Due to the different types of buildings, the failure mechanism is also different, and the interaction among foundation pit supporting structure, soil and surrounding buildings (structures) leads to complex problems such as multi-level and multi-index in the study of safety state classification of adjacent buildings in deep foundation pit. In order to solve these problems, the ideal point method is introduced into the research. Ideal point method is a classic multi-index comprehensive evaluation method, which has the advantages of intuitive analysis principle, simple calculation and small sample demand. At present, it has been well applied in geotechnical engineering, environmental quality assessment and other fields. According to the engineering practice I participated in, based on the ideal point method, the author constructed the safety grade evaluation index system of high-rise buildings adjacent to deep foundation pits, and gave a detailed quantitative evaluation benchmark with reference to common monitoring standards, thus forming a relatively complete evaluation model. At the same time, the fusion weight is obtained from both subjective and objective aspects, so as to achieve the purpose of more reasonable weight distribution.
The basic principle of 1 ideal point method
The ideal point method needs to determine the evaluation index system, determine the weight through scientific methods, define a model, and find a point as close as possible to the ideal point, so as to minimize the distance from the positive ideal point evaluation function and maximize the distance from the negative ideal point evaluation function, and finally evaluate the status of the object through the proximity of the ideal point.
1. 1 Establish an evaluation index matrix. For an evaluation object, let it have /z evaluation indexes. Take these n indicators as each objective function of the evaluation object decision, set the weight corresponding to the vector function as the evaluation object scale, and the value under the objective () is. Its index matrix is to determine the positive and negative ideal points. Evaluation indicators can be divided into two categories: positive indicators and negative indicators. If the index changes monotonously, the ideal point -f(+) and the negative ideal point (I) can be defined. When the indicator is a positive indicator, the larger the value, the better, then (+) = maxl () (1) = min; () When the indicator is an inverse indicator, the smaller the value, the better (+) = min; () (1) = max/ () where: f(+) and f (1) are the influences respectively.
1.2 Determine the optimal solution of the distance index from the evaluation object to the positive and negative ideal points. The closer to the positive and negative ideal points, the farther away from the positive and negative ideal points, the better the solution. Minkowski distance method is a common method. In this paper, Euclidean distance is used to define the distance from the evaluation object to the positive and negative ideal points. The distance to the positive ideal point is d = {∑ wiei (x)-(+)]} (4) The distance to the negative ideal point is D={∑[ ()-( 1) ]}(5).
1.3 Calculate the closeness of the ideal point. The closeness of the ideal point is: c = d2/(d 1+d2) (6) Obviously, c belongs to the interval [0, 1]. The larger C is, the closer it is to the positive ideal point and the farther it is from the negative ideal point.
2 fusion weight
2. 1 coefficient of variation method for obtaining objective weights The coefficient of variation method is an objective weighting method, which uses the information contained in index data to obtain weights. The basic calculation steps are as follows: 1) Use the coefficient of variation of each index to measure the difference of each index. The formula of coefficient of variation of each index is as follows: VI = 6 ~/(I = 1, 2, n)(7), where c is the coefficient of variation of the first index, also called standard deviation coefficient; Is the standard deviation of I index; Is the average of the first index. 2) Normalize the coefficient of variation and determine the weight of each index: nwi1=/(8) i = l.
2.2 analytic hierarchy process, also called analytic hierarchy process, is to compare and judge each evaluation item subjectively and calculate its weight. In this paper, the proportional scale method is used to weight each index, and the basic calculation steps are as follows: 1) Establish the judgment matrix. Through the expert's evaluation of the evaluation index, the initial weight forms a judgment matrix, and the elements in the rows and columns in the judgment matrix represent the scale coefficient obtained by comparing the index with xi. 2) Calculate the geometric average of each row and scale data in the judgment matrix, and record it as k..3) Normalize. Calculate the results in turn with the formula n = ki/∑ k (9) i = l, and determine the subjective weight of each index.
2.3 The objective weight is obtained by the coefficient of variation method. (i= 1, 2,, n) The subjective weight (i= 1, 2,, n) is obtained by AHP proportional scaling method. By fusing them, the final fusion weight Wi (= 1, 2, 1 1, 0) can not only reflect the judgment of subjective experience, but also reflect the information contained in objective weight. Because there are many evaluation indexes, this paper uses a relatively simple multiplication combined weighting method to calculate the final fusion weight, and the formula is: =wi. /∑ (I = 1, 2, n) I =1(1o) Ⅲ construction water displacement.
