Cement Stabilization of Black Cotton Soil Using Locust Bean Waste Ash as Admixture
CHAPTER ONE
Aim and Objectives
The aim of this research is to establish the effect of using locust bean waste ash, as an admixture, on the geotechnical properties of cement stabilized black cotton soil.
The objectives are highlighted below.
- Determination of the natural properties of the black cotton soil
- Determination of the properties/oxide composition of LBWA
- Evaluation of the effect of LBWA (0, 2, 4, 6 and 8% by dry weight of soil) on the index and strength properties of the black cotton soil stabilized with cement (0, 2, 4, 6, and 8% by dry weight of thesoil)
- Determination of the optimum amount of cement and LBWA admixtures needed for the stabilization of the soil.
CHAPTER TWO
LITERATURE REVIEW
Expansive Soils
Expansive soil refers to soil material that has the potential for swelling and shrinking due to changing moisture conditions. The major problem is that its deformations are significantly greater than elastic deformation, thereby preventing its prediction by simple classical elastic or plastic theories. The movement is usually in an uneven pattern and of such as magnitude as to cause extensive damage to the structures and pavements resting on them (Nelson and Miller, 1987). Expansive soils cause more damage to structures particularly light buildings and pavements than any other natural hazard, including earthquakes and floods. The annual cost of these damages to civil engineering structures is estimated at one billion dollars in the United States and many more billions of dollars worldwide (Gourley et al., 1993).
Expansive soils can be found anywhere in the world but they are basically confined to semi–arid and arid regions of the tropical/temperate zones. These areas are naturally characterized by marked dry and wet seasons with low rainfall, poor drainage and exceedingly great heat. The climate condition is such that the annual evapotranspiration exceeds the precipitations (Chen, 1988). They are found in the north-eastern part of Nigeria, Cameroon, Lake Chad, Sudan, Ethiopia, Kenya, South Zimbabwe, South Africa and other Eastern African countries. They are also found in India, Australia, South- Western USA and Israel (Ola, 1978).
Two groups of parent materials have been associated with the formation of expansive soils. The first group comprises of sedimentary rocks of volcanic origin which can be found in North America, South Africa and Israel (Ola, 1978) while the second group of parent materials are basic igneous rocks found in India, Nigeria and South–Western USA (Plait, 1953). The most well–known example of expansive soils is the black cotton soil which is dark–grey to black in colour and the name originated from India where locations of these soils are favourable for growing cottons. It is also the Nigerian type of expansive soils (Osinubi, 1999).
CHAPTER THREE
MATERIALS AND METHODS
Materials
Black cotton soil
The disturbed soil samples used for this study were collected at the Chad Basin Development Authority (CBDA) reserved site at New Marte (Latitude 130 27’N and longitude 130 50’E) along the Maiduguri–Gamboru Road in Borno State. The top soil was removed to a depth of 0.5m before the soil samples were taken, sealed in plastic bags and put in sack to avoid loss of moisture during transportation. The soil samples were then allowed to dry before pulverizing to obtain particles passing sieve BS No. 4.
Although it was reported by Nelson and Miller (1992) that there is no standard classifications for expansive soils, different methods are practically employed in different parts of the world. NBRRI (1983) classified the variability of black cotton soil in terms of particle size distribution, clay and silt content, liquid and plastic limits as well as swelling potentials; as summarized in Table 3.1.
CHAPTER FOUR DISCUSSION OF RESULTS
Properties of Materials Used in the Study
Natural soil
Preliminary tests conducted to determine the natural properties of the soil revealed that the soil has very high moisture content of 35%, due the period of its collection (in September during the rainy season). The index properties are summarized in Table 4.1, while its oxide compositions are summarized in Table
4.2. The soil belongs to the CH group in the Unified Soil Classification System (ASTM, 1992) or A-7-6(13) soil group of the AASHTO soil classification system (AASHTO, 1986). The soil is grayish black in colour (from wet to dry states) with a liquid limit of 63%, plastic limit of 27% and plasticity index of 36%.
The soil has a free swell of about 75%, soaked CBR values of 5%, 6% and 9% for the three energy levels of British Standard Light, West African Standard and the British Standard Heavy, respectively and UCS values of 151, 324 and 637kN/m2 for the energy levels, respectively. The soil was found to be highly plastic and falls below the standard recommendation for most geotechnical construction works especially for sub-base or base courses in highway construction (Butcher and Sailie, 1984; Osinubi and Medubi, 1997).
