Engineering Geological Characteristics And Evolutional Process Of Touzhai Landslide

The failured rockmass of Touzhai landslide is permian system nonporphyritic basalt of emeishan basalt. Touzhai basalt landslide has a volume of 900×104 m³, in which 400×104 m³ of rock and soil slid away from the source area. The oblique length, its horizontal projection length and superelevation from crown to tip of the landslide are 3423 m, 3330 m and 763 m respectively. The landslide deposit is composed mainly of basalt debris and clay mineral minerals and its gradation is discontinuous. The space change of the deposit ingredient and structure is not conspicuous. Lay mineral in the giant basalt blocks account for 9% . The surface of rupture consists of three parts from the toe of surface of rupture whose dip is 48°, 38°and 15°respectively, in which the first part is main sliding zone, that is, main scarp.Rock chemical and mineralogical characteristics of fresh basalt and its saprolite is investigated by chemical analyses, optical observation of thin sections and SEM (scanning electron microscopic). Chemical analysis showed that components of fresh basalts and corestones has few changes except that the changes in Fe²⁺ to Fe³⁺ are most noticeable. In the course of corestones changing to saprolite elements of Si, Ca, Na and Mg and the ratio of SiO₂/Al₂O₃ decrease gradually, while elements of Fe, Al and values of LOI and CIA has a significant increase. The weathering front under SEMs is tortuous, and physical weathering cracks which are easier emergence along the cleavage plane can be observed in the vicinity of the weathering front. Weathering order of main minerals is dissolution of the glass, pyroxene and feldspar, which are in agreement with the sequence of weathering-resistant minerals. The main secondary mineral is smectite and chlorite. Rock coupling chemical weathering with physical weathering processes not only exists in the macro-scale, but also in the meso-scale and micro-scale. Changes in the valence of iron takes place in the whole formation process of basalt saprolite, and leads to a volume increases, resulting in physical cracks.The observations of groundwater in the landslide area and the indoor simulation experiments show that: groundwater mainly originate from rainfall and its chemical composition is closely related to the chemical weathering of basalt in unsaturated zone, the soluble salts, adsorption cation, humic acid in weathered soil layer will enter the groundwater, at the same time, the components of the rain will be adsorbed by the weathered soil; The reaction scale between basalt in saturated zone and groundwater is not significant.Basalt crystal structure, chemical bonding, and charge balance will be damaged and ionic radius will be decreased and the porosity of the structure will be increased during the weathering processing, thus the link between crystals will be decreased, The loss of the mechanical strength of the slightly weathered basalt rock is significant.A interbed sliding zone with fault-slip cleavage in an amygdaloidal basalt thin layer made up the embryo of the main sliding zone and the coupling chemical weathering with physical weathering based on lateral unloading made the interbed sliding zone get a material prone to slide finally. The Columnar joints and structural fractures from fold movement made the potential landslide body appear a fragment-mosaic structure, and the saprolited crusts from chemical weathering taking place along discontinuity surfaces made the rockmass evolve into the structure of rock sandwiching soil, which accelerated the time-dependent deformation process of the rockmass.After the landslide occurred, the main body knocked against the slopes of Touzhai valley at a high speed and disintegrated and fine grains, especially clay minerals, from saprolited crusts pervaded among basalt corestones and acted as lubricant and encapsulant. They made the soil-rock aggregate present fluid characteristics. On the other hand, these fine component sealed the air under the soil-rock aggregates after the aggregates dropped into ground effect region, which made the materials from main body move on the air cushion at a long distance. So, chemical weathering process and its products are important to the occurrence of Touzhai landslide and the high speed, long-distance drift of the landslide body.