Effect of Failure Plane on Slope Stability and Tensile Force Distribution along Soil Nails (Case Study: Slope leading to Ilam-Salehabad Road)

Abstract Background and Instability of slopes leading to roads in steep mountainous areas is a major problem in the development of roads worldwide, causing excessive human as well as financial losses. Soil nailing is one way of in-situ soil reinforcement. The behavior of a reinforced soil system depends on different parameters including geometry of the structure, mechanical characteristics of the soil, density of the reinforcing material, and length of the soil reinforcing material as well as the angle it makes with the failure plane. Though much research has been conducted on earth slope stability, few studies have examined the effect on slope stability of the soil nailing angle and tensile force distribution along the nail. In spite of the extensive studies conducted on slope stability, no specific insight has been obtained so far on the effect of the failure plane or soil nailing angle on the tensile force distribution along the nails. In view of these facts, this study aims to examine the effect of nail angle as well as nail length on the nail safety factor with due consideration of the effect of shear strain distribution on slope stability. Selecting the slope leading to Ilam-Salehabad Road in western Iran as our case study, we studied the stability of this slope at different nail lengths and angles. The slope leading to Ilam-Saalehabad Road (after the Karbala Road tunnel) was selected as the case study. To determine the mechanical parameters of the soil, we provided soil samples from the slope site and tested them at the laboratory in accordance with ASTM code to obtain the required soil characteristics. The powerful geotechnical software FLAC2 was subsequently used for modeling the slope leading to the road. Upon completion of analysis, we compared the settlement obtained from the software at two points on the earth slope with similar measurements obtained from the instruments, and observed a good agreement between them, with an approximate maximum error of 3%. In the following, the effect of soil nailing angle (with the horizontal line) as well as the length of the nail on the nail safety factor and nail tensile force is discussed. Discussion and Our results showed that increasing the soil nailing angle (i.e., nail driving angle defined as the angle between the soil nail and the horizontal line) from 0 to 30 degrees would increase the nail safety factor by about 23%. Thereafter, increasing the nail driving angle from 30 to 45 would cause a reduction of 2.8% in the safety factor. A further increase of nail angle (with the horizon) caused a corresponding increase in the tensile force induced in the nails, so that the maximum tensile force at 30 degrees increased by about 6%. The maximum nail driving angle efficiency was observed at the points undergoing maximum shear strain: increasing the drive angle to 30 degrees led to a 10-fold increase in the tensile force developed in the nails at points with maximum shear strains. On the other hand, increasing the nail length increased nail safety factor, so that increasing the nail length by 1 meter would increase nail safety factor by 4.3%. However, increasing the nail length beyond 1 meter reduced the rate of increase of the safety factor. Accordingly, the optimum length increase in the nails was taken as 1 meter.