Abstract: Sandy pebble soil has the characteristics of large internal friction angle, poor mobility and high permeability coefficient. It will be excavation difficulties, cutter head wear, groundwater spewing and other problems in the construction of shield in sandy cobble stratum. Through foam optimization test, the foam liquid concentration is 2%-3% when the foam half-life and foam expansion ratio are optimum. Based on the sand-pebble stratum of Chengdu metro line 7 in Sichuan Province, the soil with 20%, 40%, 60% and 80% of the gravel content is prepared. Large-scale indoor shearing test, slump test and permeability test are carrie out, it is shown that with the increase of foam injection ratio, the content of foam soil wrapped around sand and pebble soil increased, the phenomenon of sand and stone separation gradually disappeared; the shear strength and internal friction angle decreased nonlinearly; and the slump gradually increased. The impermeability can be obviously improved; and the foam injection ratio of different stone contents gravel soil satisfying the requirements of “ideal state soil” in shield construction is obtained. The result can be of great importance in solving the problems encountered in the construction of shield in sand pebble stratum.

Key words: sand cobble, foam, injection rate, shear strength, fluidity, permeability