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Analysis on the suitability of exploitingoil-contaminated soils in wearing layerof Asphalt concreteEsra Aleisaa, Lubna Aljenaia & Dalal Jeraqa

a Industrial and Management Systems Engineering Department,College of Engineering and Petroleum, Kuwait University, KuwaitCity, KuwaitPublished online: 21 Nov 2013.

To cite this article: Esra Aleisa, Lubna Aljenai & Dalal Jeraq , European Journal of Environmentaland Civil Engineering (2013): Analysis on the suitability of exploiting oil-contaminated soils inwearing layer of Asphalt concrete, European Journal of Environmental and Civil Engineering, DOI:10.1080/19648189.2013.860922

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Analysis on the suitability of exploiting oil-contaminated soils inwearing layer of Asphalt concrete

Esra Aleisa*, Lubna Aljenai and Dalal Jeraq

Industrial and Management Systems Engineering Department, College of Engineeringand Petroleum, Kuwait University, Kuwait City, Kuwait

(Received 15 January 2013; accepted 25 October 2013)

This paper investigates the suitability of exploiting weathered oil-contaminated soil(OCS) that resulted from the Gulf war in creating asphalt concrete used in road con-structions. A full factorial experimentation methodology was adopted. The designfactors considered are the oil saturation percentages and the amount of OCS relativeto clean soil in concrete mix. The main response variables are the mixture stability,flow and the percentage voids in mineral aggregates (VMA) which were measuredby using sieve and Marshall Tests. The analysis showed that the OCS grain size issuitable for the application. The Analysis of Variance (ANOVA) indicated that anOCS ratio of 40% to clean soil will maintain stability and flow for the wearing layerof the asphalt concrete. However, increasing the OCS percentages to up to 50% didaffect the VMA of that layer. Quality control and assurance procedures for collectingsamples including protections against tool cross-contamination and transport chain ofcustody were adopted. The results open doors towards exploiting massive amountof existing OCS in the region in road construction as opposed to the current practiceof disposing it into landfills.

Keywords: geotechnical; oil-contaminated soil; Kuwait oil lakes; Asphalt concrete;Marshall test; ANOVA

1. Introduction

As part of the devastating Gulf war on the region in 1990, out of Kuwait 914operational oil wells, 616 were set on fire, 76 flooded, 99 were seriously damaged(Al-Sarawi & Massoud, 1998; Bader & Clarkson, 1995; Omar, Bhat, & Asem, 2009;Omar, Grealish, & Roy, 2006). Oil well fires have damaged several well heads andcasings and resulted in free flowing oil that formed large oil lakes. The total amount ofcrude oil in these lakes was estimated to be about fifty million barrels (Alsulaimi,Viswanathan, & Szekely, 1993). Omar et al. (2006) states that an estimated two millionbarrels of oil escaped from the damaged wells daily, and the amount of oil lost due toboth the fires and oil flows has been calculated to be approximately 1.0–1.5 billion bar-rels. The financial costs were estimated of about eighty billion US dollars (Shaban,1994). Seawater was used to fight fires at an estimated rate of 95,000 m³ per day(AlSenafy, Viswanathan, Senay, & Sumait, 1997). The extinguishing of fires started on20 March and ended on 2 November 1991. A total of 727 oils were extinguished withinseven months (Al-Sarawi & Massoud, 1998). According to AlSenafy et al. (1997),

*Corresponding author. Email: e.aleisa@ku.edu.kw

© 2013 Taylor & Francis

European Journal of Environmental and Civil Engineering, 2013http://dx.doi.org/10.1080/19648189.2013.860922

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approximately 200 km² was polluted by oil lakes and about 35 km² was affected by sootcreating a massive layer of oil-contaminated soil (OCS) beneath oil lakes. OCS stillexists in massive quantities and have deformed due to harsh desert weather conditions.As specified in the United Nations Compensation Committee (UNCC) decision number258 (UNCC, 2005), the UNCC have decided landfill of about 26 million highly contam-inated soil. The authors of this paper proposed a safer alternative to this option.

