Delimitación del objeto específico de examen

Instability can occur at three different times:

• During construction, before completion of the wall

• After completion of wall construction, when loads (such as on a dock quay) are first applied

• In the long term, well after completion of wall construction Global instability can also occur as a

• First-time slide

• Reactivation of instability on a preexisting shear surface

These issues are important because the designer must choose appropri-ate soil strength parameters during analysis (see Chapters 1 and 2). The parameters to be used are not the same in each and every case, and indeed

H Strength of this

soil ignored Strength of this

soil ignored

γH γH

cu

cu

c_u

c_u

Strength of embedded sheets ignored

Movement of soil into base of excavation

Figure 5.5 Instability in the base of a braced excavation.

may often be different for earth pressure calculations and for calculations of global and local instability. For example, Figure 5.3 shows a quay wall retaining gravel fill, where failure has occurred in the soft clay below.

Calculations to determine earth pressures on the wall would use effective strength parameters for the gravel, but global stability might in this case need to be estimated on the basis of the undrained strength of the clay.

5.2.1 Short-term or long-term parameters?

Long-term, peak effective angles of friction (ϕ′, with c′ = 0) should be used for all analyses in sands and gravels, because of their rapid drainage rate.

This applies whether for earth pressure calculation or for global stability estimates. The most difficult issue is whether drained (long-term, c′ ϕ′) or undrained (short-term, c_u) parameters are appropriate for calculating the strength of shear surfaces passing through clays. (See discussion in Chapter 1, Section 1.6). If in doubt, one option is to carry out separate cal-culations using undrained and drained parameters, and adopt the param-eters that give the least favourable results.

If a wall fails during, or shortly after construction, then a number of scenarios are possible:

1. Forward sliding on the base of a gravity wall, or rotation on a slightly deeper shear plane, may occur during construction as a result of back-filling and compaction (see Chapter 3). Calculation of compaction pressures should, of course, use parameters relevant to the type of backfill. Global stability should be checked using parameters relevant to the shear surface under consideration. If the wall fails by forward sliding, and has been founded on clay, the resistance of the wall/soil interface to sliding should be calculated using the preexisting undrained shear strength, as determined from undrained triaxial tests carried out on samples from the founding depth that have been obtained during ground investigation, with a reduction factor to allow for the smoothness of the wall soil interface and for any swelling of the clay that may have occurred during construction. Deeper shear surfaces, passing, for example, through a layer of clay at some depth below the wall, can use the undrained shear strength without taking into account the reduction of strength along the wall/soil interface.

2. If an embedded wall fails during excavation of the soil at the front of the toe (such as in Figure 5.5) then, depending on the time taken for construction to occur, some drainage of excess pore water pressure may have occurred. But it is by no means sure whether the mean total stress in front of the wall will have increased or decreased, as the ver-tical unloading during excavation at the base may be compensated for by the horizontal total stress increase as the soil supports the toe of

the wall. Assuming no time for drainage, an analysis using undrained strength parameters is normally used in a clay soil. This is the com-mon assumption when designing braced excavations, for example.

3. The addition of loads shortly after wall construction, for example, by stacking cargo behind a quay wall, can produce three types of failure. First, there may be a bearing capacity failure immediately below the loaded area; second, loading may lead to deformation of ducting carrying tie-back anchors, and the strain induced by this may cause the anchor stresses to exceed their ultimate values; and third, an overall global instability may be triggered, involving the wall, any anchors and the surcharge loading behind the wall (Figure 5.3).

Bearing capacity calculations should be carried out using undrained analysis (with undrained shear strength, c_u) for clay, and drained analysis (with effective angle of friction, ϕ′) for sands and gravels.

A judgement will need to be made whether to use undrained shear strength for global stability analysis of clays. It is probably wise to make an assessment using both undrained and drained analyses for clays.

4. Long after wall construction ground, water levels may have read-justed to their new boundary conditions as a result of changes in the ground profile, and the impermeability of the wall. Any excess pore pressures set up either by loading or by unloading, during regrading of the ground, will have dissipated leading either to swelling or to consolidation. Under these conditions the shear strength of cohesive soil is likely to have decreased (as a result of unloading and swell-ing), but may have increased locally (as a result of loading and con-solidation). The available strength must be calculated using effective strength parameters c′ and ϕ′. As noted in Chapter 2, great caution needs to be exercised when adopting values of effective cohesion inter-cept (c′) for analyses. In soft or firm normally consolidated clays, it is prudent to assume c′ = 0. In stiffer heavily overconsolidated clays, it is sensible to restrict c′ to 1–2 kPa (Chandler and Skempton 1974), unless there is considerable evidence (e.g. from back analyses of field case records) to support the adoption of a higher value.

5.2.2 Peak, residual or critical state parameters?

Estimates of global stability for first-time slides (i.e. slides on a surface that has not experienced failure in the past) can be carried out using peak effec-tive strength parameters for any kinematically admissible shear surfaces.

If effective shear strength parameters are poorly known, then critical state parameters may be used to obtain a conservative estimate of stability.

The development of a ‘first-time’ slide produces complex patterns of undulating failure surfaces and the peak effective stress angle of friction

(ϕ′) obtained from effective stress laboratory testing, used with a con-servative value of effective cohesion intercept (c′ = 0–2 kPa) can safely be used to analyse potential failure surfaces. However, as displacements on failure surfaces become greater, the failure surface is smoothed, and clay particles align, giving a polished and striated (slickensided) appear-ance to exhumed surfaces. The effective angle of friction in medium and high plasticity clays is gradually reduced, eventually reaching a ‘residual’

value, φ′r. The residual effective angle of friction, which can be less than 10° in ‘fatty’ clays (i.e. compressible clays of high plasticity), must be used in stability analysis to assess the effects of reactivating preexisting shear surfaces.

Such a low value of angle of friction is only relevant on the actual plane or planes upon which large movements have previously occurred, so these need to be identified and their positions mapped. Very often, these pre-existing shear surfaces are shallow (less than 10 m deep) and non-circular.

Non-circular failures require a particular form of analysis (see below).

As Lupini, Skinner and Vaughan (1981) show, rolling of particles during shearing prevents alignment of clay particles in low plasticity soils, so that the residual angle of friction for low-plasticity clays, is much the same as the peak value. Analysis of a reactivated slide in granular material should be carried out using the critical state angle of friction, equivalent to assum-ing a low relative density on the shear surface itself.

5.3 BaSE HEaVE aND LOCaL