applied sciences Article An Experimental Study of a Nailed Soil Slope: Effects of Surcharge Loading and Nails Characteristics Mahmoud H. Mohamed 1 , Mohd Ahmed 1,*, Javed Mallick 1 and Pham V. Hoa 2 1 Civil Engineeg Department, College of Engineering, K. K. University, Abha 61421, Saudi Arabia; [email protected] (M.H.M.); [email protected] (J.M.) 2 Ho Chi Minh City Institute, Vietnam Academy of Science and Technology, Ho Chi Minh City 008428, Vietnam; [email protected] * Correspondence: [email protected]; Tel.: +966-172-418-439 Abstract: The earth nailing system is a ground improvement technique used to stabilize earth slopes. The behavior of the earth nailing system is dependent on soil and nailing characteristics, such as the spacing between nails, the orientation, length, and method of installation of nails, soil properties, slope height and angle, and surcharge loading, among others. In the present study, a three-dimensional physical model was built to simulate a soil nailed slope with a model scale of 1:10 with various soil nail characteristics. The simulated models consist of Perspex strips as facing and steel bars as a reinforcing system to stabilize the soil slope. Sand beds in the model were formed, using a sand raining system. The performance of nailed soil slope models under three important nails characteristics, i.e., length, spacing and orientation, with varying surcharge loading were studied. It was observed that there is a reduction in the lateral movement of slope and footing settlements with an increase in length. It was found that the slope face horizontal pressure is non-linear with different nail characteristics. The increase in length and inclination of the soil nails decreased the Citation: Mohamed, M.H.; Ahmed, vertical, horizontal stress and footing settlement, while the increase in spacing of the nails increased M.; Mallick, J.; Hoa, P.V. An the vertical and horizontal stress behind the soil mass. Experimental Study of a Nailed Soil Slope: Effects of Surcharge Loading Keywords: soil nailing system; nails characteristics; soil slope; surcharge loading; nails orientation and Nails Characteristics. Appl. Sci. 2021, 11, 4842. https://doi.org/ 10.3390/app11114842 1. Introduction Academic Editor: Daniel Dias The soil nailing system is a technique for stabilizing earth slopes and it is constructed in successive phases, as first illustrated by Stocker et al. [1], from top to bottom as ex- Received: 24 April 2021 cavation proceeds. A soil-nailed slope consists of three main elements, namely, the soil, Accepted: 19 May 2021 Published: 25 May 2021 steel reinforcements bars (nails), and the facing. The elements’ composite interactions determine the deformations and stability behavior of a soil nailing system. The shear and Publisher’s Note: MDPI stays neutral tensile strengths of the reinforcing element will increase the overall shear strength and with regard to jurisdictional claims in self-supportability of the in situ soil. The function of the facing in a soil nailing system published maps and institutional affil- is to ensure the local stability of the ground between the nails and to limit its decom- iations. pression; the facing should be flexible enough to withstand ground displacement during excavation [2]. Sharma et al. [3] have thoroughly reviewed the advancements in the soil nailing technique, different installation processes, failure modes of soil-nailed structures, design philosophies, effects of various construction parameters on the design method, and the analytical, numerical, field and lab testing procedures in the nails’ pull-out capac- Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. ity. Lazarte et al. [4] submitted state-of-the-art practice documents (NCHRP Project 24–21) This article is an open access article related to the analysis, LRF design, and construction of soil-nailed walls for highway appli- distributed under the terms and cations. The accuracy of the pull-out capacity of the nails will result in the economical and conditions of the Creative Commons safe design of the soil-nailed structure [5]. Yelti [6] analyzed and designed soil nailed walls Attribution (CC BY) license (https:// in expansive soils and the recommended guidelines to improve the design procedures for creativecommons.org/licenses/by/ soil-nailed walls, considering the lateral pressures generated by soil expansion. A simpli- 4.0/). fied method to determine the movement at the crest of a soil nailed wall is developed by Appl. Sci. 2021, 11, 4842. https://doi.org/10.3390/app11114842 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 4842 2 of 28 Bridges et al. [7] who concluded that soil effective cohesion has the greatest effect on crest displacement of the wall. Centrifuge and numerical tests were conducted by Viswanadham and Rotte [8] to study the significance of slope facing on the stability and deformation behavior of soil-nailed slopes subjected to seepage. They found that soil-nailed slopes without