Keywords: contact model, surface interaction, skin friction, hypoplasticity, retaining wall, soil.

Granular materials, e.g. sand, show strong dependencies in shearing on the mean pressure. This is known as barotropy. Further, the shear strength is influenced by the density of the material. But shearing changes the density. Contraction and dilation can occur. Modern constitutive models for soil, like hypoplastic models, can be used to simulate these characteristics in numerical calculations [1,2]. The skin friction between soil and structures shows also some of the same qualities [3,4,5]. A part from this, interface behaviour in FEM simulations is usually modeled by using a simple Coulomb-failure criteria. This cannot cover the complex interaction between granular materials and structure, such as skin friction of a retaining wall.

Starting from a general hypoplastic constitutive law by [6], a new model is developed to describe the interface behavior between granular materials and completely rough surfaces. This new contact model makes it possible to simulate the same characteristics of the material in the interface as in the continuum. The gap between contact modeling and the continuum is closed. Further, the model is extended to take in account different roughness of surfaces. A new parameter is derived from modified direct shear tests. It is introduced in the model for simulation of varying roughness. A comparison of calculated results with data from laboratory tests shows the capabilities of the new model.

An example of a cantilever retaining wall embedded in sand emphasizes the importance of a realistic modeling of the skin friction. The advantages of the new model come out clearly comparing of the calculated results to those received by using a Coulomb-friction model.

1

G. Gudehus, "A comprehensive constitutive equation for granular materials" Soils and Foundations, 1, 1-12, 1996.

2

D. Kolymbas and W. Wu, "Introduction to Hypoplasticity" in Modern Approaches to Plasticity (D. Kolymbas, ed.), 213-223, Elsevier Science Publishers B.V., 1993.

3

M. Uesugi and H. Kishida, "Influential factors of friction between steel and dry sands"Soils and Foundations, 2, 33-46, 1986.

4

M. Uesugi and H. Kishida, "Frictional coefficient at yield between dry sand and mild steel" Soils and Foundations, 4, 139-149, 1986.

5

J. Tejchman and W. Wu, "Experimental and numerical study of sand-steel interfaces," Int. Journal for Numerical and Analytical Methods in Geomechanics, 19, 513-536, 1995. doi:10.1002/nag.1610190803

6

P.-A. v. Wolffersdorff, "A hypoplastic relation for granular materials with a predefined limit state surface" Mechanics of cohesive frictional materials, 3, 251-271, 1996. doi:10.1002/(SICI)1099-1484(199607)1:3<251::AID-CFM13>3.3.CO;2-V

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