Geotechnology - Olabisi Onabanjo...

ΟΡΥΚΤΟΣ ΠΛΟΥΤΟΣ/MINERAL WEALTH 163/2012 17

ASSESSMENT OF PLASTICITY AND COMPRESSIBILTY OF ENGINEERING

SOILS IN IBADAN, SOUTHWESTERN NIGERIA.

Geotechnology

ADEBISI, N.*

ABSTRACT

Geotechnical properties of engineering soils, which relate to road construction in Southwestern Nigeria, have been substantially published. However, characteristics governing soils properties as foundation and construction materials for light structures are still to be a subject of serious research. This study discusses detailed plasticity characteristics and compressibility to explaining the poor performance of soils derived from the variably migmatised gneisses, used for engineering purposes in Ibadan.

In a reconnaissance survey, the nature and extent of various gneisses were ascertained form outcrops, and profile thicknesses up to 3.0m were established. Thirty samples (disturbed and undisturbed soils) were obtained from three trial pits, at 0.5m interval. Standard laboratory testing of natural moisture content, consistency limits and consolidation were

* Department of Earth Sciences, Faculty of Science. Olabisi Onabanjo University, Ago-Iwoye, Nigeria.

conducted on the soils in accordance with ASTM D - 2487 and BS 1337. Variations in compression index are relative, which is an implication of small magnitude differential settlement. The Casagrande chart for the studied soils classification reveals silty clay content of low to fairly high plasticity. The consistency index (IC) liquidity index (IL), flow index (IF) and toughness index (IT) reveal semi-solid-unstable-friable soils that are susceptible to weathering. The regression plots reveal that IP, IC, IT and IF increases with increase in compression index (CC). Flow index shows the lowest correlation coefficient (r = 0.032) with compression index. The resultant effects of these on buildings are crack failures and collapsed walls.

Key words: Engineering, soils, plasticity, compressibility and index

INTRODUCTIONClay-sized particles have been identified to significantly control the engineering performance of soils in Nigeria. Ola, (1974), Balogun (1984), Madedor and Adeleke, (1987) Adeyemi et al., (2003) and Oloruntola et al., (2005) adopt some stabilization measures using cement and lime to control the behavior of the soils.Badmus (2010) evaluates the deformation characteristics of

the soil’s clay content using coefficient of consolidation (CV) and compressibility (MV). These parameters facilitate the estimation of the rate and amount of settlement. Carter and Bentley (1991) explain that application of these parameters when quoting compressibility values or correlating with derived limits, is meaningless as they vary with confining pressure. Furthermore, detailed plasticity characteristics which explain their strength at the plastic limit and susceptibility to weathering are ignored. Correlation between deformation characteristics and soil consistency has assumed a great significance in the recent past (Azzouz et al. 1976, Worth and Wood 1988; Carrier

ΟΡΥΚΤΟΣ ΠΛΟΥΤΟΣ/MINERAL WEALTH 163/201218

reveal gneisses to be the dominant bedrock in the area, with subordinate coverage of quartzite and quartz-schist of the Metasedimentary series. These rocks have since undergone physico-chemical processes of weathering into soils, forming profiles of various thicknesses that are widely employed as construction and foundation materials. For the purpose of clarity, professionals such as civil engineers, geologists, soil scientists, pedologists e.t.c., have various definitions for soils. Figure 2, provides a visual comparison of these definitions. Therefore, engineering soils include profiles developed over the crystalline bedrocks in the study area. They are usually excavated and employed as both construction and foundation materials. Geologically, this is

1985). Compression index of a soil is directly related to its plasticity index. Holtz and Kovacs (1981) quote variou In some parts of the ancient city of Ibadan, such as Oje, Beere, Mapo, Yemetu, etc. are light structures (houses) founded on soils, and rocks. They are noticed with crack failures and partly collapse of walls, resulting to occasional loss of lives and damages to properties. The entire area is a tropical environment within longitudes range of 30 50’E and 30 58’E, and a latitude range of 70 20’N -70 24’N in Southwestern part of Nigeria (Fig. 1). Geologically, the area is underlain by variably migmatised gneisses of the Precambrian Basement Complex (Jones and Hockey, 1964). Olayinka and Olayiwola (2001) and Adebisi(2010)

FIGURE 1. Map of lbadan showing the study area


(α)

(β)

FIGURE 2. The relation between pedological soil and soil in engineering sense (Adapted from BS 1957)

FIGURE 3. Flow curve obtainable in a liquid limit test from which flow index is determined

FIGURE 4. Various curves obtainable leading to estimating compression index (Cc) in a consolidation test


roughly equivalent of regolith, which may include saprolite, in-place bedrock that is chemically altered, coherent but retaining its original texture.Holtz and Kovacs (1981) quote variou s equations partially relating compression index to liquid limit. Sridharan and Nagaraj (2000) show that compression index (Cc) correlates better with shrinkage index than either of the plasticity index or liquid limit. However, plasticity index of a soil can still give information on its compressibility characteristics in absence of shrinkage index.In this study, the consistency of the residual laterised soils are determined and interpreted in terms of the plasticity index, liquidity index, flow index and toughness index. Variations in plasticity and compression indices which adequately define volume decrease of the soils are enumerated. This approach will unravel the basis for the poor performance of the soils when employed as construction or foundation materials. The results of this study will provide a database in addition to information on stabilization measures to improve the general engineering performance of the soils.

