Sensors Monitor Deep Excavations at a Nuclear Power Station

Monitoring Force sensors are being used to monitor loads at the face of some of the deepest excavations for the new reactors. They are the key early warning instrument for localized areas of high stress. The sensors are able to accurately measure any load change, without being affected by temperature changes.

The Monitoring Force sensors are being used for load monitoring in conjunction with some of the approx. 14,000 25-40mm Ø DYWIDAG Gewi Soil Nails that are installed to depths of up to 13.5m. The Smart Gewi Soil Nails feature a bonded section of 3m at the distal end of the nail, to point anchor in the stable zone, with the remaining length debonded to allow the nail to be used as a monitor for any load change resulting from movement either side of the slip circle.

With the debonded section passing through the active wedge, it is not possible to mobilize any restraining bond in this area; therefore, a heavy bearing plate is required at the face to ensure sufficient restraint. The borehole is first drilled using rotary percussion. This is followed by step drilling the top 1.5m of borehole, in order that one Monitoring Force Sensor can be placed over each DYWIDAG Gewi Soil Nail.

The elasto-magnetic Monitoring Force Sensors induce a magnetic field around the DYWIDAG Gewi Bar, which then passes through the bar, with the changes to the magnetic field picked up by a secondary coil and sent back to the read out unit. Since the magnetic permeability increases in proportion to an increase in load, the system can accurately measure load in kN.

Any movement of the active wedge or face translates into a load increase in the soil nail, which is identified by the Monitoring Force Sensor. The long debonded section, up to 10.5m, ensures that any deep-seated fault movement can be picked up by the sensor. Some of the excavations are known to cross existing fault zones. As the excavations have deepened, stresses recorded by the Monitoring Force-Sensors in some of the DYWIDAG Gewi Soil Nails have increased significantly. The Monitoring Force measurements have provided the engineers with a high level of information on the impending movement at the face, enabling the areas of high stress to be remediated with additional bolts or anchors prior to the excavation continuing. In addition, in areas where heavy cranes are working at the top of the excavations, Monitoring Force Sensors provided additional safety by an accurate monitoring of load changes.

All sensors are connected to multiplexers and separate read out units. Readings for the sensors are taken remotely and ensure continuous around the clock monitoring of the excavation slopes. DYWIDAG also supplied double corrosion protected, 40mm Ø DYWIDAG Bar Anchors for the large decoupling walls at the north end of the jobsite.