Ingenieurgeologie und Tonmineralogie vulkanogener Sedimente

Summary

Excavation of deep roadcuts in tuffitic sediments of the northern ‘Siebengebirge’ montains, W-Germany, have entailed considerable slope failures. Oligocene tuffs and tuffites of the trachytic Siebengebirge volcanism are deposited on tertiary sand and coal or directly on residual clays of the Devonian basement. Because the sliding planes (inclination 6° – 10°) cannot be explained by the easily determined Standard soil parameters, extensive geotechnical and clay-mineralogical investigations were carried out. The tuffites are of variable grain size depending on their degree of weathering and decomposition. Predominantly, however, they may be classified as fine-sandy, clayey silts.

Clay-mineralogical analyses have proved that the tuffites consist of smectites, illites, kaolinites and quartzes, while the thin gliding planes contain over 6 0 percent of highly swellable montmorillonites.

The swelling of tuffitic sediments was tested under different conditions in order to establish the relation of swell pressure and deformation. The tests have proved a significant dependence of the swelling potential on following soil parameters: initial water content, liquid limit, anddry density.

The shear strength of the material was tested in the ring-shear appa’ratus, considering the shear path and the degree of particle orientation. A new test method was developed which allows to carry out ring-shear tests on normal and overconsolidated samples and constant volume.

To eliminate the influence of surcharges the shear stress and the normal stress were standardized by an equivalent normal stress. The tests have proved that the angle of internal friction is a soil-specific parameter and that the cohesion depends upon the void ratio, increasingly with growing overconsolidation. In addition to the ring-shear tests, triaxial cu-tests were carried out and interpreted according to the critical-state-theory.

The post-failure behaviour of tuffites can be described as follows:

• the residual shear strength depends on the overconsolidation ratio

• normally Consolidated samples are deformed plastically

• overconsolidated samples show discrete shear joints

• after 8 cm shear displacement a residual shear strength of overconsolidated undisturbed samples of only 3 0 percent (8°) of the maximum strength remains; this compares well with calculations based on actual failures.

According to the investigations the main reason of slope failure in swellable overconsolidated tuffites is the decline of the shear strength after load relief by roadcutting. Usually, the failure starts at the base and expands progressively into the slope.

The orientation of clay minerals supports the decline of shear strength after initial movements.