An optimized grout mix design for EPB TBM tunneling of Shiraz metro in dry condition

In tunneling by shield TBM, there is a need to place material behind segmental linings to fill the gap (voids) created during excavation and subsequent lining process. This stabilizes liner during construction, helps reduce permeability of final liner system, put liner in full contact with surrounding ground to allow load transfer and decreases the risk of tunnel instability as well as surface settelemnt. There are two distinct void-filling methods used in underground construction, backfilling and contact grouting. The void-filling materials can be a cementitious grout, conventional concrete or cellular concrete. They are chractrized as good filler with a final strength close to environment materials and flowable, which significantly decrease groundwater flow to tunnel. In the Shiraz metro project, located in south of Iran, a grout was designed for wet condition for which bentonite used to make grout more flowable as well as reducing permeability. As tunnel was passed through a dry environment, there was no need using bentonite and a new cost effective grout should be designed to cover the needs for a dry condition. In this research, based upon 36 tests carried out, different grouts were designed and it was concluded that: increasing the value of cement in grout mixture decreases cementation time and bleedingvalue, which in return is cost effective. Also, adding bentonite more than 3% decreases the pumpability of grout. Moreover, with increasing rock powder and sand, flow and Lombardi values increase. Based on the results obtained, two mixtures, which satisfied the project criteria, were selected as optimum mix designs (Table 1). In grouts No. 1 and No. 2 the ratio of sand to rock powder are 2.6 and 2.42 respectively. Also, the ratio of sand to water for grout No. 1 is 2.37 and for grout No. 2 is 2.42, which regarding to the flow of grout No. 1 (41 s) and grout No. 2 (42.5 s) as well as Lombardi values of grout No. 1(1.6 kg/m2) and grout No. 2 (1.7 kg/m2) satisfaied the project criteria. Furthermore, two grouts obtained with small bleeding values of 3% and 5% and UCS of 215 KPa and 231 KPa, covered dry condition requirements.