5-1

Chapter 5 Walls for Excavations

Introduction

Excavation is necessary when constructing shallow footing, mat foundation, or subbasements. It

is a legal necessity to assure no loss of bearing capacity, excessive settlements, or excessive

lateral movements to adjacent properties. Thus, this retaining structure during excavation stage

is one of the important issues.

Failure in Excavations

The earth-retaining structures in excavations can be failed by structural collapse, excessive

deformation, basal instability, and inadequate groundwater exclusion.

Control of groundwater

There are four main methods used to exclude groundwater from excavations:

1. Stopping surface water from entering the excavation by using cut-off ditches

2. Allowing water to flow into the excavation and subsequently pumping it from drainage

systems

3. Pre-draining the soil by lowing the groundwater level ahead of the excavation

4. Stopping the groundwater from entering the excavation by a cut-off wall within the soil

This chapter will address the design issue of earth-retaining structures and the estimation of

deformation.

Wall Construction

The types of retaining walls used in excavation are:

1. Sheet piling

2. Soldier pile walls (Soldier beams with or without lagging)

3. Drilled-in-place concrete piles (or piers) walls

4. Caisson wall

5. Diaphragm (Slurry) walls

6. Soil cement mixed walls

7. Soil nail walls

Systems to hold the retaining wall in place include:

1. Cantilevered or unbraced wall (it is less economic except for shallow excavation)

2. Wales and struts or rakers (braced walls)

3. Compression rings

4. Tie-back (anchored walls) or soil nails

Sheet piling

Sheet piling is commonly used for retaining excavations because the highest strength/weight

ratio, reusable, easily installed and removed. It is not usable, however, when the subsoil contains

many boulders or is dense and the excavation is deep. Noise and vibration may be an issue when

existing buildings, especially medical facilities, are sensitive to disturbance by vibration.

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Seepage may be expected to pass through interlocking steel sheet piling which supports a

difference in hydraulic head.

Sheet Pile Drivability

Little is known about the necessary sections to allow sheet piles to be driven without damage.

However, Williams and Waite (1993) suggested that in granular soils the minimum section could

be judged on the basis of SPT resistance (N value), as shown in Table 5.1.

Table 5.1 Sheet Pile Drivability (CIRIA, 1993)

Dominant

SPT

Minimum wall modulus (cm3/m)

Remarks A570 Gr 40

(265 MPa)

A572 Gr 50

(345 MPa)

0 - 10 450 A572 Gr 50 for length > 10 m

11-20 450

21-25 850

26-30 850 Length > 15 m is not advisable

31-35 1300 Penetration > 5 m not advisable

36-40 1300 Penetration > 8 m not advisable

41-45 2300

46-50 2300

51-60 3000

61-70 3000 Some declutching may occur

71-80 4200 Some declutching may occur for L > 15m

81-140 4200 Increase risk of declutching

Soldier Pile Walls

Soldier pile walls have two basic components, soldier piles (vertical component) and lagging

(horizontal component). It is essential that the soldier piles be maintained in full contact with

the soil. For this reason, they are either driven or placed in pre-drilled holes, which are

backfilled to the ground surface with lean concrete. Drilled-in-place piles may be used if

problems of driving exist (noise and vibration).

H piles are driving on a convenient spacing of 2 to 3 m for using standard-length timber. As

excavation proceeds, 50 to 100 mm thick boards are inserted behind the front flanges. The

boards can be placed against the H-pile and clipped to the front flange using patented fasteners.

The method is suitable for overconsolidated clays, all soils above the water table if they have at

least some cohesion, and free-draining soils that can be dewatered effectively.

When water is not a problem, lagging for soldier piles or drilled-in-place piles is not required, as

“arching” or bridging action of the soil from the lateral pressure developed by the pile will retain

the soil across the open space. A spacing of 2 to 3 m is commonly used in strong soils, where

no sensitive structures are present. The spacing is reduced to 1 to 2 m in weaker soils or near

sensitive buildings. The pile will, of course, have to be adequately braced to provide necessary

lateral soil resistance. In addition, the embedment depth has to be sufficient.

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Drilled-in-place concrete (bored) piles (or piers) walls

The use of low-cost augers and, more particularly, continuous flight auger rigs to drill successive

unconnected piles provides an economical wall for both temporary and permanent use for

excavations to medium depth where soil conditions are favorable. Grouting or jet grouting can

be used to remedy leakage between piles.

When soil and water must be retained, the system will have to be reasonably watertight below

the water table and be capable of resisting both soil and hydrostatic pressures. The secant wall,

consisting of interlocking piles, is most suited. Primary (female) piles, piles at centers slightly

less than twice the nominal pile diameter, are drilled with a concrete guide wall. Before fully

hardening, the secant (male) piles (same or smaller diameter) are drilled: during the process the

drilling removes segments of the primary piles so interlock is obtained. The secant wall

provides a waterproof wall which can be built to a depth ranging 30 to 40 m. In some instances

reinforcement may be bunched at opposite sides of the pile diameter to ensure maximum

effectiveness.

