防渗墙钻孔压水试验渗流参数研究

Study of seepage parameters measured in water pressure test inside boreholes in cut-off wall

钻孔压水试验所得到透水率及渗透系数仅限于半无限域岩体,若条形域防渗墙钻孔压水试验仍沿用半无限域公式计算其渗流参数显然不合适。针对防渗墙几何形状和钻孔压水试验原理,在基于稳定流承压完整井的渗流场条件下,采用复变函数方法将条形域中井流问题变换到半无限域中,再代入直线补给边界稳定流承压完整井流公式求解,从而导出条形域防渗墙上钻孔压水试验渗流参数求解表达式。提出对条形域与半无限域流量比Q'/Q和渗透系数比k'/k进行分析,讨论防渗墙与钻孔几何尺寸L/r_0,r_0对其影响;当取L/r_0<104.7,r_0<75 mm,k'=k或Q'=Q时,得出条形域流量Q'和透水率q'均大于半无限域,即Q'>Q,q'>q,而渗透系数k'<k。根据条形域与防渗墙两侧的补给边界情况,给出防渗墙钻孔压水试验渗流流量表达式使用条件。经工程实例验证,在土体与墙体渗透系数比k_1/k_2>100时按提出的计算方法与芯样室内渗透试验结果基本一致。该研究方法还可将钻孔压水试验推广到其他形状有限域渗流参数求解中。

The permeability and permeability coefficient from water pressure test in boreholes are limited to the semi-infinite domain of rock.The cut-off wall is a strip domain,so it is clearly inappropriate to calculate the flow parameters using the equation for semi-infinite domain.The method of complex variable was used to transform the strip flow domain into the semi-infinite domain and permeability coefficient for strip domain was thus derived.The ratio Q’/Q of flux and k’/k of permeability coefficient for the semi-infinite and strip domains were analyzed.The influence of cut-off wall and drilling geometry L/r0 and r0 was discussed.When L/r0 〈 104.7,r0 〈 75 mm,k’=k or Q’=Q,the flux Q’ and the water permeability q’ in strip domain were greater than ones in the semi-infinite domain,namely Q’〉 Q,q’〉 q,and the permeability coefficient k’ 〈 k.The conditions of seepage flow equation for cut-off wall measured with water pressure test in borehole were set according to the recharge boundary conditions on both sides of the strip domain and cut-off wall.Engineering examples showed that the calculated results with the proposed method agreed with the measured ones when the soil and the wall permeability ratio k1/k2 〉 100.