Auflockerung und Verwitterung in der Ingenieurgeologie: Übersicht, Feldansprache, Klassifikation (Verwitterungsprofile) — Einleitender Beitrag

Summary

This introductory chapter deals with the significance and field Classification of weathering profiles in different important rock types. For a first description of rocks and soils in engineering geology, the distinction of 5 stages of weathering (apart from fresh parent rock) are recommended. The term “degree” of weathering should be used for more quantitative to semi-quantitative characterisations.

Near a land surface lowered by denudation, all rock types are affected by stress release (fig. 1). As a result, the rock mass starts to disintegrate along pre-existing and newly developed fissures, joints, and other discontinuities. In this stage, the material of sedimentary rocks, particularly that of mudrocks, is still overconsolidated and more or less diagenetically indurated. These properties are successively lost during weathering. For rock types containing clay minerals, the “inactivation” and “reactivation” of these minerals play a great part in these processes. Therefore, kind and amount of clay minerals are used to distinguish between different rock types (fig. 2) which, for example, behave differently when they are kept under water or are exposed to repeated drying and Wetting.

The weathering profiles can be subdivided into 2 groups (fig. 3). In the first one, the rock material is relatively resistent to weathering and remains hard nearly up to the land surface. In the second group, the rock is disintegrated and decomposed already at greater depth. Both groups can form either non-cohesive or cohesive matrix respectively soils in stages W4 and W5. Furthermore, one should distinguish between regulär, surface-oriented weathering profiles and irregular ones which may be caused by oxidation and dissolution of circulating groundwater and/or by special tectonic fracture zones and faults (fig. 4). Examples of surface-oriented weathering profiles including some explanations are shown for the following rock types: Granite and basalt (fig. 5), sandstones and bedded limestones (fig. 6), mudstones and tili (fig. 7).

Finally, some consequences of the weathering profile for shallow foundations, permeability, angle and stability of slope cuts, “back-weathering”, development of failure planes, ground-heave, and dam construction are briefly discussed (fig. 8).