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4 Mr. Michael Carroll -3- 30 November 2011 surface v-ditch. The erosion matting in the surface v-ditch will reduce flow velocities which in turn increases resident time of water within the v-ditch. Increased resident time leads to an increase in the amount of percolation to the GCL lined sub-grade v-ditch. Concerns: The sub-grade GCL is a low permeable cover component that prevents precipitation from reaching underlying waste. The GCL placed in a subgrade v-ditch contour creates a hydraulic path for subsurface drainage. Directly above the GCL lined v- ditch is a surface v-ditch lined with erosion control matting (Detail 4 on Sheet 5). Some precipitation runoff flowing in the surface v-ditch will percolate until it reaches the GCL lined subgrade v-ditch. The subsurface moisture will collect in the GCL lined subsurface v-ditch and directed parallel to the downward slope of the landfill until it reaches the toe of the landfill. It will then either remain subgrade or pool and reach the surface at the toe of the landfill. The major concern is that based on Detail 4 on Sheet 5 it appears the GCL material will be oriented perpendicular to the downward slope in the v-ditch. Thus overlapping seams will exist perpendicular to the downward slope in the subgrade v-ditch. The downward slope in the subgrade v-ditch varies from 1.23-7.05% (See Sheet 4). It is entirely possible that a saturated zone above the GCL will exist if low permeability earth fill is used above the GCL. It is recommended that the design engineer consider lining the surface v-ditch with a low permeable material such as concrete and placing a drainage Geonet material or a perforated drainage pipe directly above the GCL. It is also recommended that if the GCL is placed perpendicular to the downward slope the design engineer make adequate provisions to provide adequate overlap and sealing between the GCL. Furthermore, the design engineer should provide adequate structural support of the GCL perpendicular to the downward slope to prevent bowing/sagging and eventual separation at the overlaps. Staff is also concerned that aligning GCL perpendicular to the downward slope will not ensure that the GCL assembly at the subgrade v-ditch will be equivalent to the individual GCL panel specification of a maximum index flux of 1x10"$ m3/m2/sec with a maximum permeability of 5x10"9 cm/sec. 5. The design engineer specifies a GCL with similar properties to Bentomat@ ST made by Cetco®. GCL manufacturers such as Cetco provide GCL specification guidelines (See Enclosure 1) for use of their products to ensure proper performance. If a GCL such as Bentomat ST was actually used, several of the Bentomat ST general guidelines would not be adhered to or would be omitted altogether: a. Bentomat ST Guidelines 3.6.C. "End-of-roll overlapped seams should be constructed with a minimum overlap of 24 inches (600 mm) for Bentomat ST. Seams at the ends of the panels should be constructed such that they are shingled in the direction of the grade to prevent the potential for runoff flow to enter the overlap zone." Concerns: In Detail 4 on Sheet 5 it shows at the interface of the existing barrier soil and the GCL that that GCL is to be placed on top of the existing barrier soil cover. It is unorthodox that when trying to establish a low permeable seal by lapping two low permeable cover materials together for the down gradient cover,to be placed on top of