我国土工合成材料界面特性的研究和应用概况(英文)-study interaction.ppt

Study on the interaction behavior of Geosynthetics and soil in China,Bao Cheng-gangNingbo Institute of Technology,Zhejiang University,China,315100Yangtze River Scientific Research Institute,Wuhan,China,430010,contant,Prefacethe different views on reinforcement mechanismAnalysis of characteristics of interface and its effect on reinforcementStudy on interface characteristics and shear bandExperimental study of interface behaviorApplication of reinforcement in ChinaConclusion,Preface,In China, various kinds of geosynthetics are widely used for reinforcement purpose in civil engineering, water resource and hydropower engineering, environmental engineering, etc. the application of geosynthetics has brought a great benefit to a project not only in engineering aspect, but also in economic and environment aspects.,However, the mechanism of reinforcement for geosynthetics is not fully understood, the development in analysis method and design theory is backward as compared with its engineering application. the traditional analysis method is not perfect to reflect the reinforcement mechanism of geosynthetics in soils.,the calculated FS is always too small to reflect the real effect. the obtained increase of FS due to geosynthetics reinforcement with conventional circular arc method is only about 2-5%, such as, for Damudu Coast Embankment in Zhejiang Province is only 0.03. While the FEM is used for analysis, the increase of FS is only about 4%.,So, the development of reinforcement technology for geosynthetics is inhibited. the mechanism of geosynthetics reinforcement, and its failure mode should be investigated.the reasonable analysis method and design theory should be proposed.,In the past, the concern of most Chinese researchers is how the conventional circular arc method can still be used effectively in analysis of geosynthetics-reinforced structure. This effort has made some progress.when the balance of moment at the top face of reinforcement is considered, the result will increas about 7% in FS for circular arc method , and 2% for simplified Bishop method,most Chinese research,If the tensile effect is considered only at the part interesting with the slip surface, the value of FS tended to decrease If the redistribution of stress due to reinforcement is taken into account, the value of FS is increased significantly for FEM method, The potential failure form pointed by R.K.Rowe (2002) due to large deformation of soil, has not been much concerned at present in China.,the different views on reinforcement mechanism,Friction effectConstraint-induced strengthTensile membrane theoryStress-spreading effect by reinforced sand cushionChange of stress condition and deformation fieldShear-band theory,Friction effect due to friction effect :the horizontal displacement of the reinforced soil is restrained in contact-face,the stiffness and strength of the soil is increased, the stability of the soil structure is enhanced.In fact, it means that a friction force can be used to represent the reinforcement effect.,fig1,Constraint-induced strengthThere is shear stress existing at contact surface. Hence, the horizontal stress of the soil element near the contact surface is increased by3,. Thus, the 1f is much greater than the value for the un-reinforced case. It means, the reinforced soil is strengthened with respect to its shear strength and bearing capacity,Fig2 Mohr stress circles for sand and reinforced sand,Tensile membrane theoryIf the reinforcing material as a membrane lying on soil,the material will be curved under loading of overburden, the tension effect of curved membrane will bear part of vertical load, so ,the loading on the underlying soil is reduced. On the other hand, as the reinforcing material settles, the soil above reinforced soil layer will subside and the “arching effect” will also appear. It will enhance the stability of soil foundation.,Fig3,Stress-spreading effect by reinforce cushion the sand cushion is often used as a drainage for soft soil consolidation. The reinforced cushion also can spread the stress in the underlying soil. Hence, the stress delivered to the soft foundation will be reduced significantly.the increase of FS caused by friction action and stress-spreading effect is 15.7% and 44.7%, respectively( in which the pre-strain treatment is executed for reinforcing material.),Fig4,Change of stress condition and deformation fieldthe stress condition of foundation is greatly influenced by friction force between reinforced materials and soil. The circular sliding failure will be impossible to appear in the reinforced soil. The most possible mode is the subsidence of embankment accompanying with lateral spreading of foundation soil (it has been verified by a test embankment in Lianyungang