Study On Vertical Bearing Capacity Of The Long And Large Diamater In Hard Soil & Soft Rock

Research on vertical bearing capacity of pile foundation is always a difficult key subject both at home and abroad. The main point is that the interaction between pile soils is complicated; the interface between pile and soil is hard to simulate; and the parameters required for calculation of interaction between pile soils is hard to define. This paper takes Indonesia Suramadu Bridge Project as the research object. Starting with laboratory tests, mechanics parameter and nature of hard soil &soft rock in its geological condition is analyzed in the paper. What's more, orthogonal test is applied to solve the hole-collapse and the hole-shrinkage arising out of the construction of pile foundation in hard soil &soft rock area. The relationship and the change trend between the lateral friction and the end bearing of the pile upon interaction of pile soils is analyzed on the basis of site self-balancing test to define the model and the parameters. Finally the mechanism and the influencing factor for interaction of pile soils are analyzed in terms of the theory and the finite element modeling to define the vertical bearing capacity of the pile. By such methods as laboratory tests, job site tests, theoretical research &analysis and finite element modeling, the bearing capacity of pile foundation is analyzed, which mainly includes the following.(1)Starting with the geological condition (hard soil &soft rock) of pile foundation of Indonesia Suramadu Bridge Project, it is obtained that the strength weakening is caused due to lots of clay mineral components with strong water affinity in rock mass. Change of water causes the weakening of the strength, which is the major reason resulting in hole-collapse and the hole-shrinkage during the construction.(2)Starting with laboratory tests, strength weakening curve of hard soil &soft rock is analyzed to obtain this. Upon calculation of stability of hole wall and lateral friction of the pile, for the hole wall with the rock course or base rock with larger content of sand, peak value strength shall be applied for calculation. While for common hole wall, it is better to apply residual strength for calculation. Only by this way, can the maximum vertical bearing capacity of pile foundation be applied. But with regards of conservation, calculation can be fully done by residual strength.(3)The hole-collapse and the hole-shrinkage arising out of the construction of pile foundation in hard soil &soft rock area is solved by orthogonal tests. The results show that the more content of bentonite, the better the protection effect of wall by slurry. But on the other hand, the more the thickness of the slurry cake, the larger the weakening extent of the lateral friction. Therefore upon mixing slurry, with the precondition of ensuring wall-protection effect, the content of the bentonite shall be minimized. And by orthogonal tests and with consideration of the effects of all factors of slurry, the optimum mix ratio of the slurry is finally proposed to be as follows. Fresh water: bentonite: CMC: sodium carbonate = 100: 8: 0.1: 0.04. The actual application and experience proves that such mix ratio can effectively solve such problems as the hole-collapse and the hole-shrinkage.(4)Obtaining from the self-balancing test, when the extra-longer pile with larger diameter reaches the ultimate bearing capacity, its bearing capacity value at pile end occupies a larger portion, which is end bearing pile. Upon calculation of ultimate bearing capacity of the pile, the resistance of pile end shall be considered. Likewise, upon calculation of the ultimate bearing capacity of the pile foundation in similar works, it is necessary to consider the contribution of the resistance of pile end.(5)By analysis of the bored pile and the correlation with its surrounding soil mass, load-transferring mechanism of friction pile is analyzed. Summarize and analyze the previous contact model of all kinds of interface between pile and soil and the substantial relations to propose the pile-soil friction weakening model applicable for hard soil and soft rock area. What's more, according to the load-transferring differential equation, the vertical bearing capacity of the pile is theoretically developed and cut-and-try procedure for computation of the bearing capacity of the pile (P-S curve) in single layer of soil and many layers of soil.(6)By friction-weakening model, the emulation analysis of the original test pile, and the emulation analysis of the effects on the vertical bearing capacity of the single pile imposed by deformation modulus, adhesion force, internal friction angle, pile length and pile diameter of soil mass around different piles, the following conclusion are obtained.a) The lateral friction and the end resistance are not exerted in synchronization. For the pile foundation in soft and weak foundation, if its ultimate bearing capacity is required to be fully applied, there shall be larger displacement of pile end. Therefore the displacement at pile top determines the bearing capacity of the pile.b) Under the interaction between many layers of soil and the pile, the lateral friction of the part of the topsoil with good quality can be given priority for full application. In case of less difference between soil layers, upon interaction between pile and soil, the lateral friction will be gradually applied at pile end to its ultimate value from the bottom to the top. c) The application of the lateral friction is greatly related to the relative movement between pile and soil. With the relative movement between pile and soil increasing, the lateral friction of the pile will also gradually increase. But when the lateral friction reaches some value, it will not increase any more. Such value is related to the nature of the soil mass. The better, the soil nature, the larger,the value relatively.d) The deformation modulus of soil mass around the pile has larger influence on the settlement of the pile top at start of loading. The larger the deformation modulus is, the less the settlement is, and the larger the vertical bearing capacity of the pile is.e) The adhesion force of soil mass around the pile has decisive effects on the inflection point of the load settlement curve. The larger the adhesion force is, the load of pile top at the moment of deflection point is and the larger the vertical bearing capacity of the pile is.f) The internal friction angle of soil mass around the pile has little effect on the load settlement curve at start of loading (viz. before the soil mass reaches plasticity). When the soil mass around the pile reaches the plastic state, the larger the inner friction angle is, the less the descending amplitude of the load settlement curve is and the larger the vertical bearing capacity of the single pile is.g) Under the same soil condition, the longer the pile is, the larger the vertical bearing capacity. At the start of loading, for the same pile top load, the longer the pile is, the less the settlement of pile top is. But the pile length is not inversely proportional to the settlement of pile top. The longer the pile is, the less the impacts on the settlement of the pile top is.h) Under the same loading condition, the larger the pile diameter is, the less the settlement of pile top is and the larger the ultimate bearing capacity of the single pile. Basically they are proportional to each other. But the concrete volume is second-power related to the dead weight of the pile body. Additional cost will be increased. Before application, much consideration is necessary.