compactive effort and the permeability...

COMPACTIVE EFFORT AND THE PERMEABILITY CHARACTERISTICS OF RESIDUAL SOILS

Ibrahim Adewuyi OYEDIRAN*, 1 and Abimbola James OJEDIRAN1

1University of Ibadan, Faculty of Science, Department of Geology, Ibadan, Oyo State, Nigeria.

*Corresponding Author: Ibrahim Adewuyi Oyediran. E-mail: [email protected],[email protected]

ABSTRACT

An attempt to determine the permeability characteristics of compacted disturbed soils to explore

the possibility of obtaining results with no significant variation from those of undisturbed soils

and elucidate the influence of compactive effort was undertaken. The permeability coefficient of

the undisturbed soils was determined while the undisturbed soils were compacted at West Africa

(WA) and Modified AASHTO (MA) energy levels before subsequent determination of

coefficients of permeability. Significant reduction in permeability was observed particularly at

higher (MA) energy of compaction while insignificant difference exists between permeability

coefficients of undisturbed samples and those of disturbed samples after compaction at both WA

and MA energy levels, and from estimation from grain size parameters. Thus, compactive effort

has considerable influence and the potential to obtain results with no significant difference to

those obtained from undisturbed samples is possible.

Key words: permeability, compactive effort, undisturbed soils, insignificant difference

1

INTRODUCTION

Permeability characteristic of soils is an important geotechnical property relevant to the delivery

of several construction works. It is a measure of a soil's ability to transmit fluids, usually water

and is essentially the rate at which water moves through the soil. The more permeable the soil,

the greater the seepage and the greater the possibility of contamination of underlying

groundwater by leachate generated in a waste dumpsite. At the end of the day the human

population is at the receiving end and most times adversely affected. Thus, as indicated by

Oyediran and Iroegbuchu (2013), the permeability characteristics of soils are an important

parameter particularly in the design of waste disposal facilities because low permeability soils

are the overriding requirement for landfill barriers. Siddique and Safiullah (1995) opined that

permeability governs important engineering problems such as consolidation of clay foundation

under applied load and the flow of water through or around engineering structures. Moreover the

determination of the permeability coefficient is crucial for the solution of several geotechnical

engineering problems such as modeling of underground flow, determination of the hydraulic

properties of leachate water in waste disposal areas and calculation of the compressibility (Sezer

et al., 2009).

The determination of permeability can be by in-situ and laboratory tests. The more common

approach is the laboratory testing using constant or falling head permeameter with the use of

undisturbed samples. The constant and falling head permeability tests, usually performed for the

laboratory determination of permeability is easy to apply however; the cost and stress of

obtaining undisturbed samples has been discovered to be high relative to acquiring disturbed

samples. Most times, the material used in collecting the undisturbed sample (core cutters) may

not be readily available or the cost of getting it prohibitive and in some cases haulage of the core

cutters usually made of steel and the ultimate preservation of the undisturbed sample is

problematic. Furthermore in reality it is not easy to obtain undisturbed sample as it is impossible

to convincingly assert that the samples are truly undisturbed. The test is thus usually carried out

on samples assumed to have no significant difference in engineering properties to those of in-situ

soils. In addition, in several construction works, the soils used to achieve project completion may

have to be won from other sites indicating that their inherent properties may have been altered.

Apart from the in-situ and laboratory options of permeability determination, several authors

(Hazen, 1892; Zunker, 1930; Burmister, 1954; Mckinlay, 1961; Chen et al., 1977; Freeze and

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Cherry, 1979; Chapuis, 2004) have established relationships and have estimated permeability

from grain size and or consolidation parameters. This option has also yielded varying results

leading to the generation of several equations capable of estimating permeability. According to

Agus et al. (2005), the determination of permeability coefficient is tedious, time consuming and

labour intensive, hence predictive methods based on other soil properties are often preferred.

