Versuche zur Abhängigkeit einfacher bodenmechanischer Parameter bei veränderten Tonmineralgehalten am Odenwälder Buntsandstein

Summary

The layers on the boundary Röt/Muschelkalk (lower/middle Trias, Anis) are very strongly susceptible to landslides.

Water is passing through the fissured limestones to the substratum of Röt clay-/siltstones, which contain a high rate of active and inactive clay minerals.

The frequency of landslides decreases from north to south (200 km) in the same direction as the marinity of the fossil environment. It should be proved whether the content of clay minerals influences the stability of overconsolidated soils.

As the content in the northern part (Rhön) is known, two different points of investigations (A and B in fig. 1) were selected. One in the siltstone of Röt, the other in analogue materials in the boundary zone of Perm/Trias (A in fig. 1).

The clay minerals mainly consist of illites and kaolinites (fig. 2). Additionally, in some cases montmorillonite and corrensite appear in the Röt-claystone.

Grain size distribution of 110 samples (fig. 3) and the diagram of plasticity (fig. 4) of both test series mainly agree with each other, accordingly the angle of internal friction (23 – 25° in Röt material) and the cohesion (0,0025 – 0,035 MN/m²) are close together.

A dependence between these parameters and the clay content was not distinguishable.

In order to register the influence of active clay minerals a second investigation serie B was started. Different quantities of montmorillonite were now mixed with the basic material (grain size distribution see fig. 5). The specimens of both series were disturbed and homogeneous.

Samples with more than 10 percent (–80 percent) montmorillonite content showed — as expected — a distinct dependence on the consistency indices (fig. 6). Samples with > 20 percent montmorillonite belong within the ränge of TA (distinct plastic clays) according to DIN 18 196.

The water contents of the liquid limit reach > 100 percent in the end, thereby also the angle of internal friction will be influenced.

Resulting from this only the adding of 20 percent and more montmorillonite to prepared samples alters the soil-mechanic parameters so strongly that the stability of slopes is noticeably influenced. Because these differences of clay-mineral contents do not exist in the natural samples taken between north and south of Hesse the environment difference does not influence the instability of the slopes, but the height of slopeä, the water storage, the joints and the exposition will be of more influence for the plastification of the Liegendes and for sliding.