HomeMy WebLinkAboutEngineer's OWTS Design (Revised)August 6, 2025
502 Main Street, Suite A3, Carbondale, CO 81623 970-704-0311
Heathcote West LLC
5798 Sweetwater Road
Gypsum, CO 81637
c/o Chris Bendon
RE: Revised OWTS Design Report, On-site Wastewater Treatment System (OWTS), Tract G-2, Main
Residence, Heatchcote West LLC, 5798 Sweetwater Ranch, Garfield County, CO
SE Job No. 34008.01, Parcel No:186335300028
Garfield County Pending Permit # SEPT-07-25-9368
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-referenced 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
Construction Document plans titled C1-OWTS Site Plan dated 08-06-2025, included within the complete
CD Plan set.
OWTS Conditions and Design Criteria
Proposed site improvements and expected usage result in a design flow of 675 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 gravity service pipe from the
new dwelling to a new 2000-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
utilizing applicable multiplier loading rates per Table 10.2 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 transport
pipe will discharge to an automatic distribution valve (ADV) with individual distribution pipes from each
OWTS Design and Construction
Sweetwater Ranch Main Residence
Garfield County
SE Job No. 34008.01
August 6, 2025
Page 2
502 Main Street, Suite A3, Carbondale, CO 81623 970-704-0311
ADV outlet connected to the perforated distribution lateral at the head of each chamber trench. Each
chamber trench will have appropriate inspection port cleanouts and appurtenances.
Design Calculations OWTS
The proposed residential facility design includes a 4-bedroom single family residence with bunk room.
The design flow is based on a total residential population of 9 persons.
Per County adopted State Regulation 43, Design Flow Criteria: Minimum population is based on 2 persons
per bedroom for the first 3 bedrooms, 1 person per additional bedroom plus 2 persons per bunk room.
Design capacity = 3 x 2 + 1 + 2 = 9 persons @ 75 gal/person/day,
Max. Design flow (Qd) gallons/day = (# of people) x (avg. flow) gal/person/day.
Design flow Qd = 9 * 75 = 675 gpd
Septic Tank Design: 2-Day detention time = 675 x 2 = 1350 gallons required.
Use a 2000 gallon, three compartment concrete septic tank with a high head pump assembly.
Sub Surface Conditions and Testing:
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, in the area of 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 Engineering 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 3 profile pits. 3 profile pits were observed
within the Main Residence 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 terrain.
OWTS Design and Construction
Sweetwater Ranch Main Residence
Garfield County
SE Job No. 34008.01
August 6, 2025
Page 3
502 Main Street, Suite A3, Carbondale, CO 81623 970-704-0311
Profile Pit-MR-A
The soils below approximately 9 inches of topsoil consist of silty clay loam texture soil with slightly
blocky to granular structure of moderate structural grade with up to 35% gravels and cobbles to a depth of
3 feet. The underlying soils to a depth of 6' consist of cobbled rock fragments in a silty sandy clay loam
soil matrix of fine granular structure of moderate structural grade underlain by sandy clay loam to 7'
underlain by light brown clay loam to the 14' depth explored. No free water or evidence of seasonal high
groundwater was encountered in the pit excavation.
Profile Pit-MR-B
The soils below approximately 6 inches of topsoil consist of clay loam texture soil with granular structure
of moderate structural grade to a depth of 1.5 feet with no rock content. The underlying soils to a depth of
4' consist of 60% rock fragments in a clay loam soil matrix underlain by light brown clay loam with no
rock content to 6' underlain by reddish brown sandy loam to 7', underlain by gravel and rock fragments in
light brown clay loam matrix to the 11.5' depth explored. No free water or evidence of seasonal high
groundwater was encountered in the pit excavation.
Profile Pit-MH C
The soils below approximately 9 inches of topsoil consist of brown silty clay loam texture soil with
slightly blocky to granular structure of moderate structural grade to a depth of 1 foot containing no
scattered gravel or rock content. The underlying Soils to a depth of 3' consist of silty clay loam with
variable lenses of rock fragments <35% rock. The at soils 3' - 6' consist of light brown silty clay loam
containing with no rock content, underlain by tan weathered sandstone with fine sandy clay loam texture to
the 10' depth explored. No free water or evidence of seasonal high groundwater was encountered in the pit
excavation.
An additional subsurface soil investigation and site specific design location STA assessment was
performed by Sopris Engineering on June 23, 2024, to evaluate the soils conditions in the designed
location of the proposed STA for the main house. Three profile pits were observed within the main
residence STA envelope.
Profile Pit-MH TP-1
The soils below approximately 6 inches of topsoil (with 12 root zone) consists of brown slightly sandy
silty clay loam texture soil with slightly blocky to granular structure of moderate structural grade to a
depth of 4.5 feet containing no scattered gravel or rock content. Some scattered small boulders were
encountered within the 12" root zone below on and below surface. The underlying Soils to a depth of 6.5'
consist of dense clay loam with fine granular structure of moderate structural grade with <20% scattered
sand stone gravels and small cobbles, underlain by tan gravelly clay loam to the 8' depth explored. No free
water or evidence of seasonal high groundwater was encountered in the pit excavation.