Caisson wall

Hand-dug caisson retaining walls, often referred to as caisson walls, have been widely used in

Hong Kong. The caissons are usually dug in stages of about one meter depth, by husband

miners with their wives serving as winch operators at the ground surface. Each stage of

excavation is lined with a minimum of 75 mm thickness of in-situ concrete using a tapered steel

shutter suitab1y supported and designed for ease of striking. The shutter remains in place to

provide support for the fresh concrete and surrounding ground while the next stage of excavation

proceeds. Submersible electric pumps are commonly used for dewatering at the base of the

caissons. Excavation through core stones and other obstructions is carried cut by pneumatic

drilling. The main advantages of a caisson wall over a conventional retaining wall are

(a) It can be constructed without temporary soil cuts or shoring.

(b) It requires a small plan area and can be used close to site boundaries and existing

structures.

(c) It can act as both temporary and permanent retaining structures.

(d) Obstructions e.g. core stones and boulders, can be overcome without too much

difficulty.

Diaphragm (Slurry) walls

Diaphragm (slurry) walls are more expensive than other wall systems. However, frequently they

provide the best solution to many underground construction problems, and the technique can

usually be adjusted to cope with most conditions of difficult ground and adjacent surcharge by

adjustment of the panel length, properties of the slurry, level of slurry in the trench and

groundwater lowering. It can be used as a permanent part of the structural system (basement

walls). Concrete poured in a cavity retained with slurry (a dense liquid) producing a slurry wall.

This method will be discussed in later chapter.

Support Systems

The lateral loads due to earth pressures, surcharge loads, hydrostatic pressures, and seismic loads

are constantly trying to push the wall and must be restrained. The restraint can be developed

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from the inside the excavation (strut and/or rake systems) or outside (tieback systems). A strut

or raker system creates obstructions in the excavation area (working space). For narrow

excavation (less than 20 m wide), it is more economical than tiebacks. In addition, you don’t

need to bring in drilling machines, to solve the problem of encountering existing obstructions

such as tunnels, basement wall, or public utilities, and to obtain permission to trespass into the

adjacent property owner’s subsoil. However, for most excavation involving large area, tiebacks

are preferable than strutted system.

Rakers, sloping compression units, are attached to the wall and braced against either the structure

being constructed, or a footing specifically cast for the purpose of resisting the raker forces.

They impart both a lateral force and uplift force.

Struts, horizontal compression units, are braced against either an existing structure or another

portion of the shoring system.

Tiebacks, tension units, restrain the applied load from outside the excavation. Since tiebacks are

installed at a downward dipping angle, they impart a downward force to the wall as well as a

lateral force.

To insure very little lateral movement, it is essential that

1. The wall fit securely, snugly, and closely against the sides of the excavation.

2. Some elastic movement always occurs but the amount should be limited.

3. Use sufficient rigid wales so that very little movement between supported points.

4. Use vertical bracing to ensure little amount of bulging between brace points.

Analytical Methods for Support Systems Three methods have been used for analyzing earth-retaining structures in excavation. They are:

(a) limit analysis, (b) beam on elastic foundation, and (c) finite element method (full soil-

structure interaction)

(a) Limit Analysis

To design braced excavations using limit analysis, one must estimate the lateral earth pressure to

which the braced cuts wall will be subjected. The braced system subjected to the same earth

pressure forces as other retaining structures that may be calculated by Rankine or Coulomb

methods. However, the design pressures are different from those because of the manner in

which the pressures are developed.

Peck (1969), from measurements, proposed empirical pressure diagrams for wall and strut

design, as shown in Figure 5.1 and Table 5.2. The pressure envelope for soft to medium clay is

applicable for the condition when H/c > 4. For stiff clay,H/c 4. Based on the observation

of Tschebotarioff (1973), slightly different apparent design pressure envelops are suggested.

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Figure 5.1 Apparent Pressure Distribution for Braced Cut

Table 5.2 Parameters for Apparent Pressure Distribution

Soil Type Author Z1 Z2 Z3

Sand

Peck

0 1 0 0.65Ka

Soft/Medium Clay 0.25 0.75 0 0.3*

Stiff Clay 0.25 0.5 0.25 0.2~0.4

Sand

Tsch.+

0.1 0.7 0.2 0.25

Short Term (Medium Clay) 0.6 0 0.4 0.3

Long Term (Medium Clay) 0.75 0 0.25 0.375

*or 4

[1 ( )]c

H

whichever is larger;

+Tschebotarioff (1973)

Note that these diagrams are not intended to represent actual earth pressure or its distribution

with depth but load envelopes from which strut loads can be evaluated. Clay is assumed to be

undrained and only total stresses are considered. Sands are assumed to be drained (through the

sheeting) with zero pore pressure. Where drainage is precluded behind a non-permeable wall,

hydrostatically distributed water pressure should be added to strut load.

