Modelling of Random Variations in Soil Properties

Penriya, Weerapong ; Marton, Marta

4. term

Structural and Civil Engineering, Master

2015

2015-06-08

72 pages

Geotechnical data is constrained with uncertainties. Among them is the natural variability of the soil which is a type of uncertainty that cannot be reduced. When designing a structure, this unpredictability of the soil has to be taken into account. The traditional way of doing this is based on a deterministic design, where partial safety factors are applied to the characteristic value of the loads and strength parameters, leading to a design based on increased loads and decreased strengths. The purpose of the thesis is to model the random variations in soil properties using a probabilistic approach, based on the random field theory, in which soil properties are characterized in terms of their means and standard deviations, and through a realiability analysis to calibrate the partial safety factor for the undrained shear strength. The partial safety factor in Denmark is set to gcu = 1:8, which is quite high compared to the general factor proposed in Eurocode 7, gcu = 1:4. Two-dimensional cross-correlated random fields for the undrained shear strength and Young’s modulus of the soil are generated based on the matrix decomposition method. The probabilistic finite element model is created by using different packages; Abaqus, Python scripts, Fortran subroutines and Matlab scripts. 1000 Monte Carlo simulations are performed, resulting in 1000 bearing capacities of the structure. The data is then fitted and combined with the probability density function of the load a number of realizations are performed with the crude Monte Carlo simulation technique. Through this the partial safety factor of the undrained shear strength is calibrated to a corresponding target reliability index. This whole procedure is performed for two examples, one for a strip footing and one for an embankment. The reliability-based calibration shows that the partial safety factor can be reduced significantly, from gcu = 1:8 to gcu = 1:5, in both cases, by which the costs are reduced too. In addition, the effects of the correlation length on the failure mechanism is investigated, and it is found that the failure mechanism is highly dependent on the correlation length.

Geotechnics ; Random fields ; Reliability ; Partial safety factor

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