HomeMy WebLinkAboutOWTS Design Report 10.08.2020October 8, 2020
James P & Kelly Jonas Cleaver
3724 County Rd 115
Glenwood Springs, Co 81601
RE: Design Report—Onsite Wastewater Treatment System (OWTS), Cleaver Residence, 3724 C.R. 115, A Tract
in Lot 1 Sec 20, Lots 3 & 8, Section 29 T6s R88w 6th PM Subdivision, Garfield County, Colorado
SE Job No. 30169.01 Parcel No: 2187-292-00-077, Previous Garfield County ISDS Permits: 1094&1852
Dear James and Kelly:
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 Existing System Conditions
The waste flow from the existing residence has been served by an OWTS permitted at the time of building permit in
1981, under County ISDS permit No. 1094 to serve a 2-bedroom residence. The existing property was further
developed with a new residential home and associated improvements in 1990, including an upgrade OWTS, under,
County ISDS permit No. 1852 to serve a total 5-bedroom equivalent residence. The upgrade included the
installation of an additional single compartment Copeland 1000-gallon concrete septic tank in series with the
original 750 gallon septic tank. The existing 30 year old infiltrator chamber absorption field system is currently
functional however it is assumed to have partially clogged soils in need of replacement or expansion in the near
future. The previously installed septic tanks are functional however excessive root growth is observed between the
tank lid and risers that will compromise the tank integrity and function in the future. Currently this tank appears to
be sound with normal liquid levels. It is anticipated that over time the existing 1000 gallon tank may slowly
degenerate and the chamber field may exhibit slower infiltrative capacity and reduced recovery. Based on the
existing observed system conditions, the owners have decided to replace the system with a new regulatory
compliant system as a value added upgrade to the property. This design provides for an upgrade to the tank and
replacement absorption field based on treatment for a 5-bedroom capacity single family residence onsite wastewater
treatment system (OWTS).
Summary Design Criteria
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 residence in accordance with Table 6-2 values, Section 43.6, A.4. The replacement
OWTS will utilize the existing 1,000 gallon single compartment concrete septic tank and a new 1,060 gallon 2-
compartment reinforced plastic septic tank installed in series longitudinal adjacent to the existing tank within the
lawn area west of the primary dwelling structure. The existing backfill over the 1,000 gallon septic tank, outlet
pipe, raw gravity sewer inlet pipes, from primary dwelling and ADU and will be excavated to the tank lid and below
the pipe inverts as necessary to inspect, repair and install new risers and pipe connection as needed. The original
750 gallon tank and connecting pipe will be abandoned in place. New ribbed PVC risers with fiber lids will be
installed on the existing septic tank along with appropriate pipe connection to the new tank in series. The new
1,060 gallon 2-compartment reinforced plastic septic tank will be installed with appropriate sanitary tees, outlet
502 Main Street • Suite A3 • Carbondale, CO 81623 • (970) 704-0311 • Fax (970) 704-0313
SOPRIS ENGINEERING • LLC
civil consultants
Cleaver Residence
OWTS, 3724 C.R. 115
SE Job No. 30169.01
October 8, 2020
Page2
effluent filter with risers and lids at the surface. A gravity schedule-40 or SDR-35 sewer pipe section with two-way
cleanout will be installed between the existing tank inlet on the gravity sewer line and the primary dwelling. The
existing gravity sewer pipes will be maintained from the residential structures to the new connection point at the
inlet of the existing tank.
An additional cleanout will be installed on the new effluent outlet pipe from the new secondary septic tank in series
prior to the first grade break stepdown. A gravity schedule-40 or SDR-35 effluent pipe from the new septic tank will
be installed to the new replacement STU absorption field with appropriate stepped fitting and cleanouts as needed
to a new distribution box at the head of the field.
The new replacement soil treatment unit (STU) will consist of leaching chamber trenches that provide 840 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 variable 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 of 4 chamber trenches. 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 serving the residence, (Permit No. 113117) is located over 300
feet East Southeast of the proposed field area. Our design is outlined below and delineated on the attached C-1
OWTS plan.
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. 75 gal/person/day, No PF
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 the existing 1,000 gallon, single compartment concrete tank and a new
1,060 gallon 2-compartment reinforced plastic septic tank installed in series for a total septic tank capacity of 2,060
gallons. All tank to be installed with sealed ribbed PVC risers and fiber glass lids set to the finish grade surface.
The original 750 gallon tank will be pumped, breached, partially crushed and backfilled for abandonment in place.
Sub Surface Conditions and Observation
A subsurface soil investigation and site assessment was performed by Sopris Engineering in on October 15, 2020.
The soils from two profile pits, in the vicinity of the proposed STU field envelope area were sampled and
characterized by application of the USDA visual/tactile soil texture method analysis. The soils below 18- 24 inches
of topsoil consist of medium dense 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 2-5 feet below the surface are characterized as a soil type 2 to type 2A consisting of
silt loam texture with fine granular to weak massive structure. This soil has an effective loading rate for
Cleaver Residence
OWTS, 3724 C.R. 115
SE Job No. 30169.01
October 8, 2020
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conventional soil treatment of 0.5 Gal/S.F./day for a level 1 conventional treatment system. The soils from 5-7 feet
below the surface are characterized as a soil type 2A consisting of slightly sandy silt loam texture with weak
massive structure having an effective loading rate of 0.5 Gal/S.F./day for a level 1 conventional treatment system.
The soils below 7' to the depth of the 8.5 deep profile pit consist of slightly sandy silt loam with massive structure
and slight evidence of mottling speckles, indicating minor redox conditions from infrequent maximum high ground
water conditions below 7 feet from surface. No free water was encountered in the two profile pit excavations on
site near the proposed STU area. Normal Seasonal high Groundwater levels are expected to be below 10 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
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 2A loading rate for silt loam with weak massive structure is (0.5 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 = 840 S.F.
0.5 Gal/S.F./day
LTAR = 0.5 Gal/S.F./day for silt loam
Qd = flow (gal/day) = 600 gpd
L.F.1=1.0 Gravity Distribution
L.F.2=0.7 Chambers
Number of Quick-4 Infiltrator chambers 4'x3'x12"chambers:
840 S.F = 70 chambers; Use 70 chambers
12 S.F./Chamber
Cleaver Residence
OWTS, 3724 C.R. 115
SE Job No. 30169.01
October 8, 2020
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STU Layout
Use 70 Quick-4 Infiltrator chambers in two trenches 70' long and two trenches 74' long each by 3' wide with 17 and
18 chamber units per trench respectively plus two end caps as delineated on the plan or equivalent ArcBio-Diffuser
chamber trenches. The septic tank effluent will be gravity distributed to the head of each chamber trench 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
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