CHINESE TIANKENG: AN EXTREME FORM OF COLLAPSE SINKHOLE

Closed depressions formed by collapse processes divide broadly into soil piping sinkholes and bedrock collapse sinkholes. The latter are less common. Bedrock collapses may be shallow caused by truncation of the stress arch that overlies solution cavities in the bedrock. Bedrock collapse may also be initiated at depth if the proper structural weaknesses are present. Deep-seated collapse stopes its way toward the surface most effectively if the initiating cavity contains a flow of water sufficient to remove the fallen blocks. Efficient removal of fallen blocks leads to deep open shafts while inefficient removal results in the accumulation of rubble piles or breccia pipes with little surface expression. There is a distribution of sizes and at the end of the scale are the features called in Chinese tiankeng (literally "sky hole"). A number of these have been examined in South China and are compared with other karst collapse structures.

Tiankeng occur in thick limestones that have been sculptured into cone and tower karst landscapes. The tiankeng are vertical-walled depressions with depths ranging from 100 to more than 300 meters. Diameters are variable but are typically in the range of hundreds of meters. Some expose large underground rivers that serve as the removal mechanism for fallen blocks. Vertical fractures provide the structural weakness for the collapse. The vertical walls of the tiankeng slice through the karst towers when the collapse structure penetrates the land surface.

The tiankeng collapse depressions extend the scale range for collapse features but have formed according to the same basic mechanisms. Solution cavities in unfractured rock remain stable so long as the stress arch is undisturbed. Development of deep stoping features requires structural weaknesses extending from the initiating cavity at depth to the surface. Vertical dissolution along fractures, size of initiating cavity, and efficacy of lateral transport at depth are controlling mechanisms.