ABSTRACT:

Collapsible behaviour of geomaterials, which is the expression of their complex structure, may seriously modify their constitutive modelling. Different kinds of collapses are classified starting with hydrocollapse, garlandlike compression curves, branching-off and diffusion collapse. Some examples of that behaviour are presented using fragmentary (granulated) clay, sugar ballotini and silica gel. Collapse occurring in different ways at the same set of state parameters illustrate the deterministically chaotic nature of collapses. Due to the structural changes of the material induced by collapses it is recommended that collapsing soils be treated separately within structurally homogeneous phases.

INTRODUCTION

Mathematical models of constitutive behaviour usually consist of smooth functions to guarantee uniqueness. Exceptionally only corners, e.g. of the yield surface, are accepted (Olszak et al., 1964). Since soil behaviour models are mostly derived from the continuous material models and the requirement of uniqueness is imperative, the same rule applies with geomaterials. Their structure, however, substantially varies from that of continuous materials and the analogy in the constitutive behaviour has to be often abandoned. Structural units (grains, particle clusters) in the course of deformation disintegrate, their mutual bonding gets broken, geometry of structural units arrangement changes (compression and shear fabrics), and internal stresses (suction, capillary forces) may become annihilated. Thus, constitutive functions with singularities are to be expected. Figure 1 presents a set of settlement measurements of a 6.5 m high fill consisting of clayey clods (two bottom curves depict subsoil settlement). Such fragmentary clay and claystone has been produced by mining activity (Calabresi et al., 1994; Feda, 1998), by excavating foundations of structures (Nakano et al., 1998) etc. Figure 1 shows that the time-settlement curve of the fill (max surface settlement up to 35 cm) consists of two branches meeting in the point of singularity (kink).

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