The Effect of Elapse Time on the Geotechnical Properties of Lime-bagasse Ash Stabilized Black Cotton Soil
Chapter One
Research Aim and Objectives
This research aimed to establish the effect of delay between mixing and compaction on the properties of black cotton soil stabilized with lime admixed with bagasse ash. The research aim was achieved through the following objectives:
- Evaluation of the natural properties of the black cotton
- Evaluation of the engineering properties of the soil mixed with 0, 2, 4, 6 and 8 % lime admixed with 0, 2, 4, 6 and 8 % bagasse ash in
- Establishment of the engineering properties of the soil-lime-bagasse ash mixed after 0, 1, 2 and 3 hours
The parameters investigated were:
Compaction characteristics (i.e., dry density and moisture content), Atteberg limits, unconfined compressive strength (UCS), durability, and California bearing ratio (CBR).
CHAPTER TWO
LITERATURE REVIEW
Expansive soil refers to soil material that has the potential for swelling and shrinking due to changing moisture conditions. The major problem arising with regard to expansive soils is that the deformations are significantly greater than elastic deformations and they can not be predicted by classical elastic or plastic theory. The movement is usually in an uneven pattern and of such magnitude as to cause extensive damage to the structures and pavements resting on them (Nelson and Miller, 1992). Expansive soils caused more damage to structures, particularly light buildings and pavements than any other natural hazard, including earthquakes and floods. The annual cost of damages to civil engineering structures is estimated at one billion dollars in the United State and many more billions of dollars worldwide (Gourley et al. 1993).
The black cotton soil so named on account of its black colour and great suitability for growing cotton is one example of an expansive soil. It is generally found on sedimentary plains as a result of thousands of years erosion of the clay content out of the surrounding hills. The soil can also be found on level locations and in depressions. The soils are confined to the semi arid regions of the tropical and temperate climate with marked alternating wet and dry seasons and where the annual evaporation exceeds precipitation. Globally, the soil occupies about 3 % of the world land area (i.e., about 340 million hectares).
In Nigeria, this soil occupies an area of some 10.4 x 10 km2 (i.e., 400000 sq miles) in the North Eastern fringe of the country (Klinkenberg and Higgins, 1972). Near Maiduguri the Chad formation is known to be about 550 m (1800 ft) thick.
Geology of Black Cotton Soils
Generally, black cotton soils are formed in areas where the parent material is basic igneous rocks such as basalt that are made up of calcium rich feldspars and dark minerals which are high in the weathering order (i.e., unstable). All the constituents weather to form amorphous hydrous oxide and under suitable conditions clay minerals develop. The absence of quartz leads to the formation of fine-grained, mostly clay-size, plastic soil which is highly impermeable and easily becomes waterlogged. In addition, abundant magnesium and calcium present in the rock add to the possibility of forming of black cotton soils with their attendant swelling problems.
The parent materials from which these soils are generally formed are sedimentary rocks also of volcanic origin. The tuffs and ashes are made up of volcanic dust which is essentially a collection of minute particles of volcanic glass. These materials readily weather to form montmorillonite clays, the major clay mineral of the black cotton soils (Ola, 1983).
Other conditions favouring the formation of black cotton soils are evaporation exceeding precipitation, poor leaching, alkaline conditions and retention of magnesium and calcium in the soils. All these conditions prevail in the area of occurrence of the Nigerian black cotton soils (Ola, 1983).
General Characteristics of Black Cotton Soil
Black cotton soil (BCS) being an expansive soil swells excessively exerting many kilo Newton per square area of swelling pressure when wet and shrinks extremely, developing cracks, often measuring 70 mm wide and 1.0 m deep ( Adeniji,1991) and may extend up to 3.0 m in case of deep deposit. When wet, the soil has high index properties, its bearing value and strength are low. These undesirable properties of BCS are consequent of the presence of the expansive clay mineral, montmorillonite, which is abundant in the soil. Ola (1983) reported 70 % montmorillonite in the Nigerian black cotton soils.
Generally, black cotton soils have comparatively high percentage of clay, more than 90 % with substantial proportions of silt and sand. Their organic content is low and they are alkaline in composition with a pH greater than 7.0. The soil is black to grey in colour and it contains a very high percentage of humus (i.e., 3 to 15 %).
Mineralogical and Chemical Composition of Black Cotton Soil
The expansive characteristics of soils are dictated by the types of clay minerals, specific surface areas of the clay particles and the chemistry of the soil water surrounding these particles. Although there are different types of clay minerals, those that are composed of montmorillonite and bentonite are the most expansive. Ola (1983) showed that the Nigeria black cotton soil contains about 70 % montmorillonite and 30 % kaolinite. The high percentage of montmorillonite in the black cotton soil imparts the swell capacity characteristics of the black cotton soil.
