Application of GPS survey in tunnel construction?

Hamaling Tunnel is located in Tongling-Jiujiang section 1 contract section of newly-built railway. This is a single-line tunnel with a total length of 3,765 meters. The entrance of the tunnel is on a straight line, the exit is on a curve, and the middle section is on a transition curve. When the tunnel is excavated, the two working faces are opposite. The climate of the survey area belongs to subtropical monsoon climate zone, which is warm, humid and sunny. The survey area is located in the mountainous area of southern Anhui, with high vegetation coverage, dense forests and inconvenient transportation, so it is difficult to carry out routine survey of intervisibility conditions. Therefore, GPS static positioning is adopted for control measurement. Four American Tianbao 4600LS single-frequency GPS receivers are used for measurement, and the plane nominal positioning accuracy is 5mm+ 1ppm. The baseline calculation software adopts random software TGOffice.

2. Precision design of GPS network in Hamaling Tunnel.

According to the accuracy requirements of tunnel length and lateral penetration error, and referring to the Specification for Highway GPS Survey JTJ/T066-98, the control network of Hamaling Tunnel should be laid out and measured according to the accuracy requirements of secondary GPS network, and the technical indicators are as follows:

Satellite altitude angle ≥ 65438+05.

Data acquisition interval ≥15s

Observation time ≥60 minutes

Point geometric intensity factor (GDOP)≤6

The minimum baseline number of repeated measurements should be ≥5%.

Number of measuring cycles ≥2

Total number of effective observation satellites 6

3. Optimization of GPS network in Hamaling Tunnel.

Like the conventional ground plane control network, the principle of GPS network layout is to ensure that the tunnel is correctly penetrated according to the design accuracy, and give the accurate entrance direction from the entrance point to guide the tunnel excavation.

When laying the control network, there should be no less than three GPS points in each excavation hole. In order to facilitate detection, encryption and recovery by conventional survey methods, at least one of the three points in each excavation hole should be intervisibility with the other two points, and the distance between the directional sides of the tunnel should be more than 300m, so as to meet the routine detection in construction and improve the transmission accuracy of the entrance orientation. For railway straight tunnel, two control points should be set on the center line of entrance and exit survey to facilitate the establishment of construction coordinate system; For curved tunnels, two points should be set on each tangent to accurately calculate the curve deflection angle and construction lofting data. For highway tunnels, a line survey traverse point should be continuously measured at the entrance and exit to determine the relationship between tunnel GPS point and line survey traverse point, and to measure and adjust the connection error between tunnel control survey and line survey.

Due to the long tunnel construction period and long service time of control points in Hamaling Tunnel, four GPS control points are set at the entrance and exit for the convenience of routine inspection of the stability of the control points at the entrance and exit, among which E004 and EO111are traverse points for line survey. The layout of the control network is shown in Figure 1 (some baseline edges are not shown in the schematic diagram).

The selection of GPS control points has a great influence on the accuracy of GPS control network. Through on-the-spot investigation of tunnel entrances and exits, the points are all located on the stable bedrock at the top of the mountain, with a wide field of vision, no obstacles with a height angle greater than15, far away from high-voltage lines and high-power radio emission sources, and the directional side entrances and exits of the tunnel are all over 500 meters.

4. Topographic photography

4. 1 ephemeris forecast

Run TGOffice, and use the operation planning tool of the software to input the station information such as the approximate longitude, latitude, elevation, cut-off altitude angle and observation time of the satellite in the dialog box to check the visibility of the satellite. As can be seen from the schematic diagram of the number of satellites, the number of satellites visible in the survey area is at least 6 within the proposed observation time, indicating that it can be observed all day.

4.2 Work out the operation plan

According to the tunnel length, the distribution of excavation holes, the accuracy requirements, the number of receivers, ephemeris forecast, operation mode and traffic conditions, the observation scheme of GPS network is determined and the operation plan is made.

In order to ensure the measurement accuracy of the control network, the observation time of each period is more than 90 minutes. Because some unqualified observation periods need to be eliminated or deleted during baseline processing, it is very necessary to observe for 30 minutes without increasing workload.

