HomeMy WebLinkAboutRevised OWTS Design 05.04.2026*revised on May 4, 2026 to correct design LTAR to match Soil Type 3A conditions
Onsite Wastewater Treatment System Design
May 4, 2026*
Original: March 20, 2026
Cromwell Project
650 County Road 261
Silt, CO 81652
Parcel No. 2125-31-00-112
Prepared By:
K-Cronk Engineering, Inc.
P.O. Box 140
Mack, CO 81525
970-250-0572
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Site History
There is an existing residence on the site that is served by an existing onsite wastewater treatment system
(OWTS) that will remain in use. The owner of the property wishes to construct a new three-bedroom
accessory dwelling unit (A.D.U.) to be served by a separate OWTS.
The site consists of approximately 20.05 acres of cultivated native soil. Drainage is approximately 4% to
the southwest. A soils evaluation and site study were conducted on the property of reference on 01/19/26
by Kachayla Schroedera. Soils evaluation trench A was located approximately 65’ south of the north
property line and approximately 90’ west of the east 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 84" below ground surface (BGS). There was no
evidence of ground water or periodically saturated soils in the open excavation to a depth of 84" BGS. No
limiting layer was identified to a depth of 84” BGS. The soils evaluation indicates three distinct soil
horizons underlie the site. A lithological description follows:
depth (in.) description
0” – 36” sandy loam topsoil w/ organics, dark brown
(Soil Type 2A, USDA – sandy loam, massive, structureless)
36” – 60” sandy clay loam, brown
(Soil Type 3A, USDA – sandy clay loam, massive, structureless)
60” – 84” moist sandy loam w/ minor rock (<35% rock), brown
(Soil Type 2A, USDA – sandy loam, massive, structureless)
Soils evaluation trench B 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” BGS. The soils evaluation indicates three distinct soil
horizons underlie the site. A lithological description follows:
depth (in.) description
0” – 22” clay loam topsoil w/ organics, brown
(Soil Type 3A, USDA – clay loam, massive, structureless)
22” – 32” clay loam, brown
(Soil Type 3A, USDA – clay loam, massive, structureless)
32” – 96” moist clay loam, brown
(Soil Type 3A, USDA – clay loam, massive, structureless)
a Kachayla Schroeder holds a Certificate of Attendance and Examination from the CPOW Visual and Tactile
Evaluation of Soils Training.
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Based on the results from the 01/19/26 soils evaluation, Soil Type 3A and corresponding long term
acceptance rate (LTAR) of 0.30 are chosen for overall system sizing for an infiltrator bed soil treatment
area designed to discharge to subsoils below the 24" depth.
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 bed soil treatment system is proposed for discharge of septic effluent at the site. Construction
of the system will consist of excavating a level bed 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 bed 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 bed depth cannot be maintained, INFILTRATOR
chambers may be installed level at 24” minimum 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 bed 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.
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. Distribution boxes are
required to provide equal distribution of septic effluent from the septic tank to the soil treatment area
laterals. The distribution boxes must be placed on stable native soil or compacted structural fill to prevent
settling and assure equal distribution of septic effluent. The distribution boxes must be accessible for
cleaning and replacement from the ground surface.
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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 =1500 𝑆𝑄.𝐹𝑆.
SOIL TREATMENT AREA ADJUSTMENT FACTOR FOR GRAVITY BED DESIGN = 1.2
SOIL TREATMENT AREA ADJUSTMENT FACTOR FOR USE OF CHAMBERS = 0.7
ADJUSTED SOIL TREATMENT AREA = 1500 SQ.FT. X 1.2 X 0.7 = 1260 SQ. FT.
LENGTH OF 12’ WIDE BED REQ’D = 1260 SQ.FT. / 12 FT. = 105 FT.
USE TWO BEDS WITH 4 ROWS OF 14 QUICK4 STANDARD INFILTRATORS (or equivalent) EACH
FOR A TOTAL OF 112 UNITS
SOIL TREATMENT AREA SIZED AT TWO BEDS 12 FT. WIDE X 56 FT LONG = 1,344 SQ. FT.
As shown in the included graphics, the bed soil treatment system will consist of two beds 12' wide x
24” deep x 56' long with four (4) rows of fourteen (14) INFILTRATOR QUICK4 chambers (or
equivalent) each for a total of one-hundred and twelve (112) units. The soil treatment area will
encompass an area of 1,344 sq. ft.
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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 SCHD 40 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.
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• 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.
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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 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
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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 Schroeder, P.E.
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.
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