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Geophysical studies on stability assessment of volcano-capped weathering in part of Bamenda ring complex (Cameroon, Central Africa).

1. Introduction

Cameroon government heads for long-term perspective regarding landslide risk along road network of the Bamboutos and Bamenda ring complex. Presently destructive landslips are known across the region (Mundi, 1984; Njilah et al., 1999a, Fig. 1) and solutions are searched to reduce the risk of further disasters. To assess the stability of the subsurface formations (close to the roadside), it is important to collect information about passive or inherited factors that predisposed the weathering to landslide process (Ardisson et al., 1997; Faleh & Sadiki, 2002). Electric prospecting has a good potential, not only for subsurface mapping (Bernstone & Dahlin, 1996; Ardisson et al., 1997), but also to reveal the location of water flow path or drained faults and provide a basis for target aquifer (Mbida 2004).

This paper presents the field geophysical data, acquired using two methods of subsurface imaging: electrical trenching for assessing lateral variations and vertical sounding to estimate the thickness of the toposequence that may slide.

[FIGURE 1 OMITTED]

2. Regional context

The Bamboutos and Bamenda ring complex is located in the western part of Cameroon middle belt. It hillside forming the C-Bend escarpment (Fig. 2), consists of high plugs or extensive lava flows spanning a period of active volcanicity from early Cretaceous to the present-day (see Dunlop & Fitton, 1979; Moreau et al. 1987; Halliday et al., 1988; Tabod et al., 1992; Njilah et al., 1999b). Direct field observations supplemented by previous studies, led to describe (1) massive ignimbrite facies lying on slightly folded Pan-African basement and (2) Ypresian alkali basalts overlain by (3) Late Oligocene/Early Miocene trachy-rhyolitic suite (Tchoua 1974; Regnoult & Deruelle, 1983; Moreau et al., 1987; Njilah, 1991).

According to Moreau et al., (1987), there is a strong correlation between volcanic trends and the location of deep basement faults, suggesting that Mesozoic fractures/shear zones provide the conduit for the magmas to ascend to the surface.

As regard on plugs outcrop, the main lineament trend associated with shearing are N030 and N345, offsetting old basement fractures (Fig. 2). This deformation confirmed recent rejuvenation of Central African Shear deformation (see Reyre, 1984 and Mbida Yem PhD thesis).

Through the study area, it appears that shear band propagation and heavy rainfall combines to form natural conditions for rapid weathering. Both factors shape the landscape into two main morphological units: high relief zone with steep slopes (made up of trachy-rhyolitic suite) and the ignimbrite escarpment forming the shelf zone. Weathering type profile overlying the shelf (or ignimbrite facies) comprised two main sets of horizons: (1) a multicoloured alterites and (2) interbasaltic red bole mantle, capped in place by indurate carapace or pyroclastic neck.

As many high relief zones in the world, volcanic chain of Cameroon (including Bamenda escarpment) suffered from mass movement (slides, rockfall and mudflows) and regions situated along the hillside are particularly hit. To determine the destabilising factors in the release of ground-failure, geoelectric mapping was carried out along two lines of 90m total lengths and 08 vertical sounding around C-Bend field excavation (Fig. 2). This landslip, which results of over 1500[m.sup.3] debris flows, blocked the National Road 6 (NR-6) and disturbed the traffic for a long period of time.

[FIGURE 2 OMITTED]

3. Materials and methods

The TERRATEST TT800 Imaging system consisting of resistivity meter, electrode cables, steel electrodes and various connectors; was used for data acquisition. Electrical trenching (ET) was measured as continuous bulk resistivity profile using Wenner array, while vertical sounding (VES) was carried out using Schlumberger electrode configuration (Halvorson & Rhoades, 1976; Barker, 1989; Pozdnyakov et al. 1996; Banton et al., 1997).

ET station interval was set to AM=MN=NB=a=30m and VES were carried out at 12 points along a trace of line with maximum current spacing AB/2=120m.