Although these methods are capable of making reasonable predictions for permeability

coefficient, they have certain limitations. Hence the need to seek ways of determining the

permeability of soils from disturbed samples possibly without significant variation to the ones

determined from undisturbed samples is pertinent. This project is thus to determine the

permeability coefficient of some disturbed residual soils, at different compactive effort and from

estimation from grain size parameters, compare the results obtained to those obtained from

undisturbed samples of the same soil, determine the level of compaction which produces the

more suitable comparative results and show which energy of compaction has an overall effect in

terms of reducing the permeability of the soils.

MATERIALS AND METHODS

Twenty-four residual soil samples (comprising of twelve disturbed and twelve undisturbed

options) were obtained from Olubode, Ijebu-Isiwo, Ijebu-Imope and Mowe (Figure 1) areas of

southwestern Nigeria. In terms of geology, the area falls within areas underlain by the

Precambrian Basement Complex rocks (Olubode and Ijebu-Imope) and the Sedimentary rocks

(Ijebu-Ishiwo and Mowe) of southwestern Nigeria. The rocks which underlie the residual clay

soils of the study area range from migmatite to quartzite schist as seen in the northern parts and a

combination of clay, shale and sand with alluvium in the southern part.

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Figure 1. Geological map of study area showing sampling points

The undisturbed samples were collected with the use of steel core-cutters forced into the ground

in trial pits of depths between 2.0 and 3.0 m. The core-cutters were sealed at both ends with wax

to prevent moisture exchange and subsequently wrapped in polythene bags to further preserve

the samples. These samples were subjected to laboratory permeability test determination using

falling head permeameter as a result of the high amount of fines (silt and clay size fractions)

observed in the sample. The disturbed samples on the other hand were collected into sacks and

later air-dried for two weeks in the laboratory. Grain size distribution analyses and consistency

limits tests were conducted on the disturbed samples in accordance with BS: 1377 (1990) test

procedures. The disturbed samples were compacted at both the West African and Modified

AASHTO energy levels of compaction and the permeability of the compacted samples were

subsequently determined.

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RESULTS AND DISCUSSION

Index Properties

A summary of the index properties of the residual soils investigated are presented in Table 1.

The index properties are the basis for the engineering classification of the soils. They are useful

indices which give an idea of the type and condition of the soils and are related to their structural

properties. The specific gravity of the soils range from 2.42 to 2.76. Specific gravity is an

important index property of soils that is closely linked with mineralogy and/or chemical

composition. It is the weighted average of the specific gravities of the individual minerals

constituting the soil and is used in assessing the maturity of soils and thus is an indication of the

degree of laterisation. Maignien (1966) established a positive correlation between the specific

gravity of soils and their degree of laterisation. On the basis of the classification by Ramamurthy

and Sitharam (2005) the soils fall within either sand, silt or clay type of soil. The soils are also

not organic. In terms of utility, Gidigasu (1976) noted that the specific gravity of soil grains is an

important property in the identification and evaluation of aggregate parameters for construction

purposes. The higher the specific gravity of the soil, the better it is for construction purposes.

Most of the soils investigated have specific gravity >2.60 and are expected to be strong and

durable. Particle size fractions of the soils show they are well graded soils (Figure 2.) with

amounts of fines >73%. The high fines content in the samples have both negative and positive

implications on its utility. The clayey materials will act as binders when the soils are compacted

during road construction (Gidigasu, 1976). They may also make the soil more sensitive to

moisture. The soil still has sufficient coarse grains that would protect the soil from volumetric

shrinkage and influence the soil strength. Moreover the soils are not expected to display

excessive shrinkage and settlement but exhibit low hydraulic conductivity and are suitable for

use as liner materials because their plasticity index are >7% and <35%; liquid limits are >20%

and < 90%, (Daniel, 1991; Kabir and Taha, 2004; Benson et al., 1994; Declan and Paul, 2003).

However based on the specification of maximum liquid limit of 35% by the Nigerian Federal

Ministry of Works and Housing (FMWH, 1997) for soils used as highway sub-base materials,

the average liquid limit of the soils under consideration shows they are not suitable for use.

