HomeMy WebLinkAboutOWTS Installation Observations 11.29.2021November 29, 2021
502 Main Street • Suite A3 • Carbondale, CO 81623 • (9 70) 704 -0311 • Fax (970) 704 -0313
S OPRIS E NGINEERING • LLC civil consultants
Richard and Vicky Nash
965 Westbank Rd,
Glenwood Springs, CO 81601
richard@nashconstructioncompany.com;
RE: As-Constructed –Onsite Wastewater Treatment System (OWTS), Nash Residence, 965 Westbank Rd,
Glenwood Springs CO, AKA Westbank Ranch Subdivision #1 Lot 15 , Garfield County, Colorado
SEJob No. 30177.01 Parcel No: 218535301015.
Dear Richard and Vicky:
Pursuant to County requirements, this letter provides documentation that the Onsite Wastewater Treatment System
(OWTS) recently installed at the above referenced site is in compliance with the permitted design. Sopris
Engineering inspected the excavations, soils, materials and the installation of the system components during
construction, prior to final backfill and after all installations were completed. The newly constructed absorption
field and tank installations were designed and constructed to serve a single family residence, detached garage/shop.
Sopris Engineering performed site visits to inspect and document the as-constructed conditions of the OWTS.
Inspections were coordinated with the property owner that installed the system components. The as-built conditions
and installation of the new OWTS components are in compliance with Garfield County Regulations; the
recommendations and specifications delineated on the civil C1 OWTS plan drawing dated 05-25-2021 and the
OWTS design Report dated 05-25 -2020 by Sopris Engineering. The design criteria and system sizing information
is summarized below.
Summary Design Criteria
The constructed OWTS was designed for a minimum treatment capacity of 450 gallons per day based on the design
wastewater flow to serve the future 3-bedroom residence. The new OWTS utilizes a new 1,060 gallon 2 two
compartment reinforced plastic septic tank, located in the lawn area west of the residence. The tank was installed
with sealed risers and lids to the finish grade surface, an inlet sanitary tee and an effluent filter outlet sanitary tee in
the second compartment.
The new soil treatment unit (STU) consists of leaching chamber trenches that provide 525 S.F. of total absorption
area. The septic tank effluent is gravity discharged through a new 4-inch SDR-35gravity sewer pipe, installed at a
minimum 1% slope, to the STU via a distribution box. The effluent is equally distributed from the distribution box
via 4” distribution pipes connected to the head end cap of each chamber trench. Observation ports with caps were
installed on each end of the 3 chamber trenches. 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 installed system meets all required setbacks and was installed within the general location delineated
on the attached C-1 As-Constructed OWTS Record drawing.
Design Flow
The new OWTS is designed for a treatment capacity of 450 gallons per day based on the minimum design
wastewater flow to serve an equivalent 3-bedroom single family dwelling in accordance with Table 6-2 values,
Section 43.6, A.4. Regulation 43: Minimum population based on 2 person per bedroom for the minimum 3
bedroom capacity = 6 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 = 6*75=450 gpd
Nash Residence
AC-OWTS, 965 Wewstbank Rd
SE Job No. 30177.01
November 29, 2021
Page2
Septic Tank Design
The septic tank capacity required for a minimum 48 hour retention time is calculated as follows:
V = 450 gal/day * 2 = 900 gallons. Installed a 1,060 gallon 2-compartment reinforced plastic septic tank with an
inlet sanitary tee and effluent filter outlet sanitary tee in the second chamber. Tank was installed with sealed risers
and lids to the finish grade surface.
Sub Surface Conditions and Testing
A subsurface soil investigation and site assessment was performed by Sopris Engineering in on May 1, 2021. The
shallow soils in the proposed field envelope and soils in the deep crawl space area were sampled and characterized
by application of the USDA visual/tactile soil texture method analysis. A 2.5 shallow hand dug profile hole was
dug in the vicinity of the field and the approximately 6 feet deep vertical foundation excavation walls were
observed and sample at 3 and 6 foot depths. The soils below 6 inches of topsoil consist of medium dense slightly
sandy silt loam texture soil with slightly blocky to granular structure to a depth of 6 feet. The soil profile
contained less than 35% scattered gravel or rock content to the 6 -feet depth observed below the surface grades.
No evidence of free water was encountered in the excavations. Seasonal high Groundwater levels are expected to
be below 10 feet from the existing surface grades on the relatively flat terrain.
The native soils sampled from 2-3 feet below the surface, in the vicinity of the proposed field and foundation, are
characterized as a soil type 2 consisting of silt loam texture with slightly blocky 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.
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 consists 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 were installed with a maximum depth to the absorptive surface of 3 0 inches from finish
surface grades and backfilled with at least 4-inches of final grading topsoil mounded over the trenches placed to
allow positive drainage away from the surface. Chambers were installed level in native soil. Each trench was
constructed level along the contours with successive trenches installed following natural contour grades
accordingly. A minimum separation distance of 4 feet was maintained between the individual trench sidewalls.
Field Sizing: Long Term Acceptance Rate (LTAR)
Receiving Wastewater Treatment Level 1; Soil type 3 loading rate for a clay 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 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