HomeMy WebLinkAboutOWTS DesignMay 24, 2021
Richard and Vicky Nash
965 Westbank Rd,
Glenwood Springs, CO 81601
richard@nashc onstructionc omp a ny. com;
RE: Design Report-0nsite Wastewater Treatment System (OWTS), Nash Residence, 965 Westbank Rd,
Glenwood Springs CO, AKA Westbank Ranch Subdivision 41 Lot 15, Garfield County, Colorado
SE,lob No. 30177.01 Parcel No: 218535301015.
Dear Richard and Vicky_
This letter report presents the applicable findings in regard to the design and construction of a new 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, LiDAR, Survey Plat information and aerial mapping. The design criteria and system sizing
information is summarized below.
Summary Existing System Conditions
The subject property is a developed site consisting of a single family residence, detached garage, utilities, driveway
and established landscaping. The OWTS, as described here, will replace the original system which is to be abandoned.
No record of the original ISDS permit was found in the Garfield County permit database, as the existing house
construction and installation of the original system was prior to ISDS regulations . An alteration ISDS permit (40331)
was issued 09/02/1976 per Garfield County records to upgrade the original system.
Summary Design Criteria
The new OWTS is designed for a treatment capacity of 450 gallons per day based on the design wastewater flow to
serve a 3-bedroom residence in accordance with Table 6-2 values, Section 43.6, AA The new OWTS will utilize a
minimum 1,000 gallon two -compartment reinforced plastic septic tank to be installed within the lawn area west of the
proposed dwelling structure. It is recommended that ribbed PVC risers with fiber lids will be installed on the new
septic tank along with appropriate pipe connection to the new tank utilizing the appropriate fitting adaptor rings for
the tank and riser. The plastic tank manufacturer's plastic riser and lids may be utilized with careful installation of
select gravel and soil backfill free of rocks. The new 1,000 gallon 2-compartment reinforced plastic septic tank will
be installed with appropriate sanitary tees, outlet 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 tank inlet on the gravity sewer
line and the primary dwelling.
A gravity schedule-40 or SDR-35 effluent pipe from the new septic tank will be installed to the new soil treatment unit
(STU) absorption field with cleanout to a distribution box at the head of the field trenches
The STU will consist of leaching chamber trenches that provide 525 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
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SopRisENGINEERING • LLC civil consultants
Nash Residence
OWTS, 965 Wewstbank Rd
SE Job No. 30177.01
May 25, 2021
Paget
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 3 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 the residence
is served by the West Bank potable water system. Our design is outlined below and delineated on the attached C-1
OWTS plan.
Design Flaw
The design flow is calculated as follows for the anticipated future waste flows of the existing single family dwelling.
The replacement OWTS is designed for a treatment capacity of 450 gallons per day based on the design wastewater
flow to serve a 3-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
Use. 75 gal/person/day, No PF
Max. Design flow (Qd)gallonslday = (# of people) x (avg. flow)gallpersonlday.
Design flow Qd = 6*75=450 gpd
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. Use a minimum 1,000 gallon 2-compartment reinforced plastic septic tank. The
tank to be installed with risers and access lids set a minimum 3 inches above finish grade surface.
Sub Surface Conditions and Testint
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. The soils are suitable for a shallow
conventional shallow absorption field consisting of gravelless infiltration chamber units. Prior to field excavation and
placement of materials, during the time of construction, a formal profile pit will be excavated and observed to
confirm initial soil characterization assessment. A sample will be tested to verify the soil conditions in the field. The
findings from the additional test and observation will be included in the As -constructed documentations.
Nash Residence
OWTS, 965 Wewstbank Rd
SE Job No. 30177.01
May 25, 2021
Page3
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.
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 .:
LTAR
A(sf) = 450 gpd x 1.0 x 0.7 =525 S.F.
0.6 GaUS.F./day
A = Area,
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'xl2"chambers:
525 S.F. = 44 chambers; Use 44 chambers
12 S.F./Chamber
Use 445 Quick-4 Infiltrator chambers in 3 trenches 62' long by 3' wide with 15 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.
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 bacl&illing. County Regulations require that the Engineers of record perform site
inspections of the permitted system during construction and provide "As -Constructed" documentation of the installed
system to the County atier construction is complete. All installation will be inspected, field measured, 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.
Nash Residence
OWTS, 965 Wewstbank Rd
SE Job No. 30177.01
May 25, 2021
Page4
The County Environmental Health department should 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 contact is:
Ted White, P.E., Environmental Health Specialist III.Garfield County Public Health;
twhite ukgarfield-couW.com, 970-945-6614 x 8106 (main office) or 970-230-2442 (mobile)
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 he 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
X-A
2,8377
Paul E. Rutledge+li�rtxl�
Design Engineer Princi'par—