底部抽真空法在围垦工程中优越性研究

 将应用于围垦工程中的底部抽真空技术同传统的上部抽真空方法进行对比试验，研究底部抽真空的优点。经过改进的仪器能够对试样施加80 kPa的真空压力，同时能够量测不同土层深度下土体的孔隙压力、沉降及体积随时间的变化情况。试验用土取自南京秦淮河，研究结论表明：底部抽真空方法对土体的加固效果优于上部抽真空方法。与上部抽真空方法相比，底部抽真空方法加固后土体含水率低10%，干密度大11%，不排水剪强度提高35%，最终沉降则增加22%。除此之外，试验结果还显示底部抽真空方法固结更快，不同深度土层加固效果差异较小，底部法还能够有效保持真空度的恒定。底部抽真空方法与上部抽真空方法的最大区别就在于：上部抽真空过程中，土体内部水位不会发生变化，因此总保持饱和状态。相反，采用底部法时，真空压力的施加使得排水板内的水位快速下降到土层底部，土体中水位也因此随时间增加而逐渐下降，土体不再被水浸没，可以由饱和态转变成为非饱和态。

ADVANTAGES STUDY OF VACUUM PRELOADING AT BOTTOM OF SOIL LAYER FOR RECLAMATION PROJECTS

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.