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- In a contour plot; a contour line.represents the s elevation along a line A closed contour-thdrefalare ' 'cates either.a depression or a moue The presence of Close( contours representing a deep depression is an unacceptable condition in ��. the design of a liners stem. The deformed uFface elevations in e'ach grid point arse 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 friee flow of Leachate towards a eollecbion point. When the thickn s of the soil/waste layer above the void is increased to•2.25 rn (7.5 feet), the depressiOn is ced to less than 3 can (D.1 foot), as shown on Figure EX 2.' With a 2.7 m(9 foot) tick so•' waste layer over the void, the-depression disappears. The disturbance in the liner surface d 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 pc itential for grade.reversal, a characteristic curve of required soil/wastethicknesses (T) 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 grad (a) is illustrated on Figure 3. 5.0 . E i` y 4.0 p v C O u 3.0 'O c -C o 1.- OQ °f 2 .0 o'O,0 a y �\, o Cr O o: 0 1.0 O v 0.0 0 5 10 15 20 25 30 35 40 Conlainmenl Liner Grade {degree, Figure 3 Required controlled fill thickness to prevent grade reversal M versus containment liner grade (a) on. 0.9m x 0.9m x 1.8m (3'x3'x6') void Goosyndxxics V3-vancouva,Canada- 1505