Strain gauges warn of subsidence by the A2 in Maastricht

Avenue2, a consortium of Strukton and Ballast Nedam, is currently tunnelling right under Maastricht to re-route the A2 motorway. It’s a complex project, not only because the bottom of the excavation is at a depth of 22 meters below ground level, but also because of the soil characteristics and the fact that there are blocks of flats scarcely three metres from the edge of the digging work. To monitor 24/7 that the excavation is stable and that there is no danger of subsidence Avenue2, in collaboration with HBM, has installed strain gauges on the bottom layer of props in the excavation. These are used to measure the forces that the sheet piling exerts on the props.

The soil characteristics are a major cause of uncertainty in the construction of the A2. Part of the ground along the 2.3 kilometre long route consists of weak marl. This is usually fairly firm soil, but the passive resistance of the bottom of the excavation can fall when the water tension in the marl increases. Furthermore, the marl can contain pockets of gravel that are holding water, as well as hollows (karst). If the resistance becomes too low or karst is encountered, water and soil can be flushed out via the bottom of the excavation. Subsidence can occur if action is not taken in time, with the possible consequence of damage to the excavation or surrounding buildings, or delays to the project. This is why Avenue2 has developed a system that can monitor the status of the soil in and around the excavation in real time.

Real-time monitoring

“We are using a combination of techniques at the A2 for monitoring changes in the soil characteristics and to enable timely intervention for preventing subsidence”, explains Monitoring & Engineering specialist Hessel Galenkamp from Strukton. “We continuously measure the status of the outside walls of the buildings next to the excavation. We use automatic clinometers on the outside of the sheet piling and we measure the water tension on the inside, using pressure sensors in vertical tubes. Another method is used to measure the loads on the horizontal props. We do that with strain gauges that have been supplied and fitted by HBM from Waalwijk.”

Planning for the work by HBM was a considerable challenge. The schedules at the work site are tight, and the pressure of work can become quite high. The logistics for the props are critical, so reserving an entire day for installing strain gauges on each prop was difficult, which meant a degree of flexibility on the part of HBM.

Observational method

Galenkamp: “Up to four layers of props were installed in the excavation, at depths of 0, 4.5, 11.5 and 16.5 metres. The strain gauges were fitted only to the lowest layer of props, and only in that part of the excavation where we had to cope with weak limestone. One in every three props was fitted with strain gauges, with four distributed around its circumference. The data were digitised immediately and put on line, so that we were able to follow the digging. "We dig the soil out of the excavation in stages, starting in a central trench then working towards the sheet piling. On the screen of my tablet I can see the forces growing in real time; a good example of a process that is called the observational method: the optimization of the design through real-time monitoring of the starting points with predefined intervention scenarios.”

Limit values were not exceeded

The measuring system has performed excellently up to now, being available on line for more than 98 percent of the time. The loads on the props and the water tension were monitored in shifts for 24 hours a day. Automatic alerts were sent by SMS if the predefined limiting values were exceeded. A signal value of 4000 kN, equal to a force of 400,000 kilos on each prop, was established as the level at which the contractor had to act. The intervention value was set at 6000 kN, and an entire disaster organisation stands by day and night in case that happens.

"The initial phase of the observational method has now been completed”, says Galenkamp, “and the limiting values were not exceeded there. The forces on the props increased as digging progressed, but they never reached the 4000 kN level. We did see an abrupt change in the distribution of the loads on the props because of changes in the soil behind the sheet piling, but the new situation was stable immediately. We can even see the effect on the props of the sun heating them on one side.” The second and final phase of the observational method will start in December this year.

Monitoring for more safety

“We monitor in order to be certain that we can dig out the soil in the excavation safely. This is in the first place to ensure the safety of the people and the equipment on the excavation site, but also to prevent damage to the excavation and surrounding buildings as a result of subsidence. During the monitoring process we gather information that is useful for the further progress of the project. As we have seen that the forces remain below the signal values, we can consider using fewer props in the excavation. This leads to the minimization of manufacturing costs, transportation costs and road closures. And that’s without even going into the possible time savings. This optimization means that the cost of the system is soon repaid.”

Another saving came from an unexpected quarter: the marl turned out to be difficult to drain, which meant that the water tensions were unfavourable and even exceeded the signal levels. This weakening was compensated for by the encouraging prop forces. The site engineers were able to use the information we provided to release the excavation, which meant that that there were no delays.”

"I think we might possibly be able to manage future projects even more efficiently thanks to monitoring”, concludes Galenkamp. “In the construction industry we are constantly trying to find a balance between costs and safety. Contractors think that designers are too conservative, and on the other hand designers think that contractors take too many risks. Our data means that we can give the discussion a new dimension. Real-time monitoring tells us what forces are at play in such building work, so as a result we can possibly design in such a way in the future that structures remain strong and safe, while allowing us to build more quickly and with less material and at lower cost.”

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