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olyaei M N, maaf S. Finite element modelling of pull-out test for helical soil-nail. MCEJ 2020; 20 (5) :179-192
URL: http://mcej.modares.ac.ir/article-16-42460-en.html

In recent decades, the use of an efficient and cost-effective method to provide soil stability has been a major challenge for civil engineers. With increasing urban population, the need for underground spaces increases and deep excavation is an inevitable affair in civil projects. Deep tunnels and large buildings require deep excavations, which must use some techniques for stabilize it. Soil-nailing (reinforcing soil at the site) due to the fast build, is a good way to provide stability. It can also be described as a top-down construction technique for the improvement of behavioural properties of in-situ soil mass. Soil-nailed system is formed by inserting relatively slender reinforcing bars into the slope. Depending upon the project cost, site accessibility, availability of working space, and the soil and groundwater conditions, soil-nails can be inserted into the ground. Soil-nail is generally known as conventional and injectable nails but nails with screw plates or "helical soil-nails" are also important due to the faster build and no need for groutings. Helical soil-nails are new alternative to the conventional soil nails or tie-backs for stabilization of slopes, excavations and embankments due to ease of installation, minimal site disturbance and immediate loading capability. Helical soil-nails are installed by application of torque without a drill hole and derive its capacity from one or more helical plates attached to the nail. The shear strength-displacement behavior at the interface is an important parameter in design of various geotechnical engineering projects, for example, soil-nails, retaining walls, shallow foundations, pile foundations, etc. In soil-nailing, behavioure of interface between the soil and nail estimated by pull-out test. The behaviour of interface is governed by numerous factors, such as stress conditions, soil properties, method of installation and soil-nail interface boundary conditions. The pull-out resistance is measured as the most important factor in the design of the nailing system, by pull-out test. This study, because of limited learning of helical soil nail, aimed to investigate the pull-out resistance by a 3D finite element modeling with abaqus software and compare its results with laboratory data. A review of the literature for the screw soil-nails as well as a comparison of its performance with conventional soil nails is discussed and numerical results of a series of pull-out tests on a screw soil-nail are presented. And review of  the overburden pressure and plate number and plate distance effect is followed. The results show that in helical soil-nail pull-out a high overburden pressure effect can be seen. A semi-linear relationship between peak pull-out force and overburden pressure is observed for different methods of calculating the helical soil-nail capacity that it is indicating that it satisfies the Mohr-Coulomb failure criteria. Rupture surfaces occur at distances farther than the nail surface, and three times the diameter can be considered the optimal distance of the plates. Using fewer plate distance does not increase resistance, also using more plate with fewer distance does not increase resistance. Comparison of modeling and laboratory results indicates that modeling of pull-out test can model the behavior of helical soil-nail and verify its performance in a field soil slope.