5-1-Embankment dams with concrete structures
5-1-1-Physical considerations
The type and size of dam constructed depends on the topography and geology of site, stream morphology and the construction materials that are readily obtainable near the site (Emiroglu, 2008). Topography allows determining the first choice of the dam type. In this study, the dam site is a broad valley where embankment dam with central core can be constructed (Emiroglu, 2008, USACE 2004). However, the stream morphology is complex as shown on figures 1 and 2. This behaviour allows combining two concrete (fig. 7) spillways including main and auxiliary spillways with 85 and 102 m of length respectively and embankment sections into one structure (Golze 1977; Goldin and Rasskazov 1992). The dam site climate character well defined wet and dry seasons, thus it may be practicable to construct an earth fill dam only during two dry seasons. This option, if it is considered allowing of extend the construction season. However, a rock fill dam can be constructed during all the time (Fell et al., 1992). These site conditions are favourable to a composite dam including earth and rock fill dams according to the influence of climate (EM 1110-2-2300) and foundation conditions (Edris 1992; Fraser et al., 2001) with soil deposits and Ntem formations respectively as foundation materials. The geological cross sections and borehole analyses display geological units which are mainly composed of two strata namely upper quaternary deposits and lower Precambrian Ntem formations (fig. 10 and 12). The thickness of quaternary deposits increases from left toward right bank (from 1~3 to 13 m; fig. 10a and 10b (from E-E’ to H-H’)) and particularly, this thickness is weak on the river island. After stripping these upper alluvial deposits on the river islands, Ntem formation can emerged (fig. 13). These conditions of site with rocks and alluvial deposits as dam foundation materials (Fraser et al., 2001) can support embankment dams such as rock fill and homogeneous earth dams respectively (EM 1110-2-2300). The Ntem formations appear slightly weathered to fresh rock at depth where are low permeable. Moderate weathered zone in the valley section is only several meters thick generally. Thus, consolidation grout must reach down 5 m deep to enhance foundation rock quality and lower seepage (Edris 1992). Fulfilling this requirement allows that rock fill dam rests on the Ntem formations (EM 1110-2-1901). Based on distances between epicenters (79 - 280 km, tab. 4) and dam site, the dam site is located in an area not very prone to earthquake. This result is further confirmed by field investigations where evidence of intense fault activities was not seen. Consequently, concrete dam is not priority. However, a composite dam with concrete structure is recommended due to good quality and availabity of rocks. It is noteworthy that the river section is situated in the equatorial warm climate zone; in the wide valley area which provides a competent rock and alluvial deposit foundations and also this site provides suitable sand and gravel; thus composite dams or embankment dams such as homogeneous earth dam, rock fill dam with central core, rock fill dam with inclined core with concrete spillways can be constructed in this site.
5-1-2-Mechanical considerations
Previous studies on Ntem formations have revealed that they are good as construction materials (Bisso et al., 2020). This trend is further shown on geological cross sections and borehole analyses that display RQD values between 56 and 100%, weathering grade between high weathered to fresh rock and lugeon values are ranged between 1.99 and 2.90 Lu (fig. 12). These results indicate that Ntem formations require enhancement before to be used as foundation materials. However, the availability of suitable rock (Bisso et al., 2020) and enhancement foundation processes may favour a rock fill dam (Golze 1977; Bureau of Reclamation 1984; USACE 2004; EM 1110-2-2300).
As shown in table 5, the specific gravity values indicates that the soil deposit materials are good to excellent performance in the civil application works (Nwaiwu et al., 2006; Paige-Green et al., 2015; Onana et al., 2016). Wet and dry densities values are lower than those obtained in Yutiao and Da’ao dams (Hao-Feng Xing et al., 2006). This result indicates that soil deposits are medium density. Soil deposits display a low porosity character. Liquid limit values are higher than those obtained on lateritic gravels from northern Nigeria (Chuka Osadebe et al., 2011). Plasticity limit values display that soil materials are classified from low plasticity to plasticity according to Casagrande scale. Liquid limit values in the borrow areas are above 40% (table 5), indicating that soil materials were qualified “erosion resistant”. The deformation modulus values are higher than those (0.5-1.5 MPa) proposed by Messou (1980) and those obtained in lateritic gravels in Burkina-Faso (1.26 MPa; Millogo et al., 2008) and Cameroon (0.88-1.27 MPa, Onana et al., 2015 and 2016). In addition, this result further indicates the relatively clay contents in the soil deposits. The coefficient of permeability displays higher values than those obtained in Kiri dam (from 1.5E-08 to 1.00E-6 m/s; Ahmed Bafeto et al., 2019) and recommended values of 7.00E-10 to 1.00E-06 m/s. This result indicates that studied materials are lesser ability to allow the passage of seeping water if they are used as embankment materials. The breakdown of soil deposits increases the percentage of the particle size smaller than 0.005 mm. This result indicates that the studied soil materials are well-graded particle-size distribution and easy compaction so that they can be used as construction material during dam construction (Hao-Feng Xing et al., 2006). The plasticity index and the clay content of soil from three borrow areas are on the high side with an exception in the borrow area 3 where the clay content roughly met the requirement (tab. 6). However, these soils can be used in the civil application, notably in the base layer of dam and also constitute the transition, cushion and filter materials. In borrow areas B2 and B3, the natural water content is higher than the optimum water content (the investigation period fall within local rainy season), while in the borrow area B1, the natural water content is lower than the optimum water content (the investigation period fall within local dry season). These behaviours show that the studied soil materials are sensitive to water content variations. Overall, the studied Ntem formations and soil materials are good to be used as foundation and embankment materials. Thus, these site conditions may favour an embankment dam as previously retained above.
