variation of mechanical and hydraulic properties of oil-contaminated soil due to a...

Soil and Sediment Contamination, 19:531–546, 2010Copyright © Taylor & Francis Group, LLCISSN: 1532-0383 print / 1549-7887 onlineDOI: 10.1080/15320383.2010.499922

Variation of Mechanical and HydraulicProperties of Oil-Contaminated Soil Due

to a Surfactant-Enhanced Washing Process

BERNARDO VAZQUEZ,1 ERICK R. BANDALA,1

ROSEMBERG REYES,1 AND LUIS G. TORRES2

1Departamento de Ingenierıa Civil y Ambiental, Universidad de lasAmericas-Puebla, Puebla, Mexico2Unidad Profesional Interdisciplinaria de Biotecnologıa, Departamento deBioprocesos, Instituto Politecnico Nacional, Ticoman, Mexico

Soil contaminated with a high level of hydrocarbons was obtained from an area near anoil extraction facility located in the Tamiahua region, Veracruz, Mexico, and submitted toa surfactant enhanced washing (SEW) process to remove contaminants. The purpose ofthis article was to characterize the variations in the mechanical and hydraulic propertiesof the soil after the application of the SEW process, as these characteristics are importantin soil mechanics and might affect the behavior of the material for construction purposes.During the experimental assessment, each test was conducted several times for threedifferent soil conditions (contaminated, non-contaminated, and washed soil), applyingthe same specifications and features to each one to allow accurate comparison of data.Results show that the soil washing process produces a loss of fine particles that affectthe mechanical and hydraulic behavior of the tested materials. Data also indicates thatthe presence of the contaminant modifies soil characteristics generating different soilproperties under all the conditions studied.

Keywords hydrocarbons, physical and hydraulic properties, soil washing, surfactants

Introduction

Soil degradation caused by oil derivative spills is a topic of awareness worldwide. Thisproblem is very relevant in countries, such as Mexico, with huge oil exploration, produc-tion, and petrochemical areas where a large number of oil-contaminated sites have beendocumented. For decades, the release of oil derivatives, i.e. due to accidental spills andleakage of storage tanks, has been the major contributor to the degradation of soil (Clarkand Delfino, 2003) in our country. During the year 2001, hydrocarbon spills in Mexico,reported by PEMEX, included 185,203 events and involved 6,252 tons of oil derivatives(Iturbe et al., 2005). Total oil spills in our country have been estimated at up to 1.5 million

Address correspondence to Erick R. Bandala, Departamento de Ingenierıa Civil y Ambiental,Universidad de las Americas-Puebla, Sta. Catarina Martir, Cholula, 82720 Puebla, Mexico. E-mail:[email protected]

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tons per year (SEMARNAT, 2004; INE, 2005), generating an important amount of con-taminated soil. This situation is so relevant that just as an example, in the state of Tabasco,which is one of the most important oil exploration and production areas in the country, thetotal amount of oil-contaminated soil has been estimated at about 0.01% of the total surfaceof the state (Rivera-Cruz et al., 2004; Zavala-Cruz et al., 2005).

Crude oil and its derivatives are characterized as viscous liquids containing a com-plex mixture of chemical compounds, thousands of them belonging to the hydrocarbonfamily. Hydrocarbons are organic compounds consisting of carbon and hydrogen atoms,mostly extracted from fossil fuels (Flores-Puente et al., 2004). These products exhibit lowbiodegradability and high toxicity and persistence; one of the most important problemsthey pose is the possibility of their reaching underground water and affecting its qualityand availability as a supply source (Torres et al., 2007).

Soil is made up of a set of particles with a well-defined organization and propertiesthat may rapidly vary in a vertical direction; this means that they present changes verticallyfaster than horizontally (Juarez-Badillo and Rico-Rodrıguez, 2006). Contaminated soil isone in which characteristics have been modified by the presence of a hazardous substance,with a concentration such that could be considered a risk to human health and the environ-ment. In recent years, greater emphasis has been placed on the development of techniquesfor recovery and reuse of contaminated soils (Volke and Velasco, 2002). Among these tech-nologies, the surfactant-enhanced soil washing (SESW) process is a common technologyused mainly for the restoration of sites contaminated with hazardous pollutants. This tech-nology has been tested both in-situ and ex-situ in a wide range of soils contaminated withhydrocarbons (Torres et al., 2003; Lopez et al., 2004; Torres et al., 2007), metals (Iturbeet al., 2004; Ehsan et al., 2007; Zhang et al., 2007), or pesticides (Bandala et al., 2009;Wang and Keller, 2008). All these are substances that tend to easily adhere physically orchemically to silt and clay and therefore are difficult to separate from soil containing highamounts of fine particles (EPA, 1996).

