UPLIFT BEHAVIOUR OF PLATE ANCHORS WITH GEOSYNTHETICS

Abstract

The investigation deals with shallow horizontal plate anchors with reinforcement subjected to vertical pull out loads in cohesionless soil. There are many examples in civil engineering design such as tall transmission towers, radio and television towers, retaining walls, suspension bridges and offshore structures, whose foundations are subjected to large uplift forces. In such cases an economical design solution may be achieved by the use of tension members known as anchors, so that they can resist the pull out forces with adequate safety. Plate anchors are one of the most commonly used types of anchor. To improve the uplift capacity of plate anchors, a well-established technique for achieving cost effective solution is use of geosynthetics in the form of reinforcement. The increase of uplift capacity is due to frictional characteristics of soil geosynthetic system. The analysis in this work is done for circular and square shaped anchors using geogrid (Netlon CE-121) as reinforcement. Model laboratory tests were conducted under controlled condition in cohesionless soil. The width/diameter of the plate used is 100mm and depth ratios were 2, 3 and 4.2. The size of reinforcement is taken as three times the size of plate anchor. Three tests were conducted with single layer of reinforcement, one at top of the anchor and other two when placed at 0.25 and 0.5 times the width of plate. Also test with two layers and three layers were done with one reinforcement at top of anchor plate and others placed at an interval of 0.25 times the width of the plate above the earlier. The objective of the work is to study the uplift behaviour of plate anchors due to inclusion of geosynthetics, number of layers of reinforcement, shape of anchor plate with and without geosynthetics and soil density. The response of load-displacement variation is also studied and it was observed that the deformation at any stage of loading in reinforced anchor is less than the deformation in unreinforced anchor was observed. The best location for enhancing the pullout capacity with less deformation is noted when the reinforcement is placed at top of anchor plate. An analytical solution to predict the pullout capacity was used and compared with observed value. Good agreement between the two was noted.