To increase geophysical records accuracy over VES, three cable sets of potential difference measuring were used for in situ monitoring of apparent resistivity values. By using chart field, collecting values have been interpreted in terms of ground-layer resistivities and thicknesses. The resulting geoelectric sections matched by geotechnical tests and direct field observations led to deduced VES depth penetration of AB/4. Also, as implied in theoretical derivations and practical tests (Beck, 1981; Barker, 1990; Bertin, 1981; Dohr, 1981; Freeland et al., 1997; Parasnis, 1997), ET depth profiling is considered equal to 0.173 total lengths. Consequently the approximate depth investigation was 16m and 60m for ET and VES, respectively.

4. Results and discussion

The acquired subsurface resistivity structure using Electrical trenching, outlined a E-W undulating bedrock topography (Fig. 3), with highs representing resistive substratum swells and swale corresponding to conductive weathered zones or groundwater pathways (Freeland et al., 1997).

For quantification of subsurface succession, vertical pseudosections (Fig. 3) allowed mapping of seven-geoelectric layer. These data calibrated by direct field observations and boreholes tests led to interpret from top to the base level of the sliding sequence, the following horizons:

(1) soft nodular sequence; (2) indurate carapace or trachy-rhyolitic suite; (3) perched aquifer ; (4) interbasaltic red bole mantle ; (5) mottled massive clay ; (6) deep confined aquifer, and; (7) fractured ignimbrite substratum.

[FIGURE 3 OMITTED]

On the basis of the distribution of the resistivity within VES1 and VES5 field excavation tied, the maximum depth of the slip surface is around 22m [+ or -] 1 and corresponds to the top of late Cretaceous volcanic sequence (ignimbrite sheet). The combination of lateral and vertical resistivity profiling, shows that, the sliding sequence (including horizon 1 to 6) overlying the fractured ignimbrite bedrock dips approximately 7[degrees] SSE.

Important findings of our field geophysical survey and their hypothesized landslide implications are as follows.

(1) Very low resistivities from about 4-5m and 18-21m depths reflect perched and confined deep aquifer (CDA) within subsurface setting. Surface water (stream) and perched aquifers flow into CDA through fissures or fault planes. During the rainy season CDA gradually fills up and then triggers the slope prone to failure.

(2) Landslide location seems to be restricted along major fault zone and the corridors leading to mass movement extend between N065 and N345 fault trends (Fig. 4). This conjugated fault system drains groundwater that emerges in the base level of the slope and cut across the NR-6 via nozzle-bottlenecking (NBL). Given the gradual diminution of slip amplitude away from NBL location, we can conclude that groundwater saturation at the base level of the slope was probably induced by it congestion. Since the sliding sequence was saturated, loading created by the presence of volcanic blocks inside weathering has helped to reduce the strength shear of the slope prone, and therefore the onset of mass movement.

The present study carried out in the slip zone of C-Bend escarpment, provides relevant information about the geological structure, underground network and road consolidation infrastructures such as nozzle or scupper congestion that can assists inherited factors in destabilizing slope. It appears that groundwater saturation is the dominating factor in the release of the slope collapse. Apart from their social and economic impact, the results presented in this paper highlighted tectonic controlled of landslide development along the study area. According to lithological and structural complexity of volcano-capped sequences, further investigations involving electrical tomography should be undertaken in order to improve our knowledge concerning landslide process along Bamboutos and Bamenda volcanic belt.

[FIGURE 4 OMITTED]

Acknowledgments

This work was financially supported by public funds coming from Cameroon natural risks program, directed by the Ministry of Urban Development and Housing. The authors especially thank the former Minister of Public Works Bernard Messengue Avom for his contribution to the preparation and publication of the present paper.

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L. Mbida Yem * (1,2), J. Q. Yene Atangana (1) and P. Nouanga (2)

(1) Department of Earth Science University of Yaounde I

(2) National Civil Engineering Laboratory Postal addresses.: PoBox. 349, Yaounde-Cameroon

E-mail: mbida@geoazur.obs-vlfr.fr
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Author:Yem, L. Mbida; Atangana, J.Q. Yene; Nouanga, P.
Publication:International Journal of Applied Environmental Sciences
Geographic Code:6CAME
Date:Sep 1, 2012
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