The montmorillonite clay structure consists of layer sheet formed and stacked one above the other. Bonding between successive layers is by van de Waals force and by cation that may be present to balance charge deficiencies in the structure. These bonds are weak and easily separate by cleavage or adsorption of water and other liquid.
There is an extensive isomorphous substitution for aluminium and silicon with the lattice of the montmorillonite crystal. Aluminium in the octahedral sheet may be replaced by Magnesium, Zinc, Iron, Nickel, Lithium or other cations. Aluminium may replace up to 15% of the silicon ions in the tetrahedral sheet. Possibly, some of the silicon positions can be occupied by phosphorous (Grim, 1968). Isomophous substitution in the clay mineral gives the clay a net negative charge resulting in the water absorbing tendencies as there is an attraction for hydroxyl ions and water molecule to the clay surface.
Mechanism of Soil Volume Change
Most clay minerals have orderly arrangement of atoms that form characteristic crystal lattice and most of these crystals are small in size. A typical thickness can be as small as 15Å and lateral dimensions are on the order of microns. All of the clay minerals groups have a layered crystal structure. The mineral groups are:
Kaolinite group – generally non-expansive.
Mica-like group – includes illites and vermiculites, which can be expansive but generally do not pose significant problems.
Smectite group – includes montmorillonite, which are highly expansive and are most troublesome clay minerals. The clay minerals have large specific surface. For example, the smectite group has its primary surface (i.e., surface due to particle surface exclusive of interlayer zones) generally in the range of 50 to 120 m2/g. The secondary specific surface that may be exposed by expanding the lattice so that polar fluids can penetrate between layers may range from 700 to 840 m2/g.
CHAPTER THREE
MATERIALS, METHODS AND TESTS RESULTS
Materials
Black cottonsoil
The black cotton soil was obtained from Deba in Gombe State in the North Eastern part of Nigeria. The soil was collected by method of disturbed sampling. The top soil was removed to a depth of about 0.5 m and the soil samples were taken below 0.5 m, sealed in plastic bags and put in sacks. This was done to avoid loss of moisture when transporting the soil. In the laboratory, the soil was pulverized to obtain particle passing sieve through British Standard No. 4 sieve, (4.75 mm aperture).
- Bagasseash
The bagasse ash used for this work was obtained from Anchau, Kaduna State.
The fibrous residue (after the juice has been extracted) of sugar cane was obtained from the local sugar manufacturing mills and burnt in the open atmosphere. The ash was collected after complete burning sealed up and transported to the laboratory. The ash was then passed through British Standard No. 200 sieve, (75µm aperture). The material passing the sieve was then mixed up in the percentages of 0, 2, 4, 6 and 8 % with the soil and lime to obtain the required sample for the tests.
The oxide composition of the ash was determined at the Center for Energy Research and Training (CERT), A. B. U. Zaria, using the method of Energy Dispersive X-Ray Fluorescence (EDXRF). The result is shown in Table 3.1.
CHAPTER FOUR
ANALYSIS AND DISCUSSION OF RESULTS
Preliminary Tests
Index properties of the naturalsoil
The soil is predominantly fine-grained. It is greyish – black in colour with a liquid limit of 60 %, plastic limit of 22 % and plasticity index of 38 %. The soil was classified as CH or A-7-6 using the Unified Soil Classification System (USCS) and AASHTO classification, respectively. The soil has a free swell of about 40 %. The CBR values are 4, 6 and 9 % for British Standard light, West African Standard and the British Standard heavy compactive efforts, respectively. The soil is thus unsuitable for use as a sub-grade material.
CHAPTER FIVE
CONCLUSIONS AND RECOMMENDATIONS
Conclusions
This research was conducted to investigate the effect of delay between mixing and compaction on the geotechnical properties of black cotton soil-lime-bagasse ash mixes. The problem soil was treated with additives in stepped concentration of 0, 2, 4, 6 and 8 % by dry weight of the soil.
The natural soil was classified as CH and A-7-6(28) using the Unified Soil Classification System (USCS) and AASHTO soil classification, respectively. It was established that delay between mixing and compaction affect the properties of the soil-lime-bagasse ash mix.
The natural soil had a liquid limit of 60 %, plastic limit of 22 % and plasticity index of 38%. The liquid limit of the soil reduced from 60 to 37 % at 8 % lime/2 % bagasse ash treatment at three hours elapse time. The plastic limit reduced from 22 to 11 % at 8 %lime/6 % bagasse ash treatment after three hours elapse time. The plasticity index decreased with elapse time and a minimum value of 23 % was obtained from a maximum value of 38 % at 8 % lime/ 4 % bagasse ash treatment after three hours elapse time.
