Experimental study on the water blocking effect of tree head-stone in the treatment of pit collapse

An effective emergency treatment for the pit collapse is vital to slow down the flow velocity of the pit pond and prevent the further development of the pit pond. The scheme of tree head-stone has been widely used in engineering application owing to its capacity of promoting siltation. The height and the spacing distance of the tree head-stone were generally determined by engineering experience, and these empirical values were short of theoretical and experimental basis. The arrangements of tree head-stone in pit collapse were tested using the physical model at Zhinan village, along the Yangzhong embankment of the Yangtze River. In this study, three heights and three spacing distances were orthogonally combined in the testing scheme, and the flow direction and velocity were measured at 12 locations using the three point method. Based on the analysis of the experimental results, the surface velocity was greatly affected by inertia, and the bottom velocity was dominantly controlled by the topography of riverbed. In comparison of the experiments with different heights and spacing distances, the mean flow velocity increased with the spacing distance of the stone, but decreased with the increasing height of the tree head. The resulting low flow velocity increased the possibility of siltation. Because the investment cost would increase with the height of tree head and the low spacing distance of stone, the best relative height of tree head and the spacing distance of stone were 0.15 and 6 m×6 m, respectively.