Effects of Rock Flour on Some Engineering Properties of Lateritic Soil

Headnote

Abstract: For soil to be suitable for high way design and constructions, they must meet existing local requirements for index properties in addition to certain strength criteria. Samples of lateric soil from two locations along Igbatoro road in Southwestern Nigeria were differently stabilized with varying quantities of rock flour from 2% to 10%. The objective of this study was to determine the effect of rock flour on some engineering properties of lateric soil. This investigation includes evaluation of properties such as shrinkage limits, atterberg limits, natural moisture contents, compaction, California bearing ratio and unconfined compressive strength of the soil with rock flour contents of 2%, 4%, 6%, 8%, and 10% by weight of the dry soil. The results obtained shows that the addition of rock flour improved the engineering properties of the soil. This investigation also confirmed that rock flour is an appropriate stabilizer with optimum amount of stabilizer needed as 4%.

Keywords: Engineering properties, lateritic soil, rock flour.

1. Introduction:

Lateritic soils refer to highly weathered soils formed from materials with lower concentrations of oxides or hydroxides of iron and aluminium (Amu et al. 2011a). Laterization is the process whereby rocks are converted to laterite by enrichment of iron oxide and depletion of silica. Laterization is favored by a warm climate with alternating wet and dry seasons (Ushie and Anike, 2011). Soil stabilization is a means of modifying physical and chemical characteristics of a given soil with a view to improve its engineering properties. High water content and low workability of some soil used as fill materials pose difficulty for construction project. Most of the time, additives such as rock flour, cement, lime, asphalt and polymer stabilizers are used to improve their engineering properties. The choice and the effectiveness of an additive depend on the type of soil and its field conditions (Balasigam and Farid, 2008). Studies in the past, such as Oloruntola et al 2008, Akpokodje, 1985, Amu et al 2011b, Amadi , 2010, Rahaman et al, 2010, Amu et al 2010, Okunnade, 2010, Okafor and Okonkwo, 2009 have shown that engineering properties of soil can be modified by the use of chemical stabilizers

In this study, rock flour was used as the stabilization agent and was systematically examined through series of index and engineering tests to determine its effectiveness on lateritic soil in Nigeria which is used for various purposes such as high way embankment applications. In the recent past, cement treatment were widely used for the improvement of the mechanical properties of soils, but due to the high cost of Portland cement, other low cost additives such as rock flour, lime, rice husk are now been considered and their effectiveness has to be determined. Rock flour are the by products of crystalline rocks such as granite and gneissic rocks. It is the finely divided and powdery form of these rocks. Lateritic soil, are often used as imported fill material for the prepared sub grade in many road projects (Okunnade, 2010, Amadi, 2010.) The soil investigated is along Igbatoro road Southwestern Nigeria. The study area is located within longitude 40 401 and 50 401 East of Greenwich Meridian and latitude 70 021 and 70 321 North of Equator.

The drainage pattern here is dendritic, showing the resistance of the underlying crystalline basement rock to weathering. The climate is tropical and the vegetation is of the rain forest type, but in some places cultivation has altered the natural vegetation. It is characterized by undulating topography with highlands at elevation of about 200m to 250m above the sea level.

The study area is part of the Basement complex of southwestern Nigeria. Six major groups of rocks namely; Migmatite- Gneiss complex, Schist belt, Charnockitic, Gabbroic and dioritic rocks, older granite, hypabyssal rocks and metamorphosed dolerites and syenites has been identified in the study area (Rahaman, 1976 ; Rahaman ,1988). The major rock types observed in the field are Charnockites, Granites, Biotite and Granite gneiss. These rocks are often intruded by Pegmatite, quartz and Quartzo - feldspartic veins.

The purpose of this study is to determine the effects of rock flour on engineering properties of lateritic soils when used as a stabilizer.

2. Materials and Methods:

This study was conducted between August and November 2011 in the Engineering Geology Laboratory of Federal University of Technology, Akure, Nigeria. The study was conducted on the lateritic soil sample from two different locations around Igbatoro in parts of crystalline Basement Complex of South western Nigeria. After collection, the soil samples were air dried in the Laboratory for 3 weeks at room temperature before the analyses were done. Natural moisture content, Specific gravity, linear shrinkage and Atterberg limits were performed on the soil using BS 1377 1991 Standard procedures. Rock flour of 2,4,6,8 and 10% by weight were added to the soil Samples for index and engineering (Compaction, unsoaked CBR and unconfined compressive strength) tests. The effects of rock flour on the engineering properties of the lateritic soil were determined.

3. Results and Discussion:

A Summary of the preliminary results of the engineering properties of the soil sample A and B are presented in Table 1. The results of the addition of varying contents by weight of rock flour to the different soil sample are presented in Table 2 and 3 and average results in Table 4.

Linear Shrinkage

According to Schedig, (1948), linear shrinkage was classified as follows; linear shrinkage less than 5 is good, between 5 and 10 as medium, between 10 and 15 poor and greater than 15 as very poor. For Soil A and B, with 0, 2%, 4% and 6% rock flour has linear shrinkage values between 7 and 10% and will likely to swell moderately, while soil Sample A and B with additive of 8 and 10% has a linear shrinkage range between 10 and 12 which is poor and may crack and cause differential settlement in foundation and may lead to formation of pot holes when use in road construction.

Specific Gravity

The specific gravity of Sample A is 2.77 and Sample B is 2.74. The soil specific gravity of the study area is considered suitable for various engineering constructions, since it occurs within the accepted specific gravity range of 2.50 and 4.60 for lateritic soil (Gidigasu, 1976).

Compaction and California Bearing Ratio

It was observed that by increasing the rock flour contents from 0 to 10%, the optimum moisture

content decreases from 28.2 to 23.4% for sample A and from 24.3 to 19.6% for Sample B respectively, while the maximum dry density increases from 1456 to 1641 Kg/m3 for Sample A and for sample B from 1485 to 1771 Kg/m3. Similarly, the unsoaked CBR for Sample A and Sample B increases with increasing rock flour percentage by weight for the soil samples, this indicates the improvement in the engineering properties of the lateritic soil when stabilized with rock flour. According to Asphalt Institute (1962) CBR value for Soil to be use for sub base or sub grade must be between 0 and 20%. From this study, the optimum % by weight of rock flour for the stabilization is 4%, above 4% the soil cannot be use as sub base or sub base materials.

Atterberg Limits

The liquid limits (LL), Plastic limits (PL) and the plasticity index (PI) for Sample A are 58.4, 21.2 and 37.2%, and Sample B are 60.4, 23.2 and 37.2%. Liquid limit less than 35% indicates low plasticity, between 35 and 50% indicates intermediate, between 50 and 70% high plasticity, 70 and 90% very high plasticity and greater than 90% is extremely high plasticity (Whitlow 1995). Sample A and B here indicate intermediate plasticity. Addition of rock flour at increasing percentage reduces the Plasticity; this shows an improvement in the physical properties of the soil samples.

Unconfined Compressive Strength

The unconfined compressive strength is generally used to determine the consistency of clayey soil. The unconfined compressive strength for both sample A and B indicates the clay

consistency is from medium to stiff. This is shown in figures 2 and 3.

4. Conclusions:

The following conclusions can be drawn from the present study:

* The linear shrinkage, Atterberg limits, optimum moisture content, maximum dry density, unsoaked CBR, cured unconfined compressive strength of the studied soil were improved by addition of rock flour.

* Addition of more than 4% of rock flour by weight to the soil causes negative influence on the engineering properties of the soil; this indicates that addition of 4% by weight of the rock flour is the most appropriate for improving the properties of the studied soil.

References

References

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