International Journal of Geosciences, 2012, 3, 166-174 Published Online February 2012 (
Geomechanical Characterization of Sandstones Cliffs of
Segou (Senegal, West Africa) in the Madina Kouta Basin
Déthié Sarr1, Meissa Fall1, Papa Malick Ngom2, Mapathé Ndiaye1, Cheikh H. Kane1, Makhaly Ba1
1Laboratoire de Mécanique et Modélisation-UFR Sciences de l’Ingénieur, Université de Thiès, Thiès, Sénégal
2Département de Géologie, FST-UCAD, Université Cheikh Anta Diop de Dakar, Dakar, Sénégal
Received October 22, 2011; revised November 23, 2011; accepted December 26, 2011
This work presents the behavior of Segou sandstones in the laboratory and in the field conditions. Four types of sand-
stone are collected in the northern part of the Madina Kouta basin (eastern Senegal). These types of specimens are the
white sandstones, the red sandstones, the purple sandstones and the sandstones with intercalation of pelites. Uniaxial
tests are carried out on these specimens of sandstones. The Young Moduli (E) and the Uniax ial Compression Strengths
(Rc) are higher for the white sandstone. Values of the mechanical parameters decrease slightly for red sandstones due to
an increase of the amount of pelites in the composition of the rock. Decrease of mechanical parameters is more impor-
tant for the purple facies due to an important network of fractures. The facies with weaker characteristics corresponds to
the sandstones with intercalation of pelites. This is due to the soft nature of the pelites. The slope stability of the Cliff
sides depends also on to these characteristics.
Keywords: Unconfined Compression Test-Uniaxial Compression Strength (UCT, Rc);
JRC (Joint Roughness Coefficient), Young Modulus (E); Roughness; Segou-Madina Kouta Basin;
Discontinuities; Dihedral; Slope; Cliff
1. Introduction
In eastern Senegal, Upper Proterozoic sedimentary rocks
constitute the Segou-Madina Kouta basin. The forma-
tions of this area outcrop like Cliffs along the southern
border of Senegal. Because of this geomorphology, the
traffic is very difficult. To reduce this difficulty, it be-
comes necessary to adapt this domain to new techno logy.
For that, understanding of mechanical behavior of rocks
becomes a priority. In this basin, Sandstone is the facies
which is most represented. It corresponds to the Cliff
along the border between Senegal and Guinea. So, tests
will be done on four facies of this kind of rock. The four
facies can be identified by their colors depending on the
composition of the rock. The four facies are red sand-
stone, white sandstone, purplish sandstones and sand-
stone with intercalated pelites. These facies are charac-
teristic of the all sandstones described in the basin of
Segou-Madina Kouta. Geomechanical studies must be
performed on the sandstone cliff between Senegal and
Guinea. This cliff contains important fracturations net-
work. Some fractures can be observed directly in the
field but for others, microscopic studies must be done. In
this paper, we will correlate mechanical properties to
mineralogical and microstructural composition of the
rocks. At least, analysis of stability of this Cliff will be
done using the stereog ra p h i c m e t hod.
2. Geological and Geographical Contexts
Proterozoic formations are located in the far south east of
Senegal (Figure 1). This area is crossed by the Gambia
and the Faleme rivers (Senegal Rivers flowing). Its cli-
mate is soudano-sahelian. Lower Proterozoic formations
of Senegal outcrop in this area, between Senegal and
Guinea and correspond to the Segou-Madina Kouta basin.
This basin is limited on the north side by volcanic forma-
tions of Kedougou-Kenieba inlier, on the south side by
the Man ridge, on the west side by the Bassarides and the
Rokellides and on the east by the Basin of Taoudini. The
Basin of Segou-Madina Kouta consists of two super
The supergroup I consists of the Segou group and
the Madina Kouta group (Figure 2). It is subdivided
into two formations [1-6]. The Pelel formation con-
sists of sandstones and pelites rested on to conglom-
erates. The formation of Dindifelo consists of three
members: DFI (where sandstones alternate with pe-
lites), DFII (which represents the cliff and consists of
two members of sandstones with intercalated pelites
and where dimensions of layers decrease with the
elevation). Finally, DFIII consists of ruiniform sand-
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D. SARR ET AL. 167
stones. The formation of Fongolembi with limestone
with intercalates claystones. The formation of Kanta
consists of fine and medium sandstones. In the forma-
tion of Dira, we have sandstone associate with shale
and pelites.
