【 Suojia Premium Case 】 The Charm of Tunnel Automation Monitoring during Construction Period Is Presented!

With the development of surveying and mapping instruments, a total station controlled by motors and programs, combined with laser communication and CCD technology, has achieved the automation of measurement. This total station is called a measurement robot. It integrates automatic target recognition, automatic aiming, automatic Angle and distance measurement, and automatic recording. With the cooperation of the control terminal, it can formulate measurement plans, control the measurement process, achieve repeated measurement of hundreds or even thousands of targets, and realize unattended continuous monitoring.

Features of the robot deformation monitoring System ✦

The deformation monitoring system for measuring robots features high efficiency, accuracy, strong real-time performance, simple structure and convenient operation. It is particularly suitable for deformation monitoring in small-scale regional projects. At present, measurement robots have become the preferred automated technical equipment in the deformation monitoring during the operation period of various types of engineering buildings and projects.

During the construction period, the main monitoring of the tunnel's convergence and settlement, as well as the force and deformation of the surrounding rock and lining structure, is carried out. Due to numerous constraints at the construction site, such as power supply difficulties, poor network coverage, blasting vibrations, and dust from crushed stones, manual monitoring is currently the main method for tunnel monitoring during the construction period. So, is there really no room for the application of automated monitoring in tunnels during the construction period? Suojia, in collaboration with the technical center of A certain unit of China Communications Construction Company, conducted automated monitoring of settlement and convergence of measurement robots at the excavation site of Tunnel A of a certain expressway project in Yunnan.

Project Overview

This expressway construction project is located in Yunnan Province. The total length of the line is 74.78 kilometers. Among them, the A Tunnel project, which is constructed by a certain unit of China Communications Construction Company, is the overall control project of the entire line. The left and right tunnels of the tunnel are both over 3,000 meters long, and the maximum burial depth exceeds 200 meters. It is the longest and most geologically complex tunnel in the entire expressway project and also a key bottleneck restricting the completion and opening time of the expressway.

The tunnel is constructed using the "New Austrian Tunneling Method", which is the most common method for high-speed tunnel construction in mountainous areas. It mainly uses shotcrete and anchor bolt support as the main support methods. The main construction process is: drilling and blasting excavation - initial support - secondary lining - auxiliary facility construction. The key points emphasized in the construction method are "minimal disturbance, early spraying and anchoring, frequent measurement, and tight sealing".

The construction monitoring of this tunnel was previously carried out by a third-party manual monitoring method. The reflective plates adhered to the structures inside the tunnel were measured multiple times a day to obtain the settlement and convergence change data of the arch top and the two sides.

Technical pain points and difficulties

There are many drawbacks to manual monitoring of tunnels during the construction period:

During the process of manually collecting structural data, it is necessary to frequently enter the vicinity of the tunneling area, which poses a significant safety hazard and is prone to causing measurement data errors. The authenticity of the data cannot be guaranteed, and thus the true deformation of the surrounding rock cannot be obtained.

2. The volume of monitoring data collection is large, which occupies a great deal of human and material resources. The processing and transmission of data are not timely, which cannot meet the needs of information-based construction management and also cannot provide timely guidance for construction.

3. The monitoring and forecasting are lagging behind, making it difficult to predict sudden situations on site and thus unable to play the role of emergency early warning.

During the construction period of the tunnel, due to the large number of personnel, vehicles and equipment involved in the construction process, along with the complex geological conditions, the construction of the automatic monitoring system poses severe challenges, mainly in the following aspects:

There are many temporary facilities on site and personnel activities are frequent

Large-scale construction equipment on site (excavators, trolleys)

During the excavation stage, there are a large number of equipment and temporary equipment as well as implementation personnel, such as excavators, forklifts, shotcrete equipment and excavation operation personnel, etc., which put forward high requirements for the placement and protection of instruments.

2. Impact of construction process

The tunnel construction process is rather complex. For instance, blasting methods are used during the excavation of the tunnel face, and the trolley moves along with the construction progress during the installation of the arch frame and the secondary lining stage. This has caused significant difficulties in the selection of the positions of the automated monitoring instruments and the reference points. For example, it often occurs that the reference points are blocked due to the movement of the trolley. At the same time, the dust and dripping water generated during the construction period also pose a certain degree of threat to the equipment. The impact of blasting vibration and flying rocks on monitoring equipment cannot be ignored either.

