Evaluation of dynamic properties of Dhaka sub-soil for ground shaking assessment

Abstract

The main objective of this research was to evaluate dynamic properties of Dhaka sub-soil for ground shaking assessment. Dynamic properties such as shear wave velocity, shear modulus and damping ratio of Dhaka sub-soil has been determined in this study. Correlations between shear wave velocity and SPT N-value as well as correlation between initial shear modulus and shear wave velocity for Dhaka sub-soil has also been developed in this study. Thirteen locations of Dhaka city were selected for 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:35-54, PI:15-34). Mean grain size of clay varies in the range 0.006-0.04 mm and the same for sand varies in the range 0.085-0.31 mm. From the test results, it is found that Dhaka clay is medium stiff to very stiff with SPT N-value 5 to 15. Dhaka sand is medium dense to very dense with SPT N-value15-50. Filling sand is loose to medium dense with SPT N-value 6-15. Unconfined compressive strength, qu of Dhaka clay varies widely in the range between 30kPa and 280kPa. Internal friction angle, υ obtained from CU direct shear test ranges from 15°− 35°.The value of υ and cohesion obtained by tri-axial test were found to be 6° and 78 kPa, respectively.Internal friction angle obtained from tri-axial test under CU and CD condition was found to be 15° and 20° respectively. Among the selected thirteen sites, shear wave velocity was determined for ten sites using PS logging test (down-hole method).The value of Vs for clay layer was found between 40 - 404 m/s. Shear wave velocity for sand layer was found between 87 m/sand 746 m/s. For dredged fill sand the value of Vs was found to vary between 89 m/s and 198 m/s. Maximum shear modulus (Gmax) was determined by conducting (i) tri-axial test and (ii) the empirical equation, Go = ρ×Vs2. The value of maximum shear modulus was found in the range of 43-294 MPa for confining pressure from 100-300 kPa. Shear modulus G was determined using the empirical equation G = ρ × Vs2 at 26 locations for sand, clay and filling sand layers. Range of the value of shear modulus varies from 14 to 408 MPa for sand. The value of Gmax varies from 10 – 337 MPa for clay and 13 to 92 MPa for filling sand. Damping ratio (D) was determined by tri-axial test for sand at one location. The value of damping ratio varies from 0.232–0.238 for confining pressure 100-300 kPa. Correlations between shear wave velocity and SPT N-value as well as correlation between initial shear modulus and shear wave velocity were developed by using regression analysis for both sandy soil and clayey soil. Correlation between shear wave velocity and SPT N-value was found to be Vs = 147 N 0.19 for sandy soil. The correlation was found to be Vs = 84 N0.30 for clayey soil. Correlations between initial shear modulus and shear wave velocity were found G0 = 0.0015 Vs2.03 for sandy soil and G0 = 0.0028 Vs1.91 for clayey soil. Dynamic properties determined in the study are comparable with the data provided by other researchers for the similar soils. Findings of this study will be useful for ground shaking assessment for structures founded on Dhaka sub-soil.