METHOD OF STUDYThis research work involves a reconnaissance survey to ascertain the nature and extent of various gneisses underlying the study area. Pedological mapping was also carried out to establish areas where profile thicknesses are up to 3.0m. These were followed by laboratory testing. Three test pits were established before sample collection. A total of thirty disturbed and undisturbed samples were obtained from the pits. Five disturbed samples and five undisturbed soils were recovered from each pit with the aid of hand auger and core cutter respectively, at regular interval of 0.5m, commencing at depth of 1.0m. The samples were well labeled and subjected to standard laboratory testing following the American Standard for Testing and Materials standard classification of soils for engineering purposes, ASTM (2000) designation 2487-00 and British Standard, BS 1337 (1990). Natural moisture content was determined as a percentage, dividing the difference in weight of samples before and after drying in an oven by the dry weight (ASTM D4959 – 00, 1998).

TABLE 1. Plasticity characteristics of the studied soils

DEPTH (m)

CONSISTENCY LIMITS DERIVED LIMITS

Liquid limit

(LL)%

Plastic limit

(LP)%

Plasticity index (IP)%

Coefficient of

variation for plasticity

Index(CV) %

Consistency index (IC)

Liquidity index (IL)

Flow Index(IF)

ToughnessIndex(IT)

PIT 1 PIT 1 PIT 1 PIT 1 PIT 1 PIT 1 PIT 1

1.01.52.02.53.0

2830454128

1917383613

9137515

42

2.892.156.06.01.47

- 1.89- 1.15- 5.0- 5.0- 0.47

11.9019.3018.987.9817.79

0.760.670.370.630.84

1.01.52.02.53.0

PIT 2 PIT 2 PIT 2

28

PIT 2 PIT 2 PIT 2 PIT 2 4841343447

2122161435

2719182012

1.411.631.111.152.67

- 0.41- 0.63- 0.11- 0.15-1.57

32.028.9712.9610.488.89

0.842.121.391.911.35

1.01.52.02.53.0

PIT 3 PIT 3 PIT 2

38

PIT 3 PIT 3 PIT 3 PIT 3

3229324345

917181125

2312143220

0.071.081.290.811.25

0.30- 0.08- 0.290.19

- 0.25

16.9015.6110.917.5710.93

1.360.771.284.231.83


Based on the BS1377: Part2:1 (1990) 5:3, the consistency limits were determined on air-dried samples passing through British sieve No 40 (0.425mm). The liquid limit was measured form the 25th blow of three moisture contents at which a groove cut in a soil paste closes in the brass cup of Casagrade tool. The plastic limit was also determined from the moisture content of the air-dried soil samples after rolled into a thin thread of about 3mm without breaking up. The derived limits are estimated as follows;(i) Plasticity index (Ip) is the arithmetic difference between the plastic limit from the liquid limit. This corresponds to the range of water content over which a soil has a plastic consistency.(ii) Consistency index (Ic) is the difference in the water contents, in percentage of dry weight for which the two sections of a part of the soil in the Casagrande cup touch each other over a distance of 1cm, after 5 and 10 blows respectively.(iii) Liquidity index (IL) is calculated as a ratio of the difference between the natural moisture content and plastic limit to the plasticity index.(iv) Flow index (IF) for each sample is determined from the

slope of the flow curve shown in Figure 3.(v) Toughness index (IT) is estimated from the ratio IP/IF

The deformation characteristics of the studied soils are determined in an oedometer consolidation test. The measured change in thickness is plotted against the logarithm of the consolidation pressure. This enables approximation to much more closely to a straight line on the virgin compression curve. The compression index (CC) is finally calculated from the slope of change in thickness (dh) of sample to the consolidation pressure (LogP) as shown in Figure 4.

