What the instruments did: Observations on the instrumentation programme on the three wall projects.
Slope inclinometers: Installed before excavation on each of three wall projects. Inclinometers provided a means of evaluating whether unacceptable ground movements occurred during wall excavation and are currently being used to evaluate whether long term wall movement is continuing. The maximum 50mm horizontal movement recorded at the top of the walls during construction is within the acceptable limits. Postconstruction movements have been negligible.
Piezometers: Verified that the temporary dewatering systems lowered the groundwater levels to an acceptable level, before excavation. They were also able to verify that the long term performance of the horizontal drains on the second wall were successful in lowering the site groundwater levels to below the levels assumed in the wall analysis.
Temperature sensors: Installed on the first wall behind the shotcrete to verify whether the rigid insulation prevented ground freezing behind the wall. The results verified that the rigid insulation thickness was sufficient to prevent freezing, and Load cells and strain gauges: Placed on three in-line soil nails on one of the three walls. The miniature, low profile strain gauges are designed for use where space is limited. They were spot welded to the nails at Geokon's factory, and shipped to site ready for installation.They measured maximum load within the nails at 35% of the predicted maximum design load, which indicates a conservative soil nail analysis. Maximum load measured at the head of the nails with the load cells was 55% of the maximum measured load within the nail. This confirmed the design criteria which had assumed that 60% of the maximum nail load is transferred to the nail head.