HomeMy WebLinkAboutEngineer's OWTS Design PlanSEPT.5,2025
Heathcote West LLC
5798 Sweetwater Road
Gypsum, CO 81637
c/o Chris Bendon
RE:OWTS Design Report, On-site Wastewater Treatment System (OWTS), Guest Cabin 3,TractG-7
according to the Healthcote West tracts SB35 subdivision exemption plat rec. #1006809,
4894 Sweetwater Ranch, Garfield County, CO
SE Job No. 34008.01, Parcel No: G-7, New Garfield County Permit #
Dear Chris
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 a building permit
application to Garfield County. A new OWTS construction permit is required for the proposed usage on
the subject property.
The OWTS design is based on our evaluation of the site conditions for use in obtaining a Garfield County
OWTS construction permit. 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. 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
drawing is based on a recent Survey performed by True North Colorado. The design and construction
recommendations with specifications were prepared by Sopris Engineering LLC and are delineated on civil
design plans titled CI-OWTS Site Plan dated 09/05/2025, included within the complete Building Permit
PIan set.
OWTS Conditions and Desien Criteria
Proposed site improvements and expected usage for Guest House-2 result in a design flow of 300 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 gravrty service
pipe from the new dwelling to a new 1250-gallon concrete septic tank equipped with a high head effluent
pump assembly.
The system is designed based on the current design criteria for the use of a pressure dosed chamber trench
system. The STA area calculations are based on the design flow, LTAR loading rate for the in-situ soils
uttlizing applicable multiplier loading rates per Table I0.2 and 3, Category 3 distribution media consisting
of shallow pressure dosed chamber trenches installed over native soil.
The system will include the installation of an Orenco System bio tube effluent pump assembly for pressure
distribution of septic tank effluent to a new pressure dosed chamber trench system. The effluent discharge
piping will be installed to connect to a common pressurized manifold with connections to the head of each
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OWTS Design and Construction
Sweetwater Ranch Guest Cabin 3
Garfield County
E Job No. 34008.01
Sept.5,2025
Page 2
chamber trench with custom perforated distribution laterals. Each chamber trench will have appropriate
inspection port cleanouts and appurtenances.
Desisn Calculations OWTS
The proposed residential facility design includes a 2-bedroom single family residence.
The design flow is based on a total residential population of 4 persons.
Per County adopted State Regulation 43, Design Flow Criteria: Minimum population is based on 2 persons
per bedroom for the first 3 bedrooms.
Design capacity =2x2 = 4 persons @ 75 gal/petsonlday,
Max. Design flow (Qd) gallons/day = (# of people) x (avg. flow) gaVperson/day
Design flow Qd = 4 * 75 = 300 gpd
Septic Tank Design:2-Day detention time = 300 x 2 = 600 gallons required.
Use a 1250 gallon, two-compartment concrete septic tank with a high head pump assembly.
Sub Surface Conditions and Testins:
A Subsurface soil investigation was performed by CTL on December 17, 2024 to develop
recommendations for the foundation, floor slab, below grade walls and subsurface drainage design. The
subsoil conditions around the proposed dwelling structure were evaluated from soil samples obtained from
two excavated profile pits. Samples of the subsoils obtained during the field exploration were tested in the
laboratory to determine their classification, compressibility, natural moisture content, and finer than sand
size gradation analyses and other engineering characteristics. The subsurface profile encountered in the
pits generally consists of sandy clay loam.
A subsurface soil investigation and site assessment was performed by Sopris Engiireering in on November
18,2024, to evaluate the soils conditions and determine the effective Long Term Acceptance Rate (LTAR)
for onsite Wastewater Treatment System (OWTS) Soil treatment area (STA), The soils in the several
development envelopes explored were sampled and characterized by application of the USDA
visual/tactile soil texture method analysis from samples in 2 profile pits per building site. Two profile pits
were observed within the guest cabin 3 building envelope. No evidence of seasonal high ground water,
redox conditions or any other limiting condition was observed in the profile pits. Groundwater levels are
expected to be below 20 feet from the existing surface grades on the moderately sloped to flat tenain.
