressive stress (psi)

209.2. Modes of Failure

The loads exerted on wall/soil system tend to produce a variety of potential

failure modes. These failure modes, the evaluation of the loads on the

system, and selection of certain system parameters to prevent failure are

discussed next.

A deep-seated failure causes rotational failure of an entire soil mass

containing an anchored or cantilever wall. This type of failure is independent

of the structural characteristics of the wall and/or anchor and cannot be

remedied by increasing the depth of penetration or by repositioning the

anchor. The best option to reduce the likelihood of this failure is to change

the geometry of retained material or improve the soil strengths.

Rotational failure due to inadequate pile penetration exhibits itself as large

rigid body rotation of a cantilever or anchored wall due to lateral soil and/orwater pressures. This type of failure is prevented by adequate penetration of

the piling in a cantilever wall or by a proper combination of penetration and

anchor position for an anchored wall.

Strength failure of sheet pile or anchor components can occur due to

choice of structurally inadequate components. This type of failure can be

avoided by designing these components to appropriate strength levels.

209.3. Stabilization

During the planning and design stage, if analyses indicate potential slope

instability, standard means for slope stabilization or retention should be

considered. On occasion, the complexity of a situation may dictate using very

specialized stabilization methods. These may include grouting and injection,

ground freezing, deep drainage, and stabilization, such as vacuum wells orelectro osmosis.

axial


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209.4. Bottom Heave in a Cut in Clay

Figure 209.1(a) shows a cut in clay braced with vertical sheet piles. Figure

209.1(b) shows the same cut with the sheet piles driven to an added depth d

below the bottom of the cut. According to Bjerrum and Eide , the factor of

safety against heave at the bottom of a cut in clay is given by the Eq. (209.2).

Usually a safety factor of at least 1.5 is desired.

(209.2)

Figure 209.1. Heave at the bottom of a cut in clay.

where c= cohesion of the clay

H= depth of cut

= unit weight of soil

q= uniform surcharge at surface

N = factor dependent on the H/Band B/Lratios (see Fig. 209.2)

B= width of trench

L= length of trench

[1]

c
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Figure 209.2. Nc factor (Bjerrum and Eide) for evaluating bottom

heave in clay. (Based on Bjerrum and Eide's equation.)

The factor N is shown in Fig. 209.2. If the factor of safety is inadequate,

then the sheet pile is driven deeper. The force Pacting on the buried sheet

pile is given by

(209.3a)

(209.3b)

209.5. Typical Plan and Elevation of a Braced Excavation

Figure 209.3 shows a typical schematic of a vertical cut supported by a

system of vertical and horizontal structural members. The earth is supported

by interlocking sheet piles, which in turn are supported by horizontal

c
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members (wales) which are shown spaced a (vertical) distance h apart. If

the sheet pile is assumed to behave as if it is continuous over several wales,

the bending moment per unit width is given by

(209.4)

Figure 209.3. Plan and elevation of a braced excavation.

wherepis the horizontal soil pressure. Alternatively, a conservative decision

may be to assume that the steel pile is pin-connected at the wales and use.

(209.5)

If the allowable bending stress in the sheet pile is F , then the required

section modulus (per unit width) of the sheet pile is

(209.6)

The force per unit length of the wales can be approximately given by

v

a


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(209.7)

209.6. Equivalent Pressure Diagrams for Braced Cuts

The soil pressure existing behind structural elements such as sheet piles

supporting soil in an excavation can be very complex. One of the commonly

used empirical models is that due to Peck which is shown in Fig. 209.4. For

a vertical cut of depth H, Peck proposed a lateral soil pressure which is a

function of the unit weight, angle of internal friction, and/or the cohesion of

the soil.

Figure 209.4. Earth pressures in a braced cut (Peck).

209.6.1. Sand

For an excavation in cohesionless soil (sand) the active earth pressure isgiven by

(209.8)

The resultant active force (per unit length of the cut) is equal top Hand

may be assumed to act at midheight (y= 0.5Hfrom the bottom of the

excavation).

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a
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209.6.2. Soft to Medium Clay

For a soft to medium clay, for which the undrained cohesion (c ) is less than

H/4, the active earth pressure is assumed to grow linearly to the maximum

value over a depth of 0.25Hand then remain constant (see Fig. 209.4). The

maximum value of the active pressure is given by

(209.9)

The resultant active force (per unit length of the cut) is equal to 0.88p Hand

may be assumed to act at a heighty= 0.44Hfrom the bottom of the

excavation.

209.6.3. Stiff Clay

For stiff clays (unconfined cohesion greater than H/4), the active earth

pressure is assumed to grow linearly to the maximum value over a depth of

0.25H, remain constant over the middle of the depth Hand then decay to

zero at the bottom of the cut (see Fig. 209.4).

(209.10)

The resultant active force (per unit length) is equal to 0.75p Hand may be

assumed to act at midheight (y= 0.5Hfrom the bottom of the excavation).

