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

⎛ 1 ⎞⎜ ⎟⎝ csh⎠c⎡ shw ⎤e( w)= ash⎢ + 1c ⎥(1)sh⎣bsh⎦where: a sh = the minimum void ratio (e min ), b sh =slope of the line of tangency, (e.g., drying fromsaturated conditions), c sh = curvature of the shrinkagecurve, and w = gravimetric water content. Theratio,ash Gs= (2)b Sshis a constant for a specific soil; G s is the specificgravity and S is the degree of saturation.Once the minimum void ratio of the soil isknown, it is possible to estimate the remainingparameters required for the designation of theshrinkage curve. The minimum void ratio the soilcan attain is defined by the variable, a sh . The c shparameter provides the remaining shape of theshrinkage curve. The curvature of the shrinkagecurve is controlled by varying the c sh parameter.3.3 Integration of the Shrinkage Curve and thelaboratory measured SWCCA laboratory measured SWCC describes therelationship between gravimetric water content andsoil suction. The Fredlund and Xing (1994) equation(Equation 3) can be used to best-fit the SWCCwhich can then be combined with the shrinkagecurve.⎡⎤⎡ ⎛ ψ ⎞ ⎤ ⎢ ⎥⎢ ln ⎜1+⎟ ⎥ ⎢⎥hr⎢ 1 ⎥w( ψ ) = w ⎢s1− ⎝ ⎠ ⎥6m⎢f⎛ 10 ⎞ ⎥ ⎢ ⎥n fln 1⎡ ⎡ψ⎤⎤⎢ ⎜ + ⎟ ⎥ ⎢⎛ ⎞ ⎥⎢ h ⎢ln ⎢exp(1)⎥⎥⎣ ⎝+r ⎠ ⎥⎦ ⎢⎜ ⎥a ⎟⎢ ⎢ ⎢⎥f ⎥⎣ ⎣ ⎝ ⎠ ⎦⎦⎥⎣⎦(3)where: w(ψ) = gravimetric water content at anyspecified suction, ψ; w s = saturated gravimetricwater content; h r = residual soil suction; a f , n f , and m f= the fitting parameters for the Fredlund and Xing(1994) SWCC equation. Equation 3 is written for thegravimetric water content designation; however, itshould be noted that it can be best-fit to any of thedesignations of water content versus soil suctionbecause of the flexibility of the equation with threefitting soil parameters.It is possible to compute the degree of saturationversus soil suction as well as any other designationfor the amount of water in the soil by combiningEquations 1 and 3.4 MEASUREMENT AND INTERPRETATIONOF OIL SANDS TAILINGS RESULTSSoil-water characteristic curves were measured onsamples of Oil Sands tailings that were thickenedusing a pilot deep cone thickener. The tailings werecomprised of fines (< 44 microns) and 10% and 45%greater than 44 microns sand (i.e., 0.1 and 0.8 sandto fines ratios, SFR, respectively). The tailings werefrom development studies on Oil Sands tailings fromnorthern Alberta, Canada. The shrinkage curveresults were presented in Figure 3. The 0.1 SFRtailings have plastic and liquid limits of 30 and 55,respectively. The 0.8 SFR tailings have plastic andliquid limits of 15 and 38, respectively.Approximately 60% of the material classifies as claysize particles. The slurry material has a gravimetricwater content of about 100%. The intent is to depositthe thickened tailings material in lifts of varyingthicknesses that are allowed to dry.As water is removed from the thickened finetailings, the volume of the material decreases andthere is a slow increase in shear strength. As thematerial begins to desaturate near the plastic limitthere is a substantial increase in shear strength.Figure 4 shows the gravimetric water content, w,plotted versus soil suction for two samples testedwith each of the sand to fines ratios (SFR). Box #11had a SFR of 0.8 and a starting gravimetric watercontent of about 70%. Box #6 was the samematerial; however, it was dried to about 25% beforethe SWCC test was performed. Box #2 had a SFR of0.1 and the initial gravimetric water content wasnear 70%. Box #5 was the same material; however,it was dried to about 47% before the SWCC wasperformed.The initially wet samples show a break in thecurvature in the region around 1 kPa. The driedsamples show a break in the curvature around10 kPa. However, the curvature is not distinct and itshould not be taken to represent an air-entry valuefor the material.It is necessary to use the shrinkage curve resultswhen interpreting the meaning of the SWCC results.A best-fit shrinkage curve equation can be combinedwith the Fredlund and Xing (1994) equation for theSWCC. It is then possible to plot the degree ofsaturation, S, versus soil suction as shown in Figure5.The results show that there is a distinct air-entryvalue for the 0.8 SFR material at about 100 kPa. Theair-entry value for the 0.1 SFR material is about1,000 kPa. These are the true air-entry values forthese two materials and the degree of saturationSWCCs are the curves that must be used for theestimation of the unsaturated hydraulic conductivityfunction. The residual condition can also be clearlyidentified as having a residual suction of about3,000 kPa and a residual degree of saturation of 20%for the 0.8 SFR tailings. The 0.1 SFR tailings have aresidual suction of about 15,000 kPa and a residualdegree of saturation of about 20%.418