Pore-pressure response beneath a ring foundation on clay

Abstract

A 9 m diameter and 22 m high concrete silo with a ring footing was instrumented and its performance monitored. The instrumentation included 12 pressure cells, 7 piezometers, 9 settlement cells, and 12 settlement pins. Continuous sampling and vane testing, together with field determination of permeability and coefficient of earth pressure at rest, were also carried out.The subsoil conditions at the site consist of a 3 m crust overlying an 18 m thick soft to firm, slightly overconsolidated clay stratum, which is underlain by bedrock.This paper presents the results of field observation in general, with particular emphasis on the pore-pressure response. It was found that, in spite of the relatively low factor of safety against bearing capacity failure and the considerable load transfer, the maximum measured excess pore pressure amounts to only 40% of the measured applied stress at the silage–soil interface. Important dissipation of pore pressure during loading occurred. The postfilling rate of dissipation of pore pressure was also rapid, due to the high field value of the coefficient of consolidation. Consistently, the total settlement observed up to date was also small.An analysis of the results is carried out for the interpretation of the favourable performance. The implication to the design of similar foundations is also discussed.