If you design a strut force based on the apparent pressure diagram and uses simple supported

beams for the sheeting as proposed by Terzaghi and Peck, the sheeting may be somewhat over

designed, this over design was part of the intent of using these apparent pressure diagrams.

Tschebotarioff (1973) proposed another apparent pressure diagrams as shown in Figure 5.1 and

Table 5.2. It may be more nearly correct comparing with some case studies when the excavation

depth exceeds 16 m. It is found that when tieback is used, the loads developed resemble those of

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a conventional triangular earth pressure distribution, instead of those usually developed in a strut

wall.

Layered Soils

A major shortcoming of these apparent pressure diagrams is how to apply when the retained soil

is stratified.

For sand-clay layer using = 0 concept, the average unit weight and cohesion of an equivalent

clay layer can be calculated as (Peck, 1943):

claysandsandsand HHHH

1

a (5.1)

usandsandsandsanda qnHHHKH

C ')(tan2

1 2 (5.2)

where H = total height of the sand-clay layer

Hsand = height of the sand layer

sand = unit weight of sand

clay = unit weight of clay

= friction angle of sand

Ksand = a lateral earth pressure coefficient of sand (1)

n' = a coefficient of progressive failure ( 0.5 ~ 1; average value 0.75)

qu = unconfined compressive strength of clay

There is another available method by Bowles (1996) after Liao and Neff (1990).

1. Compute two Rankine-type pressure diagrams using the Rankine Ka and Ko and using

effective unit weights. Make a second pressure diagram for the GWT if applicable.

2. Plot the two pressure diagrams (use 0 for any tensile zones) on the same plot.

3. Compute the resultant Ra and Ro for the two pressure plots.

4. Average the two R-values.

5. Pick the shape of the apparent pressure envelope (a rectangle or a trapezoid) and calculate

the ordinate so that the area of the pressure envelope equals average R value (e.g. for

rectangle, ordinate = R/H).

6. Include the water pressure as a separate profile that is added to the preceding soil

pressures below the ground water table depending on the inside water level.

Conventional Design of Braced Excavation Walls

The conventional method, as shown in Figure 5.2, was for any multi-braced walls originally,

however, it is not for pile walls that may be due to the rigidity of piles.

The basic procedure is:

1. Sketch given conditions and indicate all known soil data stratification, water level, etc.

2. Compute the lateral pressure diagram by Tschebotarioff or Terzaghi and Peck method.

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3. Calculate the strut loads based on simple beams (see Figure 5.2) by assuming hinges at

the strut support locations.

4. Design struts or tiebacks

You can also calculate the strut loads at each successive construction stage. The highest value at

each strut level is used for strut and waling design purposes. Similar calculation is also applied

to maximum moment and shear.

The struts are actually horizontal columns subject to bending. The load-carrying capacity of

columns will depend on the slenderness ratio, l/r. The l/r can be reduced by providing vertical

and horizontal supports at intermediate points.

Figure 5.2 Simplified Method of Analyzing the Sheeting and Strut Forces

Location of the First Support

The location of the first wale can be estimated numerically by making a cantilever wall analysis.

For cohesive soils, the depth should not exceed the depth of the potential tension crack. Where

lateral movement and resulting ground subsidence can be tolerated, the depth to the first strut in

sandy soils may be where the allowable bending stress in the sheeting is reached from a

cantilever wall analysis

Effect of Wall Embedment

The apparent pressure diagrams do not include the effects of the toe of the sheeting or walling

extending below the final formation level. There is an empirical design method that allows this

penetration of walling to be taken into account in strut load calculation. It has been used since

the mid-1950s and has been adequately confirmed. The procedure is shown in Figure 5.3.

5-8

1. Construct gross earth pressure diagram using Rankine theory.

2. Calculate value of total active Pa and passive forces Pp.

3. Calculate trapezoidal apparent pressure envelope with an ordinate 1.6 Pa/H.

4. Calculate strut forces (F1, F2, F3, and F4) from the apparent pressure envelope by

splitting the distance between supports.

5. Check moment balance by taking moments about the F1. Rebalance support loads to

achieve moment equilibrium.

6. Calculate the factor of safety, mobilized passive resistance = 4FPp ( 1.4)

7. Calculate the simply supported moment (M = wl2/8) between each support where w is

the apparent pressure envelope.

