Optimisation of an excavation pit through large scale triaxial tests on gravel Niederterrassenschotter Basel

Abstract

For the construction of a new high-rise in Basel, a deep excavation pit was created in the middle of a pharmaceutical site. To secure the excavation pit against earth pressure and water pressure of up to ten metres in the highly permeable ground, a secant bored pile wall with a single intermediate support in the form of a strut slab was constructed. Based on experience from recent adjacent construction projects, it was reasonable to assume that the underlying soil properties had significant reserves. To determine the soil properties in a less conservative manner, large-scale triaxial tests were carried out on the Basel soil, the so-called “Niederterrassenschotter” (sandy gravel). The aim of the tests was to determine the effective shear parameters and the stiffness of the existing ground in a realistic manner. Based on the test results, the excavation concept was implemented. As demonstrated during the project’s implementation, the measured deformations remained well below the predicted values, and the excavation was completed on schedule.