Al-Rafidain Engineering Journal (AREJ) ISSN (Print) 1813-0526; Vol.25, No.2, December 2020, pp. 1-11 ISSN (Online) 2220-1720 Al-Rafidain Engineering Journal (AREJ) Vol.25, No.2, December 2020, pp. 1-11 Numerical Investigation the Failure Mechanism of Left Slope of Darbandikhan Dam Tareq H. AL-Rahal Suhail I. Khattab Bayer J. Al-Sulaifanie tareqrahal73@gmail.com suhailkhattab@yahoo.com dr.bayar.alsulayvani@uomosul.edu.iq Civil Engineering Department, Collage of Engineering, University of Mosul Received: 25/1/2020 Accepted: 25/7/2020 Abstract The aim of this research is to analyze the stability of the left shoulder slope of the Derbandikhan Dam failure due to rainfall and earthquake. This slope consists of unsaturated low plasticity clayey soil. All analyses were conducted by using the Geo-Studio program. The field survey was carried out for the study site and the soil samples were taken to perform the necessary laboratory tests to obtain the required soil properties. The field conditions (Rainfall and earthquake) were simulated, where the slope was exposed to rainfall at (24) mm / day intensity to various periods (10, 20, 30, 40, 50 and 60) days. A slope stability analysis was performed at end of each period. Moreover, another analyses were performed by exposed slope to an earthquake at end of each rainfall period. Safety factor reached to critical value (1) when slope was exposed to the earthquake after (20) days of rain. Keywords: Left slope of the derbandikhan dam, Rainfall, Earthquake. Copyright © 2020 College of Engineering, University of Mosul, Mosul, Iraq. All rights reserved. https://rengj.mosuljournals.com Email: alrafidain_engjournal1@uomosul.edu.iq ============================================================================= 1. INTRODUCTION Rainfall is important factor that reduces the slopes stability around the world. The soil slopes are unsaturated in general and when exposed to rainfall will cause changes in pore water pressure (suction), water flow and decrease shear strength of soil mass. These were main important parameters related directly with slope stability analyses of unsaturated soil which its surface influence by infiltration and evaporation [1] and [2]. The sources of a dynamic load may be natural, such as earthquakes, or they may be the result of human activities like vibration machines and railways. There are many instruments and types of equipment that can be used to monitor the response of slopes when exposed to earthquakes and to determine the degree of risk experienced by analyzing recorded data. In the absence of these instruments, the numerical analyses of slope stability by using the geotechnical software ( such as GEO-SLOPE) can be used. The laboratory models may be utilized to simulate the behavior of slopes during earthquakes such as a shaking table and centrifuge model [3], [4], [5] and [6]. Several tests to clear the effect of water content on the slopes stability of Yodogawa-levee sand soil by using the physical centrifuge model were conducted by Higo et. al.(2015)[7] . Ghayoomi and Mirshekari (2017) have calculated the effect of soil moisture content on the soil dynamic response when exposed to earthquakes. Srilatha, et al. (2016)[6] have a study to demonstrate the effect of the type and quantity of reinforcement on the seismic response (amplification of acceleration and horizontal displacement) of the unsaturated soil slope. Mohammed (2018)