Reducing swelling pressure on the freezing method of ground treatment by drilling and coring measures for tunnelling works

Abstract

The Outer Metropolitan Discharge Channel Project was implemented to relieve flooding problem at the outskirts of Tokyo. Vertical Shaft No. 2 is one of the vertical shafts constructed in diluvial sand and clay formations. The ground treatment work by Freezing Method was used to create a frozen earth wall to prevent water seepage and collapse at the launching portal during dismantling of diaphragm wall and initial launching stage of the shield machine. After considering the design strength of frozen earth, lateral pressure and safety factor, the thickness of the frozen earth during dismantling of the diaphragm wall was designed as 4.8 m while for initial launching stage of shield machine, the thickness of the frozen earth from the entrance packing was designed as 7.95 m. Totally, about 3500 m3 of frozen earth were generated. Due to the existence of clay in the frozen earth, a higher swelling pressure to the clay layer resulting from freezing was anticipated to occur during construction. As a counter measure to protect the vertical shaft and diaphragm wall, drilling and coring was carried out into the ground formation to the full depth of clay layer and an additional two metres into the underlying sand layer. The drill holes absorbed the displacement due to swelling stemming from freezing thereby reducing the swelling pressure. In order to determine the degree of impact of swelling pressure on the vertical shaft and diaphragm wall as well as the efficacy of drilling and coring into the ground formation in relieving swelling pressure, the following factors of steel reinforcement stress and deflection of the diaphragm wall as well as displacement of the internal space of vertical shaft were measured. The measurement results revealed that drilling and coring had managed to keep the stress, deflection and displacement below the respective established control values. In addition comparison was made between: (1) average swelling pressure estimated from maximum deflection of diaphragm wall; (2) average swelling pressure estimated from displacement of internal space of the vertical shaft; and (3) average swelling pressure deduced from a theoretical equation proposed by Takashi without subject to drilling and coring. The results show that the average swelling pressure estimated from maximum deflection of diaphragm wall on the dismantling date of diaphragm wall (i.e. 85th day after the freezing operation commenced) was 12 tf/m2. This value is about 70% of 17 tf/m2 i.e. the average swelling pressure without drilling and coring as deduced from Takashi's equation. Furthermore, the average swelling pressure estimated from displacement of internal space of the vertical shaft on the 153rd day after the freezing operation started (i.e. when the frozen earth for initial launching of shield machine was ready) was 18 tf/m2, which was about 80% of 23 tf/m2 i.e. the average swelling pressure without drilling and coring as deduced from Takashi's equation. In conclusion, it was found that drilling and coring in ground formation had reduced the swelling pressure resulting from freezing to 70% to 80% of the case without drilling and coring. Thus, this has proven that drilling and coring in ground formation is an effective way to reduce swelling pressure stemming from freezing method of ground treatment. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.