Analysis On The Stability Of The Surrounding Rock Of String-Beaded Karst Cave Under Piles Foundation

With the implement of the strategy of Development of the West Regions, highway and railway construction are frequently used in mountainous areas. Besides, the highway and railway construction are often at a karst region, karst cave can harm the foundation of bridge pier, and the karst region’s bridge pile foundation is also under various complicated hydro-geology conditions as well as engineering geology conditions, so it is very difficult to build a bridge pile foundation in a safe, excellent and fast way without leaving any damages to the future.In foreign countries, there is no enough researches of karst cave pile foundation design and construction, they often use plenty of concrete to fill the cave. They has not yet begin the research work of design and construction in a comprehensively and systematically way. In China, this kind of research also remains a starting period. Although there are quite a few scholars have already studied the karst cave pile foundation design and construction, most of them aimed at only one single-cave condition, few people has researched the beaded karst cave. Therefore, the research on the surrounding rock’s stability of beaded hidden cave under the load of pile foundation has an important theoretical significance as well as engineering reference value. This paper uses the method of the theoretical analysis combining with the numerical simulations, has concluded a systematic and thorough research of the stability of the surrounding rock under bridge piles foundation in string-beaded karst. The paper’s main research contents and results are as follows:(1) On account of present exploring status’ s quantitative assessment of the surrounding rock’s stability, this paper bases on The Theory of Small Deflection of Thin Plate of Elasticity, according to different forms and fracture development degrees, simplifies the cave roof which is in a horizontal occurrence and its ratio of the thickness and the minimum horizontal size is less than 1/4 or 1/5 into different mechanical models, and concludes the calculation formula of maximum stress and maximum immunit y of different models. Lastly, according to Maximum Tensile-Stress Criterion and Moore Kulun Guidelines,the paper calculates the safety thickness formula of different models. In addition, considering multiple caves may influence the calculating results which made by Small Deflection of Thin Plate model, the paper uses the numerical simulations method analyzes the condition of karst cave roof stress distribution and the change of ultimate bearing capacity under different cave number, radius and height change, as well as obtains the corresponding change rule.(2) For the small diameter and deep-buried caves, this paper regards those as the problem of stress distribution of infinite plate under bi-axial compression based on The Theory of Elasticity, and specifically analyzes the single cave and two round caves with the same equal radius, and concludes the surrounding rock’s stress calculation formula. The paper also discusses the model applicability in engineering and agrees for the single cave condition, the distance H from the foundation bottom to the cave center is greater than 5R(R is the cave radius); while for two round caves with the same equal radius, the H will be greater than 5.5R. In consequence, this conclusion can be used in calculating the distribution of the surrounding rock’s stress. Finally, the paper evaluates the stability of surrounding rock’s stress concentration parts by using the Griffith Criterion and analyzes the effect factors of the cave surrounding rock. Besides, this paper has also verified the the stress distribution rule of model by the means of numerical simulation, and analyzes the change of the stress distribution of cave’s surrounding rock under different cave number, shape, spacing, position change, and has also obtained the corresponding change rule.(3) As for the problem of safety thickness of roof during the period of karst cave pile foundation design and construction, this paper analyze how rock grade, karst cave radius, cave height, single pile load, cave spacing, pile diameter and such factors can influence the safety thickness of the roof. Finally, by using the Multiple Regression Analysis, the paper obtains the safety thickness calculation formula of the kacrst cave roof under these factors in all, which can be used for calculating the reasonable safety thickness of the roof.