Figure 15-36.-Removing sample portion for moisture content.the groove (fig. 15-36). Place this in a mois-ture content can, weigh it, and record theweight. Oven-dry and record the differencein weights. This is the weight of the watercontent.4. Transfer the remaining soil in the cup tothe evaporation dish. Wash and dry the cup andgrooving tool. Reattach the cup in preparation forthe next run.5. Run at least five tests on each soil, with twoclosures above, two closures below, and oneclosure at or near the 25-blow line. An idealspread is closures at 16, 23.5, 29, and 33 blows.If each testis perfect, the plotted line through allpoints is shown as a straight line. If some testsare imperfect, the operator can usually get goodresults by using the three plotted points lying mostnearly in a straight line.To determine the liquid limit, plot a FLOWCURVE on a graph like the one shown in figure15-37. It is a semilogarithmic graph, in which thevertical coordinates are water content and thehorizontal coordinates are number of blows. Theflow curve is a straight line plotted as nearly aspossible through three or more of the plottedpoints.In figure 15-37, the first-run sample was testedthree times for an average number of 16 hammerblows. The water content was 47.3 percent. Onthe graph, 16 and 47.3 are the coordinates of oneof the three Xs shown plotted. The second-runsample indicated 24 hammer blows and 46.6percent water content; these are the coordinatesof another of the Xs plotted to the right. Coor-dinates for the third X are the hammer blows andwater content for the third-run sample. The coor-dinates of the rest of the plotted points are as in-dicated by the hammer blows and water contentfor the succeeding runs. The plotted points in thegraph may not form a straight line; however, theliquid limit line (or flow curve) is a straight line,passing nearly through the mean of the plottedpoints (fig. 15-37). The usual recommendation isthat five or six trials be made so that the resultsare more representative.The liquid limit (LL) is the water content for25 blows; it is therefore indicated by the point ofintersection between the flow curve and the ver-tical line representing 25 blows. The water con-tent indicated is about 46.4 percent. This, whenrounded off to 46, is the liquid limit.The plastic limit of soil is the lowest water con-tent at which the soil just begins to crumble whenrolled into threads 1/8 in. in diameter, at slowlydecreasing water content. First, prepare thesample as follows:If you need only the plastic limit, takea quantity of soil weighing about 15 g fromthe prepared material in the evaporating dish.Place this air-dried soil in an evaporating15-32
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