Mechanische Eigenschaften von Störungen und Verwitterungszonen im Granitgebirge und deren genetische Charakterisierung

Summary

The Hercynian granites of the Schwarzwald area in southwestern Germany display distinct variations in the amount of disintegration and weathering. Different stages of transition ranging from unweathered rock to cohesionless material can be observed in the same level of erosion. The process of disintegration and weathering is accompanied by a continuous decrease in strength. Several material properties, such as dry-density or porosity (fig. 2, 3), creep behaviour (fig. 4, 5) and elasticity (fig. 5, 6) change in a characteristic manner correlating to the decrease in strength. Non-elastic properties of the granite become dominant during disintegration. The granites of the Schwarzwald area can be classified into six stages of weathering, where each stage is characterized by certain mechanical properties (fig. 7).

This Classification in terms of mechanical properties combines the following parameters: 1. uniaxial peak strength, ß_D, 2. dry-density, ρ_d, 3. strain rate ratio, ε_lateral/ε_axial, 4. creep strain rate per 2 minutes under one half of the uniaxial peak stress, 5. percentage of peak strain rate, % ε_BR which depends on uniaxial stress in a nonlinear manner, 6. modulus ratio, E_reloading/E_{1st loading}.

Tentatively the problem of determination of rock mass strength is investigated by the aid of laboratory tests. The results of direct shear tests on natural joints of the granites (fig. 8) were used to estimate a lower limit of the strength of the granitic rock mass. Discontinuities as well in the mineral structure as in the rock mass structure cause a similar effect on the p-wave-velocity (fig. 9). In connection with this result it appears plausible that a disintegrated specimen may react like a jointed rock mass. Accordingly the result of multi-step triaxial tests on rock specimens (fig. 10, 11, 12) were used to define an upper limit of rock mass strength of the Schwarzwald granites. The strength of the gran itic rock mass is also classified depending on the mechanical properties of the rock and its discontinuities (fig. 13).

The decrease in strength and the change in mechanical properties are Chiefly caused by increasing microcrack density, disintegration of miner al structure and increasing porosity. Mineralogical and chemical changes especially formation of phyllosilicates, promote disintegration but in all types of granite in the Schwarzwald area occur conditions of only slight chemical weathering intensity.

Disintegration and decrease in strength are mainly the results of mechanical, tectonic and thermal stresses.