Research On Bearing Capacity Of New Composite Piles Under Coral Reef Geology

Coral reef sediments are of biological origin.They are special rock and soil bodies formed by the destruction,accumulation and cementation of the remains of reefbuilding coral groups.Due to the alternating action between the elements of the seawater environment in diagenesis,they form unique sedimentary structures.The strata deposition of coral reefs in the Indian Ocean presents a binary soft and hard alternating structure,the upper part is the coral reef clastic layer as a whole,and the lower part is the alternating distribution of reef limestone layers and clastic layers.The coral reef clastic layer has high porosity,high internal friction angle and large interparticle occlusal force,and the reef limestone layer is complex in cementation,porous,and has large strength dispersion.Under coral reef stratum,the side resistance and durability of driven pile are low,and the bearing efficiency of cast-in-place pile is not high due to its self-weight.In order to improve the bearing capacity of the deep sea coral reef geological pile foundation,a new type of combined pile structure suitable for coral reef formations was invented,whose upper part of the pile is a UHPC hollow pipe pile,the lower part is a concrete pouring section,and the middle part is a combined connection section.And the segment is located below the zero bending moment point of the pile and mainly bears the axial force.Compared with the traditional cast-in-place pile,the new composite pile has the advantages of good durability,small self-weight,high material utilization rate,and low cost.This paper takes the new composite pile structure under coral reef geological conditions as the research object,and uses laboratory tests,model tests,theoretical analysis,numerical simulation and field tests to carry out in-depth research on the bearing capacity of the new composite piles under coral reef geological conditions,which provides scientific basis for capacity calculation of new composite pile.The main research results are as follows:Through the indoor triaxial test,it is revealed that coral sand has the shear strength characteristics of large internal friction angle and large occlusal force,and a shear band appears in the failure of the sample,and the direction of the shear band is consistent with the direction of the maximum shear stress.According to the Static side pressure coefficient measurement test,the static lateral pressure coefficient of the coral reef clastic layer remains unchanged along the depth,and a theoretical calculation formula is established.For the random distribution of pores in coral reef limestone,based on the existing framework of binary media constitutive model,the heterogeneity of reef limestone is considered from the mesoscale and homogenized to the macroscopic unit body scale.The damage evolution equation and the meso-macro correlation equation related to axial strain are constructed for the first time.An elastic-plastic damage constitutive model of coral reef limestone considering cementation characteristics and random pore distribution is proposed,and the model parameters are given based on statistical analysis.The rationality of the constitutive model was further verified by laboratory tests.In view of the lateral resistance bearing performance of the driven section of the new composite pile,a large-scale special direct shearing instrument for coral reef debris was developed,and the concrete-coral sand interface shear test was carried out.It shows shear shrinkage characteristics;The reef limestone pile driving test is carried out,and it is found that the particle size gradation of the broken body in the pile hole is quite different,and its interface shear mechanical properties are basically consistent with those of coral sand;On this basis,the model test of the mechanism of the lateral resistance of the driven pile in the coral reef debris layer was designed.Through the analysis of the test results of 24 working conditions,it is revealed that the lateral resistance of the driven pile is low,and the ultimate resistance of the pile side presents the distribution law of "upper triangle + lower rectangle".The formula for calculating the lateral frictional resistance of driven piles was proposed for the first time.Aiming at the lateral resistance bearing performance of the new composite pile in the cast-in-place section,7 kinds of cast-in-place pile-reef limestone interface shear tests with different interface forms were designed.Through the analysis of 21 sets of test results,it is revealed that the interface morphology has little effect on the shear strength.The shear failure surface is mostly located inside the reef limestone,and the interface strength is controlled by the comprehensive effect of the tensile strength and shear strength of the reef limestone.Based on the above-mentioned failure mechanism of cast-in-place pile-reef limestone interface,the finite element-discrete element coupling analysis method of the damaged reef limestone characteristics is proposed,and the reef limestone mechanical analysis program is developed;Comprehensively analyzing the field test pile data,an improved method for calculating the pile side resistance of the new composite pile cast-in section of the reef limestone layer is carried out.Five groups of UHPC-RC composite key teeth shear push-out tests with different material strengths and height ratios were designed.Combined with 58 sets of numerical analysis results,it was revealed the influence law of shear bearing capacity of composite key teeth under the condition of no pre-compression stress from key parameters such as material compressive strength,key tooth inclination,key tooth number,etc.The mechanism of compressive shear force and composition of shear bearing capacity of composite key teeth without pre-compression stress are clarified.And the calculation formula of shear bearing capacity of RC key teeth and UHPC key teeth,and the reasonable height of composite key teeth,ratio calculation formula and bearing capacity reduction factor of plane multi-key teeth are proposed;Using the aforementioned method,4 groups of axial compression tests of composite piles with different number of key teeth and different pipe pile wall thicknesses were designed,and 21 sets of numerical analysis were carried out to study the results.The influence of key parameters such as the number of key teeth,the inclination angle of key teeth,the wall thickness of the pipe pile and the ratio of diameter to thickness on the circumferential force characteristics of the pipe pile are researched.Combined with the derivation of the static balance equation,the calculation formula and design method of vertical shear resistance and circumferential resistance of the connecting section are proposed.This method is used for the trial design of the composite pile with a diameter of 3.7m,and the applicability of the method in practical engineering is verified by comparing the results of theoretical calculation and numerical analysis.