OWTS Design and Construction
Sweetwater Ranch Main Residence
Garfield County
SE Job No. 34008.01
August 6, 2025
Page 4
502 Main Street, Suite A3, Carbondale, CO 81623 970-704-0311
Profile Pit-MH TP-2
The soils below approximately 9 inches of topsoil consist of brown silty clay loam texture soil with
slightly blocky to granular structure of moderate structural grade to a depth of 2 feet containing no
scattered gravel or rock content. The underlying Soils to a depth of 4' consist of slightly sandy clay loam
with a variable 18-inch-thick lens of gravels and cobbles in a clay loam matrix. This variable lens deposit
is 3-4 wide traversing across the center extents of the open pit excavation perimeter and assumed to be a
localized anomaly. The underlying soils below 4' consist of slightly gravelly dense brown clay loam
containing less than 15% rock content, to the 7' depth explored. No free water or evidence of seasonal high
groundwater was encountered in the pit excavation.
Profile Pit-MH TP-3
The soils below approximately 6 inches of topsoil consist 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 undelying soils below 4' consist of slighty gravelly dense brown clay
loam containing less than 20% rock content, to the 7' depth explored. No free water or evidence of
seasonal high groundwater was encountered in the pit excavation.
The native soils sampled from 2'-4' below the surface are characterized as a type 3 with silty clay loam
texture of moderate grade with slightly blocky to fine granular structure. The tactile soil test ribbons were
1 3/4-inch long. The soil type-3 has an effective loading rate (LTAR)of 0.35 gal/s.f./day for a level 1
conventional treatment system. Based on the testing results, existing topography, vegetation, and
observation of site features, the soils are consistent in the 3 soil profile pits. The soils are suitable for the
installation of an STA consisting of multiple gravelless chamber trenches.
Chamber Treatment Unit/ Absorption System Design
The treatment/absorption trenches are designed based on 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 with custom perforated distribution laterals, in accordance with
Regulation 43 sections (43.10(C), 43.13.E.1.d, Table 10-2 and Table 10-3.
Long Term Acceptance Rate (LTAR)
Receiving septic tank effluent, a treatment level-1 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 Type 3
soil receiving TL1 effluent is 0.35 gal./S.F./day) (Table 10-1)
A(sf) = Qd x L.F.1 x L.F.2.: A = Absorption Area;
LTAR LTAR = 0.35 gal/S.F./day for silt loam
Qd = flow (gal/day) for OWTS = 675 gpd
L.F.1=0.8 pressure dosed trench application
L.F.2=0.7 Gravelless chambers
OWTS Design and Construction
Sweetwater Ranch Main Residence
Garfield County
SE Job No. 34008.01
August 6, 2025
Page 5
502 Main Street, Suite A3, Carbondale, CO 81623 970-704-0311
OWTS: A(sf) = 675 gpd x 0.8 x 0.7 = 1080 S.F.
0.35 Gal/S.F./day
Number of Quick-4 Infiltrator chambers 4'x3'x8"chambers:
1080 S.F = 90 Infiltrator Quick-4 chambers; Use 90 chambers
12 S.F./Chamber
Number of Equivalent Arc-36 chambers 5'x3'x12"chambers:
1080 S.F. = 72 chambers; Use 72 chambers
15 S.F./Chamber
Install 90 Quick-4 Infiltrator chambers in 6 trenches 62' long by 3' wide with 15 chamber units per trench
plus two end caps as delineated on the plan. Alternative use 72 Arc-36 chambers in 6 trenches 60' long by
3' wide with 12 chambers units per trench 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.
Effluent Distribution System
Filtered effluent will discharge to the chamber trench field laterals via a duplex high head pump biotube
discharge assembly to an automatic distribution valve (ADV) between the septic/dosing tank and the head
of the STA trenches.
The distribution system will be installed to allow for effluent to be alternately and equally distributed
across the absorption surface in each chamber row via individual custom perforated full pressure
distribution laterals connected to the ADV discharge pipe at the head of each chamber trench installed on
the absorptive surface along the entire length of the individual trench. A ball valve will be installed on the
terminal end of each distribution lateral centered below the chamber end cap inspection port housed in an
8" valve box centered above to the distal chamber end cap with lid set 2" above surface grades.
The minimum slope of the transport pipe from the dosing basin to the ADV to be installed at a minimum
2.0% upward slope to allow for drainback to the tank. The individual ADV discharge pipes from the ADV
to be installed at a minimum 1% downward slope and will connect to the distribution lateral at the head of
each trench.
OWTS Design and Construction
Sweetwater Ranch Main Residence
Garfield County
SE Job No. 34008.01
August 6, 2025
Page 6
502 Main Street, Suite A3, Carbondale, CO 81623 970-704-0311
Construction and Inspections
Prior to construction of the permitted system, the engineer 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 treatment/absorption
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. All 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).
OWTS Operation 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