Example 209.1

A 20-ft-deep, 10-ft-wide, and 60-ft-long trench in a silty clay is braced as

shown. Struts are placed every 15 ft (longitudinally). The soil has unconfined

compression strength S = 1000 psf and angle of internal friction = 0. The

unit weight of the soil is = 115 pcf. Assume that the sheeting is driven 5 ft

below the bottom of the cut. Calculate

1. The factor of safety against bottom heave

2. The maximum pressure on the sheet pile from the soil

3. The axial force in the bottom strut

u

a

a

uc


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Solution

1. Factor of safety against bottom heave is given by

where c= cohesion = S = 500 psf

H= depth of cut = 20 ft

= unit weight of soil = 115 pcf

q= uniform surcharge at surface = 0 (this case)

N = factor dependent on the H/B and L/B ratios

Using the parameters H/B = 20/10 = 2.0 and L/B = 60/10 = 6.0, Fig. 209.3

yields N = 7.3

2. Maximum lateral pressure: Since the cohesion is less than H/4 = 575 psf,

the soil may be classified as a soft-to-medium clay. The soil pressure (Peck)

is therefore as shown on the figure below. The maximum pressure is

calculated as

However, since this is subject to a minimum value of 0.3 H, we use

3. Using a tributary area concept, the bottom strut carries load from 4 ft

uc

c

c
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above and 3 ft below the strut. The pressure diagram is uniform (p= 690

psf). The resultant load on strut no. 3 is therefore

209.7. Design of Sheet Pile Walls

Sheet pilewall is a row of interlocking, vertical pile segments driven to form

an essentially straight wall whose longitudinal dimension is sufficiently large

such that its behavior may be based on a typical vertical slice of unit width

(usually 1 ft).

Cantilever wallis a sheet pile wall which derives its support solely through

interaction with the surrounding soil into which it is embedded. Cantileverwalls are usually used as floodwall or as earth retaining walls with low wall

heights (10 ft to 15 ft or less). Because cantilever walls derive their support

solely from the foundation soils, they may be installed in relatively close

proximity (but not less than 1.5 times the overall length of the piling) to

existing structures.

Anchored wallis a sheet pile wall which derives its support from a

combination of interaction with the surrounding soil and one (or more)

mechanical devices which inhibit motion at isolated point(s). An anchored

wall is required when the height of the wall exceeds the height suitable for a

cantilever or when lateral deflections are a consideration. The proximity of

an anchored wall to an existing structure is governed by the horizontal

distance required for installation of the anchor.

Retaining wallis a sheet pile wall (cantilever or anchored) which sustains a

difference in soil surface elevation from one side to the other. The change in

soil surface elevations may be produced by excavation, dredging, backfilling,

or a combination.

Dredge siderefers to the side of a retaining wall with the lower soil surface

elevation. For a floodwall, it refers to the side with the lower water elevation.

The dredge lineis the soil surface on the dredge side of a retaining or

floodwall. The wall height is measured from the dredge line. The retainedsiderefers to the side of a retaining wall with the higher soil surface

elevation or the higher water elevation. The backfillis the material on the

retained side of the wall.


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Anchoragerefers to a mechanical assemblage consisting of wales, tie rods,

and anchors which supplement soil support for an anchored wall (Fig. 209.5).

For a singly anchored wall, anchors are attached to the wall at only one

elevation, whereas for a multiply anchored wall, anchors are attached to the

wall at more than one elevation. The anchor force is the reaction force

(usually expressed per foot of wall) which the anchor must provide to the

wall.

Figure 209.5. Sheet pile wall anchored by tie rod and deadman.

Waleis a horizontal beam attached to the wall to transfer the anchor force

from the tie rods to the sheet piling (Fig. 209.6).


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Figure 209.6. Sheet pile wall anchored by grouted tie rod.

Tie rodsrefer to parallel bars or tendons which transfer the anchor force

from the anchor to the wales.

209.7.1. Stability Design for Cantilever Walls

It is assumed that a cantilever wall rotates as a rigid body about some point

in its embedded length. This assumption implies that the wall is subjected to

the net active pressure distribution from the top of the wall down to a point

(subsequently called the "transition point") near the point of zero

displacement. The design pressure distribution is then assumed to vary

linearly from the net active pressure at the transition point to the full net

passive pressure at the bottom of the wall. Equilibrium of the wall requiresthat both the sum of horizontal forces and the sum of moments about any

point must be equal to zero. The two equilibrium equations may be solved for

the location of the transition point (i.e., the distance zin Fig. 209.7) and the

required depth of penetration (distance d). Because the simultaneous

equations are nonlinear in zand d, a trial and error solution is required.

Figure 209.7. Earth pressures on cantilever sheet pile wall.

209.8. Ultimate Resistance of Tiebacks

The ultimate resistance of tiebacks in sand is given by
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(209.11)

where d= diameter of the grout plug

L= length of the grout plug

= average effective vertical stress for the grout plug

K= earth pressure coefficient

= angle of friction of soil

In clays, the ultimate resistance of tiebacks can be taken as

(209.12)

where c = adhesion of the clay

209.9. OSHA Regulations for Excavations

A brief summary of OSHA regulations on excavations is presented in this

section.

a

[3]
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available at http://protege.stanford.edu//

Bjerrum L. and Eide O. (1956), "Stability of Strutted Excavation in Clay,"

Geotechnique, Vol. 6, No. 1, 3247.

Peck, R.B. (1969), "Deep Excavation and Tunneling in Soft Ground,"

Proceedings, Seventh International Conference on Soil Mechanics and

Foundation Engineering, Mexico City.

OSHA 2226, U.S. Department of Labor, Occupational Safety and Health

Administration, 2002 (Revised).

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Citation

Indranil Goswami: Civil Engineering All-In-One PE Exam Guide: Breadth and Depth,

Second Edition. Support of Excavation, Chapter (McGraw-Hill Professional, 2012),

AccessEngineering

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