Engineering Physical Factor Considerations for Specifying Dam Type During Memve’ele Dam Construction (South-Cameroon).

This paper presents the results of engineering investigations used to select a best design for the Memve’ele dam. The selection of Memve’ele dam type has been based on consideration of physical factors such as topography, climate, availability of construction material, geology, geotechnical and foundation conditions and the selection of optima alternative between three dam types. According to these factors, Memve’ele valley is very wide and situated in the warm climate zone with dry and rainy seasons favorable to embankment type dam; the geology reveals very good conditions with Ntem formations and residual deposits as foundation materials and the dam site is not inuenced by earthquake; this behavior is conrmed by geotechnical data and further indicates that this site is favorable to a composite dam type (embankment and concrete dams or connecting concrete structures) and the construction materials found within or near the dam site are suitable for usage. These characteristics have contributed retaining three dam types. The alternative of these three dams is a composite dam type option including homogeneous earth dam and rock ll dam with central core link by concrete structures. Engineering geological map and general layout of Memve’ele’s dam highlight the important role plays by engineers. The main dam that is observed actually in the Memve’ele dam site is composed from left to right of left connecting dam section, ushing sluice, main spillway section, riverbed dam section, auxiliary spillway section and right bank dam section. amphibolites, of straight boundaries between amphibole crystals high grade metamorphism; k-symmetrical amphibole grain surrounded by quartz ribbon and biotite in mylonitic l-dissymmetrical amphibole the low

rock types and the ground water level. The geological surveys include a study of surface conditions (terrain excavation and transportation condition), distribution of usable and unsuitable layers, ground water level of quarry and borrow areas to evaluate the availability of required natural construction material combining with the results of testing. Many cross sections have been made along the studied dam axis and in the borrow areas ( Fig. 10) coupled with airborne techniques using drone at few meters above the dam site at the aim to realize geological map (Fig. 4) and high quality image captures (Fig. 17).
These airborne techniques particularly have led to do quantitative description and statistical distributions of discontinuities of Ntem Formations. The reserve in the borrow areas is estimated by using both parallel section and mean thickness methods. Cores of borehole, exploratory shafts and test pits were logged following by Engineering Geology Field Manual, 2001. Sampling and packer tests have been performed as directed by the Engineer. Laboratory tests of embankment material, rock and soil samples have been performed at National Central Material Testing Laboratory, Cameroon. Geological data were obtained by eld works, through scan line at the dam site (ISRM) and thin observations. The petrographic analysis was carried out in a thin observation with an electronic microscope. Scan line at the dam site is done by airborne technique using drone at few meters above of soil. The sand has been mined by grab boat. Granular samples from studied soils were collected to use for mechanical test. The geotechnical studies were carried out in detail including core drilled data and in situ testing. Many boreholes were drilled by drilling rig to assess the geological and geotechnical of the Ntem formation where is located the dam foundation. The liquid limit (LL) was measured by the method of the dish of Casagrande and the plasticity limit (PL) by the method of the roller both according to NF P94-051 standard. The speci c gravity of the materials is measured according to NF P94-054 standard. The compressive strength test is measured after NF P94-420 standard. The permeability tests include rock quality designation (RQD) and Lugeon water. The RQD index consists to degree of jointing and the water absorbed quantity in the 10 cm of borehole drilled on total drilling. The rock is distributed after RQD class (0-25, very slightly, 25-50 slightly, 50-75 moderate, 75-90 good, 90-100 excellent (Deere and Deere, 1987). The Lugeon values express the permeability of rock samples. This parameter is measured in the Lugeon scale (0-1 Lugeon impervious, 1-5 Lugeon low permeability, 5-10 Lugeon medium permeability, 10-15 Lugeon high permeability and > 15 Lugeon very high permeability). The determination of earthquake coe cient is based on the relation between intensity felt at the site as shown in table 4 and frequency of occurrence (Nc) in the period for 100 years and 250 years by ISC method, Japan Meteorological Agency (JMA) method and Munich Reinsurance (M.R.) company. Noted that intensity I = 8.0 + 1.5M -2.5lnr (Cornell, 1968) and LogAh = 0.014 + 0.30I (Trifunac and Brady, 1975). I is intensity, M corresponds to Magnitude assumed, r represents distance from the epicenter of earthquake to the dam site in kilometer and Ah is acceleration in cm/s 2 theoretically calculated. Additional data used for evaluating and selecting the appropriate dam type option during Memve'ele dam construction include meteorological data and the concrete tests ( Table 3). The meteorological data is not obtained directly in the dam site. Therefore, we have used meteorological data during 1996-2005 (Sinohydro, 2012) observed at Ambam station in Ntem river basin which have been collected daily and average of each month has been calculated. Parameters such as mean monthly temperatures, humidity and winds have been obtained considering cumulated year of each one of parameters respectively.

