The construction of an in situ embedded wall, whether from bored piles or diaphragms, involves excavation of the soil and replacement with reinforced concrete. Excavation inevitably reduces the in situ total stresses in the soil to either zero (if the hole is unsupported), or some value smaller than origi-nally present, for example, when a bentonite slurry is used for temporary support. In heavily over-consolidated cohesive soils, the placement of fluid concrete in the ground does not bring the horizontal total stresses back to their original values—lateral stress relief occurs in the vicinity of the wall.
These total stress reductions are known as ‘installation effects’.
The construction of walls in inner-city sites routinely requires that lat-eral wall displacements must be minimised in order to prevent damage to adjacent buildings. If lateral wall movements are kept small then it is pos-sible that the wall will have to support much higher than active pressures, perhaps approaching earth-pressure-at-rest (K0) values. Thus, the price to be paid for restricting ground movements adjacent to a construction exca-vation will be the need to design a stronger and stiffer wall and support system.
In normally consolidated soil, in situ horizontal total stresses may be sig-nificantly higher than active stresses. Taking the Rankine and Jaky values for Ka and K0 respectively, i.e. suggest-ing a 50% increase in earth pressure. But as noted in Section 3.3.2 above, in heavily over-consolidated clays, such as the London Clay, K0 may rise to well in excess of 2 at the surface (Bishop et al. 1965; Skempton 1961;
Simpson et al. 1979). If it is assumed that, as a result of the rigidity of a wall and its support system, such stresses remain at the end of construction,
then prop forces and bending moments will be calculated that are many times those derived on the basis of active pressure (Potts and Fourie 1985).
Yet, despite the fact that few designers have, until recently, taken in situ earth pressure into account, walls have not failed.
This difference between predicted and observed behaviour is probably due to the influence of wall installation processes on in situ stresses. Gunn and Clayton (1992) have noted that retaining walls may be divided into two types: filled walls, where the wall is constructed above ground and backfill is subsequently placed against it, and embedded walls, where the wall is constructed within the soil mass and the ground is subsequently removed from the front of it. Wall construction (installation) effects are only relevant in the case of embedded walls, although the construction method (particularly the method of backfill placement) is important for back filled walls, as will be described below.
We have seen that embedded walls may be of either a displacement or a replacement type. Displacement type walls are typically placed by driving either steel or pre-cast concrete sections into the soil. For the section to be driven, it must be relatively slender—it is not likely that the driving of com-monly used steel sheet-pile sections will lead to a significant increase in in situ horizontal stress conditions. However, the excavation of diaphragm wall panels or bored piles is certain to result in significant total stress reduc-tion, because during formation of the wall sections, a hole (which may be unsupported or may be supported by bentonite slurry) must be excavated in the soil. The total horizontal stress on the boundary of this hole will reduce from the initial in situ horizontal total stress in the undisturbed soil to either zero (if the hole is unsupported) or to a value which approximates to the pressure exerted by a fluid with the same bulk density as bentonite. The total stress acting on the soil is then increased to a value approximating to the pressure applied by wet concrete, at least in the upper 10 m of the wall (Figure 3.26). If the soil subsequently swells against the concrete, before bulk excavation takes place, horizontal pressures may rise somewhat.
A field experiment on a small-diameter bored pile in London Clay, reported by Milititsky (1983), also showed little increase in the measured total stress over a period of 150 days following concreting (Figure 3.27).
But this may have resulted from the rather long time taken in excavat-ing the bore, placexcavat-ing the instrumentation, and subsequently concretexcavat-ing.
Field observations of the Bell Common Tunnel, both during construction and afterward, showed not only that 30% of the measured ground surface settlements adjacent to the wall occurred during the process of the installa-tion of the secant-bored pile, but also that this was associated with a signifi-cant reduction in horizontal total stresses, particularly in the London Clay, where K0 was initially higher (Figure 3.28). Measured bending moments over a period of five years after construction were much lower than was predicted during design, perhaps partly as a result of this effect.
