Performance of using PVD with and without Vacuum Preloading

摘要

This paper presents the comparison of test results involving prefabricated vertical drain (PVD) with and without vacuum preloading. Laboratory tests were conducted in large scale consolidometer having diameter of 300 mm and height of 500 mm with reconstituted specimens using PVD with and without vacuum. In addition, field data were collected from the site of the Second Bangkok International Airport (SBIA) improved by PVDs with and without vacuum preloading. Analyses were carried out to compare the compressibility parameters by back-calculation of laboratory and field settlements using Asaoka (1978) and Hansbo (1979) methods. From the laboratory tests, the C_h value of reconstituted specimens were 1.05 and 2.20 m~2/yr for PVD without and with vacuum preloading, respectively. In addition, k_h/k_s value of reconstituted specimens for PVD without and with vacuum preloading were 1.50 and 1.45, respectively. The horizontal coefficient of consolidation of PVD with vacuum preloading was higher than PVD without vacuum preloading. After the improvement, the water content was reduced which consequent increase in the undrained shear strength. The field data analysis based from back-calculated results showed that the k_h/k_s amounted to 4 for both PVD with and without vacuum preloading. The C_h values amounted to 4.36 m~2/yr and 6.21 m~2/yr for PVDs without and with vacuum preloading, respectively. The time to reach 90% degree of consolidation for PVD with vacuum preloading was shorter than PVD without vacuum preloading by about one-third because of higher C_h value. The settlement predictions by Asaoka (1978) graphical method agreed with the observed values. Thus, the addition of vacuum pressure leads to the increase of the horizontal coefficient of consolidation which shortened the time of preloading. Moreover, the k_h/k_s values were almost the same with and without vacuum preloading.