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Analysis of monitoring method of vault subsidence value in subway tunnel construction?

For tunnel construction, how to effectively "prevent collapse" is the technical difficulty of the whole subway tunnel construction, and one of the most important contents in on-site monitoring is to monitor the vault subsidence value, so as to ensure the safety of construction operation and future delivery through effective control. A great deal of experimental research and practical experience show that the vault subsidence process and subsidence value of subway tunnel will directly affect the tunnel support design and stratum control. The establishment of monitoring datum point and several monitoring methods of vault subsidence value are briefly analyzed.

A, monitoring datum and its establishment

According to practical experience, we can summarize the problems related to the establishment of monitoring benchmarks as follows:

1. From the distance, the linear distance between the position of the monitoring datum point and the tunnel excavation should be controlled between 500 ~ 1000 m, and the monitoring datum point should be composed of three leveling points, and the distance between them should not be too far. Three leveling points are divided into main points and auxiliary points, of which 1 leveling point is set as main point and the other two leveling points are set as auxiliary points. The purpose of this exercise is to ensure and check the stability and reliability of the reference system with the help of leveling points.

2. From the point of view of stability, the three leveling points must be set in a place with stable geological structure, and special reinforced concrete piers should be set to connect the three leveling points into a closed leveling route. The height difference between three points should be measured back and forth with the help of a high-precision level (not less than) and strictly controlled.

3. Practice shows that the selection of working base points is related to the distribution of monitoring points, which are usually arranged near the observation section. In practice, it is necessary to ensure that the working datum point and the leveling datum point outside the tunnel are jointly measured regularly.

4, deformation monitoring points should be combined with engineering practice, uniform layout in the tunnel vault, usually different construction practices, its layout method is also different:

(1) If it is a small-section tunnel, its layout method is as follows: once the construction face is excavated, the initial support will be implemented immediately, and special stainless steel balls will be welded at the outer end of the vault anchor at regular intervals to mark the deformation points, and the settlement of the vault will be reflected by monitoring the settlement of the stainless steel balls. From the practical experience, the bottom of the steel ball should be slightly higher than the outer surface of the secondary lining concrete to facilitate the monitoring results.

(2) If it is a tunnel with a high vault, the actual situation will seriously limit the use of leveling staff or placing prisms. Therefore, in the actual construction process, the monitoring point must be driven into the rock mass with the aid of the anchor rod in advance, and a small steel plate should be welded at the end of the anchor rod, and a reflector should be attached to it to ensure the convenience of total station ranging. In addition to welding a small steel plate, the end of the anchor rod needs to be designed with a hook for hanging the steel ruler and horizontal operation.

Second, the vault settlement monitoring method analysis

1, precision leveling method

This technology belongs to the traditional monitoring method, which is very suitable for the monitoring process of vault subsidence in small cross-section tunnels. The specific monitoring process has the following links:

(1) After the excavation of the working face and the welding of stainless steel balls are completed, the elevation of the bottom of the steel balls shall be measured from the main point of the reference system with a level not lower than that of the second-class level immediately.

(2) The initial support deformation is basically stable. Before the secondary lining concrete construction is about to be implemented, according to the accuracy of second-class leveling, repeat the above operations from the main point and measure the elevation of the steel ball bottom at this time.

(3) Calculate the sinking amount of each steel ball according to the two measurement results.

(4) Once the elevation measurement of deformation points (several points) is completed, the elevation difference between three datum points should be measured in time, the stability of main and auxiliary points should be strictly controlled, the unstable datum points should be monitored in time, and the observation results should be scientifically corrected.

2, trigonometric leveling method

This technology is also a common method to measure vault subsidence. In fact, the front view uses a reflector, and the back view uses a leveling rod or prism. In principle, this method is based on the principle of trigonometric leveling, and the height difference between the front view and the back view of the total station and the axis is measured by using the total station, and the absolute elevation and relative elevation of the measuring point are calculated according to the measurement results. Its application conditions are quite extensive. As long as the vertical angle is less than 300 and the station is within the reflection distance of the reflector, the vault subsidence can be measured by this method.

The specific measurement principle is shown in figure 1: neither the height of the instrument nor the height of the monitoring point is needed for measurement. In the figure, p is the center of the total station, point A is the working base point with known elevation, and point B is the monitoring point. With the help of the total station, directly read the sum of the height difference from the center of the total station to the center of the prism, and combine the height v of the rear-view prism, then:

The calculation method and formula of settlement are the same as those of precision leveling.

3. Automatic total station dynamic monitoring system

This measurement method realizes the functions of automatic observation, recording, processing, storage, report preparation, early warning and forecasting, etc., without the intervention of operators. It consists of a series of software and hardware. The whole system configuration includes TCA automatic total station, prism, communication cable and power supply cable, computer and special software.

The essence of the system is to change the scanning of any point on the fixed section into the scanning of any fixed point on the fixed section in the tunnel vault subsidence deformation system of the non-reflective prism automatic tracking profiler. According to the format required by the contractor, the system directly expresses the displacement change of monitoring points in the form of standard charts, draws monitoring reports and displacement curves, automatically realizes the functions of data analysis, alarm and report generation, and can provide report forms according to user requirements.

Third, the conclusion

A large number of engineering practices show that different measurement methods have different advantages and disadvantages. For example, the traditional leveling method can reach the accuracy of 0.5 ~ lmm, but it is not suitable for measuring the points of side walls and super-large sections because of the conditions of erecting leveling ruler or hanging steel ruler. The relative accuracy of trigonometric leveling method can also reach 65438±0mm, which is more obvious in the monitoring of vault subsidence of extra-large cross-section tunnel. However, this method is seriously restricted by light. If the dust is too large, the reflector can't even reflect light. The automatic total station dynamic monitoring system is very convenient and can be continuously monitored. But if you want to arrange a large number of road sections, the cost is quite high. Therefore, in the actual monitoring, we should consider many aspects and choose scientifically.

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