3. Difficulties in power supply and network connection

During the construction period of the tunnel, it is impossible to achieve power and network coverage near the current construction section. Therefore, appropriate means need to be used to transmit the network and power supply to the position of the measurement robot to ensure the normal operation of the monitoring system.

4. High precision requirements

According to the requirements of the engineering surveying code, the mean error of the monitoring should be less than 1/20 to 1/10 of the allowable deformation value. For tunnels supported by composite lining, the minimum reserved deformation of the surrounding rock as required by the tunnel design code is 10mm. Therefore, the current mainstream monitoring accuracy for tunnel convergence and arch top settlement is set at 0.5 to 1mm. High requirements have been put forward for the measurement accuracy of the equipment.

5. The lithology inside the tunnel is complex and the risk is high

This tunnel is mainly composed of V-grade surrounding rock, with V-grade surrounding rock accounting for 70%. The tunnel passes through multiple broken zones, fault broken zones and the interface of surrounding rock. The rock mass inside the tunnel is fragmented and loose, and the adverse geological conditions such as landslide deposits are dense and widespread, posing a significant challenge to the safety of construction workers, monitoring personnel and equipment.

Solution

In order to overcome various difficulties on site, the technicians of Suojia discussed with the personnel of China Communications Construction Technology Center and the technical department of the project department the feasibility plan for the automatic collection of measurement robots. Eventually, the following equipment plan was formulated after discussion:

I. Measuring Robot

The hardware uses Sojia's NET05 0.5-second measurement robot, which has the ability to automatically focus on and track prisms and reflectors, with a distance measurement accuracy of 0.8mm+1ppm (prism) /0.5mm+1ppm (reflector), fully meeting the accuracy requirements for on-site settlement and convergence observations. It also features an IP65 protection rating and tilt correction function, making it suitable for harsh on-site monitoring environments. When combined with dedicated monitoring terminals, it can achieve unattended automated observation inside tunnels.

The monitoring terminal used in this pilot project is of a certain brand. It requires a power supply voltage of 12V and has multiple network access methods such as wireless network, RJ45, and 4G. The data supports local storage. It also features an IP65 protection rating, supports diverse configurations, and meets complex on-site conditions.

Ii. Network Power Equipment

"Network

The site where the tunnel was implemented did not have a public network environment. Eventually, it was decided to use a 4G router and a wireless bridge for network supply. The reason for not using optical fibers is, on the one hand, that there are many vehicles and personnel on site, which makes it easy for optical fibers to be damaged. On the other hand, the automated observation distance of the measurement robot inside the tunnel generally should not exceed 300 meters. During the tunnel excavation process, the position of the measurement robot needs to advance according to the on-site construction progress, and it is difficult to extend and transfer the optical fiber.

Wireless Bridges can convert network signals into wireless electromagnetic wave signals at the transmitting end and throw them into the air, and receive the electromagnetic wave signals at the receiving end and convert them back into network signals. They are very suitable for data transmission in environments where it is difficult to install network lines.

A certain brand of wireless bridge and its wiring method

Power supply

During the tunnel excavation process, the laying of power equipment is relatively lagging behind. On the one hand, the power line drawn from the power supply point is long, and on the other hand, the construction site is prone to cause damage to the line.

There are two options for power supply equipment on site. The first one is to use the power supply from the construction trolley. The trolley will move forward as the construction progresses. The drawback of this method is that the trolley will frequently move, so a certain length of wires need to be reserved. In addition, attention should be paid to the protection of the on-site lines to avoid wire damage and cause casualties.

The second option is to use large-capacity lithium batteries for power supply. Devices such as measurement robot control terminals and wireless Bridges can be powered by 12V batteries. The battery can be directly placed on the device without the need to lay power lines. Under normal circumstances, a 100ah lithium battery can meet the continuous use of a measurement robot for more than a week.