This paper’s objectives are twofolds to: first, to investigate the geotechnical proper-ties of weathered OCS; and secondly, from the results of the geotechnical part to inves-tigate the ability of using the OCS as a replacement of certain percentage of clean soilin creating wearing layer of asphalt concrete. The method for the first objective con-sisted of extraction, sieve analysis and Marshall Lab test. These showed that the soildistribution represented a poorly graded sand grains. Therefore, based on the Marshalldesign’s standards of the aggregates size, the wearing course layer of asphalt concrete isthe most suitable layer in which such oil-contaminated soils can be exploited. The sec-ond objective was achieved using full factorial design of experiment. Where, the factorsconsidered were the oil saturation percentages in OCS, and the percentage of contami-nated soil present in the sand mixture. The main response variable is the mixture stabil-ity and other responses are the amount of flow and the percentage voids in mineralaggregates (VMA). Samples were obtained from Bahra oil lake/spill located in theNorth of Kuwait City. Quality control and assurance procedures for collecting samplesincluding protections against tool cross-contamination and transport chain of custodywere adopted. Randomization of treatment combination was conducted to ensure nocontamination of nuisance factors. The model was subject to statistical adequacy check-ing using residual analysis and normal probability plots. The tests conducted in thisresearch was prescribed in accordance to the general specifications for Kuwait motor-way/expressway system of the Ministry of Public Works manual, Road Administration,of the State of Kuwait, second edition (MPW, 2008). Hence, providing a complete proofof exact requirements could be rapidly utilized.

2. Historical Background and Condition of Local Oil-Contaminated Soils

Studies indicate that the Greater Burgan, Raudatain and Sabriya oil fields suffered mostof the damage attributed to the Gulf war in 1990. Almost 70% of the operational oilwells in Kuwait occur in these areas. This includes the Maqwa, Ahmadi and Burgan oilfields (Omar et al., 2006). According to AlSenafy et al. (1997), approximately 200 km²was polluted by oil lakes and about 35 km² was affected by soot. Products ofcombustion were spread over an area of about 900 km². In his study, Al-Besharah(1991) has listed type of damage associated with each oil field in Kuwait.

2.1. Recovered oil

At earlier stages of the oil lakes, many of the flooded oils was actually skimmed andrecovered. In fact, Elgibaly (1999) indicates that about 95% of the initial oil was recov-ered by these mechanisms. AlSenafy et al. (1997) stated that this is equivalent to recov-ering approximately 21 million barrels from the oil lakes since the liberation of Kuwait.In another study, it is stated that the entire amount of oil recovered from oil lakes andpools is approximately 320 million (AlSenafy et al., 1997).

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2.2. Weathered oil lakes

Research indicates that the atmospheric conditions and elapsed time are two key issuesaffecting the viability of remedial methods of the oil lakes and the contaminated soilbeneath. As of today, the post-Gulf war oil lakes have settled for over two decades, thustermed weathered. Natural factors contributing to change in oil lakes structure andchange in distribution and elaboration of soil contaminates include (AlSenafy et al.,1997): rainfall, high temperature, wind and dust storms. In general, weathering for longperiods, weathered crude oil can be subject to the following (Shaban, 1994):

(1) There is a continuous loss of water from the oil stock due to evaporation.(2) Volatile material form from the crude oil due to evaporation as well.(3) The settled water and oil gradually sink into the sandy soil.(4) The evaporation of water leaves the residual water saturated in salt which even-

tually precipitated as crystalline salt.

2.2.1. Rainfall

Although rain is relatively scarce in Kuwait, it plays an important role to the extent ofdepth of contaminates to the soil beneath oil lakes and the water table. During the years1992–1993 and 1994–1995, Kuwait had above average rainfall, resulting in substantialinfiltration of rainwater. In fact, rainfall of 233.7 mm during 1992–1993 was the secondhighest recorded in the last 40 years. Annual rainfall is approximately 110 mm(AlSenafy et al., 1997). The migration capability of oil lake-associated polycyclicaromatic hydrocarbons (PAHs) in the soil during rainy periods and their possible infiltra-tion into the groundwater aquifer raised serious concerns (Alyakoob & Saeed, 1994).

2.2.2. Man-made factors effecting oil lake structure and spread of condiments

AlSenafy et al. (1997) also list other factors that are man-made, and have also contrib-uted to change the structure and distribution of contaminate. These included:

(1) The process of oil recovery from the oil from oil lakes.(2) The process of removing sludge and stockpiling them at selected areas.(3) Transferring oil from its original oil lake locations to man-made storage lakes

with plastic lined bottoms.(4) The mixing of residual oil in lake beds with soils to from oil-contaminated

sludge.(5) The fire extinguishing process.

