HomeMy WebLinkAboutOWTS Design Report 04.15.2020April 15, 2020
Lowell Walter
Locabebrem Family Trust
6538 331 County Rd
Silt Co 81652
RE: Design Report—Onsite Wastewater Treatment System (OWTS), Walter Ranch Residence, 6538 C.R. 331,
A Parcel In Sec 10 & 11 T7s R92w 6th PM Subdivision, Garfield County, Colorado
SE Job No. 30060.01 Parcel Number: 2401-112-00-206, Garfield County Permit:
Dear Lowell:
This letter report presents the applicable findings in regard to the design and construction of a replacement Onsite
Wastewater Treatment System (OWTS) at the above referenced Site, in support of your OWTS permit application
to Garfield County. The OWTS design is based on recent site assessment observations by Sopris Engineering of
existing physical site features, conditions geotechnical soils evaluation with respect to the proposed residential
usage and applicable regulatory design criteria in accordance with State and County OWTS Regulations. The
design recommendations, specifications and construction details are delineated on the attached OWTS
permit/construction plan that was prepared with GIS, Survey Plat information and aerial mapping. The design
criteria and system sizing information is summarized below.
Summary Design Criteria
The waste flow from the existing residence has been served by an OWTS permitted at the time of building permit in
1995, under County ISDS permit No. 2324 to serve a 3-bedroom residence. The existing chamber trench system is
currently compromised due to assumed partially clogged soils. The system is exhibiting slower infiltrative capacity
and reduced recovery. The existing functioning poly septic tank has been pumped a few times in recent years and is
equipped with sanitary tees, however it is undersized for current and anticipated future usage. This design provides
for a replacement absorption field based on treatment for a 5-bedroom capacity single family residence onsite
wastewater treatment system (OWTS).
The OWTS is designed for a treatment capacity of 600 gallons per day based on the design wastewater flow to serve
a 5-bedroom residence in accordance with Table 6-2 values, Section 43.6, A.4. The replacement OWTS will utilize
a new 1,500 gallon 2-compartment concrete septic tank, to be located in the native meadow area west of the
dwelling adjacent to the existing septic tank.. The tank will be installed with risers and lids at the surface, an inlet
sanitary tee and an outlet effluent filter sanitary tee in the second compartment. A gravity schedule-40 or SDR-35
sewer pipe with a cleanout will be installed from the existing gravity sewer line from the dwelling upstream of the
existing tank to route raw sewage to the new tank. The existing gravity sewer pipe will be maintained from the
residential structure to the new connection point.
The new replacement soil treatment unit (STU) will consist of leaching chamber trenches that provide 700 S.F. of
total absorption area. The septic tank effluent will be gravity discharged through a new 4-inch schedule-40 or SDR-
35, installed at 2% slope, to the STU via a distribution box. The effluent will be equally distributed from the
distribution box via 4" distribution pipes connected to the head end cap of each chamber trench. Observation ports
with caps will be installed on each end chamber unit. The design is in compliance with the current County
regulations with design calculations based on the soil type, texture and structure with an appropriate long term
acceptance rate (LTAR). The designed system meets all required setbacks and will be installed within the general
location indicated on the plan. The existing well is located over 200 feet northwest of the field area. Our design is
outlined below and delineated on the attached C-1 OWTS plan.
502 Main Street • Suite A3 • Carbondale, CO 81623 • (970) 704-0311 • Fax (970) 704-0313
SOPRIS ENGINEERING • LLC
civil consultants
Lowell Walter, Locabebrem Family Trust
OWTS, 6538 C.R. 331
SE Job No. 30060.01
April 15, 2020
Page2
Design Flow
The design flow is calculated as follows for the total current and anticipated future waste flows of the existing
single family dwelling. The replacement OWTS is designed for a treatment capacity of 600 gallons per day based
on the design wastewater flow to serve a 5-bedroom capacity residence in accordance with Table 6-2 values,
Section 43.6, A.4. Regulation 43:
Minimum population based on 2 person per bedroom for the first 3 bedrooms = 6 persons and 1 person per
bedroom for each additional 2 bedrooms = 2 persons for a total of 8 persons.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 = 8*75=600 gpd
Septic Tank Design
The septic tank capacity required for a minimum 48 hour retention time is calculated as follows:
V = 600 gal/day * 2 = 1,200 gallons. Use a 1,500 gallon, two compartment concrete septic tank with an inlet
sanitary tee and effluent filter outlet sanitary tee in the second chamber. Tank shall be installed with sealed concrete
risers and lids to the finish grade surface.Pump the existing 1,000 gallon septic tank and remove or breach and back
fill to abandon in place.