The total reserve of soil deposits is 182.39 x 104 m3. Natural sand is to about 30.000 m3. The excavated quantity of rock material from the structure foundation is nearly 300 x 104 m3. In the borrow area B3, the thickness of upper residual soil is weak (fig. 10c3) and consequently, this borrow area has been also retained as quarry area after excavating the upper residual soil. The reserves of underlying Ntem formations in this borrow area B3 has been estimated to 237.37 x 104 m3. These results indicate that materials are abundantly available on the dam site and may be used for an embankment dam construction such as homogeneous earth and rock fill dams linked to concrete structures (Golze 1977; Bureau of Reclamation 1984; EM 1110-2-2300). Elastic modulus, Axial compressive and tensile strengths increase with concrete grade whereas poisson’s ratio and linear expansion coefficient remain stable (tab. 3). These results suggest that Ntem formations can be used for processing the concrete aggregate; rip rap materials and crushing rocks (Emiroglu 2008; Bisso et al., 2020). Thus local supply of concrete aggregate, sand and riprap contributed to propose concrete structures associated with a rock fill dam (Golze 1977; Bureau of Reclamation 1984; Ghafoori et al., 2011).
5-2-Memve’ele’s composite dam type: types of embankment dams selected
5-2-1-Comparison, advantage and disadvantage of dam types
Three dams including homogeneous earth dam, rock fill dam with central core and rock fill dam with inclined core (fig. 14) have been selected previously on the base of factors such as topography, geological, geotechnical and foundation conditions, availability of construction materials (Hunter 1979; Emiroglu 2008). However, the comparison between them is required in order to fully select dam types.
General characters of dam types
The dam crest of the earth rock section is 1260 m in length, 10 m in width, and 13.5 m in average height, suggesting that Memve’ele’s dam is classed among high dams (ICOLD). The dam foundation is located on the Ntem formations and soil deposits. The reinforced concrete wave wall has a top elevation of 396.2 m and a dam crest elevation is of 395 m. Except homogeneous earth dam, the dam facing materials are available from the excavation of foundation pit or exploitation of borrow areas (DS-13, 2011).
Characteristics and discrimination of each dam type
Homogeneous earth dam
The upstream and downstream side slopes of earth dam have such a gradient of 1:3 (fig.14a). The upstream dam slope is of dry-laid masonry revetment, and a dry-laid masonry has a thickness of 80 cm, under which the thick transition layer and inverted filter are laid; the downstream dam slope features, 30 cm transition material for protection, slope toe lays upon the arris of body drainage (ICOLD 1989).
Advantages of the homogeneous earth dam consist in singular material, simple construction procedure and dry disturbance; large thickness of impervious part of dam body and relatively small seepage gradient is conductive to stabilize seepage and reduce seepage flowing through the dam body (EM 1110-2-1901; Brian 1989); the impervious treatment measures of dam foundation can be simplified because of long contact seepage path between dam body and dam foundation, and that between bank slope and concrete structure. This dam type can be adapted to a weak foundation such as quaternary deposit soils (EM 1110-2-2300).
Unfortunately, this dam type has some disadvantage aspects including the shear strength of soil aggregates which is less than that of rock aggregate, graves, sands used for the other dam types, so its upstream and downstream dam slopes are gentler than those of other dam types, and the filling quantity is relatively large; the dam body construction is affected by weather and rainfall, which may result in effective workdays and extension of construction time.
Rock fill dam with central core
The clay core wall is arranged on the upstream side of the dam axis, with the horizontal width of 5.0 m on the top, and a bottom connected to the bed rocks of dam foundation. Upstream and downstream side slopes of the clay core wall have a gradient of 1:2 and 1:1.8 (fig. 14b) respectively. On the upstream side of the core wall is the 1.5 m thick inverted filter and the transition layer. On the downstream side thereof is 1.5 m thick inverted filter and the transition layer (Narita 2000). The dam facing is filled using Ntem formation block stones (Golze 1977).