2.3. The effect of seawater on soil properties

As indicated earlier, seawater was used to fight oil fires at an estimated rate of95,000 m³ per day. The quantity used at Raudhatain and Umm Al-Aish alone was about15,000 m³ per day (AlSenafy et al., 1997). A study by Massoud, Al-Sarawi, and Wahba(2000) illustrates that both Al-Ahmadi and Burgan oil alkaline arid soils with similarPH, electrical conductivity and sodium absorption ratio for the two soil profiles as salinesoils. The observed high and similar proportions of sodium and chloride found in thetopsoils and the high salinity of both profiles are most likely attributed to the

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accumulation of salts due to the flooding of soils seawater used in extinguishing wellfires. Their study shows that the downward oil/water movement and salt accumulationare restricted to the upper (25–95 cm). Such a saline environment could slow or inhibitthe biodegradation process of the crude oil (10), particularly the more biodegradationportions (Bader & Clarkson, 1995).

3. Types of Contaminants in oil-contaminated soils

In general, reconnaissance field studies investigated contamination depth, total petroleumhydrocarbon concentrations (TPH), oil contamination concentration and more. Othermeans of investigation included using laser-induced fluorescence to examine soil color,soil texture, soil consistence and site location (Omar et al., 2006). Malallah et al. (1996)used reduction in chlorophyll content in certain plants as an indicator for extent environ-mental pollution due to oil lakes. Contaminates measure include: TPH, total organic car-bon (TOC) (Al-Sarawi & Massoud, 1998), PAH and heavy metals (AlAwadhi, AlDaher,ElNawawy, & Balba, 1996; Baca, Al-Sarawi, & Kana, 2005). The investigation and thepotential hazard associated with PAHs based on local environmental processes and PAHlevels in weathered crude oil from Kuwait’s north and central oil lakes have been alsomeasured in (Alyakoob & Saeed, 1994) during the period September 1992 to June1993, over a year after their formation. Other studies have also investigated to amountof SO2 in the soil that resulted from soot from burnt oil (Massoud et al., 2000). Benz(a)pyrene and pyrene elements were investigated in (AlAwadhi et al., 1996). In fact, theyshowed higher concentrations in the extracts of deeper soil samples collected from theBurgan oil field. Soils were also investigated for the existence of trace and some heavymetals including: vanadium (V), nickel (Ni), cadmium (Cd) and lead (Pb) (Al-Sarawi &Massoud, 1998). Al-Sulaimi, Viswanathan, and Szekely (1993) have also investigatedthe existence of trace metals and found the concentration of vanadium and nickel inKuwait crude oil is about 22.5 and 6.0 ppm, respectively.

3.1. Geotechnical properties of oil-contaminated soil

Most research conducted on oil-contaminated soil have focused on comparison amongphysical properties of clean soil and contaminated one. Table 1 shows a comparison ofresearch conducted on geotechnical properties of OCS of weathered oil lakes of Kuwait.Most studies have focused on the Direct Shear test and Compaction and Permeabilitytest as they investigated its aptness for the general geotechnical use. A study byRahman, Hamzah, Taha, Ithnain, and Ahmad (2010) showed that contaminates in OCSwould have lower Maximum Dry Density and optimum water content if compared withuncontaminated soils, in general, a decreased permeability and kPa. They claim thatdespite the adverse effects, contaminated residual might be used for geotechnicalpurposes.

3.2. The use of oil-contaminated sand in road construction

AlSanad, Eid, and Ismael (1995) investigated the geotechnical properties of oil-contami-nated soil of Kuwait. Their laboratory analyses indicated that oil contamination resultedin a small reduction in strength and permeability and increased in compressibility. Theysuggest that the preferred method of disposal of this material is to use it as a stabilizingmaterial for other projects such as road construction, road base or backfill.

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Table

1.Com

parisonon

relatedresearch

cond

uctedon

geotechn

ical

prop

ertieson

OCSsof

post-G

ulfwar.

