Study on the Stability of the Hole Wall of Ultra-deep Piles Under Excess Water Pressure in Deep Silty Formation
Based on the bored pile project under excess water pressure conditions after the construction in a subway depot,the stability conditions of the borehole wall were studied.The critical hydraulic slope formula for hole wall collapse and the critical consolidation formula to ensure the stability of the hole wall were derived.The influence of soil consolidation on the stability of the hole wall was analyzed;Under excess pore water pressure,the application of New Austrian Tunneling Method(NATM)in the process of piles formation in silty soft soil is discussed.The results show that the seepage force generated by excess pore water in saturated silty strata after the construction of the pipe pile is the main factor of the deformation and instability of the hole walls during the drilling of the pile;The soil co-hesion is the key parameter affecting the necking and collapse of the borehole;The stability conditions of borehole walls are mainly the supporting force and the penetration force of the round arch;During the drilling,the large creep of hole wall,short penetrating path and the soil consolidation in short time make the hole wall in stable state;New Austrian principle can be applied in bored pile for the soft soil under excess pore water pressure.
silty formationexcess pore water pressurebored pilehole wall stability