HomeMy WebLinkAboutOWTS Design Report 11.16.2023
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September 24, 2019 (DESIGN UPDATE DUE TO AS-BUILT CONDITIONS-11/2023)
Mr. Andy Schwaller, Building Official (Mr. Ted White, Env Health)
Garfield County Building Department
108 8th Street
Glenwood Springs, CO 81601
RE: Cody OWTS
Section:20 Township:6 Range: 89 TR of Land in SW Cont
Glenwood Springs, CO 81601
Dear Ted,
The purpose of this letter is to provide you soils and design information relative to a tactile soils analysis that
SGM has performed on behalf of Paul Cody in support of installing a new OWTS at his proposed residence at the
above location. The site sits east of S Marsh Lane above Three Mile Creek on a 32.32-acre parcel in Section:20
Township:6 Range: 89 TR of Land in SW Cont. The proposed OWTS will serve a 3-bedroom single family
residence with the potential for expansion to handle capacity for a second residence on the property totaling 7
bedrooms. The following figure identifies the location of the property from the County’s GIS website.
Figure 1 - Site Location
From the NRCS Websoil Survey, the on-site soils (for the STA) are identified as a Cochetopa-Jerry complex. A
soils study was performed by Kumar & Associates on August 2, 2019. The findings from the soils study generally
agrees with the Websoil Survey. We have attached a soil report from the NRCS Websoil Survey to describe
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further the expected characteristics of the soils determined through the pre-site investigation. Our preliminary
findings of the soil concur with those characteristics identified by NRCS in their soil investigation for the Websoil
information. Specifically, findings for the site are as follows:
1. Groundwater was not encountered at the time the test pits were dug.
2. Depth to limiting layer (ie., ground water or bedrock) is greater than 8 feet.
3. The sample contained less than 35% rock, >2mm.
4. Two test pits have shown that other than a limited depth of topsoil, the soil horizon(s) below the topsoil
consists of very gravelly sandy loam or clay.
5. Differing soil textures prompted an analysis of percolation rates to determine the soil type. An average
of the percolation rates for all three percolation tests indicate a Soil Type 3 in Table 10-1 of Regulation
43 from CDPHE.
Based upon our understanding of the project and the above parameters, a standard OWTS design is sufficient
containing a 1,250 gallon tank for each home, a single pump vault and a single STA utilizing a pressurized dosing
system. Therefore, this design can suffice for the future added home as long as these parameters are adhered
to.
The system is sized for 7 4 (FOUR) bedrooms with a soils percolation rate averaged at 51.3 minutes per inch. As
noted, this is a proposed design with the potential for a second residence and septic tank to be added at a later
time. Calculations have shown that two trenches will be required for the initial three-bedroom residence with
three additional trenches shown for the future four-bedroom residence (indicated as Alternate 1 in the
calculations). It is recommended, and the plan shows to install 5 trenches now, utilizing 3 (which is slightly
oversized for the first home, indicated as Alternate 2 in the calculations), then connect the last two trenches
from the ADV to the full field. It will be determined by the owner to install the additional trenches now or when
the second residence is built. Soil condition and type will be confirmed again at time of installation.
NOVEMBER 2023: The contractor was already near completion and had revisions. The site has a
single home with 4-bedrooms (therefore 7-occupants). The field and tank volume can be reduced. The
1,250 gallon tank suffices for the loading. A pumped system (below) is still utilized from the main tank
to a highpoint ADV for distribution to a reduced field size. The field calculates out to 840 square feet.
Provided by the contractor are 4 Chambered Trenches, 72-feet long, providing 864 square feet. The
attached site plan provides the updated calculation.
Pressure Dosed Trench System (NO CHANGE)
As you will see on the schematic drawings for this design and the information attached, the homes will each need
to utilize a proposed 1,250 gallon septic tank. From the septic tank, the effluent would then flow by gravity to the
pump and dosing box and then to the distribution box for the Soil Treatment Area (STA) which is proposed to be
Infiltrator Quick4 chambers constructed in a trench format. A variety of bends and clean outs would need to be
installed from the tanks to the distribution box. Should the trenches for the future residence be installed, only
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two of the ports for the automatic distribution valve will be connected, while the three future trenches will have
their ports closed until such time as they are needed with the four additional bedrooms. Each of the chambered
trenches are to be installed at a constant level (ie., 24” to 48” below grade) as equal flow distribution is being
accomplished through the pressurized system. Trench lengths are indicated on the plans as associated with the
two homes.