Some papers dealing with the chemical and microbiological characterization of thesoil after application of SESW processes have been reported in the past. Iturbe et al. (2008),for example, reported that some microbiological and psysicochemical changes occurred incontaminated soil after the SESW process. These researchers studied changes due to soilwashing when using both ionic (with and without salt addition) and nonionic surfactantsfor soil characterization before and after the SESW process. The authors concluded thatonly electrical conductivity and phosphorous concentration were significantly affected(p < 0.05). Some changes regarding Na, Ca and Mg were observed due to the ion exchange,especially when using SDS solutions. These ion exchanges lead to loss of fine particles, asalso reported by Liu and Roy (1995).

Despite the fact that chemical and microbiological changes have not been observedin soil after the SESW process, the question of how the process may affect other soilcharacteristics remains. Little data has been reported about changes in the hydraulic andmechanic characteristics of the soil as a result of the application of SESW technology.Allred and Brown (1994) found that hydraulic conductivity reduction in washed soil isinfluenced by surfactant type, concentration, soil organic content, and added electrolytes.Mulligan et al. (2001) proposed that efficiency of contaminant extraction in the SESWprocesses depends on the hydraulic conductivity of the soil and that high permeability ofsoils is favorable to SESW processes since numerous pore volumes can be passed throughthe contaminated area. Other authors (Liu and Roy, 1995) determined that variations in thehydraulic conductivity of washed soils can also be due to other physical effects, such asclay expansion or fine particle mobilization.

Variation of Mechanical and Hydraulic Properties of Oil-contaminated Soil 533

Lee and Cody (2001) reported the variation of soil’s hydraulic conductivity due toanionic surfactants in the soil column. Reduction in hydraulic conductivity due to surfactantleaching ranged from 9 to 85% relative to the initial values measured with deionized water.The most severe reductions were observed when clay loam was leached with trideceth-19-carboxylic acid (TDCA). They found that the surfactant used (sodium diphenyl oxidedisulfonate, DOSL) is a good candidate for surfactant-assisted remediation based on itsrelatively insignificant effects on the hydraulic conductivity of soil.

As stated, estimation of changes in the characteristics of soils is a very important taskif potentially adverse effects of the use of the SESW process on site restoration are to beassessed. However, the effect of the SESW process on many important mechanical andhydraulic properties of the soil remains practically unknown.

The aim of this article is to analyze the changes in some mechanical and hydraulicproperties of hydrocarbon-contaminated soil before and after their restoration with the ap-plication of the SESW process, as well as to estimate the feasibility of the proposed method-ology for the restoration of contaminated sites. An additional goal is to assess the potentialdamage to soil characteristics as a result of the application of this restoration technology.This study has the premise of generating knowledge on the effects produced by hydrocar-bon contamination on soil as well as the effects of the application of the SESW restorationprocess on characteristics such as permeability, resistance to shear, and compaction. By de-termining the changes to the mechanical and hydraulic behavior of different soil conditions,modifications to avoid possible failure in structures and foundations can be determined.

Methodology

Soil Sampling

The soil samples (contaminated and non-contaminated) were obtained from the HuastecaVeracruzana region in northeastern Mexico, near an oil extraction facility located in the areasurrounding the Paso Palomas bridge in Tamiahua, Veracruz. The materials were obtainedat a depth of between 30 and 50 cm from two different areas close to each other, one wherespilled oil was mixed with soil and the other one, as a reference, in an adjacent area whereno traces of the contaminant were visible.

Both samples were placed in tightly closed plastic bags to prevent loss of moisture.Moisture content was determined in both soils using the standard procedure specified inthe ASTM D2216-98 Standard Test (ASTM, 1998). Soil samples were placed in trays andplaced in an oven at 100◦C for 24 hours. After drying, soil samples were weighed to deter-mine their moisture content. Two different tests were performed for both contaminated andnon-contaminated soil in order to determine the average moisture content at the sample site.

