Study On The Engineering Behavior Of Stiffened DCM Pile

Stiffened DCM pile is a new type of composite material pile appeared recently, which is commonly used in the treatment of soft soil foundation and support of deep pit in the ground of silt, clay and so on. Cement soil plays a key role in the load transfer of composite pile. It exist the inevitable relationship between the bearing capacity of composite pile and the strength of cement soil. However, the current understanding of its working mechanism is not perfect, but also the lack of a feasible and reliable method of calculation and analysis, which restricts the application of stiffened DCM pile.In order to further reveal the working properties of stiffened DCM pile, this dissertation takes the cement soil and stiffened DCM pile as the research object, and investigates the strength of cement soil strengthened by cement, fly ash, etc, the bearing capacity of stiffened DCM pile using the methods of unconfined compressive strength test and static load test. The failure mode of composite pile is optimal designed and analyzed. The main conclusions are as follows:1. Through the unconfined compressivie strength test, it obtains that:(1) When mixing only with cement, the strength of cement soil increases with the increase of cement content, but the increasing trend is not entirely linear. When the content is more than 22%, the increase of cement content is not obvious for the improvement of cement strength. The increase of age has effect on the strength of cement soil:the strength of cement soil increases slowly for the age of 14 day to28 day, then increases rapidly for the age of 28 day to60 day, finally maintains after passing 60 day. (2) The activity of fly ash is stimulated by adding fly ash, lime and sodium sulfate. The optimum mixing ratio is 77:20:3. After incorporation, the early strength increases slowly and later strength-increases rapidly. (3) Under the condition of same strength,50% cement content can be saved by adding sodium sulfate when the incorporation ratio of cement and sodium sulfate is 1:2. (4) The optimal mixing ratio of gypsum is 2%. The effect of adding gypsum on the strength of cement soil is not obvious, but it can improve the early strength.2. Through the model test of composite pile, it obtains that:(1) the inflection point of the composite pile load-settlement curve is obvious. When the load is less than the ultimate bearing capacity load, the curve changes approximately linear, but when the load is bigger than the ultimate bearing capacity load, the settlement increases rappidly. (2) Long pile concrete core can transmit the load downward effectively, and can make use of pile side friction resistance effectively. When the load is the same, the settlement of the long pile is smaller than that of the short pile. (3) At the same cement soil, compared with short pile, the bearing capacity of long pile increases 1.5-1.8 times. (4) The distribution of axial force of pile core shows bigger at upper part and smaller at lower part. The load of short pile reduced 50% when transmitted downward 30cm, and reduced 70% when transmitted downward 40cm. The axial load decreases fastest in the upper 5cm to 25cm, and decreases slower in the lower part. The change of axial force distribution of long pile is same as that of short pile.3. The optimal design and analysis is made on the composite pile under failure mode. From the analysis, it can be obtained that:(1) The failure mode of the composite pile mainly has four kinds:the crush of the pile cap of the core pile, the penetration of core pile into cement soil, the crush of non core segment and the penetration of composite pile into pile end soil. The fourth kind of failure mode can make the side resistance of composite pile be brought into full play, thus the highest bearing capacity of single pile can be obtained, of which is the most ideal failure mode. (2) The three optimal design ideas is brought out based on ideal failure mode:(â…°) Increases with the length of core pile ultimate bearing capacity of single pile, the length of core pile can make more appropriate load transfer of pile body and give full play to the lower part ofthe lateral friction resistance and end resistance of pile pile, lime quantity is small, the larger thecoreless segment. (â…±) The geometry of core pile of the composite pile can be determined by the core pile length, the section area of the pile top and pile bottom, (iii) Under the condition that the composite pile can be made into the soil smoothly, the strength of cement pile can be improved by reducing the water cement ratio, increasing the cement content near the pile bottom, or by adding additives in cement paste, such as gypsum.