In-situ tests of the hydraulic performance of grout borehole seals.

Abstract

Three tests are proposed for determining the hydraulic properties of in-situ borehole seals. Two consist of monitoring the rate of injection of water at constant pressure into an injection zone at one end of a seal and monitoring the collection rate or rate of flow out of a free-draining collection zone at the other end. The third test is performed by shutting in the collection zone and monitoring the buildup in hydraulic head. One-dimensional and axisymmetric three-dimensional flow models are presented for analyzing test results. In the one-dimensional models, the seal is assumed to be a homogeneous and isotropic porous medium. In the axisymmetric models, the seal and surrounding rock mass are taken as homogeneous and isotropic porous media. The equation for saturated, confined ground-water flow is assumed to apply. The hydraulic properties of the seal are expressed by its hydraulic conductivity and specific storage. In the axisymmetric models, the conductivity and specific storage of the rock mass are included in the formulation. Closed-form solutions are presented for the analysis of tests using the one-dimensional models. Analysis with the axisymmetric models is numerical using an available computer code for ground-water flow. The code is used to examine the effects of variations in hydraulic parameters on the measured quantities in the tests (i.e. flow rates or head) and to compare the one-dimensional and axisymmetric models. Methods are presented for obtaining the hydraulic properties of the seal and/or rock mass by analysis of test results. A fourth test, a tracer travel-time test, is presented as a means for detecting the existence of a high-velocity flow path through or around the seal. The test methods are applied to cement grout borehole seals from 10 to 36 cm in length and 10 cm in diameter in two rock types, a recrystallized limestone and a dense basalt.