Once dried, soil samples were characterized and, in the case of the contaminatedsoil, submitted to the SESW process. By means of this procedure, three different soilconditions were obtained: contaminated soil (CS), which is dry, contaminated soil withoutany restoration treatment; non-contaminated soil (NCS), which is the reference soil withoutoil contamination; and washed soil (WS), which is the contaminated soil after its submissionto the SESW procedure.

SESW Assessments

The SESW conditions for optimal contaminant removal reported by Castillo-Espinoza(2008) were used for the development of this paper and are described as follows: non-ionic


Table 1Some chemical and physical characteristics of the surfactant used in the experimental

assessments

Chemical Ionic Molecular weightname nature (g/g mol) HLB CMC

Etoxylated nonylphenol (Poe = 10)

Non-ionic 660 13.5 49.5

From Torres et al., 2005.

surfactant Nonyl phenol (technical grade, Proquipusa, Mexico; Poe = 10, other surfactantcharacteristics are shown in Table 1) was used in the soil-washing procedures. The washingprocess was carried out using an acrylic tank (total volume of 2.5 L) and a mechanicallydriven mixer, including a A200 impeller with 4 blades with a 45◦ inclined, 5 cm diameter,and stirring speed of 1700 rpm.

The washing procedure was as follows: 200 g of contaminated soil and 600 ml ofsurfactant solution (0.5% v/v) were mixed in the acrylic tank for a period of 90 min.The engine used for mixing was a Lightning Lab Master R©(tm) Mixer BK223116. The kitincludes a 60 cm steel shaft and a mixing tank with 10 cm diameter with four 1′′-thickbaffles placed every 90◦ within the circumference of the tank. The impeller was located inthe central part of the mixing tank, to avoid contact with the baffles, and 5 cm from thebottom.

Total Petroleum Hydrocarbon (TPHs) Determination

To determine the concentration of contaminants in the soil, a gravimetric method wasused for the quantification of total petroleum hydrocarbons, which were extracted usingHexanes (J.T. Baker) as previously reported by Torres et al. (2007). A porcelain capsulewas placed in an oven for 24 hours at 100◦C; after this period its weight was registered. 10grams of dry soil (contaminated and washed) were subjected to the hydrocarbon extractionby reflux with hexanes. After sedimentation, the extract was decanted, filtered through apaper filter (Whatman glass microfiber filter, grade GF 6) to avoid the presence of fineparticles, transferred to the porcelain capsule, and subjected to evaporation in the oven at atemperature of 60◦C for 24 hours. The difference in the weight of the capsule before andafter the evaporation of the extract was quantified as the total petroleum hydrocarbons inthe soil sample.

This procedure was applied to the contaminated and washed soil samples in order todetermine the rate of removal as described in equation (1):

%Re moval = 100 − [100TPH(W.S)/TPHs(C.S.)] (1)

where TPHs(W.S.) is the total petroleum hydrocarbon content in the washed soil andTPHs(C.S.) is the total petroleum hydrocarbon content in the initial contaminated soil.Previous experiments showed that results for this technique vary between 4 and 5% (Torreset al., 2007).


Particle Size

To determine soil particle size, each sample was analyzed by screening, shaking the samplematerial through a set of meshes with openings that gradually decrease in compliance withthe ASTM D422-63 method (ASTM, 2002).

Consistency Limits

The procedure to determine the consistency of fine-grained soils based on variable watercontent was used. By using this procedure, a soil sample can be classified into four basicstates: solid, semisolid, plastic, and liquid. The water content between the transition ofthese states is known as the Atterberg limits (Das, 2001). Based on the results from theconsistency limits, the identification and classification of a soil can be determined using theparameters established in the Unified System of Soil Classification (USSC). The tests werecarried out following the specifications of the sampling and testing methods for materialsincluded in the ASTM D4318-05 method (ASTM, 2005)

Compressibility is closely linked to the characteristics of plasticity, in particular to thevalue of liquid limit, since this property increases by increasing the value of liquid limit ifother factors remain constant. The plasticity of a material is due to the laminar form of thecolloidal particles that constitute it; therefore, the characteristics of plasticity of the soil arean indirect measurement of the amount of colloidal particles it contains and, as a result, ofits compressibility (Juarez-Badillo and Rico-Rodrıguez, 2006).

