Experimental On Pull-Out Behavior Of Geogrid Under Rockfill Materials And Its Application

Geogrid is a kind of high strength and good flexible reinforced material, mainly composed of transverse ribs, longitudinal ribs and unique mesh structure, which can improve the strength of the soil by the friction and interlocking force between geogrids and soil. Now it's increasingly applied in rockfill dam slopes strengthening, it has been used to reinforce the top area of dam, such as Qingfengling dam, Yele dam et al. The soil in present study about the Soil-geogrid interaction mainly is sand or clay. At the same time, the study about interaction between rockfill materials and geogrids is not enough, and the mechanical property of rockfill materials is obvious different from sand and clay. So it's necessary to study the interaction between rockfill materials and georids by pullout test.Based on the literatures and specifications about pullout test, writer redesign and improvement the existing large direct shear apparatus in lab. To better understand the mechanism and effect geogrids in the reinforcement rockfill, the pullout tests were conducted using three types of geogrid specimens and large-scale pullout devices. Thses three types of geogrid specimens are plastics-welded geogrid, stretch plastic geogrid and warp-knitted geogrid. At last we advice the best geogrid and evaluating the effect of reinforcement on dam through numerical analysis method based on the test result. The main research works and conclusion in this thesis are listed as follow:(1) The pullout box and clamp were designed based on ASTM(D6706-01), which suitable for large size soil and geogrids with large large aperture. And we drawing up the test methods.(2) Results of pullout test show that seismic strengthening using this kind of geogrid in larger particle sized rockfill is not suggested since low stretch plastic geogrid intensity could cause overall fracture failure; Joint strength of plastics-welded geogrid is a key factor for reinforcement effect, the value could be characterized with csg which is independent of normal load. The effect of joint failure towards anti-sliding stability should be considered when strengthening using this kind of geogrid. Thus, seismic strengthening using high joint strength geogrid is suggested; As for warp-knitted geogrid, after large displacement pullout test, no debonding was observed in joints. Resistance comes mostly from friction resistance of longitudinal geogrid ribs and resistance of the geogrid nodes.(3) The results of static and dynamic FEM simulation show that the effect of reinforcing was not obvious under the impact of static force and middle or low seismic motions in Lianghekou dam. Plastic strain and permanent deformation can be obviously reduced in reinforcing areas by reinforcing under strong earthquake.(4) Through FEM analysis we can find that dam slope safety factor is increased while slippage is decreased after reinforcement; Stable calculation of the dam is highly influenced by joint strength of plastics-welded geogrid; Using of warp-knitted geogrid reinforcing crest area is suggested when mechanical and structure parameters are close to those of plastics-welded geogrid's.