Pore pressures in cut slopes

When clay ground is excavated to form a cut slope the reduction in effective stress can induce negative pore water pressures if the ground remains undrained. This can give the slope a temporary stability in the short term, which can be lost if the pore water pressures increase. Delayed failure of clay slopes is a common occurence in the UK.

The time to failure is frequently many decades and can be influenced by the presence of surface vegetation. The potential for instability can be assessed by measuring the insitu pore water pressures (which could still be negative) and comparing them with the long-term expected pore water pressures. Analysis will reveal the expected time to failure.

GeO flushable piezometers and suction probes are ideal for monitoring the pore water pressures in cut slopes. The figure below shows the pore water pressures recorded 40 years after the construction of a cutting formed in stiff clay and with dense surface vegetation on the upper part of the slope.

Geotechnical Observations Limited
Brunel Science Centre
Coopers Hill Lane, Egham
Surrey, TW20 0JZ, United Kingdom

Tel: +44 (0)1784 473733
Fax: +44 (0)1784 473734
Email: info@geo-observations.com
Web: www.geo-observations.com