Landslides - Causes, Types and Effects.pdf

264 Jiří Nedomaods for numerical studies of potential landslides originated in different regions in the worldas results of geodynamic and seismic effects or creep movements on wetting slopes and ofmost water-saturated masses of soils and rocks after the bigger torrential rains and ensuingbigger deluges. The geological structure is obviously one of the most important factorsgoverning the stability of slopes. By the term geological structure is meant both rock typeand discontinuities which together form the rock mass. Discontinuities are presented ingreat scales, from microcracks which have lengths less than 10 mm, to the deep faults inthe earth crust with lengths of thousands of kilometers. Boundaries between different typesof rocks can be also viewed as discontinuities. Similarly, bedding planes which can extendfor kilometers and schistosity which can be several tens of meters long can be viewed asdiscontinuities. Other geological discontinuities include joints which vary in size from approx.1 meter to 100 meters and faults which can be from approx. tens of meters to severalhundreds of kilometers in length. Faults are distinguished from joints in that faults have undergoneshear movements. Several parallel joints and faults form a shear zone (see Fig. 1).The actual shapes of discontinuities are not well known and in reality all discontinuitiesare two- or three-dimensional surfaces. Failures occur when the loads or stresses acting onthe rocks exceed the (compressive or tensile) strength of the rock materials. By the failuremode is meant a macroscopic description of the manner in which failure occurs. Based onthe geological structure and the stress state in the rock mass, certain failure modes appear tobe more likely than others in large scale slopes. The failure surface, defined as the surfacealong which failure develops, could be a single discontinuity (plane failure) or the so-calledwedge failure when two discontinuities intersect each other and the so-called step path andstep wedge failures when a combination of several discontinuities are connecting together.A common feature of most failure modes is the formation of a tension crack at the slopecrest. The significant group of failure modes are rotational shear failures, which occur inslopes without critically oriented discontinuities or planes of weakness. These types of failuresare the typical modes of failures in soils. Furthermore, blasting-induced failures are amarginal problem for large scale slopes. Seismic events, and similarly other low frequencyvibrations, could be more dangerous for large scale slopes. Several seismic-induced failuresof natural slopes have been observed in great mountains, like the large landslide in theHuascaran area in the North Andes forty years ago. The interesting question is where suchfailures are initiated and the question about the mechanisms of such failures (see [26, 75]).In some cases the failure is initiated at the toe of the slope, where the highest shear stressesare found. In many cases the failure surface forming at the crest of the slope will be a tensioncrack. Failure initiation in clayey soils depends also upon the groundwater conditions.For more details see [26, 70, 75]. A few failures are presented in Figs 2-7.The facts, which should be necessary mentioned here, will also show how to obtainapproximately the geometry of the investigated areas, the dimensions of the potential failuremasses and the mechanisms of landslide origins. The investigated slope mainly consists ofintact up to weathered or otherwise destroyed rocks and has been affected by recurrentinstability phenomena.In order to analyse and evaluate the potential hazard related to unstable rock slopes,related to effects of weather like the great torrential rains with resulting great deluges and,moreover, related to consequences of geodynamic and geomechanic processes in these regionswith ensuing earthquakes or shocks, it is essential to understand the processes and