The grain size distribution and composition of the Touzhai rock avalanche deposit in Yunnan, China

The September 23, 1991 rock avalanche that occurred in Touzhai, Zhaotong, Yunnan province of China claimed 216 lives and caused heavy property losses. The Touzhai avalanche possesses typical and representative feature of high-mobility. Field surveys are applied in the exploration of the grain size distribution and deposit composition, and the lab analyses provide important information about the occurrence mechanism of the rock avalanche flow. The Touzhai rock avalanche has a volume of 9 million m3 that traveled about 3.4 km along Touzhai valley within 3 min, with an effective coefficient of friction of 0.22. The deposit of the Touzhai rock avalanche contains a wide spectrum of grain sizes (< 5 μm-7.82 m) and the proportion of the boulders (> 50 cm) outcropped in the deposit surface is 0.2%. Compared with the upper reaches, a greater number of boulders are found to exist in the middle- and lower reaches of the rock avalanche flow. The non-uniformity coefficient, curvature coefficient and fractal dimension of the deposit (< 150 mm) are 21.1, 2.93 and 2.53, respectively, indicating that the particles are poorly sorted. The changes in the grain size distribution from the upper part of the rock avalanche flow to the terminus are not obvious. There is no sorting or stratification in terms of structure and composition of the deposit profiles. The source rock mass is highly fragmented, having been subjected to initial discontinuities, tectonic movement and weathering, but undisaggregated, which is responsible for the broad grain size distribution of the deposit. The montmorillonite and kaolinite formed by the weathering of Emeishan basalt, together with the fine particles created by the fragmentation of Emeishan basalt saprolite, might have pervaded the spaces among the coarse debris and trapped the compressed air, which may have supported the debris in the rapid rock avalanche flow.