HomeMy WebLinkAboutOWTS Design Report 04.07.2020April 7, 2020
William Rice
T & T LLC
Po Box 1252,
Carbondale, CO, 81623
"BillRice" billrice@sopris.net
RE: Design Report—Onsite Wastewater Treatment System (OWTS), T & T, Commercial Storage
Facility, 0544 C.R. 100, Garfield County, Colorado
SE Job No. 18197.01 Parcel Number: 2393-353-02-002
Dear Bill:
This letter report presents the applicable findings in regard to the design and construction of an Onsite Wastewater
Treatment System (OWTS) at the above referenced Site, in support of your OWTS permit application to Garfield
County. The design recommendations, specifications and construction details are delineated on the attached OWTS
permit/construction plan based on Sopris Engineering survey plat drawing.
The Sopris Engineering OWTS design is based on evaluation of existing physical site features, soils investigation
and site conditions with respect to the proposed facility usage, building layout and applicable regulatory design
criteria that is in accordance with Garfield County OWTS Regulations. The system on site is designed to serve the
needs of a commercial storage facility currently under construction by IRMW of Colorado, Inc. The design criteria
and system sizing information is summarized below.
Summary Design Criteria
The OWTS is designed with a maximum treatment capacity of 200 gallons per day based on the estimated daily
wastewater flow based on base fixture counts for the 1/2 baths per unit and additional employee population usage in
accordance with Table 6-2 values, Section 43.6, A.4. The OWTS will include the installation of a 1000 gallon 2-
compartment concrete septic tank with concrete risers and lids installed at the surface. The tank will be installed
with an inlet sanitary tee and an outlet effluent filter sanitary tee. A gravity schedule-40 or SDR-26 sewer pipe with
a cleanout will be installed from the warehouse facility structure to the tank. The soil treatment unit (STU) will
consist of two leaching chamber trenches that provide 240 S.F. of total absorption area. The septic tank effluent will
be distributed by gravity via a 4" distribution pipe connected to distribution box with individual 4" pipes connected
to the head end cap on 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).
Existing Site Location Existing and Proposed Site Conditions
The subject site is located at 0544 County Road 100, on a parcel of land situated in SE Section: 35 Township: 7
Range: 88 Subdivision: IRMW LLC', Parcel 1 Lot: 1B as per Kminor Subdivision Plat reception number 893462
Garfield County, Colorado. The site is bounded to the north by C.R. 100, to the South by steep undeveloped lands.
The site development evelope is located on relatively flat dry land area with steep sloped forested terrain toward the
South. Vegetation consists of sagebrush, shrubs and grasses. The Site comprises approximately 2.1 acres. The site
is partially developed with gravel parking, storage and associated driveway areas. The new OWTS will serve the
502 Main Street • Suite A3 • Carbondale, CO 81623 • (970) 704-0311 • Fax (970) 704-031J
SOPRIS ENGINEERING • LLC
civil consultants
Bill Rice, T & T LLC OWTS
0544 C.R. 100
SE Job No. 11174.01
April 7, 2020
new 3840 S.F. storage building being constructed in the northwest portion of the site on the lower benched area
with the field to be located east of the building in a flat area adj acent to and along the toe of the Southwesterly slope
terrain. The existing ground surface in the proposed absorption field area has average 3%. Slope. Domestic water
is supplied by an existing domestic water system. The site has an existing gravel surfaced driveway that traverses
through the property from the primary access off C.R. 100.
The proposed site improvements include the construction of a 3840 S.F. commercial building structure that will
have four 1/2 bathe restrooms. The new OWTS will be constructed designed for limited commercial usage capacity
to serve potential lease holders. The proposed site improvements will include appropriate site grading around the
structure, OWTS, and driveway improvements within the development area.
Sub Surface Conditions and observation
Subsurface soil investigations and USDA soil texture method tests were performed by Sopris Engineering in May,
2018. 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 to confirm
the appropriate absorptive surface suitable for conventional absorption systems in native soils. A shallow and deep
excavated pit was observed in the Easterly portion of the site adjacent to the proposed field location. The soils
below 4 to 9 inches of mixed surface fill & topsoil consist of medium dense sandy silt loam with moderate granular
to slightly blocky structure with some cobbles and gravels. The percentage of cobbles and gravest below 24 inches
was less than 20% in the sandy silt loam to the 7 foot depth explored. The soil appears to be consistent across the
site the easterly side of the relatively flat development envelope.
The native soils sampled from 2-3 feet below the surface are characterized as a soil type 2 consisting of silt loam
texture with moderate 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 assumed to be
approximately 16-25 minutes per inch. No free water was encountered in the excavations on site. Groundwater
levels are expected to be well below 10 feet from the existing surface grades. The soils appear suitable for a
conventional shallow absorption field consisting of gravelless infiltration chamber units.
Design Flow
The design flow is calculated as follows for low useage commercial facility onsite wastewater treatment system
(OWTS)
From the Regulation 43 usage Table 6-2 :
Max. Design flow (Qd) = # of people x (avg.per capita and/or per base fixture flow-GPD)
Each unit has 1/2 bath with single water closet (24.8 gpd) and Lavatory (8.4 gpd)
Qd1=Base fixture load = (24.8 gpd + 8.4 gpd)=33.2 gpd x 4 units = 33.2 x 132.8 gpd;
Qd2= Assume 1 additional daily worker per unit: 1 persons @ 15 gpd = 4 x 15 = 60 gpd;
Qd=Qd1 + Qd2 = 132.8 + 60 = 192.8 gpd
Total Design flow Qd = 200 gpd
The septic tank capacity required for a minimum 48 hour detention time is calculated as follows:
V = 200 gal/day * 2 = 400 gallons (Per Regulation a minimum 1,000 gallon capacity, two compartment is required).
Use a 1,000 gallon 2-compartment concrete septic tank. The tank shall have inlet and outlet sanitary tees with an
effluent filter. The tank access shall be maintained with risers and lids installed at the finish surface grade.
Treatment -Soil Treatment Unit/ Absorption System Design
Bill Rice, T & T LLC OWTS
0544 C.R. 100
SE Job No. 11174.01
April 7, 2020
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
scarified native soil absorptive surface.
Long Term Acceptance Rate (LTAR)
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 silt loam
Qd = flow (gal/day) = 200 gpd
L.F.1=1.0 Gravity Distribution
L.F.2=0.7 Chambers
A(sf) = 200 gpd x 1.0 x 0.7 = 233 S.F.
0.60 Gal/S.F./day
Number of quick-4 chambers:
233 S.F = 19.4 chambers, Use 20 Quick-4 Infiltrator or 18 Biodiffuser chambers
12 S.F./Chamber
Use 20 Quick-4 Infiltrator chambers in two trenches 42' long by 3' wide with two end caps as delineated on the plan.
The septic tank effluent will be gravity discharge to a distribution box with two 4" distribution pipe routed to the
head of each chamber row from the D-box. Additional soil evaluation will be performed during construction to
verify the soils in the excavation.
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 need 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.
Bill Rice, T & T LLC OWTS
0544 C.R. 100
SE Job No. 11174 01
April 7, 2020
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