Abstract: During tunneling in fully weathered granite formations, the earth pressure balance shield faces various challenges such as the accumulation of muck cake on the cutterhead and the blockage of the screw conveyor. To address these challenges, the east extension project of the Shenyang metro line 1 is used as a case study to systematically examine the compressive and shear properties of foam-conditioned soil. First, slump tests are performed based on a central composite rotational design to clarify the influence of water content, the volume fraction of the foaming agent, and the foam-injection ratio on the flow plasticity of the conditioned soil. Using a self-developed pressurized rotary shear apparatus and Box-Behnken response surface experiments, the effects of variations in the water content, foam-injection ratio, and vertical pressure on the porosity, compression coefficient, and tangential adhesion strength of the conditioned soil are examined. Finally, regression models are established for variance analysis and multiobjective optimization, and the results are validated through field tests. The experimental results indicate the following: (1) The addition of foam markedly increases the porosity of the conditioned soil. When the water content is 18% and the foam-injection ratio exceeds 40%, the conditioned soil exhibits strong compressibility, with the compression coefficient at each stage exceeding 0.5 MPa^－1. (2) Increasing the water content enhances the conditioning effect of foam. Under conditions of 18% water content and 50% foam-injection ratio, the shear adhesion strength can be reduced to below 5 kPa. (3) When the pressure exceeds 200 kPa, the conditioning effect of foam gradually decreases; however, further improvement can be achieved by increasing the water content to 19.1% and the foam-injection ratio to 51.6%. (4) Field tests show that the optimized conditioning scheme reduces the cutterhead torque by 22.7% and the thrust force by 25.8%, which effectively lowers the shear strength of the conditioned soil and addresses the problem of poor muck discharge.

Key words: fully weathered granite, earth pressure balance shield, foam-conditioned soil, compression coefficient, tangential adhesion strength, shear characteristics