1 Page 1-11 © MAT Journals 2019. All Rights Reserved Journal of Geotechnical Studies Volume 4 Issue 1 Effect of Width, Length and Position of Cutoff Wall on the Seepage Characteristics of Earth Dam 1 Md. Mahmud Sazzad, 2 Md. Mohayminul Islam 1 Professor, 2 Undergraduate student Department of Civil Engineering Rajshahi University of Engineering & Technology, Rajshahi, Bangladesh Email: 1 mmsruet@gmail.com, 2 mohayminul.islam37@gmail.com Abstract Seepage is one of the main causes of failure of earthen dam. To encounter the seepage through the foundation of dam, construction of cut off wall is a frequently used and a conventional technique. In this study, several numbers of numerical analysis are performed against different widths and lengths of the cut off wall for different positions along the cross section of the dam to find out the appropriate one. All these analyses are performed for a homogeneous dam with foundation soil being clay and silty clay. Numerical analyses are conducted by SEEP/W (a sub program of geo studio). The numerical results depict that the best possible position of the cut off wall is at the downstream toe of the dam. This is because, in this position, most number of flow lines gets encountered by cutoff wall. It is also observed that the effect of the width of cutoff wall on the total head and pore water pressure (PWP) is negligible but it affects the velocity of seepage and total discharge. On the other hand, the length of the cutoff wall controls all these flow properties. Clay foundation provided a better result than silty clay foundation for all the cases to control the seepage through the foundation of the earth dam. Keywords: Cutoff, Dam foundation, Seepage control, earthen dam. INTRODUCTION Dam is a structure to retain water in the reservoir to protect dwellings from flooding, changing the direction of stream, hydro electricity project, holds back water for irrigation for the dry season etc. Out of numerous types of dam, earthen dam may be the most popular, economical and conventionally used technique. According to Zhang et al. [1], 66% of the dam is earth dam among the different types of dam like earth dams, concrete dams, masonry dams, rockfill dams, and so on. As the soil is a permeable material, the seepage through the dam is not at all an unnatural event but unnatural is uncontrolled seepage. Uncontrolled seepage may lead the dam to piping, internal erosion, sliding and sloughing failure, develop excessive internal pressures and/or saturation and excessive uplift, heave, or blowout [6]. For homogeneous earth fill dams and zoned earthfill dams, piping in the dam body/foundation is a dominant failure. Piping through dam body and foundation is responsible for 58.3% of dam failure [1]. According to Lukman et al. [3], dam failure occurs due to leakage and piping (35%), overtopping (25%), spillway erosion (14%), excessive deformation (11%), sliding (10%). Garg [4] reported that 35% of failures of earth dams are due toseepage failures. Investigations result by Arora [5] also showed that about 35%, 30% and 20% of failures of earth dams are due to hydraulic, seepage and structural failures, respectively. To mitigate the failure due to seepage, the researchers all over the world suggested numerous types of solutions. For example,