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OWTS Design and Construction
Sweetwater Ranch Guest Cabin 3
Garfield County
SE Job No. 34008.01
SePt.5,2025
Page 3
Profile Pit-GC3-A
The soils below approximately 12 inches of topsoil consists of brown silty clay loam texture soil with
slightly blocky to granular structure of moderate structural grade to a depth of 5 feet containing no
scattered gravel or rock content. The underlying Soits to a depth of 9' consist of tan silty clay loam with
fine granular structure of moderate structural grade with rock content less than 307o, underlain by brown
clay to the 12'depth explored. No free water or evidence of seasonal high groundwater was encountered in
the pit excavation.
Profile Pit-GC3-B
The soils below approximately 12 inches of topsoil consists of brown silty clay loam texture soil with
slightly blocky to granular structure of moderate structural grade to a depth of 4 feet containing no
scattered gravel or rock content. The underlying soils at 4'-7' consist of gravelly silty clay loam with fine
granular structure of moderate structural grade with rock content <307o, underlain by dense silty clay loam
to 8.5', underlain by rocky soils with silt loam matrix underlain by weathered sandstone to the refusal 10.5'
depth explored. No free water or evidence of seasonal high groundwater was encountered in the pit
excavation.
The native soils sampled from 2-4below the surface, in both profile pits, are characterrzed as a soil type 3
consisting of silty clay loam texture of moderate grade with granular structure. The tactile soil test ribbons
were over l3l|-inchlong . The Soil Type-3 has an effective loading rate (LTAR)of 0.35 GaUS.F./day for a
level 1 conventional treatment system. Based on the testing results, existing topography, vegetation and
observation of site features the soils appear and are assumed to be relatively consistent across the site.
The soils are suitable for the installation of an STA consisting of multiple gravelless chamber trenches that
will be fully pressure dosed meeting the requirements under sections (43.lO.C,D,E,F&H and 43.11.C.).
Additional soil evaluations will be performed by the engineer during construction to verify the soils in the
entire STA fietd excavation prior to installation of the OWTS components.
Chamber Treatment Unit/ Absorption Svstem Design
The treatment/absorption trenches are designed based on utilizing the effective Long Term Acceptance
Rate (LTAR) for native soils and the application of appropriate loading factors for a treatment level-1
system utilizing pressure dosed chamber trenches. The new treatment system will consist of pressure dosed
chamber trenches with custom perforated distribution laterals, in accordance with Regulation 43 sections
(43.10(C), 43.I3.8.1.d, Table l0-2 andTable 10-3 as approved by CDPHE for this technology.
Long Term Acceptance Rate (LTAR)
Receiving septic tank effluent, a treatment level-l system for chambers requires the infiltrative surface
being at least 4-feet above a limiting layer (43.8, Table 7-2.4); Maximum hydraulic loading rate for TL1
effluent is 0.35 ga1./S.F./day) (Table 10-1) being the long-term acceptance rate of the Type 2 receiving
soil.
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OWTS Design and Construction
Sweetwater Ranch Guest Cabin 3
Garfield CountY
SE Job No. 34008.01
Sept.5,2025
Page 4
A(sf) = Od x L.F.1 x L.F.2.:
LTAR
A = Area;
LTAR = 0.35 gallS.F./day for silt loam
Qd = flow (gallday) for OWTS = 675 gpd
L.F.1=0.8 pressure dosed trench application
L.F .2=0.7 Gravelless chambers
OWTS: A(sf) = 300 gpd x 0.8 x 0.7 = 480 S.F.
0.35 GaVS.F./day
Number of Quick-4 Infiltrator chambers 4'x3'xS "chambers :
480 S.F = 40 Infiltrator Quick-4 chambers; Use 40 chambers
12 S.F./Chamber
Number of Equivalent Arc-36 chambers 5'x3'x12"chambers:
480 S.F. = 32 chambers; Use 32 chambers
15 S.F./Chamber
Install 40 Quick-4 Infiltrator chambers in 2 trenches 82' long by 3' wide with 20 chamber units per trench
plus two end caps as delineated on the plan. Alternative use 32 Arc-36 chambers in 2 trenches 80' long by
3' wide with 16 chambers units per trenih with ends caps. A 4" vertical inspection port riser to be installed
from the molded penetration port on each chamber end cap or each end chamber in the trench.
Effl uent Distribution Svstem
The septic tank effluent will be pressure discharged to a common manifold pipe, installed at the head of
the trenches, via a 1.5" transport pipe from the high head pump discharge assembly in a biotube filter vault
set in the second compartment of the 1,250 gallon tank The minimum slope of the transpoft pipe from the
tank's dosing compartment to the manifold shall be installed at a minimtm 27o 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 trench via individual custom perforated pressure distribution laterals connected to the manifold
pipe. The distribution laterals will be installed on the infiltrative surface, under the chambers, along the
entire length of each trench. The 5/32-inch orifices wilt be drilled on 2.5' centers oriented upward along
the length of the distribution lateral. A ball valve will be installed on the terminal end of each distribution
lateral centered below the 4" inspection port, housed in a 6" valve box with lid set 2" above surface grades'
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OWTS Design and Construction
Sweetwater Ranch Guest Cabin 3
Garfield County
SE Job No. 34008.01
Sept".5,2025
Page 5
Construction and Inspections
Prior to construction of the permitted system, the elngineer should be contacted by the contractor and
construction project manager well in advance to provide adequate time to discuss the system components
with the contractor, answer questions, resolve any conflict issues and schedule inspection site visits based
on construction progress. A pre-OWTS construction meeting and calls are essential and required prior to
installation of the OWTS.
The engineer, prior to excavation shall inspect the staked location, of the proposed treatmentlabsorption
bed and tank. All septic system components shall be staked in the field for approval by the Engineer prior
to excavation. During initial construction the engineer will evaluate the soils in the location of the
absorption treatment field to confirm design assumptions made and existing conditions. The engineer will
provide construction recommendations and adjust the field layout and sizing if needed with discussion of
construction procedures.
As necessary a revised Construction document drawing will be submitted to the County as needed if
changes are required. A11 installations shall be inspected, surveyed, documented and reported/delineated in
the Certification letter and As-Constructed drawing.
County Regulations require that the Design Engineer of record perform site inspections of the permitted
system during construction and provide "As-Built" documentation of the installed system to the County
after construction is complete. Photo documentation during construction and site system inspections is
required to be performed by the contractor and Engineer. System component installation inspection and
review of photo documentation is required of the exposed system components prior to backfilling. Photo
documentation will be included with the As-Built Record drawing submittal package to finalize the OWTS
permit. Therefore, the Engineer should be contacted as needed and notified of the construction progress
and scheduling to allow time to schedule specific inspection times.
The county environmental health department needs to be contacted at least 48 hours prior to final backfill
of components to provide advanced notice of construction schedule in order to perform a final inspection
of the in place components. The Garfield County environmental health contact number is 970-945-6614 x
8106 (main office).
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OWTS Design and Construction
Sweetwater Ranch Guest Cabin 3
Gart'ield County
SE Job No. 34008.01
Sept. 5, 2025
Page 6
OWTS Oneration and Maintenance
All components of the engineered OWTS shall be inspected on a regular basis and be properly maintained.
The responsibility for repair and maintenance of the system will remain with the property Owner' The
owner shall retain the services of qualified personnel to inspect the OWTS and to perform all maintenance
and repairs necessary to ensure that the system is in good operating condition and follows the
manufacturer's performance requirements. The operating components of the OWTS shall be inspected by
the engineer within 30 days of being placed into operation and thereafter every six months by the owner or
qualified service provider. In general, for a properly utilized system, septic tanks should be pumped every
3-5 years. The effluent filter should be cleaned every six months and at the time of pumping. Absorption
fields should be maintained with suitable vegetative cover and kept free of root invasive plants. Positive
surface drainage away from the absorption field should be maintained.
If you have any questions or need any additional information, please call
Sincerely,
SOPRIS ENGINEERING, LLC
Paul E. Rutledge
Design Engineer
2tt377
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