Thesis (M.Eng.) - Asian Institute of Technology

A thesis submitted in partial fulfillment of the requirement for the degree of Master of Engineering, School of Engineering and Technology

In order to study the feasibility of improving the soft Bangkok clay, two full scale, instrumented, test embankments were constructed and have been constantly monitored since their construction in December, 1985. One embankment was improved by using 0.3 m diameter granular piles with 1.5 m spacing in triangular pattern. The other was improved by Mebra vertical drains with the same spacing of granular piles. The granular piles and vertical drains were penetrated to the stiff clay layer at the depth of 8m. Both embankments had the same final height of 4 m and situated in the campus of Asian Institute of Technology (AIT). Previous studies have been done to analyzed the settlement and stability behavior of the composite grounds using analytical methods. In this study, for the first time, Finite Element Method (FEM) based on revised Cam clay model has been used for the consolidation analysis of these test embankments. It was found that the best agreements between the observed and predicted settlements were obtained for both embankments, in which, the smear effects due to installation have been considered by using the previous findings of BERGADO et al. (1990), wherein dJd.n = 2.5 with kJks = 10. For the case of granular piles, dJ'4 = 2 was a very good fitting parameter. Comparison with the predicted settlements using analytical methods obtained by previous investigators (BOONKIAT, 1987; APOLLO, 1989), both the ASAOKA (1978) and SKEMPTON-BJERRUM (1957) method gave good agreements for the embankment on vertical drains. However, for embankment on granular piles, only FEM and ASAOKA·s method yielded good results. The conventional method (ABOSHI et al., 1979; BARKSDALE, 1981) resulted in overestimation of settlement. The predicted stress concentration factor in granular pile improvement ranged from 2.4 ·to 8 depending on the degree of consolidation and locations within the piles. The results shown that the shear strength-to-shear stress ratio of ground improved by vertical drains increased to 1.3 times in the period of 600 days from the end of construction and almost remained constantly after that time. On the other hand, the shear strength-to-shear stress ratio of ground improved by granular piles was at least 1.25 times greater than that of the ground improved by vertical drains. The effects of ground subsiding on different depths of the ground improvement have been evaluated. For the condition of the test site, penetration of vertical drains and granular piles to the depth of 8 m, where the pore pressure drawdown about 1.3 m from the current hydrostatic pore pressure, additional settlements were obtained in both improved grounds. The results suggested that it was better to use 6 m of granular piles or vertical drains down to the top of the medium stiff clay, where the pore pressure was almost hydrostatic. An equivalent permeability for plane strain model of improved ground that has been introduced, yielded the time rate of consolidation in good agreement with observed data in both embankments. Other behavior of composite grounds, such as stress changes due to installation, failure stress of granular piles and surrounding soils have also been analyzed.