Experimental Study On Dewatering Technique Of Vacuum Tube Well And Mechanism Discussion

To ensure the safety of foundation pit construction, we should take some measures for water drainage which called engineering dewatering. Currently,conventional dewatering can easily drain off the gravity water, but can hardly do anything for saturated water in aquitard or water in unsaturated zone.Vacuum tube well compound dewatering method achieved good efficiency in current construction, while the relative theory research is always at a standstill.Therefore, to replenish theory basis for vacuum tube well compound dewatering method is a pressing problem.In this paper, according to the principle of the vacuum tube well compound dewatering design in Beijing No. 10 subway project, I designed some indoor vacuum tube well compound dewatering experiments which use saturated silty soil as material.This series of experiments include 5 times of air extraction, 2 times of water pumping and 2 times of air extraction with water pumping.Based on these sufficient test data, it is easy to reflect the difference between vaccum dewatering and conventional dewatering when applied to saturated silty soil.Through deep analysis on indoor simulated test data, this paper summarizes the lateral and vertical change law in vacuum degree and pore water pressure,deduces the numerical relation between vacuum degree and pore water pressure of the aquitard which take silty soil as the major. Besides, this paper does some comparative analysis on water table,water quantity,water drawdown and water ratio between vacuum tube dewatering and conventional tube well dewatering. This paper alse does some brief analysis on seepage mechanism of vacuum tube compound dewatering. According to all the achivements based on data analysis,I provide some solutions and technical parameter when encounter problems in construction. The main research achievements in this paper can summarized as below:(1) The vacuum degree in the main well of the sily soil layer is larger than in the sandy layer.That is vacuum dewatering can play a more important role in aquitard;(2) When the vacuum test point is buried above the groundwater table, then the vacuum degree equals to the pore water pressure,while if the vacuum test point is buried below the groundwater table,then the vacuum degree has a determined numerical relation with the pore water pressure;(3) This paper deduces one-dimensional saturated flow equation under vacuum condition.I conclude that the change rate of water ratio increases under vacuum dewatering, so that rapid water pumping can be done;(4) The permeability coefficient decreases after vacuum dewatering to silty layer, which differs from sandy layer.To calculate the permeability, we can use the line S ? lg t /t′in water level recovery method.