EVALUATION OF SEEPAGE, SLOPE STABILITY AND DEFORMATION IN MBANKMENT DAM UNDER STATIC CONDITION (CASE STUDY: GRINDEHO EARTHEN DAM EASTERN TIGRAY, ETHIOPIA)

Abstract

Nowadays, monitoring seepage, slope stability and deformations is the primary and most important reason to ascertain the overall safety of earthen dams. This research study was conducted, with the objective to evaluate static condition seepage, slope stability, and stress deformation performances of Grindho earthen dam which is found in eastern Tigray, Northern part of Ethiopia and to come up with possible helpful remedial measures for the identified safety incidents. The AutoCAD product Geostudio-2007 software was employed to analyse the seepage using SEEP/W, slope stability safety factors using SLOPE/W and stress deformation using SIGMA/W under different cases of operations. Results of seepage analysis using SEEP/W was utilized as a parental input for the analysis of SLOPE/W and SIGMA/W models. Additional actual field data measurements and observatory investigation also carried out. In this study, Seepage analysis such as leakage flow rate, pore water distribution and location of phreatic line are determined using the SEEP/W. From the simulated results the average flow rate of leakage for the entire length of the dam at full reservoir condition is equal to 0.001163 m3 /s whereas the actually measured quantity of seepage accumulated at downstream toe of the dam is 0.00647m3/s. The seepage source has been identified using the electrical conductivity and temperature taken from the reservoir and downstream toe drained water. Slope stability analysis was performed for the critical slopes using the limit equilibrium SLOPE/W software based on the Ordinary, Bishop, Janbu, Morgenstern-Price and Spencer methods safety factors. The stability analysis results indicated that the critical upstream and downstream slope of the dam are safe within the prescribed USACE and BDS rang of factors of safety for the possible loading and operation cases. The result of stress-deformation was analysed using the Finite Element model (SIGMA/W) in the two cases; during the empty or after end of construction and full reservoir conditions and the maximum horizontal and vertical displacements are calculated. The resulted maximum horizontal and vertical settlement for full reservoir conditions is 0.21% and 0.92% of the dam height respectively. According to the (USBR, 2014) standards and (Hunter and Fell, 2003) recommendation the impervious core material is not safe against the inside cracking and hydraulic fracturing due to the horizontal displacement and the dam couldn’t be affected by overtopping problem through the impact of current vertical settlements. Based on the research findings recommendations and possible remedial measures that helps to strengthen the safety of the dam are also provided: