HomeMy WebLinkAboutEngineer's Observation LetterApril 10, 2023
502 Main Street • Suite A3 • Carbondale, CO 81623 • (970) 704 -0311 • Fax (970) 704 -0313
S OPRIS E NGINEERING • LLC civil consultants
Murray, Edward & Cherri
PO Box 551
New Castle, CO 81647
"Eddie Murray" <emurray@teampsm.com>
RE: As-Constructed–Report–Onsite Wastewater Treatment System (OWTS), 331 Rapids View Ln, New Castle, CO , LOT
13, Rapids On The Colorado
SE Job No. 32019, Parcel No: 218104307013, Garfield County Permit #: SEPT-06-22-7597 Issued 06-24-22
Dear Eddie:
Pursuant to County requirements, this letter provides documentation that the Onsite Wastewater Treatment System (OWTS)
upgraded components 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 replacement absorption field and tank installations were designed
and constructed to serve the proposed usage on the subject property.
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-constructed conditions and installation of the
new OWTS components is in compliance with Garfield County Regulations; the recommendations and specifications delineated on
the civil design plans, C1-OWTS Plan revision dated 03-27-2023, by Sopris Engineering. The design criteria and system sizing
information is summarized below.
OWTS Design Criteria and Sizing
The residential structure has 3 bedrooms. In order to ensure adequate OWTS capacity for existing usage and potential future usage
the new system is designed for the maximum design flow for a 4-bedroom single family residence. The system is designed based
on the current County design criteria and the design criteria approved for the use of a pressure dosed sand filter (SF) system. The
design is in compliance with the current County Regulation No. 43 with area calculations based on the LTAR loading rate for
ASTM-33 filter sand per a minimum equivalent 4-bedroom residential design flow capacity.
A a two-compartment 1,500 gallon concrete septic tank was installed equipped with an Orenco Systems simplex high head pump
in a Biotube vault installed in the second compartment with associated piping and controls installed. A full pressure dosed Sand
Filter (SF) dual bed system was installed to provide for equal pressure distribution of effluent across a 670 S.F.
treatment/absorption SF field with 24-inches of filter sand below gravelless chamber spray shield units. The effluent is dosed
across the two distribution rows via an individual custom perforated pressure distribution pipe running the length of each 42’ feet
by 8’ feet sand filter beds.
Design Calculations OWTS
Minimum population is based on 2 persons per bedroom for the first 3 bedrooms = 6 persons and 1 person per bedroom for each
additional bedroom. Therefore: design population is 3 bedrooms x 2 persons + 1 bedroom x 1 person = 7 persons total.
Gallons per day per person =75 gal/person/day, No peaking factor
Design capacity = Max. Design flow (Qd)gallons/day = (# of people) x (avg. flow)gal/person/day.
Design flow Qd = 7 + 75 = 525 gpd
Septic Tank Design: 2-Day detention time = 525 x 2 = 1,050 gallons required.
Installed a 1,500 gallon two-compartment tank with a bio-tube high head pump dosing assembly in second compartment of the
tank. Tank was installed with 24” diameter ribbed PVC risers 44” tall over each access with lids set slightly above finish surface
grades.
Sub Surface Conditions and Testing
OWTS As-Constructed Certification
331 Rapids View LN, New Castle
SE Job No. 320191.01
April 10, 2023
Page 2
A subsurface soil investigation and site assessment was performed by Sopris Engineering in on February 4, 2022. The soils in the
proposed field envelope area were sampled and characterized by application of the USDA visual/tactile soil texture method
analysis from samples in shallow hand dug excavations and nearby embankment cuts. The shallow soils in these excavations
were sampled, observed and tested at 1, 2 and 3 foot depths. The soils below nil to 3 inches of topsoil consist of medium dense
gravelly sandy silt loam with scattered cobbles to 3 feet below the surface.
Two profile pits were excavated on site in support of the foundation design near the northwest and southeast corners of the
proposed building envelope. The medium dense soils observed from 1 to 7 feet in the NW test pit consists of loamy sand to sandy
loam with some gravels. The medium dense soils observed from 1 to 7 feet in the NE test pit consists of sandy silt loam with
some gravels and cobbles. The primari ly Type 2 soils in both pits have rock content less than 35%.
The native soils sampled from 1-3 feet below the surface near the proposed field are characterized as a soil type 2 soil consisting
of slightly gravelly sandy silt loam texture with moderate granular structure containing approximately 30% rock content. 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 10-20 minutes per inch. The soils become increasingly rocky at depths below 5.5
feet. Soils below 5.5 feet are characterized as a USDA rocky soil R1 type 2 consisting of dense rocky sandy loam texture of
granular structure with approximately 40% rock and gravel being less than 3/4 inch and containing less than 20% cobbles and
small boulders in the sandy loam matrix. In order to provide optimal treatment with respect to soil conditions, a pressure dosed
STA utilizing a 24-inch depth of filter sand media will be installed to provide for an enhanced treatment level TL-3 system. No
free water is expected to be encountered within the field excavations. Groundwater levels are expected to be below 6 feet from
the existing surface grades.
Sand Filter Bed Treatment Unit/ Absorption System Design
The treatment/absorption field is designed based on utilizing the effective Long Term Acceptance Rate (LTAR) for an intermittent
sand filter treatment system utilizing pressure dosed distribution laterals in beds and gravelless chamber units as spray shields.
The new treatment system consists of a fully pressure dosed sand filter treatment System with a 24-inch layer of filter sand
(ASTM-33, "washed ASTM-33 Concrete sand") in a dual bed configuration.
Long Term Acceptance Rate (LTAR)
Considering the application of the new state regulation 43:
Receiving septic tank effluent, the treatment level for intermittent sand filters is considered TL3 for the minimum 24" depth of
treatment media with the upper infiltrative surface being at least 4-feet above a limiting layer;
Maximum hydraulic loading rate for TL1 effluent applied to “Secondary Sand Media” in an unlined sand filter is 0.8
gal./sq.ft./day, or the long term acceptance rate of the receiving soil Type 2 and or R-1 for TL3 (1.1 gal./S.F./day) (Table 10-1
&10-1A) whichever results in the larger area.
The maximum Loading rate is therefore 0.8 gal./sq.ft./day
Loading factors; (Table 10-2, pressure dosed bed application = 1.0 )
A(sf) = Qd x LF1 = A = Area;
LTAR LTAR = 0.8 Gal/S.F./day for 2nd sand, ASTM-33, " Concrete sand"
Qd = flow (gal/day) = 525 gpd
LF1=1.0 pressure dosed beds
Bed A (sf )= 525 gpd x 1.0 = 656.25 S.F.
0.8 Gal/S.F./day
As-Constructed Soil Treatemnt Area (STA)