The supergroup II is composed by the formation of
Hassanah Diallo and the formation of Nandoumari.
The Hassanah Diallo formation consists of Eocam-
brian tillite associated to sandstone. The Nandoumari
formation consists of three members. The Tanague
member with quartz-arenite, the Bowal member with
dolomites and the Fougon member with Cherts and
3. Geological Characterization of the Cliff
The geological data are obtained in the sector from Wali-
diala to Yamoussa. In this domain, we have sandstones
alternated with pelites. In Tanague, the beds of pelites are
decimetric to metric. The beds of sandstone are centimet-
ric (Figure 3).
The cliff in the border between Senegal and The Re-
public of Guinea consists of sandstones alternating with
pelites. Sandstones form centimetric to metric beds while
pelites are centimetric to subordinate or absent (Figure
4). It appears like two members (Member 1 and Member
2). The member 1 is characterized by an alternance of
centimetric to decimetric beds of sandstone with interca-
lation of pelites. The member 2 constitutes the Cliff along
the boundary between Senegal and Guinea (Conakry).
Figure 1. Geological map of Segou-Madina Kouta Basin [1].
Figure 2. Lithostrat igraphy of Se gou-Madina Kouta basin [5] .
Figure 3. Lithostratigraphy of Tanague.
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These fractures show high dips (Figures 5-7). Because of
this fact, they show pole con centration at NW-SE,
WNW-ESE and NNE-SSW for the side between Din-
difelo and Pelel. For the side between Dindifelo and Se-
gou, pole concentrations vary slightly and are NNW, NE
and ENE. These discontinuities show high dip angle
(Figures 6 and 7).
4.2. JRC, Spacing, Frequencies and Opening of
The JRC (Joint Roughness Coefficient) is estimated by
comparing the profile of the discontinuities with Barton
and Choubey ch arter [7-11] for labor atory data. The JRC
in the field is given by the following equation:
Figure 4. Lithostratigraphy of the formation of Dindifelo.
This second member consists of two terms. The term A is
composed of centimetric beds of sandstones with inter-
calated pelites. The term B is composed of decimetric to
metric beds where pelites are subordinate or absent.
4. Mechanical Characterization
4.1. Structural Data
Three directions of fractures characterize this area. These
directions are the NE-SW, the ESE-WNW and ENE-
WSW. Fractures are represented by cracks and faults.
JRCn is the exact value, (JRC0) the reference value, (Ln)
the measured length and (L0) the reference length
Dr awing a scanline and measuring the distance betwe en
different discontinuities define the spacing between dis-
continuities. From these data, we deduce the spacing as
the average distance between discontinuities. Frequency
(f) is defined by the following function where X is the
average distance:
X 1
Given the range of these JRC (Table 1), we can see
that we have smooth to slightly rough discontinuities.
This more or less smooth aspect of the discontinuities
confer them remarkable power of shear. So a small tan-
gential strength can caused displacement of joint. For
surfaces of stratification (Table 2), the range of JRC
Figure 5. Map of lineaments of the sector from Walidiala to Tempere.
D. SARR ET AL. 169
Figure 6. Panoramic view of Cliff between Dindifelo to Pelel.
Figure 7. Panoramic view of the cliff between Dindifelo and Segou.
Table 1. JRC values for sandstone fractures.
L0 (Length in charter (cm)) 5 5 5 5 5 5 5
Ln (length in the field (cm)) 75 50 45 205 150 100 20
JRC0 (JRC in the charter) 1 1.5 1.5 1.5 1.5 0.8 1
JRCn (Calculated JRC) 0.947 1.399 1.404 1.341 1.354 0.762 0.972
Table 2. JRC values for surface of stratification.
L0 (Length in charter (cm)) 10 10 10 10 10 10 10
Ln (length in the field (cm)) 100 48 30 37 30 95 10
JRC0 (JRC in the charter) 10 15 17 15 17 15 16
JRCn (Calculated JRC) 6.309 9.369 11.701 10.130 11.701 7.6341 16
L0 (cm) 10 10 10 10 10 10 10 10 10 10 10
Ln (cm) 14 13 17 20 35 120 95 95 70 50 25
JRC0 16 17 17 11 16 8 1 1 0.5 0.2 0.2
JRCn 14.366 15.549 14.193 9.444 10.7165.375 0.956 0.956 0.490 0.199 0.199
shows values with very remarkable differences. Values
are less than 1 for the surfaces of pelites beds and can
reach some values around 16.
