On the effects of unloading and reloading of excavations in clay and silt

Abstract

In connection with pipe laying in an excavated trench supported by steel sheet piles in clay with underlying layered silty soil, large settlements were measured on a newly laid pipe in a project in Varberg, Sweden. This work aims to investigate possible causes of the settlement by numerical modelling of the geotechnical design. The work also aims to draw conclusions that can help in future excavation work in similar soil profiles. The project initially identified relevant sounding points with in-situ geotechnical tests and laboratory tests. A soil profile was developed from the empirical interpretation of CPTu logs using soil behaviour type (SBT) classification systems. Soil parameters were determined from field and laboratory tests. A numerical modelling was performed of the unloading and reloading effects of a supported excavation. The numerical analysis shows that the excavation bottom heaves during unloading and mechanical swelling. During reloading and pipe laying, there is an immediate settlement and a consolidation settlement. The settlement of the excavation bottom (and the pipe) during reloading in the numerical model may be due to an elastic rebound effect of the bottom under the pre-consolidation pressure in the soil. A difference arises between the total heave and total settlement corresponding to the magnitude of the mechanical swelling. The work also identifies other potential causes of the actual settlement of the excavation bottom (and the pipe) but which were omitted in the numerical modelling. These factors are briefly addressed and include hydraulic uplift, water flows along the soil-sheet pile interface due to penetration of permeable water-bearing subsoil layers, installation and removal effects of sheet piles, and the influence on the strength properties of the soil from vibrations during sheet pile driving in layered silty soils and compaction works in the shafts.