Undrained behaviour of brecciated Upper Lias Clay at Empingham Dam

Abstract

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...

Numerical study of the effect of soil–atmosphere interaction on the stability and serviceability of cut slopes in London clay

“…The parameters fit the measured permeability profile with depth employed in previous studies of Kovacevic et al (2007) and Tsiampousi et al (2014). Under the effect of suction, tensile total stresses may lead to instigation of desiccation cracks within the soil, temporarily increasing its overall mass permeability.…”

“…A cohesion of 7 kPa and an angle of shearing resistance of 23°was adopted, while zero dilation was assumed (see also Table 1). Similar values have been employed in the past by Kovacevic et al (2007). The constitutive model was used in combination with the Imperial College generalized small-strain stiffness model (ICG3SM) of Taborda et al (2016) (model parameters also in Table 1), which is required for the assessment of the slope's serviceability.…”

Stiff sedimentary clays: geological origins and engineering properties

“…There would have been limited change in slope geometry initially, so the stress path moves horizontally; the slope material then failed at peak strength, the slope inclination fell, and the direction of the stress path is reversed to a condition at (or close to) critical state. The critical state parameters for the Upper Lias are probably about c9 ¼ 0, ö9 ¼ 238, the values that Kovacevic et al (2007) found gave the best fit in their modelling of the Empingham dam foundation deformations. At this point there would have been large deformations within the slope as the camber process occurred, and the present-day slope was formed.…”

“…The construction of Empingham dam resulted in considerable movement within the dam foundations, which has been analysed by Kovacevic et al (2007), who successfully modelled the movements of the dam, but to do so used strength parameters close to critical state values. The reason why critical state parameters should be appropriate seems to lie in the properties of the brecciated Lias, whose behaviour can be examined in the context of its stress state and strength.…”

Current research in embankment dam engineering at Imperial College London

“…Imperial College has a strong tradition, spanning nearly six decades, of research into embankment dam behaviour, from Bishop's early work on the stability of embankment slopes (Bishop, 1955) to the post-failure analysis and redesign of Carsington dam (Potts et al, 1990;Skempton and Vaughan, 1993) and Vaughan's research into the 'perfect filter' (Vaughan and Soares, 1982), to more recent analysis and redesign of the main dam embankments at Empingham and at Abberton reservoir (Kovacevic et al, 2007(Kovacevic et al, , 2013.…”