Authors: Thut, A; Rabaiotti, C; Wörsching, H

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Thut, A, Rabaiotti, C & Wörsching, H 2015, 'Instrumented test shaft in soft ground', in PM Dight (ed.), Proceedings of the Ninth Symposium on Field Measurements in Geomechanics, Australian Centre for Geomechanics, Perth, pp. 423-435,

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A large underground structure is planned under the existing main railway station of Lucerne (Switzerland) to expand the capacity of the station. An instrumented test shaft was built to study the proposed construction technique. The test shaft was 5.6 m wide and 13.6 m long consisting of reinforced diaphragm wall with a thickness of 80 cm and a depth of 24.0 m. Three levels of struts were planned to support of the excavation to a depth of 18.0 m. Prior to excavation of the test shaft jet grouting columns were built between 18.0 and 23.5 m to support the lower part of the wall and provide a seal against piping. The instrumentation used to monitor the test shaft included four TRIVEC systems (high precision measurements of the three displacement vectors), fixed to the reinforcement cage and imbedded in the concrete, inclinometers installed in the soil 1.5 m from the shaft, and strain gauges to measure forces on the struts during excavation.The effect of jet grouting at the base of the narrow excavation in soft soil led to high lateral pressures acting outward on the walls pushing the diaphragm wall against the soil outside of the shaft. The TRIVEC system indicated bending of the wall with maximum displacement of 130 mm at a depth of 12.0 m. Horizontal displacements of the same order of magnitude were observed in the upper soil layers. Increases in the measured forces in the struts during excavation showed that the buckling load on the struts would have been exceeded by the final level so additional struts were added. The results and interpretation of the measurements are presented and discussed in the paper.

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