Inherent stiffness anisotropy of clay and its usage in the numerical back-analysis of displacements in the unsupported cavern during the process of K0 estimation in Brno "tegel"
Abstract
Determination of a complete set of material parameters of inherent anisotropic stiffness in the very-small strain range requires realization of the laboratory tests using small sensors (Local Displacement Transducers - LDT) placed on the sample, in combination with piezo-elements sending and receiving the seismic longitudinal or shear waves. On the basis of clay particles sedimentation idea, the clays are considered a cross-anisotropic material with 5 independent parameters.
For the purposes of implementation of anisotropy into hypoplastic model, the tensorial formulation had to be developed. The modification of the hypoplastic model was subsequently used in the analysis of the coefficient of lateral earth pressure at rest of overconsolidated clayey soil.
The coefficient of earth pressure at rest K0 of fine-grained soils is often being estimated empirically from the overconsolidation ratio (OCR). The relationships adopted in this estimation, however, assume that K0 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 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 the back-analysis of convergence measurements of an 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 maximal shear moduli αG was 1.45, the ratio of horizontal and vertical maximal Young moduli αE was 1.67 and the value of Poisson ratio νtp was close to 0.
The soil was described using a hypoplastic model considering small-strain stiffness anisotropy. The back-analysed value of K0 was 0.75.