1 Introduction
More and more attention has been paid to the dynamic design method of deep foundation pit with deformation as the control means, because the deformation of supporting structure is an intuitive reflection of the interaction between supporting structure and soil during foundation pit excavation, and also a concrete reflection of the interaction between supporting design and site geology and construction. If the deformation of the supporting structure can be predicted in advance according to the parameters of the surrounding soil and the related parameters of the supporting structure, it is of great significance to ensure the safe construction of the foundation pit to take the predicted value of the most unfavorable situation as the early warning value to control the deformation of the supporting structure.
Based on the monitoring information of working conditions obtained during the excavation of deep foundation pit, the mechanical parameters of soil and supporting structure are inversed by optimized back analysis, and then the deformation, internal force and supporting force of pile wall under the next working condition are predicted by finite element calculation, and the calculated values are taken as the control values of supporting structure. By adjusting the construction scheme and supporting parameters according to the actual situation on site, with the progress of construction, the corresponding information of the next construction stage is continuously collected, and the pile wall deformation under the next working condition is predicted by inverse parameter calculation, and so on.
2 prediction principle
In fact, the principle of prediction is to do back analysis first, and then do positive analysis, that is, according to the displacement monitoring information of each working condition, select the appropriate soil mechanics model and corresponding boundary conditions, then establish the objective function, search the soil parameters and mechanical parameters of the supporting structure close to the measured values by the optimization method, and then use these parameters to calculate the parameters of the next working condition, so as to predict the deformation of the supporting system, and control the deformation of the supporting system in combination with monitoring.
2. 1 Establish the objective function
According to the monitoring information of each working condition of foundation pit excavation, the objective function of back analysis method is generally:
Where uci(x) is the calculated horizontal displacement of the measuring point I on the supporting structure, and uti is the measured horizontal displacement of the measuring point I on the supporting structure; X represents the m value of soil, the stiffness coefficient of support, the stiffness of pile wall, etc. N is the total number of measuring points.
2.2 Displacement calculation of any position of pile body
The displacement of supporting structure is calculated by finite element method of elastic foundation beam. The final result of the calculation is the internal force and deformation at the node of the unit, but the position of the measuring point may not be at the node. In order to reflect the dynamic response of the construction process and solve the objective function value, it is necessary to give the monitoring information increment at any position and any construction stage of the monitoring point, so the horizontal displacement uci of the measuring point I on any unit can be obtained by linear interpolation method. The calculation formula is as follows:
Where x 1 and x2 are the coordinates of both ends of the unit where the measuring point I is located; Uc 1, uc2 is the calculated value of horizontal displacement at both ends of the unit where point I is located; Uci is the horizontal displacement of measuring point I; Xi is the coordinate of measuring point I (coordinate origin is the vertex of pile wall).
2.3 Monitoring data collection
The inclinometer for measuring the inclination of the pile body on site reads from bottom to top at the point distance of 0.5m, that is, the inclinometer tube is divided into n measuring sections (see figure 1), and the length of each measuring section is li=500mm. By calculation, the inclination θi between two pairs of guide wheels (li) measured at a certain depth can be obtained.
The calculation formula is: δ I = Lisinθ I (3)
The horizontal displacement Δ i of a certain depth can be obtained by the accumulation of section displacement Δ i, that is:
In the calculation, it is assumed that the pipe bottom is taken as the reference point, and the displacement value δI of a certain depth is calculated from bottom to top, and then the horizontal displacement value is corrected according to the measured displacement of the pile top at this point. However, no matter whether the datum point is located at the top or bottom of the pipeline, the calculated displacement δi is always positive to the side of the foundation pit, and vice versa. The horizontal displacement (H ~ δi) curve of the pile can be drawn by connecting the displacement values δ I measured by the same inclinometer at different depths in the retaining structure on the coordinate paper.
2.4 data optimization processing
(1) Due to the interference and destruction of measuring instruments, operators, construction status or measuring points, the number of monitoring points is often not ideal and sufficient, so lagrange interpolation or spline function interpolation is often used in data processing.
