Bruchmechanismus steifer geklüfteter Tone bei einaxialer Kompression

Summary

Recent results in fracture analysis have suggested that the Coulomb-Mohr criterion appears to be best suited for soils, while failure criteria based on microcrack nucleation may be best suited for rocks that have a great number of inherent ‘Griffith-cracks’ in the form of joints or grain boundaries, required by these theories.

In this context stiff fissured clays take up an intermediate position. They are substantial soils, but due to their sharp physical discontinuities they exhibit structural attributes of a rock mass.

A lot of efforts have been devoted to investigate the strength behavior of stiff fissured clays based on the Coulomb-Mohr criterion, yet the understanding of long-term as well as short-term strength and failure mechanism still remains insufficient.

The main objective of the investigations presented was to determine, whether dilatancy mechanics similar to those that are described in the rock mechanics literature may control the strength behaviour of stiff fissured clays in the lower isotropic stress region.

In order to obtain the stress-dilatancy relationship of a soil specimen under uniaxial compression, a new method was developed based on optographic trace recording of location points attached to the sample surface (fig. 6).

The results of dilatancy measurements offer a progressive Volumetrie strain rate with temporal intensity in various sample regions (fig. 9).

Comparing test readings with microfabric studies in scanning electron micrographs and thin section analysis (fig. 11, 12 and 13), dilatancy obviously results from microcrack nucleation.

New cracks were found to appear first at large components, such as quartz grains, close to inherent fissures and bending or forking of fissures (fig. 14).

At peak stress no faulting was obtained. Collapse of specimen occurs in post-failure region either as longitudinal Splitting or inclined shear-like fracturing (fig. 10).

Analytical energy calculations revealed that failure mechanisms obtainedcan be divided into six sequential events

• i) initiation of microcracks

• ii) stable propagation of microcracks

• iii) critical energy release

• iv) unstable microcrack propagation including coalescence of separate cracks leading to

• v) ultimate strength

• vi) formation of macroscopic fracture.

Thus failure mechanism of stiff fissured clays under uniaxial compression stands in contrast to soil mechanical understanding of shear failure.

Taking into account that failure mechanism below the brittle-ductile transition operates in the same manner, new findings for solving longterm strength behaviour may be expected.