HomeMy WebLinkAboutOWTS Design Report
Onsite Wastewater Treatment System Design
October 26, 2023
Deter Project
6186 CR 320
Rifle, CO 81650
Parcel No. 2175-261-00-200
Prepared By:
K-Cronk Engineering, Inc.
P.O. Box 140
Mack, CO 81525
970-250-0572
- 1 -
Site History
There is an existing three-bedroom residence on the site that is served by an existing onsite wastewater
treatment system (OWTS). The owner of the property has noticed signs of failure and wishes to replace the
system with a new OWTS.
The site consists of approximately 6.849 acres of uncultivated native soil. Drainage is approximately 10%
to the west. A percolation (perc) test/soils evaluation and site study were conducted on the property of
reference on 10/24/2023 by Kachayla R. Cronk, registered professional engineer (R.P.E.). The perc
excavation trench was located approximately 175’ north of the south property line and approximately 375’
east of the west property line.
This OWTS design has been prepared for the project based on information disclosed during the site study
mentioned above. A discussion of the engineered OWTS design follows. The perc test/soils evaluation is
attached for reference as Appendix A. The location of the perc excavation trench is shown on the Plot Plan
included in this report.
Development of Design Parameters
The perc excavation trench was extended to a depth of 84" below ground surface (BGS). There was no
evidence of ground water or high seasonal water table in the open excavation to a depth of 84" BGS. A
lithological description follows:
depth (in.) description
0” – 20” sandy clay loam, brown
(Soil Type 3, USDA – sandy clay loam, granular, moderate structure)
20” – 28” sandy loam, red
(Soil Type 2A, USDA – sandy loam, granular, weak structure)
28” – 84” weathered sandstone, white
(Soil Type 1, USDA – loamy sand, single grained, structureless)
Based on the results from the 10/24/23 perc test/soils evaluation (Appendix A), a design perc rate of 36
min/in. and corresponding long term acceptance rate (LTAR) of 0.50 are chosen for overall system sizing
for an infiltrator trench soil treatment area designed to discharge to subsoils below the 36" depth. An LTAR
corresponding to Soil Type 2A is used instead of Soil Type 3 because the infiltrative surface will be placed
below the layer of Type 3 Soil.
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System Design
An INFILTRATOR trench soil treatment system is proposed for discharge of septic effluent at the site.
Construction of the system will consist of excavating level trenches in the soil treatment area. Grading
during construction may affect slope, therefore contractor shall verify slope at time of installation.
Soil treatment area orientation may be adjusted per site variations to facilitate consistent trench
depth running parallel to final contours. Each trench excavation shall be continued to a level depth of
36" BGS at the uphill side of the trench (see Soil Treatment Area Cross Section). If consistent trench depth
cannot be maintained, INFILTRATOR chambers may be installed level at 24” minimum and 36” maximum
BGS. After removal of surface soils, the open excavations will be disked or scarified to promote subsurface
absorption.
Following completion of the trench excavations, QUICK4 STANDARD INFILTRATORS (or equivalent)
will be used to construct a septic effluent distribution system in the open excavations. The INFILTRATOR
effluent distribution system will be installed in accordance with the "Infiltrator Technical Manual",
available from Infiltrator Systems Inc., 123 Elm Street, Suite 12, Old Saybrook, Connecticut 06475.
Adjacent INFILTRATORS shall be fastened with a minimum of three (3) ¾” self-tapping screws to prevent
movement and separation during backfilling.
Septic effluent will be transferred from the upper trench to each of the adjacent downhill trenches through
a serial distribution network as shown in the included graphics. The serial distribution connections between
adjacent trenches will facilitate the saturation of the uphill trench before septic effluent is allowed to flow
to each of the adjacent downhill trenches.
After the INFILTRATORS are installed, synthetic filter fabric (140-200 mesh) will be placed over the
INFILTRATORS and the system will be covered with a soil cap. The soil cap will consist of approximately
24” of native soil backfill. The soil cap will be mounded 5% above the existing ground surface to promote
surface run off away from the soil treatment area.
The installer should confirm the feasibility of gravity discharge of sewage effluent from the foundation line
to the soil treatment area by field verification of assumed design parameters. Assumed design parameters
with respect to gravity discharge include:
• If the distance from the sewage pipe at exit from the existing septic tank to the soil treatment
area is 20' or less,
• The elevation of the ground surface at entry to the soil treatment area shall be no more than 27”
above the invert elevation of the sewage pipe at exit from the existing septic tank.