Literature

(Alsanad

etal.,19

95)

(AlSanad

and

Ismael,19

97)

(Al-Saraw

i&

Massoud

,19

98)

(Puri,20

00)

(Hassanet

al.,

2008)

(Nasr,20

09)

Directshear

test

Measuredthefrictio

nangle

with

respectto

different

oiltypesand

contam

ination%

atall

relativ

edensities

Measuredthe

streng

thand

frictio

nwith

differentaged

samples

Studied

both

theshear

streng

thandangleof

frictio

nwith

varying

theoilsaturatio

nlevels

Measuredthestreng

that

differentoilsaturatio

nlevels.And

measuredthe

angleof

frictio

nof

different

oiltypes

Com

paction

& perm

eability

test

Studied

thecompactionat

differentoilsaturatio

nlevels

Studied

the

perm

eability

with

respect

todepth

Studied

thecompaction

effect

ofsamples

atdifferentoiltypesand

with

water

aswell

Studied

the

perm

eability

with

the

additio

nof

oil-

contam

inated

soil

Triaxialtest

Measuredthestreng

thof

samples

ofdifferentoil

type

Measuredthe

elasticity

mod

ulus

with

differentaged

samples

Con

solid

ation

test

Studied

thecompressibility

with

thepresence

ofoilin

thesoil

Studied

the

compressibility

indexof

differentaged

samples

Studied

therelatio

nbetweensaturatio

n%

andtheconstrained

mod

ulus

EC

Extractiontest

Measuredthe

difference

ofthe

saturatio

nlevel

insamples

with

aging

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AlSanad et al. (1995) resolute to this idea as most of the remediation methods are con-sidered expensive and uneconomical, given the large quantities of OCS around. Hassanet al. (2008) investigated the use of OCS in making asphalt concrete using the Marshalldeign method by adding 0–40% of fine aggregate to OCS and accordingly reducing thetrial rang of asphalt 3.5 to 5.5% of mix total weight. Hassan et al. (2008) have also con-sidered evaluating the following properties: stability (failure load), flow (correspondingdeformation), bulk specific gravity (BSG), air voids (AV), VMA and voids filled withbitumen (VFA).

4. Design of the experiment

4.1. Design variables

Atmospheric conditions and elapsed time are two key issues affecting the viability ofremedial methods of the oil geotechnical properties of oil-contaminated soil. As oftoday, the post-Gulf war oil lakes have settled for over two decades, thus termed weath-ered. This is a full factorial experiment with two replications required to account forsuitability despite soil variability. Two factors are considered, one with two levels andthe other with three levels. The six treatment combinations with two replications make atotal of 12 experiments. The two design factors considered are: the saturation percent ofoil in the contaminated soil and fraction of contaminated soil to clean soil present in theasphalt mixture. Oil saturation levels considered are: low (2%) and high (4%); whilecontaminated soil fractions are: 30, 40, and 50%. Hassan (2008) investigated thereplacement of 0–40%. It is worth mentioning that the total mass of sand in general isobtained from the general specifications for Kuwait motorway/expressway system of theMinistry of Public Works (MPW) manual, Road Administration, of the State of Kuwait,second edition (MPW, 2008). However, the percentage of clean soils to oil-contaminatedsoils was obtained through conducting initial pilot studies that indicated that oil-contam-inated soil ratio of 40% to clean soil will maintain stability and flow for the wearinglayer of the asphalt concrete. And, increasing the OCS percentages to up to 50% didaffect the VMA of that layer.

4.2. Response variables

The response variables of interest include: stability, flow value, AV in mixture and per-cent VMA. Stability is the maximum load carried by the specimen at standard test tem-perature of 60 °C. It is achieved through stability density and flow tests. A target valueof over 600 g is required for our analysis. The same test can be applied to measure theflow of a target value of 8–16 g. AV and percent void mineral aggregates are testedusing density and void analysis, a target value of over 15 g and 4–6 g are required,respectively. We apply the Marshall Test method to achieve optimum binder content.

5. Samples

Samples were collected from Bahra oil lake/spill. Bahra oil lake is dry weathered oilspill that resulted from a damaged well head south of Al Bahrah oil field during theIraqi aggression. A picture of Bahra oil spill is provided in Figure 1. It is located atN:29°37′0.4″ and E:47°54′43.1″. Bahra oil lake was chosen due to its proven clearanceform unexploded ordinates that still remain in many other oil lakes around Kuwait.

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Quality control and assurance procedures for collecting samples including protectionsagainst tool cross-contamination and transport chain of custody were adopted. Elaboratespecifications on sampling methodology are found in (Alyakoob & Saeed, 1994). Physi-cal properties of the soil profiles and contaminants were investigated through typicaltopsoil sampling. OCS samples were collected from just below the surface at a 20 cmdepth while avoiding any visible extraneous material. Each sample has a total weight of1200 gm.

6. Lab analysis

6.1. Extraction

The amount of toluene extricable hydrocarbon is provided in Table 2. The soil sampleswere demoisturized and cleaned with continuous hot toluene extraction method. TheToluene soluble hydrocarbon is calculated as weight fraction. This step is a preparationstep to sieve analysis and also to measure extent of contamination.