facing experience face failure due to bearing failure at the slope surface and the nail head surface as well as due to the build-up of excess pore water pressure at the toe region. The influence of the bending stiffness of the facing material—installed on a nailed slope surface of dense sand—on deformation and bearing capacity characteristics were studied by Kotake and Sato [9] by conducting tests on rigid and flexible models of footings. Several nailed soil-retaining structures were instrumented to establish a database for the evaluation of structure performance and development of reliable design methods. The material point method-based numerical model was developed by Ceccato et al. [10] to study the application of plate anchors for landslide stabilization. The numerical model was validated with the results of some small-scale laboratory tests. Sahoo et al. [11] caried out the shaking table tests to study the seismic behavior of nailed soil slopes. They found that the model nailed slope with a nail inclination of 15◦ provided a better reinforcement action than the horizontal and 30◦ inclined nails when stabilizing soil slopes. The failure modes of the slope stabilized by a frame beam with prestressed anchors were studied by Zhang et al. [12] on a large-scale shaking table. They observed that the seismic damage of the slope stabilized by the frame beam with prestressed anchors mainly occurs at the top of the sliding mass and on the free surface of the slope. A simplified limit equilibrium method is presented by Deng et al. [13] to analyze the stability of slopes reinforced with anchor cables, considering the nonlinear Mohr–Coulomb strength criterion for shear failure of the slip surface. The seismic design of the soil nailing was carried out by Villalobos et al. [14] with varying geometrical and mechanical parameters for the nail and soil of a slope. They observed that the global factor of safety increased with soil cohesion and friction as well as the nail length, diameter and inclination; nail inclinations higher than 15◦ reduced the factor of safety. Yuan et al. [15] proposed a new approach for the stability analysis of soil-nailed walls and concluded that the shear strength of soil has a significant effect on the factor of safety and the reliability index of soil-nailed walls. Durguno˘gluet al. [16] presented the performance of very deep, temporary soil-nailed walls constructed in a very seismically active region. The displacement data for various projects were evaluated in terms of various design parameters of the soil-nailed walls and the excavation depth. Babu and Singh [17] studied the performance of a soil-nailed wall supporting a vertical slope under both static and seismic conditions by observing and computing the maximum lateral displacements, development of nail forces and failure modes of soil-nailed walls. Numerical analyses of the stabilizing mechanisms of loose fill slopes were carried out by Cheuk et al. [18] to study the influence of hybrid nail orientations on the behavior of the ground nailing system and concluded that the hybrid nail arrangement would limit slope movement and enhance the robustness of the system. Taule [19] applied Bishop’s Simplified Method to evaluate and rank the sensitivity of the soil properties, namely cohesion, unit weight and internal friction angle, with respect to the global factor of safety, using the Monte Carlo simulation in a soil nailing design. He showed that the internal friction angle is the most sensitive parameter, while cohesion and unit weight are less sensitive with respect to the global safety factor. Jayawickrama et al. [20] developed non-destructive test methods for evaluation and checking the integrity of installed soil nails. Villalobos et al. [21] present the re-assessment stability analysis of soil nailing design and construction in heavily weathered granite (residual soil), using a limit equilibrium sliding block method (bi-linear failure surface). They found that even after a maximum acceleration of 0.63 g of a stronger earthquake, the nailed wall did not show any damage, probably due to the use of undrained shear strength parameters. Alhabshi [22] studied the hybrid, mechanically stabilized earth and soil nail wall system and proposed the optimum length of nails required to stabilize the structure Appl. Sci. 2021, 11, 4842 3 of 28 and reduce the wall deformation. A slope stability analysis of a combined system of soil nails and stabilization piles on loess soil is presented by Wu et al. [23]. Benayoun et al. [24] proposed the optimization of soil nailing parameters for soil-nailed structure design, using the genetic algorithm method. A composite silt–soil nailed symmetrical foundation pit was numerically investigated by Han et al. [25] for stability and deformation characteristics. They found the critical values of inclination and spacing of the soil nails, and the diameter and embedded depth of the mixing pile for stability of the foundation pit.