HomeMy WebLinkAboutOWTS Design ReportAugust 2, 2022
502 Main Street • Suite A3 • Carbondale, CO 81623 • (970) 704 -0311 • Fax (970) 704 -0313
S OPRIS E NGINEERING • LLC civil consultants
Steven & Christine Nilsson,
787 Canyon Creek Dr,
Glenwood Springs CO 8160
"Client EM address”
RE: Design Report–Onsite Wastewater Treatment System (OWTS), Nilsson Residence, 167 Shore Dr, Silt, CO,
Lot 2, Riverview Ranch Subdivision, Garfield County, Colorado
SE Job No. 32029.01 Parcel No: 217912301002. Garfield County OWTS Permit No._____________
Dear Steven:
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 Construction permit
application to Garfield County. The OWTS design is based on recent site assessment observations by Sopris
Engineering of existing physical site features, conditions, review of previous 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 current LIS Survey Plat information and
architectural plans. The design criteria and system sizing information is summarized below.
Existing and Proposed Conditions
The proposed development area is situated on an undeveloped 5.6-acre lot in the existing residential Subdivision.
The subject parcel’s northerly boundary is adjacent to the Colorado River to the North. The proposed OWTS will
serve a new single family residential structure with a design capacity of up to 4-bedrooms.
The subject residential development area’s existing topography and site conditions are derived from Lidar
topography and existing subdivision survey plat information. The site is situated on gently sloped (3% to 6 %.)
agricultural land that generally trends down gradient to the West Northwest. The property’s water supply is from
an existing well, permit number: 66673 -F.
The proposed development will consist of the construction of two-level residential structure with attached garages
on a 3700 S.F. building footprint. The proposed site improvements will include associated driveway, auto court,
utilities and a new onsite wastewater treatment system. The development will be based on applicable site design
criteria and site construction management. The proposed development improvements and site layout plan is shown
on the attached OWTS plan.
Summary Design Criteria
The new OWTS is designed for a treatment capacity of 525 gallons per day based on the design wastewater flow to
serve a 4-bedroom residence in accordance with Table 6 -2 values, Section 43.6, A.4. The new OWTS will utilize a
minimum 1,25 0 gallon two-compartment concrete septic tank to be installed off the south westerly corner of the
proposed structure. Sealed concrete or ribbed PVC risers with 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 new 1,25 0 gallon 2-compartment 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 accessory structure.
OWTS Nilsson Residence,
167 Shore Dr, Silt, CO
SE Job No. 32029.01
August 2, 2022
Page2
The STU will consist of leaching chamber trenches that provide 613 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 minimum 1.5% slope,
to the STU via a concrete distribution box. The effluent will be equally distributed from the distribution box via 4”
distribution pipes connected to the head end cap of each chamber trench. Observation ports with caps will be
installed on the proximal and distal ends of each chamber trench. 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. 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 minimum design capacity of an equivalent 4-bedroom residential
dwelling. The OWTS is designed for a treatment capacity of 525 gallons per day based on the design wastewater
flow in accordance with Table 10-1 values, Section 43.6, A.4. Regulation 43:
Minimum population based on 3 bedrooms at 2 persons per bedroom and a fourth bedroom at 1 person per
each additional bedroom over 3, equating to 7 persons.
The Design flow is calculated based on residential usage of 75 gal/person/day, No PF
Max. Design flow (Qd)gallons/day = (# of people) x (avg. flow)gal/person/day.
Design flow Qd = 7*75=525 gpd
Septic Tank Design
Use a minimum 1250 gallon, two compartment concrete septic tank with an effluent filter outlet sanitary tee in the
second chamber. Tank shall be installed with sealed concrete risers and lids to the finish grade surface.
Sub Surface Conditions and Testing
A previous subsurface soil investigation was performed by Kumar & Associates, Inc june 9th, 2022. The result of
the soils evaluation is presented in the subsoil study for foundation design and percolation test, proposed residence
of lot 2, river view ranch subdivision at 167 shore drive in Garfield County Colorado. The subsurface conditions
were evaluated by excavation of three exploratory borings in the building envelope. The soils encountered in the
borings consisted very stiff, slightly clayey, slightly sandy to sandy silt in all three test borings . Based on the
geotechnical report the soils are assumed to be a sandy silt loam with slightly blocky and granular structure with
moderate structural grade having an LTAR of 0.60 g/sf/d.
A subsurface soil investigation and site assessment was performed by Sopris Engineering in on July 20, 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 two profile pits. Generally the soils below approximately 18 -24
inches of previously placed fill consist of medium dense silt loam texture soil with slightly blocky to granular
structure to the profile pit depth of 8 feet. The soils contain less than 5% scattered gravel or rock content to the 8-
feet depth explored below the surface grades. No free water was encountered in the pit excavations. No evidence of
seasonal high ground water or redox conditions was observed in the profile pits. Seasonal high Groundwater levels
are expected to be below 20 feet from the existing surface grades on the relatively flat terrain.
The west central portion of the lot was previously filled and graded with the placement of approximately 18 -24
inches of clay loam soils. The depth is variable with evidence of this imported fill overlying the previous vegetated
OWTS Nilsson Residence,
167 Shore Dr, Silt, CO
SE Job No. 32029.01
August 2, 2022
Page3
surface and remnants of a topsoil layer. The native soils sampled from the pits, 2-3 feet below the surface , in the
vicinity of the proposed field, are characterized as a soil type 2 consisting of silt loam texture with granular
structure. This soil has an effective loading rate for conventi onal 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 absorption field consisting of gravelless infiltration chamber units.
Additional observations and soil evaluation will be performed during construction, by Sopris Engineering in the
area of the proposed absorption field and tank to confirm the previous and recent soil evaluations and design
criteria If soil conditions encountered require adjustments to design criteria or the design plan, notice will be made
to county EHD and applicable revisions to design/construction plans and report will be submitted. The results of
any additional soil evaluation and soil characterization will be included in the as-constructed documentation.
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;
LTAR; (Table 10-1 Soil type 2 loading rate for a silt loam is (0.6 gal./S.F./day);
Design Flow; Qd = flow (gal/day) = 525 gpd
Loading factors; (Table 10-2, Gravity application trenches, L.F.1 = 1.0 )
(Table 10-3, Gravelless chambers, L.F.2 = 0.7)
STU absorption treatment area; A = Area S.F.;
A(sf) = Qd x L.F.1 x L.F.2 .: A(sf) = 525 gpd x 1.0 x 0.7 = 612.5 S.F.
LTAR 0.6 Gal/S.F./day
Number of Quick-4 Infiltrator chambers 4'x3'x12"chambers:
612.5 S.F. = 51 chambers; Use 52 chambers
12 S.F./Chamber
Number of Equivalent Arc chambers 5'x3'x12"chambers:
612.5 S.F. = 41 chambers; Use 44 chambers
15 S.F./Chamber
Use 52 Quick-4 Infiltrator chambers in 4 trenches 54' long by 3' wide with 13 chambers units per trench plus two
end caps as delineated on the plan to provide approximately 625 S.F. of absorptive area.. Alternative use 44 Arc
chambers in 4 trenches 55' long by 3' wide with 11 chambers units per trench with end caps to provide
approximately 660 S.F. of absorptive area. 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