CHAPTER FIVE CONCLUSION AND RECOMMENDATION
Conclusion
The preliminary investigation conducted on the natural black cotton soil collected at Chad Basin Development Authority (CBDA), New Marte, Borno State shows that it falls under A-7-6(13) classification for AASHTO (1986) and CH according to Unified Soil Classification System (ASTM, 1992). The natural soil has high moisture content of 35% because it was collected during the rainy period. It has liquid limit of 63%, plastic limit of 27%, plasticity index of 36%, linear shrinkage of 17%, free swell of 75%, specific gravity of 1.94 and NBRRI Classification of high swell potential. All these values indicate that the soil is highly plastic with about 71% of the soil particles passing the B. S. No 200 sieve. The strength characteristics are also very low, thereby rendering the soil unfit for sub-base or road base courses.
In an effort to raise the soil’s suitability for engineering use, the air dried samples were treated with OPC/LBWA in stepped concentration of 0, 2, 4, 6, and 8% by dry weights of the soil. The tests conducted showed that the liquid limit of the natural soil increased from 63% to 77% at 6% OPC/6% LBWA. The plastic limit, however, decreased from 26.6% for the natural soil to 21.4% at 6% OPC/4% LBWA. The plasticity index values for all the concentration of the additive exceeded the 30% value prescribed for sub-grade materials by the Nigeria General Specification (1997).
The MDD increased with higher additive blends and compactive efforts; which is in conformity with the same trend reported by Osinubi (1999a), Osinubi et al. (2007b), Staphen (2005) and Akinmade (2008). The peak MDD values recorded for BSL, WAS and BSH compactive efforts are respectively 1.4, 1.5 and 1.6Mg/m3 at 6% OPC/6% LBWA treatment. The OMC, on the other hand, decreased with higher compactive efforts but increased with higher LBWA contents. The decrease may be due to the effect of breakdown of the flocculated aggregates and elimination of large pores, when higher compactive energies were used. The optimum moisture content values at the natural states increased from 24, 21 and 19% to 40, 38 and 33% when compacted using BSL, WAS and BSH energies at 8% OPC/6% LBWA, 6% OPC/8% LBWA and 6% OPC/6% LBWA, respectively.
The unconfined compressive strength (UCS) values for natural soil compacted with BSL, WAS and BSH energies at 7 days curing period are 179, 381 and 750kN/m2 respectively and increased to 986, 1436 and 1650kN/m2 at 6% OPC/6% LBWA treatment. The values of UCS for both BSL and WAS fell short of the requirement based on Road Note 31 (TRRI, 1977) requirement for economic range of OPC stabilization. However, the UCS value of the BSH compaction could be acceptable for base courses of pavements.
The 28 days curing period UCS values obtained showed that the OPC/LBWA blend has a long term advantage in terms of strength gain. There were tremendous increments in the values of UCS from their natural states. The 28 days curing period UCS produced a peak value of 2616kN/m2 at 6% OPC/6% LBWA for BSH compaction, showing that the soil treated with this blend can be used (at BSH compaction) as base course of pavement material.
The unsoaked CBR values of 5, 7 and 11% (for the natural soil) compacted with BSL, WAS and BSH energy efforts, respectively, increased to 46, 77 and 83% at 6% OPC/6% LBWA. The 24 hours soaked CBR values recorded peak values of 42, 66 and 66% with BSL, WAS, BSH energies, respectively, which showed about 10-15% decrease from the unsoaked CBR values. The CBR values of 66% with BSH compaction at 6% OPC/6% LBWA blend can be used as sub base material because it meets the 29% recommended values for sub-base by the Nigerian General Specification (1997). Also the 42% recorded with BSL compaction at 6% OPC/6% LBWA treatment meets the 15% recommended for subgrade material by the Nigerian General Specification (1997).
The resistance to loss in strength of the soil increased from 13, 7, and 15% for the natural soil to peak values of 42, 13 and 71% for BSL, WAS and BSH energies at 6% OPC/8% LBWA, 8% OPC/0% LBWA and 6% OPC/6% LBWA, respectively. Only 71% resistance to loss in strength (29% loss in strength) at 6% OPC/6% LBWA with BSH compaction is close to the limiting value of 80% resistance to loss in strength (Ola, 1983) based on 4 days soaking. The 6% OPC/6% LBWA treatment of the soil can be used, at BSH compaction, for sub- base material because the soil was subjected to a harsher condition (of 7 days soaking) and due to the time dependent gain in strength advantage of the pozzolana.
RECOMMENDATION
Based on the results obtained, the optimum blend of 6% OPC/6% LBWA treatment on the black cotton soil is to be used as sub-base material when compacted with the British Standard heavy energy.
REFERENCES
- AASHTO (1986). Standard Specifications for Transport Materials and Methods of Sampling and Testing. 14th Edition, American Association of State Highway and Transport Officials (AASHTO), Washington, D.C
- Aitken, D. H. (1971). “Transportation of geomechanics to roads and bridges.” Proc. Of the 1st Australia-New Zealand Conference on Geomechanics, Melbourne. Vol. 2, p. 489.
- Akinmade (2008). The Effect 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.
- 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.
