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HomeMy WebLinkAboutEngineer's OWTS Design Plan Onsite Wastewater Treatment System Design September 24, 2025 Robbins Project -TBD- County Road 202 DeBeque, CO 81630 Parcel No. 2415-041-00-001 Prepared By: K-Cronk Engineering, Inc. P.O. Box 140 Mack, CO 81525 970-250-0572 - 1 - Site History The owner of the property wishes to construct a new one-bedroom residence to be served by a new OWTS. The residence will be greater than 400 sq. ft., therefore the OWTS will be designed to accommodate a two- bedroom minimum residential loading (Garfield County OWTS Regulations 43.6.A.2.C & Table 6-2, footnote 3). The site consists of approximately 10,282 acres of uncultivated native soil. Drainage in the local area of the proposed residence is approximately 3% to the east/southeast. A soils evaluation and site study were conducted on the property of reference on 09/22/25 by Kachayla R. Cronka. Soils evaluation trench A was located approximately 520’ north of the south property line and approximately 820’ east of the west property line. This OWTS design has been prepared for the project based on information disclosed during the site study mentioned above. A discussion of the engineered OWTS design follows. The location of the soils evaluation trench is shown on the Plot Plan included in this report. Development of Design Parameters Soils evaluation trench A was extended to a depth of 96" below ground surface (BGS). There was no evidence of ground water or periodically saturated soils in the open excavation to a depth of 96" BGS. No limiting layer was identified to a depth of 96”. The soils evaluation indicates three distinct soil horizons underlie the site. A lithological description follows: depth (in.) description 0” – 32” clay loam w/ minor gravel (<35% rock), gray (Soil Type 3, USDA – clay loam, blocky, strong structure) 32”-38” clay loam, gray (Soil Type 3A, USDA - clay loam, massive, structureless) 38”-96” clay loam w/ minor gravel (<35% rock), gray (Soil Type 3, USDA – clay loam, blocky, strong structure) Soils evaluation trench B was extended to a depth of 36" below ground surface (BGS) and showed the same soils lithology as described for trench A above. Based on the results from the 09/22/25 soils evaluation, Soil Type 3 and corresponding long term acceptance rate (LTAR) of 0.35 are chosen for overall system sizing for an infiltrator trench soil treatment area designed to discharge to subsoils below the 42" depth. The system will require excavating below the Type 3A material so effluent is discharged into the Type 3 soils. a Kachayla R. Cronk holds a Certificate of Attendance and Examination from the CPOW Visual and Tactile Evaluation of Soils Training. - 2 - System Design The installer shall confirm the feasibility of gravity discharge of sewage effluent from the foundation line to the soil treatment area by field verification of required parameters. Gravity sewer lines that transport solid waste material must maintain a minimum fall of 1/4” per foot (2% slope); gravity effluent lines that transport only liquid waste material must maintain a minimum of 1/8” per foot (1% slope). If site conditions do not allow for the requirements mentioned above to be maintained, gravity discharge of septic effluent cannot be assured, and a lift station may be required. Please contact the design engineer and Garfield County to address any design modifications necessary if these design assumptions are not upheld. A chamber trench soil treatment system is proposed for discharge of septic effluent at the site. Construction of the system will consist of excavating level trenches in the area comprising the soil treatment area. Grading during construction may affect slope, therefore contractor shall verify slope at time of installation. Soil treatment area orientation may be adjusted per site variations to facilitate consistent trench depth running parallel to final contours. The initial excavation shall be continued to a level depth of 42" BGS (see Soil Treatment Area Cross Section). If consistent trench depth cannot be maintained, INFILTRATOR chambers may be installed level at 38” minimum (or to Soil Type 3 conditions, whichever is deeper) and 48” maximum BGS. After removal of surface soils, the open excavations shall be disked or scarified to promote subsurface absorption Following completion of the trench excavations, chambers will be used to construct