harbor),the direction of shear stress will be changed along the ground surface due to existence of reinforcing material, and the bearing capacity of foundation is increased remarkably. the displacement field in foundation is changed significantly as a result of reinforcement.In fact, a “self-support bearing system” (i.e. “quasi-rigid zone”) is basically formed in the composite soil foundation due to the existence of reinforced cushion,Shear-band theorythe shear-band will appear in the zone adjacent to the contact surface when the shear strain reaching a certain extent.The anti-sliding effect to contact surface by the localized deformation is an important aspect of reinforcement mechanism.,Analysis of characteristics of interface and its effect on reinforcement,all the reinforcement mechanism can be divided into two aspects: One is “The Direct Reinforcement Effect”, which is induced by interaction between soil and geosynthetics; the other is “The Indirect Reinforcement Effect”, which is resulted from the composite soil mass (quasi-rigid mass) formed by reinforcement material and adjacent soil layer.,The former one is: the friction and interlocking effect, the “tensile membrane” effect at contact surface and the “shear band” developed at the adjacent soil,It should be emphasized that the concept of “INTERFACE” is a zone with a certain thickness, including the contact face and its adjacent thin shear band. The ”interface effect” is a generalized term to express the comprehensive effect occurred at the interface.(to complete),For the latter one “indirect reinforcement effect is owing to the re-distribution of stress and strain condition in foundation, the change in the direction of shear stress, the spreading effect of stress, the “arching effect” , the “quasi-rigid zone” ,changing the failure mode of soil, increasing bearing capacity, improving stability of soil structure greatly .,Study on interface characteristics and shear band,(In this section some test results will be present for verifying the above point of views),The interface effect is firstly caused by the friction and interlocking effect at contact surface. The friction is induced by the relative displacement between reinforcement material and soil, while the interlocking is caused by the interaction of soil and geogrid , and the interlocking can only be appeared at large displacement condition. the two effects will probably be mobilized at different time. It was indicated by the pullout test by Yan,Fig 5,the development of shear band is attributed to the friction and interlocking effect, and another one is related to the non-uniformity of reinforcing materials in quality.when the displacement of reinforcing material develops to a certain extent, the soil grain will be slid, rotated, crashed, etc, and then, the shear band is formed.The thickness of shear band is about 56 times of average size of soil grain.Then, the “interface” would include the “contact surface ” and the “shear band”.,The behavior of interface is influenced by several factors: The roughness of contact surface. The value of normal stress. The properties of reinforced soil., The roughness of contact surface. The rough contact surface will lead to anisotropic response of the stress-strain behavior in interface, The value of normal stress. If the normal stress is small, the deformation of interface exhibits a positive dilatancy, otherwise, it shows a negative dilatancy, The properties of reinforced soil. The strength envelope line tends to change with the shape and friction angle of soil grains,Because of the localization phenomenon of deformation, the soil state in various part are rather different, the stress and strain behavior are also different, so the constitutive relationships are quite complicated. Hu and Zhang presented a constitutive model(EPDI Model)based on the Elasto-plastic Damage Theory. The FEM analysis result from this model is comparable with the result from a large scale pull-out test.This model can reflect the behavior of strain softening, dilatancy and anisotropy etc.,Fig6 Comparison of model prediction and experimental result for shearing along a interface between gravel and geotextile,Fig7 Modelling result for the relationship between pull-out resistance and normal stress of pull-out test (based on EPDI model with geotextil),For the tensile membrane theory, Wang-Tao & Wang-Zhao have proposed an analysis model for reinforced pavement and obtained a general solution under symmetry and non-symmetry loading,Experimental study of interface behavior,Direct shear test and pull-out test Triaxial test for reinforced soil with geogrid and crushed stone soilStatic model test for spread foundation on aeolian sand Centrifugal testField test and in-situ test,Case 1: Direct shear test and pull-out testthree types of