Research On Key Technology Of Ground Treatment In The Karst Areas

The karst geology situation of Guangdong Province is very complicated and of greatdisadvantage of engineering, especially in the northern area such as Shaoguan, Qingyuan,Zhaoqing. As a result, a more suitable foundation style and way of ground treatment isurgently needed. The School of Civil engineering and transportation of South ChinaUniversity of technology has conducted a series of experiments and engineering practices inthe northern area of Guangdong Province under the financial support provided by Key Projectof the State Key Laboratory of Subtropical Building Science South China University ofTechnology.Nowadays the bearing capacity of composite foundation is determined by the single-pilestatic loading test which ignores the pile-group effect and is not able to offer the forcedistribution suitably. And the interaction between the superstructure and the foundation isdifficult to be considered because of the restriction of computers. So further research shouldbe done to simplify the analyzing method to make it feasible. Moreover the field experimentalresults show that there is a remarkable difference between the experimental stress of raft steelrebar and the calculated value, and the measured values are smaller. It is indicated that theCalculation Model exists insufficiently and the design method is conservative. So the designmethod should give a more accurate steel rebar stress in the raft. Furthermore, with the wideuse of composite foundation technology in earthquake region, it is urgent to study thedynamic interaction of composite foundation and superstructure. This thesis adopts themethod of theoretical analysis and experiments to study the bearing and settlementperformance of the rigid-pile composite ground-raft foundation-superstructure system, with apractical engineering background. The main contents and innovative works are listed asfollows:1) The results of twenty-four load tests of single-pile composite foundation incomplicated karst geology show that the bearing capacity of soil between piles exerts earlyand it’s difficult for both pile and soil to reach their design value under the characteristic load.With0mm thick sand-gravel cushion and high soil’s bearing capacity, part of the soil’sbearing capacity still works as long as the foundation is in close contact with the soil. With100mm thick sand-gravel cushion, under the characteristic load, the mean stress of soil is0.31MPa, the pile-soil stress ratio is about19, the load bearing ratio of soil is46.5%, and thebearing capacity factors of pile and soil is about0.72and1.78respectively. With300mmthick sand-gravel cushion. Under the characteristic load, the mean stress of soil is0.38MPa, the pile-soil stress ratio is about14.6and the load bearing ratio of soil is61%. The analysisshows that this kind of treatment is feasible.2) Compared with the single pile composite foundation loading test, the correctness ofeffectiveness of the adopted soil elastic-plastic model is veirified. The contact relationship andthe selected parameters can be used to analyze the stress of the rigid pile compositefoundation, and the deformation performance more accurately. On this basis, the size effect ofgroup piles composite foundation loading plate is researched and the formulas for calculatingthe final settlement of large size loading plate is worked out. The thickness of the Cushionlayer is100mm, the final settlements of the foundations with the side length of30m,40m and50m,is respectively16.7mm,17.5mm and18.1mm; while the thickness of the cushion is300mm, the final settlements of the foundations with the side length of30m,40m and50m,isrespectively32.4mm,34.5mm and36.1mm. The results match well with the measured values.The results of analyzing the macroscopic constitutive relation of the group piles compositefoundation show that with the increase of the number of the piles, the settlement of theloading plate increases, the bearing capacity of the soil decreases. In this geologicalconditions, under the effect of characteristic value of loads. As the thickness of cushions isrespectively100mm,200mm and300mm, the average value of49piles compositefoundation settlement is respectively about1.6,2.18and2.2times the foundation settlementof a single pile composite on average and the soil stress is respectively46%,65%,78%ofsingle pile composite foundation soil stress. In early loading period, the single and multiplepiles were of the similar penetration into the cushion, while the penetration of pile groupproved to be larger as the load increases. Thickness of the cushion layer is respectively100mm,200mm and300mm, on the characteristic loading value, the penetration of the singlepile into the cushion is3.9mm,6.8mm and8.4mm respectively. The penetration of themultiple piles is6.8mm、18.7mm、19.2mm respectively. When the pile number reaches theamount of49, and cushion layer of foundation up to300mm, the soil stress of the piles,foundation settlement, penetration of rigid pile, change little. Regulation of the cushion layertends to be stable.3) According to the test results and FEM analysis, combining engineering experiences,the bilinear spring macro constitutive relation of composite foundation is put forward. Thecorresponding settlement of characteristic load is12mm, and the corresponding settlement ofultimate load is40mm. The interaction between superstructure and composite foundation canbe simulated by SAP2000, and the corresponding foundation settlement of characteristic loadchanged from4mm to20mm. In addition to changes of raft moment in some individual area are close to9%, others are negligible. The results show that the method can not only considerthe effect of the stiffness of superstructure, but also can reflect the nonlinear property of theground as well as the overall settlement.4) Solid element model of the foundation has been established, considering the frictionbetween the raft and cushion. Elastoplastic analysis was done with ABAQUS to consider theinteraction of superstructure with composite foundation. The results showed that under thedesign load of elastoplastic calculation, section stress distribution is appropriately uniform,cross-sectional maximum tensile stress is about1.63Mpa, the maximum stress ofreinforcement is about80Mpa; the maximum stress of reinforcement under standard load is48Mpa，the calculated results agree well with the measured values to verify the correctness ofthis analysis in this article. By calculation and analysis, compared to calculation results withcommon methods, it shows that ground reaction force model, and the friction between raftand cushion are major factors that affect the calculation results of the forces in the raft.Ground reaction force at the edge of the raft surges, the maximum value reaches3.5times theaverage value, which can easily cause damage to the foundation of local area in advance.Finally, the foundation soil deformation modulus, thickness cushion, cushion modulus,thickness and other parameters on the raft force and deformation have been analyzed. Theresults show that the ground reaction force and stress in the raft are related to the relativestiffness of the raft and ground.5)The modeling method of FEM-Infinite element coupling is adopted to set up theintegral three-dimension analyzing model for the dynamic elastoplastic time-history analysis.By comparing with the results of ordinary pile raft foundation dynamic time-history analysis,the results show that the composite foundation has no seismic reduction effect at the stage offrequent earthquake of intensity of7; but at the same degree of rare earthquake intensity, themaximum pile moment and shear force of ordinary pile raft foundation is1.8times of thosevalues in the rigid pile composite foundation, and there is a large plastic deformation and sliprelative to the substrate in the cushion, which shows a certain role in energy dissipation andseismic reduction. The dynamic response of raft foundation in the composite foundation andsuperstructure is90％of the corresponding amount in ordinary pile raft foundation, theseismic decrease coefficient lies between0.8and0.9.