Casagrande chart classification (Figure 3.) of all the soils shows that, they all possess medium

plasticity and hence compressibility except JL2C which displays low plasticity. In terms of the

Unified Soil Classification System (USCS) the soils are regarded as clays (CL).

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Table 1. Index properties of residual soils

SAMPLE

CODE

SPECIFIC

GRAVITY

PARTICLE SIZE DISTRIBUTION (%) CONSISTENCY LIMITS (%) CHART CLASSIFICATION

SAND

SIZE

SILT

SIZE

CLAY

SIZE

AMOUNTS

OF FINES

LIQUID

LIMIT

PLASTIC

LIMIT

PLASTICITY

INDEX CASAGRANDE USCS

JL1A 2.57 25 43 32 75 42 24 18 MEDIUM PLASTICITY CL

JL1B 2.75 24 33 43 76 48 22 26 MEDIUM PLASTICITY CL

JL1C 2.75 22 35 43 78 46 23 23 MEDIUM PLASTICITY CL



JL2C 2.52 27 39 34 73 29 15 14 LOW PLASTICITY CL







Figure 2. Grading curves of the residual soils

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Figure 3. Casagrande chart classification of the residual soils

Compaction Parameters

The soils under investigation were compacted at both the Modified AASHTO (MA) and West

African (WA) levels of compaction and the Maximum Dry Density (MDD) and Optimum

Moisture Content (OMC) obtained are displayed in Table 2. The MDD for all the soils increased

with increase in the energy of compaction from WA to MA while the OMC reduced with

increase in energy of compaction from WA to MA confirming observations by (Ladd et al.,

1960; Oyediran and Kalejaiye, 2011) thus increasing the chances of the soils as a suitable sub

base material (Adeyemi, 2003) and as general filling and embankment materials (FMWH, 1997).

This is because during compaction, the number of particles per unit volume is increased as the

fluids which filled up voids are displaced hence increasing the density of the soil. The soils can

be said to better compacted at the Modified AASHTO level of compaction and hence more

improved denser soils are produced.

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Table 2. Compaction parameters of the residual soils SAMPLE

CODE

MAXIMUM DRY DENSITY (Kg/m3) OPTIMUM MOISTURE CONTENT (%)

West African

Level

Modified AASHTO

Level

West African

Level

Modified AASHTO

Level

JL1A 1990 2020 12.50 11.30

JL1B 1860 2132 14.20 9.50

JL1C 1900 2070 12.36 10.20

JL2A 1890 2178 12.30 8.00

JL2B 1882 2130 12.50 8.80

JL2C 1870 1968 14.70 12.30

JL3A 1736 1979 17.10 15.50

JL3B 1679 1818 18.40 17.30

JL3C 1977 2017 14.50 13.70

JL4A 1720 1978 13.60 11.30

JL4B 1729 1924 16.30 14.50

JL4C 1740 2130 14.00 11.30

Permeability

The permeability coefficients obtained for undisturbed samples and those from compacted

disturbed samples at both West Africa (WA) and Modified AASHTO (MA) levels of compaction

are shown in Table 3.; alongside permeability coefficient estimated from grain size parameters

based on Mckinlay (1961) established equation [k=c(D502) where c =0.00357].

Compactive effort has considerable influence on soil coefficients of permeability. The results

show a significant reduction in permeability after compaction particularly at the MA level.

Compaction further lowers coefficient of permeability of the soil and thus improvement in soil

property is expected. The decrease in permeability with increase in energy of compaction is due

to the reduction in the occurrence of large pores as compaction energy increases and results in

lower permeability (Acar and Oliveri, 1989). Hence the higher the energy of compaction the

better results obtained in terms of reducing permeability and can be adopted for in-situ

compaction of soils to achieve a significant reduction in permeability.