If field measurements do not confirm these design assumptions, gravity discharge of septic effluent from
the proposed structure cannot be assured and a lift station may be required. Please contact the design
engineer and Garfield County to address any design modifications necessary if these design assumptions
are not upheld.
There is an existing 1,000 gallon, two compartment septic tank that is in good working order and will
remain in use to provide a minimum of 48 hours retention time for sewage generated from the proposed
project. A non-corrodible Orenco filter (model # FTW0436-28) shall be installed at the final outlet tee of
the septic tank to limit the size of solids and sludge passing into the soil treatment area. The filter must be
accessible for cleaning and replacement from the ground surface.
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As shown in the included graphics, the trench soil treatment system will consist of three (3) trenches 3'
wide x 36” deep x 72’ long with a total of fifty-four (54) INFILTRATOR (or equivalent) units. The soil
treatment area will encompass an effective area of 925.71 sq. ft.
DESIGN CALCULATIONS
DESIGN LOADING RATE OF 3 BEDROOMS @ 150 GAL./BEDROOM-DAY = 450 gal./day
DESIGN PERCOLATION RATE = 36 MIN./INCH
DESIGN LONG TERM ACCEPTANCE RATE (LTAR) = 0.50 GAL./SQ.FT./DAY
A = 𝑄
𝐿𝑇𝐴𝑄, WHERE,
A = SOIL TREATMENT AREA (SQ. FT.)
LTAR = LONG TERM ACCEPTANCE RATE (GAL./SQ. FT./DAY)
A = 450
0.50 =900 𝑆𝑄.𝐹𝑆.
SOIL TREATMENT AREA ADJUSTMENT FACTOR FOR GRAVITY TRENCH DESIGN = 1.0
SOIL TREATMENT AREA ADJUSTMENT FACTOR FOR USE OF CHAMBERS = 0.7
ADJUSTED SOIL TREATMENT AREA = 900 X 1.0 X 0.7 = 630 SQ. FT.
LENGTH OF 3’ WIDE TRENCH REQ’D = 630 SQ.FT. / 3 FT. = 210 FT.
USE 3 TRENCHES WITH 18 QUICK4 STANDARD INFILTRATORS (or equivalent) EACH,
CONFIGURED IN SERIAL DISTRIBUTION FOR A TOTAL OF 54 UNITS.
SOIL TREATMENT AREA SIZED AT 3 TRENCHES X 3FT WIDE X 72 FT LONG = 648 SQ. FT.
- 4 -
Installation - Setbacks, Notifications, and Inspections
The owner and installer shall be aware of and comply with the following installation and system operation
requirements.
• The installer must be approved and licensed by Garfield County for installation of onsite
wastewater treatment systems.
• All installation activities shall be conducted in accordance with current Garfield County OWTS
Regulations. If at any time during construction, subsurface site conditions are encountered
which differ from the design parameters previously described, construction activities will stop,
and the design engineer and Garfield County will be contacted to address any necessary design
modifications.
• The final cover shall not be placed on sewer lines, septic tank, or the soil treatment area until
the system has been inspected and approved by the design engineer and Garfield County. The
installer shall provide 48-hour notice for all required inspections.
• Installation procedures including grade, location, setbacks, septic tank size, and soil treatment
size shall conform with the attached graphic details. Construction activities and system
components will not encroach upon existing easements or utility corridors. A minimum of 5
ft of undisturbed soil shall be maintained between individual absorption elements and the septic
tank. A minimum of 4’ of undisturbed soil shall be maintained between adjacent absorption
elements.
• The installer must maintain all setbacks to utility lines, easements, property lines, or other
adverse conditions, whether they are known and shown on the attached graphics or have been
disclosed during construction. Vehicle traffic and parking is to be prevented over the soil
treatment and repair area. Minimum setbacks for system components are:
Source Septic Tank Soil Treatment Area
domestic water line 10' 25'
domestic well 50' 100'
domestic water cistern 50' 100'
property lines 10' 10'
structure w/crawl space 5' 20'
structure w/o crawl space 5' 10'
irrigation ditch
open 50' 50'
intermittent irrigation 10' 25'
gated 10' 25'
solid pipe/lined 10' 10'
• All gravity flow sewer lines and effluent distribution piping shall be 4 inches in diameter and
have glued joints. All lines discharging sewage from the residence to the septic tank shall
maintain a minimum fall of 1/8 in. per foot and shall employ sweep 90's or 2-45's at all turns.