6.2. Sieve analysis

Sieve analysis is used to determine the grain size distribution needed in designingMarshall test trials. The process of implementing the analysis begins with the collec-tion of a representative oven dry soil sample, then determining the mass of the sampleaccurately to 0.1 gm. Then, the soil samples are poured into the stack of sievesaccording to size. Next, the stack of sieves is run through a sieve shaker for about10–15 min (Das, 2002). Sieve analysis results are provided in Figure 2. The distribu-tion represents poorly graded sand grains. Therefore, based on the Marshall design’sstandards of the aggregates size, the wearing course layer of asphalt concrete is themost suitable layer in which such oil-contaminated soils can be exploited. This resultwas obtained while reviewing initial pilot experiments with standards of the Ministry

Table 2. The amount of toluene extricable hydrocarbon.

Level Contamination Result %wt

1 Cont. Low 2.4712 Cont. High 2.788

Figure 1. Bahra dry weathered oil lake.

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of Public Works manual, Road Administration, of the State of Kuwait, second edition(MPW, 2008).

6.3. Specific gravity

Specific gravity (SG) of the samples was found using the pcynometer. First a 10 g ofa dry soil sample was placed in a clean and dry pycnometer and weighted (WPS).Then, distilled water is added to fill about half to three-fourth of the pycnometer; atthis point WB is calculated. The pycnometer is then emptied and cleaned. Then, fillingit with distilled water only (to the mark) to measure the WA value (Das, 2002). TheSpecific gravity is calculated according to Equation (1). The results are provided inTable 3.

SG ¼ Wps

Wps þ ðWA þWBÞ (1)

6.4. Preparation of the Marshall test

A 1200 g sample that contains coarse, fine and filler aggregates with OCS was preparedby taking the midpoint of each range of the aggregate as shown in Table 4. A 30, 40,and 50% of grain size from 4.75 mm to <75 mm was replaced with the high and lowOCS. The choice of the sieve size was based on the result of the sieve analysis of theOCS. Equipment used are: 175–190 °C oven, round mold of diameter of 10 and 7.5 cm

Figure 2. Sieve analysis result for both types of low- and high-contaminated samples.

Table 3. The results of specific gravity for low- and high-contaminated soils obtained fromBahra oil lake.

Weights at mark Low contamination High contamination

WPS 18.46 g 22.87 gWA 156.78 g 155.62 gWB 167.57 g 168.43 gSG 2.4 2.2797

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height consisting of a base metal and collar extension, compaction pedestal, sampleextractor and a hammer (Ali, 2003).

Preparing the samples for the Marshall test involved the following steps:

(1) Thorough blending of samples while heating to obtain a homogenous sample.(2) Pouring blend into the test mold one-third at a time and compressing each part

20 times using a cylindrical metal bole.(3) Placing the mold on a compaction pedestal facing 75 blows. This was done on

both sides of the mold.(4) Extruding samples from their molds and allowing them to cool to room

temperature.

6.5. The Marshall test

The samples were subjected to plastic deformation by using Marshall stability testingmachine to get the stability and flow values (Ali, 2003). Calculations and analysis werefurther conducted to get the VMA, and hence the optimum binder content of each trial.Bitumen is added to produce physical properties required for the industrial use of thefinal product (Wess, Olsen, & Sweeney, 2004). Each replication contained four sampleseach with different percentage of bitumen. Percent of bitumen and respective bitumenweight per sample are provided in Table 5.

The Marshall test involved the following procedure:

(1) Weighing the samples in air and water in order to calculate the specific gravityGbcm.

(2) Immersing the sample in the water at a temperature of 60 °C for a period of 30min.

(3) Placing the sample in Marshall stability testing machine and load at constantrate of deformation of 5 mm/min until failure.

Table 4. Pre-Marshall test analysis.

SieveID

Sievesize

(passing)Range %(pass)

Testselection(approx.mid-point)

%Retained Sample weight (g)

0 3/4″ 100 100 0 01 1/2″ 66–95 80 20 2402 3/8″ 54–88 70 10 1203 No.4 37–70 55 15 1804 No.8 26–52 40 15 (30% of

OCS) 126(40% ofOCS) 108

(50% ofOCS) 90

5 No.16 18–40 30 10 84 72 606 No.30 13–30 20 10 84 72 607 No.50 8–23 15 5 42 36 308 No.100 6–16 10 5 42 36 309 No.200 4–10 5 5 42 36 3010 Filler 0 0 5 42 36 30Weight of the oil-contaminated soil 198 264 330

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(4) Recording Marshall stability which is the total maximum in KN that causes thefailure, the stability values are corrected for volume.