(2) In the actual monitoring process, errors caused by environmental and artificial readings are inevitable. In order to eliminate the influence of this error on the accuracy of back analysis results, the monitoring data must be smoothed. See reference [3] for the specific process.
3 engineering application
The buried depth of the foundation pit of Yingkou Road Station of Tianjin Metro Line 3 is 14.753m, and the enclosure form of reinforced concrete bored piles and cement mixing piles is adopted. The specification of the bored pile is φ 1000 @ 1200, the pile length is 25.6m, and the foundation pit is located at the center of the capping beam with heights of 3.75m, 6.75m and 1650.
The project is located in the bustling urban area, surrounded by important buildings and many pipelines, such as cables, gas pipes and water pipes. The excavation construction steps are shown in Figure 2: excavation to -3.0m at working condition 1, erection of the first support to -0.5m at working condition 2, erection of the second support to -6.8m at working condition 3, erection of the second support to -3.75m at working condition 4, excavation to-10.7m at working condition 5 and erection of the third support to-6.77m at working condition 6. Limited by space, this paper only introduces the comparison between the predicted curves and the measured curves of working conditions 3, 6 and 8.
The method of deformation prediction in the process of foundation pit excavation is to invert the soil parameter m value according to the monitoring information of working condition 3. According to the retrieved parameters, the wall deformation under working condition 6 is predicted. Similarly, according to the monitoring information of working condition 6, the value of soil parameter m is inversed, and the wall deformation under working condition 8 is predicted accordingly. The parameter values determined by inversion are shown in Table 2.
See fig. 3 for the comparison between the measured curve and the predicted curve of working conditions 3 and 6. See Figure 4 for the predicted and measured curves of Working Condition 8.
Description 1: In the figure, 3- 1 is the deformation curve predicted according to design specifications and empirical values, 3-2 is the actually measured deformation curve of field monitoring, 3-3 is the deformation curve after back analysis and parameter optimization, and 6- 1 is the predicted deformation curve of working condition 6.
Note 2: As shown in Figure 4, 6- 1 is the predicted curve under working condition 3, 6-2 is the measured deformation curve, 8- 1 is the predicted curve and 8-2 is the measured curve.
As can be seen from Figure 3 and Figure 4, the prediction error after inversion based on monitoring data is reduced from the maximum error of empirical prediction and actual measurement of 3.75mm in Case 3 to the maximum error of 3. 1mm in Case 6. With the further inversion and optimization of parameters by using field monitoring data, the prediction error of working condition 8 is 2.2 mm, and the predicted deformation curve is more and more consistent with the actual deformation curve. The value of the proportional coefficient m of foundation reaction is constantly changing with the excavation process. The inversion method can be used to accurately invert the M value in different working conditions, accurately predict the excavation information in the previous working condition and accurately predict the pile deformation in the next working condition. Moreover, the prediction curve fits well and the prediction accuracy is greatly improved.
4 conclusion
(1) In the deep foundation pit construction of Tianjin subway, the deformation prediction method of foundation pit excavation and maintenance structure is accurate and successful.
(2) Through the inversion and optimization of the parameters of the maintenance structure, the maximum deformation of the maintenance structure can be accurately predicted, and the predicted value can be used as an early warning value to guide the construction.
(3) Parameter optimization back analysis technology, according to the field monitoring information, can accurately invert the actual soil parameters and supporting structure mechanical parameters under various working conditions. By optimizing the inversion parameter values of the previous working condition, not only the next working condition can be accurately predicted, but also the deformation prediction curve is in good agreement with the measured curve.
(4) In the process of foundation pit excavation, soil parameters change constantly with the excavation process. The soil parameters and the mechanical parameters of the supporting structure determined by experiments and experience are random, so it is not accurate to predict the lateral displacement of the wall according to the initial values.
For more information about project/service/procurement bidding, and to improve the winning rate, please click on the bottom of official website Customer Service for free consultation:/#/? source=bdzd