Like conventional ground control survey, GPS network also requires enough redundant observation values, that is, it has certain reliability. The reliability of GPS network is expressed by the average reliability index R, where r=nr/n, where N is the total number of independent baseline vectors selected, nr is the number of redundant baseline vectors, nr is equal to the total number of baseline vectors N minus the necessary number of baseline vectors nt, and the necessary number of baseline vectors nt is equal to the total number of points np minus 1, so R = (n-NP+1)/n. According to the tunnel length and accuracy requirements,

It can be seen from the table 1 that the total number of independent vectors in the GPS network of Hamaling Tunnel is 2 1, and each point is observed for three periods. The average reliability index r, r =(2 1-7)/2 1 = 0.67, the r value is greater than 0.3, and the reliability index is good.

Because four GPS receivers are used for observation, two receivers are set at the entrance and exit respectively, the dispatching plan is strictly implemented, and the synchronous observation operation is carried out according to the specified time. According to the observation plan, the whole field work lasts for one day.

4.3GPS receiver static operation

Place the GPS receiver in the center and keep it horizontal, measure the height of the antenna, turn it on and observe. After measurement, power off first, then move the station, and then measure the antenna height before moving the station. The error between the two measurements is less than 2mm.

Before the end of observation at each station, it is necessary to record the antenna height, opening and closing time, receiver serial number, antenna type, date, receiver type and high volume mode for data post-processing.

5.GPS measurement data processing

The field measurement of GPS has a high degree of automation, and the data acquisition is completed automatically by the receiver, with almost no manual intervention. However, the data collected by GPS field survey cannot be directly applied to engineering practice, so the corresponding calculation software must be used to process the GPS observation data. Only through a series of data processing processes, such as baseline processing, GPS baseline vector network adjustment solution, unconstrained adjustment, constrained adjustment, coordinate transformation, etc., can the survey results available for construction be obtained.

5. 1 Establish coordinate system

The static data of field observation is based on WGS-84 ellipsoid and adopts WGS-84 global geodetic coordinate system. However, the route design of Hamaling Tunnel is carried out in Beijing 54 coordinate system, and the longitude of the central meridian is 120 degrees 45 minutes. Therefore, the local coordinate system consistent with the design unit coordinate system should be established by using the solution software, so as to facilitate the constrained adjustment of GPS network and transform the measurement results from WGS-84 coordinate system to construction coordinate system.

5.2 Download and import data

Firstly, a folder named "Hamaling Tunnel" is established to store the files in the process of GPS data processing, and then the GPS receiver is connected to the computer through a cable for data transmission. According to the automatic file naming rules of the 4600Ls receiver, the observation data file of the GPS network of Hamaling Tunnel is selected to be downloaded, and the station information (such as roll call, antenna height and volume height) in each time period of the station is input according to the station records. At this time, you need to carefully check the roll call, file name, start time and download. Download the observation data of four GPS receivers to the "Hamaling Tunnel" folder one by one.

After the data download is completed, the downloaded data should be backed up immediately to save the transmitted original data.

5.3 baseline processing

Firstly, the quality of observation data is analyzed and preprocessed, and the satellite signals with large jitter in each satellite observation period are prohibited by using the time axis tool, and all baselines are processed by TGOffice one by one.

After baseline processing, check the evaluation index of baseline processing quality. The best solution type is L 1 definite solution, and the best ratio is greater than 3, and the greater the better. The empirical value of the reference variable is about 2, and the mean square error RMS is less than 3 cm, and the smaller the better. Among these indicators, the type and mean square deviation of the solution are the most critical.

In baseline processing, deleting satellite data under bad working conditions is an important way to improve the accuracy of pseudo-range positioning. If the treated baseline changes from ink to yellow, it indicates good quality. If individual baselines are red or rejected, it means that the quality of baselines has problems, so it is necessary to eliminate the observation signals and select the baselines with quality problems one by one for fine processing.

Check the satellite residuals in the baseline processing report, and remember the satellites with large satellite residuals and the time periods with large satellite residuals. Satellites with large residuals and time periods with large residuals are prohibited from participating in baseline processing. The satellite period with large residual has a very significant influence on the baseline processing results, so it is particularly important to eliminate the satellite period with large residual in baseline processing.

