Geological and Geotechnical Characteristics of Ntem Formations: Insight of its Applications in the Memve’ele Dam Construction (Southern Cameroon)

Abstract

This paper discusses the geological and geotechnical potentialities of Ntem formations from the Memve’ele dam construction. Stratigraphic formations on the dam site consist from top to bottom sand, silty sand, silty, silty clay, very to moderate weathered rocks with a thickness of about 0.5–3 m above a sound bed rock which are represented by Ntem formations. These rocks have main mineral associations represented by orthopyroxene, clinopyroxene, quartz, plagioclase and garnet, display reaction textures, idiomorphic crystals, straight boundaries between crystals, the iron bearing amphibole, biotite and epidote, quartz ribbons and iron minerals (ilmenite and magnetite) are also present. All these permit to class them from granulite to amphibolite facies metamorphic conditions; therefore they are very hard and appear good for civil applications. The permeability has thickness from 0 to 13 m in the depth with Lugeon values higher than 15 UL. The RQD values are permitted to class these rocks as being moderated to good resistant. These character features which improve their geophysical properties (e.i. hard quality) have permitted that the dam and powerhouse foundations refitting on them. According to geophysical properties and rock mass rating, the rock mass is fair and good and is classified to class II and III. All these indicate that the rock mass is well compacted and sound with sparsely spaced discontinuities. The Ntem formations have uniaxial compressive strength under saturated condition generally higher than 40 Mpa, dry density or specific weight not less than 2.7 g/cm3, low porosity (average from 0.69 to 0.83%), low coefficient of water absorption (average from 0.012 to 0.04%), deformation modulus from 82.3 to 87.86 Gpa, Los Angeles and Micro Deval values are lower than 40% and 20% respectively, swelling risks are very limited with low values (1.07–1.11%) of sulphate, under 4.5% of cement and low values (0–0.07%) obtained during alkali-reaction test. All these have permitted that to obtain concrete aggregate and rip rap by crushing and processing these rocks from quarry during the dam, spillway, powerhouse and road constructions. These results confirm use of these formations during Memve’ele dam constructions.