(5) The total amount of deformation is unit of 0.25 mm that occurs at maximumload and is recorded as flow value.

7. Calculation of binder values

The main responses that we need to calculate are: stability, which is the maximum loadcarried by the specimen at a standard test temperature of 60 °C, and the flow value,which is the deformation that the test specimen undergoes during loading up to themaximum load. While the required standards are shown in Table 6, the procedurerequires testing each trial for all four binder contents with respect to the following tests:

(1) Bulk density determination.(2) Stability and flow test.(3) Density and voids analysis.

7.1. Bulk density determination

The Bulk density determination, Gbcm, is determined by weighing the sample in air, Wa

and in water, Ww as in Equation (2). The bulk density determination is done as a mea-sure of soil compaction. In general, the higher the density, the less pore space for watermovement (NRCS, 2012).

Gbcm ¼ Wa

Wa �Ww(2)

7.2. Stability and flow test

The stability and flow test require the calculation of: BSG of aggregates in pavingmixture (Gbam, see Equation (3)), maximum specific gravity of the compacted pavingmixture, Percent voids in compacted mineral aggregate (VMA) and percent AV incompacted mixture (Pav):

Table 6. Minimum and maximum standard values for stability and flow.

Description Min Max

Stability, KN 6 –Flow, 0.25 mm 8 16VMA 15 –

Table 5. Percent of bitumen and respective bitumen weight per sample.

% Bitumen Weight of bitumen (g)

4 484.5 545 605.5 66

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Gbam ¼ Pca þ Pfa þ Pmf þ PconPcaGbca

þ PfaGbfa

þ PmfGbmf

þ PconGcon

(3)

where, Pca, Pfa, Pmf, and Pcon are percent by weight of coarse aggregate, fine aggregate,mineral filler and contaminated soil in paving mixture, respectively. And Gbca, Gbfa,Gbmf, and Gcon are bulk specific gravities of coarse aggregate, fine aggregate, mineralfiller and contaminated soil, respectively.

7.3. Maximum specific gravity of the compacted paving mixture

The maximum specific gravity of the compacted paving mixture Gmp is calculated usingEquation (4):

Gmp ¼ Pca þ Pfa þ Pmf þ Pcon þ PasPcaGbca

þ PfaGbfa

þ PmfGbmf

þ PconGcon

þ PasGas

(4)

where Pas corresponds to percent asphalt content by total weight of paving mixture andGas corresponds to specific gravity of the asphalt. Then, the percent VMA will be calcu-lated using Equation (5):

VMA ¼ 100� Gbcm � Pta

Gbam(5)

where Pta correspond to aggregate percent by weight of total paving mixture. Then, thepercent AV in compacted mixture (Pav) is calculated as follows (see Equation (6)):

Pav ¼ 100� Gmp � Gbcm

Gmp(6)

7.4. Determination of optimum binder content

Finally, the optimum binder content, B0, is selected as the average binder content formaximum density, maximum stability and specified percent AV in the total mix (seeEquation (7)):

B0 ¼ B1 þ B2 þ B3

3(7)

where, B1, B2, and B3 correspond to percent asphalt content at maximum unit weight,percent asphalt content at maximum stability and percent asphalt content at specifiedpercent AV in the total mix, respectively. The final results of the optimum binder con-tent are shown in Table 7.

8. Statistical analysis

8.1. Randomization

The description of the resultant 12 treatment combinations is provided also in Table 7.The order column depicts the sequence of experimentation. Randomization of treatmentcombinations achieves data independence that prevents nuisance factors from contami-nating the results.

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8.2. The hypothesis test

Hypothesis tests are used to examine if difference in treatment combination of oil satu-ration percentage, the contaminated soil percentage and their interaction do have aneffect on soil properties. Let τi be the effect of the saturation percentage level i and βjbe the effect of the contaminated soil percentage level j. ðsbÞij, is interaction effectbetween the two factors.

Then, H0: τ1 = τ2 = 0 and H1: at least one si 6¼ 0; i= 1, 2, 3. Also, H0 = β1 = β2 = β3 =0 and H1: at least one bj 6¼ 0; j= 1, 2, 3. For the interaction effect: H0: (τβ)ij = 0 for alli, j and H1: at least one ðsbÞij 6¼ 0.

