Mikrogefüge und bodenmechanische Eigenschaften überkonsolidierter Tone

Summary

In a research work, financially supported by the Deutsche Forschungsgemeinschaft, an attempt has been made to interpret the mechanical behavior of overconsolidated clays from the stratigraphic periods Cretaceous to Quarternary by investigating their classifications, mineralogical and geological characteristics (RAABE 1984).

Oedoemter tests on undisturbed and remoulded samples of different orientations were used to analyse the anisotropic behaviour and to prepare samplet for the following testing program.

A micro-computer controlled triaxial testing system developed as part of the work was used to study the mechanical behaviour of undisturbed samples under working and limit loads.

Stress path dependence of deformation, pore pressure and strength characteristics were evaluated. Mineral content and microfabric features were analyzed by x-ray diffraction methods. These results were verified by scanning microscope investigations.

Mineral content, size, and orientation of clay particles, diagenetic bonds, and void ratio were related to geotechnical properties.

In this paper results of microscope investigations are presented and discussed with regard to the observed mechanical behaviour under working loads.

The Classification results (table 2) show the wide ränge of basic parameters of the investigated clays. Clay mineral composition was quantitatively estimated from x-ray-diffraction traces of oriented aggregates. The investigated sediments have different mineral components.

The high glaukonite content of the Miocene-clays and the high content of carbonates in some other clays are important results and lead to characteristic properties. Swelling clay minerals like smectites and mixedlayer- minerals were found in the Quarternary, Oligocene and Cretaceous clays.

Clay fabric was investigated by scanning microscope and x-ray-diffraction methods. A particle orientation parameter T was defined which expresses the degree of clay mineral orientation in relation to the horizontal plane in situ (undisturbed samples) or reconsolidation plane (remoulded samples).

The Quarternary clay ED shows the highest degree of orientation. The Miocene clays have randomly orientated particles. The results of the xray- diffraction analysis were verified by scanning microscope techniques.

The degree of orientation of remoulded and reconsolidated samples generally increased with the applied consolidation pressure (fig. 4). Anisotropie behaviour under working loads was investigated in oedometer and triaxial tests. In oedometer tests the undisturbed samples fehowed a high anisotropy which decreases with increasing load. The stiffness in horizontal direction was considerably higher than in vertical direction (direction of Sedimentation and pre-loading).

The remoulded and reconsolidated samples however reacted upon loads with higher stiffness in the direction of reconsolidation (fig. 7).

Swelling behaviour of the clays generally corresponded with the results of compression tests (table 5) as well as the behaviour in triaxial consolidation tests.

High anisotropy, observed in oedometer and triaxial tests was related to

• structural anisotropy in the microfabric

• the last stress State

• type of new loading.

The dependence of anisotropy on stress level and the high degree of anisotropy for small stress changes was pointed out.

The observed influence of diagenetic bonds on mechanical behaviour is of great importance and should be a topic of further investigation.