a septic effluent distribution system in the open excavations. The chambers will be installed in accordance with the manufacturer’s specifications, available here for Infiltrator Quick4 Standard Chambers: https://www.infiltratorwater.com/Customer-Content/ www/CMS/files/chamber-manuals/PLUS09_ 052913.pdf After the chamber distribution system is installed, non-woven geotextile filter fabric will be placed over the chambers and the system will be covered with a soil cap. The soil cap will consist of approximately 30” (12” min.) of sandy loam. The soil cap will be mounded 5% above the existing ground surface to promote surface run off away from the soil treatment area. A 1,000 gallon, two compartment septic tank is required to provide a minimum of 48 hours retention time for sewage generated from the proposed project. The tank must be approved and accepted by CDPHE for use as a septic tank. A list of such tanks may be found her e: https://cdphe.colorado.gov/OWTS under “Regulation 43: product acceptance lists”. Water-tight risers to surface with securely fitted lids for access to all compartments of the septic tank will be provided. A non-corrodible filter shall be installed at the final outlet tee of the septic tank to limit the size of solids and sludge passing into the soil treatment area. The filter must be accessible for cleaning and replacement from the ground surface. A distribution box is required to provide equal distribution of septic effluent from the septic tank to the soil treatment area laterals. The distribution box must be placed on stable native soil or compacted structural fill to prevent settling and assure equal distribution of septic effluent. The distribution box must be accessible for cleaning and replacement from the ground surface. - 3 - DESIGN CALCULATIONS DESIGN LOADING RATE OF 2 BEDROOMS @ 150 GAL./BEDROOM-DAY = 300 gal./day DESIGN SOIL TYPE 3 DESIGN LONG TERM ACCEPTANCE RATE (LTAR) = 0.35 GAL./SQ.FT./DAY A = 𝑄 𝐿𝑇𝐴𝑄, WHERE, A = SOIL TREATMENT AREA (SQ. FT.) Q= DESIGN FLOW (GAL./DAY) LTAR = LONG TERM ACCEPTANCE RATE (GAL./SQ.FT./DAY) A = 300 0.35 =857.14 𝑆𝑄.𝐹𝑆. SOIL TREATMENT AREA ADJUSTMENT FACTOR FOR GRAVITY TRENCH DESIGN = 1.0 SOIL TREATMENT AREA ADJUSTMENT FACTOR FOR USE OF CHAMBERS = 0.7 ADJUSTED SOIL TREATMENT AREA = 857.14 SQ.FT. X 1.0 X 0.7 = 600 SQ. FT. LENGTH OF 3’ WIDE TRENCH REQ’D = 600 SQ.FT. / 3 FT. = 200 FT. USE 3 TRENCHES WITH 17 QUICK4 STANDARD INFILTRATORS (or equivalent) EACH, FOR A TOTAL OF 51 UNITS SOIL TREATMENT AREA SIZED AT 3 TRENCHES 3 FT. WIDE X 68 FT LONG = 612 SQ. FT. As shown in the included graphics, the trench soil treatment system will consist of three (3) trenches 3' wide x 42” deep x 68' long with seventeen (17) INFILTRATOR QUICK4 chambers (or equivalent) each for a total of fifty-one (51) units. The soil treatment area will encompass an area of 612 sq. ft. - 4 - Installation - Setbacks, Notifications, and Inspections The owner and installer shall be aware of and comply with the following installation and system operation requirements. • The final cover shall not be placed on sewer lines, septic tank, or the soil treatment area until the system has been inspected and approved by the design engineer and Garfield County. The installer shall provide 48-hour notice for all required inspections. • The installer must be approved and licensed by Garfield County for the installation of onsite wastewater treatment systems. • All installation activities shall be conducted in accordance with current Garfield County OWTS Regulations. If at any time during construction, subsurface site conditions are encountered which differ from the design parameters previously described, construction activities will stop, and the design engineer and Garfield County will be contacted to address any necessary design modifications. • Installation procedures including grade, location, setbacks, septic tank size, and soil treatment size shall conform with the attached graphic details. Construction activities and system components will not encroach upon existing easements or utility corridors. • The installer must maintain all setbacks to utility lines, easements, property lines, or other adverse conditions, whether they are known and shown on the attached graphics or have been disclosed during construction. Minimum setbacks for system components are: Septic Tank, Dosing Tank Building Sewer, Effluent Lines STA spring/well/cistern 50 50 100 potable