Permeability

Permeability tests were done using a falling head permeameter, which measures the amountof water passing through the soil sample applying different levels of hydraulic head in amanometric tube. Tests were performed based on the ASTM-D5084 methodology (ASTM,2003a). The soil samples were prepared using 150 g of material sifted through mesh No.4, providing the humidity found at the sampling site (28%) and forming a tablet inside thepermeameter in three 50 g. layers of wet soil by providing 50 blows using a load with thefollowing characteristics: contact surface area, 9.62 cm2; weight, 526 g. The blows weredelivered by dropping the load from a height of approximately 5 cm above the level of eachlayer. A porous stone was placed at the bottom of the permeameter and another one at thetop of the tablet. A saturation time of 1 hour was allowed for each sample test. Each testwas repeated three times with three replicates of each using different soil conditions (CS,NCS, and WS). With the data obtained the hydraulic conductivity was calculated usingequation (2):

k = (aL/At) ln(h1/h2) (2)

where A is the cross-sectional area of the soil sample, a is the cross-sectional area of theburette, L is the length of the sample, h1 is the initial hydraulic head, h2 is the final hydraulichead, and t is the time elapsed from h1 to h2.

Compaction

As water content is gradually increased and the same compaction energy is applied, theweight of solids gradually increases for an established volume. After a certain water content,any increase in moisture tends to reduce the dry weight because water replaces space that


could have been occupied by solid particles (Das, 2001). The methodology used for thistask was the ASTM D 698 (ASTM, 2003b). This test method was used to determine therelationship between molding water content and dry unit weight of soils (compaction curve)compacted in a 4 in. (101.6 mm) diameter mold with a 5.50-lbf (24.5-N) rammer droppedfrom a height of 12.0 in. (305 mm), producing a compactive effort of 12,400 ft-lbf/ft3 (600kN-m/m3). Three layers of soil and 25 blows per layer were used.

Resistance to Shear

The resistance to shear of a mass of soil can be defined as the internal resistance per unit areathat the material presents to resist failure and sliding along any plane within it (Das, 2001).The study of this property is fundamental to understanding various problems involving soilstability. Coulomb’s law states that the resistance of the soil on a potential failure plane isaffected by the characteristics of cohesion (c) and internal friction (ϕ); this relationship isexpressed in equation (3):

τ = c + σ tan ϕ (3)

where τ is the shear force on the failure plane, σ is the normal stress on the failure plane,C is the cohesion, and ϕ is the internal friction angle.

Direct testing to determine the resistance to shear strength in a soil sample is one of theoldest tests in soil mechanics. The test can be performed controlling strain or deformation.For the present paper, deformation control was used. Normal stress was calculated dividingthe axial strength applied by the cross-sectional area of the specimen. Shear resistance wasobtained dividing shear strength by the cross-sectional area of the specimen. In total, 3different axial strengths were used (10, 30, and 100 kN/m2) for each soil condition (CS,NCS, and WS). Each test was repeated 5 times; in the results and discussion section onlythe average values and their standard deviation from the five repetitions are shown.

Results and Discussion

SEWS Assessments

During the SEWS process it was observed that foam is formed at the top of the containerwhere the mixture was sedimented. After a while, many soil particles sedimented; however,some particles adhered to the walls of the container as a residue of the foam. It was observedthat the particles that adhered to the walls of the container were fine particles. Furthermore,some other fine particles remained in suspension and were lost during the decantationprocess in order to obtain only the sediment. Castillo-Espinoza (2008) mentioned thatnon-ionic surfactants present slow precipitation and also that medium-sized particles weresuspended in the upper part of the solution. By keeping soil particles in suspension it ispossible to verify that the original hydrophobicity of the material is reduced due to thecharacteristics of the contaminant affecting it. Oil pollutants form a solution with the waterthrough the surfactant.

For the contaminated soil, TPHs concentration was 50,600 mgkg−1 of dry soil whereasfor the washed soil TPHs concentration was 26,000 mgkg−1. Therefore, a removal percent-age of about 49% was obtained. It is important to highlight that the efficiency of the surfac-tant depends on soil characteristics, so different removal percentages can be obtained withdifferent types of soil, although the same concentrations and washing conditions are used.


