Observations of a strutted diaphragm wall in London clay: a preliminary assessment

Abstract

A preliminary assessment of the results obtained from a comprehensive field instrumentation and monitoring scheme of a deep basement in London clay, initiated in 1981 and expected to continue until at least 1987 and probably beyond, is presented. The aspect of particular interest is the performance of the 18 m deep perimeter diaphragm wall and the associated temporary works during construction of the basement. Details of the recorded wall movements, strut forces and strains developed within the wall are given together with observations of water pressures in the ground behind the wall. In addition the results of line and level surveys of the wall and the surrounding area are presented. Soils information extracted from the original site investigation report, together with results obtained from samples and in situ tests conducted during the installation of a deep borehole extensometer, are given. A comparison is made with the original design predictions of wall and ground movements, bending moments and strut loads. Tentative conclusions are drawn which suggest that the forces developed within the struts are within the design values; similarly strains in the wall derived from the design bending moments have not been exceeded. The dependence of the wall deformations on the speed and sequence of excavation is illustrated. The measurements to date suggest that the zone of influence with respect to movements of the surrounding ground is confined to the immediate vicinity of the excavation. L'article présente une évaluation préliminaire des résultats d'un projet d'ensemble contrôlé à instrumentation sur place qui a été entrepris dans un sous sol profond dans l'argile de Londres en 1981 et qui va continuer probablement jusqu'à 1987 voire plus longtemps. On s'intéresse en particulier au comportement d'une paroi périphérique d'une profondeur de 18 m et aux travaux associés temporaires au cours de la construction du sous sols. Des détails sont présentés concernant les mouvements enregistrés de la paroi, les forces dans les étais et les déformations crées dans la paroi, aussi bien que les pressions d'eau dans le sol derrière la paroi. L'article donne en outre les résultats des études d'alignement et de niveau de la paroi et du terrain environnant et présente des renseignements au sujet des sols contenus dans le rapport original concernant la reconnaissance du sol initiale aussi bien que les résultats obtenus à partir d'échantillons et d'essais effectués in situ au cours de l'installation d'un extensomètre dans un sondage profond. Une comparaison est faite avec les prévisions originales des mouvements de la paroi et du sol, des moments de flexion et des charges dans les étais. Les conclusions tirées suggestent que les forces développées dans les étais n'excèdent pas les valeurs du projet, et de façon analogue les déformations dans la paroi qui découlent des moments de flexion du projet n'ont pas été dépassées. On démontre la dépendance des déformations de la paroi avec la vitesse et le cours de l'excavation. Les mesures faites jusqu'à présent donnent l'impression que la zone d'influence des mouvements du sol environnant est confinée au voisinage immédiat de l'excavation.

Fundamentals of the coupled thermo-hydro-mechanical behaviour of thermo-active retaining walls

“…The analysed case study is presented in Wood & Perrin (1984a) and Wood & Perrin (1984b): an 11 m deep excavation for an underground car park, 100 m x 60 m in plan, of a six storey building located in central London is supported by an 18 m deep, 800 mm thick diaphragm wall ( Figure 1). During excavation, the wall was supported by a propping system consisting of a steel frame supported by concrete soldier piles.…”

“…In order to limit the impact of modelling simplifications (Zdravković et al, 2005), 2D plane strain analyses were carried out on a section perpendicular to the walls of greatest length (east and west walls), as this type of analyses is typically employed to simulate structures of large dimension in the out-of-plane direction (Potts & Zdravković, 1999). The ground profile was determined from the site investigation information reported in Wood & Perrin (1984a) and from available borehole data nearby the site. These indicated the level of ground surface to be at +22.0 mOD and a ground profile consisting of 4.8 m of Made Ground (MG), 2.0 m of Terrace Gravel Deposits (TGD), 40.0 m of London Clay (LC), 12.0 m of Lambeth Group Clay (LGC) and 7.0 m of Thanet Sand (TS) overlying Chalk (CH), as displayed in Figure 2.…”

“…Concrete was modelled as a linear-elastic material with a stiffness of 30 GPa, as indicated in Wood & Perrin (1984a). Excluding the Made Ground, which was modelled as a linear elasto-plastic material with a Mohr-Coulomb failure surface, all other soil layers were modelled as non-linear elasto-plastic materials, coupling a Mohr-Coulomb failure surface with the IC.G3S non-linear elastic stiffness model (Taborda et al, 2016).…”

“…The first part of this paper simulates the excavation sequence of a deep basement in London (Wood & Perrin, 1984a;Wood & Perrin, 1984b), for which detailed field data is available, with the aim of validating the employed mechanical and hydraulic soil models and properties. Subsequently, different FE analyses based on the same geometry of the analysed case study, hypothesising the use of the wall as a heat exchanger, are presented.…”

Monitoring and analysis of the bending behaviour of discrete piles used to stabilise a railway embankment

“…Estimating the bending moment using equation (1) is not straightforward, because the flexural rigidity EI of the piles varies with bending moment when the concrete is partly cracked. Uncracked behaviour is normally assumed in the analysis of field measurements of retaining walls (Tedd et al, 1984;Wood and Perrin, 1984), and has been used in the analysis of the results here. Thus the composite flexural rigidity, EI, was calculated as 187 3 10 3 kN m 2 for the lower 7 m of the pile and 171 3 10 3 kN m 2 for the top 3 m, both using E ¼ 25 3 10 6 kN/m 2 for uncracked concrete (British Standards Institution, 1985).…”