Research on deep foundation pit excavation based on numerical simulation

The safety of deep foundation pit excavation has always been an important topic of urban construction safety. The traditional method is to control the construction safety by monitoring and adjusting the construction process, but the timeliness of monitoring makes it limited in the control of construction safety. In this paper, MIDAS / GTS software is used to simulate the excavation process of a hospital foundation pit in Guiyang. The maximum horizontal displacement in X direction is 4.03mm, and the minimum is -4.78mm. The maximum horizontal displacement in Y direction is 4.41mm, and the minimum horizontal displacement is - 4.9mm. Through the study of the common horizontal X and Y displacement nephogram of crown beam, waist beam and anchor, the synergistic effect can be well expressed. The maximum horizontal displacement in X direction is 5.56mm. The minimum horizontal displacement is - 3.54mm. The maximum horizontal displacement in Y direction is 6.56mm. The minimum horizontal displacement is - 4.45mm. By comparing the numerical simulation results with the field measurement results, the reliability of the numerical simulation in the evaluation of the foundation pit is fully proved.


Preface
Numerical simulation is used to simulate the excavation process of foundation pit, and good results are obtained. Yin Shengbin, Ding Hongyan, etc. through numerical simulation of foundation pit excavation process, think that the surface settlement has obvious distribution law when the retaining structure rotates forward and backward and three basic deformations in downtown area, which can be simulated by exponential curve and empirical formula can be obtained. Luomei and Huahua used Midas GTS software to simulate a foundation pit in Shijiazhuang, analyzed the soil stress, supporting structure deformation and anchor axial force, and compared with the monitoring value. It was found that the anchor axial force was at the maximum value in the anchorage section near the free section. Jiang Yu used Midas GTS software to analyze the pile anchor supporting structure of a foundation pit in Wuhan. Through the analysis, it is concluded that the change trend of the surface outside the foundation pit is "spoon" shape, and the change trend of the water particle movement of the double row column is "big in the middle, small at both ends". The influence of various factors on the horizontal displacement and internal force of the supporting structure is analyzed, and it is found that the row spacing has the greatest influence. The purpose of this paper is to evaluate the safety of foundation pit excavation by studying the numerical simulation of specific excavation process, and verify the reliability of this method in evaluating the safety of foundation pit by means of on-site monitoring.
Where E is the modulus of elasticity and ν is Poisson's ratio.

Orthotropic elastic model
The independent model parameters of orthotropic elastic model are young's modulus E 1 , E 2 and E 3 in three orthogonal directions, Poisson's ratio ν 12 , ν 13 and ν 23 in three orthogonal directions, shear modulus G 12 , G 13 and G 23 in three orthogonal directions, and the stress-strain expression is 2.2.

Working condition setting and
According to the construction scheme of on-site construction, the excavation is simplified into six working conditions, and the working condition information is shown in table 1. The second -6 The first anchor rod and the first waist beam The third -9 Second bolt The fourth -12 The third anchor rod and the second waist beam The fifth -15 The fourth bolt The sixth -20 The fifth bolt   In the construction stage-5, the simulation results have great changes, which are quite different from the actual situation, but the settlement trend is more consistent with the measured results. At this time, the influence of the surrounding buildings on the settlement is very small, and the fourth anchor has a great influence on the stability of the foundation pit.Showed in Figure 3. In the construction Stage-6, the fifth anchor rod and the third waist beam participate in the work. At this time, the surrounding buildings and the whole foundation pit area are stable, but there is a big gap between the simulation results and the actual measurement data, but the overall trend is in line with the change trend of the foundation pit excavation process.  Figure 4. , it can be seen that the displacement of the component is consistent with the displacement of the pile as a whole, the maximum horizontal displacement in X direction is 0.38mm, the minimum is -0.81mm; the maximum horizontal displacement in Y direction is 0.56mm, the minimum is 0.63mm.  Figure 5. , it can be seen that all the supporting structures in this stage have entered the working state, so the displacement level in this stage has not increased to a large extent, the maximum horizontal displacement in the X direction is 5.56mm, and the minimum horizontal displacement is -3.54mm; the maximum horizontal displacement in the Y direction is 6.56mm, and the minimum horizontal displacement is -4.45mm.

Summary
Through the analysis of the cloud image of soil settlement direction, in the process of excavation, the influence of foundation pit excavation on the surrounding soil can be seen clearly. Because there are shallow foundation buildings on the back of the foundation pit, in the first three layers of the foundation pit excavation, the influence on the surrounding of the foundation pit is still large. But with the completion of the fourth excavation, the cloud image shows that the place with the largest settlement at this time is no longer the back, but the foundation The record of the maximum and minimum value of each excavation shows that the settlement of the surrounding soil is a process of change and then gradually stable.
By studying the displacement nephogram of horizontal X and y of enclosing pile, the force of surrounding soil on enclosing pile during excavation can be clearly expressed, showing the overall role of pile in maintaining the stability of foundation pit. The maximum horizontal displacement in X direction is 4.03mm, and the minimum is -4.78mm; the maximum horizontal displacement in Y direction is 4.41mm, and the minimum is -4.9mm.
Through the study of the common horizontal X and Y displacement nephogram of crown beam, waist beam and anchor, the synergistic effect can be well expressed. The maximum horizontal displacement in X direction is 5.56mm, and the minimum horizontal displacement is -3.54mm; the maximum horizontal displacement in Y direction is 6.56mm, and the minimum horizontal displacement is -4.45mm.
Through the comparison between the numerical simulation results and the actual monitoring data, it is found that the error between the actual monitoring results and the numerical simulation results of each monitoring point is small, which fully verifies the reliability of the numerical simulation for the safety assessment of foundation pit excavation