8.3. Analysis of Variance with respect to stability

Table 8 shows the results of the Analysis of Variance (ANOVA) for the precedinghypothesis tests with respect to stability.

From the ANOVA in Table 8, the P-value of the stability is greater than α= 0.05.Therefore, we fail to reject the null hypothesis which indicates that the stability meanvalues do not change at different oil saturation percentages or contaminated soil percent-ages. This indicates that using either high oil saturation values or low does not influencethe stability in creating asphalt concrete and the same applies on the contaminated soilpercentages. Furthermore, increasing the contaminated soil percentages up to 50% willnot affect the stability of the asphalt concrete. This result is manifested in the maineffect plot provided in Figure 3. Hence, increasing the percentage of oil from low to

Table 8. ANOVA table for stability.

Source DF Seq SS Addj SS Adj MS F P

% of oil 1 3.887 3.887 3.887 3.01 0.133% of OCS 2 1.062 1.062 0.531 0.41 0.680% of oil*% of soil 2 1.286 1.286 0.643 0.50 0.631Error 6 7.751 7.751 1.292Total 11 13.986

Table 7. Response data for stability, flow and VMA.

Factor A Factor B Sample ID Order Stability kN Flow 0.25 mm VMA

1st replication−1 −1 4L30 5 8.2 11.8 9.5−1 0 5L40 11 6 30 10.4−1 1 6L50 8 7.37 8.84 9.21 −1 1H30 12 5.6 13 91 0 2H40 6 7.4 25 8.51 1 3H50 1 6.3 10.5 7.5

2nd replication−1 −1 10L30 10 7.8 14 10.1−1 0 11L40 4 7.84 15 10−1 1 12L50 2 8.73 10.4 91 −1 7H30 3 8.25 22 8.71 0 8H40 7 5.7 13.6 8.61 1 9H50 9 5.86 11.92 8

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high decreases stability. In seeking high stability, low saturation of oil is better. It alsoshows that increasing the contaminated soil percentage from 30 to 40% decreases thestability and it increases when increasing the contaminated soil percentage from 40 to50%. The max stability was found with both low oil % and 30% soil.

8.4. ANOVA with respect to flow

The ANOVA results for the flow response are provided in Table 9. Again, the P-valueis greater than α = 0.05, then again, we fail to reject the null hypothesis, which indicatesthat the flow mean values do not change at different oil saturation percentages or con-taminated soil percentages and the same stability conclusion applies here.

From the main effect plot of flow Figure 4, it is evadable that increasing the oilpercentage from low to high increases the flow, and the high oil percentage has out ofthe range value. When moving the contaminated soil percentage from 30 to 40%, theflow increases, while it decreases when moving the soil percentage from 40 to 50%,40% yields value out of the acceptable range.

8.5. ANOVA with respect to VMA

As for the VMA in Table VI, since P-value is lower than α = 0.05, then the null hypoth-esis is rejected. Therefore, there is a strong evidence to indicate that changing the levelof the percentage of oil and percentage of soil affects the VMA values. The main effects

Figure 3. ANOVA main effect plot for stability.

Table 9. ANOVA table for flow.

Source DF Seq SS Addj SS Adj MS F P

% of oil 1 5.2008 5.2008 5.2008 68.58 0% of OCS 2 2.2867 2.2867 1.1433 15.08 .005% of oil*% of soil 2 0.2467 0.2467 0.1233 1.63 .273Error 6 0.4550 0.4550 0.0758Total 11 8.1892

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plot of VMA in Figure 5 indicates that increasing the percentage of oil from low to highdecreases the VMA. It also indicates that moving the contaminated soil percentage from30 to 40% increases the VMA, while it decreases when moving the soil percentage from40 to 50%.

It is noteworthy to mention that the P-values of all interaction effects were found tobe greater than α=0.05. Then, we fail to reject that interaction effect oil saturation per-centage and the contaminated soil percentage is significant on the stability, flow andVMA (Table 10).

8.6. Residual analysis

Residuals are an estimate of the experimental error. They represent the differencebetween observed and predicted values of the sample properties (stability, flow andVMA). Residual analysis is a model adequacy checking technique by which theunderlying ANOVA assumptions can be validated (Montgomery, 2008). Verified

Figure 5. ANOVA main effect plot for VMA.

Figure 4. ANOVA main effect plot for flow.

