Ivo Herle

Hypoplastic model for cohesionless soils with elastic strain range

SUMMARY In order to eliminate ratcheting a so-called intergranular strain has been added to a hypoplastic constitutive model. This additional state variable represents the deformation of the interface layer between the grains. The new concept is outlined and comparisons with and without intergranular strain are presented. Some comments on numerical implementation and determination of material constants are made. A discussion on the uniqueness of the solution and objectivity of the rate of intergranular strain is added. # 1997 John Wiley & Sons, Ltd. Mech. of Cohes.-Frict. Mater., 2, 279‐299 (1997)

Determination of parameters of a hypoplastic constitutive model from properties of grain assemblies

The stress–strain behaviour of granular materials can be modelled with hypoplastic constitutive relations. A hypoplastic model is briefly introduced for the axially symmetric case, and a procedure for the determination of its parameters is described in detail. It is shown, for several sands and one gravel, that all parameters of the hypoplastic model are closely related to the granulometric properties of grain assemblies. Recalculations of some element tests are presented in order to verify the proposed procedure. Copyright

FE-studies on the influence of initial void ratio, pressure level and mean grain diameter on shear localization

The paper is concerned with shear localization in the form of a spontaneous shear zone inside a granular material during a plane strain compression test. The influence of an initial void ratio, pressure and a mean grain diameter on the thickness of a shear zone is investigated. A plane strain compression test with dry sand is numerically modelled with a finite element method taking into account a polar hypoplastic constitutive relation which was laid down within a polar (Cosserat) continuum. The relation was obtained through an extension of a non-polar hypoplastic constitutive law according to Gudehus and Bauer by polar quantities: rotations, curvatures, couple stresses and a characteristic length. It can reproduce the essential features of granular bodies during shear localization. The material constants can be easily calibrated. The FE-calculations demonstrate an increase in the thickness of the shear zone with increasing initial void ratio, pressure level and mean grain diameter. Polar effects manifested by the appearance of grain rotations and couple stresses are only significant in the shear zone. A comparison between numerical calculations and experimental results shows a satisfying agreement. Copyright

Stationary states in hypoplasticity

The present paper shows briefly several developments of hypoplastic models of the Kolymbas type. In particular the embedding of stationary states or so-called critical states is discussed more comprehensively. The hypoplastic concept is based on a single constitutive equation of the rate type which is apt to model a non-linear and anelastic material behavior starting from any initial state up to a so-called failure. Stationary states are included as asymptotical states in which the memory of the material on the initial state is swept out. By including a pressure dependent density factor the hypoplastic model describes the essential properties of initially dense and initially loose granular materials for a wide range of pressures using a single set of constitutive constants. Polar effects within shear zones are investigated by an extension of the basic hypoplastic concept to a polar continuum. The performance of recently developed hypoplastic models is discussed and the results of numerical simulations are compared with experiments.

Compression of collapsed loess: Studies on bonded and unbonded soils

Abstract Compression of bonded (collapsible) and debonded (collapsed) loess is studied in the laboratory using reconstituted samples of Sedlec loess. Oedometric and hydrostatic (triaxial) compression curves yield identical compression indices. Intrinsic (ICL) and semilogarithmic (LCL) compression lines are favourably compared with the test results of reconstituted samples. Owing to the aging of reconstituted samples, the bonded structure that develops seems to be qualitatively identical to that of natural collapsible loess. In the course of loading, both natural and aged samples undergo gradual destruction of the structural bonds. The only difference is in the initial porosity, which in the case of natural samples is much higher (these are metastable). Reconstituted samples, even if aged, and consequently bonded, cannot, therefore, be collapsible.

Geotechnical Problems of Cultural Heritage due to Floods

Flood events demonstrate devastating effects not only on materials and structures in contact with flowing surface water. Equally important is the behavior of foundations in interaction with subsoil. This contribution gives an overview of different phenomena which arise in subsoil and at the foundation level during the groundwater rise accompanied by several case histories related to cultural heritage. Possible geotechnical measures are discussed as well.

Hypoplasticity: A Framework to Model Granular Materials

To solve a series of problems, engineers are interested in the mechanical behaviour of materials (and geomaterials are, probably, the most fascinating materials). What are our approaches to detect the mechanical behaviour of materials? We approach by observation, experiments and theories. Constitutive models are theories. A common opinion is that experiments are of primary importance. As soon as the experimental results are available, the formulation of an appropriate theory is straightforward. Another common opinion is that constitutive theories (models) are only valuable to feed sophisticated FEM-codes. Both opinions are not completely true and are, thus, misleading. It is very important to realize that a theory (besides serving as a tool for better FEM-simulations) helps to understand nature. C. H. Darwin said that ‘All observation must be for or against some view, if it is to be of any service’. Besides of this, we have to recognize that experiments are burdened with a series of errors.

Comparison of hypoplastic and elastoplastic modelling of undrained triaxial tests on loose sand

The capabilities of two different constitutive relations for the modelling of undrained triaxial tests on water-saturated loose sand are compared. The hypoplastic model represents a relatively simple model based on rational mechanics whereas the chosen elastoplastic model by Di Prisco belongs to the non-associative plasticity with combined hardening. Both models perform qualitatively well in spite of some discrepancies in comparison with experiments. A modification of the hypoplastic model is tested showing both an improvement as well as additional malfunctions in the model behaviour.

Shear resistance of fissured Neogene clays

Abstract Two types of fissured Neogene clays (SM and WB) were tested in the laboratory using undisturbed and reconstituted specimens. Although of similar age, the clays differ in their geological history: SM clays were deposited in a marine environment, WB are lacustrine clays with a complex tectonic history. The index properties of both clays are similar and, accordingly, their strength parameters are mutually close. Test results of the more compact and less wet clay (WB) are affected by the sampling and trimming procedures, especially for 38 mm diameter specimens. Although the behaviour is similar, the origin of fissures is different. For the SM clays, weathering is the main cause and fissuring reaches a depth of about 20 m. WB clays are dissected by macro- and microfissures originating as the result of tectonic processes and volumetric changes (effect of the coal formation underneath the investigated massif). Cementation of some samples was found and the transition from clays through cemented clays to claystones (soft rocks) is speculated.

Suction and Collapse of Lumpy Spoilheaps in Northwestern Bohemia

Large spoilheaps of overburden from open-cast coal mining in Northwestern Bohemia offer a space for development. However, they exhibit unfavourable features such as collapse potential caused by the lumpy structure with high overall void ratios. Suction oscillations contribute to the structure degradation by the intervoids closure. The paper presents in-situ suction monitoring and laboratory investigation of the collapse potential of spoilheaps.