Mechanical Anisotropy of Diatomaceous Mudstone at Small Strain

Abstract

Recently, Neogene mudstone is being adopted as foundation of fill-type dam due to the lack of competent rock site in Japan. In this case, engineering problems such as the settlement of foundation due to the weight of embankment would occur. Hence the elastic moduli of material should be carefully evaluated from a very small strain level up to 10^-3 in order to calculate the deformation of the foundation during the embankment construction.
To investigate the deformation characteristics in small strain region, both triaxial compression (UU) and dynamic tests are performed in cylindrical specimens of Diatomaceous mudstone prepared in vertical and horizontal directions with mechanical anisotropy. The triaxial compression tests make it possible to evaluate not only the difference of Young's moduli due to cross-anisotropy, but also the variation of those with strain level at 10^-5-10^-3 obtained by measuring the axial displacements over the central half of a specimen by gap sensors. The dynamic tests comprising both resonant column and pulse methods, five independent elastic constants of cross-anisotropy including Young's moduli are established at small strain (about 10^-6) by measuring the resonant frequencies, compressive (P) and shear (S) wave velocities in vertical and horizontal directions.
The results obtained are summarized as follows:
(1) Specimens with the higher strain level exhibit the greater anisotropy as reflected by the ratio between the horizontal to vertical Young's moduli.
(2) The dynamic test results indicate that the strain dependency of Young's modulus in vertical direction exists to some degree even at 10^-6 strain level.
(3) The ratio between the horizontal to vertical shear modulus is greater than that of Young's modulus; it suggests that the Diatomaceous mudstone behaves as a cross-anisotropic material and the small strain Young's modulus in vertical direction as inferred from down-hole measurements assuming isotropy can be underestimated.
(4) Using laboratory dynamic test data, all elastic parameters of Diatomaceous mudstone with cross-anisotropy are determined.