INFLUENCE OF ENGINEERING PROPERTIES OF THE SOIL, GEOLOGICAL AND HYDROMETEROLOGY FACTORS ON FAILURES AND DISTRESS OF EARTH DAMS IN NORTH-EASTERN NIGERIA

Abstract

Earth dam failures could result in the loss of lives, damage to properties, health, environmental and social problems. Distressed dams cost a lot of resources and inconveniences to remediate. There is paucity of data on failures and distresses of many earth dams located in the north-eastern part of Nigeria. This study was designed to determine the geological, hydrometeorological, engineering factors and soil properties responsible for the failures and distresses of earth dams. A total of 42 randomly selected earth dams spread across various geologic formations and constructed with different soil materials in north-eastern Nigeria were studied. Data were obtained on failure modes, design and construction features, operation and maintenance, dam safety instrumentations and operations using the Association of State Dam Safety Officials method. Geological and hydrometeorological data related to dam failures, distresses and functionality were obtained from Upper Benue River Basin Development Authority, States Ministries of Water Resources and Nigerian Meteorological Agency. Soil samples collected were analysed for specific gravity (Gs), particle size distribution, Atterberg limits, compaction, California Bearing Ratio (CBR), permeability, triaxial compression and consolidation tests according to BS1377. The results were analysed using descriptive statistics. The proportions (%) of failed, distressed, uncompleted and functional dams were 27, 12, 12 and 49 respectively. The failure modes were; seepage (5%), piping (8%), structural (1%), hydraulic (50%) and a combination of two or more modes in a complex manner (36%). The main causes of failure were inadequate maintenance (71%), lapses in design (9%) and poor construction (15%). On the Basement complex formations, 62, 27 and 11% of the dams were functional, failed and distressed respectively. All the dams on Gombe sandstones and Pindiga formations are functional. The status of the dams were affected by peak monthly total rainfall (327.1–478.8mm) where 75% of the failures and distresses occurred due to high runoff, erosion, siltation and overtopping, while 20% of the failures occurred due to excessive water loss as influenced by peak monthly total evaporation ranging from 354.6-409.7mm coupled with relatively high temperatures (39–43oC). Soil groups for constructing the earth dams in the study area ranged from poorly graded sands to silty/clayey sands. Seventy-nine percent of the failed and distressed dams have embankment materials with Coefficient of uniformity of less than 5. Sixty-five percent of failed and distressed dams have Plasticity Index of 0-7. Eighty percent of functional dams have highly compacted soils with maximum dry density ranging from 1.84 to 2.01Mg/m3. High permeability ranging from 0.018 to 0.110 m/day influenced 33% of dam failures. Consolidation tests showed a settlement of 1.18mm and 2.29mm for functional and failed dam respectively. The Gs (2.41-2.70) and CBR values (11-46%) as well as cohesion (35-215kN/m2) and angle of internal friction (3-18o) influenced particular incidents without a trend. Geologic formations, weather conditions, lapses in design, poor construction, maintenance, operation and poor soil characteristics influenced the status of earth dams in north-eastern Nigeria. Grouting, soil stabilisation, use of rock ripraps, impervious blankets and maintenance scheduling are suggested to minimise failures and ameliorate distresses.