Research On The Consolidation And Settlement Law Of Soft Soil Foundation Reinforcement Plastic Drainage Board Method

Soft soil foundation is widespread in our country. There are many researches on the settlement of soft soil foundation in the eastern coastal provinces, but there are less research on the settlement of soil foundation for inland especially the west, This paper combines Suizi highway soft soil foundation treatment, mainly researches the consolidation settlement law of the soft soil foundation treated by plastic drainage board by means of in-situ measured data, laboratory test, theoretical research and numerical analysis in the several ways below:(1)The indoor test reveals the variation law of soft soil porosity, compression coefficient, compression modulus, and consolidation coefficient along with consolidation pressure:compression coefficient decreases along with consolidation pressure increasing while compression modulus increases and consolidation coefficient Cv decreases, consolidation coefficient and pressure are fitted well by power function. In triaxial rheology test, the output of pore water increases with time passing by, which shows the deformation, exists from the beginning to the end. The relationship of principal stress deviator and axial strain shows strong nonlinearity of both soft soil layers and the principal stress deviator bore by first later is obviously lower than the second, reflects the soft soil dilatancy and riction.(2)The subside process of Suizi highway TJ1-11construction area consists three stages:the settlement is small before subgrade reaches the filling early days, soil mass mainly contains elastic deformation in settlement-occurring stage. After the end of the filling, settlement continues though the rate decreases gradually to zero. The compression of the area is small, the total measuring settlement only accounts1.9%of the thickness of soft soil. Besides, the differential settlement is not obvious. The measuring data of settlement in layers show that the settlement which mainly formed by soft soil in upper layer is large before5.5m deep, taking up70%of the whole. In the settlement theory calculation consider each level after loading soft soil compression modulus changes can better agreement with the actual measurement.(3)Lateral displacement decreases with depth increasing when the depth of sub grade exceeds3.0m and the maximum occurs at the depth from lm to4m. Lateral displacement develops slowly in the initial stage of preloading and the ratio of max lateral displacement and the middle settlement gets only0.027-0.094. In the middle stage, lateral displacement develops quickly and the ratio reaches0.10-0.33. In the final stage, lateral displacement decreases and the ratio gets only0.008-0.11. On the whole, the average ratio of is0.11.(4)Pore pressure regularly increases and dissipates with the change of load and time. Pore water pressure is originally large when loading quickly, but the subgrade doesn’t lose stability due to the plastic drainage board which accelerates the speed of drainage and the effective stress increase quickly. Pore pressure under step loading consists three obviously different stages:in the first loading stage, the soil skeleton bears the load, excess pore water pressure remains small and settlement rate remains slow. In the middle stage, pore water mainly bears the load increment, excess pore water pressure increases quickly and settlement develops rapidly. In the final stage, due to the drainage of pore water, the effective stress increases and mainly bears the load, excess pore water pressure increases little. Settlement will gradually convergent to a stable value with excess pore water pressure dissipates to zero.(5)After three months of preloading period, the average degree of consolidation of each layers in the area reaches the same level, above85%. Settlement has a strong linear relationship with the product of the thickness of soft soil and the filling height. The settlement when the consolidation basically finished could be estimated exactly according to the statistic formula in this paper. Using plastic drainage board to treat soft soil foundation could contribute to the decrease of post-construction settlement. From the two indexes:permissible post-construction settlement and average degree of consolidation, we could judge that plastic drainage board treats well.(6)Through FLAC3D the author analyses fluid-solid coupling of soft soil foundation. The settlement simulated from FLAC3D could reflect the actual deformation trend in general and well reflect the settlement law. Considering the influence of plastic drainage board and drainage of sand cushion in calculation compares to the calculation without plastic drainage board, the result shows that the settlement of the former calculation is larger and the lateral is smaller while both settlements of the two calculations are larger than the measured of which the former one fits well. The lateral displacement of shallow foundation increases with the depth increasing and reaches the maximum at about3m, and then it gradually decreases when going deeper. The max lateral displacement increase with loading and the maximum position changes from deep under the slope toe to the middle of subgrade. Plastic drainage board could accelerate the consolidation and during the consolidation, pore water nearby all goes to board area of high coefficient of permeability, and through the board drain off water to sand cushion. Pore water centralizes around the board, so it accelerate the consolidation and make the settlement finished in loading period. The author suggests that consider the influence of plastic drainage board when calculating in order to make the calculation and prediction more exact, guide the control of subgrade deformation in highway construction.