Laboratory Investigation On The Effect Of Particle Size Distribution On Mechanical Properties Of Calcareous Sands Improved By Polyurethane Foam Adhesive

Calcareous sand is a reef clastic deposit with loose and weakly cemented.The mineral composition is mainly aragonite and high magnesium calcite,and the content of calcium carbonate is as high as 96%,namely calcareous sand.Since the last century,the coral reefs and calcareous sand were used to build large,modern military,medical and other civil works in the South China Sea.Due to the special geotechnical characteristics of calcareous sand such as porous(containing internal pores),irregular shape,easy to be broken and so on,some problems have appeared after many years of operation existing projects,such as cracking,leakage,corrosion and so on.Therefore,it is of great significance to study the improvement of calcareous sand in the South China Sea.Polyurethane Foam Adhesive(PFA)is a kind of lightweight material with excellent mechanical properties,good waterproof and impermeability,environmental protection and durability,high adhesion.It is widely used in railway subgrade,slope,embankment and other engineering reinforcement,which is used to improve the uneven settlement of soil,improve the bearing capacity and impermeability.Because of the special geotechnical properties of calcareous sand,the excellent strengthening performance of PFA and the influence of the mechanical properties of particle size distribution of sand,this paper uses PFA to improve calcareous sand with different particle gradation in South China Sea.Through scanning electron microscope tests(SEM),unconfined compressive strength test,consolidation drainage triaxial test and California bearing ratio(CBR)test,the curing mechanism of calcareous sand improved by PFA and the effect of particle gradation on strength characteristics under static load of calcareous sand improved by PFA are studied.The relevant work and conclusions are as follows:(1)The specific gravity bottle test of calcareous sand particles shows that the relative density of calcareous sand ranges from 2.73~2.78.The fractal analysis of calcareous sand with three particle gradation shows that the particle fractal dimension D of calcareous sand increases with the increase of fine grain content,and the particle fractal dimension D of natural gradation calcareous sand,calcareous coarse sand and calcareous medium sand is 2.109,2.233,2.338 respectively.(2)The strengthening mechanism of PFA to calcareous sand can be explained.On the one hand,when the PFA is mixed with calcareous sand particles,the polymer can be filled in the structural gap of calcareous sand sample.On the other hand,PFA are filled in the pores of calcareous sand samples after curing,forming a large skeleton-like network on the whole by cementing the originally presented as loose calcareous sand particles into a complete structure.(3)Unconfined compressive strength test of PFA-cured calcareous sand with different particle gradation was carried out.The experimental results show theunconfined compressive strength of calcareous sand solidified samples decreases with the increase of particle fractal dimension.(4)The triaxial test of the PFA solidified calcareous sand with different particle gradation was carried on.The results show that PFA have the best effect on increasing shear strength of natural graded calcareous sand,followed by calcareous coarse sand and the worst medium sand.(5)California bearing ratio(CBR)test of calcareous sand under different particle gradation conditions improved by PFA was carried out.The test results show with the same PFA content,CBR of calcareous medium sand samples is the largest,but the improved effect on bearing capacity of natural graded calcareous sand is the best.Considering the improvement effect and economic reasons,for natural graded calcareous sand,the optimal PFA content is 7.5%;for calcareous coarse sand,the optimal PFA is 6%;for calcareous medium sand,the optimal PFA is 7.5%.