VARIATIONS OF IN-SITU FORCES IN SOIL NAILS AND PRE-STRESSED GROUND ANCHORS AS COMBINED STRUCTURE FOR EXCAVATION OF A SUBWAY STATION FOUNDATION PIT IN BEIJING

This field test used the supporting structure for excavation of foundation pit at the PANDA annular island subway station in Beijing. The supporting structure combined pre-stressed ground anchor and soil nails. The field test results included the axial force variation of the soil nails with time, the axial force distribution of the soil nails along its length, the maximum axial force distribution of the soil nails, the pre-stress variation of the cable and so on. The principle of the pre-stressed composite soil nailed reinforcement structure is studied from the aspect of mechanical mechanisms. The results show that: (1) The tensile force in soil nails is variable with time and excavation; (2) The development and distribution of the axial force in soil nails are relevant to the potential slide trend of the foundation pit; (3) The influence of the pre-stress ground anchor on the axial force distribution of the soil nails is rather large; (4) The usage of the pre-stress ground anchor can reduce the displacement and improve the stress state of the pit slope, as well as reduce the axial force of the soil nails; (5) in the initial stage of being used, the variation of the load condition in the soil nailing has less effect on that of pre-stress ground anchor. They function independently.