water supply line 10 5 25 structure w/ crawlspace 5 0 20 structure w/o crawlspace 5 0 10 property lines, piped or lined irrigation 10 10 10 subsurface drain, intermittent irrigation 10 10 25 lake, water course, irrigation ditch, stream 50 50 50 cut slope steeper than 3H:1V 10 10 25 septic tank -- -- 5 • Four-inch, 2-way clean out must be installed within 5 ft of the outside of the building. • Gravity sewer lines that transport solid waste material must maintain a minimum fall of 1/4” per foot (2% slope); gravity effluent lines that transport only liquid waste material must maintain a minimum of 1/8” per foot (1% slope). • All gravity sewer/effluent piping shall meet minimum ASTM-3034 PVC standard, be 4-inch in diameter, and have glued joints. Gravity sewer lines that transport solid waste material shall employ sweep 90's or 2-45's at all turns. Sewer/effluent lines under driveways shall meet minimum SCHD 40 PVC standards. Additionally, sewer/effluent lines in traffic areas with less than 24" of cover shall be encased in 6" CMP or flow fill and covered with minimum 2" high density blue board insulation. - 5 - • Sewer/effluent lines or domestic water lines shall be encased in minimum SCHD 40 with water- tight end caps or a minimum of 6” of flow fill at all points with less than 5’ separation between sewer and domestic water lines. • Four-inch clean outs shall be installed at maximum 100' intervals in all gravity sewer lines that transport solid waste material that exceeds 100' in length. • A minimum of 12" of soil cover (18" recommended) shall be maintained over all gravity draining OWTS components to prevent freezing of septic effluent (excepting septic tank access ports which must be extended to the ground surface). • The system contractor shall be aware of the potential for construction activities to reduce soil permeabilities at the site through compaction, smearing, and shearing. The following precautions and construction procedures should be employed during installation to minimize disturbance to native soils: i. Excavation should proceed only when the moisture content of the soil is below the plastic limit. If a sample of soil forms a rope instead of crumbling when rolled between the hands it is too wet and should be allowed to dry before excavation continues. ii. Excavation and backfill equipment should work from the surface where at all practical to avoid compaction of the soils at depth. iii. The bottom and sidewalls of the excavation should be left with a rough, open surface. The appearance should be that of broken or ripped soil as opposed to a sheared, smeared, or troweled surface. Any smoothed or smeared surfaces should be removed with a toothed rake or shallow ripper, taking care to remove loose residues from the bottom of the excavation by hand if necessary. iv. Care should be taken in placing fill materials in the excavation to avoid damaging the exposed soil surfaces. - 6 - Operation - Maintenance and Inspections • The owner shall install a structural barrier if necessary and take precautions to prevent vehicular traffic, excessive surface watering, accidental flooding, or other activities in the vicinity of the soil treatment area which may compact, saturate, or otherwise alter the subsurface soil parameters used in designing the septic system. • The owner will plant and maintain grass or other shallow rooted cover crop to prevent erosion and promote evapotranspiration over the soil treatment area. Every month: • Inspect backfill over the septic tank and soil treatment area for signs of settling – provide additional mounding as needed. • Inspect area of septic tank and soil treatment area for signs of intrusion by burrowing animals and deep rooted plants and take measures to prevent future intrusions as needed. Every year: • Remove tank lids and inspect outlet effluent filter for damage and clean as needed. • Inspect soil treatment area through the ports provided at the beginning and ends of each lateral trench for signs of excessive moisture or pooling water. • Inspect tank risers for signs of high water level markings that may indicate slow drainage Every four years: • Remove accumulated sludge from the septic tank by pumping all compartments – interval may be adjusted to less than four years or more than four years depending on family specific usage and habits. OWTS Component Descriptions: Component: Model: Notes: Septic Tank Infiltrator IM-1060-2CP, or