Loading...
HomeMy WebLinkAboutEngineers OWTS Design Plan Onsite Wastewater Treatment System Design July 2, 2025 Bashon Project 6180 CR 320 Rifle, CO 81650 Parcel No. 2175-261-00-199 Prepared By: K-Cronk Engineering, Inc. P.O. Box 140 Mack, CO 81525 970-250-0572 -1 - Site History There is an existing three-bedroom residence on site that is served by an existing onsite wastewater treatment system. During a recent inspection, the owners discovered that the existing OWTS was only sized for a one-bedroom residence, and they wish to upgrade the system to accommodate a three-bedroom residential loading. The site consists of approximately 6.83 acres of uncultivated native soil. Drainage is approximately 4% to the west. A soils evaluation and site study were conducted on the property of reference on 07/02/25 by Kachayla R. Cronk. Soils evaluation trench A was located approximately 90’ north of the south property line and approximately 210’ 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 108" below ground surface (BGS). There was no evidence of ground water or periodically saturated soils in the open excavation to a depth of 108" BGS. No limiting layer was identified to 108” BGS. The soils evaluation indicates three distinct soil horizons underlie the site. A lithological description follows: depth (in.) description 0” – 30” silty clay loam, brownish/white (Soil Type 3, USDA – silty clay loam, granular, strong structure) 30” – 60” silty clay loam, white (Soil Type 3A, USDA – silty clay loam, granular, weak structure) 60” – 108” sandy clay loam, brown (Soil Type 3A, USDA – sandy clay loam, blocky, weak structure) Soils evaluation trench B was extended to a depth of 54" below ground surface (BGS). There was no evidence of ground water or periodically saturated soils in the open excavation to a depth of 54" BGS. No limiting layer was identified to 54” BGS. The soils evaluation indicates two distinct soil horizons underlie the site. A lithological description follows: depth (in.) description 0” – 24” silty clay loam, brownish/white (Soil Type 3, USDA – silty clay loam, granular, strong structure) 30” – 54” silty clay loam, white (Soil Type 3A, USDA – silty clay loam, granular, weak structure) Based on the results from the 07/02/25 soils evaluation, Soil Type 3A and corresponding long term acceptance rate (LTAR) of 0.30 are chosen for overall system sizing for an infiltrator trench soil treatment area designed to discharge to subsoils below the 24" depth. - 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 24" 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, 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 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. An ISDS inspected was performed by Down Valley Septic & Drain, Inc. on 06/13/25. The results of the inspection indicate the septic tank is 1,000-gallon, concrete tank with two compartments that is in good condition with mild erosion of the liquid side. A copy of this ISDS Inspection is included as Appendix A. The existing septic tank will remain in place and in use. 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 3 BEDROOMS @ 150 GAL./BEDROOM-DAY = 450 gal./day DESIGN SOIL TYPE 3A DESIGN LONG TERM ACCEPTANCE RATE (LTAR) = 0.30 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 = 450 0.30 = 1,500 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 = 1,500 SQ.FT. X 1.0 X 0.7 = 1,050 SQ. FT. LENGTH OF 3’ WIDE TRENCH REQ’D = 1,050 SQ.FT. / 3 FT. = 350 FT. USE 6 TRENCHES WITH 15 QUICK4 STANDARD INFILTRATORS (or equivalent) EACH, FOR A TOTAL OF 90 UNITS SOIL TREATMENT AREA SIZED AT 6 TRENCHES 3 FT. WIDE X 60 FT LONG = 1,080 SQ. FT. As shown in the included graphics, the trench soil treatment system will consist of six (6) trenches 3' wide x 24” deep x 60' long with fifteen (15) INFILTRATOR QUICK4 chambers (or equivalent) each for a total of ninety (90) units. The soil treatment area will encompass an area of 1,080 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 ft. 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 ft of the outside of the building. • 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. 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 - OWTS Component Descriptions: Component: Model: Notes: 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) 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. BASHON PROJECT 6180 -CR-320 2175-261-00-199 PLOT PLAN JULY 2, 2025 SCALE, 1 '=60' I I o.reo. of reference 6.83 o.cres A.� o.pprox,__/ �� loco.tlon � of well � 41/. slope lnsto.ller to verify slope direction drive ,!, oppcos. loco<loa of o.pprox, � , loco.tlon of / existing OIJTS --- I NORTH tree line _____,,, soils evo.luo.tlon trenches I / ('\ \ \ 1\ \ I \ \ CJ 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 Appendix A Inspection Report by Others