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HomeMy WebLinkAboutEngineer's OWTS Design Plan Onsite Wastewater Treatment System Design March 7, 2025 Daley Project 632 County Road 228 Silt, CO 81652 Parcel No. 2125-311-00-046 Prepared By: K-Cronk Engineering, Inc. P.O. Box 140 Mack, CO 81525 970-250-0572 - 1 - Site History According to the owners, there is an existing residence on the site that is served by an existing onsite wastewater treatment system (OWTS) that will remain in use. There is an existing two-bedroom accessory dwelling unit (A.D.U.) currently served by a separate, unpermitted system that has failed and needs replacements. The existing A.D.U. system consists of a 500-gallon, single compartment septic tank followed by what appears to be a metal culvert that was modified to act as a seepage pit. The proposed OWTS will be designed to accommodate a two-bedroom residential loading. The site consists of approximately 286 acres of cultivated native soil. Drainage is approximately 5% to the south. A soils evaluation and site study were conducted on the property of reference on 03/05/25 by Kachayla R. Cronka. Soils evaluation trench A was located approximately 125’ south of the existing A.D.U. 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 that depth. The soils evaluation indicates two distinct soil horizons underlie the site. A lithological description follows: depth (in.) description 0” – 18” clay loam w/ organics, dark brown (Soil Type 3, USDA – clay loam, blocky, moderate structure) 18” – 96” sandy loam, light brown/red (Soil Type 2A, USDA – sandy loam, massive, structureless) Soils evaluation trench B was extended to a depth of 48" below ground surface (BGS). The soils lithology matched that described for Trench A above. Based on the results from the 03/05/25 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 36" depth. a Kachayla R. Cronk holds a Certificate of Attendance and Examination from the CPOW Visual and Tactile Evaluation of Soils Training. - 2 - System Design An INFILTRATOR 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 36" BGS (see Soil Treatment Area Cross Section). If consistent trench depth cannot be maintained, INFILTRATOR chambers may be installed level at 24” minimum and 48” maximum BGS. Following completion of the trench excavations, QUICK4 STANDARD INFILTRATORS (or equivalent) will be used to construct a septic effluent distribution system in the open excavations. The INFILTRATOR effluent distribution system will be installed in accordance with the "Infiltrator Technical Manual", available from Infiltrator Systems Inc., 123 Elm Street, Suite 12, Old Saybrook, Connecticut 06475. Adjacent INFILTRATORS shall be fastened with a minimum of three (3) ¾” self-tapping screws to prevent movement and separation during backfilling. After the INFILTRATORS are installed, synthetic filter fabric (140-200 mesh) will be placed over the INFILTRATORS and the system will be covered with a soil cap. The soil cap will consist of approximately 24” of sandy loam backfill. The soil cap will be mounded 5% above the existing ground surface to promote surface run off away from the soil treatment area. The installer should confirm the feasibility of gravity discharge of sewage effluent from the foundation line to the soil treatment area by field verification of assumed design parameters. Assumed design parameters with respect to gravity discharge include: • If the distance from the sewage pipe at exit of existing line to the soil treatment area is 40' or less, • The elevation of the ground surface at entry to the soil treatment area shall be no more than 16” above the invert elevation of the sewage pipe at exiting of existing line. If field measurements do not confirm these design assumptions, gravity discharge of septic effluent from the proposed structure 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. The existing 500-gallon septic tank was pumped and it, as well as the existing 4-inch sewer line to the seepage pit, were recently inspected by Voskuil Excavation and found to be in good working order. The existing tank will remain in place and act as the first compartment. An additional 500-gallon minimum, single 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 here: https://cdphe.colorado.gov/OWTS under “Regulation 43: product acceptance lists”. Provide water-tight risers to surface with securely fitted lids for access to both compartments of the septic tank. A non- corrodible Orenco filter (model # FTW0436-28) 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 - 3 - 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. As shown in the included graphics, the trench soil treatment system will consist of three (3) trenches 3' wide x 36” deep x 68' long with seventeen (17) INFILTRATOR (or equivalent) units each for a total of fifty- one (51) units. The soil treatment area will encompass an area of 612 sq. ft. 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. - 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 installer must be approved and licensed by Garfield County for 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. • 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. • 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. A minimum of 5 ft of undisturbed soil shall be maintained between individual absorption elements and the septic tank. A minimum of 4’ of undisturbed soil shall be maintained between adjacent absorption elements. • 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. Vehicle traffic and parking is to be prevented over the soil treatment and repair area. 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’ of the outside of the building. • All gravity flow sewer lines and effluent distribution piping shall be 4 inches in diameter and have glued joints. All lines discharging sewage from the residence to the septic tank shall maintain a minimum fall of 1/8 in. per foot and shall employ sweep 90's or 2-45's at all turns. All unperforated pipe used for gravity flow of effluent, including pipe from the foundation to the septic tank, septic tank to distribution box, and distribution box into chambers, must meet minimum standard ASTM-3034. Lines discharging from the septic tank to the soil treatment - 5 - area shall maintain a minimum fall of 1/8 in. per foot. Sewer lines under driveways shall meet the minimum Schedule 40 PVC standards. Additionally, effluent piping 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. • Sewer 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 flow effluent lines exceeding 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. 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. • The owner will inspect and maintain the required mounding and drainage away from the soil treatment area to prevent saturation from precipitation and surface flows. • To mitigate the generation of preferential flow channels which may compromise the operation of the system, the owner will inspect and prevent intrusion of burrowing animals and deep - rooted plants into the soil treatment area. • The septic effluent filter shall be inspected and cleaned as necessary every six (6) months. • The owner will conduct periodic maintenance of the septic system by removing accumulated sludge from the septic tank every 3-4 years to prevent clogging of the soil treatment area. - 6 - Limitations This report is a site-specific design for 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 in tended 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) notes for your installer 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