Geotechnical Instrumentation of Pile Foundations Embedded in Molasse of a High-Rise Building-Paper

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The paper describes the geotechnical instrumentation program for the foundations of Silex2, a 135 m high-rise building under construction in Lyon, France. The site has a shallow groundwater table, Rhône River alluvium overlying Miocene molasse, whose properties are not fully understood. The building’s 1.2 m diameter continuous-flight-auger piles extend to approximately 17.5 m and are anchored in the molasse.<br /><br />Six of the project’s 20 piles were instrumented to study pile behavior and soil–structure interaction under construction and service loads. Monitoring equipment included vibrating-wire strain gauges and electrical-resistance strain gauges to measure pile strain; concrete pressure cells to record loads at pile heads; and distributed optical-fiber sensors to measure deformation in piles and the raft. A multipoint fiberglass-rod extensometer, with six measurement depths down to 35 m, was installed in a borehole to track soil displacement.<br /><br />The paper outlines sensor selection, placement, installation on reinforcing cages and in the raft, and connection to data-logging systems. Sensors were checked after installation, and readings were scheduled more frequently during early construction, then reduced after measurements stabilized. Long-term monitoring for ten years is planned. Most sensors survived pile casting, although some were damaged during construction of the transfer structure.<br /><br />At the time of publication, construction and data collection were ongoing. The measurements are intended to assess foundation performance under the tower’s full loads and, ultimately, help calibrate a numerical model of the soil–foundation system.

Keywords

geotechnical instrumentation

high-rise foundations

pile monitoring

soil-structure interaction

vibrating-wire strain gauges

optical fiber sensors

groundwater table

Rhône River alluvium

Miocene molasse

foundation performance