Experimental Study On CFG Pile Composite Foundation Of Railway Subgrade In Inland Saline Soil Area

Inland saline soil is refer to the soil that is distributed in the inland lakes, depressions around, and the soluble salt content in average is more than0.5%in1.0m depth below the surface of the earth. This soil not only has the nature of saline soil, and has some characteristics of the soft soil. When improving this foundation, it must consider how to increase the bearing capacity, reduce the settlement, and also consider the corrosiveness and the salt soluble and expansion to apply influence to this measure.The DK.821+600～900of new second double of the Lanzhou-xinjiang is test section of inland saline soil foundation processing, the foundation improving scheme as follows:Firstly digging up the saline soil that the salinization is strong in1m depth below the earth, and making the CFG pile composite foundation, its'cushion is made up of cement soil layer that the thickness is1m, the layer also serve as the gap fault of capillary water=.Adopting CFG pile composite foundation form are rare in saline soil regions, the studying about this theory is rare. The paper based on the test section, studied systematically the bearing and deformation characteristics of the CFG pile composite foundation in saline soil area, and discussed the bearing mechanism and the deformation characteristics when salt dissolve and salt expansion occurred, the paper specifically do some parts as follows.(1) Studying the single pile and single pile composite foundation's bearing capacity and the relation between load and settlement of pile top by load test.(2) Measuring salt content, water content, temperature, pile side stress, pile top stress and soil stress between piles at regular intervals above underground water level and then studying the changing regularity of water content, salt content and temperature and they exert the influence on bearing capacity of the CFG pile composite foundation.(3) Measuring pile top and soil stress between pile of CFG pile composite foundation's cushion when filling the embankment by burying the test equipment in site, and then studying the changing regularity of the pile-soil stress ratio with the filling height of embankment, and CFG pile composite foundation's bearing mechanism under embankment. (4) Simulation studying the distribution of the pile side friction by transfer coefficient method, and analyzing the variation of the pile side friction combined with the site measure when salt dissolve and salt expansion, and analyzing salt dissolve and salt expansion produce an effect on bearing capacity and deformation of the CFG pile composite foundation, discussing bearing mechanism and deformation characteristics of the CFG pile composite foundation in the condition of the dissolution and salt expansion deformation in saline soil.(5) Studying the cushion layer parameters' variations produce the effect on pile-soil stress ratio by computer simulation, putting forward to change pile-soil stress ratio by adjusting the cushion layer's thickness and elastic modulus, achieving the purpose that controlling the total settlement of composite foundation.(6) Confirming that the cement soil has the very good partition effect by the laboratory test and field test, putting forward CFG pile composite foundation's settlement calculation model by theoretical analysis, then calculating and analyzing the settlement deformation to test section, and discussing the additional deformation when salt expansion and dissolve happened.(7) Anaylzing pile length, diameter, pile spacing exert influence on the settlement by computer simulation, studying optimization parameter design of CFG pile composite foundation based on the purpose of controlling the settlement.(8) Measuring and observing the subgrade settlement by burying the settlement apparatus, concluding the settlement and deformation characteristics of the saline soil subgrade and putting forward prediction method of post-construction settlement on the basis of the observation results in site.The biggest bearing characteristic of CFG pile composite foundation is pile and soil to share load together, the pile-soil stress ratio is one of the important parameters that reflecting this characteristic, and is the key that solve the composite foundation bearing and deformation. By changing the cushion layer parameters and CFG pile's design parameters to adjust the pile-soil stress ratio, and finally achieving the aim of controlling the settlement.CFG pile composite foundation can more dramatically improve the natural foundation soil bearing capacity and a significant reduction in the total settlement and differential settlement, and not sensitive to changes in environmental factors of inland saline soil, to ensure long-term stability of the railway subgrade basement. Therefore, the CFG pile composite foundation can be used as effective improving measure for railway subgrade basement in inland saline soil area.