Verdichtungs-, Verformungs- und Sättigungsverhalten von Schüttungen in Abhängigkeit von der geologischen Gesteinsentfestigung

Summary

The object of this research project was to determine the influence of geological disintegration of rock material on the mechanical properties of granitic rockfills. As a first step an appropriate method to quantify the degree of geological disintegration of granites was developed. Comparative investigations led to a characterization based on micro-mechanical and chemical properties, i.e. on the frequency of microfractures and the content of newly formed clay minerals in the rock material (fig. 2).

Numerous oedometric and triaxial compression tests on various granite fills with different degrees of disintegration (ranging from sound to severely decomposed) revealed the influence of disintegration on the strength and stress strain characteristics.

The preparation of realistic samples of grain breakage sensitive rockfill materials with the help of dynamic compaction was used in this project. The method and the intensity of compaction has a great influence on the mechanical behaviour of such materials.

The Settlements of fills due to first flooding, an important aspect in embankment dam engineering, and the necessary pre-compaction to avoid such Saturation Settlements can both be estimated using the methods developed in this research project.

The main conclusions from this research project may be summarized as follows:

• a) Dependent on the geological disintegration, rockfill materials show a characteristic particle breakdown behaviour during compaction (fig. 5). The structure of such a fill is mainly determined by the process of breakdown and this structure governs the mechanical behavior of such fills.

• b) Under consolidation or shear stresses, dry rockfills and rockfills with natural moisture content exhibit a stiffer behaviour and a higher shear strength than those which are watersaturated (fig. 8). Saturation of a dry sample, under a certain constant load results in a Saturation strain. This strain was found to be equal to the difference between the strains obtained from a dry and a saturated test at the same load (fig. 7).

• c) Saturation strains, mainly caused by particle breakdown effects during Saturation, are considerably larger under shear than under isotropic compression for the same mean stress (fig. 11).

• d) Strains due to Saturation can be anticipated by “breakdown compaction”. The necessary specific compaction energy to achieve this, increases as the rock grains become more disintegrated (fig. 6).

• e) By applying the developed test technique, it is possible to estimate the necessary compaction intensity to avoid Saturation strains (fig.6).

The results of this investigation show that it does not seem to make much sense to improve the existing numerical solutions for the calculation of stresses and strains in rockfill dams unless the techniques for testing the material behaviour under conditions as close as possible to those in situ are also improved at the same time.