Geotechnical Engineering: Earth Dam Construction and Failure - Case Study Example

 This paper seeks to investigate why earth dams fail, even before their expected life span.The study puts more emphasis on the dams in Nigeria with a major focus on Cham Dam in Gombe, to be used as a case study. The causes of its failure resulted from engineering problems. This dam lasted for only six years. The main reasons of its failure could be attributed to poor planning, insufficient study, poor design, coupled with the lack of proper maintenance. To prevent such drastic outcomes in the future, it has been suggested that good design, appropriate planning and proper maintenance programs be put in place (Umaru, Ogedengbe, & Omobowale 2010: 1). This paper, therefore, seeks to identify these factors, right from dam design, and construction up to the time it fails. Earth dams are constructed out of layers of tightly compacted earth, rock and clay, making them quite economical to construct. This is because they are constructed from materials which are close to the site, drastically reducing the construction cost. The earth dams are used for storage of water, generation of hydro-electric power or even flood control. Water being a very valuable natural resource, and as researched by Umaru et al., (2010), water forms a fundamental resource in that humans have to use water in their entire production activities (p. 47). The world fresh water represents only a 3% of the world’s total amount of water. The need to conserve this water creates the need for constructing earth dams, which are also called embankment dams because they are built in the shape of the embankment (Raul & Norma 2011: 1). The dams are constructed for the interception of runoff and creation of a reservoir. A dam is constructed with the aim of lasting for a long time. Properly constructed dams have a life span of more than 25 years (Kosamu 2013:177).To ensure the safety of an earth dam, and then a well thought and calculated design should be followed to ensure safety in case of a draw down (Umaru et al., 2010: 47). In Nigeria, 81% of the dams constructed by the Ministry of Agriculture and Water Resources are earth dams. The dams gained popularity after the creation of River Basin Development Authorities (RBDA), under the same ministry by decree number 25 of 1976. Cham multipurpose dam was constructed by the upper Benue RBDA in the Cham village of Gombe state. The dam was constructed through direct labour at a cost of N8.9 million and had a storage capacity of 8 million cubic meters. It was constructed to supply irrigation water to Cham village, provide a reliable source of water to the village people, and to contain floods for the people downstream. The dam.lying between latitude 8 degrees North and 12 degrees of the Greenwich Meridian was commissioned in December 1992, only to fail in September 1998 (Umaru et al., 2010: 47). Most earth dams fail resulting from embankment and foundation piping, overtopping, differential settlement and cracks, embankment and foundation slides, slides during construction, earthquake damage, reservoir wave construction, damage from burrowing animals, damage from water soluble material, flow slides from liquefaction and damage from surface drying (Umaru et al., 2010: 47). In general, earth dam failures result from three main categories; overtopping failures, seepage failures and structural failures. These types of failure are however interrelated in a complex manner. About 35% of the earth dam failures are hydraulic failures, seepage failures amount to 30% and 20% are from structural failure.7% of the failure result from other miscellaneous causes such as accidents and natural disasters (Pagano, Sica, & Desideri 2006: 88). From two design drawings of the Cham dam, of reference numbers, UBRBDA-HQ072/95 and UBRBDA-HQ081/96, it was noted that there were marked differences between them. Comparing the two, it showed that there was a difference between them in that the crest width was reduced from 10m to 8m which reduced its retention capacity (Umaru et al., 2010: 46). The crest of the dam was designed to have a lateritic material overlying a layer of compacted gravel stones. The main trench was designed in such a way that it was to be constructed to as much low as 2.5 below the original ground surface. It had an impervious core and a key trench both made up of clay. It however had a semi-pervious material bordered by mass boulders of rock embedded in clay soil. This eliminated the need for a key trench since it already had a rocky foundation. The dam also had a non-uniform crest level, ranging from about 522.2 meters to 524.78 meters above sea level, instead of the uniform crest level of 523 meters above sea level. All these shortcomings in the design of the dam led to the following weaknesses resulting to the eventual failure. About 500 meters away from the crest crack; there was seepage at the downstream toe. As a result, excess pore water pressure induced by shock vibration from a landslide resulted to seepage and foundation movement. Incomplete construction of the spillway and the incomplete crest also led to major weaknesses in the dam. Human activities could also be attributed to the failure of the dam. At the upper side of the dam, farmers would harvest water, store it in small ponds for farming, and when the floods of 1998 came, the farmers released the stored water, resulting in increased volume of the water which eventually led to overtopping of the dam leading to a spill over. The above mentioned failures of Cham dam can be summarized into three main categories as; hydraulic failure which resulted from overtopping. The second one was seepage failure, which resulted from piping at the downstream of the dam. The third one was structural failure which resulted from the crest breaking at various points. Resulting to a complete breach of the embankment. Dam failure should be avoided at all costs since its results are just destructive. This is because this failure results in land degradation in that it causes soil erosion, which normally results from breaching, or poorly designed and constructed spillways it was the case with Cham dam. Breaching may also result from seepage, construction flaws, overtopping or siltation (Zhang, Xu, & Jia 2009: 186). To sum this discussion up, from the above two mentioned case studies, it is, therefore, very important that earth dams be regularly inspected and maintained by qualified engineers. This is because many dams fail as a result of human errors or natural factors such as earthquakes. Human errors include inadequate preliminary study of the area of dam construction, loopholes in the design, and poor engineering applications during construction or poor maintenance culture. A case in point is the Cham dam in Nigeria, which was attributed to hurried commissioning of the dam even before the achievement of the desired crest level (Umaru et al., 2010: 52). To avoid dam failures in the future, adequate study should be done about the area in terms of the hydro-meteorology, geology and also the soil type. Moreover, there ought to be strict adherence to the project right from its conception, implementation and maintenance. Bibliography KOSAMU NYONI (2013),Environmental Impacts of Earth Dam Failures and Spillway Malfunctions. Greener Journal of Physical Sciences. Vol. 3(5), 177-186. ISSN: 2276-7851 PAGANO, L., SICA, S., & DESIDERI, A. (2006). Representativeness of measurements in the interpretation of earth dam behaviour. Canadian geotechnical journal, 43(1), 87-99. RAUL FLORES-BERRONES & NORMA PATRICIA LOPEZ-ACOSTA(2011)Internal Erosion Due to Water Flow Through Earth Dams and Earth,Mexican Institute of Water Technology Institute of Engineering, National University of Mexico(UNAM)Mexico . ISBN: 978-953-307-710-9 UMARU, A., OGEDENGBE, K., & OMOBOWALE, M. O. (2010). Structural failures of earth dams in Nigeria: A case study of Cham Dam in Gombe State. Journal of Engineering & Applied Sciences, 5(11). ZHANG, L. M., XU, Y., & JIA, J. S. (2009). Analysis of earth dam failures: A database approach. Georisk, 3(3), 184-189. Read More