Investigating the Suitability of Coconut Husk Ash as a Road Soil Stabilizer

There is a pressing need to locate cheaper alternatives to traditional stabilizers such as Portland cement and lime which will reduce the cost of stabilized roads and make the practice of treating local soil materials very attractive to road development agencies in poor countries of the underdeveloped world, where deficient soils are often used without treatment, the consequence of which is premature deterioration of roads. This paper presents a study that was conducted to investigate the suitability of coconut husk ash (CHA), a waste product from crop plants, as a road soil stabilizer. The oxide composition of CHA was determined to establish its suitability as a pozzolanic material. It was then mixed with a lateritic soil (classified as A-2-6(1) using the AASHTO system of soil classification) in varying proportions, ranging from 0–20% by dry weight of soil at increments of 2%. The physical and strength properties of each of the soil-CHA blends was then determined in the laboratory. The results show that oxides of K₂O, SiO₂, Cl, CaO, P₂O₅, MgO and Al₂O₃ constitute 92% of CHA, indicating that it is a pozzolanic material. The optimum moisture content (OMC) of the soil increased, while its maximum dry density (MDD) decreased, with increasing CHA content. The CBR and UCS of the mixes increased with CHA content up to 8%, but decreased with a further increase in CHA content. However, the increase in the strength of the soil obtained at the optimum CHA content was not significant enough to warrant its usage as a lone stabilizer for sub-base and base materials, but it can be used for subgrade stabilization. For sub-base and base stabilization, CHA should be admixed with conventional stabilizers for improved performance.

California bearing ratio; Coconut husk ash; Compaction characteristics; Lateritic soil; Pozzolanic material; Unconfined compressive strength

Soils are routinely used for pavement layer construction in developing nations of the tropics, where good quality aggregates are not readily available in all locations as a result of the intensive weathering in the region. In Nigeria, for instance, there is hardly any road project that does not employ lateritic soils for sub-base, or for both sub-base and base, construction. This heavy application of lateritic soils in road construction derives from their widespread availability in the country.

However, many soils in their natural state do not possess the requisite engineering properties that qualify them to be used for pavement layer construction (Chittaranjan et al., 2011; Chhachhia & Mital, 2015). This deficiency renders soils wholly or partially incapable of meeting construction requirements. In the event that the local soil fails to meet the requirements of construction, the options available to the road engineer are to source good quality materials from where they are available and transport them over long distances, or to use soil stabilization techniques to improve or modify the properties of the local soil to bring it to the standard required for pavement layer materials. The first option is often uneconomical, as hauling materials over long distances adds considerably to the final construction cost (Ekeocha & Agwuncha, 2014). Soil stabilization provides cheap materials for the construction of low-cost roads, given that unsuitable locally available materials can be improved and used effectively (Pundir & Prakash, 2015).

The most common and widely used soil stabilizers, known as traditional stabilizers, are Portland cement, lime and bitumen (Roy, 2014; Barasa et al., 2015). However, over-dependence on these industrially manufactured traditional soil stabilizers has resulted in an increase in the construction cost of stabilized soil roads (Oriola & Moses, 2010; Roy, 2014; Zahra et al., 2015). Moreover, the industrial process of the production of cement and lime is associated with hazardous greenhouse gas emissions that threaten the environment (Rahman et al., 2014). Therefore, there is a pressing need to locate cheaper and environmentally friendly alternatives to traditional stabilizers, which will reduce the cost of stabilized roads and make the practice of treating local soil materials very attractive to road development agencies in poor countries such as Nigeria.

In a bid to address the challenges that have been thrown up by total dependence on industrially produced soil stabilizers and the associated hike in cost of construction of stabilized soil roads, researchers over the years have been busy searching for suitable alternatives that can replace cement and lime, partially or wholly, as soil stabilizers. Successes have been recorded in the area of partial replacement of cement and lime with pozzolanic materials, some of which were obtained from plant ashes. These successes have been reported in the works of Basha et al. (2005); Aparna (2014); and Barasa et al. (2015).

The knowledge that traditional stabilizers generally rely on pozzolanic reactions and cation exchange to modify and/or stabilize soils (Little & Nair, 2009) and the successes recorded in the use of known pozzolanic materials like fly ash for soil stabilization have prompted investigations into the feasibility of using plant ashes as soil additives, considering that the ashes have been classified as pozzolans. According to Chmeisse (1992), plant ashes have high silica content and can be made to be pozzolanic by suitable treatment.

In recent years considerable research has gone into identifying agricultural wastes whose ashes produce good pozzolans and which are available in exploitable quantity (Tsado et al., 2014). Locally available agricultural wastes that have received serious attention from researchers include:  Rice Husk Ash, Groundnut Shell Ash, Sugarcane Bagasse Ash, Sugarcane Straw Ash, Coconut Shell Ash, Corn-cob Ash, Coco Pod Ash, Locust Bean Ash, Palm Kernel Shell Ash, Saw Dust Ash etc.

Investigation of the suitability of ash remains of agricultural wastes to completely take the place of cement and lime as road soil stabilizers has received the focus of many researchers in Nigeria and other underdeveloped countries in recent years. The use of agricultural waste products as soil stabilizers will help to solve two very important engineering problems: reducing cost of road construction and managing environmental waste. Agricultural waste products are viable sources of environmental pollution. Recycling and putting them to productive use will help to sanitize the environment.

This paper presents a study that was carried out to determine the possibility of stabilizing road soils with the ash remains of coconut husks. Coconut husk is the outermost layer of the coconut fruit that houses the coconut seed. Farmers harvest coconut for the seed, while the husk and the shell are usually discarded as waste. Coconut husks, like many other agricultural waste products, are found dumped indiscriminately in areas where coconut is harvested, posing serious environmental sanitation challenges. The scope of the study involved the determination of the compaction and strength characteristics (in terms of CBR and UCS) of different combinations of lateritic soil and coconut husk ash. The scope also covered the determination of chemical composition of CHA and the index properties of the lateritic soil.

Based on the results of the study, the following conclusions can be drawn: (1) The lateritic soil in its natural state is weak and unsuitable for pavement layer construction; (2) CHA can be classified as a pozzolanic material; (3) CHA altered the compaction characteristics of the soil. The OMC of the CHA-treated soil increased with CHA content, while the MDD decreased with an increase in the CHA content of the soil. This trend is similar to that established for soil stabilization projects involving pozzolanic materials; (4) CHA also produced measurable effects on the strength characteristics of the soil measured in terms of CBR and UCS. The CBR and UCS values of the CHA-treated soil increased with CHA content up to a maximum of 8% CHA, beyond which the values decreased with an increase in CHA. The optimum CHA content was therefore found to be 8% of the soil’s dry mass; and (5) CHA, as a lone additive, can be used to improve the strength of subgrade materials. If admixed with conventional stabilizers (such as Portland cement and lime), it can be used for the improvement of sub-base and base soil materials.

This paper could not have been completed without the tremendous amount of background information made available by various research workers, and authors of excellent books and articles, which have been referred to and are listed in the references. We thank them.

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