The Use of Weathered Ignimbrites for the Production of Stabilized Earth Blocks (Seb): Case of Dschang (Western Cameroon)
Abstract
The weathered mantle developed on welded ignimbritic formations of Dschang (Western Cameroon) constitute for population living in the vicinity, the raw material use to manufacture building blocks. However, many defects have been noticed on houses built using these blocks such as cracking, swelling and low compressive resistance of blocks.
The purpose of this study is to investigate the physical and geotechnical properties of these soil materials and their suitability in manufacturing Stabilized Earth Blocks (SEB) for optimized local constructions by testing their physico-mechanical characteristics. Results of these investigations, reveal two major types of soil profiles: the soil profile from top hill (Meka’a Profile) and the soil profile of downhill (Mingou Profile). These soil profiles are well-developed with three main horizons (A, B and C). The surrounding population has been using materials from B horizons (middle unit of soil profiles) for the manufacture of SEB. These materials are mainly composed of quartz (12 – 31%), goethite (17 – 22%), kaolinite (8 –18%) and gibbsite (23 – 44%). They display high value of moisture content (48 – 61%) and are very porous (47%). Grain size analysis classified these materials as fine-grained particles (47 – 65%) and so do other geotechnical classifications systems such as HRB, GTR and LCPC. The Atterberg limits reveal that they have high plasticity in nature with plasticity index ranges from 39 to 53.8%.
Satisfactory results were obtained when the weathered materials was stabilized with cement (CPJ 35) above 6%. Flexural strength tests ranged from 2.11 – 2.44 MPa and compressive strength ranged from 2.68 – 3.53 MPa. Nevertheless, some corrections are necessary to enhance these characteristics. This can be carried out by grain size distribution correction or the use of any type of chemical stabilizer.
However, some more detailed specific tests are still required in a bid to optimize the suitability of weathered ignimbrites for stabilized earth blocks production.
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Introduction
Southwestern area of Mt Bambouto has been subjected to several volcanic episodes. Those episodes have respectively laid: basalts (21 Myrs), ignimbrites and trachytes (18.5 - 15.3 Myrs), phonolites (15 – 4.5 Myrs) and pyroclastic dejections (0.5 Myrs) (Kagou et al., 2010).
Ignimbrites (17%) represent the major volcanic rocks of Mt Bambouto. The outcrop of ignimbrites covers an area of about 135 km2 (Tchoua, 1968; 1974) where they are underlaid by granitic basement at Dschang area. They crop out around localities of Menouet, Makong II and Tsintsé. They are mainly composed of acidic minerals (Youmen, 1994; Gountie Dedzo, 1994; Gountie Dedzo et al., 2011).
Once exposed to the atmosphere, those rocks undergo some physical and chemical weathering. Because of the prevailing equatorial climatic conditions, they encounter a high degree of weathering, and the weathered products are used for several applications (agriculture, civil engineering, ceramics and others).
These loose materials after being been classified in terms of their suitability for civil engineering works through GTR (A4 and A1), HRB (A-6 and A-7-5) and LCPC (silty fine sands), have been found to be inappropriate for civil engineering works due of their high water content and predominance of fine particles (Poueme Djueyep, 2012). Due to the rapid rate of urbanization, the consequence is the rapid rate of erecting building. But, with the low purchasing power, the acquisition of modern construction materials (cement, concrete, gravel, sand…) is not really affordable to a major segment of the population. That is why the local population have to turn to local materials: weathered materials developed on ignimbrites. It has been discovered that sandy materials quarried at the surroundings have been found to be of poor quality when used for construction (Tchop, 2000; Ananbe Njitsop, 2008; Fobeu Nguemo, 2011). Also, because of the socio-political prescription encouraging the use of local resources found within the vicinity of any locality for its development. With this scenario in place, this also raises with some environmental concerns.
More so, it has been observed that houses built with these materials displayed some inconveniences such as cracking, swelling and poor resistance of blocks. Previous studies by Tematio (2005) and Ananfouet Djeufack (2012) have characterized these materials as „fine laterites‟. Soils of similar characteristics located in Odza (Centre Cameroon) have been used to produce SEB of appropriate quality (Dtawagap Doumtsop, 2005). However, there is still an important need of scientific information concerning the case of Dschang and particularly, its outcrop of ignimbrite. The objective of this paper is in a bid to fill the gap of the existing documentation on the use of local construction materials in Cameroon.
The highlight is to set out the roles played by the clay mineralogy in the behavior of stabilized earth blocks.
Conclusion
The local construction materials obtained from weathered materials developed on welded ignimbrites of Dschang display convenient preliminary characteristics which permit them to be used for Stabilized Earth Blocks (SEB) manufacture. However, they still present some few shortcomings in terms of water absorption. The compressive resistances, flexural resistances and water absorption ratio can be improved upon by physical and chemical corrections as well as improved techniques used to manufacture blocks.