Stabilization of Black Cotton Soil With Locust Bean Waste Ash
Chapter One
Aim and Objectives of the Study
This study was aimed at establishing the effect of locust bean waste ash on cement modified black cotton soil. This was achieved through the following specific objectives:
- Determination of the properties of the natural properties
- Modification of black cotton soil using known quantities of cement and locust bean waste ash as admixture when compacted with the British Standard light (BSL), West African Standard (WAS) and British Standard heavy (BSH) conpaction
- Evaluation of the effect of locust bean waste ash on the cement modified black cotton soil using particle size distribution, Atterberg limits, compaction tests and shear strength
- Determination of the optimum mix ratio of cement – locust bean waste ash for optimum workability of black cotton
CHAPTER TWO
LITERATURE REVIEW
Soil Modification /Stabilization
Generally, geotechnical structures are to be founded on good and sound engineering soil in order for them to attain their design life span. If these structure are founded on soil with low bearing capacity, they are likely to fail either during or after construction, with or without application of wheel load on them. The black cotton soil is an expansive soil with low bearing capacity, has the ability to absorb and dissipate water with subsequent change in volume. Construction of any structure on this type of soil requires either replacement of the soil by importing a better foreign one or by addition of chemical(s) that will improve the desired properties of the soil. But if the construction involves a large area of land like road construction, replacement of soil by importing new materials or avoiding them is impossible and the soil will therefore have to be stabilized in-situ (Osinubi, 1995).
Well built and maintained roads play major role in the development of a nation. Sub-grade soil form the integral part of the road pavement structure as it provides the support to the pavement from beneath. Therefore, the knowledge of properties of sub- grade soil are very important in the design of road pavement as well as other engineering structures. The main function of the sub-grade is to give adequate support to the pavement and for this, the sub-grade should posses sufficient stability under adverse climate and loading condition. If the weak sub-grade is stabilized, the required crust (outer layer) thickness will be less, rutting is restricted resulting in less repair and overall economy. Basically, there are three majors ways of modifying/stabilizing soils for engineering purposes. These are mechanical, geosynthetic and chemical modification. Modification is restricted to clayey soils of the AASHTO A-4, A-5, A-6, and A-7 (Office of geotechnical Engineering, 2008).
Modification occurs as a result of calcium cations supplied by modifier replacing the cations present on the surface of clay minerals, promoted by the high pH environment of the modifier – water system. Thus altered, the clay surface mineralogy results in plasticity reduction, reduction in moisture holding capacity, swell reduction, improved stability and the ability to construct working platform as benefit (National Lime Association, 2001).
Mechanical modification / stabilization
Mechanical stabilization is the process of altering soil properties by changing the gradation through mixing with other soil, densifying the soil using compactive efforts, or undercutting the existing soil and replacing them with granular material to improve the soil engineering properties of strength, permeability and compressibility. An existing soil may have poor engineering properties perhaps because of excess clay, silt or fine sand. If a suitable soil is located within a reasonable haul distance, blending the soil together could effect an improvement in the existing soil.
A common remedial procedure for wet and soft grade is to cover it with granular materials or to partially remove and replace the wet sub-grade with a granular material to a predetermined depth below the grade line. The compacted granular layers distribute the wheel loads over a wider area and serve as a working platform (Thompson, 1977).
To provide a firm working platform with granular material, the following condition shall be met. (Office of Geotechnical Engineering, 2008):
- The thickness of the granular material must be sufficient to develop acceptable pressure distribution over the wet
- The backfill material must be able to withstand the wheel load without
- The compaction of the backfill material should be in accordance withthe standard Based on experience, 300 to 600 mm granular material should be adequate for sub-grade modification or stabilization.
Chemical Modification / Stabilization
For close to half a decade, the concern of the geotechnical engineer has been to make poor engineering soil much better. This stimulated research into the available chemical or mechanical means of modifying the soil. These chemicals are mainly industrial waste which pose environmental problems. In the absence of organic matter, when a soil contain a certain amount of fines that cause plastic behaviour of the soil, modification is often recommended (Osinubi and Katte, 1997). Modification is a broad term used to describe any technology or operation that is used to improve soil characteristics and as with stabilization, it involves the use of different kinds of agents. These agents include cement, lime, bitumen, fly ash, etc. These chemicals have been used either single or in combination with one another.
