Predicting the stand-up time of temporary London Clay slopes at Terminal 5, Heathrow Airport

Abstract

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.

Analysis of tunnel excavation in London Clay incorporating soil structure

“…They can be seen to span the range of available data. The base case lies on the upper limit of the experimentally supported measures, close to that assumed by previous analysts of the St James's case, but still well below that applied by Kovacevic et al (2007) at T5. The low elastic stiffness case roughly matches the boundary between the previous lower bound for experimental values and the upper value deduced from the T5 measurements.…”

“…As a further reference, the initial modulus values employed in previous numerical analyses are also plotted in the figure. It is apparent that the backanalysed values of initial stiffness tend to lie on the upper range of experimental values, and in some cases much higher stiffness values have been used (Kovacevic et al, 2007). There are several possible reasons for this discrepancy, including testing procedures, sample quality and soil anisotropy; but until a convincing explanation is produced, a measure of engineering judgement is required.…”

A numerical simulation of underwater slope failures generated by salt diapirism combined with active sedimentation

“…Orange et al (2003) describe how tectonic conditions along the Sigsbee escarpment generated discontinuities and slides. Existing discontinuities can impact on slope stability when their orientations reinforce, or even override, progressive failure mechanisms that might otherwise develop within the soil mass (Kovacevic et al, 2007). Potts et al's (1997) analyses concentrate on cases where pre-existing discontinuities do not dominate, although the effects of fissures can be addressed when assigning operational shear strength parameters.…”

Use of recycled crushed concrete and Secudrain in capillary barriers for slope stabilization

“…Numerous studies on the impact of heavy rainfall on slope stability using monitoring devices and numerical modelling have been carried out by Chipp et al (1982), Sweeney (1982), Pitts (1985), Krahn et al (1989), Fredlund and Rahardjo (1993), Lim et al (1996), Rahardjo et al (1998), Ng et al (2003Ng et al ( , 2008, Li et al (2005), Kovacevic et al (2007), Charles and Bromhead (2008), and Rouainia et al (2009). The effect of climate change in Singapore and elsewhere on slope stability may be detrimental to some public transport infrastructures (Anderson and Kneale 1980;Leroueil 2001;Toll 2001;Borga et al 2002;Brooks et al 2004;Smethurst et al 2006;O'Brien 2007;Kilsby et al 2009).…”