This paper aims at testing and numerical modelling of soil slopes reinforced by using screw nails. Two model soil slopes at 45° and 90° with the horizontal are stabilized by inclusion of six screw nails at 0° inclination with horizontal. The screw nailed soil slopes are subjected to an increasing surcharge load at slope crest and their failure mechanism is studied. The present investigation also examines the failure load and displacement, failure surface and volumetric deformation of screw nailed slopes. Numerical modelling of the two screw nailed slopes using limit equilibrium method and generalized finite element analysis method has also been carried out. Factor of safety along with failure slip surfaces has been analysed by limit equilibrium method. Generalized finite element analysis is used with strength reduction method to determine factor of safety and slip surfaces. The slope displacement, failure load and volumetric deformation of these two screw nailed slopes are also studied by generalized finite element analysis method. The model testing results are validated by its numerical modelling. It is found that factor of safety, slip surfaces, load–displacements from testing and modelling are in accordance with slight variations. A comparative study between factor of safety and load–displacement of slopes with smooth nails and screw nails under study is also done. A higher factor of safety and a decrease in slope displacement is observed for screw nailed slopes. With the advantage of ease of installation, the screw nails are found to provide sufficient slope stability for the two modelled slopes.

Testing and Modelling of Screw Nailed Soil Slopes

The slope stability was researched over the past decades, understandings the behavior ofnailed soil slopes, with their properties and characteristics, have guided to development of enhancedrecognizing of the variation in nailed soil slopes parameters. Failures can arise over time, hence theidentification of the necessities and the restrictions of laboratory and in situ testing for evaluating nailingparameter have to be deeply understood. Many researcher have enriched and refined the knowledges of theprinciples of soil nailing mechanics that associate soil behavior to slope stability and factor of safety. Thispaper is aimed to explore some of important state of the art development of nailed soil slope and analyticaltechniques extended by extensive investigation of the mechanics of slope stability analyses. The mostsignificant effective parameter that play important role in slope safety factor in almost of many works arelength, nailing angle, and distance between center to center of nails. Slope angle, connection of nailed head,the distance with toe and nail diameter were also the points that cannot be neglected in their study. Choosingthe proper cable or pile element in the finite element modeling and the mobilized both axial and shear forcesare also reviewed in this paper.

On the nailed soil slopes research development

Stabilization of soil nailed slope by flexible materials

Because geotechnical engineering requires that the materials and their configuration be investigated for each new project, the profession has always placed great emphasis on judgment. In this environment the computer, and mathematical methods in general, have traditionally been viewed with some suspicion. The ready availability of microcomputers has given engineers access to a very powerful tool, but the most popular uses remain word processing and spreadsheets, even for engineers heavily involved in analytical work. Primarily because of economic constraints, the geotechnical community has not developed much modern software designed for its peculiar needs. Other recent developments in computer applications are the practical application of artificial intelligence, the expansion of capacity for three-dimensional graphics, and the development of relational databases. These are now being used in practice in many fields, and they will have an impact on the practice of geotechnical engineering before long. Examples of recent applications in practice are illustrated.

Geotechnical Engineering Design in the Age of the Modern Computer

Although a variety of research works have been carried out to investigate the behavior of nailed slopes and nail walls, only a few studies are devoted to the optimum design of soil nail walls. In this study, the limit equilibrium-based approach presented in FHWA manual, which is the prevalent design method in conjunction with nonlinear programming, has been used. Based on this approach, the optimum nail inclination angle (η) leading to the maximum safety (FS) factor against overall failure was determined for typical soil nail walls. Effects of some primary contributing factors such as nail diameter, nail length, soil friction, slope angle, back slope angle, and layout of nails on the optimum design indices were investigated thoroughly and presented in dimensionless graphs. Results indicate that increase in the nail diameter gives rise to an increase in both FS and ηopt. In addition, increasing the length of nails up to 1.875H leads to the most significant improvement in slope stability. Furthermore, the increase in soil friction results in an improvement in FS and a limited rise in ηopt. Steeper nail walls are less stable and require higher ηopt, a linear function of slope orientation (α). A higher back slope angle was found to reduce FS. Furthermore, it was found that inserting the nails in the lower 1/3 part of the slope leads to the highest efficiency, and the nails have a minor influence on the stability of short walls or highly cohesive slopes.

Optimization of Nail Inclination Angle in Soil Nail Walls Based on a Prevalent Limit Equilibrium Method

Slope failures occur when the shear resistance along the slip plane is exceeded. This can be caused by excessive load imposed at the slope crest or compromised stability of the slope, e.g. disturbed dimensions of the slope. In order to prevent slope failure, stabilisation or reinforcement measures need to be taken. A common solution is to intercept the slope failure plane with reinforcement elements, such as soil nails and ground anchors. In soil nailing, reinforcement bars are installed on the slope to effectively resist the additional shear forces from the imposed loads, hence reducing the probability of failure in the long run. This paper describes the innovation of soil nail with screw-in installation mechanism instead of the conventional push-in approach. The screw-in installation ensures better soil-nail grip and less disturbance during the slope stabilisation procedure, especially in terms of noise and spoils. In addition, the novel nail has a hollow stem which improves shear resistance with greater soil-nail surface contact on the inner wall. The opening at the nail head also enables displaced air to escape as the nail is screwed into the slope and soil pushed into the inner hollow cavity. The prototype nails were tested in a slope model with different configurations, and were found to reduce the Angular Distortion Ratio by 37 % and the Volumetric Deformation Index as much as 33 % respectively. The novel screw-in soil nail could be potentially used to stabilize natural and man-made slopes, though full-scale simulations are recommended to formulate the installation procedure and to validate the effectiveness.

Screw-in soil nail for slope reinforcement against slip failure: a lab-based model study

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Screw-in soil nail for slope reinforcement against slip failure: a lab-based model study