Enhancement of efficiency of prefabricated vertical drains using surcharge, vacuum and heat preloading

Abstract

ABSTRACT: This paper focuses on increasing the efficiency in accelerating the rate of consolidation of prefabricated vertical drains (PVDs) in improved soft Bangkok clay using vacuum and heat preloading. Laboratory model tests were performed in new, large-scale consolidometers with PVD-improved reconstituted specimens using the following four schemes: surcharge preloading (PVD only), surcharge combined with vacuum pressure preloading (vacuum-PVD), surcharge combined with heat (thermo-PVD), and surcharge combined with heat and vacuum pressure (thermo-vacuum-PVD). Subsequently, the flow parameters in terms of the horizontal coefficient of consolidation (Ch) and the ratio between the horizontal hydraulic conductivity in the undisturbed zone (kh) to the horizontal hydraulic conductivity in the smear zone (ks), based on Hansbo's method. were investigated by back-calculation. The results show that the values of Ch were 1.93 m2/yr, 2.23 m2/yr, 4.17 m2/yr and 4.37 m2/yr for the PVD, vacuum-PVD, thermo-PVD and thermo-vacuum-PVD, respectively, with corresponding kh/ks values of 3.0, 2.7, 1.4 and 1.1, respectively. On the other hand, the increase in Ch of reconstituted specimens using the vacuum-PVD, thermo-PVD and thermo-vacuum-PVD were 15.54%, 116.06% and 126.42%, and the increases in kh/ks were 10.00%, 53.33% and 63.33%, respectively. Thus the vacuum-PVD can increase the horizontal coefficient of consolidation, Ch, resulting in a faster rate of settlement at the same magnitudes of settlement, compared with the PVD. The thermo-PVD and thermo-vacuum-PVD can further increase the coefficient of horizontal consolidation, Ch, with associated reduction of kh/ks values, by reducing the drainage retardation effects in the smear zone around the PVD, and consequently faster rates of consolidation were obtained. However, higher magnitudes of settlement were obtained upon application of heat owing to the associated collapse of the clay structure, similar to the results of previous investigations.