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experimental assumptions were randomization, independence and normality of error. Theerrors follow a normal distribution with mean 0 and share the same variance N (0, σ2).

From the Normal probability plot of residuals of Figure 6, the residuals along thediagonal line indicate that the normality assumption is adequate. Figure 7 shows the plotof residuals with respect to experiment order. The plot depicts that the randomizationand independence of experiment assumption was adequate as well.

Table 10. ANOVA table for VMA.

Source DF Seq SS Addj SS Adj MS F P

% of oil 1 2.98 2.98 2.98 0.08 0.786% of OCS 2 220.43 220.43 110.21 2.97 0.127% of oil*% of soil 2 30.95 30.95 15.47 0.42 0.677Error 6 222.63 222.63 37.10Total 11 476.98

Figure 6. Normality plot of the residuals.

Figure 7. Plot of residual vs. observation experimentation order.

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9. Conclusion

This paper analysed some geotechnical properties of oil-contaminated soil that resulteddue to the Gulf war in 1990. Samples were obtained from Bahra oil lake/spill located inthe North of Kuwait City. Quality control and assurance procedures for collecting sam-ples including protections against tool cross-contamination and transport chain of cus-tody were adopted. The tests included in this study are: stability, flow value, bulkdensity determination, percent VMA, sieve and Marshall test. The aforementioned testswere prescribed in accordance to the minster of public works (MPW), as these tests areto authorize the utilization of existing oil-contaminated soils for wearing layer of asphaltconcrete. Especially, that the current practice is to landfill the oil-contaminated soils, adamaging option from an environmental and economical perspectives, as it brings thecontaminants closer to the water table. Other auxiliary tests may also be conducted toinvestigate oil-contaminated soils to be used in other construction work types, as build-ing material for example. These could include: Atterberg limit, compaction, permeabilityand unconsolidated undrained triaxial tests. However, since this study is not intended toinvestigate the aptness to use oil-contaminated soils in buildings, it is not prerequisite.The paper reports findings that the grain size of oil-contaminated soil distribution repre-sents poorly graded sand grains. Therefore, based on the Marshall design’s standards ofthe aggregates size, the wearing course layer of asphalt concrete is the most suitablelayer in which such oil-contaminated soils can be exploited. However, atmospheric con-ditions and elapsed time are two key issues affecting the viability of remedial methodsof the oil geotechnical properties of oil-contaminated soil. As of today, the post-gulfwar oil lakes have settled for over two decades, thus termed weathered. Therefore, fullfactorial experiment with two replications was conducted to test variable conditions ofsoil properties and ratio of oil-contaminated soil to clean soil in asphalt mix. Hence,two design factors considered are: the saturation percent of oil in the contaminated soiland fraction of contaminated soil to clean soil present in the asphalt mixture. Theresponse variables of interest include: stability, flow value and percent VMA. Marshalltest method was used to achieve optimum binder content. Soil stability values wereacceptable when compared to standard values. In general, the resulted stability for lowsaturation yielded better values than high saturation level. The high oil saturation levelwith 30, 40, and 50% were below the standard value in one replicate only. As for theflow, values found were acceptable. The results also showed that increasing the percent-age of oil from low to high decreases the VMA .It also indicates that moving the con-taminated soil percentage from 30 to 40% increases the VMA, while it decreases whenmoving the soil percentage from 40 to 50%. Randomization of treatment combinationwas conducted to ensure no contamination of nuisance factors. The model was subjectto statistical adequacy checking using residual analysis and normal probability plots.Future research is directed towards comparing the amounts of pollutant released whenusing contaminated soils in asphalt concrete with the existing natural situation on thelong run using life cycle analysis (LCA), in addition to investigating the roads infiltra-tions (see Jullien & Francois, 2006) with and without the use of oil-contaminated soilsin asphalt concrete.

AcknowledgementsThis research was supported by the office of career, consultants and development (OCCD) ofKuwait University for supporting this project financially and Dr Adam Almulla head of OCCDfor facilitating Kuwait University labs. The Authors would like to thank Dr Rawa Al-Jarallah,

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Engineer Dalal Althuwainy, and Fatma Al-Wazzan, Dr Hassan Alsanad, Dr Rana Al Fares,Dr Waled Abdullah, and Dr Amar Ben Nakhi and lab technicians Reda Amin, Sharif Al-Saqti,Qoddos Abo Al-Kalam, and Sulaiman Abdulhajj.

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