The core-wall lies in the middle of the dam body without relying on the permeable dam facing (Brian 1989), with its dead weight passing to the foundation in itself, safe from sedimentation of dam facing. The core wall depends on the dead weight of core wall filling earth to enable the contact surface between core-wall and foundation to produce a relatively big contact stress, which helps to strengthen the connection between core wall and foundation and improve the permeable stability of contact surface along the dam foundation. In case of drop reservoir level, the water contained in upstream dam facing will be discharge rapidly, conductive to the stabilization of upstream dam slope and homogenizing the upstream dam slope gradient of clay core wall dam or steepen the slope core wall. In view of the relatively low seepage line of the downstream dam, the downstream dam slope can be designed to be relatively steep (ICOLD). Under the condition of the same impervious effect, the clay core-wall dam consumes less earth aggregate than that the sloping core dam does, and the climate has a little impact on construction. It is relatively easy to connect the core-wall on the dam axis to the bank slope and the concrete structures (Golze 1977).
Because the earth aggregates of core-wall and the permeable materials are on the same horizon, different from the sloping core dam, the dam facing of the clay core-wall dam cannot be filled to meet the schedule in the case that the climate has an adverse impact on construction.
Rock fill dam with inclined core
The top of sloping core-wall has a horizontal thickness of 5.0 m, and the bottom thereof is connected to the bedrocks of the dam foundation with 5 rows of consolidation grouting. The upstream and downstream side slopes have a gradient of 1:3 and 1:1.5 (fig. 14c) respectively, filled with inverted filter and transition layer. The dam facing is filled using Ntem formation block stones (Golze 1977).
Where there is difficulty in filling in rainy seasons, the clay sloping core dam is employed, and the permeable materials of the downstream dam facing shall be first filled to meet the schedule. The relatively low seepage line of the downstream dam facing is conductive to the stabilization of the downstream dam slope (ICOLD).
Given the fact that the clay sloping core wall relies on the permeable dam facing, the too much sedimentation of dam facing will lead to the crack of the core wall. The connection between the clay sloping core dam and the bank slope and concrete structures is not easy as that between the core wall dam and the bank slope and concrete structures. The contact stress between the sloping core wall and the foundation is less than that between the core wall and the foundation. Additionally the conditions of connection are not as good as that of core wall dam.
The homogeneous earth dam, the dam with inclined core and the dam with central core have total quantity of impervious material of 675,000 m3, 307,000 m3 and 198,000 m3 respectively, of which the dam with central core has the least filling quantity, and the homogeneous earth dam has the most filling quantity. In view of the impact of rainfalls in rainy seasons upon filling progress and quality of earth and rock fill dam, it is better to reduce the quantity of impervious material as possible (Emiroglu 2008) so the dam with central core is priority to be selected.
The homogeneous dam or dam with central core, which leads easily to junction of embankment with concrete structures such as main/auxiliary spillways (EM 1110-2-2300, Golze 1977) while the dam with inclined core is to join with the concrete structures through a transition zone of homogeneous dam. In such case it seriously interfering the construction.
Comparing with the dam with inclined core, the dam with central core provides higher contact pressure between the core and foundation to prevent leakage and greater stability under loading (EM 1110-2-1901).
Overall, a composite dam including homogeneous earth and rock fill with central core dams linked at concrete structures (fig. 16) has been selected for Memve’ele dam.
5-2-2-Engineering plan map of the Memve’ele main dam
Assuming that the dam axis is topographically flat and the river is divided mainly into the left and right stream (fig. 15). Each stream section is occupied by weak thickness of alluvial deposits very fine sand overlying moderately weathered granitic gneiss of Ntem formation as it is the same in the island river (fig. 1). These behaviours are suitable for arrangement of main spillway, flushing sluice, auxiliary spillway on the right and left side of the main river course upon the weathered granitic gneiss. The geometrical characteristics of Memve’ele main dam are shown in figure 16. The layout of dam axis has both a curved shape with radius of about 400 m and a right line (fig. 16 and fig. 17). The dam has about of 20 m height with design reservoir storage of 1.3 x 108 m3 and effective storage of 0.08 x 108 m3. From right to left banks, the general layout of Memve’ele is main spillway, flushing sluice, rock fill dam with central core (MD 1+695.511 to MD 0+438.283), auxiliary spillway (MD 0+438.283 to MD 0+332.283) and homogeneous earth dam (MD 0+332.283 to MD 0+000.000) (fig. 16). In the figure 16, engineers have provided for that homogeneous earth dam rests on weaker foundation at the left bank and have lower length than rock fill dam with central core. All these suggest that the erosive action of water flow is limited and the large quantity of construction materials was never used respecting economic factors and safety of entire dam (Emiroglu 2008). This plan has been adopted and is materialized actually by the dam structure (fig. 17).