4-1-Environmental context and local geology
The dam site lies at Memve'ele's island in the southern plateau area of Cameroon, at where the ground generally descends from north towards south, at an elevation varied between 400 m and 700 m above the sea level. The river course at the ferry is expanded of about 300 m wide with both the gentle riverbed and elevation of about 381-382 m. The upstream topography is gentle and the water ow is slow, while at the dam axis and its downstream, a ground elevation is of about 382-387 m. The ridges on the left embankment and on the right embankment have elevations of about 420 m and 400 m respectively. The bank slope on the left embankment is relatively steep and the natural side slope is about 35° whereas, on the right embankment, the natural bank slope is relatively gentle of about 5-10°. The dam site belongs to the equatorial warm climate zone. The air temperatures vary from 16.5 °C to 39.5°C with an average annual of ~ 27.7°C. The wind speed is ranged between 1.5 and 25 m/s with average annual of 3.64 m/s. This area is covered with dense rain forest, featured by four seasons divided in two dry seasons and two rainy seasons. The average annual rainfall is 2294.4 mm. The rst rainy season falls in March to June, relatively frequent rainfall with moderate intensity accounting for about 27% of the annual rainfall. The rst dry season falls in July to September. The second rainy season falls in mid-September to mid-December, very frequent rainfall with high intensity accounting for 55% of the annual rainfall. The second dry season falls in mid-December to February with high temperature (tab. 7 and g. 11). The river is divided into the left and right stream with twists and turns, distributed with several curved branches and three relatively big islands ( g. 1). The Ntem river ows through the quaternary deposits including alluvial deposits; lacustrine/swamp deposits and residual deposits which are found above of lower Precambrian Ntem formations ( g. 4 and 5a; Bisso et al., 2020).
The quaternary deposits display clay content between 21.3 and 59.7%, plasticity index ranged between 24 and 36, permeability coe cient between 1.77E-6 and 8.44E-9 cm/s and natural water content between 8 and 33% (table 6). The Ntem formations are mainly composed of migmatite, gneiss, and limestone to magnetite which are intruded by granite, garnet bearing amphibolites, diabase, granodiorite and orthogneiss with pyroxenes ( g. 5), distributed both downstream and upstream of the Memve'ele dam site. These formations were originally of sedimentary rocks of argilla-calcareous and sandstone having vast geosynclines. However, rocks derived from igneous origin are present. These formations have been metamorphosed in the Mesoarchean (2927± 8.9 Ma, U-Pb/Zrn, unpublished data) and folded in generally NE-SW direction. The foliation is variably oriented NE-SW, E-W and N-S mainly ( g. 5b), and consists both to an oblique and a steep dips ( g. 5e) in widespread directions. Mineral lineation is outlined by aggregate boudinaged amphibole and quartz while stretching lineation is illustrated by stretched quartz. A fault is observed along the river course of the Ntem in the downstream of the waterfalls. Precisely in 120 m line distance away from the powerhouse area. Two movements of this fault are observed including the northeast side of the fault line is sunk and the southeast side of the fault is raised, also the northwest side of the fault is raised and the southwest side is sunk. These features suggest a hinge fault. The Ntem river course is linearly running along the fault line through the Ntem trough or gorge ( g. 5i) at the Ntem downstream of the waterfalls whereas, its course of the upstream is meandering to the northeast direction (NE-SW to ENE-WSW, g. 3c), due to developing depression zone here by fault movements. However, no sense of shear indicators could be associated with these fractures. The conglomerate or breccias near and along the fault line is well consolidated and matrix is tightly cemented ( g. 5h and j) due by the fault activity. It is inferred that the conglomerate might be formed in Mesozoic to Palaeogene time. As seen during eld investigations, notably both in the waterfall and near the powerhouse areas, Ntem fault is composed of many brittle normal and horizontal faults and display polished surfaces or slickensides printed on epidotes ( g. 5f). The brittle normal faults generally strike NE30°~ 40°, dip northwesterly at a moderate to steep angles of 50° ~ 80°. Densely jointed zones are found at rocky outcrops, dominantly striking NE30°~ 40° and E-W with moderate to steep dipping angles ( g. 5j, k and l), indicating an outline sub-parallel for all tectonic features (gneissosity, shear zone, fault and jointed rocks). Overall these behaviors of tectonic features imply local deformations. The microscopic features of petrographic and tectonic elements are shown in the gure 6. An earthquake can obviously have catastrophic effects on the dam if it is not designed accordingly (Seed et al., 1975;Narita 2000). From table 9, the calculation of earthquake coe cient (k = gal/980) is resulted as k = 0.0006, say k = 0.01 for the return period of 100 years and k = 0.03 for 250 years respectively. The value of k = 0.01 is the proposed earthquake coe cient for the Nachtigal among hydropower project locating some 250 km northeast of Memve'ele site. Operating Basic Earthquake (OBE) is an earthquake that can reasonably be expected to occur with 50% probability of exceedance during the service life (ER 1110-2-1806, American regulation for operation basic earthquake (OBE)). This corresponds to a return period of 144 years for a project with a service life of 100 years. For conservative design, OBE is recommended to be 0.03 g which corresponds to a return period of 250 years and MCE to be double of OBE for Memve'ele hydroelectric project (0.06 g).

