Modelling the effect of horizontal sand blanket on the stability of a heterogenous earth dam against piping failure under steady seepage condition

GB3 Water storage Earth Dam is a 301m long, 101m wide and 18m high geotechnical structure constructed by Sierra Rutile Mining Company in Sierra Leone. This is a zoned earth filled heterogeneous embankment dam with an upstream slope of 1:3 and downstream slope of 1:2 with a horizontal sand blanket. The reservoir capacity of the dam is 5,386,000m3. Over the years, piping effect has been the most prevalent cause of failure of most heterogeneous Earth dams. Existing earth dams have suffered a lot from this effect.  As a result of this a comprehensive analysis in modelling the effect of horizontal sand blanket on the stability of heterogeneous earth dams against piping failure was carried out. The quantity of water seeping through the dam was estimated using SEEP/W software alongside Slide 7 software which helps in estimating the critical hydraulic gradient used in evaluating the stability nature of the dam against piping. The heterogeneous earth dam was initially modelled without the inclusion of a horizontal sand blanket. The seepage quantity estimated was0.00511×10-6m3s   per metre length of dam with a maximum exit gradient of 0.28. This gives a safety factor against piping failure of 3.1.  It was then modelled with the presence of a horizontal sand blanket. In this case, the total seepage quantity was estimated to be 0.141×10-6m3s  per metre length of dam with a maximum exit gradient of 0.15. This resulted in a factor of safety against piping failure of 5.8. The results obtained from the study indicate that the inclusion of a sand blanket gradually increases the quantity of water seeping through the dam but there by providing a substantial safety factor against piping failure.