Bentonite is an essential component of drilling fluids and geosynthetic clay liners, which are mostly used in the geotechnical engineering industry for oil well drilling and lining of tailings storage facilities as well as landfill sites. The Ghana Environmental Protection Agency (EPA) is advocating for a shift from the compacted clay liners to geosynthetic clay liners since the latter has superior hydraulic properties and longer service life in terms of preventing groundwater and soil pollution than the former. In Ghana, several metric tonnes of bentonite are imported for drilling purposes costing millions of dollars. In order to save much needed foreign exchange for other sectors of the economy, there is the need to source for local material to substitute commercial bentonite. Bentonite is used due to its unique properties, which are dictated by its mineralogical composition. It is mainly composed of the clay mineral montmorillonite. Montmorillonite is reported to be the major clay mineral in black cotton soils occurring in Ghana. Black cotton soils are reported to occur widely in Ghana covering an area of over 168,000 hectors, which are largely unexploited. This study therefore looked at black cotton soil with the aim of using it as substitute for bentonite in liner and drilling applications. Sample of black cotton soils were collected from Dawhenya, Prampram and Tsopoli which are located within the Accra Plains of Ghana. The samples were air dried, crushed and sieved to -0.075mm. The physical, chemical, mineralogical properties as well as the plastic viscosity, apparent viscosity, gel strength and yield point were determined and the results compared to those of commercial bentonite and some local and international standards. The physical properties included textural characteristics, colour, lithological characteristics by visual inspection and some index properties such as particle size distribution, Atterberg’s limits, specific gravity and moisture content by BS 1377. The chemical and mineralogical properties were determined using x-ray fluorescence and x-ray diffractometry respectively. The organic matter content, cation ion exchange capacity (CEC) and exchangeable ions were also determined by the Walkley and Black method and ammonium displacement method respectively. The pH of the soils was determined using the glass electrode. Swell index and permeability were determined in accordance with IS 1498 and ASTM D5887 respectively. The plastic viscosity, gel strength and yield point were determined as stipulated in the API 13B-1 standard. The rheological properties were determined for concentrations of 22.5g/350ml, 32.5g/350ml, 42.5g/350ml, 52.5g/350ml and 62.5g/350ml. In order to improve the rheological properties, the tests were repeated for the same concentrations dosed with 10% Na2CO3. The results of physical, chemical and mineralogical test showed that the black cotton soils contain calcium montmorillonite as the dominant clay mineral. The permeability results obtained were of the order of 10-9cm/s, which compare well with those of commercial bentonite and are within the Ghana Minerals Commission LI 2182 requirements as well as the USA EPA (most widely used in other parts of the world) requirements. The plastic viscosity, gel strength and yield point before the addition of Na2CO3 were 5 mPa.s, 4.8 mPa for 10sec and 10mins, 5-10 mPa respectively for a concentration of 22.5g/350ml. Upon the addition of 10% of Na2CO3, the plastic viscosity, gel strength and yield point increased to between 24-28.8 mPa.s, 5-12 mPa for 10sec, 11-26 mPa for 10mins, and 4.8-33.6 mPa respectively at the concentration of 22.5g/350ml and these values fulfill the requirements of the API 13B-1. An economic evaluation indicated a possible cost savings of about 58% when black cotton soil is used as drilling fluid and a cost saving of about 94% when used to substitute bentonite in geosynthetic clay liners.

1.1 General
All over the world, hydrocarbons and minerals industries are very lucrative sectors and international companies are always looking to invest in these sectors. The discovery of oil in commercial quantities in Ghana brought in many international companies who obtained licenses for offshore exploration. Since the hydrocarbons can only be extracted through the drilling of wells, clay will continue to be used in drilling activities (Omole et al., 2013). The clay component used in drilling fluid is bentonite and the cost of importing commercial bentonite for drilling purposes runs into millions of dollars. The total expenditure in drilling an oil well is estimated to be between 1 million to 100 million dollars out of which 5 to 15% is spent on drilling fluids (Bloys et al., 1994). There is therefore the need for a local substitute that may reduce cost.

In Ghana, huge quantities of rock are mined in open pit mines and processed to extract precious metals, which results in the generation of a lot of waste known as tailings. The waste material generated by mining activities contains chemicals that are harmful to the environment and therefore need to be stored in a manner that the environment will not be adversely affected.

It has therefore become important to safely dispose tailings; hence, mining companies are compelled to contain them behind specifically designed dams with basin liner systems.

The purpose of the liner system is to prevent hazardous chemicals from reaching the soil and ground water (Akayuli et al, 2013).

Natural soil barriers containing clay minerals have been the most widely used material in containment facilities in Ghana. These barriers are referred to as compacted clay liners (CCLs) or recompacted clay liners. Rowe et al, (1995) stated however that, a clay’s ability to act as a barrier to chemical, municipal and mining waste could not be determined only from laboratory permeability measurements. They further explained that the permeability of compacted clay depends on factors such as placement and compaction of the clay, the mineral composition of the clay, or the effect of leachate on hydraulic conductivity resulting from the mineral composition.

The use of geosynthetics in Ghana is increasing rapidly. While the use of geosynthetics in civil engineering industry is low, the mineral industry long appreciated the advantages associated with the use of these geosynthetics (Bouazza et al. 2013).

Geosynthetic Clay Liners (GCLs) are made of bentonite glued or stitched between two geotextiles or glued to a geomembrane. The permeability of bentonite dictates the permeability of geosynthetic clay liners except those with geomembranes (Bouazza et al. 2013).

Bentonite is any clay, which has smectite as the most abundant clay mineral and the two main classes of bentonite that are mostly used in industry, are Sodium and Calcium bentonite. Bentonite is also used as a component of drilling fluids for the purposes of lubrication, cooling cutting tools and removal of cuttings from wells. The flow and deformation properties of bentonite are very important in geotechnical engineering (Hosterman et al, 1992). These properties stem from the swelling ability of the bentonite that also depends on the type of smectite clay within the bentonite.

Black cotton soils are defined as being black or greyish black in colour and containing clay of over 50%. The predominant clay mineral in the soil is the smectite group (Morin, 1971).

Black cotton soils are generally expansive; therefore, pose serious engineering challenges when engineering structures are to be founded on them. The presence of the smectite group in Black cotton soil makes it capable of possessing some of the properties of bentonite that is widely used as drilling mud and in Geosynthetic clay liners. There is therefore the need to evaluate these properties of the black cotton soils with the aim of using them as substitute for bentonite in Geosynthetic clay liners and drilling mud. This study considers the physical, chemical, mineralogical, geotechnical and rheological properties of some typical black cotton soils found in Ghana.

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