Sewer lines from the dwelling to the septic tank and at least 6 ft from the septic tank outlet
must meet minimum standard ASTM-3034. Lines discharging from the septic tank to the soil
treatment area shall maintain a minimum fall of 1/8 in. per foot. Sewer lines under driveways
shall meet the minimum Schedule 40 PVC standards. Additionally, effluent piping in traffic
areas with less than 24" of cover shall be encased in 6" CMP or flow fill and covered with
minimum 2" high density blue board insulation.
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• Sewer lines or domestic water lines shall be encased in minimum SCHD 40 with water -tight
end caps or a minimum of 6” of flow fill at all points with less than 5’ separation between sewer
and domestic water lines.
• Four-inch clean outs shall be installed at maximum 100' intervals in all gravity flow effluent
lines exceeding 100' in length.
• A minimum of 12" of soil cover (18" recommended) shall be maintained over all gravity
draining OWTS components to prevent freezing of septic effluent (excepting septic tank access
ports which must be extended to the ground surface).
• The system contractor shall be aware of the potential for construction activities to reduce soil
permeabilities at the site through compaction, smearing, and shearing. The following
precautions and construction procedures should be employed during installation to minimize
disturbance to native soils:
i. Excavation should proceed only when the moisture content of the soil is below the
plastic limit. If a sample of soil forms a rope instead of crumbling when rolled
between the hands it is too wet and should be allowed to dry before excavation
continues.
ii. Excavation and backfill equipment should work from the surface where at all
practical to avoid compaction of the soils at depth.
iii. The bottom and sidewalls of the excavation should be left with a rough, open
surface. The appearance should be that of broken or ripped soil as opposed to a
sheared, smeared, or troweled surface. Any smoothed or smeared surfaces should
be removed with a toothed rake or shallow ripper, taking care to remove loose
residues from the bottom of the excavation by hand if necessary.
iv. Care should be taken in placing fill materials in the excavation to avoid damaging
the exposed soil surfaces.
Operation - Maintenance and Inspections
• The owner shall install a structural barrier if necessary and take precautions to prevent vehicular
traffic, excessive surface watering, accidental flooding, or other activities in the vicinity of the
soil treatment area which may compact, saturate, or otherwise alter the subsurface soil
parameters used in designing the septic system.
• The owner will plant and maintain grass or other shallow rooted cover crop to prevent erosion
and promote evapotranspiration over the soil treatment area.
• The owner will inspect and maintain the required mounding and drainage away from the soil
treatment area to prevent saturation from precipitation and surface flows.
• To mitigate the generation of preferential flow channels which may compromise the operation
of the system, the owner will inspect and prevent intrusion of burrowing animals and deep-
rooted plants into the soil treatment area.
• The septic effluent filter shall be inspected and cleaned as necessary every six (6) months.
• The owner will conduct periodic maintenance of the septic system by removing accumulated
sludge from the septic tank every 3-4 years to prevent clogging of the soil treatment area.
- 6 -
Limitations
This report is a site-specific design for installation of an onsite wastewater treatment system and is
applicable only for the client for whom our work was performed. Use of this report under other
circumstances is not an appropriate application of this document. This report is a product of K-Cronk
Engineering, Inc. and is to be taken in its entirety. Excerpts from this report may be taken out of context
and may not convey the true intent of the report. It is the owner's and owner's agent’s responsibility to read
this report and become familiar with the recommendations and design guidelines contained herein.
The recommendations and design guidelines outlined in this report are based on: 1) the proposed site
development and plot plan as furnished to K-Cronk Engineering, Inc. by the client, and 2) the site conditions
disclosed at the specific time of the site investigation of reference. K-Cronk Engineering, Inc. assumes no
liability for the accuracy or completeness of information furnished by the client. Site conditions are subject
to external environmental effects and may change over time. Use of this plan under different site conditions
is inappropriate. If it becomes apparent that current site conditions vary from those anticipated, the design
engineer and Garfield County should be contacted to develop any required design modifications. K-Cronk
Engineering, Inc. is not responsible and accepts no liability for any variation in assumed design parameters.