Secondly, select the baselines with quality problems one by one and check the PDOP value. If the PDOP value in a certain time period is abrupt or too large, the satellite signal in that time period is prohibited.

In addition, open the sky map to see the altitude angle of the satellite. Write down the low-altitude satellite and low-altitude period, and prohibit the signals of low-altitude satellite and low-altitude period.

Repeat this process until all baselines are yellow after processing, or the number of baselines rejected when processing baselines is zero.

Check the closing error of GPS ring. The closing error reflects the internal coincidence accuracy. If the internal coincidence accuracy is poor, the baseline will be reprocessed, and the internal coincidence accuracy will directly affect the final accuracy.

The closure error of the GPS control network of Hamaling Tunnel is 88, 88 pass, and all pass, which shows that the coincidence accuracy in the baseline is good, and the baseline processing is completed, so the unconstrained adjustment can be carried out.

5.4 Unconstrained adjustment

Unconstrained adjustment is carried out to check the accuracy of GPS baseline vector network itself, and to check whether there is obvious systematic error between baseline vectors, so as to eliminate baseline edges with gross errors.

When making unconstrained adjustment, first make sure that the datum plane is WGS84 and select WGS-84, and then click "Adjustment" in the adjustment toolbar to make unconstrained adjustment.

After adjustment, check the network adjustment report and statistical summary to see whether the network reference factor is near 1 and whether the X-side test has passed. If it fails, the weighted strategy is used to continue the protocol-free adjustment until it passes and the change of plane coordinates is zero.

5.5 Constraint adjustment

Because the tunnel control survey mainly emphasizes the relative accuracy between the control points of each excavation face, it is not important whether the GPS network is accurately located in the geocentric coordinate system, so the classical fixed point free network adjustment method is usually used for minimum constraint adjustment.

Firstly, the datum is converted into local datum (Beijing 1954 coordinate system of Hamaling Tunnel with the center longitude of120 45'), that is, the datum is selected as "projection datum-Beijing 1954", and the minimum constraint adjustment is carried out with the 54 Beijing coordinate system coordinates of line traverse point E004 as the known coordinates.

After adjustment, check whether the reference factor in the network adjustment report is near 1, whether the X-side test passes, and whether the change of plane coordinates is zero. Otherwise, the weighting strategy should be adopted to continue the plane constraint adjustment. Until the network reference factor is about 1, the x test passes and the plane coordinate changes to zero. Whether there are red warning data, residual histogram, error ellipsoid and the ratio of horizontal accuracy to elevation accuracy in the network adjustment report can support the quality of the whole observation results.

5.6 Result output

All the information of the adjustment results are included in the "Network Adjustment Report", such as residual histogram, point error ellipse, adjustment result table, covariance, ring closure difference, and observation result table after adjustment.

The result of "adjustment grid coordinates" in the "network adjustment report" is the construction coordinates of each control point in the outer plane control survey of Hamaling tunnel.

The report can be printed directly or copied to the Word word processing system for typesetting and output.

The field survey of Hamaling tunnel by GPS needs one day of rock burying, one day of field observation and one day of data processing, while the conventional survey needs more than ten days. Through the practice of GPS control survey in Hamaling tunnel, the following conclusions can be drawn.

(1)GPS tunnel control survey has high accuracy, and the accuracy can reach millimeter level by using GPS static relative positioning technology.

(2) The labor intensity of 2)GPS measurement is low, and there is no need to climb mountains. A transition point was set at the top of the mountain. Field measurement is a fool, just press the start measurement button to measure. Data collection is completed automatically by GPS receiver without too much manual intervention.

(3) 3) The efficiency and economy of GPS survey are usually more than four times that of conventional survey. The longer the tunnel is, the more significant the efficiency and economic benefits are.

(4) Control points are easy to recover. If a control point is lost, it can be recovered in just a few hours.

(5)GPS measurement is an advanced positioning technology, which can improve the scientific and technological level of enterprises and create good economic and social benefits for enterprises.

The above contents were collected and sorted by Zhong Da Consulting Company.

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