Sub Surface Conditions and Observation
A subsurface soil investigation and site assessment was performed by Sopris Engineering in on April 15, 2020. The
soils in the proposed field envelope area were sampled and characterized by application of the USDA visual/tactile
soil texture method analysis. The soils below 6 inches of topsoil consist of medium dense sandy silt loam soil with
less than 2% scattered gravel or rock content to 8 feet below the surface grades. The soils appear to be consistent
across the site from observations in the excavated profile pits.
The native soils sampled from 2-3 feet below the surface are characterized as a soil type 2 consisting of sandy silt
loam texture with fine granular structure. This soil has an effective loading rate for conventional soil treatment of
0.6 Gal/S.F./day for a level 1 conventional treatment system. The equivalent percolation rate is assumed to be 20-30
minutes per inch. No free water was encountered in excavations on site. Seasonal high Groundwater levels are
expected to be below 20 feet from the existing surface grades. The soils are suitable for a shallow conventional
shallow absorption field consisting of gravelless infiltration chamber units.
Treatment -Soil Treatment Unit/ Absorption System Design
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 system utilizing gravity
distribution to gravelless chamber trenches. The new treatment system and absorption field will consist of gravity
distribution of septic tank effluent via a distribution box to individual chamber trenches with treatment across the
native soil absorptive surface.
The shallow STU trenches shall be installed with a maximum depth to the absorptive surface of 36 inches from
finish surface grades and backfilled with approximately 3-inches of final grading topsoil mounded over the trenches
Lowell Walter, Locabebrem Family Trust
OWTS, 6538 C.R. 331
SE Job No. 30060.01
April 15, 2020
Page3
placed to allow positive drainage away from the surface. Chambers shall be installed level in native soil. Each
trench will be constructed level along the contours with successive trenches installed following natural contour
grades accordingly. A minimum separation distance of 4 feet will be maintained between the individual trench
sidewalls.
The engineer, prior to installation shall inspect the trench excavation to confirm soil conditions and installation
procedures. All septic system components and trench installations are to be approved by the Engineer prior to
backfilling.
Field Sizing:
Long Term Acceptance Rate (LTAR)
Considering the application of the state regulation 43:
Receiving Wastewater Treatment Level 1; Soil type 2 loading rate for a silt loam is (0.6 gal./S.F./day).; Loading
factors; (Table 10-2, Gravity application trenches = 1.0 ) and
(Table 10-3, gravelless chambers = 0.7)
A(sf) = Qd x L.F.1 x L.F.2 .: A = Area;
LTAR
A(sf) = 600 gpd x 1.0 x 0.7 = 700 S.F.
0.6 Gal/S.F./day
LTAR = 0.6 Gal/S.F./day for silt loam
Qd = flow (gal/day) = 450 gpd
L.F.1=1.0 Gravity Distribution
L.F.2=0.7 Chambers
Number of Quick-4 Infiltrator chambers 4'x3'x12"chambers:
700 S.F = 58.3 chambers; Use 60 chambers
12 S.F./Chamber
Number of 5'x3' Bio-Diffuser chambers:
700 S.F = 47c chambers; Use 48 chambers
15 S.F./Chamber
Use 60 Quick-4 Infiltrator chambers in two trenches 42' long by 3' wide with 10 chambers units per trench plus two
end caps as delineated on the plan or equivalent Bio-Diffuser chamber trenches. The septic tank effluent will be
gravity distributed to the head of each chamber row via individual distribution pipes from a distribution box.
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. A pre-OWTS construction
meeting is essential and required prior to installation of the OWTS. 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 septic system components and trench installations are
to be approved by the Engineer prior to backfilling. All installation will be inspected, surveyed, reported and
Lowell Walter, Locabebrem Family Trust
OWTS, 6538 C.R. 331
SE Job No. 30060.01
April 15, 2020
Page4
delineated in the Certification letter and record drawing. The contractor needs to photo document all OWTS
construction phases and supply photos to the Engineer.
County Regulations require that the 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
Ownership of the system and responsibility for maintenance and repair will remain with the property Owner. The
Owner is encouraged to retain the services of qualified personnel to inspect the OWTS and to perform all
maintenance and repairs necessary to ensure that the system components are maintained in good operating condition
and suitable vegetative cover is established on the fields. The components of the OWTS system should inspected
within 30 days of being placed into operation and should been inspected and maintained bi-annually. The tank,
absorption field and other system components should be visually inspected bi-annually for debris, damage, leaks, or
other potential problems. In general, for a properly utilized system, septic tanks should be pumped every 3 - 5years.
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 question or need any additional information, please call.
Sincerely,
SOPRIS ENGINEERING, LLC
Paul E. Rutledge
Design Engineer
Yancy Nichol, PE
Principal