reinforcing materials: warp-knitting geogrid, plastic geogrid with one direction tension, woven geotxtile Three kinds of soils : sand- gravel (35.0, c=0),coarse-sand (31.5, c=0) granite residual soil (=23.929c=14.6kPa)The size of the specimensfor direct shear tests is 100100120 mm (i.e., lengthwidthheight), for pull-out test is 20020060mm.,Test result (table 1)Table1 the result of the direct-shear test and pull-out test,Table 1 the result of the direct-shear test and pull-out test,Fig8 Shear stress versus displacement curves and shear strength envelope from laboratory tests,(b) Warp-knitted geogrid to gravel, pull-out shear test,(a) Warp-knitted geogrid to gravel, direct shear test,The surface of warp-knitted geogrid is the roughest one amony three of the reinfording materials, so, the interlocking effect is most significant, the shear-band is the thickest. the strength is the greatest. on the other hand, the greater the grain size, the higher the shear strength is. Also, more grains are mobilized in the shearing process.,Case 2: Triaxial test for reinforced soil with geogrid and crushed stone soil the size of specimen is 300 mm in diameter and 655 mm in height. the maximum grain size of the soil is 60 mm, and d60 is equal to 26 mm.The placement of reinforcing material is 2 and 3 layers,The test result shows:the reinforced soil is no longer a pure disperse material, but somewhat continuum material. the shear strength is increased by 29% for the specimens with 3 layers, and by about 43% for 5 layers The cohesion is also increased greatly. The stress-strain curve exhibits a strain-hardening behavior instead of strain-softening,the extensibility is improvedthe lateral and axial strain of soil is reduced. In this triaxial test, the deformation is decreased by 30%60% for 3 layerThe total compression, particularly the differential settlement is reduced and the bearing capacity is increased,Fig 9 Stress-strain curves under different ambient pressures,Case 3: Static model test for spread foundation on aeolian sand The aeoline sand is a fine sand, the grain size of which is very uniform, d80=0.250.1mm. The tensile strength of reinfording material- geogrid is about 60 kN.m-1, and the extensibility is 16.5%. Four types of placement are selected for comparison :one layer, two layers, rectangle box type and long rectangle box type,Fig10 P-S curve for different types of placement of reinforcing material,The result shows:for long rectangle box type, the reinforcement effect is more significant than the others. It is the failure occurring tardily, and the reinforced soil become more integral. As the loading increases, geogrid starts to bend and settle, the tense membrane effect of geogrid appears due to the tension of the reinforcing material. At the same time, the “arching effect” occurs in the upper soil layer. Then, the stress in soil is spread beyond the “bending-settling plate”,Case IV: Centrifugal testCentrifugal test is an effective way to study the reinforcement mechanism of geosynthetics and the reasonable distribution form for reinforcing material. The test results obtained by Yang, X. W. indicated the stretching path of crack and fissure in soil will be changed as a result of reinforcement.The reinforcement tends to inhibit the joining-up of cracks, and changed the direction, at that time the stress is also redistributed (if the material is not pull-out or broken),then, the possibility of forming a throughout sliding surface would be reduced It means the soil is more stabilized and the circular sliding failure is impossible.,Fig11 The change of stretching path of crack due to reinforcement,Application of reinforcement function in China,The geosynthetics reinforcement has been widely used in engineering practice and yield great benefit in China. The main fields include reinforcement of soft foundation, reinforced retaining wall (the highest one is 55m, preventing road pavement from reflection crack, reinforced stone-column with geogrid, vehicles jumped at bridge approach, reinforced permafrost soil in Tibet regions. reinforced expansive soil slope, and so on.,case 1: The natural foundation improvement for two ancient city gates of Quenzhou cityThe reinforced foundation with warp-knitting geogrid is selected instead of pile foundation and reinforced concrete slab foundation.The comparison of settlement by monitoring date is as follows,Fig12 P-S curves for natural and reinforced foundation, field loading test,Case2: Retaining wallReinforced retaining wall is very popular in China now, the highest one is 55m, mostly is less 30m.Some research work has been performed, including the calculation method and monitoring.It means the prediction of reinforced structure behavior is possible(e.g.Rankine theory),Fig13 Comparision of horizontal pressure for observational