FAILURE INVESTGATION OF EARTH FILL DAM, THE CASE OF ZANA DAM IN AMHARA RIGEN, ETHIOPIA

Abstract

construction of dams serves a number of purposes such as water supply, irrigation, hydroelectric power generation, flood control and navigation etc.Zana dam is anEarth Fill Dam which islocated on Zana River used for irrigation purpose. The main problem of this dam is seepage and downstream slope failure.To address this problem evaluate the current dam conditionofdetermination of seepage and slope stability by Geo-studio software and identification of downstream slope failure and seepage through the body of the dam were assessed . Analysisofseepagewasdoneusingseepage and slope analysismethods which integrate Geo-studiosoftwareof Seep/w and slop/wtoolsatnormalandcurrentpoollevelcondition, the study was conducted mainly based on the laboratory investigation of materials used for construction. The result demonstrated that there wasa material property gap between what was stated in the design report and actually used in construction. The amount of seepage generated from the analysis was found to be1.406*10-7m3/s/mhowever from the actual constructed was found to be1.683 *10- 3 m3/s/m.Accordinglyfrom slope stability analysis the factor of safety of=1.335 and FS=1.193 for the designed and constructed sections respectivelywhich were less than 1.5.On the other handusing the newly proposed embankment section and the material property analyzed the seepage quantity through the embankment body found to be = 2.9218x10-7 m3/sec/m and the minimum factor of safety of = 2.127 and 2.285 with steady state condition upstream and downstream slop respectively, and factor of safety of =1.963 for using both horizontal and vertical seismic action. Similarly the major finding of the cause of failure is absence of proper filter and drainage materials. Result of gradation analysis of both base and shell materialsdemonstrated thatD15 (shell) =3 mm & D85 (core) =0.2mm this resulted 0.6mm>0.2mm which yielded that there was piping or internal erosion of the base material. Consequently the maximum and minimum bounds of filter materials obtained were: D60min=0.5mm, D15min=0.1 mm, D5min=0.075 mm, D100max=75 mm, D90max=20 mm, D60mm=2.5 mm & D15max=0.5mm obtained.Thereforecheminefilter material design and provision is mandatory for the safe life of the zoned type earth fill dam.