equivalent Tank Riser Infiltrator EZsnap Riser SNAPIS-24## & Lid, or equivalent 2”, 6”, & 12” available Optional Safety Star: SNAPSAFT- 2400 Effluent Filter Orenco Model FTW0436-28 Distribution Media Infiltrator Quick4 Standard Chambers + endcaps, or equivalent Filter Fabric Non-woven permeable geotextile fabric (max. 2 oz. / sq. yd), or equivalent - 7 - Limitations This report is a site-specific design for the installation of an onsite wastewater treatment system and is applicable only for the client for whom our work was performed. Use of this report under other circumstances is not an appropriate application of this document. This report is a product of K-Cronk Engineering, Inc. and is to be taken in its entirety. Excerpts from this report may be taken out of context and may not convey the true intent of the report. It is the owner's and owner's agent’s responsibility to read this report and become familiar with the recommendations and design guidelines contained herein. The recommendations and design guidelines outlined in this report are based on: 1) the proposed site development and plot plan as furnished to K-Cronk Engineering, Inc. by the client, and 2) the site conditions disclosed at the specific time of the site investigation of reference. K-Cronk Engineering, Inc. assumes no liability for the accuracy or completeness of information furnished by the client. Site conditions are subject to external environmental effects and may change over time. Use of this plan under different site conditions is inappropriate. If it becomes apparent that current site conditions vary from those anticipated, the design engineer and Garfield County should be contacted to develop any required design modifications. K-Cronk Engineering, Inc. is not responsible and accepts no liability for any variation in assumed design parameters. K-Cronk Engineering, Inc. represents this report has been prepared within the limits prescribed by the owner and in accordance with the current accepted practice of professional engineering in the area. No warranty or representation, either expressed or implied, is included or intended in this report or in any of our contracts. SEAL Kachayla R. Cronk, P.E. Date NOTE: This OWTS design is meant to include the following five pages: 1) plot plan 2) septic layout plan 3) project notes 4) soil treatment area plan view 5) soil treatment area cross section. The plan is not to be implemented in the absence of these sheets. NOTES FOR YOUR INSTALLER This is a STAMPED, ENGINEERED design – yes, septics are “simple” and water runs downhill. However, like anything that is engineered, it is critical that you follow the design so the system performs as intended – I cannot approve a system that was not installed to plans! If, FOR ANY REASON, you need to: 1. Change the location/rotation of the septic tank (i.e. plumbing was stubbed out of different wall, etc.) 2. Change the location/rotation of the soil treatment area (i.e. slope differed from the design, owner has changed the site plan, etc.) YOU MUST CONTACT THIS OFFICE FOR A REVISED DESIGN! The revision must be submitted and approved through the County (with a County $ revision fee) prior to calling in for inspection. Revisions take time, so please be prompt when contacting us to ensure the project is not delayed. Next, and I cannot stress this enough - DO NOT install the chambers DEEPER than the maximum depth stated in the design – the depth is determined by factors that the design engineer cannot control. This includes SATURATED/POOR SOIL CONDITIONS & COUNTY OWTS CODE. Our designs give the widest range of depth possible for the specific site – if the required depth cannot be met, other options may need to be explored. If the soil treatment area is installed at a depth greater than allowed, there are very few solutions to resolve the problem. Examples include: • Removing all components and reinstalling them in a different location at correct depth. • Backfilling trenches to an acceptable depth (using WASHED CONCRETE SAND ONLY), installing a pump chamber, effluent pump, and pressurizing the system. Please reach out at any time during the installation process if you have any questions – this office has experienced several installations that have failed County inspections due to issues described above – I would like to see every system PASS without comment as this makes everyone’s job easier. Kachayla Cronk, P.E. – (970) 250-0572