Delayed failure of railway cutting slopes in stiff clays excavated in the nineteenth and early twentieth centuries has been studied for some forty years. Known failures have generally been deep-seated. More recently failures have been a problem in the slopes of motorway cuttings and embankments, although generally these have been shallow. The average operational strength at failure in these slides has been significantly less than the peak strength, and progressive failure has been postulated as the probable cause of this. Progressive failure can now be analysed using advanced numerical techniques. A series of coupled finite element analyses have been conducted assuming strain-softening soil with properties based on the Brown London Clay, and the results are reported. They show that progressive failure is considerable, and fully explain the observed field behaviour. The delays experienced in the field are also recovered by the analyses. Progressive failure is generated primarily by the high lateral stresses in the soil prior to excavation. The rupture surface generated spreads horizontally from the toe as the soil swells, and differs significantly from the critical surface predicted by limit equilibrium analysis. The average strength on it at collapse is significantly lower than that obtained by back analysis by limit equilibrium methods. It becomes lower as lateral stress increases, but the effect is compensated by the increasing depth of rupture. The roll of the hydraulic surface boundary condition, which is controlled by climate, is important in controlling collapse. The implications of the findings for monitoring and for remedial works in existing slopes is discussed.
The paper describes the results of finite element analyses of the temporary slope geometries in London Clay at London Heathrow Airport's Terminal 5. The aims of the analyses were to examine the times before failures developed, and to identify the failure mechanisms involved. The brittle behaviour of the London Clay was modelled, and the effects of progressive failure were taken into account. The possible presence of tectonic shears with their strength close to residual was considered by comparing analyses with and without a tectonic shear zone. The predicted time to failure of the slopes and the form of the failure, whether shallow or deep-seated, were determined by a combination of the assumed permeability profile and whether or not allowances were made for increases in permeability as the clay swelled, the average surface suction, the in situ K0 profile, the depth of excavation, and whether or not a low-strength tectonic shear surface was present in the slope. The analyses fall into the category of Class A predictions, and were used in the assessment of how long temporary slopes to deep excavations could be left open before backfilling, and how the slopes should be monitored.
Empingham Dam was built in the early 1970s on a brecciated Upper Lias Clay foundation of fill derived from it. When the dam was designed, the undrained behaviour of intact stiff plastic clay in the field was poorly understood. Thus, a conservative bound to conventional strength tests was adopted to provide an undrained strength profile in the foundation. The shearing resistance of the foundation was also investigated by incorporating an instrumented 24 m high trial bank in the upstream berm of the main dam. As the trial bank was nearing completion, two slips occurred in temporary borrow pit slopes. It was considered that this field experience, together with undrained triaxial tests with pore pressure measurement on large diameter samples carried out in the laboratory, justified the original design assumptions. However, the end lengths of the main embankment on the undrained foundation started to spread laterally when the fill was some 2 m below the final level. This was in spite of the fact that the conventional factor of safety, calculated using the original design assumptions, was in excess of two. This paper describes the finite element (FE) back analysis of the observed behaviour. For this purpose two different constitutive models have been employed: a kinematically hardening 'bubble' model accounting for pre-peak plasticity, and a non-linear elastic Mohr-Coulomb plastic model accounting for strain-softening. It is demonstrated that the observed undrained behaviour of the Empingham foundation can be reproduced by FE analyses, but only if the undrained shearing resistance varies from section to section.Le barrage d'Empingham a été construit au début des années 1970 sur une fondation bréchiforme d'argile d'Upper Lias et le remblayage utilisé provenait de la même argile. Quand le barrage a été conçu, on comprenait mal le comportement non drainé sur le terrain de l'argile plastique ferme intacte. Ainsi, pour la conception, on a adopté une limite conservatrice pour les essais de résistance conventionnels de sorte à fournir un profil de résistance non drainé dans la fondation. On a fait des recherches sur la résistance au cisaillement de la fondation en incorporant un talus d'essais de 24m de hauteur, muni d'instruments, dans la banquette en amont du barrage principal. Alors que la construction du talus d'essais était presque à son comble, il y a eu deux glissements de terrain sur les pentes de l'emprunt temporaire. On a considéré les hypothèses de conception de départ se justifiaient au vu de cette expéri-ence sur le terrain ainsi que des essais triaxiaux non drainés effectués en laboratoire sur des échantillons de grand diamètre, avec mesure de la pression interstitielle. Il a été surprenant de constater que la largeur des extrémités du remblai sur la fondation non drainée commençait à augmenter latéralement lorsque la hauteur du remblayage était à 2m de la hauteur finale. Cela s'est produit en dépit du fait que le facteur de sécurité conventionnel d'équilibre limite (EL), calculé à l'aide des hypothèses de conce...
Compaction grouting involves the injection of stiff grout that does not penetrate the ground. Bulbs of grout are formed. It is applied to free-draining granular soils. The aim is to increase the density of the soil being treated by applying high local pressure. The design of compaction grouting is based largely on experience and empiricism. If the soil is not sufficiently permeable for consolidation to occur as it is treated, excess pore pressures may be generated, which will dissipate after treatment. The potential effect of such excess pore pressures on the compaction achieved is considered here. It is found that the efficiency of treatment may be reduced substantially.
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