Soil modification or stabilization started since the 1960s in Europe and has spread world-wide, since problematic soils are wide-spread. According to Osinubi (1995) cement and lime, mostly used for modification or stabilization, change the water film on the soil particles, modify the clay minerals to some extent and decrease the soil plasticity index. The main purpose of soil modification is to improve the particles size, plasticity index and durability under adverse moisture and stress conditions (Osinubi, 2002). Traces of the modifying agent(s) are added to the soil mass in appropriate proportions. The modifier and soil must be mixed thoroughly in order to achieve the desired strength and durability. To achieve this goal in the laboratory, the natural soil structure is destroyed by grinding when preparing the soil. In the field however, successful modification depends on the ability of the modifiers to penetrate large lumps of soil which may contain coarse aggregates and to modify the active constituents (Osinubi, 2002). The most appropriate method used for any situation depends on the economics, engineering requirement and the soil characteristics which have to be determined.
CHAPTER THREE
MATERIALS AND METHODS
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Materials
Black Cotton Soil
The soil sample used for this study was collected at the Chad Basin Development Authority (CBDA) reserved site at New Marte (Latitude 13o27IN and longitude 13o50IE) along Maiduguri – Gamboru Road in Borno State. It was collected as disturbed sample at a depth between 0.5 m to 1.5 m after removing the top soil to avoid organic matter. The soil sample were taken, sealed in plastic bags and put in sack for determination of natural moisture contents. The soil samples were then allowed to dry before pulverizing to obtain particle passing sieve BS No. 4. A study of geological and soil maps of Nigeria after Akintola (1982) and Areola (1982), respectively, shows that the parent materials in the study area is basic igneous rock which when weathered formed weakly developed black cotton soil. This location lies within north eastern part of Nigeria extensively covered by black cotton soil.
Locust Bean Waste Ash
The locust bean waste ash (LBWA) used for the study was obtained locally from the burning of the locust bean husks source from Doko, Vunchi and Agaie villages around Bida area in Niger State. The husk of locust bean generated through human activities were collected from dumps in the villages and stockpiled. Smith (1992) as well as Mohammedbhai and Baguant (1990) reported that heat treatment plays a vital role in the production of pozzolanas from agriculture waste. Therefore stockpiled husk of locust bean generated was burnt to ash in open air on a galvanized iron roofing sheet and cooled before being passed through BS No 200 sieve and kept to be mixed with the soil cement in the appropriate percentages.
The oxide composition of the locust bean ash was determined at the Centre for Energy Research and Training (CERT) A.B.U, Zaria by the method of Energy Dispersive X – Ray Fluorescence (XRF). The specification for pozzolanas are given in
CHAPTER FOUR
RESULTS AND DISCUSSION
Properties of Material Used in the Study
Natural Soil
The result of the test for the identification of the natural soil and the properties revealed that the soil has very high moisture content of 35%, this is attributed to the period of sample collection (during rainy season). The index properties and oxide composition of the black cotton soil are summarized in Tables 4.1 and 4.2, respectively. Based on the American Association of State Highway Transportation Officials (AASHTO, 1986) classification, the soil is classified as an A-7-6 (24) and based on the Unified Soil Classification System (USCS) the soil is a CL soil. The soil is greyish black in colour (from wet to dry) with a liquid limit of 63%, plastic limit of 27% and plasticity index of 36%. Details of test result are shown in Tables A4.1 to A4.4 in the appendix.
Based on the Nigerian General Specification (1997) and the Highway Research Board (1943), suitability limit of 50% passing BS No. 200 sieve, 40% liquid limit and 18% Plasticity Index requirements the soil is found to be unsuitable for direct use as a base course or sub-base course and would therefore require initial modification to improve its workability.
