Experimental Study And Numerical Analysis Of Bearing Capacity Of Statical Drill And Rooted Pile

Statical-Drill-and-Rooted-Pile (shorted as " SDRP ") is composite pile consists of mixed soil-cement and prestressed high strength concrete pipe pile. SDRP has a sort of advantages, such as high bearing capacity, strong adaptability, less construction difficulty, low noise and mud pollution, etc. This paper reviewed the history of cement-soil, pipe pile, composite pile and SDRP, focused on the study of vertical compression behavior, and observed lateral bearing performance.Study was on the basis of three statical load tests —— two situs and one model. Situ One compared 6 piles, Situ Two surveyed the load transfer characteristics of SDRP by means of embeding strain sensors into the body, and Model Test studied the differences of load transfering on pile-soil interface by elimiting toe resistance. In Situ One piles could not be compressed to damage in consideration of carrying out both vertical and lateral load tests, and in Situ Two the failure mode was pile head but not soil destroyed, so the measured results were not as ideal as hoped. In order to cover this shortage, two methods were used to predict the ultimate bearing capacity.Cavity Expansion Theory was referred to calculate the compaction effect of SDRP. The comparation of calculated results and measured values showed that they were fit well in the upper but bad in the lower of pile. The reason was that upper soil compressibility was high and confining compressure was lower, so compaction effect could be generated and pile shaft resistance was promoted. Lower soil was just on the opposite, so compaction effect could not be generated.Thick Wall Cylinder Theory was combined with deformation coordinating conditions to analyze both internal and external interfaces of SDRP. Results showed that pile-soil interface was the weakest if some conditions were met. Hoop effect was negligible, so the strength, stiffness and bearing capacity of whole pile could be simply caculated by superposition of each part separately.With aid of numerical method the situ tests were complemented, and the axial force, stress of both soil-cement core and shell were achieved. Then it was found that although shaft resistance was prior and toe resistance was minor in long pile, the toe of pile needed to be specially strengthened. Further more, be the diameter of SDRP certain, the thicker the soil-cement shell (or the smaller the diameter of pipe pile) was, the lower the SDRP bearing capacity was. Soil-cement had little contribution to cross-section compressive stiffness, so increasing the cost of soil-cement was not necessary as soon as its strength was enough to designing load.Orthogonal Experiment Method was employed to design the finite element model of long SDRP, and four factors —— pipe pile diameter, wall thickness, concrete strength, and soil-cement strength were taken into account. Results showed that pipe pile diameter had the greatest impacts on compressive ultimate bearing capacity, shaft resistance and toe resistance, so it was the most important parameter in SDRP. When pipe pile diameter was certain, wall thickness had great impact on shaft resistance, so frictional pile should treat thicker wall pipe pile preferentially; soil-cement strength had great impact on toe resistance, so end-bearing pile should strenghthen the soil-cement preferentially. Optimization suggestion of SDRP is that pipe pile diameter should be large (slightly less than the whole diameter is advisable), and wall should be thicker.Put forward a method to predict bearing capacity of pile:if pile bearing capacities of any two certain diameters under unified settlements and one end-bearing ratio were known, pile bearing capacities and end-bearing ratios of other diameters could be inferred, and shaft resistances and toe resistances could be obtained consequently. Note that the bearing capacities were not limited in ultimate; they could be in any loading phases.Finally lateral bearing performances of SDRP and cast-in-place bored (shorted as " CPBP ") pile were compared on the basis on 6 piles lateral statical load tests. Their mechanical characteristics were analyzed, then the conclusions were drawn out:In terms of reducing lateral deformation, the priority was CPBP, so the better way was substituting CPBP for SDRP with same diameter but not enlarging SDRP diameter; but if deformation resilience was more important, the priority was SDRP.