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' In a contour plot, a contour line.represents the s elevation along a line A closed contour thcrefom indicates either.a depression or a mounts Thcpresence of closec contours representing a deep depression is an unacceptable condition.in the design of a liner system. The deformed 4 urface elevations in each grid point are calculated by the computer Program. The deform surface contours on the 7 percent sloping surface are then plotted for each case, as shown or Figures EX 1 - EX 4. Where the thic ess-of the soil/waste layer is 1.8 m (6 feet), as shown on Figure EX 1, the presence of the void will caseate a depression 9 em (0.3 feet) deep. This depression could trap and accumulate liquids, hindering the free flow of leachate towards a collection point. When the thicimess of the soil/waste layer above the Void is Increased to-2.25 m (7.5 feet), the depression is used to less than 3 cm (0.1 foot), as shown on Figure EX 2.- With a 2.7 m (-9 foot) thick so waste layer over the void, the-depression disappears. The disturbance in the liner surface dem a= as the depth to the void increases, as seen when the thielmess of the soil/waste layer is increased to 3.6 m (12 feet) figure EX 3 and EX 4). To evaluate the potential for grade.reversal, a characteristic curve of required soil/waste thicknesses (T) over a 0. m (3 feet) wide by 0.9 m (3 feet) deep by 1.8 m (6 feet) long void versus liner surface grad (a) is illustrated on Figure 3. s.o . N 4.0 0 C 4 Y 3.0 , C s o U ~ 00 a h 2_0 ®.0`0 . o \ o`'�-o CL, o t.0 N o v 0.0 0 5 10 15 20 25 30 35 40 Contoinrnenf Liner Grade (degree) - Figure 3 Required canrolled fill thickness to prevent grade reversal (T) versus containment liner grade (a) on. 0.9m x 0.9m x 1.8m (3'x3'x6') void Gcasyntbetics'93•Vancouver.Canada-1505