Generally, maximum dry density (MDD) increased with higher compactive effort but decreased with higher lime/bagasse ash treatment as well as elapse time between mixing and compaction. The MDD of the natural soil compacted at the energies of the British Standard light (BSL), West African Standard (WAS) and British Standard heavy (BSH) were 1.47, 1.60 and
1.83 Mg/m3, respectively. For specimens treated with 6 % lime/8 % bagasse ash and compacted
at the energy levels of BSL, WAS and BSH three hours after mixing, the MDDs reduced from the natural values obtained for soil to 1.34, 1.29 and 1.42 Mg/m3, respectively.
A general trend of increasing optimum moisture content (OMC) with increasing lime/bagasse ash treatment as well as elapse time between mixing and compaction for all the compactive efforts was observed. For specimens compacted at the energy levels of the BSL, WAS and BSH the OMCs increased from 21, 18 and 13 %, respectively for the natural soil to 32, 26 and 24 %, respectively, when specimens were treated with 6 % lime/8 % bagasse ash and compacted 3 hours after mixing.
The 7 day unconfined compressive strength (UCS) values of the soil in its natural state were 173, 343 and 633 kN/m2 for specimens prepared at the BSL, WAS and BSH compactive efforts, respectively. UCS values increased with higher lime and bagasse ash treatments for all the compactive efforts and curing periods considered. However, UCS of specimens decreased with elapse time for all compactive efforts. There was a decrease in 7 day UCS from a value of 410 to 311 kN/m2 for specimen treated with 6% lime/8% bagasse ash and compacted at the BSL energy level 3 hours after mixing. At the same energy level for 14 and 28 days curing periods, the UCS decreased from 510 and 540 kN/m2 to 360 kN/m2 and 380 kN/m2 respectively when specimens were treated with 6 % lime/8 % bagasse ash. At the same lime-bagasse ash ratio, the peak UCS for West African Standard energy level decrease from 680, 750 and 850 kN/m2 to 490, 600 and 620 kN/m2, and from 1160, 1210 and 1330 kN/m2 to 780, 960 and 1020 kN/m2 at the British Standard heavy energy level at 7, 14 and 28 days curing periods after three hours elapse time respectively.
Although, there is no established strength criterion for soil-lime/bagasse ash mix, using the 7 day UCS value of 1034.25 kN/m2 normally utilized as criterion for adequate lime stabilization, it is observed that sample stabilized with 6 % lime/8 %bagasse ash did not achieve the required strength. The strength at 28 days however showed that the strength development of lime/bagasse ash is a slow process and a longer period would be required to attain the specified strength.
Durability assessment of the soil-lime-bagasse ash mix showed a decline in the resistance to loss in strength of the mixes with elapse time for all compactive efforts. There was however some increase in the UCS value of the mixes with increase in lime/bagasse ash content. A peak value of 50 kN/m2 was attained at 6 % lime/8 % bagasse ash for BSL compactive effort. This declined to 20 kN/m2 after three hours. Similarly, peak values of 120 and 200 kN/m2 were attained for WAS and BSH energy levels at 6 % lime/8 % bagasse ash treatment. These values decreased to 58 and 60 kN/m2 after three hours elapse time. The resistance to loss in strength values of tested specimens fell far short of the acceptable conventional 80 %, as reported by Ola (1974).
The CBR of the soil increased with increase in lime/bagasse ash content for all compactive efforts. Peak values of 14, 24, and 35 % were achieved at 6 % lime/8 % bagasse ash treatments for compaction BSL, WAS and BSH energy levels, respectively. These values represent increases of about 10, 18 and 25 % over the values of the natural soil of 4, 6, and 9 % respectively. The CBR values also showed gradual and steady decreases in values with elapse time. A decrease from 14 to 5 % after three hours elapse time was observed at 6 % lime/8 % bagasse ash treatment for BSL energy level. At the WAS and BSH compactive efforts, decreases from 24 to 11 % and 34 to 20 %, respectively, at 6 % lime/8 % bagasse ash treatment after three hours were recorded. None of these mixes fulfilled 30 % soaked CBR requirement for sub-base reported by Gidigasu and Dogbey (1980) as well as Osinubi (2001).
Recommendations
Based on the results obtained from this study, it is recommended that an optimal mix of 6 % lime/8 % bagasse ash be used for the treatment of black cotton soil compacted at British Standard heavy energy level. The delay between mixing and compaction should not exceed one and half hour.
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