Study On Grouting Reinforcement Technology For Sandy Gravel Stratum Of Pingnan Third Bridge

Widely distributed in China,sandy gravel stratum is a kind of loose coarse clastic accumulation soil layer of quaternary period sediments.Natural sandy gravel is composed of pebble(gravel),sand,silt and clay,with wide range of particle size.Also,it has the characteristics of easy water seepage,weak cementation and poor self-stability.Therefore,the bearing capacity of sand gravel stratum may not meet the engineering requirements of structure foundation.Grouting reinforcement technology is the top preference to deal with geological problems mentioned within overburden layer of sand gravel soil.It is of great significance for grouting construction in complex foundation(such as sand and gravel)to predict the grouting feasibility and grouting reinforcement effect of sand gravel stratum based on limited geological survey data and construction technology parameters.The research background of the study is the sand gravel foundation grouting reinforcement project of the north bank arch base of Pingnan No.3 Bridge.To meet the high requirements of the long-span arch bridge on the bearing capacity and stability of the foundation,the static pressure grouting method is used to seal the bottom of the sand gravel stratum within the closed range of the wall after the construction of the diaphragm retaining wall.The study aims to demonstrate the feasibility of grouting reinforcement construction technology in sandy gravel stratum and evaluate the grouting reinforcement effect,considering different fine particle content and slurry water cement ratio.Physical and mechanical properties test,grouting model test and finite element numerical simulation were conducted.And main research contents and achievements are listed as follows:Firstly,the soil sampling analysis and mechanical properties test of the natural sand gravel stratum are carried out.Then,based on the mechanical properties test results,the mechanical properties of the natural sand gravel stratum concrete foundation system are analyzed by numerical analysis method.The results show that:the strength index(φ)of the natural sand-gravel soil is about 36.7 ° and the parameter K of Duncan model is about 207.4;the test results of contact surface characteristics between concrete block and gravel sample show that the strength index(φ)is about 32.7 ° and the stiffness coefficient K value is 2244.0;the foundation settlement deformation of sand gravel layer without grouting reinforcement is large,the maximum value of overall vertical displacement of foundation is about 7.5 cm,and its value is significant greater than the displacement limits of superstructure.The peak acceleration of GM-0 model(not grouted)reaches the maximum value of 0.87g,and the peak acceleration of the surface soil shows obvious amplification effect.In order to estimate the feasibility and effect of grouting within sand gravel stratum,the visual grouting model tests of sand gravel stratum were carried out.The grouted medium with different fine particle content(particle content less than 5mm)and water cement ratio of slurry were taken as the main factors.It is found that the grouting feasibility decreases with the increase of fine particle content,and the grouting effect of sample with 10%fine particle content is better than samples with more than 30%fine particle content.And great difference between grouting diffusion distances of 20%and 30%fine particle samples is observed.Moreover,the grouting reinforcement mode is changed from infiltrating grouting to splitting one,with the increase of fine particle content.Based on the visual grouting model test results,the best water cement ratio of slurry was used to prepare the sand gravel samples after grouting,and the mechanical properties of the samples after grouting were studied.Based on the test results,the numerical analyses of bearing capacity,foundation settlement and dynamic response of sand gravel foundation after grouting treatment were carried out.It is found that the anti-deformation ability of sand gravel layer is significantly enhanced after grouting reinforcement.And the overall vertical displacement of foundation is reduced to 1cm,meeting the settlement requirements of buildings and foundation.After grouting in the diaphragm wall,the equivalent plastic strain of sand gravel layer is significantly decreased,and no high plastic zone is found.Further,the maximum peak acceleration response of GM-1 model(3 m grouting)is 0.15g,and that of GM-2(6 m grouting)is only 0.06g.The dynamic response of sand gravel layer after grouting treatment is significantly reduced.A kind of axial compression cylinder grouting test model system is designed to evaluate grouting within different depths in Pingnan No.3 Bridge.The system is used to restore the original stress state of the sample,explore feasibility and effect of grouting reinforcement within different depths.It is found that the effective porosity and slurry injection rate in different depths of sandy gravel stratum are affected by both overburden pressure and particle size distribution,and the stratum gradation is the main factor for relatively deep stratum.The grouting amount calculated according to the current norm is greater than the actual one.Acceptable feasibility is ensured by combining the modified formula with fractal theory in sandy gravel stratum to calculate the grouting volume.Results obtained by model test is fully consistent with those measured in situ,providing guilds for grouting engineering practice in similar sand gravel stratum.