A Study On Properties Of Bridge Piles In Karst Areas

The karst is common in many cases of bridge construction. As the karst topography is very complex and the piles' properties in them are influenced by many factors, there are many divergent opinions on the pile's bearing properties and design methods in karst areas. In practice, it is usual to regard the karsts pile as the conventional rock-socketed piles during design and construction, and check the thickness of the aven's roof-supporting. Since there are some hollow spaces under the bottom of piles, whose characteristic is affected seriously by the groundwater, the bearing mechanism of the karsts pile obviously differs from the conventional rock-socketed piles. So it is apparently unreasonable to design and construct karsts pile according to the conventional rock-socketed piles' theory. As a result, it is necessary to study systematically the karsts pile's bearing properties and come up with the reasonable and sophisticated design methods.The paper analyzes rocks' properties around the bottom of pile and generalizes the study on the karsts pile on the basis of the references to literature search. It sets up ten mechanical models to simulate the mechanism properties of pile-rock under loading in different geologic conditions according the theory of the elastic mechanics and mechanics of materials, and gives each model's applicability. Then, the paper computes the result of each model and compares them with each other according to field engineering.According to Kraft's loads transfer theory, this paper respectively makes a study of the ultimate bearing capacity of karsts pile and gives the ultimate bearing formula by each model, then analyzes the influence of the socket length on the ultimate bearing capacity. The formular results show: (1) the each model's result differs from each other noticeably, so it is the key to explore the geologic condition accurately and choose suitable model in nailing down the ultimate bearing capacity of the karsts pile in design and construction. (2) on the condition of making sure the necessary socket length, it should be followed to adopt the socket length as short as possible in designing.After studying the bearing properties of the karsts pile, this paper checks the theoretical result according to field engineering data, and simulates the mechanism properties of the pile in karsts areas under loading by computer simulation techniques. The emulational results shows: (1) the mechanical properties and damage patterns of the karsts pile differ considerably from the counterparts of the conventional rock-socketed piles, so it is necessary to make special design method; (2) the karsts cave's height has little influence on the bearing properties of the karsts pile, but the cave's span and the roof-supporting' thickness of karsts cave do it sharply. (3) the karsts pile's ultimate bearing capacity decreases with the advance of karsts cave's span, and increases with the advance of roof-supporting' thickness. But the trends become unobvious when the cave's span or the roof-supporting's thickness is in excess of three times of pile diameter.Finally, this paper makes out the bridge pile's design methods in karsts area, which gives the procedure of reasonable socket length, ultimate bearing capacity and admissible settlement, and offers some suggestions about design and construction methods of karsts pile in special conditions.