4.3. Laboratory Data
4.3.1. Petrographic Data
Analysis of pr epar ation s of sandstone shows that they are
in the major part made up by quartz minerals and small
portion of pelites. There are also fragments of rock rep-
resented by fragments of granits. So we deduce from this
composition that the sandstone has the granit of the socle
as protolite. All the four facies have the same composi-
tion. But, red ones (Figure 8(a)) and white on es (Figure
8(b)) are must own. The sandstones with intercalation of
pelites (Figure 8(c)) are characterized by the important
part of pelites and contain also some oxides. The purple
sandstones are characterized by their fractures network
(Figure 8(d)).
Analyses of JRC show that microdiscontinuities are
smooth (Table 3). Their JRC are lower than one (1). This
characteristic causes in some part a low supported char-
ging of the rock. For th e con tacts of minerals, JRC shows
large intervals o f values (Table 4). So, these values vary
Copyright © 2012 SciRes. IJG
(a) (b)
(c) (d)
Figure 8. Preparation of sandstones. (a) Red sandstone s; (b) White sandstones; (c) Sandstones w ith intercalation of pelites; (d)
Purple sandstones
Table 3. JRC values for microfractures.
L0 (Length in the charter (cm)) 5 5 5 5 5 5 5 5 5
Ln (length in the preparation (cm) 0.02 0.3 1 0.01 0.009 0.1 0.07 1.5 0.04
JRC0 (JRC in the charter) 0.5 0.6 0.5 0.5 0.7 0.9 0.6 0.8 0.5
JRCn (Calculate JRC) 0.528 0.620 0.508 0.532 0.764 0.965 0.631 0.815 0.525
Table 4. JRC values of contacts between minerals.
L0 (Length in the charter (cm)) 5 5 5 5 5 5 5 5 5
Ln (length in the preparation (cm) 0.03 0.201 0.015 0.02 0.06 0.105 0.06 0.014 1.05
JRC0 (JRC in the charter) 6.7 8 6.5 3 1 0.5 0.5 6.5 10
JRCn (Calculate JRC) 13.298 13.37813.832 4.178 1.092 0.519 0.523 13.95613.663
between 0.5 to 14. Those values correspond to smooth to
rough contacts.
4.3.2. Laboratory Data
Mechanical tests are carried on different facies of sands-
tones. The stress-strain curves (Figure 9) of the sand-
stones showthat mechanical parameters of this rock are
different from one specimen to another one. The values
of parameters deduced to uni-axial test are represented in
the following table (Ta b le 5). Those parameters corre-
spond to the Young Moduli (E) and the Uni-axial com-
pressive strengths (Rc).
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D. SARR ET AL. 171
Figure 9. Strain-stress curves of the sandstones samples.
Table 5. Uni-axial compression tests results.
Red Sandstones White Sandstones Purple Sandstones Sandstones with Intercalation of pelites
E (MPa) 7999 10,540 7500 2897
Rc (MPa) 62.87 73.31 50.25 39.83
According to the tests results (Figure 9 and Table 5),
best characteristics (E or Rc) are obtained for the white
sandstones facies. This is caused by the fact that this fa-
cies is less rich in pelites and is constitu ted essentially of
quartz (hard mineral). Minerals of this facies are also
embedded causing their very good cohesion. Portion of
pelites aren’t very important in this facies also.
The second facies correspond to red sandstones. Like
the first facies, this one is also characterized by embedd-
ed of the minerals, more proportion of pelites and micro-
fractures compared to the white facies. It contains also
more part of fragments of rock with granit’s minerals.
The third facies corresponds to the purple one. This fa-
cies shows a very important part of microfractures. Also,
their smooth aspect characterizes those microfractures.
The worst characteristics are obtained for the sandstones
with intercalation of pelites. His high proportion of pe-
lites characterizes this facies. This confers them very low
uni-axial compressive strengths and Young moduli.
5. Slope Stability of the Cliff
5.1. Analysis of Dip and Poles of Discontinuities
Analysis of maps of dip direction (Figure 11) and dia-
gram densities (Figure 10) show that:
In Walidiala (S1) the discontinuities are concentered
in all the area of the stereogram. Dip directions are
low and sens of pandage are variable. In Tanague (S2
and S3), poles of discontinuities show concentrations
along the E-W axe and the dip direction sense are
East, W, NNW and NE.
