A. S. Balasubramaniam

INTERACTION BETWEEN COHESIVE-FRICTIONAL SOIL AND VARIOUS GRID REINFORCEMENTS

Abstract A total of 52 large-scale laboratory pullout and 24 large-scale direct-shear tests were conducted to investigate the interaction behavior between the different reinforcements and cohesive-frictional soil. The reinforcements used were steel grids, bamboo grids, and polymer geogrids. The backfill material used was locally available weathered Bangkok clay. The test results show that the inextensible reinforcements, such as steel grids, move approximately as a rigid body during the pullout test, and the maximum pullout resistance was reached within a relatively small pullout displacement. For extensible reinforcements, such as Tensar geogrids, the degree of resistance mobilization along the reinforcement varies, and the pullout-resistance achieved in the tests was controlled by the stiffness of the reinforcement. For steel grids, the friction resistance from the longitudinal member contributed only to about 10% of the total pullout resistance of the grids. The pullout of the bamboo and Tensar geogrids without transverse members yields 80–90% of the pullout resistance of the corresponding grids with transverse members, attributed to the nodes or ribs on longitudinal members. The bond coefficient as calculated for steel and bamboo grids demonstrated that the steel grids yielded a higher bond coefficient than that of the bamboo grids with the same grid size. However, for a polymer geogrid, the bond coefficient cannot be calculated from a pullout test because of the complicated pullout-resistance-mobilization mechanism along the reinforcement. The large-scale direct-shear-test results showed that, for the soil/grid-reinforcement interfaces, shear resistance can exceed the direct-shear resistance of the soil itself owing to the influence of the apertures on the grids. Finally, for compacted weathered clay, the strength parameters obtained from large-scale direct-shear tests were found to be substantially smaller than the results of triaxial UU tests. This may be because the failure plane in the large-scale direct-shear test was formed progressively, and the peak soil strength along the predetermined shear plane may not have been mobilized simultaneously.

IMPROVEMENT OF SOFT BANGKOK CLAY USING VERTICAL DRAINS

Abstract This paper presents case records demonstrating the use of vertical drains on soft Bangkok clay. The documented cases include four sand drains and two prefabricated vertical drains. The observed performances of the vertical drains are evaluated. The effects of soil flow parameters, including smear effects due to the installation of the drains, in particular the prefabricated vertical drains, are also evaluated in the light of the current practice of using vertical drains for ground improvement. It is concluded that the use of vertical drains is a viable ground improvement method for soft Bangkok clay.

PROPOSED CRITERIA FOR DISCHARGE CAPACITY OF PREFABRICATED VERTICAL DRAINS

Abstract Modified triaxial tests and ASTM-based discharge capacity tests were carried out to obtain specifications of discharge capacity. The test results indicate that, at straight condition, ASTM-based discharge capacity decreases with increasing lateral pressure, time and hydraulic gradient. With lateral pressure application, the filter jacket of a prefabricated vertical drain (PVD) is pressed into the channel system of the core, thereby reducing the flow area. Furthermore, the discharge capacity decreases at a high hydraulic gradient due to loss of flow energy as a result of turbulent flow. Moreover, discharge capacity has been investigated by simulating the different possible drain deformations in the field. The average percentage reductions of the discharge capacity from the straight condition to various PVD deformed conditions have been found as follows: 26, 32, 33, 43, 48 and 78%.for conditions of 10% bent, 20% bent, 90° twisted, 180° twisted, 20% bent with one clamp and 30% bent with two clamps, respectively. Subsequently, reduction factors for discharge capacity have been formulated. Moreover, a simple method is proposed to evaluate specifications of discharge capacity with some established relationship considering the length of PVD, .spacing, time required for consolidation and the magnitude of the horizontal coefficient of consolidation.

Prediction of vertical-band-drain performance by the finite-element method

Abstract This paper presents the predictions of the performance of vertical-band drains (i.e. Alidrains) on soft Bangkok clay under full-scale embankment loading in the field and during large-scale consolidation with a drain by using a reconstituted sample in the laboratory. A finite-element model of transient- and axisymmetric-flow problems considering equal strain was used. The numerical model also included the smear effects. The predicted data were compared with the laboratory- and field-test results. Laboratory consolidation tests using a Rowe cell and an oedometer apparatus were performed to obtain the compressibility parameters. By using a value of kh/ks of together with ds=2dm, the predicted results agreed with the field performance of the vertical drains. In the laboratory, by using a reconstituted clay specimen, better predictions were obtained with kh/ks=2 and ds=2ds. Thus, the prediction by means of an equal-strain finite-element model was found to be satisfactory for the performance of vertical-band drains on soft Bangkok clay provided that the disturbance due to its installation was taken into account.

Improvement of soft Bangkok clay using vertical geotextile band drains compared with granular piles

Abstract For the first time, improvement of the soft Bangkok clay by accelerated consolidation has been studied by constructing two full-scale, instrumented, test embankments. In one, consolidation was accelerated by vertical band drains and in the other, by granular piles. The embankment on granular piles, initially 2·4 m high, was later raised to a height of 4 m after 345 days, to impose the same level of applied stress as the other embankment on vertical band drains. The actual settlements and pore pressure build-up have been constantly monitored for both embankments since their construction in 1985. Relative comparison with previous embankment performance on umimproved ground demonstrated the effectiveness of the vertical drains to accelerate the consolidation process and increase the shear strength of the soft clay. The installation of vertical band drains at 1·5 m spacing on a triangular pattern, resulted in achieving 90% consolidation after 430 days using 4 m high embankment preloading. In contrast, the embankment on granular piles indicated reduction in settlements by as much as 20–40%. The results suggested that the granular piles seemed to reinforce the soft clay rather than draining it, increasing the bearing capacity up to four times and increasing the slope stability safety factor by up to 25%. Comparison of actual and predicted values using different methods of settlement predictions are presented. It was found that for the embankment on vertical drains, best agreement between the observed and predicted settlments were obtained using the Asaoka method adopting back-analyzed parameters. The Skempton & Bjerrum method yielded good agreement with the observed data except at the initial stages of loading, where overestimation of settlement could be seen. In addition, underestimation of settlements resulted using the one-dimensional method. Settlement reduction due to the presence of granular piles were calculated using the equilibrium (Aboshi et al.; Barksdale) and finite element (Balaam et al.) methods. The equilibrium method resulted in overestimation of settlements while Balaams finite element formulation yielded good agreement with the observed data.

Laboratory pull-out tests using bamboo and polymer geogrids including a case study

Abstract This study investigates the interaction between soil and geogrids by using both direct shear and pull-out tests and applied the results to a case study. A polymer geogrid and bamboo grids were used with clayey sand and weathered clay as backfill since these materials are readily available in Thailand. The results indicated that the interaction between soil and reinforcement consists of: (a) the adhesion between soil and reinforcement on the solid surface area of the geogrid; and (b) the bearing capacity of soil in front of all transverse members of the geogrids which behaved as a strip footing embedded in the soil. The proposed design procedure for pull-out resistance agreed fairly well with the laboratory pull-out test results. In addition, it was observed that bamboo grids have higher pull-out resistance per unit area than the polymer geogrids. Moreover, the cohesive fill proved to be quite effective when used with geogrid reinforcement. Finally, the proposed design procedure and test results were applied to a case study on an irrigation canal bank repaired by the Public Works Department of Thailand using cohesive backfill and Tensar SS2 geogrids resulting in much improved slope stability.