4-2-Geotechnical conditions
Three borrow areas including borrow areas B1 and B2 on the right bank and borrow area B3 on the left bank ( g. 8) have been used. These borrow areas are . The thickness of quaternary deposits increases from left toward right bank (from 1~3 to 13 m; g. 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 ( g. 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 ll 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). Ful lling this requirement allows that rock ll 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 con rmed by eld 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 ll dam with central core, rock ll 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  were quali ed "erosion resistant". The deformation modulus values are higher than those (0.5-1.5 MPa) proposed by Messou (1980)  indicates the relatively clay contents in the soil deposits. The coe cient 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 lter 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. 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 owing through the dam body (EM 1110-2-1901; Brian 1989); the impervious treatment measures of dam foundation can be simpli ed 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 lling 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 ll 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 ( g. 14b) respectively. On the upstream side of the core wall is the 1.5 m thick inverted lter and the transition layer. On the downstream side thereof is 1.5 m thick inverted lter and the transition layer (Narita 2000). The dam facing is lled 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 lling earth to enable the contact surface between corewall 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 lled to meet the schedule in the case that the climate has an adverse impact on construction.
Rock ll 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 ( g. 14c) respectively, lled with inverted lter and transition layer. The dam facing is lled using Ntem formation block stones (Golze 1977).
Where there is di culty in lling in rainy seasons, the clay sloping core dam is employed, and the permeable materials of the downstream dam facing shall be rst lled 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 m 3 , 307,000 m 3 and 198,000 m 3 respectively, of which the dam with central core has the least lling quantity, and the homogeneous earth dam has the most lling quantity. In view of the impact of rainfalls in rainy seasons upon lling progress and quality of earth and rock ll 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 ll with central core dams linked at concrete structures ( g. 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 at and the river is divided mainly into the left and right stream ( g. 15). Each stream section is occupied by weak thickness of alluvial deposits very ne sand overlying moderately weathered granitic gneiss of Ntem formation as it is the same in the island river ( g.  16). In the gure 16, engineers have provided for that homogeneous earth dam rests on weaker foundation at the left bank and have lower length than rock ll dam with central core. All these suggest that the erosive action of water ow 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 ( g. 17).

6-conclusion
(1)-The application of airborne as new investigation techniques coupled with eld works of dam site have been allowed to realize a geological map and a high quality image captures have been done by drone. These approaches may have application in other areas (2)-A composite dam (homogeneous earth dam + rock ll dam with central core) option was found to be the best solution to retain water for hydroelectric purposes (3)-The engineering geologist and design engineer work together while dam planning. The results from this study show their contributions to supply a general layout of main dam. Long and at (low)

Geological conditions
Two depression zones to be acrossed lowered the rock line at the right bank. Thick soil at the right bank.
One depression zone to be acrossed. Rock line of the right bank is elev.383 m.
One depression zone to be acrossed. Rock line of the right bank is below elev.390 m.
One depression zone at the 100 m upstream of the axis. Rock line lowered at the right bank (El. 375 m).   Poisson's ratio 0.167 Linear expansion coe cient (C°-1 ) 9.0*106       Figure 1 As viewed near the ferry station (on the right bank), the front of dam axis.

Figure 2
The spatial distribution of control points showing the delineate of dam axis. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.  Engineering geological map of dam project area . Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.   Main spillway (concrete structure) under construction.  Geological maps: a-in the dam site; b-in the borrow areas (b1-borrow area B1, b2-borrow area B2 and b3-borrow area B3). Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Figure 10
Geological cross sections: a-longitudinal section of dam; b-transversal section of dam; c-geological section in the borrow areas (c1-borrow area B1, c2-borrow area B2 and c3-borrow area B3).

Figure 11
Ombrothermic diagram of 2005 year from Ambam station.

Figure 13
The foundation pit of dam under construction.

Figure 14
Typical cross sections of dam types primarily selected according to physical factor considerations; a-homogeneous dam; b-rock ll dam with central core and c-rock ll dam with inclined core.

Figure 15
Topography of dam site.

Figure 16
Engineering sketch map showing the layout of main dam. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors. Figure 17 a-As viewed near the main spillway (on the left bank), the Memve'ele main dam, b-as viewed from downstream position, the secondary spillway (structure concrete) on the right hand of the Ntem stream.