The side between Dindifelo and Pelel Kindessa (S4 to
S6) shows three discontinuities families. The first one
is oriented NNE-SSW, the second one NE-SW and
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the third one in E-W. It shows high dip preferentially
in ENE but also in N and SE. The side between Din-
difelo and Segou (S7 to S10) shows the same concen-
tration with dip direction essentially on the NNW and
ENE. From Afia II to Segou (S12 to S15), discontinui-
ties show same the characteristics. Dip directions are
NNE and SSW.
The side in front of Segou (S16, S17) shows three fami-
lies of discontinuities. The first family is concentered
N with dip direction sense in the N and in the S. the
poles of the second family are concentered NNW
with dip sen se to the NNW and SSE. The po les of th e
third family are concentered to East with dip direc-
tions to N, S, S SW, WNW and WSW.
The side in face of Yamoussa (S18, S19) shows also
three families of discontinuities. The first one shows
concentration of pole to the SW with dip direction to
the NE. The poles of the second family are concen-
tered to NE with high dips to the SW. The third fam-
ily shows a concentration of poles of WNW and their
dip directions are oriented into ESE to WSW, N to
NE and SE.
5.2. Slope Stability of the Cliffsides
This section studies the unstable po ssibilities of rocks by
stereographic method (Figure 12). For this, all fractura-
tions crossed are represented in a stereogram. So, for this
analysis, we must have the orientation of the cliffsid e an d
of the bedrock.
The area of Walidiala shows possibilities of dihedral
and plane sliding. These slides have done to SE sense
(S1). In Tanague (S2 and S3) there are possibilities of di-
hedral sliding to WNW associate to switching instabili-
ties to NW. The side between Pelel Kindéssa and Din-
difelo shows possibilities of dihedral sliding into the
WSW, East and NNW (S4, Figures 10-12). The side S5;6
show possibilities show also a possibility of plan sliding
and dihedral sliding. The Cliffside between Dindifelo
and Segou shows switching instabilities associated to
Tipping rock. These instabilities have done into N t o NW.
Between Segou and the cascade of Segou we have
possibilities of plane instabilities associate to dihedral
instabilities. These displacements have done into the N
and the NW (S11 to S14, Figures 10-12). The side on face
to Segou (S15 to S16, Figures 10-12) shows the possibili-
ties of instabilities are represen ted by switching, dihedral
and plane sliding. Concerning the Yamoussa side (S18 to
S19; Figures 10 and 11), instabilities are represented by
dihedral, plan slidin g. There are also possibilities of stip-
ping of rock.
6. Conclusion
Previous studies allow us to conclude that the area of
Segou is characterized by discontinuities and microdis-
continuities forming three families. These families are
oriented NNW-SSE, NE-SW and ENE-WSW. Finest
studies of this Cliff show the sandstones and pelites are
crossed by discontinuities. These formations show vari-
ables values of JRC n. These values of JRC are low for
fractures, microfractures and surfaces of pelites beds. For
Figure 10. Densities of pole map of discontinuities.
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D. SARR ET AL. 173
Figure 11. Dip direction map of discontinuities.
Figure 12. Stereographic plots of discontinuities cross on different hillsides.
the surface of sandstone beds and the contacts between
quartz minerals, there are high values for JRC. The uni-
axial compression tests done on to different facies of
sandstones shows these mechanical parameters (Young
modulus and Uni-axial compressive strength) vary from
the white sandstone to the san dstone with intercalation of
pelites. The greatest values are obtained for white sand-
stones which are less rich in pelites. The second one is
the red sandstone also clean but with most important part
of pelites than the white one. Purple one, very fractured,
represents the third facies. The facies with worst charac-
teristic corresponds to the sandstones with intercalation
of pelites. The Young Modulus and Uni-axial compres-
sive strength decrease with the proportion of pelites and
Copyright © 2012 SciRes. IJG
also with the microfractures. The cliff shows a very
complex instability with sliding plane and dihedral asso-
ciate to switching.
7. Acknowledgements
Authors would like to Acknowledge Mr. Amsata Thiam,
technical Director of SENBUS industries sa for his great
contribution in this work, allowing to us the supplies of
his manufactory for the samplings. We acknowledge also
Dr. Edmond Dioh, Head of the Department of Geology
of the IFAN-Cheikh Anta Diop (Dakar), most of the mi-
croscopic analysis had been done in his laboratory. The
authors would like to acknowledge Mouhamadou Lamine
Lo (Assistant Professor—EPT/Thiès) for his guidance
and valuable input in this research project.
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