Tunneling Ground Reinforcement by TAM Grouting: A Case History

摘要

The objectives of this project were to reinforce a section of the decomposed rock above a16.5?foot diameter tunnel and to improve the permeability of the zone above the tunnel to reduce seepage intothe tunnel during boring.The work was performed in two phases. Phase one consisted of installing seventy five (75), fifty footlong, 1-in. diameter fiberglass rods, spaced 3.5-feet apart in a fifty-foot section. Phase two consisted of injectinggrout from the ground level through twenty seven (27) TAM pipes into the rock, over a section of about225 feet over the tunnel.To install the fiberglass soil nails, 6-in diameter cased holes were advanced to about 50 feet below grade.The soil nails were 1-in. diameter fiberglass bars and 50 feet long. The soil nails were placed in the holes and theboreholes were filled with 4000-psi grout. The installation of seventy five (75) soil nails took about 10 weeks.The grouting operation involved injecting grout into pre-determined pattern locations horizontally andvertically into the rock formation using tube-a-manchette (TAM) methods, also known as permeation groutingtechnique. The grout was injected into the rock, below ground water table, starting from the spring line of thetunnel to about 30' above the spring line of the tunnel using cement grout material.The TAM pipes were constructed of 1.5-inch diameter PVC material and were about 50 feet long. TheTAM grout pipes were installed by advancing 6-in diameter holes, then setting the pipes in the boreholes. Theannular space of each of the borehole was grouted with a brittle sheathing grout.To determine the approximate in-situ permeability of the rock, a Falling Head permeability test was performedin each borehole prior to the TAM pipe installation.The grouting operation typically began with a 3:l, water cement ratio. Grout was injected and thickened to2:1, 1:1, and 0.5:1 until refusal was reached. A total of 250 cubic yards of grout was injected into the rock. Thegrouting operation took about 12 weeks.A detailed quality control procedure was accomplished through the use of an automatic data recordingsystem (HANY) connected to the grouting equipment. This equipment monitored grout intake and pressureover time, and provided a detailed record of the grouting operation.Three months after the completion of the grouting operation a series of exploration was performed in thegrouted area to determine and measure the effectiveness of the grouting operation. That exploration testing reveledpresence of grout in the overburden soil and in some accessible joints within the rock and the permeabilityof the grouted zone were reduced by up to 500 times. The grouting operation achieved its objectives.