Standard end caps are proposed on the end of each trench along with inspection ports. The piping from the tank,
distribution box and infiltrator trenches shall be a minimum diameter of 1-1/2” diameter ASTM 3034 PVC, with
field flow testing of the pump to establish the number of 1/8” holes needed for 3-5’ of spray head and maximize
the potential for even distribution in the field and ensure the flush valve at the end of each trench is installed to
appropriate height. The location of the tank, distribution box, gravity pipe route and infiltrator trenches are
approximated in the attached drawing plan of this package. It is likely that each trench will be at a different
elevation requiring the field spray test for each. Note that the drawing package is prepared in a schematic format
that relies on the contractor provide adequate grading to accomplish the intent of the design. In this manner, it
is anticipated that field modifications will be necessary but not alter the resultant performance design.
NOVEMBER 2023: Contractor provided photos, diagram, and video of pump test showing 3+ feet of spray
consistently along the trench length. The site plan has been amended per field notes and photos to the
record location installed.
The component construction for the system will need to follow the
requirements of Sections 43.8, 43.9 and 43.10 of regulation 43 (as
adopted by Garfield County) for the tank, pressure system, piping
and STA as applicable. We have attached a copy of each of these
sections for reference by the contractor to assure each component
construction is adequately addressed. Upon your receipt and
review, if you have any questions, please don’t hesitate to call.
Respectfully,
SGM
Rick L Barth, PE
Senior Engineer
Attachments:
· OWTS plan drawing
· Calculation sheets for OWTS design
· Cut sheets for installed materials
Client:Jack Cody Project Location:Section 20
Lot 32, Mountain Springs Ranch Township 6S
GWS, CO 81601 Range 89W
Date:9-Nov-23
Flow Data for the OWTS Design:: Current Application
STEP # Bedr flow per BR
1 Home Use BR count 3 150 450
Add'l Bedrooms 1 75 75
Total=525 gpd
Home Use 525 gpd
Totals:525 gpd
Desigining one STA for all contributing flows
Soil Data for the OWTS
2 Data from on-site soil observations (Data from HP Kumar):
Appears to be a gravelly sandy loam, dense at depths of 3' - 6' and below.
At a depth of 8', neither bedrock or groundwater have been encountered.
Web Soil Survey Classification(s):Cochetopa-Jerry complex
stony loam / cobbly clay loam / cobbly clay
Web Soil Survey Classification(s):Cochetopa-Jerry complex
Field Soils PP1 PP2 (gravel removed)
Gravels 57 9
Clay 3 7%51 56%
Silt 10 23%22 24%
Sand 30 70%18 20%
PERC 1 80
PERC 2 30
PERC 3 44
Average of 3 percolation holes:51.3 mpi Soil classification provided by HP
Given the consideration of all data, the Long Term Acceptance Rate to use is 0.35 gallons/sf/day (soil type 3)
Septic Tank Sizing
Current Application
3 Flow calculated from above:525 gpd
48 hour detention time for septic tank sizing;Volume= 1050 gpd
Install 1250 gallon tank, affirms GC Code section 43.9.B.2
OWTS Design Report and Calculations
Home use, 2 pp BR for first 3 BR, then 1 per above 3 BR
Dosing Tank Sizing
4 NO SEPARATE DOSING TANK. PUMP IN MAIN TANK.
Sizing of Absorption Field or Soil Treatment Area
5 Going with a soil type 3 and Treatment Level 1, LTAR = 0.35 g/d/sf
For a pressure dosed system, size adjustment factor is 1.0 for a bed configuration
For a gravity system, the size adjustment factor shall be 1.2 for a bed configuration
For a gravity trench system, adjustment factor = 1.0
For a siphon or pump dosed system, adjustment factor= 0.9
For a pressure dosed trench system, adjustment factor = 0.8
For use of chambers: size adustment factor is 0.7
STA= Flow/LTAR 1500 square feet (unfactored)
1500 TOTAL THIS PROJECT
T.A.
For a chamber system, adjust size to 0.7 = 1050 square feet
Incorporating a pressure dosed trench system, adjust size to 0.8=840 square feet
Incorporating a siphon or pump dosed system adjust size to 0.9=NA square feet
Utilizing a 36" wide chamber for the above selected system.3 feet
Therefore:: Current Application
For a pressure dosed chamber system in a trench configuration, length=280 feet total length of chambers
Number of trenches selected (max 150' length)4 # of trenches
Length of trenches 70 feet each trench
Alternate 1:Construct 4 chambered trenches, 72 feet long
Alternate 2:
Alternate 3:NONE