DYNAMIC PULLOUT RESISTANCE OF GEOSYNTHETICS

Abstract

Soil is an important construction material. It is strong in compression but weak in withstanding tension. In earthquake engineering, ductility is an important material property and which helps in earthquake resistant design of structures. Fortunately soil can be reinforced with ductile materials, so that, the reinforced earth as a composite material becomes ductile in nature. Hence reinforced earth is ideal for application of geotechnical earthquake engineering. Since reinforcements basically take - up tension, tensile forces in reinforcement get transferred to soil in the form of compressive confining stress leading to increased strength when compared with plain earth. The process of transfer of these tensile stresses to soil is done through the friction resistance between the reinforcement and soil. Therefore, pullout resistance of reinforcement is a critical material property on which design of reinforced earth depends. The strain control apparatus popularly used to determine static pullout resistance consists of a test box similar to that of direct shear test apparatus. A geosynthetics buried in the soil within the box is pulled out at a constant rate of strain under a normal stress to determine the pullout resistance. Since the mobilized pullout resistance at any pullout displacement keeps on changing with time, the resisting force developed does not last sufficiently long time to cause pullout displacement to its fullest capacity. This leads to an underestimation of pullout displacement and overestimation of pullout stress, which is unsafe. Till now there have been no satisfactory methods for determination of pullout resistance under dynamic condition. Its determination by carrying out back analysis of reinforced earth structures under dynamic loading indicates that dynamic pullout resistance is considerably smaller than that under static condition. Mehdi (1998) developed stress control apparatus to determine static pullout resistance. It consists of applying pullout force to geosynthetics reinforcement buried in test box similar to that used in direct shear test.