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HomeMy WebLinkAboutEngineer's OWTS Design PlanJanuary 8, 2026 502 Main Street • Suite A3 • Carbondale, CO 81623 • (970) 704-0311 • Fax (970) 704-0313 S OPRIS E NGINEERING • LLC civil consultants Thompson, Family Joint Rev Trust 0073 Native Lane Glenwood Springs CO 81601 C/o Kelli Broughton RE/MAX Country PO Box 1010 New Castle, CO 81647 kelli@rmcrealestate.com RE: OWTS Design Report, On-site Wastewater Treatment System (OWTS), 315 Westbank Rd, Glenwood Springs, Westbank Ranch Sub. #3, Garfield County, CO SE Job No. 36000, Parcel No: 239502103001. New Garfield County Permit # ____________ Dear Kenia: This letter report presents the applicable findings regarding the design and construction of a new Onsite Wastewater Treatment System (OWTS) at the above-mentioned Site, in support of an OWTS construction permit application to Garfield County. A new OWTS is recommended to replace the existing compromised system that currently serves a 2,268 S.F., 2-story, wood frame house built in 1978. The OWTS was completed in 1983, under County ISDS permit No. 530 to serve a 3-bedroom residence. This design is for the replacement of the tank and seepage pit (drywell well) system that is currently compromised due to deteriorating tank, partially clogged sewer pipes and soils in the seepage pit. The existing 40-year-old seepage pit system is functional however is exhibiting slower infiltrative capacity and reduced recovery, therefore in need of replacement by new system in compliance with current design criteria. The existing 1000-gallon septic tank was pumped and the sewer/effluent pipes have been jetted recently. The system is undersized for current and anticipated future usage. The treatment capacity for the replacement system is designed for a minimum 5-bedroom residence adequate for the existing 4-bedroom single family home that will provide additional capacity for future usage as needed. It is assumed that in the future the residence could undergo a major remodel that may include the addition of another bedroom. We have evaluated the existing and proposed site conditions with respect to the operation and installation of a new OWTS in accordance with County OWTS Regulations and State of Colorado Regulation 43. A site assessment and soil evaluation was performed by Sopris Engineering in coordination with Voskuil Operations that performed inspections and service to the exiting system. Voskuil will install the new replacement system. The new system meets all required setback requirements and operational demands for the proposed usage. Based on the physical site features, soils, and proposed building layout, we have prepared a civil OWTS design plan with construction details for permit approval. The design base drawing is based on recent site assessment observations, field measurements, aerial, County GIS mapping and LIDAR topography. The design and construction recommendations with specifications are delineated on the civil design plans, C1.0-OWTS Plan dated 01-08-2026, by Sopris Engineering. OWTS Design and Construction 315 Westbank Rd, Garfield County SE Job No. 36000.01 January 8, 2026 Page 2 OWTS Conditions and Design Criteria Existing and expected future usage results in a design flow of 600 gpd. The system will be constructed to provide for demand dosing of septic tank effluent to an engineered soil treatment area (STA), pursuant to Regulation 43 design criteria. The OWTS will include a new gravity service pipe from the existing iron sewer pipe at the exterior dwelling wall to a 1,500 gallon, 2-compartment reinforced plastic septic tank in series with a 500 gallon, single-compartment dosing tank. with a bio-tube duplex high head pump dosing assembly. The system will include the installation of an Orenco System bio tube duplex high head effluent pump assembly for pressure distribution of septic tank effluent to a new pressure dosed chamber bed system. The STA area calculations are based on the design flow, and the LTAR loading rate for the in-situ soils utilizing applicable multiplier loading rates. Individual effluent transport pipe will installed to a pressurized manifold at the head of each bed to connect each chamber row’s custom perforated distribution laterals. Each distribution lateral will have appropriate inspection port cleanouts and appurtenances. Design Calculations OWTS The design flow is calculated as follows for the total current and anticipated future waste flows of the existing single family dwelling. The replacement OWTS is designed for a treatment capacity of 600 gallons per day based on the design wastewater flow to serve a 5-bedroom capacity residence in accordance with Table 6-2 values, Section 43.6, A.4. Regulation 43: Minimum population based on 2 person per bedroom for the first 3 bedrooms = 6 persons and 1 person per bedroom for each additional 2 bedrooms = 2 persons for a total of 8 persons.Gallons per day per person =75 gal/person/day, No peaking factor Max. Design flow (Qd)gallons/day = (# of people) x (avg. flow)gal/person/day. Design flow Qd = 8*75=600 gpd Septic Tank Design The septic tank capacity for a minimum 48 hour retention time is calculated as follows: V = 600 gal/day * 2 = 1,200 gallons. Use a 1,500 gallon, 2-compartment reinforced plastic septic tank with an inlet sanitary tee and effluent filter outlet sanitary tee in the second chamber in series with a 500 gallon, single-compartment dosing tank with a bio-tube duplex high head pump dosing assembly. The tank will be installed with a sealed 30" diameter PVC or plastic access risers and lid set at the finish grade surface. The existing 1,000 gallon septic tank will be pumped, breached with crushed lid and backfilled to abandon in place. Sub Surface Conditions and Testing: A subsurface soil investigation and site assessment was performed by Sopris Engineering on January 5, 2026. The soils were sampled and characterized by application of the USDA visual/tactile soil texture method analysis. The soils below 12 inches of topsoil consist of medium dense silt loam soil with less than 1% scattered gravel or rock content to 7-8 feet below the surface grades. The soils appear to be