DISCUSSION OF RESULTSA summary of consistency limits characteristics of the studied soils is presented in Table 1.Variation in plasticity index is as high as 40 %. Rag (1972) adjudged variation above 10% as measure of low precision, which implies that the moisture content at which the studied soils are plastic is unpredictable. Clay-sized particles have been revealed to strongly influence the behaviour of soils in the field (Means and Parcher, 1963; Holtz and Kvacs, 1981; CEG, 2002). Plots of plasticity index (IP) against liquid limit (LL) on the Casagrande chart (Fig. 5) for classification mostly fall

TABLE 2. Deformation characteristics of the studied soils

Depth Compression Index(Cc)

Range Average Standarddeviation

Coefficient of

variation(CV)%

1.01.52.02.53.0

PIT 1

0.147 -0.264 0.191 0.055 28.80

0.1470.1610.2640.2380.147

1.01.52.02.53.0

PIT 2

0.189 -0.287 0.234 0.047 20.09

0.2870.2380.1890.1890.28

1.01.52.02.53.0

PIT 3

0.154 -0.264 0.204 0.050 24.510.1750.1540.1750.2520.264


above the A-line, and a few below the U-line, which marks the approximate upper limit for natural soils within the LL range of 28% – 48 %. In accordance with the Unified Soil Classification System (USCS), the studied soils can be said to have silty clay content of low to fairly high plasticity. The consistency limits characteristics of the studied soils are further interpreted in terms of the plasticity index (IP), consistency index (IC), liquidity index (IL) toughness index (IT) and flow index (IF). C. E. G, (2002) categorises soils, which incorporate cohesive property based on the consistency index (IC) taking cognisance of their states. The studied soils range in IC between 0.07 and 6.0. Therefore, the studied soils can be said to contain a variety of stiff-

hard-solid character. Lujan (2003) employs the consistency index to estimate the ability of soil structure to persist stability. This also explains the susceptibility of tropical soils to weathering and erosion. The majority of the studied soils have IC values less than 2.5, which is characteristic of weak cohesive strength thus make the soil unstable. Liquidity index (IL) values for most of the soil are less than zero. This shows moisture content lower than the plastic limit, and it is indicative of semi-solid to solid soils (Davison and Springman, 2000). Toughness index (IT) values for most of the soils are within the range of 0 - 3 with flow index (IF) ranging between 7.57 and 32.02. Means and Parcher, (1963) attribute IT values within this range to friable soils. Variation in compression index (CC) of the soils is relative, generally less than 30%, as it ranges between 0.147 and 0.287 (Table 2). This implies that structures founded on the soils may undergo differential settlement of small magnitude, which varies unpredictably. The regression plots of CC against the derived limits (Figures 6 to 9) reveal that each of the plasticity index (IP), consistency index (IC), toughness index (IT) and flow index (IF) increases with increase in compression index. Flow index shows the lowest correlation coefficient (r = 0.032) with compression index. The coefficients of correlation

FIGURE 5. Plasticity characteristics of the studied soils

FIGURE 7. Regression plot of compression index against consistency index

FIGURE 9. Regression plot of compression index against flow index

FIGURE 8. Regression plot of compression index against toghness index

FIGURE 6. Regression plot of compression index against plasticity index


between the compression index and the derived limits are generally low. This implies that foundation settlement cannot be reliably estimated from any of the derived limits.

CONCLUSIONSSoils developed over the gneisses used for engineering purposes in parts of Southwestern Nigeria have proved to be of poor performance when employed for construction purposes. They contain silty clay which exhibits low to fairly high plasticity. Their consistency limits further reveal that they have soils have weak cohesive strength and unstable. Therefore, the soils are expected to be friable and susceptible to weathering. Consolidation characteristics which measures decrease in the soils thickness with applied pressure vary relatively from pit to pit. The implication of this is that settlement due to building load will vary and decrease with the moisture content over which they are expected to be plastic. This study affirms that the rate of loss of their shearing strength of the soils developed over gneisses increases with increasing moisture content.

REFRENCESADEBISI, N.O. 2010. Geotechnical investigation of

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ADEYEMI, G.O., OWOSANYA, R.A. AND ANOKWU, F.C. 2003. Some geotechnical properties of cement stabilized granite-gneiss-derived soils from lle-lfe, SW Nigeria. Journal App. Sci. Tec.,. Vol.3, .No.2, pp. 28 – 72.

AMERICA SOCIETY FOR TESTING AND MATERIALS (ASTM) 1998. D 4959-00, Standard test for determination of moisture content of soils by direct heating.

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CENTER FOR EXPERIMENTAL GEOTECHNICS (CEG). 2002. Basic hydrophysical properties of soil. Retrieved 2nd March 2006, from DAVISON, L. AND SPRINGMAN S., 2000. Soil description and classification. University of West England. Retrieved 13th Sept., 2002 from pp. 7- 90.

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OLORUNTOLA, M.O., ADEYEMI, G.O., ADEBISI, N.O. AND ODUNEYE, O.C. 2005. Geotechnical properties


Manuscript received from:- the author on 4.7.2011- the Review Committee on 8.9.2011

Παραλαβή εργασίας:- αρχική από τον συγγραφέα στις 4.7.2011- τελική από την Κριτική Επιτροπή στις 8.9.2011

of cement stabilized soils developed over pegmatite banded gneiss and porphyroblastic gneiss from Agi-Iwoye, Southwestern Nigeria. Mineral Wealth, pp. 26-36.

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