CORRESPONDENCE_2011 - Laserfiche WebLink

Home Browse Search

Mr. Michael Carroll -4- 30 November 2011 the up gradient one. In this configuration there is a possibility that moisture moving on top of and within the up gradient cover will flow under the down gradient cover. Conventional construction requires that down gradient low permeable cover material be placed below up gradient low permeable cover material(shingled) to "catch" moisture as it flows downslope. It has been suggested that placing bentonite powder between the GCL and existing barrier soil will rectify the unorthodox configuration. This does not solve the issue of providing a conduit between the clay material and the GCL. b. Bentomat ST Guidelines 3.4.E. "The GCL panels shall be placed parallel to the direction of the slope." Concerns: Drawing Sheet 3 shows a 9% downward slope intersecting an upgradient 3:1 slope which is also seen in Section A on Sheet 4. Based on the proposed orientation of the GCL shown in Detail 4 on Sheet 5 (perpendicular to the direction of slope) it appears that the design engineer envisions continuing the GCL transversely across the 9% downward slope to the anchor trench located on the north side of the 9% slope. In this scenario the GCL overlap seams will be oriented near perpendicular to the downward 9% slope. Section 3.03 of the GCL specification states that "Overlaps or seams are not allowed perpendicular to slopes greater than 10 percent. In these areas GCLs must be placed in one piece along the entire slope, unless otherwise approved by the Engineer." The seam perpendicular to the 9% slope can be as long as 60 ft. in this panel layout configuration. Alternatively, If the design engineer envisioned terminating the GCL that starts at the existing barrier soil, continues perpendicular to the downward slope under the perimeter road at the toe of the downward 3:1 slope where it intersects the 9% slope, and then runs GCL panels parallel to the downward 9% slope, beginning at the anchor trench at the top of the 9% slope, the continuous GCL panel length would have to be at least 310 feet long in order to avoid a seam perpendicular to the 9% slope. However, GCL panels rarely exceed 200 feet. The concern is that the complexity of the GCL panel layout required in this site-specific application has not been adequately addressed. The existence of a subgrade v-ditch, 3:1 slopes up gradient and down gradient of the perimeter road, as well as intersection of the 3:1 slope with the 9% slope complicates panel layout when the constructor must try to comply with the following competing requirements: 1. GCL rolls shall be of adequate lengths to reach from the top of the slope to the bottom of the slope. (2.01.A) 2. Overlap shall be a minimum of 6 inches along length. (3.03.A.1) 3. Overlap a minimum of 12 inches along width. (3.03.A.2) Bentomat ST recommended end-of-roll overlap is 24 inches. 4. Overlaps or seams are not allowed perpendicular to slopes greater than 10 percent. In these areas GCLs must be placed in one piece along the entire slope, unless otherwise approved by the Engineer. (3.03.A.3) 5. As directed by the project drawings and specifications, the end of the GCL roll shall be placed in an anchor trench at the top of the slope or an equivalent runout design shall be utilized.