HomeMy WebLinkAboutEngineer's Design PlanFebruary 20,2025
Larry and Susie Brannon
PO Box 2834
Santa Maria, California 93547
B arehunterl 7 @ hotmail.com
RE: OWTS Design Report, On-site Wastewater Treatment System (OWTS), Larry and Susie Brannon
Family Trust, Brannon Residence, 253 Shore Drive, Silt, Garfield County, CO
SE Job No. 34132, Parcel No:217912401004. New Garfield County Permit #
Dear Larry:
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 your building permit
application to Garfield County. A new OWTS construction permit is required for the proposed usage on
the subject propefiy.
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 Topographic Survey, by Sopris Engineering, dated ll-ll-2024. The design
and construction recommendations with specifications are delineated on the civil design plans, C1.0-
OWTS Plan dated 02-20-2025, by Sopris Engineering.
OWTS Conditions and Desisn Criteria
Proposed OWTS System
Proposed site improvements and expected usage result in a design flow of 525 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 150O-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 for Table 10.3, 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 pressurized manifold connecting to the head of each chamber trench
row with custom perforated distribution laterals. Each chamber trench will have appropriate inspection
port cleanouts and appurtenances.
502 Main Street a Suite A3 a Carbondale, C0 B1 623 a (970) 704-031 1 a Fax (970) 704-031 3
SoPRrs tnGmtIRmG o 11G civil consu ltants
OWTS Design and Construction
Brannon, 253 Shore Drive, Silt, Garfield County
SE Job No. 34132.01
February 20,2025
Page2
Desien Calculations OWTS
The proposed residential facility design includes a 4-bedroom single family residence.
The design flow is based on a total residential population of 7 persons.
From County adopted State Regulation 43, Design Flow Criteria: Minimum population is based on 2
persons per bedroom for the first 3 bedrooms = 6 persons and 1 person per bedroom for each additional
bedroom.
Design capacity =3 x2+ I =7 persons @ 75 gal/person/day,
Max. Design flow (Qd) gallons/day - (# of people) x (avg. flow) gallperson/day.
Design flow Qd =7 * 75 = 525 gpd
Septic Tank Design: 2-Day detention time = 525 x 2 = 1050 gallons required.
Use a 1500 gallon, two compartment concrete septic tank with a high head pump assembly in the second
compartment.
Sub Surface Conditions and Testing:
A Subsurface soil investigation was performed by Kumar USA on December l1 ,2024 in order to develop
recommendations for the foundation, floor slab and potential groundwater mitigation design. The subsoil
conditions in the area of the proposed dwelling structure were evaluated from soil samples obtained from
two drilled exploratory boring holes. Samples of the subsoils obtained during the field exploration were
tested in the laboratory to determine their classification, compressibility, natural moisture content, density,
and finer than sand size gradation analyses and other engineering characteristics. The subsurface profile
encountered in the borings consists of very stiff, slightly clayey, slightly sandy to sandy silt down to the
maximum depth drilled of about 25 to 30 feet. No limiting soil or hydrological conditions were
encountered. The soils below 6 inches of topsoil consist of stiff sandy silt. Based on the geotechnicai
testing results, existing topography, vegetation and observation of site features, the soils appear and are
assumed to be consistent across the site.
From the results of the tests performed and presented in Kumar's subsoil study report, the soils from 2-3
feet below the surface are consistent with a USDA soil type 2 consisting of sandy silt texture with granular
structure and moderate to strong grade. This soil has an effective loading rate for conventional soil
fteatment of 0.60 Gal/S.F./day for a level 1 conventional treatment system. No rocky soils or change of
texture was encountered at depth. Observation and sampling of the boring spoils by the engineer indicate
presence of soil type 2 sandy silt with granular structure and moderate grade. In order to provide optimal
treatment a pressure dosed soil treatment area (STA) utilizing shallow chamber trenches is recommended
to provide for a treatment level TL-l OWTS system. No free water was encountered in the borings on site.
Normal Groundwater levels are expected to be below 10 feet from the existing surface grades. 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 (43.10.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 field excavation prior to installation of the OWTS components.
OWTS Design and Construction
Brannon, 253 Shore Drive, Silt, Garfield County
SE Job No. 34132.01
February 20,2025
Page 3
Chamber Treatment Unit/Ahsorntion vstem DesisnS
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- I
system utllizing 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.E.1.d, Table l0-2 and Table 10-3 as approved by CDPHE for this technology.
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 1-2.4); Maximum hydraulic loading rate for TL1
effluent is 0.6 gal./S.F./day) (Table 10-1) being the long term acceptance rate of the Type 2 receiving soil.
A(sf = QdxL.F.lxL.F.2.A = Area;
LTAR = 0.6 gallS.F.lday for silt loam
Qd = flow (gallday) for OWTS = 525 gpd
L.F.1=0.8 pressure dosed trench application
L.F.2=0.7 Gravelless chambers
LTAR
OWTS: A(sD = 525 Epd x 0.8 x 0.7 = 490 S.F
0.6 Gal/S.F./day
Number of Quick-4 Infiltrator chambers 4'x3'x8 "chambers :
490 S.F = 41 Infiltrator Quick-4 chambers;Use 44 chambers
12 S.F./Chamber
Number of Equivalent Arc-36 chambers 5'x3'x12"chambers
490 S.F. = 33 chambers; Use 36 chambers
15 S.F./Chamber
Install 44 Quick-4 Infiltrator chambers in 4 trenches 46' long by 3' wide with 1i chamber units per trench
plus two end caps as delineated on the plan. Alternative use 36 Arc-36 chambers in 4 trenches 45' long by
3' wide with 9 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
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 1,500 gallon tank The minimum slope of the transport pipe from the
tank's dosing compartment to the manifold shall be installed at a minimum2Vo slope to allow for drain
back to the tank.
OWTS Design and Construction
Brannon, 253 Shore Drive, Silt, Garfield County
SE Job No. 34132.01
February 20,2025
Page 4
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 3/16 inch orifices will be drilled on 2' 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.
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 ffeatment field to confirm design assumptions made and existing conditions. The engineer will
provide construction recommendations and make adjustments to 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 rs 970-945-6614 x
8106 (main office).
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
OWTS D"ttgo and Construction
Branrcn,253 Shorc Drive, Silt, Garfi€ld County
SEJob lt{o. 34132.01
FsmarY ttiff"T
and repairs necesary to ensure that the system is in good operating condition and is in compliane with the
manufacturer's performance requirements. The operating components of the OWTS shall [6 inspected by
the enginwr within 3O days of being placed into operation and thsreafter every six months by the owner or
qualifred senrice provider. In geneml, for a properly utilizd 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 oover and kept frree of root invasive plants. Positive
surface drain4ge away from the absorption field should be maintained.
If you have any question or ned any addifional information" please call.
Sincerely,
SOPRIS ENGINEERING, LT,c'
Paul E. Rutledge
Design Engineer
Yarcy
Principal
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