Stability Analysis Of Small Transverse Size Fabriform Sand Cofferdam In Soft Soil Foundation

Small transverse size fabriform sand cofferdam, that is, the cofferdam piled up by fabriform sand which lateral dimension(in width direction) is small, such as 2, 4, 5 meters in width direction, and which axial dimension(in length direction) is over tens meters. Because of its low engineering cost, wide working platform, high-efficiency, adaptability for soft soil foundation, easy of removal, etc, small size fabriform sand cofferdam are widely used in the temporary cofferdam project of artificial island, submarine tunnel, sea reclamation, channel regulation, cushion cap of pier. Since its complexity of mechanical characteristics, the mechanical behavior and stability control mechanism of the fabriform sand cofferdam needs further study and its design and construction mainly depend on existing experiences or specification of other types of dam. Therefore, the method which numerical simulation and theory analysis were combined was used in the analysis of stability of the small size fabriform sand cofferdam in soft soil foundation. The main contents and results are as follows:(1) With the aid of the finite element software ABAQUS, the 2D model of the small size fabriform sand cofferdam in soft soil foundation were established to simulate the mechanical behavior of heap and dewatering construction conditions based on strength reduction method. The results show that the stability of fabriform sand cofferdam is based on the stability of soft soil foundation rather than the slope stability. The fabriform sand cofferdam can be considered as masonry structure with poor bonding performance. The failure of fabriform sand cofferdam is detachment from each other of fabriform sand for the plastic shear failure of the soft soil foundation instead of rotational slide cutting through fabriform sand.(2) The 2D model of the big size fabriform sand cofferdam in soft soil foundation were established to compare with the small size. The differences of mechanical properties revealed are as follows: Firstly,the displacement of big size fabriform is bigger than the small size; Secondly, the requirements for the ultimate tensile strength of fabriform of the big size is higher than the small size.(3) Based on the data of the fabriform sand cofferdam in hard soil foundation, comparation with the fabriform sand cofferdam in soft soil foundation shows that the failure of cofferdam in hard and soft soil foundation both derive from instability. The failure of cofferdam in hard soil foundation results from the interfacial slip and vertical crushing damage, while the failure of cofferdam in soft soil foundation results from the plastic shear failure of the soft soil foundation.(4) As to the problem of low strength of the soft soil foundation, different heights and widths of loading berm were simulated to reveal the law of influence on the stability of cofferdam. Besides, the stress and deformation characteristics of cofferdam were analyzed by means of the ground treatment of cement mixing piles, and optimization scheme of the piles arrangement was proposed to relieve the level of shear stress in the seam of fabriform sand cofferdam. What’s more, the law of shear strength increment of foundation by vacuum preloading was explored and some suggestions for design and construction were advanced on account of low demand of fabriform sand cofferdam for ground strength. In addition, the failure mode of cofferdam was studied after the foundation treatment. Finally, suggestions for ratio of slope were given.(5) On the basis of soil mechanics, the theory of bearing capacity of foundation soil were introduced to gain the limit height of cofferdam in the thick soft soil foundation and theoretical formula between the height of cofferdam and foundation depth in the finite thickness soft soil foundation.