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Innovative measurement techniques for underground infrastructure
Tunnel boring machine type Closed Front Boring.

Innovative measurement techniques for underground infrastructure

Closed roads, traffic problems, disruption of the streetscape and hefty costs due to moving large amounts of soil are common problems when installing underground pipes using traditional methods. Therefore, trenchless techniques are increasingly being used to install underground pipes and cables with minimal surface disturbance. These techniques offer a smart solution to the problems, but require a high degree of precision, as small deviations can have major consequences for the stability of underground structures. Iv applies a specific approach to ensure this precision. 

Measurement setupi tachypeter for automatic deformation measurements
Measurement setup tachymeter for automatic deformation measurements.

The power of the closed front technique 

A popular method is Pipe Jacking, also known as jacking. This technique involves laying pipes underground without digging a large trench. A specific type of Pipe Jacking is closed front drilling, or GFT (Closed Front Technique). This technique uses a drill shield with a rotating drill wheel at the front that excavates the ground to create a tunnel. The drill shield supports the tunnel wall while the drill wheel excavates the soil. Directly behind the drill shield, the pipes, also called drill pipes, are placed. The process begins in a press well, where the drill shield is pushed away with hydraulic jacks. The pressing jacks press against a support front at the back of the press well. Once the drill shield is moved, the augers are retracted and a drill pipe is connected to the drill shield. This combination is then pushed further away by the ground. By adding new drill pipes each time, long distances can be covered. Both horizontal and vertical bends can be made during drilling. This drilling method makes it possible to drill through various soil types, from sand and clay to hard rock, and provides an efficient way to install pipes without much surface disturbance. 

Extensometers.

Application of trenchless techniques 

Trenchless techniques are used for a variety of applications, such as the construction of sewer systems and cable and pipe tunnels in urban areas. The technique also provides an ideal solution for crossing highways, railroads and rivers underground. A recent application is the construction of a buffer to collect rainwater underground. With more frequent extreme weather conditions due to climate change, directional drilling has become increasingly important. In urban areas where many surfaces are paved, it is difficult for rainwater to be absorbed by the soil. This often leads to flooding. To address this problem, underground water collection systems, such as storm basins, are used in many places.  

Continuous positioning for tunnel boring machines 

Even small deviations in soil movement can cause major problems, such as soil subsidence. This is where Iv's CoMeT system offers added value. CoMeT (Continuous Measuring Technology) is a self-developed measuring system for continuous and automatic positioning. This system uses existing Geomos monitoring software to control the measuring instruments, supplemented by proprietary software to perform data-driven processing.  

CoMeT visualization
CoMeT visualization.

How does it work? 

In the terminology of surveying, it seems simple: absolute control of the position of the tunnel boring machine is carried out with a polygon measurement through the already drilled tunnel, starting from known reference points in the starting shaft. By performing this polygon measurement every minute, you get continuous absolute positioning.  

In practice, this is achieved with serially placed tachymeters that perform polygon measurement at a fixed frequency. Since the tunnel tubes are continuously moving during drilling, all tachymeters are equipped with automatic adjustment screw blocks to ensure that the measuring instruments always remain level. CoMeT continuously processes the polygon measurement data and calculates the deviation of the tunnel boring machine from the pre-planned theoretical boring path of the jacking. In addition to the horizontal and vertical deviation of the tunnel boring machine, other important data are also visible. 

Effective monitoring and risk management 

Positioning TBMs can be quite challenging. Especially in urban areas or when crossing major infrastructure. This poses risks, such as settlements of overlying structures. 

Sometimes the client therefore requires that settlement measurements be carried out. This was the case with the Strandboulevarden project in Copenhagen. To limit subsurface settlements in such a way that no damage could occur to the surrounding buildings, Iv devised a monitoring campaign. 

In order to identify all possible adverse effects of the tunnel project, Iv deployed a wide range of monitoring resources. This was done both during the construction of the departure and receiving shafts and during the drilling of the tunnels. The resources deployed included noise and vibration measurements around the construction pits, continuous inclinometers in the walls of the construction pits, continuous settlement measurements over the entire area of the settlement trough above the drilling tunnels, continuous deformation measurements of facades and surrounding buildings, and continuous monitoring well measurements for groundwater level monitoring.

While performing the jacking operations, it was crucial to be able to adjust all drilling parameters in a timely manner. This included parameters such as water supply, bentonite supply, pressure and rotation speed of the drilling front and jacking pressure from the start shaft. To adjust it accurately and prevent settlements, the monitoring results were permanently available in the TBM's control cabin via the online platform Vision42. Especially for this project the tool was developed herein to have the position of the TBM visible in real time on the map with the monitoring points.  

So applying trenchless techniques offers many advantages, but it involves many measurement and monitoring aspects. Both before and during the work. For this to go well and safely, the aspects mentioned above are indispensable.     

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