Evaluation of K0 in stiff clay by back-analysis of convergence measurements from unsupported cylindrical cavity
Abstrakt
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, K0 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 aG was 1.45, the ratio of horizontal and vertical Young's moduli aE was 1.67, and the value of Poisson ratio mtp 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 K0 1/4 1:3, while Ja'ky formula estimated K0 1/4 0:63 for the state of normal consolidation.
The back-analysed value of K0 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 K0 estimation. K0 of many clays may actually be lower than often assumed.