Obtaining unsaturated soil properties for high volume change oil ...

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2 ROLE OF THE SOIL-WATERCHARACTERISTIC CURVE, SWCCThe SWCC shows the relationship between theamount of water in a soil and various applied soilsuctions (Fredlund and Rahardjo, 1993). There aretwo primary reference points on the SWCC; namely,the air-entry value and the residual suction. Theapparent air-entry value for a soil is dependent onhow the amount of water in the soil is quantified.When a soil undergoes volume change as soilsuction is increased, the air-entry value appears tooccur at different values. The differing air-entryvalues are related to how the amount of water in thesoil is defined. Each variable to designate theamount of water in the soil has significance but it isimportant that the correct interpretations be appliedat various stages of the analysis (Fredlund, 2002).The variables used to quantify the amount ofwater in the soil are:i) gravimetric water content, w;ii) volumetric water content, θ; with theinstantaneous total volume used in calculatingvolumetric water content;iii) volumetric water content, θ, with the volume ofwater, V w , referenced to original total volume ofthe specimen, V o , (i.e., V w / V o ); andiv) degree of saturation, S.Each of the above designation for the amount ofwater in the soil can be used to plot a soil-watercharacteristic curve. Each of the SWCCs wouldprovide similar information to the geotechnicalengineer provided the soil does not undergo volumechange as soil suction is increased. However, whenthe soil undergoes volume change, as is the caseunder consideration, then the geotechnical engineermust be able to plot each of the SWCCs and use theappropriate curves in the proper manner forevaluating unsaturated soil property functions to beused in performing numerical simulations ofphysical processes.Gravimetric water content is the most basicmeasurement of the amount of water in a soilbecause it requires only the measurement of mass.Volumetric water content, θ, has commonly beenused in agriculture-related disciplines and is definedas the amount of water in the soil referenced to theinstantaneous total volume of the soil specimen. Thetotal volume of the soil specimen must be knownwhen computing the volumetric water content in thismanner. Therefore, it is necessary to know the totalvolume of the soil specimen corresponding toequilibrium soil suction conditions. When modelingwater seepage through a soil, it is the change involumetric water content that defines the change inwater storage in the soil as suction changes.However, there are two possible ways to computevolumetric water content. If the overall volume ofthe soil changes by a small amount during theincrease in soil suction, then either designation ofvolumetric water content can be used. However, ifthe volume changes are substantial, then theinstantaneous total volume should be used for thecalculation of the water storage function.The degree of saturation, S, references thevolume of water in the soil to the instantaneousvolume of voids and therefore also requires the totalvolume of the specimen. When a soil undergoesvolume change as soil suction increases, then the airentryvalue and residual conditions need to beevaluated from the plot of degree of saturationversus soil suction. In addition, it is the degree ofsaturation versus soil suction plot that must be usedduring the integration process to calculate thepermeability function. The permeability functionmay also need to be computed in a different mannerto accommodate changes in void ratio prior toreaching the air-entry value of the soil.It is important to note that the unsaturated soilproperty functions need to be computed in distinctlydifferent manners when the soil undergoessubstantial volume change as soil suction increases.It is the instantaneous volumetric water contentdesignation that must be used to compute waterstorage and the degree of saturation designation thatmust be used to define the air-entry value of the soil.The degree of saturation designation plays the majorrole involved in estimating the unsaturated hydraulicconductivity function of the soil.The soils of primary concern in this paper are thehigh water content tailings that are a by-productfrom Oil Sands extraction. These materials may startwith a natural water content that is well above theliquid limit of the material and undergo a largevolume change upon drying.The difficulties associated with the testing ofsoils that undergo large volume changes withincreasing soil suctions are not a recent discovery.Fredlund (1964) showed that shrinkage curvemeasurements needed to be used in conjunction witha conventional measurement of the SWCC in orderto properly interpret the unsaturated soil behaviourof high volume change soils. Fredlund (loc. cit.)performed a series of tests on highly plastic Reginaclay that had a liquid limit of 75, a plastic limit of25, and had 50% clay size particles.3 SHRINKAGE LIMIT AND THE SHRINKAGECURVEThe shrinkage limit of a soil has been one of theclassification properties since the inception of soilmechanics (ASTM D427). Mercury immersion wasoriginally used for the measurement of the volume416
of the soil specimen. This technique is no longerconsidered acceptable in most countries.The shrinkage limit is defined as the watercontent corresponding to the minimum volume that asoil can attain upon drying to zero water content. Itis a fictitious water content that generally fallsslightly below the plastic limit of the soil.It is the entire shrinkage curve from an initiallysaturated soil condition to completely dry conditionsthat is called the shrinkage curve and is significantfor the interpretation of SWCC data. Figure 1 showsthe drying curve for an initially saturated soil. As aclay soil dries, a point is reached where the soilstarts to desaturate. This point is generally quiteclose to the plastic limit of the soil. Consequently,there is an approximate correlation between theplastic limit of a soil and its air-entry value. Uponfurther drying, another point is reached where thesoil dries without any further change in overallvolume. This can be referred to as the true“shrinkage limit” of the soil and the gravimetricwater content appears to approximately correlatewith the residual soil conditions.Void Ratio3.02.52.01.51.00.50.0ResidualWater ContentPlasticLimitShrinkage LimitLiquid LimitSaturation Line0% 20% 40% 60% 80% 100%Gravimetric Water ContentFigure 1. Shrinkage curve with its relationship to the AtterbergLimit classification properties.3.1 Measurement of the shrinkage curveAn experimental procedure was developed for themeasurement of the entire shrinkage curve frominitial high water content conditions to completelydry conditions. A digital micrometer was used forthe measurement of the volume at various stages ofdrying as shown in Figure 2. Brass rings weremachined to contain the soil specimens (i.e., therings have no bottom). The rings with the soil wereplaced onto wax paper and drying was allowed. Thedimensions of the soil specimens were selected suchthat cracking of the soil was unlikely to occur duringthe drying process. The initial dimensions selectedfor the shrinkage curve specimens were a diameterof 3.7 cm and a thickness of 1.2 cm.Figure 2. Digital micrometer for the measurement of thediameter and thickness of shrinkage specimens.The mass and volume of each soil specimen weremeasured on a daily basis. Four to six measurementsof the diameter and thickness of the specimen weremade at differing locations on the specimens. Figure3 shows typical measurements of water content andvoid ratio as the soil dried. It was observed that asthe specimen diameter began to decrease, with thespecimen pulling away from the brass ring, the rateof evaporation increased significantly (i.e., abouttwice as fast).The increase in the evaporation rate was relatedto the increased surface area from which evaporationwas occurring. Consequently, it is recommendedthat the measurements of mass and volume shouldbe increased to once every two to three hours oncethe soil shows signs of pulling away from the sidesof the ring.Void Ratio2.52.01.51.00.50.0Plastic LimitLiquid Limit0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%Gravimetric Water ContentBox #11 - S1Box #11 - S2Box #6 - S1Box #6 - S2Aged TT - S1Aged TT - S2Aged TT - S3100% SaturationBest FitFigure 3. Typical shrinkage curve for an oil sand clay soil.3.2 Equation for the Shrinkage CurveThe shrinkage curve has the form of a hyperboliccurve. Fredlund et al. (1997, 2002) proposed anequation to best-fit data for the shrinkage curve. Theequation has parameters with physical meaning andis of the following form:417
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