Comparison of foundation settlements using soil parameter from consolidation test and drained triaxial test

Abstract

Shallow foundations are extensively used to support structures in order to safely transmit the structural load to the ground without exceeding the bearing capacity of the ground and causing excessive settlement. They are typically embedded up to a few meters into the soil. While designing foundations, two requirements need to be satisfied: complete failure of the foundation must be avoided with adequate margin of safety (bearing capacity) and relative settlement should be within limits that can be tolerated by superstructure, often the design is governed by allowable settlement criteria. The thesis aims to calculate the settlement of a foundation using soil parameters obtained from Oedometer Tests and Consolidated Drained Triaxial Tests (CD-Triaxial Tests) of over-consolidated Dhaka clay. Oedometer (consolidation) test results of Dhaka clay were collected from various sources such as private Sub-soil Investigation Company and BRTC, BUET. Similarly CD-Triaxial test results of Dhaka clay were also collected from BRTC, BUET. For this study, Oedometer and CD-Triaxial test were also performed using samples collected from Kurmitola, Dhaka and the tests were conducted at Geotechnical Laboratory, Department of Civil Engineering, BUET. In Dhaka city there is about 6-10 m over-consolidated clay under lain by dense sand in most of the places. Consolidation parameters of over-consolidated clay have been obtained from laboratory test results and results collected from different sources. Laboratory test results and results collected from various sources are similar in nature. The water contents ranges from 20% to 25%. The past maximum over burden pressure ranges from 150 to 200 kPa. The Cr (re-compressive index) value ranges from 0.02 to 0.05. The Cc (compressive index) value ranges from 0.10 to 0.15. The drained modulus E50 obtained from CD-Triaxial Test ranges between 15 MPa and 20 MPa. Settlement was calculated for a typical 3m squre footing resting at 3m below ground level with 150 kPa net pressure using soil parameters obtained from Oedometer tests and using soil parameters obtained from CD-Triaxial tests. It was observed that the settlement (13m) obtained from CD-Triaxial test results is smaller than the settlement (24m) obtained from Oedometer test results. Thus the parameters obtained from Oedometer test over estimates settlement. Using Geological Factor, (μg) from Skempton and Bjerrum (1957) with the settlement obtained using Oedometer test results is about 90% of the settlement calculated using CD-triaxial test results. Using settlements correction factor (λ) from HRB (1973) with Settlements obtained using Oedometer test results is about 70% of settlement calculated from CD-triaxial test results. For a typical raft foundation, settlement was also calculated and similar observation were observed. Therefore for settlement calculation it is recommend to use soil parameters from CDtriaxial test results. On the other hand Oedometer test parameters may be used with Skempton and Bjerrum (1957) and HBR(1973) correction factor.