Monitoring and Analysis of Deformation Laws of Deep Foundation Pit Considering Spatial Effect
Abstract
Starting with the analysis of the foundation pit design, the support structure of foundation pit was designed and checked combined with a large deep foundation pit project in Shijiazhuang; According to the current codes, the monitoring scheme considering spatial effect is established; The fiber grating sensor was introduced in the monitoring of the lateral pressure of foundation pit slope, which could realize the on-line, dynamic and real-time monitoring. The field test results showed that spatial effect was significantly reflected in the deformation, earth pressure and other aspects: the foundation pit slope of central settlement is 40%~65% bigger than corner ,horizontal displacement in the middle is 28%~42% larger than corner; The amplification of the positive angle to the settlement and horizontal displacement reached 48% and 11% respectively; The maximum horizontal displacement of the deep soil is located at the depth of 15%~35% below the excavation face; lateral earth pressure of Slope changed with volatility. The lateral earth pressure value of each measuring point increased with the increase of time. At a distance of 0.2 times of the excavation depth below the ground, the measured values of the center, external corner and inside corner were smaller than the calculated values of 42.10%,82.46% and 108.77% respectively; At a distance of 0.5 times of the excavation depth below the ground, the measured value of the middle was 35.21% greater than the calculated. The values of external corner and inside corner were 49.06% and 29.21% smaller than the calculated value respectively. At a distance of 0.8 times of the excavation depth below the ground, the measured values of the center, external corner and inside corner were greater than the calculated values of 6.29%, 40.41% and 23.27% respectively; The settlement of surrounding ground surface was larger when the foundation pit slope settlement was larger.
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Introduction
The foundation pit construction project is developed to deep and large direction. The geological conditions and the surrounding environment of the foundation pit are more and more complex. The design theory on deep foundation pit engineering is not perfect at home and abroad. The design method based on of plane strain problem makes supporting structure design of deep foundation pit be not reasonable and result in waste. The finite element method is used to study the spatial effect of deep foundation pit. Because the constitutive model of soil and its calculation parameters are difficult to be determined accurately, the finite element method cannot be used as a practical engineering design method. Dysli et al. [1] (1982) got the conclusion of the ground settlement in the middle part of the retaining wall by the field observation. Ou et al. [2] (1993) Confirmed that the calculated results of the two dimensional were larger than the calculated results and measured values of the three dimensional, and the results of the three dimensional were close to the measured values. It is mainly the existence of pit angle to restrain the development of the displacement of the adjacent area. Liu et al.（1995）[3] considered that the calculation results of the three dimensional finite element could well predict the lateral deformation of the retaining wall below the excavation face, and the two dimensional calculation result was too large, and the pit angle effect could only occur under the excavation face. Fook-Hou Lee et al.（1998）[4] monitored the foundation pit of high-rise building in soft soil region and got the results that the corner of the strengthening effect could significantly reduce the deformation of the supporting structure and ground settlement. Roboski (2006)[5] proposed the method of fitting error function to describe the ground displacement distribution based on the observation data of deep foundation pit supported by an anchor cable. Blackburn et al.(2007) monitored the excavation process of deep foundation pit of the different soft clay ground, and obtained the results that the support forces, soil displacement, and surface displacement had spatial effect.
Conclusion
(1) The monitoring results show that the roof settlement and horizontal displacement of the foundation pit have obvious spatial effect. The middle part of the settlement is larger 40%~65% than that of the corner, and the horizontal displacement of the middle part is larger 28%~42% than that of the side corner. The amplification of the positive angle to the settlement and horizontal displacement is greater than 48% and 11% respectively.
(2) The maximum horizontal displacement of the foundation pit is 15% to 35% in the depth of the foundation pit. Compared to the displacement of foundation pit on the north and south side, the horizontal displacement of the central part is higher than that in the corner. The influence of space effect not only exists in the surface and the deep soil layer is also affected by the space effect.
(3) The lateral earth pressure changes with volatility. The lateral earth pressure value of each measuring point increases with the increase of time. at 0.2 times the depth of excavation of the ground, the central foundation pit, the positive and negative angle position of the measured values are smaller than the calculated value of 42.10%, 82.46%, 108.77%; at 0.5 times of the excavation depth of the ground, the middle of the foundation pit position of the measured value is greater than the calculated value of 35.21%; the measured values of positive and negative are 49.06% and 29.21 respectively; at 0.8 times the depth of excavation of the ground, the measured values of the the center of the foundation pit, the positive and negative angle position are greater 6.29%, 40.41%, 23.27%than the calculated value respectively