Laboratory Investigation On Strength And Deformation Characteristics Of Calcareous Sand Improved By Polyurethane Foam Adhesive

In order to safeguard China's ocean rights and accelerate the construction of the ocean power,the islands and reefs in the Nansha Islands are currently under construction.In the South China Sea,calcareous sand is widely distributed.If it is used as a foundation backfill,it can greatly reduce the project cost and the serious economic burden of the island and reef project construction,shorten the construction period and accelerate the construction pace.However,if the calcareous sand is directly used as the foundation soil,uneven foundation settlement is easy to occur.Therefore,it is necessary to strengthen the foundation of the South China Sea calcareous sand foundation.The use of flexible and crack-resistant cementitious materials can significantly improve the strength of the improved soil without changing the stiffness.In recent years,non-aqueous reactive polymers represented by polyurethane have been increasingly used in anti-seepage reinforcement and foundation treatment projects in water conservancy,civil engineering,and transportation engineering fields.It has the characteristics of light weight and high strength,superior waterproof and impermeability,environmental protection and durability.If it can be applied to the calcareous sandy backfill project,it is expected to improve the poor engineering characteristics of calcareous sand,and thus it will be widely used as a backfill material in the construction of South Island reef.In view of the engineering problems caused by the mechanical properties of calcareous sand and the good characteristics of polymer-based cementitious materials,this article uses the high polymers to solidify and improve the calcareous sand of an island reef in the South China Sea.A series of consolidation compression tests,large-scale triaxial tests,and scanning electron microscope tests were conducted to study the strength and deformation characteristics of solidified calcareous sand under static loading and the mechanism of curing improvement.The purpose is to provide a certain technical reference for the application of polymer-based gelling materials in the construction of the South Island Reef calcareous sand backfill project.The related work and conclusions are as follows:?1?Through the compression test of calcareous sand,the particle size distribution of the specimen before and after compression was compared,and the breaking rules of calcareous sand particles were analyzed.It is found that in the small pressure range?below 400kPa?,the main crushing form is grinding and crushing;when the pressure reaches a higher pressure?above 400kPa?,compression crushing is the main,and the volume of the soil sample changes significantly.Based on Hardin's crushing potential model,the crushing behavior of calcareous sand was evaluated by using the relative crushing rate B_r.It was concluded that the termination pressure P had a significant effect on the relative crushing rate B_r and the increase in B_r has slowed down.Curve fitting was performed on the relative crushing rate B_r and the average particle diameter d₅₀0 at each termination pressure P.It was found that the relative crushing rate B_r had a good linear negative correlation with the average particle diameter d.The particle diameter d₅₀0 gradually decreases.?2?In view of the particle breakability of calcareous sand particles,this article uses the high polymer to improve the curing of calcareous sand,and uses SU8010high-resolution field emission scanning electron microscope to observe the microscopic morphology of the polymerized calcareous sand,explained the polymer improvement mechanism for calcareous sand.On the one hand,the polymer reactant can provide strong cohesive force,so that the calcareous sand particles are bonded into a whole;On the other hand,the foaming effect of the polymer fills the internal pores of the calcareous sand particles and increases the contact surface between the particles,thus improving the integrity of the sandy soil and its related mechanical properties.?3?A series of consolidation and compression tests of polymer-solidified calcareous sand were carried out,and the deformation characteristics of samples with different amounts of polymer-solidified calcareous sand under different consolidation pressures were studied.The relationship between the polymer dosage and the compression coefficient of the polymer-calcified sand samples was established.The compression coefficient a_1-2-2 at low pressure levels was negatively correlated with the polymer content.?4?In order to analyze the effect of calcareous sand particle size on the curing effect of high polymer,four other different grades of calcareous sand were designed for comparative test.When the polymer content is the same,the improvement effect of the polymer on the calcareous sand with a larger proportion of coarse particles in the gradation?gradation 1?is better than that of the calcareous sand with a higher proportion of finer particles?gradation 5?Improvement effect.?5?The ISO standard sand was introduced for comparative tests to indirectly reflect the important influence of calcareous sand particle crushing on its compression deformation characteristics.By comparing the compressive deformation characteristics of the two,the standard sand sample shows better resistance to compressive deformation.At the same time,the main factors that affect the curing effect of the polymer are verified:when the polymer content??3%?is low,the particle size distribution of the calcareous sand is the main factor;when the polymer content?>3%?At higher temperatures,the particle breaking characteristics of calcareous sand are the main influencing factors.?6?In order to reflect the improvement effect of polymer-calcified calcareous sand,a trial of ordinary portland cement solidified calcareous sand under the same dosage is introduced.It is concluded that the cement-modified calcareous sand exhibits the same volumetric strain law as the polymer,and the improvement effect of the polymer on the deformation resistance of the calcareous sand is better than that of cement.?7?A large-scale triaxial test of polymer-cured calcareous sand was carried out,and the stress-strain curves of polymer-cured calcareous sand with different dosages were obtained.The effect of polymer content on strength characteristics was studied,and the relationship between peak strength and polymer content was obtained.The relationship between the shear strength index and polymer content was analyzed.The test results show that high polymers can significantly improve the cohesiveness of calcareous sandy samples.?8?In order to study the deformation characteristics of high-polymer-solidified calcareous sand,the Duncan-Chang model was used to describe the relationship between the stress and strain of different high-polymer-doped calcareous sand samples.According to the fitting results,it is found that only the calcareous sand can use this model to describe its constitutive relationship,the modified sample of calcareous sand with high polymer is only partially applicable.It shows that the addition of polymer changed the original mechanical properties of calcareous sand material.