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Table 3. Permeability Coefficients of the residual soils

SAMPLE

CODE

PERMEABILITY COEFFICIENT (m/s)

UNDISTURBED

SAMPLE

SAMPLE COMPACTED AT

West African (WA) Level

SAMPLE COMPACTED AT

Modified AASHTO (MA) Level

GRAINSIZE

ESTIMATION

JL1A 2.19 x 10-8 2.19 x 10-9 1.14 x 10-9 1.43 x 10-8

JL1B 3.04 x 10-8 3.04 x 10-9 2.00 x 10-9 1.73 x 10-8

JL1C 2.33 x 10-8 2.48 x 10-9 2.78 x 10-9 1.57 x 10-8

JL2A 6.47 x 10-9 2.36 x 10-9 2.41 x 10-9 1.43 x 10-8

JL2B 1.46 x 10-9 3.83 x 10-9 1.61 x 10-9 2.60 x 10-8

JL2C 4.10 x 10-10 1.97 x 10-8 4.85 x 10-9 5.71 x 10-8

JL3A 2.53 x 10-8 1.92 x 10-8 1.10 x 10-9 1.00 x 10-10

JL3B 1.30 x 10-8 8.41 x 10-9 6.84 x 10-9 6.13 x 10-9

JL3C 1.66 x 10-8 2.26 x 10-9 1.57 x 10-9 1.40 x 10-10

JL4A 7.01 x 10-9 1.30 x 10-8 4.39 x 10-9 5.72 x 10-8

JL4B 5.41 x 10-7 8.42 x 10-9 4.36 x 10-9 6.00 x 10-8

JL4C 3.31 x 10-8 4.21 x 10-9 1.71 x 10-9 8.93 x 10-8

Statistical T-test revealed that no significant difference exists between permeability coefficients

obtained directly from undisturbed samples and those obtained from disturbed samples after

compaction at both WA and MA energy levels. Furthermore no significant difference was also

observed from comparing permeability coefficients determined from undisturbed samples and

those determined from estimation from grain size parameters. Thus it may be safe to assume that

the potential to determine permeability coefficient from disturbed samples and obtain results

with no significant difference from those obtained from undisturbed samples is possible.

However significant difference was observed between permeability coefficients of samples

compacted at WA and those compacted at MA energy levels. This may not be unconnected with

the difference in compaction energy dissipated at both levels and the effect of significant

reduction in permeability noticed at the MA energy of compaction. A glance (Figure 4.) at the

correlation coefficient calculated for the different data sets shows that although positive

9

correlation was obtained between permeability coefficients determined from undisturbed

samples and those from disturbed samples at both WA and MA energy levels, the correlation

coefficients were weak. However while it was 0.23 and 0.29 between permeability determined

from undisturbed samples and MA energy compacted samples on one hand as well as

permeability estimated from grain size parameters on the other respectively, it was very low

(0.04) between permeability determined from undisturbed samples and WA energy compacted

samples. Hence permeability determination from disturbed samples compacted at MA energy

level and those estimated from grain size parameters give a better and show a closer relationship

with those obtained directly from undisturbed samples than from samples compacted at WA

energy level. It must be noted that, although significant difference exists between the

permeability coefficients of WA and MA energy compacted soils, a positive correlation

coefficient (0.37) was observed.

Figure 4. Relationship between permeability coefficients of the residual soils

10

CONCLUSIONS

An attempt to determine permeability of residual soils from compacted disturbed samples

possibly without significant variation to determination from undisturbed samples has led to the

following conclusions;

I. Compactive effort has considerable influence on soil coefficients of permeability with the

observation of significant reduction in permeability after compaction particularly at the

MA higher energy of compaction.

II. The effect of higher compactive effort may have resulted in the significant difference

between permeability coefficients of soils compacted at WA and those compacted at MA

energy levels.

III. Insignificant difference exists between permeability coefficients obtained directly from

undisturbed samples and those obtained from disturbed samples after compaction at both

WA and MA energy levels, and from estimation from grain size parameters.

IV. Permeability determination from disturbed samples compacted at MA energy level and

those estimated from grain size parameters give a better and show a closer relationship

with those obtained directly from undisturbed samples.

V. Thus, the potential to determine permeability coefficient from disturbed samples and

obtain results with no significant difference from those obtained from undisturbed

samples is possible.

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Benson, C.H., Zhai, H. and Wang, X., 1994. Estimating hydraulic conductivity of clay liners.

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