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HomeMy WebLinkAboutOWTS Installation Observations 08.04.2025August 4, 2025 502 Main Street • Suite A3 • Carbondale, CO 81623 • (970) 704-0311 • Fax (970) 704-0313 S OPRIS E NGINEERING • LLC civil consultants Richard and Vicky Nash 965 Westbank Road, Glenwood Springs, CO 81601 richard@nashconstructioncompany.com; RE: As-Constructed–Onsite Wastewater Treatment System (OWTS), Nash Residence, 965 Westbank Rd, Glenwood Springs CO, AKA Westbank Ranch Subdivision #1 Lot 15, Garfield County, Colorado SE Job No. 30177.01 Parcel No: 218535301015. Garfield County OWTS Permit No. SEPT-9134 Dear Richard and Vicky: Pursuant to County requirements, this letter provides documentation that the Onsite Wastewater Treatment System (OWTS) recently installed at the above referenced site is in compliance with the permitted design. Sopris Engineering inspected the excavations, soils, materials and the installation of the system components during construction, prior to final backfill and after all installations were completed. The newly constructed absorption field and tank installations were designed and constructed to serve a single bedroom small ADU constructed as an addition on the existing detached garage/shop. Sopris Engineering performed site visits to inspect and document the as-constructed conditions of the OWTS. Inspections were coordinated with the property owner that installed the system components. The as-built conditions and installation of the new OWTS components are in compliance with Garfield County Regulations; the recommendations and specifications delineated on the civil C1 OWTS plan drawing dated 04-24-2025, that was prepared with GIS, LIDAR, Survey Plat information, Previous OWTS Record drawing under Pemit #SEPT-05-216880 and the OWTS design Report dated 04-24-2025, by Sopris Engineering. The design criteria and system sizing information is summarized below. Summary Design Criteria The new OWTS was designed for a treatment capacity of 150 gallons per day based on the design wastewater flow to serve a minimum 1-bedroom ADU in accordance with Table 6-2 values, Section 43.6, A.4. A new 1,000 gallon two-compartment reinforced plastic septic tank was installed off the west side of the proposed ADU structure. Plastic risers with lids were installed on the new septic tank along with appropriate pipe connection to the new tank. The new 1,000 gallon 2-compartment septic tank was installed with appropriate sanitary tees, outlet effluent filter with extension handle. A gravity schedule-40 sewer pipe section with two-way cleanout was installed between the tank inlet on the gravity sewer line from the accessory structure. The existing garage bathroom ejector pump transport line was recovered, cut and connected to the new sewer service pipe with full body wye fitting. The new water service to the ADU was encased in the trench with the existing ejector pump transport pipe. The new STA consist of leaching chamber trenches that provide 180 S.F. of total absorption area. The septic tank effluent is gravity discharged through a new 4-inch schedule-40, installed at a minimum 1.5% slope, to the STA via a plastic distribution box. The effluent is equally distributed from the distribution box via 4” distribution pipes connected to the head end cap of each chamber trench. Speed levelers were installed on the d- box outlet pipes. Observation ports with caps were installed on the proximal and distal ends of each chamber trench. The design is in compliance with the current County regulations with design calculations based on the soil type, texture and structure with an appropriate long term acceptance rate (LTAR). The designed system meets all required setbacks and will be installed within the general location indicated on the plan. Our design is outlined below and delineated on the attached C-1 OWTS record drawing. Nash Residence ADU OWTS Permit No. SEPT-9134 965 Westbank Road, Glenwood Springs, CO SE Job No. 30177.01 August 4, 2025 Page2 Design Flow The design flow is calculated as follows for the minimum design capacity of an equivalent 1-bedroom additional dwelling unit. The OWTS is designed for a treatment capacity of 150 gallons per day based on the design wastewater flow in accordance with Table 10-1 values, Section 43.6, A.4. Regulation 43: Minimum population based on 1 bedrooms at 2 persons per bedroom equating to 2 persons. The Design flow is calculated based on residential usage of 75 gal/person/day, No PF Max. Design flow (Qd)gallons/day = (# of people) x (avg. flow) gal/person/day. Design flow Qd = 2*75=150 gpd Septic Tank Design The septic tank capacity required for a minimum 48 hour retention time is calculated as follows: V = 150 gal/day * 2 = 300 gallons. Use a minimum 1,000 gallon 2-compartment septic tank. The tank was installed with risers and access lids set a minimum 2 inches above finish grade surface. Sub Surface Conditions and Testing A subsurface soil investigation and site assessments were performed by Sopris Engineering on May 1, 2021 and June 7 2021. The shallow soils in the previously excavated chamber trench field, excavation for the replacement tank and soils observed in the deep crawl space area were sampled and characterized by application of the USDA visual/tactile soil texture method analysis. Soils were observed in the 2-3 feet deep trenches excavated for the original replacement field. A 2.5 shallow hand dug profile hole was dug in the vicinity of the field and the approximately 6 feet deep vertical foundation excavation walls were observed and sampled at 3 and 6 foot depths. The soils below 6 inches of topsoil consist of medium dense slightly sandy silt loam texture soil with slightly blocky to granular structure to a depth of 6 feet. The soil profile contained less than 35% scattered gravel or rock content to the 6-feet depth observed below the surface grades. No evidence of free water was encountered in the excavations. Seasonal high Groundwater levels are expected to be below 10 feet from the existing surface grades on the relatively flat terrain. The native soils sampled from 2-3 feet below the surface, in the vicinity of the proposed field and foundation, are characterized as a soil type 2 consisting of silt loam texture with slightly blocky granular structure. This soil has an effective loading rate for conventional soil treatment of 0.6 Gal/S.F./day for a level 1 conventional treatment system. The equivalent percolation rate is assumed to be 20-30 minutes per inch. The soils are suitable for a shallow conventional shallow absorption field consisting of gravelless infiltration chamber units. Additional soil evaluations were performed during construction to verify the soils in the entire field excavation. The soils in the tank, pipline and STA trench excavations were observed and found to have consistent relatively dense silt loam soils with fine granular structure shape of moderate grade without the presence of excessive rock or gravels at the infiltrative surface. Treatment-Soil Treatment Unit/ Absorption System Design The treatment/absorption field is designed based on utilizing the effective Long Term Acceptance Rate (LTAR) for the native soils and the application of appropriate loading factors for a soil treatment unit system utilizing gravity distribution to gravelless chamber trenches.