Figure 5.3 Empirical Method for Wall Embedment

Beam on elastic foundation and finite element method

Numerical analyses based on beam on elastic foundation are popular due to the advent of

powerful personal computers. They are efficient means to obtain the strut force and moment in

the sheeting. The soil is represented by springs attached to the piling. With the easy access of

finite element program, finite element method has been commonly used to analyze walls in

excavation. For linear FEM, the soil is characterized with an elastic modulus and a Poisson’s

ratio. Nonlinear soil models have been used. Although they cannot produce accurate results,

good designs are achieved constantly with good experience and engineering judgment.

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Instability Due to Heave of Bottom of Excavation Braced cuts in clays may become unstable as a result of the clay flows beneath the sheeting into

the excavation. The failure is analogous to a bearing capacity failure of foundation, only in

reverse; the failure is a shear failure in the soil below formation level, but caused by relief of

load and not by the application of load as occurs in a conventional foundation bearing failure.

For deep excavations with H/B > 1, Bjerrum and Eide (1956) can be applied to calculate the

factor of safety against base failure. For shallow or wide excavation with H/B < 1, the method

by Terzaghi (1943) can be used. These methods are shown in the Figure 5.4.

If the factor of safety against base heave is less than about 1.5, substantial soil deformation is

likely. Therefore, the embedment depth should be deeper. Many designers believe the FS

cannot be achieved by the use of a flexible retaining system, and that it may require a more rigid

wall (slurry walls or secant walls). Usually the embedment depth is kept less than or equal to

B/2. If lesser soil movement is necessary, a minimum factor of safety of 2 is required.

Figure 5.4 Instability of Braced Cuts

(a) Bjerrum and Eide (1956)

(b) Terzaghi (1943)

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Stability of the Bottom of the Cut in Sand

Although it is unlikely, instability cohesionless soils can also be analyzed using NAVFAC (DM-

7, 1986). The stability is independent of the depth and the width of the excavation. The factor of

safety against base instability can be estimated.

1

1

2 tan2

akNFS

(5.3)

where N is the bearing capacity factor of soil beneath the excavation level

1 is the unit weight of soil above the excavation level

1 is the friction angle of soil above the excavation level

ka is the active earth pressure coefficient of soil above the excavation level

2 is the unit weight of soil below the excavation level

Stability of Braced Cut in Sand Due to Piping

Instability of cut in sand mostly will be due to piping (hydraulic failure). When the ground water

table is encountered and the water level inside the cut is lowered below the ground water level by

pumping, instability may be created as a result of the upward seepage of water into the cut. The

designer must ensure that basal instability will not occur because of the flow of water.

As shown in Figure 5.5, design charts (DM-7, 1986) for penetration of cut-off walls to prevent

piping in sand can be used to estimate the stability.

Factor of safety against piping can be calculated as:

ww

wsub

h

dHFS

)2( (5.4)

Where

Hw = the height of water in the wall above the dredge line

d = the embedment depth

sub = submerge unit weight of the soil beneath the base (formation level)

hw = the total hydraulic head loss

For stratified soils, the method of flow net and numerical analysis (e.g. program SEEP) are more

appropriate to estimate the FS against piping.

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Figure 5.5 Instability Due to Piping

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Ground Movements of In-situ Walls In earlier portion of this chapter the analysis focuses on the adequacy of the strut loads,

anchoring, and sheeting or walling. The risk of excessive deformation of the walls is higher

when the excavation is deeper and the site has a greater plan area. This section will address

those factors which cause ground movement around an excavation and the amount of ground

movement. An excellent review on this topic was given by Clough & Schmidt (1981). Ground

movements caused by pile driving and dewatering were discussed in the report by D'Appolonia

(1971).

Movements of in-situ walls are a function of many factors:

soil and groundwater conditions

groundwater level

initial horizontal Stress

depth and shape of excavation

preloading of the support system

type and stiffness of the wall and its support

methods of construction of the wall and adjacent facilities

Soil and Groundwater Conditions

In clays, the maximum lateral wall movement could be correlated with the factor of safety

against basal heave (Clough et al, 1979; Mana & Clough, 1981), which in turn depends on the

shear strength of the soil. The rate and magnitude of movement increase rapidly as the factor of

safety approaches one. If a clay is anisotropic instead of the presumed isotropic, the basal heave

factor of safety may be much lower and lateral wall movements as well as ground surface

settlements may be larger than expected (Clough & Hansen, 1981).

Wall movements and ground settlements are smaller in stiff soils, such as granular soils and stiff

clays, than in soft soils, such as soft and medium clays and compressible silts (Peck, 1969).

Initial Horizontal Stress

For excavated walls in soils with a high initial k0 value, large soil and wall movements are

experienced even at shallow depths of excavation. The wall behavior is dominated by vertical

unloading caused by the excavation process and large movements still occur even if the wall is

fully restrained from horizontal movement. For walls in soils with a low k0 value the

displacements are much smaller in magnitude (Potts and Fourie, 1984).

