Evaluation of K 0 in stiff clay by back-analysis of convergence measurements from unsupported cylindrical cavity

Abstract

The coefficient of earth pressure at rest K 0 of fine-grained soils is often being estimated empirically from the overconsolidation ratio (OCR). The relationships adopted in this estimation, however, assume that K 0 is caused by pure mechanical unloading and do not consider that a significant proportion of the apparent preconsolidation pressure may be caused by the effects of ageing, in particular by secondary compression. In this work, K 0 of Brno Tegel, which is a clay of stiff to hard consistency (apparent vertical preconsolidation pressure of 1800 kPa, apparent OCR of 7), was estimated based on back-analysis of convergence measurements from unsupported cylindrical cavity. The values were subsequently verified by analysing a supported exploratory adit and a two-lane road tunnel. As the simulation results are primarily influenced by soil anisotropy, it was quantified in an experimental programme. The ratio of shear moduli $$\alpha _G$$ was 1.45, the ratio of horizontal and vertical Young's moduli $$\alpha _E$$ was 1.67, and the value of Poisson ratio $$\nu _{tp}$$ was close to 0. The soil was described using a hypoplastic model considering very small strain stiffness anisotropy. For the given soil, the OCR-based estimation yielded $$K_0=1.3$$ , while Jaky formula estimated $$K_0=0.63$$ for the state of normal consolidation. The back-analysed value of K 0 was 0.75. The predicted tunnel displacements agreed well with the monitoring data, giving additional confidence into the selected modelling approach. It was concluded that OCR-based equations should not be used automatically for K 0 estimation. K 0 of many clays may actually be lower than often assumed.