On the effects of unloading and reloading of excavations in clay and silt
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Type
Examensarbete för masterexamen
Master's Thesis
Master's Thesis
Model builders
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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.
Description
Keywords
excavation, unloading effects, reloading effects,, layered soil, vertical displacement, heave, mechanical swelling, settlement, embedded retaining structures, sheet piling