Groundwater Conditions

In practice, a perfectly watertight wall penetrating an impermeable soil layer at the bottom of the

excavation does not exist. Where water flows into the excavation, a decrease in groundwater

pressure will occur. This will cause an increase in effective stress and settlement of the soil

surrounding the excavation. Where groundwater has not been brought under complete control,

large, erratic and damaging settlement due to the flow or the migration of fines into the

excavation is not uncommon.

Depth and shape of the excavation

The deeper the excavation, the greater is the decrease in total stress, and thus the larger are the

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movements of the surrounding soil. The shape of the excavation affects the basal stability in

clays. Thus, it affects the movement of surrounding soils.

Preloading of the support system

Preloading removes slack from the support system, and thus eliminates this potential source of

movement. Each application of preload reduces the shear stresses set up in the soil due to

previous excavation activities. This means that the soil is partially unloaded, and its stress-strain

response is stiffened until the next excavation step generates shear stresses that reload the soil

beyond its maximum previous level of shear stress. This temporary stiffening of the soil also

leads to reduce movements. Use of preloads in the struts or tiebacks reduces movement,

although there are diminishing returns at higher preloads. Very high preloads may, in fact, be

counter-productive, since local outward movements at support levels can damage adjacent

utilities.

Prestress loads calculated using the apparent trapezoidal pressure diagrams (Terzaghi and Peck,

1967) is more effective in reducing movements than the prestress loads in tiebacks calculated in

accordance with a triangular at-rest pressure diagram (Clough and Tsui, 1974). Using levels

slightly greater than those recommended by Terzaghi and Peck (1967) reduced the movements

Clough (1975). Hanna & Kurdi (1974) supported this finding in their model tests. Clough &

Tsui (1974) showed that movements of the tied-back wall, and the soil settlements behind the

wall, could be substantially reduced by a judicious choice of prestress load and support system

stiffness. In most cases, cross-lot struts prestressed to 50% of their design load will be

sufficiently rigid to restrict further movement at the level of the supports, and sufficiently low in

load to avoid being overstressed as additional excavation occurs (O’Rourke, 1981).

Type and Stiffness of the Wall and Its Support

The support system stiffness depends on the stiffness of the wall and its supports, the spacing

between supports, and the length of wall embedded below the excavation bottom. Goldberg et al

(1976) produced a valuable study into the effect of wall stiffness and support spacing and the

results are presented in Figure 5.6. The instability number of the base (H/Cu) is plotted versus

the stiffness parameter (EwIw/h4), where EwIw is the flexural stiffness of the wall per unit length,

'h' is the vertical spacing between supports, H is the effective overburden pressure, and Cu is the

undrained shear strength of the soil. A non-dimensional effective stiffness parameter was also

proposed (Koutsoftas et al. 2000, Clough at el. 1989):

4

EIS

H (5.5)

where is either the unit weight of soil or unit weight of water.

Secant and tangent pile walls and structural slurry walls (diaphragm walls) are considered stiff on the basis of the rigidity of the wall element. Walls that are considered flexible include steel sheet pile walls and soldier pile and lagging walls

5-14

Figure 5.6 Effect of Basal Stability and Wall’s Stiffness on Lateral Wall Movement

Various boundary lines are drawn to establish zones of expected lateral wall movements. These

data suggested that an increase in EwIw/h4

has a significant effect in reducing movements. The

movement is also a function of the factor of safety against basal heave, being more significant at

lower factors of safety than at higher ones. Increasing the stiffness of the strut or the tieback

decreases movements, but this effect shows diminishing returns at very high values of strut or

tieback stiffness.

Movements are also reduced as the depth to an underlying firm layer decreases and when the

wall toe is embedded into the underlying firm layer.

Methods of Construction of Walls and Adjacent Facilities

Associated Site Preparation Works

Site preparation may include the following activities

(a) Relocation and underpinning of utilities,

(b) Dewatering of aquifers above and below the base of the excavation,

(c) Construction of the excavation support system, and

(d) The installation of deep foundations

In some cases, the movements associated with the site preparation works will exceed those that

occur as a result of the excavation and support process. The relocation of utilities, for example,

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may have a locally severe impact on an adjacent property, especially when trenching is carried

out close to pipelines and communication conduits.

Dewatering may consolidate the soil over an area, which substantially exceeds the area affected

by excavation-induced movements. Also, it often causes settlements well in excess of

excavation settlements. However, in areas that have been subjected to earlier dewatering

activities, settlements due to further dewatering are smaller than those in virgin ground due to the

stiffer response of the preconsolidated soil.

Wall construction may require predrilling for soldier piles, the use of vibratory hammers to

install sheet piles, or the excavation of slurry panels for concrete diaphragm walls. Each of

these can cause permanent movements, the magnitude and distribution of which will vary

according to the soil conditions, site history and details of the construction procedures.

