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Abstract An experimental program is conducted to study the advantages of vacuum preloading applied at the bottom of soil layer，named VAB method，in comparison with vacuum preloading conventionally applied to the top of soil layer，named VOT method，for reclamation projects like new slurry ponds. The modified apparatus has capacity of applying designated vacuum pressure of 80 kPa to soil body；otherwise it is also possible to measure the pore pressure at different depths of the soil column，the settlement and the volume change along elapsed time. Two series of test are implemented on silty clay dredged from Qinhuai River in Nanjing，China. The study concludes that the vacuum preloading has generally remarkable effects on soil improvement. However，the effects by using the VAB method are much better than that by using VOT method. It is found that the water content more decreased by about 10%；the dry density more increased by about 11%. The undrained shear strength has increased by about 35%. The final settlement is about 22% greater. The results also demonstrate that the VAB method is less time consuming and the improvement is steadier along the soil depths. In addition，the maintenance of vacuity is much more convenient. The essential difference between the two methods is that for the VOT method，the relative water table level remains unchanged in soil mass during vacuum procedure. The soil is therefore always in submerged and saturated states. On the contrary，with the VAB method the water table level inside the PVDs falls down very quickly to bottom of soil layer after the vacuum pressure is fully applied. The water table level in soil mass also drops with elapsed time. The soil is not in submerged state anymore and may change from saturated to unsaturated state.
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Received: 19 January 2009
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