Stiffness and strength anisotropy of overconsolidated Bootlegger Cove clays

This paper presents the evaluation of the stiffness and strength anisotropy of overconsolidated (OC) Bootlegger Cove Formation (BCF) clays at the Port of Alaska, formerly known as the Port of Anchorage. The stiffness and strength anisotropic material response was evaluated based on triaxial samples equipped with internal instrumentation including a submersible load cell and three subminiature linear variable displacement transducers (LVDTs). Three sets of bender elements were used in this research to measure shear wave velocities for different propagation and polarization directions. The effects of reproducing the stress history of the soil deposit on the stiffness cross-anisotropic behavior of the material are discussed. The laboratory test results are compared with in situ measurements of shear wave velocities based on suspension logging and crosshole and downhole soundings. The results of the experimental program showed that BCF clay is a cross-anisotropic material. Mean stiffness anisotropy ratios ranged from 0.90 to 1.22 and 0.93 to 1.46 for lightly OC and OC conditions, respectively. Strength anisotropy ratios, defined as the ratio of undrained shear strength under triaxial extension to compression, varied between 0.8 and 0.5. It is found that reproducing the stress history of the OC soil deposit during the laboratory reconsolidation stage did not have a significant impact on the initial stiffness anisotropy ratios of the BCF clay.

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Determination of small-strain stiffness of Shanghai clay on prismatic soil specimen

Canadian Geotechnical Journal ◽

10.1139/t2012-050 ◽

2012 ◽

Vol 49 (8) ◽

pp. 986-993 ◽

Cited By ~ 14

Author(s):

Q. Li ◽

C.W.W. Ng ◽

G.B. Liu

Keyword(s):

Stress State ◽

Shear Wave ◽

Small Strain ◽

Triaxial Tests ◽

Shear Moduli ◽

Small Strain Stiffness ◽

Wave Velocities ◽

Shear Wave Velocities ◽

Stiffness Anisotropy ◽

Elastic Shear

Although a large number of tunnels and deep excavations have been constructed in Shanghai, small-strain stiffness properties of natural Shanghai clay have rarely been reported in the literature. In this study, the degree of inherent stiffness anisotropy of natural Shanghai clay was investigated in a triaxial apparatus equipped with local strain transducers and a shear-wave velocity measurement system. Three sets of side-mounted bender elements, consisting of one transmitter and two receivers each, were installed on a prismatic specimen. Two series of triaxial tests on prismatic specimens of intact Shanghai clay were carried out under an isotropic stress state. Shear-wave velocities and hence elastic shear moduli in different planes were determined from bender element measurements. The cross-correlation method using two received signals gives rise to the most objective and repeatable results on shear-wave velocities in comparison with other commonly used methods. Intact Shanghai clay clearly exhibits inherent stiffness anisotropy in terms of its elastic shear modulus ratio (G0(hh)/G0(hv)) of about 1.2 for a mean effective stress varying from 50 to 400 kPa. The measured higher stiffness in the horizontal plane may be attributed to the stronger layering structure in the horizontal bedding plane. A unique relationship is found that relates the normalized shear moduli to the stress state in each plane by incorporating a void ratio function in the form of F(e) = e–2.6.

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