Bottom-heave control of a deep sensitive clay excavation in Ottawa, Canada

Abstract

For a new Police Headquarters Complex in Ottawa, an excavation was required to a depth of 10 m in medium to stiff sensitive silty clays and clayey silts. The risk of a bottom-heave failure due to uplift caused by significant groundwater pressures in the silty soil a few metres below the bottom of the excavation was recognized, and contract requirements included a wellpoint installation assisted by electro-osmosis. The successful bidder was able to develop an alternative method based on the observational approach and completed the foundation at a lesser cost. A small test excavation was instrumented, and digging proceeded cautiously to full depth. The observed bottom heave and pore pressure changes were encouraging, so the test area was enlarged in steps until a workable size of excavation was safely made, then the structural raft was able to be placed. A backup system of pore pressure relief wells using wick drains in the bottom of the excavation was also developed. A large pore pressure reduction, due to the release of overburden pressure, was expected by the contractor's consultant, and a slow regain of the groundwater pressure was also expected due to the low horizontal permeabilities involved. The test excavation instrumentation and observations confirmed these expectations. Key words: bottom heave, excavation, clay, pore pressure.