The influence of earthworks construction on pore water pressures in clays and mudstones of the Lias Group

Abstract

Monitoring the changes in pore water pressure associated with the construction of earthworks can yield information on the stiffness and permeability of the ground, as well as how the natural groundwater regime might be impacted. This paper presents three years of pore water pressure measurements in weathered Lias Group mudstone, obtained from a trial cutting and a trial embankment constructed for the UK's High Speed Two (HS2) railway. The immediate changes in pore water pressure were small in relation to the changes in total stress imposed. This can be explained by the consolidation or swelling during the period of construction, combined with the sensitivity of very stiff clays and mudstones to a very small (0.5%) reduction in the degree of saturation. In the longer-term, pore water pressures reduced across the site owing to the reduction in ground level at the trial cutting. Rates of pore water pressure change were accelerated by more permeable limestone within the ground profile reducing drainage path lengths. It is concluded that construction-induced pore water pressure changes may be smaller, and their rate of dissipation more rapid, in weathered clays and mudstones such as those of the Lias Group than in younger, more compressible clay deposits.