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HomeMy WebLinkAboutAmbientAirTrenchPlugDeviationLetter_06232021 1091 Stoneridge Drive • Bozeman, Montana • Phone (406) 587-1115 • Fax (406) 587-9768 www.chengineers.com • E-Mail: info@chengineers.com Civil/Structural Engineering and Surveying June 24, 2021 Cody Flammond City of Bozeman Engineering Department 20 E. Olive Street Bozeman, MT 59718 Re: Ambient Air Solutions, Lot 7, Block 2, Nelson Meadows Subdivision (COB - #21063; C&H #200217) Subject: Deviation from trench plug requirement Dear Mr. Flammond, The purpose of this letter is to respond to comment #4 of the Water and Wastewater section of the site plan review comment letter dated April 13, 2021 for the Ambient Air Solutions project. The comment states “…The applicant must install a trench plug in the water and sewer service trenches between the stormwater retention facilities and the water and sewer mains.” It is our understanding that the rationale for this requirement is to prevent stormwater runoff that percolates beneath the pond and into the water and sewer trenches that pass beneath said ponds from increasing the volume of water flowing through the pipe bedding of the adjacent water and sewer mains in Prince Lane. It is the opinion of C&H Engineering that this requirement should be relaxed in this particular instance. The following numbered list describes the reasons as to why we believe this requirement is not necessary from a technical standpoint and also causes undue strain on the overall project budget for the owners of this established Bozeman business: 1. A trench plug that meets the MPWSS standards essentially requires the use of bentonite clay. The standard does give an alternate option to using bentonite clay if the soil materials are of 1-inch maximum size with more than 50% passing the No. 200 sieve and with liquid and plastic index values of at least 50% and 25% respectively. In our experience, these standards are difficult if not impossible to meet with native silty clays that are found on-site through Bozeman and in this subdivision in particular. With that said, bentonite clay chips are extremely expensive at about $1,140 per pallet of forty-eight 50-pound bags (0.7 cubic feet in each bag). Per MPWSS Detail No 02222-1 the minimum trench plug size for typical 4-foot-wide service trench approximately 7 feet deep is 10’ wide by 7’ deep by 10’ long, which equates to 700 cubic feet of material. This quantity of bentonite clay chips would cost approximately $23,750 per trench plug (not including delivery or installation). The services proposed with this plan are likely to cost approximately $5,000 to $10,000 each if no plugs are required. The trench plug requirement would increase the cost of the typical service by a minimum of 2 to 4 times. Civil/Structural Engineering and Surveying 2. The requirement of the trench plug is only preventing additional water from flowing down the service trench to the main trench in a very low probability event. Given the groundwater elevation in this subdivision is between 4 and 8 feet below grade for half of the year (March through August) the mains and services are inundated with groundwater in their respective trenches already. According to the Nelson Meadows As-Builts, no trench plugs were installed in any of the sewer or water main trenches. A 25-year storm event is most likely to occur during this 6-month timeframe in which the trenches are already inundated with groundwater; therefore, the trench plug that is designed to stop the flow of water from the pond above produces negligible effect on the overall quantity of water in the trench because during the time at which the pond is full of stormwater and infiltrating into the soil below, the groundwater has likely already inundated the trench. 3. The native soil in this subdivision, and on the subject property in particular, consists of a deep layer of silty clay, which is not considered a well-draining soil type. This layer persists until about 10 feet below grade where native gravels were found during the subdivision geotechnical exploration by Allied Engineering. Based on DEQ Circular 4 and C&H Engineering experience with test pits in this subdivision, the silty clay has a percolation rate of approximately 121 minutes per inch (infiltration rate of 0.5 inches per hour). For comparison, a well-draining native pit run would have an infiltration of 75 to 100 inches per hour. Because the soils where the retention ponds are located are not well-draining, we have designed a well-draining column in the low point of the pond to act as a conduit for stormwater that accumulates in the pond to easily percolate through the silty clay. See detail 3/C5.1 on the revised civil plan sheet with this re-submittal site plan application. This column of well-draining material positioned at the low point of the pond is located away from the service trench that passes under the pond. The sewer service passes 12 feet from its well-draining column in Retention Pond #3. The water service passes 45 feet from its well-draining column in Retention Pond #1. These well-draining columns will infiltrate most if not all of the stormwater from the pond into the groundwater table, thereby, mitigating the concern of excessive groundwater in the service trenches that pass under the retention ponds. 4. The sewer service is deep and maintains the minimum 5 feet of cover as it passes under Retention Pond #3. The water service; however, will require two sheets of 4’x8’ rigid foam insulation where it passes under Retention Pond #1. This layer of rigid foam will act as a water barrier for any stormwater (in the unlikely event) that would percolate through the silty clay. The stormwater will be diverted from the underlying trench and forced to infiltrate around the trench due to the rigid foam above. If you require any further information, please give me a call at (406) 404-7520. Sincerely, Tim Staub, P.E. Enclosures: Groundwater information for Nelson Meadows Subdivision Geotechnical information for Nelson Meadows Subdivision Circular DEQ 4 Page 19 of 159 2013 Edition TABLE 2.1-1 Soil Texture Descriptions are found in Appendix B Texture Percolation Rate (minutes per inch) Application rate (gpd/ft2) (a) (b) Gravel, gravelly sand, or very coarse sand (c) <3 0.8 Loamy sand, coarse sand (d) 3-<6 0.8 Medium sand, sandy loam 6- <10 0.6 Fine sandy loam, loam 10- <16 0.5 Very fine sand, sandy clay loam, silt loam 16-<31 0.4 Clay loam, silty clay loam 31-<51 0.3 Sandy clay 51-<121 0.2 Clays, silts, silty clays (e) 121- <240 0.15 Clays, silts, silty clays (f) >240 Additional Soil Information Required (a) If more than 500 lineal feet, or 1,000 square feet of distribution line, calculated before applying any reductions, are needed, then pressure distribution must be provided. (b) Comparison of the soil profile report, percolation rate, and USDA-NRCS soils report should be reviewed. If the information shows a variable application rate, additional site specific information may be required by the reviewing authority. (c) Systems installed in gravel or coarser textured soils with percolation rates faster than 3 mpi must be pressure dosed and sand lined. (d) Pressure distribution must be provided for these soils if there is less than 6 feet from the bottom of the trench to a limiting layer. (e) Percolation tests must be conducted in accordance with Appendix A. (f) Soils with initial percolation rates greater than 240 mpi must be reevaluated using the double-ring infiltrometer procedure outlined in ASTM D 5093-02. Systems may be proposed for these soils only if the double-ring infiltrometer procedure shows a percolation rate of 240 mpi or less. All calculations and results must be reported to the reviewing authority. Only ETA or ET systems designed in accordance with Subchapter 6.8 may be used. 2.1.8. Site Factors The land slope, potential for flooding, and amount of suitable area must be evaluated. 2.1.8.1. Type and Percent of Land Slope The type (concave, convex, or plane), percent, and direction of land slope must be reported along with the method of determination. The reviewing authority may require a 2-foot contour map of the area for sites having slopes exceeding 15 percent within 25 feet of the absorption system or replacement area.