**2.1 Geological results**

The in situ investigation in the Memve'ele dam site, comprised detail observation, description of geological units observed in the field and in the borehole cores and the laboratory tests include tests for petrographic analysis and on rock samples. The investigation resulted in the geology of dam site display two geological units based on borehole information and surface investigation (**Figures 1** and **2a**) [4]. These two geological units are named quaternary deposits and lower Precambrian Ntem Formations. Sometimes, Ntem Formations emerge and appear highly to moderately weathered down to elevation 386 m below which is slightly weathered diabase. The quaternary deposits are composed of full weathered soil covers with uneven thickness underlain by Ntem Formations in the dam site (**Figure 2a**). Full weathered soil covers with weak thickness occur up to Ntem Formations along the dam axis particularly. The plasticity index (15 < Ip < 40) and the clay content (A-7-5 type) of soil are generally on the high side, but the clay content roughly met the requirement. The subsurface strata are described in the **Figure 3**. The mineral assemblages of Ntem Formations indicate a high grade metamorphism which corresponds to granulite to amphibolite facies metamorphic conditions. Slightly

#### **Figure 1.**

*Geological units in the Memve'ele displaying quaternary deposit underlain by Ntem formation based on borehole information. Note uneven thickness of quaternary deposit between (a) and (b).*

*Construction Materials and Dam Foundation While Memve'ele Dam Building in the Craton's… DOI: http://dx.doi.org/10.5772/intechopen.93758*

#### **Figure 2.**

*(a) Quaternary soil deposits underlain by Ntem formations that represent two geological units found in the whole dam site and (b) fractured and slightly weathered rock mass.*


**Figure 3.** *Description of strata.*

(**Figure 2b**) and densely jointed zones are found at rocky outcrops, dominantly striking NE30° ~ 40° and E-W with moderate to steep dipping angles (**Figure 4a** and **b**), indicating an outline sub-parallel for all tectonic features (gneissosity, shear zone, fault and jointed rocks). The quantitative description and statistical distributions of discontinuities of Ntem Formations derived from boreholes and those obtained from the geomechanical mapping through scan line at the dam site [5]. The stereographic projection can give an overall view of the number of dominant discontinuity sets of joint and bedding directions within the rock mass. A plot of contoured pole concentration is dominant along the mean direction of N034.44 SE (**Figure 4c**). Quantitative description of discontinuities shows that spacing is extremely close to moderate (0–0.1 m and 30–40 m), their length vary from 1 to 50 m with small

#### **Figure 4.**

*(a) and (b) Engineering discontinuity map, displaying some discontinuity natures, fold and gneissossity; (c) plot of contoured concentration displaying great circle direction.*


*Construction Materials and Dam Foundation While Memve'ele Dam Building in the Craton's… DOI: http://dx.doi.org/10.5772/intechopen.93758*


**Table 1.**

*Geotechnical properties of soil deposits in the Memve'ele dam site.*

discontinuity dominants [5]. The dam site area is considered to be passive tectonically and geological disasters are mainly composed by weathering, other phenomena such as landslide, collapsing and debris flow are rare found.
