Abstract：The main types of shield machine used recently in tunnel construction are earth pressure balance(EPB) shield and slurry shield. In order to keep muck discharged fluently and lighten machine burden，the soils in chamber of EPB shield should be plastic and fluid. This is fundamentally critical to ensure the shield to advance continuously and smoothly. Sandy cobble without water is a typically unstable stratum and its mechanical properties are unfavorable to force’s transitivity and spread. Sandy cobble is of unfavorable plasticity and fluidity，which is sure to bring serious wear of cutter wheel and screw. Moreover，earth pressure balance of face is not easily maintained and the face is easy to collapse. As a case of EPB shield construction，a sewage tunnel in Beijing is presented. In the beginning 200 m of the shield tunnel，field driving tests are carried out. Firstly mud is injected into the chamber to sustain the plasticity and fluidity，but perfect effects are not acquired. Then foam technology is studied experimentally and used in field shield construction，in which calculation method and dosage of the foam are recommended. Then test results between injection of mud individual and combined injection of mud and foam are compared；and oil pressure of the cutter wheel and the screwer of the later is about 1/3 of the former；surface settlements also decrease from 20 mm to 5 mm. Finally，the mechanism of foam effects is analyzed and its effects can be summed up to two aspects as following：(1) forming a admixture level in the tunnel face composed of sandy cobbles，mud and foam；(2) forming a thin membrane around every granules. Study and practice prove that the mud and foam are not only beneficial to earth pressure balance，but also can lighten mechanical load and abrasion，which are remarkably significant to smooth cutting，soil discharge and high-speed advance of the EPB shield.
