Research On Enhancement Mechanism Of Bearing Capacity Of Large-diameter Post-grouted Piles And Design Method Based On Settlement Criterion

Post-grouting techniques refer to the technology that the grouting pipe is preinstalled in the bored pile and the cement grout is pressed into the pile by the grouting pump to enhance the strength of the soil at the pile side and end,thereby improving the bearing capacity and decreasing the settlement of the pile foundation.At present,it has been widely used in foundation projects such as super high-rise buildings,long-span bridges and high-speed railways because of its simple process,low cost and reliable reinforcement effect.The applicable objects of post-grouting have developed from medium-and-small diameter and medium-and-short pile to large-diameter and super-long pile.However,due to the limitation of research means of large-diameter piles and the lack of complete field measured data,there is still a lack of systematic research on the reinforcement mechanism,bearing characteristics and design methods of large-diameter post-grouted piles,which makes its theoretical research lag behind engineering practice.The strengthening mechanism of bearing capacity and the design method of settlement control of large-diameter post-grouted piles need further study using theoretical analyses,laboratory tests,field tests,and mathematical statistics.The main research achievements are as follows:(1)Considering the enhancement effect of the reinforcement effect of grouting on the soil at the pile end and augmented head effect on the base resistance of post-grouted pile,the hyperbolic function is used to capture the base resistance-displacement response,the initial stiffness of the soil at the pile end and the improvement coefficient of the base resistance are introduced.The analytical solution of the grout bulb radius is proposed on the basis of the spherical cavity expansion theory,which provides a theoretical calculation basis for the mechanism of enlarged head reinforcement.Moreover,considering the enhancement effect of the grout migration upward on the side resistance of post-grouted pile,a theoretical model for calculating the grout migration upward along the pile side is proposed with consideration of the time-dependent behaviors of viscosity,the methods to determine the parameters and to iteratively determine the grout migration height in the layered soils are presented,respectively.A case history is cited to demonstrate the validity of the model.In addition,the influence of post-grouting on the interaction of pile resistance is studied by field test,and the interaction mechanism of post-grouting on pile resistance is theoretically analyzed.Finally,the mobilization of the base and side resistances of post-grouted pile is deeply analyzed through the field static loading test,the strengthening effect of post-grouting on the pile resistance is verified,and the important influence of the preloading effect on the pile resistance is analyzed,and then the mechanism of strengthening effect of post-grouted pile is fully revealed.(2)The vertical and horizontal loading tests of steel pipe piles under static pressure piling were carried out in two different model foundations of siliceous sand and calcareous sand,and the influence of post-grouting on the bearing characteristics of single pile in two different sand soils under vertical and horizontal loads was studied.The results show that the vertical and horizontal bearing characteristics of ungrouted single pile in calcareous sand are weaker than those of siliceous sand.The main reason is that the calcareous sand is easy to cause lateral compression during pile sinking,and the change of lateral frictional resistance caused by lateral compression is smaller than that the negative effect caused by particle breakage.After grouting,the vertical and horizontal bearing capacities of single pile are greatly improved in two different model foundations and show roughly the same bearing characteristics.The mechanism of pile-soil-slurry interaction in sandy soil is revealed by analyzing the distribution of slurry of single pile through excavation.(3)The influence of different grouting types on the bearing characteristics of large-diameter piles was revealed by the field comparison test of large-diameter combined grouting pile and side grouting pile.The results indicate that the bearing capacity of combined grouting pile is obviously better than that of side grouting pile.Under the service load,the percentage of the settlement at the large-diameter and super-long pile head due to the compression of pile shaft is about 90%.Therefore,the settlement caused by the compression of pile shaft should be considered in the design of super-long pile.The large-diameter and super-long piles behave as friction piles at the ultimate load.Meanwhile,field load tests were conducted on three large-diameter post-grouted piles in coral reef limestone formations,and the bearing capacity of pile foundation,the axial force transfer characteristics and the mobilization of pile resistance were deeply analyzed.It is found that the post-grouting technique can be applied to coral reef limestone formation,and can effectively improve the bearing capacities of piles foundation and reduce their settlements.Additionally,the effect of combined post-grouting on controlling settlement of pile head is better than that of post-grouting at pile end.Finally,combined with the field long-term static load test,the long-term bearing behavior of post-grouted piles and the variation of pile resistance with time were studied.The results show that the bearing capacity of the post-grouted pile has a time effect,and the base resistance and the side resistance are further enhanced with time.(4)The reinforcement effect of combined post-grouting was comprehensively evaluated by borehole coring test,standard penetration test and electromagnetic wave CT test.The results show that cement slurry infiltration downward,migration upward and transverse permeation are mixed with fine sand stratum to form cement stabilized soil,which enhances the strength and stiffness of soil layer at pile side and end.The SPT blowcounts of the soil around pile after grouting is significantly higher than that before grouting,and an empirical method is given to predict the side resistance of the piles before and after grouting based on the SPT blowcounts before grouting.It is feasible to carry out an electromagnetic wave CT after grouting to detect the grouting effect.The spatial distribution pattern of the pile,injected grout and soil can be observed by drawing the inversion image of apparent absorption coefficient of each section,and the diffusion range of the grout of the soil at pile end and side can be determined.(5)Based on the static load test data of 716 test piles collected from 139 projects,the related parameters of grouted piles and ungrouted piles were statistically analyzed,and the empirical estimation approach for the bearing capacity of large-diameter post-grouted piles were proposed by using the improvement coefficient method of ultimate bearing capacity.The calculation approach for the bearing capacity of large-diameter post-grouted piles suitable for different grouting types was established by using the enhancement coefficient method of base resistance classified by soil layer.The value range of the empirical coefficients of grouting amount of pile side and end classified by soil layer were summarized,and the formula for calculating grouting amount of large-diameter pile applicable to different grouting types was proposed.The applicability of the calculation formulas for the bearing capacity and grouting amount of post-grouted piles given in this paper was evaluated through a large number of measured data.The research results are incorporated into the industry standard of the People's Republic of China "Code for design of ground base and foundation of highway bridges and culverts"(Revised version 2017)and the industry standard of the engineering construction "Technical specification for post-grouting of cast-in-place pile of Highway Bridges"(T/CECS G: D67-01-2018).(6)Two different calculation approaches for the settlement of a single pile after grouting were presented in this paper.In the first method,the influence coefficient of post-grouting settlement was introduced on the basis of the settlement calculation method of large-diameter pile foundation without grouting.Based on the statistical analysis,the recommended range of influence coefficient of post-grouting settlement was given,and a method for calculating settlement of large-diameter post-grouted piles suitable for different soil layers was obtained.The hyperbolic load transfer function was used on the basis of the load transfer method,empirical range of improvement factors for shaft and base resistance in different soil deposits was given,and the calculation method of load-settlement relationship of post-grouted pile considering the radius of the grout bulb and the thickness of cement grouting along the pile was established in the second method.Finally,the rationality of the two design approaches is verified by engineering examples.