Figure 1. Particle size distribution for contaminated, washed and non-contaminated soils.

Particle Size

Using the results of granulometric analysis, granulometric distribution curves in half loggraphs were conducted. The diameters of the particles are plotted in logarithmic scale andthe percentages of material passing through each mesh in arithmetic scale. Figure 1 showsthe average results obtained for each soil condition.

From Figure 1, a significant difference in the percentages of particle size between 1.0and 0.1 mm for contaminated and non-contaminated soils can be observed. This changemay be due to the presence of the contaminant that causes fine particles to adhere to largerones, modifying their size and shape; thus, the percentage of material passing through themeshes in that range decreases because it is more easily retained.

According to the results reported by Adams and Morales (2008), hydrocarbons act as abinder paste that sticks fine particles (clay) to coarse particles (sand). In the tests conducted,they noticed a reduction in clay particles that was directly proportional to the increase in theamount of sand and were able to determine that, apparently, hydrocarbons have the abilityof agglomerating fine particles to larger ones.

During the experiments, larger soil particles in the SESW process were removed inorder to avoid damaging the mixing equipment. A decrease in the content of fine particles inthe washed soil was observed, and its behavior was different from the other two conditions:it begins almost equal to the non-contaminated soil and then falls in a steeper slope thanthe contaminated soil.

Consistency Limits

Liquid limit. Three tests for every soil condition were conducted and the results were plottedplacing the number of blows in a logarithmic scale on the x-axis and their correspondingmoisture content on the y-axis using an arithmetic scale. The liquid limit is the water content


Figure 2. Moisture content as a function of blows number.

corresponding to 25 blows. Figure 2 shows the results obtained for the liquid limit in thedifferent soil types tested. From the figure, it can be observed that the contaminated soilrequires more water to stop behaving plastically. It is possible that hydrophobic propertiesof the contaminant are related to this phenomenon. Adams and Morales (2008) suggestedthat soil particles generally have a negative charge, allowing moisture retention causedby its electrostatic interaction with water due to its polar characteristic and the partialpositive charge of the hydrogen atom in the molecule. On the other hand, hydrocarbonsare essentially non-polar or have very low polarity; therefore, when petroleum covers thesurface of the soil particle it disrupts the electrostatic interaction between soil particles andwater, reducing its capacity to retain moisture. This phenomenon produces an increase inwater needed to moisturize soil particles and presents the behavior sought in the test todetermine the liquid limit, and is likely to contribute by increasing the plasticity of thematerial.

The loss of fine particles of soil during the SESW process, and certain amounts ofsurfactant and contaminant remaining in the soil, leads to a change in liquid behavior, thisbeing completely different from those of the other two soil conditions. The washed soilpresents a sandier consistency that may be caused by the fact that this type of soil requireda smaller amount of water to behave as a liquid.

Plastic limit. The plastic limit was determined by the average value of moisture content atwhich a roll of soil split into pieces when it reached 3 mm in diameter, as shown in Figure3a. When it is not possible to determine the plastic limit with any water content using theconventional method (see Figure 3b), the soil sample is reported as non-plastic (NP); thisphenomenon occurred with the washed soil due to its sandy consistency.

Once the liquid and plastic limits were obtained, the plastic index was calculated bysubtracting the plastic limit value from the liquid limit value. Table 2 contains the resultsfor the three soil conditions studied.


Figure 3. a and b. Roll of washed soil for plastic limit determination.

With the results of the granulometric analysis and the consistency limits, the classifica-tion of the soil can be made using the USSC system. An alternative procedure to determinethe plastic limit of the washed soil was performed; this method consisted of graduallyincreasing the water content of the soil sample and trying to make the 3 mm diameter rolls.By using this alternative methodology the plastic limit obtained was 34.76, which is a valuevery close to the liquid limit for this soil condition. This explains the reason why the plasticlimit could not be determined with the conventional procedure.

Soil Classification

Using the information from Table 2 and with the knowledge that the soil studied presentedmore than 50% fine particles as per the USSC plasticity chart, it can be determined that forboth contaminated and non-contaminated soils the classification for low compressibilityinorganic silt (ML) is obtained. Using the value obtained for the washed soil applyingthe alternative procedure, ML classification is also obtained; however, its location on theplasticity chart is separated from the other two soil conditions and also from line A in theUSSC plasticity chart, which means that its behavior is quite different from that of thecontaminated and non-contaminated soils.

