Study On Deformation Property And Settlement Prediction Of Soft Clay In Hebei Coastal Expressway

By using settlement plates, inclinometer, piezometer, earth pressure gauge, test project of soft clay ground was designed in cement mixing pile composite foundation and plastic drainage plate foundation in the Hebei coastal expressway, and observation has been lasting more than two years from construction, surcharge preloading to unloading and loading term. The main results are obtained by theoretical analysis and experimental research as follows:1. The property of foundation settlement, horizontal displacement and pore water pressures in cement mixing pile composite foundation and plastic drainage plate foundation are summarized by analyzing field experimental dates. In construction term, the settlement of cement mixing pile composite foundation is only 7.64%~25.70% of plastic drainage plate foundation and the final prediction settlement is 8.76%~28.9% of plastic drainage plate foundation. The post-construction settlement of cement mixing pile composite foundation is 6.78%-25.01% of plastic drainage plate foundation having no precompression term, and 8.51%-36.46% of plastic drainage plate foundation having two months precompression term. In construction term, the maximum settlement is 3.7-4.1 times higher than the maximum horizontal displacement in the cement mixing pile composite foundation, however its 8.6~15.3 times in plastic drainage plate foundation. For less post-construction settlement and horizontal displacement, the cement mixing pile composite foundation fits for embankment at bridge abutment or fast filling.2. The settlement in construction term, the final and post-construction prediction settlement increase significantly with the increase of pile spacing in cement mixing pile composite foundation. In construction term, the settlement of the pile spacing of 1.4m is 1.79 times higher than the pile spacing of 1.2m, and 2.91 times higher than the pile spacing of 1.0m. The final prediction settlement of the pile spacing of 1.4m is 1.67 times higher than the pile spacing of 1.2m, and 2.79 times higher than the pile spacing of 1.0m. The post-construction settlement of the pile spacing of 1.4m is 1.58 times higher than the pile spacing of 1.2m, and 2.63 times higher than the pile spacing of 1.0m. So the reasonable adjustment of pile spacing can form settlement transition section to avoid or reduce the jump at bridge abutment.3. The stress on the pile tip and in the soil and the pile-soil stress ratio have good linear relation with the increase of load in cement mixing pile composite foundation, but the average increase amplitude is 8.65-12.3 times on pile tip than in the soil. The pile-soil stress ratio of the pile spacing of 1.4m is 1.39 times higher than the pile spacing of 1.2m, and 2.57 times higher than the pile spacing of 1.0m at the critical load. The pile-soil stress ratio is about 2-6 at the pile spacing of 1.0m-1.4m in the coastal-marine soft clay of Hebei.4.The foundation treatment factor value in the cement mixing pile composite foundation is about 0.85-0.95 in the settlement coefficient calculation formula; The settlement coefficient calculated by the calculation formula is a little bigger than the back calculation value, and the settlement coefficient value is about 1.05-1.15 in the coastal-marine soft clay of Hebei.5. By adding the exponent of the time factor of the verhulst curve model, the exponential type model is put forward in the paper. The prediction settlement of the improved verhulst model is very close to the measured value, and the final prediction settlement is also satisfactory. By analyzing the calculation result, the exponent is about 1.5.6. The BP neural network prediction model based on MATLAB are established in the cement mixing pile composite foundation and plastic drainage plate foundation. The final settlement prediction results of BP neural network model are very close to the improved verhulst model, but 6.4%~42.87% higher than the hyperbola model and the Xinye model. By applying the BP neural network model and the improved verhulst model, it solve better the problem of smaller final settlement predictions by other settlement curves.7. By calculation the post-construction settlement, the precompression term in the general embankment treated with plastic drainage plate is not needed, but it demands at least two moths precompression near atculvert or box channel in equal loading or surcharge preloading. And the precompression term put forward in the paper about plastic drainage plate foundation breaks through ―the precompression term is no smaller than 6 months in vertical drainage‖ in the Specifications for Design of Highway Subgrades.