0
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Depth (m)
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γc = 2.3 t/m3
γb = 1.2 t/m3
Lateral concrete pressure (kN/m2)
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Di Biagio and Roti (1972)
Uriel and Otero (1977) After 24 hours
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Figure 3.26 Total horizontal stress measured in diaphragm wall panels. (From Clayton, C.R.I. and Milititsky, J., Ground Engineering 16(2), 17–22, 1983.)
7001 80
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Time after installation (days) Horizontal stress (kN/m2)
Figure 3.27 Measured total horizontal stress as a function of time since concreting, for a small-diameter bored pile installed in the London clay. (From Milititsky, J.M., Installation of Bored Piles in Stiff Clays: An Experimental Study of Local Changes in Soil Conditions. Ph.D. thesis, University of Surrey, 1983.)
Numerical analyses of retaining walls, using finite element methods, have shown the important implications of both construction detail and installation effects. Potts and Fourie (1985) and Fourie and Potts (1989) have demonstrated that for a propped cantilever wall in clay, finite ele-ment analyses can provide predictions of equilibrium depths of embedele-ment which agree with those calculated using simple limit equilibrium solutions, regardless of the initial value of horizontal total stress assumed for the soil.
But, in contrast, the values of prop force and maximum bending moment will be much higher than are predicted by limit equilibrium methods, when the assumed initial horizontal in situ stresses in the finite element analyses are high.
Figure 3.29 shows how this effect can be visualized on the basis of sim-ple limit equilibrium analyses. For normally and lightly over-consolidated soils, in situ horizontal stresses are closer to the active than the passive state. Conventional analyses of a uniform soil deposit, using a factor of safety on passive pressure, predicts that the prop force will be 302 kN/m run of wall, while the bending moment will be 1916 kNm/m run. For a heavily over-consolidated soil, however, the in situ horizontal stress close to the ground surface will approach the passive value. Full passive pressures will therefore remain after excavation at the front of the wall, and the pres-sures behind the wall will not fall to active (see, for example, the measured earth pressures given by Carder and Symons 1989). For this scenario, the prop force and maximum bending moments will be doubled if moment
400
Total horizontal stress (kN/m2) 100 0 0
Figure 3.28 Total horizontal stress measured adjacent to the Bell Common retaining wall, after installation of secant piles and before main excavation. (From Tedd, P. et al., Géotechnique 34 (4), 513–532, 1984.)
equilibrium is maintained as before, because the horizontal stresses to be supported must be twice the active value. However, in reality, the stresses on the wall will be a function of installation procedures and, in addition, of wall flexibility.
Finite element analyses reported by Kutmen (1986), Higgins et al. (1989) and Gunn et al. (1992) have all shown the sensitivity of the calculated wall bending moments to the details of the construction procedure. In a coupled-consolidation analysis, modelling in a simple way the complete construction process, Gunn et al. (1992) have demonstrated the following:
3
Figure 3.29 Example limit equilibrium calculation to show effects of initial K0 value on calculated prop force and maximum bending moment. (From Gunn, M.J. and Clayton, C.R.I., Géotechnique 42, 137–141, 1992.)
• The impact of installation effects will depend upon the position of the groundwater table. When the groundwater table is high, installation has little effect, since most of the total active pressure is as a result of the water pressures.
• The greater the restraint imposed at the top of the wall, for example by propping or anchoring, the larger will be the effects of installation.
This results from the fact that if the wall is relatively unrestrained then the effective horizontal stresses will fall to their active values.
Higgins et al. (1989) modelled the installation effects of the Bell Common Tunnel using high-quality soil data obtained after the wall was constructed. Wall construction was modelled as an undrained event, which was followed by the application of seepage forces to determine long-term conditions. A number of slightly different (but plausible) assumptions were made for strength and stiffness parameters, according to whether the soil was expected to experience compression or extension stress paths. The results showed that analyses based upon non-linear elastic soil proper-ties, measured in high-quality laboratory tests, gave predictions as good as those made using parameters derived from back-analysis of structures in similar ground conditions. Modelling all construction phases, including wall installation, brought the computed behaviour closer to that observed, although agreement between observed and calculated displacements and bending moments still remained poor.