Abstract:  To control the stability of sinking of pressin caisson in soft soil layer, a case study is conducted on an ultradeep underground garage constructed by pressin caisson method in Wenzhou. The change characteristics of tread pressure, slope pressure and ratio of end resistance to total resistance during the sinking process of pressin caisson are studied by field measurement. The effects of two main processes of static pressure sinking and soil extraction on the end resistance are analyzed and compared with the standard theoretical values. The results show the following: (1) The pressingsinking has little effect on the pore water pressure, and the pore water pressure is significantly affected by the properties of the formation beneath the foot blade, which is mainly concentrated in the early stage of static pressure. (2) There is a positive correlation between tread pressure and static pressure. Moreover, the influence of static pressure on tread pressure becomes more pronounced with increasing height of the soil plug in the well. (3) The slope pressure increases with the static pressure. It is greatly affected by soil plug height and decreases with the soil beneath blade foot hollowed. When the soil core rate is in the range of 0.47－0.69, the sinking attitude has a great influence on the slope pressure. (4) The vertical difference between the tread pressure and the slope pressure is basically kept within 5－20 kPa, which is minimally affected by the sinking depth. The working condition and state of caisson can be preliminarily judged by the pressure difference curve. (5) The end resistance increases first and then becomes stable with the sinking depth, and the increase is mainly concentrated near the formation interface, which is basically consistent with the standard calculation value. The distribution value of end resistance with depth is less affected by static pressing and soil extraction, but the value is generally lower than the standard value.

Key words: soft ground, press-in caisson, tread pressure, slope pressure, foot blade, insitu monitoring, soil plug, end resistance