CHAPTER FIVE
CONCLUSION AND RECOMMENDATIONS
Conclusion
The preliminary investigation conducted on the natural black cotton soil collected at the Chad basin Development Authority (CBDA) New Marte, Borno State shows that the soil falls under A-7- 6 (24) classification for AASHTO(1986) and CL using the Unified Soil Classification System (USCS). The natural soil has high moisture content of 35% because it was collected during the rainy season. It has a liquid limit value of 63%, plastic limit of 27% and plasticity index of 36% specific, gravity of 2.01 and the predominant clay mineral is montmorillonite. All these properties indicate that the soil is highly plastic with about 80% of the soil particles passing the BS.No. 200 sieve. The workability of the soil is very low and this makes it unsuitable for geotechnical engineering use.
In an effort to improve the workability of the soil for engineering purpose, the air dried sample were treated with up to 4% OPC to 8% LBWA blend in stepped concentration of 0,2,4,6 and 8% by dry weight of the soil. The test conducted showed that the percentage of particles passing the BS No. 200 seive size tremendously reduced from 80% for the natural soil to almost zero for all the energy levels considered. Also the liquid limit of the natural soil increased from 63 to 79% at 4%OPC / 6% LBWA treatment. The plastic limit however decreased from 26.6% for the natural soil to 24.6% at 4%OPC/6% LBWA treatment. The plasticity index for all the concentration of additive exceeded 12% value prescribed for sub-base and base courses by Nigeria General Specifications (1997).
The MDD increased for soil compacted using British Standard light energy level from 1.30Mg/m3 for the natural soil to 1.48 Mg/m3 at 4% OPC / 6%LBWA treatment. For WAS compaction the MDD increased from 1.42 Mg/m3 for natural soil to 1.55 Mg/m3 at 4% OPC / 6% LBWA treatment. For BSH compaction, the value increased from 1.50 Mg/m3 for the natural soil to 1.62 Mg/m3 at 4%OPC/6% LBWA treatment. The OMC on the other hand decreased with higher compactive efforts, but increased with high LBWA content. The optimum moisture content values at the natural states increased from 24, 21 and 19% to 39,33 and 30% when compacted using BSL, WAS and BSH energies at 4%OPC/6%LBWA treatment, respectively.
The angle of internal friction for the cement modified soil increase while the cohesion decreased. The angle of internal friction values increased from 180,140 and 160 for the natural soil to 35.5o,45o and 40o at 4%OPC/6% LBWA blend for BSL, WAS and BSH compaction, respectively. The cohesion value decreased from 75, 70 and 60kN/m2 for natural soil to 22, 15 and 20 kN/m2 treatment for BSL, WAS and BSH compactions, respectively.
Recommendation
Based on the results of the investigation of the effect of locust bean waste ash on cement black cotton soil, an optimum of 4% OPC/6% LBWA is recommended for the modification of black cotton soil used in road construction.
REFERENCES
- AASHTO (1986). Standard Specifications for Transportation Materials and Methods of Sampling and Testing. 14th Edition, American Association of State Highway and Transport Officials (AASHTO), Washington, D.C
- Abdullahi M. (2003) The Use of Rice Husk Ash in Low – cost Sandcrete Production.
- Unpublished M. Eng. Thesis, Department of Civil Engineering, Federal University of Technology, Minna
- Akinmade (2008). The Effects of Locust Bean Waste Ash on the Geotechnical Properties of Black Cotton Soil. Unpublished M.Sc. Thesis, Department of Civil Engineering, Ahmadu Bello University, Zaria.
- Akintola, F. A. (1982). “Geology and Geomorphology.” In : Nigeria in Maps edited by M. Barbous, Hodder and Stoughton, London.
- Areola, O. (1982). “Soil” In: Barbous, K.M. (eds) Nigeria in Maps Hodder and Stoughton, London.
- ASTM C618 – 78(1978). Specification for Fly Ash and Raw or Calcined Natural Pozzolanas for Use as a Mineral Admixture in Portland Cement Concrete. American Society for Testing and Materials, Philadelphia.
- ASTM (1992). Annual Book of Standards Vol. 04.08, American Society for Testing and Materials, Philadelphia.
- BS, 1377 (1990). Methods of Testing Soil for Civil Engineering Purposes. British Standards Institute, London.