Influence of Construction Factors

Additional movements of excavations, and even local failure, have been produced by late

installation of supports, over-excavation, pile driving, caisson construction, loss of water through

holes for tie-backs and joints or interlocks of slurry or sheet pile walls, remolding and

undercutting of clay berms, and surcharge loads from spoil heaps and construction equipment. A

review of the influence of construction factors on excavation movements was made by Clough &

Davidson (1977), and useful specific case history accounts can be found in Broms & Stille

(1976), Hansbo et al (1973), Lambe et al (1970), O'Rourke et al (1976), White (1976) and

Zeevart (1972).

Lambe (1970, 1972) demonstrated that variations in wedging techniques between the walings

and struts, and differences in excavation procedure, can result in doubling of the wall and soil

movements. Clough & Tsui (1974) used a finite element analysis to show that over-excavation

could lead to a 100% increase in movement.

Because these factors cannot always be quantified, it is difficult to make accurate prediction of

movements. However, many of the undesirable effects of these factors have been identified, and

they can therefore be anticipated and controlled through good specifications, well-planned

construction procedures and close supervision and monitoring. The designer must consider how

the excavation and subsequent construction should be carried out, identify critical construction

factors and, where possible, allow for them in performance estimates and specifications.

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Prediction of Soil Movements

Prediction of soil movements behind a supported excavation can be made by the following

methods:

(a) Empirical methods

(b) Semi-empirical methods

(c) Numerical methods

Empirical Methods

Figure 5.7 is the first practical approach for estimating movements for in-situ wall systems

proposed by Peck (1969). The data used to derive the three zones shown in this figure were

taken from excavations supported by soldier piles or sheet piles with cross-lot struts or tiebacks.

(1) Zone I – Sand and stiff to hard clay, average workmanship

(2) Zone II –

(a) Very soft to soft clay

Limited depth of clay below bottom of excavation.

Significant depth of clay below bottom of excavation but H/Cu < 5.14

(b) Settlement affected by construction difficulties.

(3) Zone III – Very soft to soft clay to a significant depth below bottom of excavation and

with H/Cu > 5.14.

Generally, where workmanship is average or above average and soil conditions are not especially

difficult, settlements should not exceed one percent of the excavation depth. In cases where

seepage can occur and soil consolidation results, the one percent figure can be exceeded (Lambe

et a1., 1970). It should also be noted that construction technique could have a strong influence

on movements of strutted systems.

Figure 5.7 Observed Settlements behind Excavations (Peck, 1969)

Peck’s recommendation has been found to be very conservative for stiff clays. Clough and O’Rourke (1990) found that the average horizontal and vertical movements of support systems in stiff clays were roughly 0.2 percent and 0.15 percent of the total excavated depth, respectively. Their findings agree with guidance established in Canadian Foundation Engineering Manual (1985), which

5-17

states that lateral movements of temporary support systems in stiff clay are less than 0.2 percent of the excavation depth. This compares to guidance established in NAVFAC DM-7.2 (1982) that suggests in stiff fissured clays lateral movements may reach 0.5 percent of the total excavated depth or higher depending on quality of construction.

Semi-empirical methods

Semi-empirical methods have been developed based on the estimated lateral wall deflection

profile obtained from simple numerical programs. The volume of the soil (Vs) in the lateral

displacement zone is obtained by integrate from the wall deflection profile. Then Vs is related to

the amount of soil movement vertically near the excavation area. Two methods are presented

here. The Caspe’s method is for excavation in clays, and the Bauer’s method is for excavation in

sands.

Caspe’s method

The lateral distance of the settlement influence is calculated as follow (Caspe, 1966) for the case

of the base soil being clay:

1. Compute a distance Hp below the excavation level as (B = width of excavation area):

0

1 tan(45 ) ; 0

2 2

p

B when

HB

(5.6)

2. Compute H' = H + Hp, where H is the excavation depth.

3. Compute the approximate distance D [= H' tan (45 - /2)] from the edge of the wall over

which ground loss occurs

4. Compute the surface settlement at the edge of the excavation wall as So = 2 Vs/D

5. Compute remaining ground loss settlements assuming a parabolic variation of Si from the

edge of the wall as Si = So (1 - x/D)2, where x is the horizontal distance from the edge of

the wall.

Bauer’s method

A semi-empirical method for excavation in sands was proposed by Bauer (1984) as shown in

Figure 5.8. The settlement is related to the friction angle of the sand. The influence distance is a

function of the friction angle of sand, workmanship, and construction difficulty. The effects of

workmanship and construction difficulty on the influence distance are described by factors f1 and

f2 as listed in Table 5.4.

