Reinforcement Mechanism And Arching Effect Of Piles Improved Subsoil

The main components of piled embankment include the piles, the sand blanket, the embankment fill and the soft subsoil. Compared with other traditional soil-ground improvement techniques, the piled embankment has merits of the little post-construction settlements and differential settlments, the saving cost and the less time-consuming, which iswidely used by engineers, especially in soft soil areas. In the design of piled embankment, the attention is paid to the development of the soil arching effect in embankment and the height of embankment. If the embankment is not high enough, the soil arching effect could not develop fully, resulting in the occurrence of large differential settlement in the embankment surface. On the other hand, the larger height of the embankment could give rise to some economic problems.In this paper, the behavior of embankment improved by rigid piles and the calculation methods for soil stress subjected to surrounding soil were given firstly. Then, laborotary model tests were conducted to investigate the failure surface and the soil arching effect in embankment. Based on the test results, a new calculation method was proposed to obtain the soil stress applied on surrounding soil. At last, the height of the equial settlement plane and the stress transfered to piles were investigated by the simplified 3D numerical modeling.(1) Compared experimental values with theoretical values, it is obvious that load applied on the surrounding soil can be reasonably predicted by H-R theory, Terzaghi theory and mutli-arching theory. In terms of piled embankment with geosynthetic, the tensile stiffness of geosynthetics, the size of pile cap, the thickness of subsoil as well as the modulus of compressibility of subsoil can affect the membrane effect provided by geosynthetics. For example, the stiffer geosynthetics will result in more stress transferred to piles, while the larger modulus of subsoil will induce less stress applied onto piles.(2) Trapdoor model test and piled embankment model test are carried out to investigate the failure plane in the embankment under the consolidation of soft subsoil, which is simulated by the descent of movable plate. In the Trapdoor model test, a couple of symmetrical triangular sliding surfaces appear initially, called triangular arch model. Two different evolution models are revealed with the descent of movable plate with respect to the density of embankment fill are as follows. In the case of ρ=1500 kg/m3, the initial triangular arch model turns to be pagoda-shaped multi-arch model; in the case of ρ=1650 kg/m3, the initial triangular arch model evolves into vertical arch model.The failure plane form of the piled embankment model test differs from that of Trapdoor model test. With the settlement of movable plate increasing, a wedge zone is revealed at the pile top, through which embankment load is transferred to piles. The height of the wedge zone is related to the internal friction angle and the width of pile. When the ratio of the height of the embankment fill to the width of the movable plate is greater than 3, there is no differential settlement on the embankment surface. Based on the configuration of the wedge zone above piles, an equation has been derived to predict the vertical stress applied on surrounding soil in the piled embankment. The proposed method is proved to be reasonable compared with the test results.(3) The settlement in embankment surface increases with the descent of movable plate, the development of which is similar as the load-settlement curve for the bearing capacity of foundation soil. There is a critical settlement. Once the descent of movable plate is larger than this critical value, the embankement surface settlement increases sharply, denoting that embankment is unstable under this condtion.(4) Some simplified simulation models are performed to investigate the height of equal settlement plane and the pile efficacy, neglecting the interaction between the piles and the soft subsoil. From the results, it can be obtained that when the relative height of embankment(i.e., the ratio of the embankment height to the net pile spacing) is greater than 4, the differential settlement at the bottom of embankment cannot be reflected on the embankment surface, meaning the occurrence of equal settlement plane in embankment. The height of equal settlement plane is approximately 1.5~2 times the net pile spacing. Besides, the height of equal settlement plane and the pile efficacy increase linearly with the property of soft subsoil.(5) 3D numerical models are condutcted to investigate the performance of embankment supported by rigid piles, considering the pile modulus, the rate of embankment construction, the pile length and the pile spacing. The increasement of pile modulus can reduce the settlement of subsoil, as most embankment loads are transferred to piles. The behavior of piled embankment will be scarcely improved with the increasement of pile modulus after the pile modulus increases to a certain value. The influence of construction rate on embankment stability is enormous. The great construction rate will give rise to the increment of pore water pressure within the subsoil, and the larger pore water pressure will result in the unstability of embankment when the ratio of excess pore water pressure to the applied load is greater than 0.34. The larger the pile length is, the less the subsoil settlement will be. However, the influence of pile spacing on subsoil settlement is on the opposite way. Comparisons of simulation results with those based on previous theories for the stress show that Terzaghi theory and multi-arch theory can be uitilised to predict the stress applied on subsoil.