Civil Engineering April 2021 | Vol 29 No 3

18 April 2021 Civil Engineering In 1986, a limnic eruption at Lake Nyos produced a large cloud of carbon dioxide (CO 2 ), killing 1 746 people and 3 500 livestock. Limnic eruptions, also known as lake overturns, are a rare type of natural disaster in which dissolved CO 2 suddenly erupts from deep lake waters, forming a gas cloud capable of suffocating wildlife, livestock, and humans. The dam’s purpose was later expanded from pure flood protection to include water storage for domestic and agricul- tural purposes as well as hydropower generation, which resulted in the dam’s storage capacity being increased. The pro- ject also included network integration that comprised approximately 210 km of trans- mission lines plus six new substations. Kashimbila Dam is a 32 m high composite dam comprising a 1 530 m long clay core rockfill embankment, with a 150 m long central mass concrete gravity spillway. A 40 MW hydropower station is included. The general layout is shown in Figure 3. GENERAL GEOLOGY AND SEISMICITY The dam is located in an area underlain by Precambrian undifferentiated Basement Complex, predominantly com- prising granite gneiss which is intruded by younger dolerite dykes (Figure 4). Characteristic orthogonal linea- ments present structural controls on the drainage pattern that are clearly visible in satellite imagery (Figure 5). GGS (2009) indicated that the area of interest is located between the Benue Trough, an ancient and non-active rift system, and the Oku Volcanic Field in Cameroon. The Precambrian crystalline basement rocks on which Kashimbila Dam is located are typically stable, non- seismic areas. In any event, the seismic activity in Nigeria is considered low (GYA, 2009). Peak ground accelerations (PGA) for the area of interest are less than 0.2 m.s -2 , with a 10% probability of exceed- ance in a 50 year period. This equates to a very low seismic hazard. FOUNDING CONDITIONS The first phase of construction focussed on constructing the spillway and appur- tenant works, as well as the embankment sections outside of the river course. Once the spillway was completed the river was diverted through this area, allowing the embankment to be completed in the original river course. Conditions within the original river were not as favourable as might usually be encountered in such a major river. The granite gneiss proved to be highly frac- tured and quite weak in places (Figure 6). Foundation preparation included signifi- cant excavation, and a comprehensive programme of foundation grouting. The footprint of the hydropower station is located wholly within the boundaries of an intrusive dolerite dyke, and the entire excavation was within the dolerite. In terms of geological structure, the dolerite dyke was intruded verti- cally into the gneiss host rock, and the contacts were noted to be vertical and tightly sealed. The dolerite rock mass is generally unweathered, although during the initial foundation clearing, early-stage core-stone development was noted in the upper, weathered zones. Conditions Figure 3 General layout of Kashimbila Dam Figure 4 Regional geology of undifferentiated granite gneiss basement (excerpt from Geological Map of Nigeria, 2009) Kashimbila Dam is a 32 m high composite dam comprising a 1 530 m long clay core rockfill embankment, with a 150 m long central mass concrete gravity spillway. A 40 MW hydropower station is included. Return channel Hydropower station Spillway with diversion culverts Embankment