Three centrifuged concrete piles were measured in a static-loading test. These 45 cm piles will be used to support a cut-and-cover tunnel. They are the floating type and they are installed in clay-silt-sand soil with layers of gravel. The 3 and 4 m long SOFO sensors were arranged in chains of 7-8 thus forming a 22 m to 26 m long multi-point extensometer that was installed in the central hole of the piles. The hole was subsequently grouted.
Aim of monitoring:
The Horw project aimed to measure the lateral friction and the load-bearing capacity, the piles were subjected to charges up to 2000 kN.
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The SOFO sensors were selected because of their reliability, precision, ease of installation, and the possibility of carrying out automatic measurements. Furthermore, the measurement did not require access to the pile head which greatly simplified the loading setup. Before, during, and a few days after the static loading test the SOFO sensors were automatically measured using an optical switching unit. The measurement allowed the determination of the pile settlement as a function of the load level and the time, the calculation of the lateral friction, the ultimate load-bearing capacity, and the evaluation of the soil parameters. The measurements showed that shorter piles could be used to carry the forecasted loads. Figure 1 shows the measured deformation as a function of the applied loads. Each curve represents a sensor. Figure 2 shows the axial deformations for different depths. The reduction in the deformation of the lower sensors indicates that part of the load is dissipated through lateral friction.