A numerical simulation study on the chemical reinforcement area at shield start shaft

When the ends of shield tunnel in soft soil regions are reinforced in a chemical manner, it is vital to solve some urgent key problems, such as determining the range of shield start shaft which requires reinforcement, the reinforcement time of various reinforcement techniques. Frequently-adopted methods to reinforce the soil at the shield tunnel ends were summarized in this paper first, then by combining the case of shield launching project (exclusive of aquifer) of Fenhu Road Station's west end in Suzhou Subway, ADINA program was applied here to conduct simulation analysis of different reinforcement areas at shield start shaft under the most unfavorable construction condition-removal of the sealed door. The research indicates: a) Frequently-adopted methods to reinforce shield tunnel ends include deep mixing method, high pressurized jet grouting, artificial freezing and precipitation technique, etc. Soil reinforcement may adopt one method or combination of many methods; b) According to the numerical simulation, when longitudinal reinforcement length reaches 3mm, the soil shall move towards the working well in the tunneling direction of shield tunnel, the maximum displacement occurs in the center of exposed tunnel face, up to 12.92mm. Surface soil above the sealed door becomes deformed most, settling by approximately 3.0mm and soil within the strong reinforcement area is stressed in designed strength range; c) At the shield start shaft which is exclusive of aquifer, if the longitudinal reinforcement length reaches 3m, requirements in both strength and deformation can be met, and it is considerably safe; d) Most of the shield tunnel ends in Suzhou Subway are distributed in rich water-bearing layer (especially water-bearing sand layer), therefore, the longitudinal reinforcement length is usually designed to be 9m.