Iii. Other auxiliary equipment

To ensure the stable operation of the measurement robot in the tunnel, a custom-made stainless steel bracket should be used to install and fix it inside the tunnel. The feet of the bracket should be fixed with expansion screws to suit the transfer of the measurement robot. In addition, to ensure the safe use of wireless Bridges and lithium batteries in the field, waterproof protective covers have been custom-made to prevent the impact of water vapor condensation inside the tunnel on them. In necessary cases, protective covers can also be custom-made for the monitoring prism to prevent damage from on-site stone impact or equipment damage.

Workflow

The basic process of tunnel monitoring should include selecting and setting up stations, establishing a control network, constructing network power and protection facilities, installing monitoring points (collecting initial values), automated observation, and setting up and transferring stations, etc.

Implementation process

Measure the automated monitoring process of the tunnel during the construction period of the robot

Step One

In the areas where the initial masonry has been completed, select an appropriate line-of-sight location as the setting point for the measurement robot. This location should not affect the subsequent construction, and at the same time, the field of vision should ensure the monitoring requirements for the structures that have just completed blasting excavation and initial masonry. Install the measurement robot using a customized tripod.

Step Two

Select the working base points to establish the control network. The working base points can be the reference points of tunnel construction or the survey points of the reference points of tunnel construction. The reference points should be able to meet the requirements of the rear intersection of the measurement robot.

Step Three

Install the supporting network and power supply equipment as well as the corresponding protective equipment.

Step Four

At the target points that need to be monitored, install reflective plates or monitoring prisms according to the on-site conditions. The orientation of the monitoring prism should be as close as possible to the position of the measuring robot. After the installation is completed, the measurement robot is used to collect the initial coordinates of all monitoring points as the basis for monitoring the surface changes of the tunnel. At this point, the initial coordinates of the monitoring point and the coordinates of the reference point are in the same coordinate system.

Step Five

Configure the monitoring cycle, monitoring limit difference, early warning mode, etc., and enter the automated collection process.

Step 6

If the data collected during the automated process conforms to the design parameters and no warning is issued, the station can be transferred according to the progress of the tunnel excavation. If a warning phenomenon occurs during the automated collection process, immediate communication with the construction party is necessary to suspend construction to investigate the cause. At the same time, the frequency of automated collection by the measurement robot should be increased, and the density of observation points should be increased if necessary.

Equipment installation location

Schematic diagram of tunnel monitoring equipment layout during construction period

Monitoring prisms can be set up at five monitoring points at each cross-section inside the tunnel (among which the monitoring points at the two sides and the top can calculate the convergence and settlement of the tunnel). The internal cross-section intervals of the tunnel can be set at intervals of 5 to 10 meters (according to the surrounding rock grade), and the layout of monitoring points should be densified in areas with poor geological conditions or severe deformation.

Monitoring data processing

Tunnels are generally configured with a monitoring cycle of 30 minutes or 1 hour based on the number of monitoring points. After setting the observation time on the monitoring software, the control terminal monitoring can send instructions on time to control the measurement robot for measurement. The Angle measurement data and distance measurement data are sent back to the monitoring software on the server in real time through the network. The monitoring software uses advanced adjustment algorithms for calculation to obtain the three-dimensional spatial coordinate changes of the points inside the tunnel. When the data exceeds the set warning value, it promptly notifies the relevant monitoring personnel via email, wechat, phone, etc.

This technical pilot project utilized Soga measurement robots and their corresponding monitoring and control terminals, combined with novel network transmission technology, and customized a variety of accessory tools for on-site response, thus overcoming the complex and changeable internal environmental problems during the tunnel construction period. An unmanned and automated deformation monitoring system integrating data collection, processing, analysis, reporting and early warning has been realized.

This system is replicable and scalable. Compared with conventional convergent sensors and manual measurement, it not only improves the accuracy and efficiency of monitoring data, but also reduces the on-site input and operation risks of monitoring personnel. After a long period of on-site comparison and testing in this project, the monitoring data of this system is highly consistent with the manual observation data and design data. It is efficient, high-precision and reliable, making a certain contribution to the tunneling progress and receiving high praise from the technical center of the project department.