Table 2Liquid and plastic limits for contaminated, non-contaminated and washed soils

Contaminated soil Non-contaminated soil Washed soil

Liquid limit 46.50 42.00 38.50Plastic limit 30.37 26.36 N.D.Plastic index 16.13 15.64 N.D.


Table 3k values for the three assessments with different soils

k (cm/min)/105 k (cm/min)/105 k (cm/min)/105

Soil condition First assessment Second assessment Third assessment

Non-contaminated 1.84 3.37 1.67Contaminated 11.20 7.61 16.70Washed soil 43.40 14.20 24.20

Permeability

In Table 3, the average hydraulic conductivity values for each soil condition are depicted.Three successive determinations were made using the same soil sample and three differentsoil samples were used for each type of soil.

Changes in the hydraulic conductivity in successive determinations of the same soilsample were observed. In altered samples that have been remolded, some soil particles arefree and when water flows through the sample, those particles are dragged and rearranged.Sometimes these particles clog the channels through which water passes and it is for thisreason that successive measurements of the hydraulic conductivity in the same sampledecrease a little. Juarez-Badillo and Rico-Rodrıguez (2006) noted that sometimes theseparticles are dragged out of the soil sample, causing turbidity in the outlet water. Thiscondition was observed in the tests undertaken, and more significantly in the washedand contaminated soil. These authors also stated that this phenomenon can occur even inundisturbed samples due to the combination of layers with different soil characteristics.

Although all of the values in the test for the same soil condition are different, apattern can be observed: non-contaminated soil always presented the lowest hydraulicconductivity; in contrast, washed soil showed the highest values. This might be related tothe sandy consistency of the former that did not allow the application of the test to obtainits plastic limit with the conventional method. It can be anticipated that the arrangementamong washed soil particles permits the formation of a greater number of voids; as a result,the soil becomes more permeable.

Hydraulic conductivity values for the contaminated soil are in a range between thevalues for non-contaminated and washed soil; however, all the measurements indicate thatthis type of soil becomes more permeable than the soil with an absence of contaminants.This condition can be due to petroleum acting as an agglutinant that adheres fine particlesto those of larger size and also to other fine particles, modifying their shape and size. With agreater amount of larger soil particles an arrangement among them that allows more voidsthrough which water can move easily could be expected.

In addition, it was possible to observe that by providing moisture to the contaminatedsoil sample some clumps were formed, possibly due to the hydrophobicity of the contam-inant. Saturation time for the samples analyzed was 1 hour; it is likely that by providingan extended saturation time, results different from those presented in this paper may beobtained.

Compaction

The results of the test performed for the three different soil conditions are depicted in Figure4. For a long time, the idea that particle size was the determining factor in many important


Figure 4. Soil compaction for the three soil conditions studied.

mechanical properties in soil, particularly compressibility, was unchanged. However, it hasnow been determined that the shape of particles plays an even more important role in thatproperty.

According to different authors (Juarez-Badillo and Rico-Rodrıguez, 2006), fine soilparticles tend to be crushed, laminar-shaped. For this reason compressibility of non-contaminated soil is greater than the other two conditions; because the accommodationof its particles is denser, there is a lower volume of voids. On the other hand, contaminatedsoil particles whose shape has been altered by the presence of petroleum have lower com-pressibility, which indicates that the arrangement among its particles is not the same asthat which the non-contaminated soil presents. By supplying moisture to the contaminatedsoil sample, clumps were formed due to the hydrophobic properties of the contaminant,producing larger particles and leading to a reduction of the compaction capacity.

In the washed soil, the amount of contaminant is reduced, allowing the separation of theparticles that were stuck to each other. However, loss of fine particles during the restorationprocess impeded the same arrangement occurring in the non-contaminated soil. Despite thefact that compressibility of the washed soil is improved compared to that presented in thecontaminated soil, it cannot reach the dry bulk density of the non-contaminated soil.

