| Abstract | 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. |