Page - 975 - in Book of Full Papers - Symposium Hydro Engineering

2. UNCERTAINTY AND RISK In contrast with some engineering fields dealing with man-made materials, geotechnical engineers deal with geometries and materials provided by nature. These natural conditions are unknown to the designer and should be inferred from limited and costly observations. The principal uncertainties have to do with the accuracy and completeness with which subsurface conditions are known and with the resistances that the materials will be able to mobilize. The uncertainties in geotechnical engineering are largely inductive: starting from limited observations, judgment, knowledge of geology and ecology, and statistical reasoning are employed to infer the behavior of a poorly-defined universe [3]. In engineering contexts, risk is commonly defined as the product of probability and consequence, or expressed another way, risk is taken as the expectation of adverse outcome. Risk assessment provides proper opportunity to experts with considering uncertain data related to slope safety decisions, to evaluate qualitative and quantitative assessment of slope safety. Hence, experts finally take appropriate economical and practical decisions. Quantitative risk assessment includes risk analysis, risk assessment and management. Risk management is consideration of the risk analysis along with risk control that threatens safety. Risk control is one of important parts of safety management, which includes review of alternatives in dealing with risks such as risk mitigation, risk acceptance and risk avoidance[4,5]. 3. VEGETATION EFFECT To assess the safety of a slope to prevent human casualties and economic losses, it is necessary to find out the ways in which soil and vegetation interact. The most important and general problem is a shallow seated instability of a slope, that is at depth of around 0.5-2 m below the ground surface and that this is in fact the most widespread form of slope failure particularly in embankments [2]. Depending on the potential slip surface, the factor of safety (FOS) varied from 2.8- 3.7 and 1.8-2.0 for unrooted soil. When the mean value of the FOS increased significantly for surface depth of 0.3m, albeit as distance progresses to a depth of 1.2m these benefits diminish [6]. The stability of slopes is governed by the load, which is the driving force that causes failure, and the resistance, which is the strength of the soil-root system [7]. Based on Coppin and Richards (1990), main influences of vegetation on the stability of the slope segment are from enhanced soil cohesion due to soil reinforcement by roots and tensile root force acting at the base of the slip plane [8]. The FOS for a slope may be calculated using the infinite slope model [1] by Eq. 1. 𝐹 𝑂 𝑆 = 𝑐 +(𝛾 𝑧 −𝛾 𝑤 ℎ)𝑐 𝑜 𝑠 2𝛽 𝑡 𝑎 𝑛 𝜑 𝛾 𝑧 𝑠 𝑖 𝑛 𝛽 𝑐 𝑜 𝑠 𝛽 [1] 975