3. Safety evaluation model of high-rise buildings near deep foundation pit based on ideal point method of fusion weight.
3. 1 Establishment of multi-level building safety diagnosis index system Based on the failure mechanism analysis and engineering practice of adjacent buildings in deep foundation pit, combined with the protection measures index system of adjacent buildings in the process of deep foundation pit construction and the requirements of relevant standards and specifications, the safety evaluation elements of adjacent buildings in deep foundation pit are divided into three categories by analytic hierarchy process: building stability information, soil disturbance information and foundation anti-disturbance ability. Considering the representativeness and feasibility of the evaluation index, the correlation coefficient method is used to select the bottom index which is representative and easy to measure in engineering practice. The safety evaluation index system of high-rise building adjacent to deep foundation pit is shown in figure 1.
3.2 Establish a set of safety evaluation indicators for high-rise buildings adjacent to deep foundation pits. Referring to the current domestic and international codes and standards and measured engineering data, supplemented by mechanical calculation and numerical simulation, the quantitative evaluation criteria of all 14 measurable bottom indicators are determined. In order to calculate and eliminate the influence of different measurement methods, units and properties of the original data on the comprehensive evaluation, the membership values of each index are obtained through fuzzy transformation, and the standardized values of all the bottom indicators are obtained, as shown in table 1. Anti-disturbance ability of soil horizontal displacement rapid room wind building foundation C foundation coefficient Cl prats coefficient c2 permeability coefficient c3.
3.3 Calculating the fusion weight of evaluation indicators According to the above method, the objective weight is calculated by the variation coefficient method, the subjective weight is calculated by the AHP- product scale method, and the objective weight and the subjective weight are combined by the combination weighting method to get the final fusion weight of each evaluation index.
3.4 Establish positive ideal points and negative ideal points. After normalization, all evaluation indexes are positive. Taking the upper and lower limits of the grading standards of each evaluation index in table 1 as positive ideal points and negative ideal points, the positive ideal point matrix (+) and negative ideal point matrix (I) of damage grading I-IV are obtained according to (2).
3.5 Determining the safety level of the building to be evaluated According to the relevant standards and comprehensive expert opinions, the safety level of the high-rise building adjacent to the deep foundation pit is divided into four levels, and the influence of each level of 8383 on the construction personnel, surveyors and managers of the foundation pit is given, which provides a reference for taking reasonable countermeasures in time. The specific classification results are shown in Table 2.
Four engineering examples
4. 1 project overview
A residential high-rise building was built in 20 13 years, with a height of about 33m and a depth of about14.5m. The building foundation is precast concrete pipe piles. The circular ramp of river tunnel is used for foundation pit excavation. The foundation pit is curved and close to the east side of the target building, with a gentle slope and a buried depth of about16.4m. The underground aquifer is thick and rich in water, and its stable water level is related to the buried depth of the aquifer, which is basically consistent with its topographic slope. The plan of the building and foundation pit is shown in Figure 2.
4.2 Access to evaluation indicators
In this paper, three buildings with equal time intervals after foundation pit excavation are selected as evaluation objects for monitoring, and the detailed change data of building deformation index and soil disturbance index are obtained. In the geological survey report before the foundation pit project starts, there are detailed geotechnical test parameters.
4.3 Calculate the index weight
According to formula (7)-( 10), the objective weight, subjective weight and fusion weight of each evaluation index are calculated.
4.4 Determination of building safety level
The fusion weight and matrix of positive and negative ideal points are calculated. According to formulas (4) and (5), the closeness between the three monitoring results and the ideal points of each judgment level can be calculated, as shown in Table 6. Comparing the judgment results of the model with the actual judgment results of the project, the judgment results are basically the same, which shows that the evaluation model has good judgment effect.
5 Results and discussion
1) Based on the mechanism of foundation pit affecting adjacent buildings, combined with engineering practice and referring to the current monitoring specifications, the easily available and representative evaluation index of 14 is determined by using the correlation coefficient method, and the safety grade evaluation system of high-rise buildings adjacent to foundation pit is constructed. Based on the existing standards and specifications, engineering measured data as the basic reference, supplemented by mechanical calculation and numerical simulation methods, through the standardization of dimensions, the safety grade evaluation model of high-rise buildings adjacent to the foundation pit is obtained based on the ideal point method, which provides a theoretical basis for judging the safety hidden dangers of buildings adjacent to the foundation pit and taking control measures in time.
2) The safety grade of the deep foundation pit adjacent to the high-rise building on the ramp is evaluated, and a good judgment result is obtained, which shows that the model is reliable.
3) From this result, it can be seen that the tilt index of high-rise buildings has a very prominent influence on the safety evaluation results, so the value range and scientific weighting of this index need to be improved. It is necessary to further study and improve the division and value of evaluation indicators in various environments.
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