Motorway slip report warns ‘go easy’ with clay embankments : Byrd, T New Civ Engr, N516, 11 Nov 1982, P18–20

Abstract

The final report on the failure of a massive M25 embankment has not blamed a terram separation membrane, although the fabric possibly promoted a soft clay layer. Failure involved a 200 M long section of the batter slope breaking away with secondary rotation of the fill behind. It was concluded that the ground probably contained shear surfaces, that it was saturated with rain at the time of loading and the drainage blanket was not draining freely above it. A loss of strength due to softening of the clay immediately above the separation membrane occurred and possible segregation of the finer size clay particles during migration of moisture through the membrane made the problem worse. Tendency of the original ground to spread all contributed to the collapse. Very careful sampling and evaluation of the top-most layer of the foundation are needed to assess the influence of discontinuities and pre-existing shear surfaces. The effect of rainfall on the natural ground and the effectiveness of any drainage blanket need to be carefully assessed. Also a short break in construction of an embankment on saturated clay should allow local excess pore water pressures to dissipate through fissures and drainage blankets. (TRRL)