Deltares - R&D Highlights 2009

Contact: lars.beuth@deltares.nl +31 (0)88 335 7523 john.vanesch@deltares.nl +31 (0)88 335 7457 pieter.vermeer@deltares.nl +31 (0)88 335 7307 Material Point Method for large deformations Quasi-static MPM simulation of shear stress from a CPT in clay In geomechanics various processes like the installation of piles and hazards such as landslides involve large deformations of soil. Obtaining reliable predictions of such complex deformation processes proved cumbersome due to the computational eff ort imposed by available numerical methods and the limits of applicability of these methods. Further complexity is added to large deformation problems by the highly non-linear mechanical properties of granular materials. The drawback of the widely used Finite Element Method (FEM) lies in its reliance on a mesh. Strong mesh distortions during large deformation analyses render the FEM prone to numerical problems that are diffi cult to overcome. The disadvantage of fully mesh-less methods such as the Smoothed Particle Hydrodynamics appears to be the involved high computational cost. In the framework of an EU funded research project named GeoInstall, a mesh-free method called the Material Point Method (MPM) is being developed at Deltares with support of universities in Stuttgart, Stellenbosch, Glasgow and Delſt for the analysis of both dynamic and quasi-static problems. At present, the emphasis lies on modelling cone penetration and pile driving. Static cone penetration testing (CPT) was invented in the Netherlands for the purpose of fi nding sand layers for pile foundations, but meanwhile it has become an in-situ test for soil profi ling as well as estimating soil properties on strength and stiff ness. Many fully empirical correlations have been proposed for the extrapolation of CPT measurements to soil properties, but as yet many of them require independent verifi cation as might be done by numerical simulations. Obviously, this requires a large-strain analysis as possible with the MPM. Shear stresses for a deep CPT penetration into clay have been computed in a large area around the CPT tip. At the shaſt just above the tip of the cone, the computed stresses can directly be compared to measurements at the so called friction sleeve. Similarly, this can be done for the resistance at the tip of the cone. 80

Dynamic MPM simulation of shear strains from a sphere shot into clay At this very moment the MPM works nicely for clay penetration. The challenge is now to perform these computations also for sand. Also dynamic processes can be modelled, like a projectile being shot into soil. The problems encountered in this analysis are very similar to the ones that occur in pile driving, the second focal point of the ongoing research. As for quasi-static penetration, soil-metal contact has to be modelled, but on top of this, inertia forces create stress waves so that a fully dynamic approach is required. Meanwhile, fi rst simulations have been done for piledriving in dry soil, but the challenge is now to extend these to saturated soil and to predict pore water pressure waves. Indeed, commercially available soſt ware exists for large deformation problems, but not in the case of dynamically loaded saturated porous media. Funding GeoInstall (EU) 81

lars.beuth@deltares.nl, john.vanesch@deltares.nl, pieter.vermeer@deltares.nl,
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