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*a s : Frescntation and Discussion of Roul s Ina contour plot, a contour line represents the same elevation along a line. A closed contour therefore indicates either a depression or a mound. The presence of closed contours representing a deep depression is an unacceptable condition injo - the design of a liner system. The deformed surface elevations in each grid point are calculated by the computer program. The deformed surface contours on the 7 percent sloping surface are then plotted for each case, as shown on Figures EX -1 - EX 4. Where the thickness -of the soil/waste layer is 1.8 m (6 feet), as shown on Figure EX 1, the presence of the void will create a depression 9 cm (0.3 feet) deep. This depression could trap and accumulate liquids, hindering•the free flow of leachate towards a collection point. When the thickness of the soil/waste layer above the void is increased to -2.25 m (7.5 feet), the depression is reduced to less than 3 cm (0.1 foot), as shown on Figure EX 2. ' With a 2.7 m (9 foot) thick soil/waste layer over the void, the depression disappears. The disturbance in the liner surface decreases as the depth to the void increases, as seen when the thickness 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 M over a 0.9 m (3 feet) wide by 0.9 m (3 feet) deep by 1.8 m (6 feet) long void versus liner surface grades (a) is illustrated on Figure 3. 5.0 L7 4.0 N � N C o u 3.0 C L O 1— U .. n; 2 ..0 0 Cr d 0 1.0 Y v 0.0 11 n 0 5 10 15 20 25 30 35 40 Con#oinment Liner Grode (degree) Figure 3 Required controlled 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 Geosynthctics '93 - Vancouver, Cans& -1505