2012-09-10_REVISION - M1982090 (5)

Home Browse Search

Mr. Oswald/M -1982- 090 /TR -04 September 10, 2012 Page 2 of 6 DRMS: Please provide more information about the buried pipeline, such as depth, pipe bedding, CMP joints, compaction of backfill, outlet structure, etc. CBCI: Due to corrosion resistance and aesthetics, CBCI has chosen to use 30" N -12 Low Head Pipe (LH pipe) manufactured by Advanced Drainage Systems, Inc. (ADS) instead of CMP. LH pipe is a high density polyethylene pipe with a smooth interior and corrugated exterior. It was selected by engineers at Harris Water Engineers (HWE) in Durango to be suitable for the low pressure, gravity flow conveyance that is needed from the portal discharge to the sedimentation pond. Enclosed with this response letter, are ADS' detailed specifications of the LH pipe and joint specifications. As per installation recommendations by ADS, CBCI will use the materials and compaction guidelines in ASTM D2321 where practicable (see Figure 4): • LH pipe with a 30" inside diameter has a 36" outside diameter, therefore the pipe trench should be at least 54" deep and 56" wide (ASTM D2321 recommendation) • bedding material must be a minimum of 6" thick using Class 1, 2, or 3 material (ASTM D2321 recommendation) • initial backfill must cover the pipe by a minimum of 6" (42" thick) using Class 1 or 2 material (ASTM D2321 recommendation) • final backfill must be a minimum of 6" thick above initial backfill using Class 1 or 2 material (ASTM D2321 recommendation) • in non - traffic areas minimum cover above the pipe must be 12" (ADS recommendation) • in traffic areas where vehicles up to 25 tons will be passing, minimum cover above the pipe must also be 12" thick (ADS recommendation) • in traffic areas where vehicles up to 75 tons will be passing, minimum cover above the pipe must be 24" thick (ADS recommendation) Class 1 material is composed of angular crushed rock that is <1.5 ", where 15% passes through a #4 sieve, 25% passes through a 3/8" sieve, and 12% passes through a #200 sieve. The materials to be excavated from the sedimentation pond and most of the pipe trench consist of course blast rock with a wide range of sizes and angularity. During excavation of the pond and the trench, CBCI will screen the excavated material to obtain 400 cubic yards of materials that meet Class 1 specifications. If the Class 1 material cannot be obtained onsite, it will be trucked to the project from a local supplier on an as needed basis. ASTM D2321 states that suitable compaction for Class 1 materials is achieved by dumped placement then worked by hand and vibration into voids (e.g. the haunch zone). Compaction equipment will be used on an as needed basis in areas where the backfill does not appear to be settling well. The upper course of the discharge pipe is approximately 315 feet westerly from the point of intake (elev. 9,821 feet) to the northerly turn (elev. of 9,809 feet) with a total drop of approximately 12 feet. The second course of the discharge pipe is approximately 460 feet northerly with a drop of approximately 60 feet to the discharge pipe outfall (elev. 9755 feet). Given the slopes and size of pipe, the flow velocity at the discharge outfall was calculated by HWE to be between 0.45 to 1.36