Comprehending the behaviour of a Muller Verpress pile under tensional loading

Abstract

Müller Verpress piles are driven displacement piles which inject grout at the enlarged tip of the pile into the surrounding soil. Performed full-scale tests were not able to be loaded the pile upon failure, as the pile can resist forces which are significantly larger than its design capacity. This type of pile is known for its large tensile capacity and it is used often to bear major loads in large structures, but the ultimate true capacity has not been identified yet. A better understanding in the behaviour of a MV pile can be crucial in an optimal design of such structures, as construction costs and obtained safety level can be reduced. From provided full-scale tests the uncertainties about the shape of the circumference of the injected grout body and the acting soil pressures have been reduced. Further a different relation is found between the mobilized shaft friction and the cone resistance. As the soil becomes stiffer and holds a larger cone resistance, more shaft friction is mobilized.A two-dimensional numerical model of a cross section of the pile is generated to obtain the actual shape of the circumference and the maximum shaft friction after installation. The measured grout discharges and mobilized shaft friction from pile tests were in the same range as the results obtained from this model, validating that the results of the model are within reason. All available full-scale tests were extrapolated by application of the model and generated similar capacities as the method by measured shaft friction, setting a minimum boundary for α_t at 1.4%. A second numerical model of a complete full-scale tested pile is successfully generated, including the adaptations of obtained parameters from the other model. The adaptations consist of a new circumference, different relation for EA and larger values for the maximum shaft friction. The model showed results which are in the same range as the analytical derivations regarding the course of forces and mobilized shaft friction. The modelled results do however differ slightly in value. This difference in results can be explained by the absence of detailed soil investigation. The lack of detailed geotechnical parameters would increase the accuracy of the model, as the model is very sensitive to variation in the geotechnical parameters. The produced adaptations of multiple parameters have proven however to be a better indication of the total capacity than the current norms. Therefore the norm can be adapted into a more progressive code based on the produced relations of this thesis. The adaptations in EA relation, circumference and mobilized shaft friction comprehend the behaviour of the Muller Verpress pile with a better understanding. Changing these parameters are preferred over the adaptation of the α_t factor, as the theoretical background remains in the design rule.