HomeMy WebLinkAboutEngineer's Design Report 03.10.2018March 10, 2016
Wirkler, Norman E & Margaret & Chris
730 PO BOX 506
Carbondale, CO, 81623
nwirkler@yahoo.com
RE: Engineering Report—Onsite Wastewater Treatment System (OWTS) — Wirkler Residence,
9663 State Highway 82, Garfield County, Colorado
SE Job No. 18020.01 Parcel Number: Parcel Number: 2393-202-00-111
Dear Norm:
This letter report presents the applicable findings in regard to the design and construction of an Onsite
Wastewater Treatment System (OWTS) and residential site improvements at the above referenced Site, in
support of a Garfield County OWTS permit Application. .
The OWTS design is based on our evaluation of the site conditions, the existing OWTS and with
information provided by others 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 field, tank and retrofit of the existing tank for use as a sewage pump station, in accordance
with Garfield County OWTS Regulations and State of Colorado Regulation 43. Based on the physical site
features, soils, and existing components, we have prepared civil OWTS design plans with construction
details for permit approval and construction.
Conclusions
The new OWTS and upgraded components will serve the existing residential structure with a combined
total dwelling space of approximately 3000 S.F. The basis of our design is for a 3 -bedroom residence with
the potential addition of future bedrooms and ability to upgrade the system to serve another single family
dwelling. The existing 1250 gallon single compartment septic tank will be converted to a sewage pump
station that will pump raw sewage to the new septic tank. A 1500 -gallon septic tank will be installed that
will discharge effluent to a 650 square foot soil treatment unit (STU) absorption trench system. The soil
absorption system will consist of gravelless leaching chambers installed in trenches. The leaching
chambers will be installed over native soils in the trench excavations. An effluent filter will be installed on
the sanitary outlet tee in the secondary chamber of the septic tank. The filtered tank effluent will be gravity
discharged to the STU via a distribution box. The effluent will be equally distributed from the distribution
box to the end caps at the first leaching chamber in each trench. The designed system meets all required
setbacks and will be installed within the general location indicated on the plan. Our design is outlined
below and delineated on the attached Cl and C2 OWTS plan drawings.
Existing Site Location and Conditions
The subject site is located at 9663 State HIighway 82, Carbondale in Lots 4 & 5 of the Martin Parcel in
Section: 20 Township: 7 Range: 88 of the 6th P.M., Garfield County, Colorado.
502 Main Street • Suite A3 • Carbondale, CO 81623 • (970) 704-0311• Fax (970) 704-0313
SOPRIS ENGINEERING • LLC
civil consultants
Norman Wirkler
SE Job No. 18020.01
April 11, 2018
Page 2
The site is located on a relatively flat land area that is irrigated with a slight slope downward toward the
West Southwest. Vegetation consists of shrubs and grasses. The Site comprises approximately 7.0 acres.
Irrigation ditch traverses through the property along the Easterly and western property boundary. The
existing ground surface in the proposed absorption field area has average slope of less than 4%. Domestic
water is supplied by an existing well located approximately 200 feet from the proposed absorption field.
Subsurface Conditions
Subsurface soil investigations and USDA soil texture method tests were performed by Sopris Engineering.
The soils in the area of the proposed absorption field were sampled and characterized by application of the
USDA soil texture method analysis. The soils were tested to determine the hydraulic loading rate and the
appropriate absorptive surface depth in the native soils. The soils below 6 to 8 inches of topsoil consist of
medium dense silt loam with moderate blocky to granular structure overlying deep cobbles and gravel in a
sandy clay loam matrix to the 7 foot depth explored. These soils appear to consist of glacial outwash
materials consisting of sand, silt, clay, gravels, cobbles and small boulders.
The native soils sampled from 1-2 feet below the surface are characterized as a soil type 2 consisting of silt
loam texture with moderate blocky to granular structure. This soil has an effective loading rate for
conventional soil treatment of 0.60 gal/S.F./day for a level 1 conventional treatment system. The
equivalent percolation rate is approximately 16-25 minutes per inch. Increased quantities of cobbles and
gravel are encountered at depths below 5 feet. No free water was encountered in excavations on site.
Groundwater levels are expected to be well below 10 feet below the existing surface grades. The soils
appear suitable for a conventional shallow absorption field consisting of gravelless infiltration chamber
units.
OWTS Design
The new system was designed based the current County design criteria and the design criteria approved in
the adopted Regulation 43. The design is in compliance with the current County regulations with area
calculations based on the soil type and texture with an appropriate long term acceptance rate (LTAR).