consistent across the site from observations in the excavated profile pit and existing tank/effluent pipe exploratory excavation. OWTS Design and Construction 315 Westbank Rd, Garfield County SE Job No. 36000.01 January 8, 2026 Page 3 Septic Tank Excavation Pits: The soils encountered in the existing tank excavation below approximately 12 inches of topsoil consist of slightly sandy silt loam to the excavation depths of about 7-feet below the surface grades. No limiting soil or hydrological conditions were encountered. The slightly dense silt loam texture soil has blocky to granular structure with moderate grade with no scattered gravel or rock content.. No change of texture was encountered at depth. No free water was encountered in the pit excavations. No evidence of seasonal high ground water or redox conditions was observed in the excavation. PROFILE PIT-PP-1 The soils below approximately 12 inches of topsoil consists of brown silt loam texture soil with blocky to granular structure of moderate structural grade to a depth of 5 feet containing no scattered gravel or rock content. The underlying soils to a depth of 8' consist of friable slightly sandy silt loam with slightly blocky to fine granular structure of moderate structural grade with no scattered gravel or rock content to the 8' depth explored. No free water or evidence of seasonal high groundwater was encountered in the pit excavation. The native soils sampled from 2'-4' below the surface are characterized as a type 2 with silt loam texture of moderate grade with blocky granular structure. The tactile soil test ribbons were ¾ to 1-inch long. The soil type-2 has an effective loading rate (LTAR)of 0.60 gal/s.f./day for a level 1 conventional treatment system. Based on the testing results, existing topography, vegetation, and observation of site features, the soils are generally consistent in the 2 soil profile pits. The soils are suitable for the installation of an STA consisting of multiple pressure dosed gravelless chamber beds. Additional observations will be performed by Sopris Engineering in the area of the proposed treatment/absorption field and tank during initial stages of construction prior to placement of the distribution materials. Sopris Engineering will verify the soil conditions, OWTS components and provide installation guidance to the contractor. If any adjustments to design criteria or component modification are necessary, notice will be made to County EHD and applicable revisions to design/construction plans and report will be submitted. All findings, conditions and installation documentation will be included in the As-Constructed plan and report. Soil Treatment Area/Absorption System Design The soil treatment area (STA) is designed based on utilizing the effective Long Term Acceptance Rate (LTAR) for the in-situ soils at the absorptive surface in a chamber bed system. The STA will utilize pressure dosed distribution laterals to equally pressure dose each chamber row in each chamber bed. Long Term Acceptance Rate (LTAR) Considering the application of regulation 43 design criteria for treatment level 1 system consisting of gravelless chamber beds receiving pressure dosed septic tank effluent on native soils: The absorptive area and corresponding number of chambers for a treatment level 1 STA is sized per the LTAR of (0.6 gal./S.F./day) for type 2 receiving soils; The STA is designed utilizing the allowable loading factors: (Table 10-2, pressure dosed application bed = 1.0 ) and (Table 10-3, L.F.2=0.7 for Gravelless chambers Category 23) OWTS Design and Construction 315 Westbank Rd, Garfield County SE Job No. 36000.01 January 8, 2026 Page 4 A(sf) = Qd x L.F. A = Area; LTAR LTAR = 0.6 gal/S.F./day for soil type 2 silt loam, " Qd = flow (gal/day) = 600 gpd L.F.1=1.0 pressure dosed bed L.F.2=1.0 leaching chambers A(sf) =600 gpd x 1.0x 0.7 = 700 S.F. 06Gal/S.F./day Number of Arc chambers 5'x3'x12"chambers: 700 S.F. = 47 chambers; Use 48 chambers 15 S.F./Chamber Construct two fully pressure dosed 40.5' by 9' chamber beds. Install 48 ARC-36 chambers in two beds with 3 rows of 6 chambers per row for a total absorption area of 700 S.F. The bottom of each bed to be excavated to an average depth of 30 inches below the existing surface grades. The absorptive surface to be scarified at level grade. Chambers to be covered with filter fabric and backfilled to a minimum 3" above existing perimeter grades. Effluent Distribution System The septic tank effluent will be pressure discharged via two independent 1.5” transport pipe from the high head pump discharge assembly in a biotube filter vault installed in the 500 gallon tank to a manifold pipe, installed at the head of the lateral rows at each bed. The minimum slope of the transport pipe from the tank's dosing compartment to the manifold shall be installed at a minimum 2.5% slope to allow for drain back to the tank. The distribution system will be installed for effluent to be equally distributed across the absorptive surface in each row via individual custom perforated pressure distribution laterals connected to the manifold pipe in each bed. The duplex pump system will alternate dosing between the two beds. The distribution laterals will be installed on the infiltrative surface, under the chambers, along the length of each bed. The 5/32- inch orifices will be drilled on 2.5’centers oriented upward along the length of the distribution lateral except for the first and last orifice that will be oriented downward with an attached orifice sheild to allow for drainage of the lateral. A ball valve will be installed on the distal end of each distribution lateral centered below the 4” inspection port, housed in a 6" valve box with lid set minimum 1" above surface grades. Construction and Inspections Prior to construction of the permitted system, the engineer should be contacted by the contractor well in advance to provide adequate time to discuss the system components, answer questions, resolve any conflict issues and schedule inspection site visits based on construction progress. If you have any questions and to schedule inspections, please call Paul Rutledge or Carter Page, Sopris Engineering, 970 704 0311.