Table 5.4 Factors of workmanship and construction difficulty

Factor Workmanship Factor Construction Difficulty

Excellent Good Average Poor None Average Severe

f1 0.8 0.9 1.0 1.1 f2 1.0 1.02 1.05

Numerical methods

Finite element method has been used to study and calculate both the horizontal and vertical soil

movements adjacent to the excavation. Due to the complexity of the soil behavior, the

unpredicted construction factors, quantitative predication of soil movements is very difficult.

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However, it has provided valuable qualitative studies. With the appropriate engineering

judgment, reasonable predication can be obtained.

Figure 5.8 Semi-empirical Method to Estimated Settlement (Bauer, 1984)

Base Movement Trends

Maximum movement - Stiff Clays, Residual Soils and Sands

As shown in Figure 5.7, Peck’s data suggested that the movements of excavation support

systems in these soils were limited to 1% H. Goldberg et al (1976) showed that the maximum

horizontal movements for in-situ walls and settlements of the retained soil masses in such

materials were usually less than 0.5% H. O’Rourke (1990) found that the average horizontal and

vertical movements of support systems in stiff clays were roughly 0.2 percent of the total

excavated depth.

Excluding those special points, Figures 5.9 and 5.10 show that:

The horizontal movements tend to average about 0.2% H

The vertical movements tend to average about 0.15% H

There is ample scatter in the data, with the horizontal movements showing more than the

vertical movements.

There is no significant difference between trends of the maximum movements of

different types of walls, and this includes even the new soil nail and soil cement walls.

These two figures are useful to understand movement patterns and also can be used as design

tools to estimate maximum wall and soil movements. Although we don’t know the maximum

movement for a particular soil, the linear trend between movement and excavation depth is

obvious.

A finite element study also predicted maximum lateral wall movements versus H also follow an

approximately linear response with excavation depth, centered around a trend line of 0.2% H.

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This is consistent with the average behavior observed in Figure 5.9. The parameters wall

stiffness and strut spacing were found to have only a small influence on predicted movement

because the soil in these circumstances is stiff enough to minimize the need for the structure.

However, soil modulus and Ko had a more significant impact (Clough and O’Rourke, 1990).

Figure 5.9 Observed Maximum Lateral Movements for Insitu Walls in Stiff Clays, Residual

Soils and Sands (Clough and O’Rouke, 1990)

Figure 5.10 Observed Maximum Soil Settlements by In-situ Walls (Clough and O’Rouke, 1990)

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It is suggested here that variations in soil stiffness have a more profound effect on wall behavior

than system stiffness.

Maximum Movements - Soft and Medium Clays

As opposed to stiffer soils, basal stability in soft and medium clays may be at issue, and as a

result, movement patterns in these conditions can be dominated by deflections beneath the

excavation. Figure 5.11, a plot of maximum lateral movement of the wall versus FS of basal

heave, shows that as FS falls below 1.5, movements increase rapidly. It also illustrates the

influence that wall stiffness and support spacing. Caution is needed when using this chart for

design.

Figure 5.11 Design Curves to Obtain Maximum Lateral Wall Movement for Soft to Medium

Clays (Clough et al., 1989)

Clough et al (1979) and Mana & Clough (1981) reviewed case histories of sheet pile and soldier

pile walls in clays supported primarily by cross-lot struts. They found that the ratio of maximum

settlement movements ranges from 0.5 to 1.0 times the lateral wall movements.

Figure 5.12 shows that the maximum lateral movement can be correlated with the factor of safety

against basal heave defined by Terzaghi (1943, see Figure 5.4b). The movements increase

rapidly below a factor of safety of 1.4 to 1.5, while at higher factors of safety the non-

dimensional movements lie within a narrow range of 0.2% to 0.8%. Moreover, there do not

appear to be any significant differences in lateral movements between sheet pile walls whose tips

are embedded in an underlying stiff layer and those whose tips remain in the moving clay mass.

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Figure 5.12 Soil Movements versus Basal Stability for Soft to Medium Clays

Displacement Profile

General patterns of ground movement are illustrated in Figure 5.13. They are obtained from

inclinometer and settlement measurements for braced and tied-back excavations (Milligan, 1984,

Clough and O’Rouke, 1990, Finno et al., 1989).

Figure 2.13 Typical Lateral Movement Profiles

Initial excavation before strutting

The excavation was deepened before supports were installed. Thus, deformation of the wall

occurred primarily as a cantilever-type movement. The horizontal strains produced in this mode

of deformation form a triangular pattern of contours that decrease in magnitude with depth and

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distance away from the wall. Settlements during this stage of construction may be bounded

within the triangular distribution of displacement as shown in Figure 5.13a.

When the excavation advances to deeper elevations, upper wall movement is restrained by

installation of new support or stiffening of existing support members. In the deeper portion of

the excavation, inward bulging of' the wall caused tensile strains, the contours for which were

inclined at approximately 45o to the vertical. Deep inward movement of the wall occurs, which

is shown as an incremental component of the total displacement in Figure 5.13b.

