Research On The Behavior Of Composite With Rammed Soil-Cement Pile

Research on the rammed soil-cement piles composite foundation with cushion in unsaturated clay and silt drops obviously behind its practice application at present and the current design mainly consult the design method of CFG pile composite foundation. Due to setting cushion and limited strength, its behaviors are different from that of the cement-soil mixing piles composite foundation without cushion in soft ground and the rigid pile composite foundation with cushion. It is of great significance to study load transfer mechanism, stress and displacement field distribution for the development of rammed soil-cement piles composite foundation.The technology, which is suitable for multi-layer and low high-rise building ground treatment, has the advantage of simple construction,low cost and shorter construction cycles and the need for construction equipment is relatively simple. In this dissertation, the behaviors of the rammed soil-cement piles composite foundation with cushion in unsaturated clay and silt are studied via systematic field loading tests and theoretical analysis for development of the technology. Main content are as follow:1. Systematic test study:Field loading tests on single piles in which lengths and strengths various and field loading tests of composite foundation with single pile,four piles,nine piles in different length and area displacement ratio have been carried out. Experimental tests include loading and deformation, pile-soil pressure, pile stress, pile and soil surface deformation, stratified settlement of soil between piles. Aiming at special method of installation of rammed soil-cement piles, a new type of stress and deformation of pile testing methods was proposed. On this basis, load transfer and deformation characteristics of pile and composite foundation, bearing capacity, pile-soil stress ratio, load sharing ratio,pile-soil deformation characteristics of composite foundation have been analyzed. Preliminary summarization is made to basic law of load transferring mechanism and deformation characteristics.2. Theoretical analysis:First, theoretical analysis, which combined load-transfer method and Geddes' formula, are carried out for further study load and deformation characteristics of rammed soil-cement piles; second, basing on filed loading test results of rammed cement-soil pile composite foundation with cushion, a new pile shaft resistance distribution pattern was proposed by considering the cushion and pile compression. Analyzing method is presented by considering harmony working of piles, surrounding soil and cushion in composite foundation and theoretical solution is given. Compared with theoretical solution and in-situ test results, a pronounced agreement is found. What's more, the solution can provide a guide to determine pile-soil load share ratio. Considering the influences of pile length,cushion stiffness,shaft resistance,soil modulus,pile strength, variation rule of pile-soil stress ratio,load sharing ratio,distribution of the axial force,pile stress along pile shaft,soil stress along depth,the position of neutral point,the size of pile penetrating cushion are studied.3. Settlement calculation of rammed soil-cement piles composite foundation with cushion:The soil stress distribution of rammed soil-cement piles composite foundation with cushion are studied on the basis of the above theory, which combined Mindlin solution and Boussinesq solution. Simplify soil stress distribution pattern was given and a way to determine deformation modulus was proposed. A new improved method for calculating the settlement of composite foundation, modified stress method and modified stress and modulus method, is put forward. The comparison between calculated results and measured data show that improved methods enjoy merits of simplified calculation,clear physical concept and certain precision.4. Reliability index of rammed cement-soil pile composite foundation:Based on the large amounts of data of rammed cement-soil pile composite foundation loading tests, the settlement rate of composite foundation is used as an analytical variable considering its randomness and fuzziness of failure. The membership function of the failure state is obtained by fuzzy statistic method. The calculation method of fuzzy random reliability of composite foundation lost its stability have been established and reliability index of rammed cement-soil pile composite foundation based on load test have been analyzed through examples for current design standards.