Experimental Study On Side Compaction Grouting Strengthening Of Piles In Silty Clay-Silty Fine Sand Stratum

Pile-side grouting is the main method to enhance the bearing capacity of in-service pile foundation,but the design of pile-side grouting parameters and the evaluation of pile foundation reinforcement effect are difficult to study due to the concealment of grouting works.Aiming at the in-service friction pile pile-side grouting reinforcement project in silty clay-fine silty sand stratum,indoor grouting test,pile model test,finite element simulation and field test are adopted to reveal the mechanism of the action of pile side extrusion grouting to enhance the bearing capacity of in-service friction pile.The design method of pile-side grouting parameters and pile bearing capacity analysis method in powder clay-fine sand stratum are proposed.The pile foundation is reinforced by grouting in silty clay-silty sand stratum.The main research contents and results are as follows:(1)Through the development of indoor grouting reinforcement tests in silty clay and fine sand strata,the diffusion law of cement and water glass grout under different grouting pressures has been clarified,and the difference of grouting reinforcement effects in silty clay and fine sand strata has been analyzed.The test shows that cement-water glass grout in silty clay,silty sand,fine sand stratum reinforcement mode is compaction-splitting.The slurry flow field exerts pressure on the side wall of the sand splitting channel and compresses the formation to improve the formation mechanical parameters.Small grouting pressure can improve the mechanical properties of sand stratum because fine sand has compressive rigidity.Silty clay requires higher grouting pressure to achieve load transfer.(2)The test device of pile side compaction grouting is developed and its reliability is verified.Based on the increase of pile bearing capacity and the development of pile side friction,the grouting parameter design method of silty clay-fine sand composite stratum is proposed.When the pile side grouting pressure is 1.25 MPa,the maximum increase of the bearing capacity of the reinforced friction pile is 167.27%.At the same time,the relative position of grouting hole and pile has significant influence on the bearing capacity of pile foundation.The distance between grouting hole and pile side should be less than 15cm,and the best reinforcement effect is at 3/5-4/5 depth of pile foundation.When the depth of the interface decreases from 25cm to 75cm and the thickness ratio of silty clay and fine sand layer changes from 1:3 to 3:1,the bearing capacity of pile foundation decreases by about 50～100 N.(3)The numerical calculation of pile-side grouting on site is realized by embedding exponential load transfer model of pile side grouting reinforcement.The influence regularity of stratum,reinforcement length,length and diameter of pile reinforced by grouting on the reinforcement effect of pile foundation in service is studied.With the increase of silty sand thickness,the bearing capacity of pile foundation strengthened by grouting increases,and the optimal position of pile foundation strengthened by grouting gradually moves upward.When the reinforcement length is 4 m,the best reinforcement effect is located at 40～90%of the pile,and the bearing capacity increases by 10.7～13.7%.When the interlayer interface is located at the upper part of the pile,the bearing capacity improvement effect is the best,while when the interface is located at the lower part of the pile,the effect is better.With the increase of reinforcement length,the ultimate bearing capacity of pile foundation increases gradually.When the reinforcement length exceeds 16 m(about 70%of the pile length),the ultimate friction resistance in the middle and lower part of the pile can be greatly increased.(4)According to the results of model test and numerical analysis,the strengthening coefficients of pile side resistance under different working conditions in silty clay,silty sand and fine sand stratum with 90%confidence are obtained by mathematical statistical method,and the theoretical calculation method of pile side compaction grouting to enhance pile foundation bearing capacity is established.The bearing capacity of pile foundation obtained by theoretical calculation,numerical calculation and field test is 11,979 kN,11,190 kN and 12,600 kN respectively.An error of less than 12%indicates confidence.