The SDR -26 or schedule 40 PVC sewer pipes will be installed with respect to existing grades. The pressure
and gravity sewer pipes routed to the 2- compartment septic tank or STU will be bedded and backfilled
with Class -6 base material and encased in blue board insulation if depths are less than 3 feet within any
drive area. Any additional installation techniques not shown will be documented and included with the
AS -Built certification drawing to be submitted after construction is completed. The system will meet all
required setbacks and be installed within the general location indicated on the plan. Our design is outlined
below and delineated on the attached design and construction plan.
Design Flow Criteria
The existing OWTS is being upgraded with a replacement absorption field to serve an existing three bed -
bedroom residence. The onsite wastewater treatment system (OWTS) is designed based on the design flow
for a 3 -bedroom residencial capacity with expansion potential.
Norman Wirkler
SE Job No. 18020.01
April 11, 2018
Page 3
From the Regulation 43 usage table :
Minimum population based on 3 bedrooms = 6 persons.
Additional population based on bedrooms assuming 1 person per bedroom.
Gallons per day per person =75 gal/person/day, No peaking factor
Max. Design flow (Qd)gallons/day = (# of people) x (avg. flow)gal/person/day.
Design flow Qd = 6*75=450 gpd
Septic Tank Resign
Use a 1,500 gallon, two compartment concrete septic tank with an effluent filter outlet sanitary tee in the
second chamber. Tank shall be installed with sealed concrete risers and lids to the finish grade surface.
Continue use of existing septic tank to serve as a raw sewage pump station to lift sewage to the new septic
tank. The treatment/absorption field is designed based on utilizing the effective Long Term Acceptance
Rate (LTAR) for the native soils and the application of appropriate loading factors for a soil treatment unit
(STU) system utilizing gravity distribution to gravelless infiltrative chamber units in a trench
configuration, for treatment across the native soil absorptive surface. The new treatment system will
consist of sewage collection and conveyance via a sewage pump station that will transport raw sewage to a
new septic tank for detention and gravity distribution to the new STU via a distribution box.
Soil Treatment Unit/ Absorption System Design
Long Term Acceptance Rate (LTAR)
Considering the application of the new state regulation 43:
Receiving Wastewater Treatment Level 1; loading rate for silt loam is (0.60 gal./S.F./day) for soil type 2.;
Loading factors; (Table 10-2, Gravity application trenches = 1.0 ) and
(Table 10-3, gravelless chambers = 0.7)
A(sf) = Qd .: A = Area;
LTAR LTAR = 0.60 Gal/S.F./day for soil type 2, silt loam
Qd = flow (gal/day) = 450 gpd
L.F.1=1.0 Gravity Distribution
L.F.2=0.7 Chambers
A(sf) = 450 gpd x 1.0 x 0.7 = 525 S.F.
0.60 Gal/S.F./day
Number of quick -4 chambers:
525 S.F = 44 chambers Use 54 chambers
12 S.F./Chamber
54 chambers x 12 S.F./chamber provides 648 S.F. of absorption/treatment area that equates to a Design
Capacity of 555 gpd suitable for an additional bedroom on the the existing residence or to be used as
partial capacity for a future dwelling up to a 3 bedroom capacity with the addition of 2 additional trenches
and tank.
Norman Wirkler
SE Job No. 18020.01
April 11, 2018
Page 4
Use 54 Quick -4 chambers in three trenches 74' long by 3' wide with 18 chambers units per trench plus two
end caps as delineated on the plan. The septic tank effluent will be gravity distributed to the head of each
chamber row via individual distribution pipes from a distribution box. Additional soil evaluation will be
performed during construction to verify the soils in the excavations.
Construction and Inspections
Prior to construction of the permitted system, the engineer should be contacted by the contractor and owner
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.
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 absorption treatment
field excavations to confirm soil conditions and make adjustments as needed. All installation will be
inspected, surveyed, reported and delineated in the Certification letter and As -built drawing.
County Regulations require that the Design Engineers 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.
OWS Operation and Maintenance
All components of the engineered OWTS shall be inspected on a regular basis and be properly maintained.
The system and responsibility for repair and maintenance of the system will remain with the Lot 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 is in
compliance with the manufacturer's performance requirements. 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 draina 7- awa from the absorption field
should be maintained.irf
Sincerely,
SOPRIS ENGINEERING, LLC
28377
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Paul E. Rutledge Yan
Design Engineer Principal