The cumulative wall and ground surface displacement profiles are shown in Figure 5.13c.

Therefore, the displacement profile depends on the predominant movement pattern.

In sand, stiff to very hard clay, cantilever movements predominate and the settlements

tend to follow a triangular pattern.

In soft to medium clay, the deep inward movement is predominant and the settlements

tend to be bounded by a trapezoidal displacement.

Field measurements of horizontal strains at excavations in different types of soil showed similar

patterns, with triangular contours of strain caused by cantilever wall deformation and deep

concentric contours of strain bounding zones of maximum settlement caused by walls subject to

deep inward displacement.

Excavations in Sand

Figure 5.14 summarized the settlements adjacent to the excavations in predominantly sand and

granular soil profiles. The excavations were in granular soils above the water table by

dewatering. The settlement profiles are very consistent for different kind of supporting systems

(cross-lot struts, soldier pile and lagging with tiebacks, sheet piles with tiebacks, and slurry wall

with cross-lot struts).

Figure 5.14 Observed Settlement Adjacent to Excavations in Sands (Clough and O’Rourke,

1990)

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Maximum settlements were typically less than 0.3% H. It is a triangular profile and distributes

over 2 times the excavation depth from the edge of the cut.

Excavations in Stiff to Very Hard Clays

Figure 5.15 summarized settlements for excavation sites in stiff to very hard clays for different

retaining systems. The displacements caused by ancillary construction activities were removed.

The settlements are only 0.3% H but are distributed over three times the excavation depth from

the edge of the cut.

Figure 5.15 Observed Soil Movements Adjacent to Excavations in Stiff and Very Hard Clays

In the horizontal movement, the majority falls within a triangular boundary with the same

dimensions as those pertaining to the observed settlements. A second zone was drawn which

contains highway excavations of London clay. They were affected by the low horizontal

stiffness of the support systems.

Excavations in Soft to Medium Clays

Figure 5.16 summarizes settlements for excavations in soft to medium clay, involving cross-lot

struts supporting sheet pile, soldier pile and lagging, and concrete diaphragm walls. Due to the

scatter of the data, it is difficult to identify the maximum settlement. However, if the settlements

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were plotted as fractions of maximum settlement, a relative well-defined grouping of the data is

evident. The settlement distribution is bounded by a trapezoidal envelope in which two zones of

movement can be identified. At 0 d/H 0.75, there is a zone in which the maximum

settlement occurs. At 0.75 < d/H 2, there is a transition zone in which settlements decrease

from maximum to negligible values.

Figure 5.16 Observed Soil Movements Adjacent to Excavation in Soft and Medium Clays

The largest differential soil movements occur near or within the transition zone. In almost all

cases, the maximum angular distortion was located between 0.5 H and 1.25 H from the edge of

cut. There is a linear relationship between the maximum settlement and the logarithmic of

angular distortion. Angular distortion appears to be relatively small when maximum settlement

is less than 50 mm. It increases exponentially with maximum settlement.

Figure 5.17 presents dimensionless settlement profiles recommended as a basis for estimating

vertical movement patterns adjacent to excavations in three different kinds of soils.

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Figure 5.17 Dimensionless Design Settlement Profiles for Excavations in Different Soil Types

Be careful that these diagrams pertain to settlements caused during the excavation and bracing

stages of construction. Movements associated with other activities, such as dewatering, deep

foundation removal or construction, and wall installation, should be estimated separately.

Vertical settlements have been observed during the installation of the diaphragm wall at different

soils (granular soil, soft to medium clay, stiff to very hard clay). The maximum settlement ratio

is less than 0.15% H. The influence distance from the edge of the wall is about 2.5 H.

Driving of sheet-pile in a loose sand layer will also cause the layer to settle. Thus, the

geotechnical engineer should know how in-situ walls are constructed since otherwise he cannot

properly predict how the soil will behave.

General comments

In stiff clays, in-situ walls will exhibit creep, a prompt installation of supports if movements

are to be minimized.

Minimize struts spacing (strut stiffness is less important within normal range).

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Try to enforce the excavation limit such that no over excavation without the installation of

supports.

It is advisable to penetrate the wall to a bearing layer where possible. Using sheet-pile, every

fifth sheet-pile can be penetrated to the bearing layer.

Summary

System stiffness and basal stiffness affect excavation in soft to medium clays but not in stiff

clays and sands.

Additional movements can also be generated by

(a) poor construction technique

(b) slow installation of supports after the excavation level is reached

(c) construction and removal of foundations within the excavation and,

(d) removal of soil below the design level of a support.

Increasing wall stiffness tends to reduce system movements, but this is most effective in soft to

medium clays. Support spacing is more important than wall stiffness in defining system stiffness

and helping control movements.

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