Two component backfilling in shield tunneling: laboratory procedure and results of a test campaign

The two-component backfilling system is becoming the most frequently used method to fill the annular gap created during shield machine advancement. It is based on the mixing of two fluids (mortar and accelerator) that flow through separated pipelines from the batching station to the machine tail and are mixed few centimeters before the output noz-zles. The induced turbulence allows a good mixing and the obtained new material gels in few seconds. The mortar is easy to be pumped, stable and preserves its workability for long time. The mixing process allows an easy management of the gelling time and the hardening speed of the final product. The check of the material properties is important and should be carried out before starting tunneling to assess both which mechanical properties can be obtained and how to manage the final resistance and the gelling time. Since no standardized laboratory schemes are available, laboratory procedures for both mortar manufacturing and testing the obtained samples are presented. The results of tests on hardened two-component grout produced with different mix design are also presented and discussed.