Hydraulic uplift of the base of a deep excavation in Oxford Clay

Abstract

A local hydraulic uplift failure has been observed in the base of a 29 m deep, large brick pit excavation in Oxford Clay. The failure was preceded by at least a 150 mm rise of the pit base as water accumulated under pressure beneath impermeable clay surface layers. Hydrogeological studies have indicated that the source of water was a thin underlying bed of limestone. Rupture of the capping clay finally resulted in the rapid release of about 7OGU m3 of water which flooded part of the pit and caused an immediate settlement of up to 100 mm of the pit base. Precise survey observations begun in the year before the failure have provided a unique record of the rate and extent of the uplift and settlement movements.

Dewatering of a deep excavation undertaken in a layered soil

“…Nevertheless, these alternatives may pose a risk if the soil has not been adequately characterised. Unexpected transmissive layers within the low hydraulic conductivity materials may lead to instability at the bottom of the excavation (Ramaswamy, 1979, Moore and Longworth, 1979and Davies, 1984. A detailed hydrogeological characterisation is therefore essential.…”

Undrained stability of pit-in-pit braced excavations under hydraulic uplift

“…For example, Milligan and Lo (1970) observed the field performance of eight excavations destabilized by hydraulic uplift and reported the large basal upheaval displacement and ground settlement behind supporting walls. Moore and Longworth (1979) carried out the field measurement to observe the construction process of deep excavations in Oxford clays above a confined aquifer. Shi et al (2018) investigated the far-ground response of groundwater to pumping artesian water in deep excavations.…”

Initiation and failure mechanism of base instability of excavations in clay triggered by hydraulic uplift

“…For excavations carried out in clay overlying an aquifer, catastrophic basal failures due to hydraulic uplift are reported from time to time all over the world (Milligan and Lo 1970;Moore and Longworth 1979;Ramaswamy 1979;Clough and Reed 1984;Davies 1984;Qu et al 2002;Gue and Tan 2004). In spite of some reported field observations, initiation of base instability (i.e., when downthrust of clay inside excavation became insufficient to resist upthrust by hydraulic pressure) cannot be assessed in each reported case history, due to insufficient measurements made in the field.…”