**2.3 Geotechnical results investigated on lower Ntem Formations**

A geotechnical investigation into mechanical behavior of Ntem Formations at Memve'ele has been described in detail by Bisso et al. [4]. The rock mass qualities of Ntem formations of the dam site were assessed using sonic well-logging in boreholes and sonic sounding on core samples. Acoustic logging is carried out of from bottom to top of a borehole; the spacing between two receiving detectors and the point distance of detections are 0.2 m respectively. Acoustic test of core consists to penetrate the couple of detector and core by butter. The compressional wave velocity and density results with low values at top and high at the bottom respectively help to interpret the geophysical layers (**Figure 6**). Layer at top is identified as quaternary deposits and layer in the bottom corresponds to Ntem Formations as previously shown in **Figure 2a** and description of strata (**Figure 3**). Thus, the compressional wave velocity and density in the rock mass

**Figure 5.** *Slope stability affected by raining water.*

indicate that there is not only a correlation between velocity and formation layers, but also between rock conditions encountered in the Memve'ele dam site (**Figure 7**), velocities of <5000 m/s indicating highly fractured zone, low density and moderately weathered Ntem Formations with both widely and closely

**Figure 6.** *Seismic velocities, density measurements and geophysical layers in the Memve'ele dam site.*

**Figure 7.** *Drillhole structure and cross plot of seismic velocity measurements.*

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

spaced discontinuities, while velocities of >5000 m/s correlate with fresh to slightly weathered and high density Ntem Formations with very closely spaced discontinuities.

During core drilling, permeability tests were performed and the permeability of the rock mass is expressed in term of Lugeon values. According to Lugeon scale, the Ntem Formations display high, medium and low Lugeon values in the boreholes. However, evidence of two permeability areas is seen and consists to a permeability area with UL > 15, located between 0 and 13 m of depth and an impervious area with UL ≤ 5, beyond of 13 m of depth. The high Lugeon values observed to surface are because of highly jointed rock masses whereas the low Lugeon values correspond to sound rock with moderate fracturation rate. More detailed examination of the competence, softening, density; alkali reactive, hardening of Ntem Formations is displayed by Bisso et al. [4].
