Determination of bearing capacity of shallow foundation on Dhaka sub-soil using subloading tij model

Abstract

Elasto-plastic constitutive model parameter identification is an important task for proper modeling of any soil. At first, static and dynamic properties of Dhaka sub-soil were determined in this study. Six sites were selected in this research. It is found that Dhaka sub-soil mainly consists of top red clay layer underlying by a fine sand layer up to infinite extent. Dhaka clay shows low to medium plasticity (liquid limit: 38 to 60; plasticity index: 15 to 38). Strength properties were determined for Dhaka sub-soil. From the test results it is observed that Dhaka clay is stiff to very stiff with SPT N-value 5 to 15 and Dhaka sand is medium dense to very dense with SPT N-value 15 to 50. Unconfined compressive strength, qu of Dhaka clay is found to vary in the range between 60 kPa and 700 kPa. From direct shear tests, it is found that angle of internal friction of Dhaka sand varies between 300 and 360. Tri-axial tests were conducted for Dhaka clay samples. From this test results, value of cohesion, c is found to be 78 kPa under consolidated undrained condition. Tri-axial tests were also conducted for Dhaka sand under consolidated drained and undrained condition from which the angle of internal friction is found to be 200 and 150 respectively. 1-D consolidation tests were conducted to determine the compressibility properties of Dhaka clay. Compression index, Cc; swelling index, Cs and OCR value are found to vary in the range from 0.11 to 0.28; 0.04 to 0.06 and 1.2 to 3.3 respectively. Dynamic properties were determined for the reconstituted samples of Dhaka fine sand by conducting cyclic tri-axial tests at three different confining pressure i.e., 100 kPa, 200 kPa and 300 kPa. Value of maximum shear modulus (G0) of Dhaka fine sand varies in the range between 10 and 50 MPa. It is found that stiffness of Dhaka sand increases with the increases of confining pressure. The value of damping ratio is found to vary between 0.227 and 0.238. From the value of damping ratio, it can be said that Dhaka sand is under-damped. In this study, an elasto-plastic constitutive model i.e., Subloading tij model (Nakai and Hinokio, 2004) was used for numerical analysis. The parameters of this model were obtained from the simulation of the results obtained from tri-axial and consolidation tests. Value of λ, κ, Rcs, β, a and b for clay are 0.080, 0.0078, 3.82, 1.5, 600 and 2.5 respectively. Value of λ, κ, Rcs, β and a for sand are 0.088, 0.015, 1.64, 2.0 and 600 respectively. Using these parameters, bearing capacity of Dhaka sub-soil has been estimated for different OCRs and considering bonding effect which cannot be obtained using classical theories. Considering the effect of shape factor for shallow foundation 2D and 3D FE analyses has been conducted. It is found that bearing capacity determined by the conventional methods match well with the results of the numerical simulations.