Resistance to Shear Stress

Figures 5a through 5c show the resistance to shear stress in the different soil conditionstested. An increase in shear resistance can be observed for the contaminated and washedsoil. In the first (Figure 5a), the presence of the contaminant and also the formation ofclumps due to the effect of the petroleum may help increase this resistance. Larger soilparticles in the washed soil are likely to increase friction among themselves, and thereforeimprove shear resistance. The increase in resistance can be observed more easily withincreasing values of normal stress; however, this increase is not very substantial. Different


results are expected using saturated samples and also with the natural and optimum watercontent obtained from the proctor compaction test.

The maximum shear stress (τ ) obtained was plotted against normal stress (σ ) inFigures 5a, b and c, from where it is possible to determine the internal friction angle (ϕ)and the cohesion value (c) for each soil condition. In the literature review, establishedranges of values for the type of soil obtained (ML) were found; however, values obtained

Figure 5. a, b and c. Resistance to shear stress for washed, non-contaminated and contaminatedsoils.


Figure 5. (Continued)

experimentally in this paper are outside these ranges (26–35 Das, 2001). For this reason itis impossible to reach a final conclusion regarding this aspect.

Conclusions

After the SESW process, during the sedimentation of the mixture and decantation of thewash water a loss of fine particles was observed. A contaminant removal of 48.6% wasachieved.

A proportional relationship between the decreasing amounts of fine particles and theincreasing amount of larger particles in the granulometric analysis for the contaminatedsoil was observed. The contaminant seems to act as an agglutinant, bonding fine particlesto larger particles and also forming clumps of soil particles. Washed soil presented a lesseramount of fine particles in comparison with the other two soil conditions, due to the loss ofthese particles during the restoration process. This condition increases the amount of largeparticles providing the soil with a sandy consistency.

The contaminated soil required more water to stop behaving plastically. The liquidlimit for the washed soil was the lowest; this condition must be related to the loss of fineparticles, since it is these that provide the plastic properties to the material.

Due to its sandy consistency, washed soil presented greater difficulties in performingthe test to determine its plastic limit, as a result of the loss of fine particles. The valueobtained for the contaminated soil was higher than for the non-contaminated soil; the sameproportional relationship was presented in the liquid limits; therefore, the plasticity indexis almost the same for both contaminated and non-contaminated soil conditions.

Non-contaminated soil presented the lowest hydraulic conductivity, meaning that itwas less permeable, probably caused by a greater amount of fine particles providing adenser arrangement of soil particles. Washed soil was the most permeable due to the loss


of fine particles. Contaminated soil presented an intermediate behavior, closer to washedsoil; however, all of the measurements indicated that it is more permeable than the non-contaminated soil, due to the effect of the contaminant on the soil particles that has beendiscussed.

The soil compaction capacity is reduced by the presence of the hydrocarbon. Non-contaminated soil reaches a maximum dry bulk density (DBD) of 1.6 ton/m3, with anoptimum moisture content of 19%. Optimum moisture content for contaminated soil in-creases to 22% but the DBD decreases to 1.35 ton/m3. This behavior may be due to the highconcentration of hydrocarbon in the soil sample, since it weighs less than solid particlesof soil, contaminated soil bulk density tends to decrease. In addition, the test showed thatby applying water to the soil, larger clumps were formed; so it is likely that the number ofvoids increases and thus the compressibility of the soil decreases. In the washed soil thereis a reduction of the contaminant, so the maximum dry bulk density increases; nevertheless,the loss of fine particles restricts the increase of the soil compaction capacity.

In the test for direct resistance to shear a small increase in resistance in both con-taminated and washed soil was observed. The contaminant must directly contribute to thisresistance and also to the formation of lumps/clumps. The loss of fine particles increasesfriction among larger-sized particles in the material, which may lead to an increase ofits resistance. However, despite the fact that improvement in the resistance was observedexperimentally, it was not significant. These tests were conducted with dry material and itis likely that the behavior may change with the application of certain moisture content.

There are modifications to the mechanical and hydraulic properties; however, thesechanges do not lead to a decline in soil quality, thus the soil resulting from the SEWSprocess can only be considered different. Depending on the desired use of the soil it can bedetermined if it is convenient to apply the restoration process or whether the mechanical andhydraulic characteristics of the contaminated soil are more suitable for future construction.There is a need for more tests with different soil types in order to draw conclusions thatcan be generalized.

Acknowledgments

The financial support of the National Council for Science and Technology (CONACyT)Mexico (Project No. 37557) is acknowledged.

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