This thesis presents a nonlinear numerical analysis of the excavation and support of a construction pit on the northern edge of the Ljubljana Marshland. The approximately 9.2 m deep excavation reaches into highly compressible marsh sediments and will be supported by a 60 cm thick reinforced concrete diaphragm wall and three levels of steel struts. Four buildings stand in the immediate vicinity of the pit. The response of such ground is markedly nonlinear and constitutive models describe it differently. We analysed how the choice of the constitutive model for the marsh sediment layer affects the displacements of the support structure and of the adjacent buildings, and the internal forces in the diaphragm wall. The analyses were carried out in PLAXIS 2D under plane strain conditions, in two cross sections with different strutting arrangements. For each cross section we prepared four calculation variants, differing only in the constitutive model of the marsh sediments: Mohr-Coulomb, Soft Soil, Hardening Soil and Hardening Soil with Small-Strain Stiffness. The choice of model does not change the shape of the displacement field or of the internal force envelopes, but it does change their magnitude. The largest values are obtained with the Hardening Soil and Hardening Soil with Small-Strain Stiffness models, which differ by up to 12 % in internal forces and by up to 25 % in displacements. The values computed with the Soft Soil and Mohr-Coulomb models are systematically smaller: the heave of the excavation base by 5 to 32 %, the displacements of the diaphragm wall by 15 to 37 %, the settlements of the adjacent buildings by 16 to 67 %, and the internal forces in the diaphragm wall and struts by 14 to 34 %. Characteristic values of displacements and internal forces are compared, without partial safety factors. Since no field measurements of displacements are available, the computed values are only comparative predictions.
|