K-Cronk Engineering, Inc. represents this report has been prepared within the limits prescribed by the
owner and in accordance with the current accepted practice of professional engineering in the area. No
warranty or representation, either expressed or implied, is included or intended in this report or in any of
our contracts.
SEAL
Kachayla R. Cronk, P.E.
Date
NOTE: This OWTS design is meant to include the following five pages:
1) plot plan
2) septic layout plan
3) notes for your installer
4) soil treatment area plan view
5) soil treatment area cross section.
The plan is not to be implemented in the absence of these sheets. In addition, results from the
percolation test and soils evaluation are included for reference as Appendix A.
NOTES FOR YOUR INSTALLER
• Please verify slope at the time of system installation – the orientation of the
STA shown in this design has been determined based on existing site
conditions at the time of the original site investigation, but that may have
changed during construction and pre-construction grading of the site.
• The depth of the trenches is determined by percolation rates and soil
conditions relative to the existing ground surface at the time of the soils
investigation. If the site slope or grading of the ground surface at the time of
installation differs significantly from what was initially observed, the trench
depths may have to be adjusted to make sure the infiltrative surface remains
in the desired soil layer, and portions of the trenches don’t end up too deep or
too shallow. The depth of the soil cap relative to finished grading is not part
of the OWTS design criteria.
• Please provide K-Cronk Engineering with a copy of the septic tank
manufacturer’s specification sheet verifying maximum backfill depth. Tanks
must be backfilled according to manufacturer’s specification.
Pressurized System Notes
• Please provide K-Cronk Engineering a copy of the sand sieve analysis, if
relevant, dated within 30 DAYS of installation before purchasing/installing
the sand so we can check that it meets Garfield County OWTS standards.
• Pump floats do not come pre-set from the factory. Please refer to the Pump
Vault Detail for the distance needed between the on/off floats.
Please reach out at any time during the installation process if you have any questions!
____________________________
Kachayla Cronk – (970) 250-0572
Appendix A
Soils and Percolation Report
Soils and Percolation Report
October 24, 2023
Deter Project
6186 CR 320
Rifle, CO 81650
Parcel No. 2175-261-00-200
Prepared By:
K-Cronk Engineering, Inc.
P.O. Box 140
Mack, CO 81525
970-250-0572
- 1 -
Soils Evaluation
The site consists of approximately 6.849 acres of uncultivated native soil. Drainage is
approximately 10% to the west. A percolation (perc) test/soils evaluation and site study were
conducted on the property of reference on 10/24/2023 by Kachayla R. Cronk, registered
professional engineer (R.P.E.). The perc excavation trench was located approximately 175’ north
of the south property line and approximately 375’ east of the west property line.
The perc excavation trench was extended to a depth of 84" below ground surface (BGS). There
was no evidence of ground water or high seasonal water table in the open excavation to a depth of
84" BGS. A lithological description follows:
depth (in.) description
0” – 20” sandy clay loam, brown
(Soil Type 3, USDA – sandy clay loam, granular, moderate structure)
20” – 28” sandy loam, red
(Soil Type 2A, USDA – sandy loam, granular, weak structure)
28” – 84” weathered sandstone, white
(Soil Type 1, USDA – loamy sand, single grained, structureless)
Perc holes were constructed at the surface and at depths of approximately 24 in. and 36 in. The
holes appeared to be well saturated at the time of the test. Results of the percolation test are shown
in Table 1.
TABLE 1
Percolation Test Results
6186 CR 320
Depth Time on 10/23/2023 Time
Drop
Perc
Rate
min/in 10:09 10:40 10:50
0”-12” 8.5 9.375 9.625 41/1.125 36
24”-36” 5.625 6.625 7 41/1.375 30
36”-48” 4.875 6.375 6.625 41/1.75 23
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Limitations
This document is representative of the site conditions disclosed at the specific time of the site
investigation. Site conditions are subject to change from external events both manmade (irrigation
or water feature construction) and naturally occurring (flooding or excessive precipitation).
K-Cronk Engineering, Inc. is not responsible and accepts no liability for any future variation in
site conditions.
K-Cronk Engineering, Inc. represents this report has been prepared within the limits prescribed by
the owner and in accordance with the current accepted practice of professional engineering in the
area. No warranty or representation, either expressed or implied, is included or intended in this
report or in any of our contracts.
SEAL
Kachayla R. Cronk, P.E.
Date