DESIGN, FABRICATION AND PERFORMANCE OF STRESS CONTROL DYNAMIC PULLOUT RESISTANCE APPARATUS FOR GEOTEXTILES

Abstract

Soil in 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 resistance design of structures. Fortunately soil can be reinforced with ductile materials, so that, the reinforced earth becomes ductile. The strength of reinforced earth is considerably larger than plain earth. 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 increase 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 geotextile 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 for sufficiently long time to cause pullout displacement to its fullest capacity. This leads to underestimation of pullout displacement and overestimation of pullout stress, which is unsafe. There are 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 geotextile reinforcement buried in test box similar to that used in direct shear test. The pullout force is applied through a flexible wire passing over the pulley and to which weights are suspended. The pullout displacement is measured after allowing sufficient time for the pullout force to cause pullout displacement. This is continued till failure. However, with increase in pullout displacement, the length of the geotextile, developing pullout resistance continuously reduces, which affects the test conditions adversely. Besides, it leads to arching action in soil within the test box, which is undesirable. Hence, there is an urgent need for development of an improved stress control apparatus for determination of static/dynamic pullout resistance of reinforcements using stress control apparatus for a more precise estimation of the same. The aim of this investigation is to design and fabricate a stress-control apparatus to determine static/dynamic pullout resistance. It uses a test box proposed by Mehdi in which the geotextile is buried. The length of the sample is sufficiently longer than the length of the box so that when estimated pullout displacement occur, there is adequate extra length of the geotextile which enters into the test box to keep the length of the geotextile within the box to remain constant. To apply dynamic pullout force, the entire test box assembly is supported on rollers and excited sinusoidal by using an eccentric drive powered by a DC motor whose speed may be varied. The inertia force generated by the weights suspended to the string (iv) supply the dynamic pullout force. Dynamic loading parameters like frequency and amplitude may be suitably controlled. Various time dependent test parameters are recorded by using acceleration pickup, LVDT and force transfer and stored with the help of data acquisition system. The test data may be analysed to obtain the dynamic pullout resistance. Limited number of tests carried out indicates that equipment performs satisfaction. A detailed experiment parametric study is not within the scope of this investigation. (