Analysis of collapse mechanism for mantled karst collapse

Tushan lake of Nanshan street in Nan’an district of Chongqing municipality is a low mountain area characterised by karst valley landforms where developing 7 karst zones and ever taking place a mantled karst collapse at the VHSC bridge. In order to provide theoretical basis for the governance, monitoring and early warning of mantled type karst collapse, and to ensure that human life and property are secured in karst collapse zones, this paper took mantled karst collapse as the research object. At first, the evolution mechanism of mantled karst collapse is analyzed. Then the calculation expressions of sliding force and anti-sliding force of mantled karst collapse are obtained considering the joint interaction of soil load, vacuum negative pressure, rainfall and blasting. At the same time, based on the limit equilibrium theory,the methods for calculating collapse soil stability coefficient and determining possible collapse extent of the mantled karst collapses were established; and these methods were applied to the collapses on the left of VHSC bridge. The calculated result of collapse extent is basically consistent with the field observation. In addition, soil thickness that affects stability coefficient and combined effect of soil load, vacuum negative pressure, rainfall and blasting on the collapse extent are were. The results shows that the stability coefficient can increase by 46% when the thickness of the soil increases, and when the depth of water increases from 0.1 m to 1 m and the vacuum pressure increases from 0 kPa to 50 kPa, the stability coefficient can reduce by 30% to 35%. Collapse extent also increases with the increase of soil thickness and explosive amount; and it decreases with the increase of explosive distance and internal friction angle. The reduction of collapse range is larger with the explosive distance increase at start, when the explosive distance increase to 110 m, the collapse diameter decreases by 17%. The collapse range decreases exponentially with the increase of internal friction angle, when the internal friction angle increase from 10° to 26°, the collapse diameter decreased 1%. At last, considering the timeliness of blasting, the soil slump’s change law of stability coefficient with blasting vibration is analyzed. After blasting, the stability coefficient decreased 4%. Blasting effect the soil duration is about 6 s.