HomeMy WebLinkAboutOWTS Engineer's Report 03.10.2016Andre J. Ulrych
730 Latigo Loop
Carbondale, CO, 81623
ajulrych@ gmail.com
March 10, 20161
RE: Engineering Report-Onsite Wastewater Treatment System (OWTS) -Ulrych Residence,
9183 State Highway 82, Garfield County, Colorado
SE Job No. 26297.01 Parcel Number: Parcel Number: 2393-202-00-110
Dear Andre:
This letter report presents the applicable findings in regard to the design and construction of an Onsite
Wastewater Treatment System (OWTS) and residential site improvements at the above referenced Site, in
support of your building pennit application to Garfield County. The proposed improvements discussed in
this report address grading, drainage, potable water, sewer, electric and other utilities to service the
proposed ranch cabin residence shown on the Site Plan prepared by SE in coordination with the property
owner.
The OWTS design is based on our evaluation of the site conditions with infonnation provided by others for
use in obtaining a Garfield County OWTS construction permit. We have evaluated the existing and
proposed site conditions with respect to the operation and installation of a new OWTS in accordance with
Garfield County OWTS Regulations and State of Colorado Regulation 43. The proposed site specific
improvements have been designed in accordance with County Regulations and general engineering
guidelines regarding grading, drainage, utilities and construction best management practices. Based on the
physical site features, soils, and proposed building layout, we have prepared civil site and OWTS design
plans with construction details for building permit approval.
Conclusions
The new OWTS will serve a small ranch style residential structure with a combined total dwelling space of
approximately 1864 S.F. The basis of our design is for a 3-bedroom residence. A I 000-gallon septic tank
be installed that will discharge effluent to a 630 square foot soil treatment unit (STU) absorption trench
system. The soil absorption system will consist of gravelless leaching chambers installed in trenches. The
leaching chambers will be installed over a 4" filter sand levelling course overlying the native soils in the
trench excavations. An effluent filter will be installed on the sanitary outlet tee in the secondary chamber
of the septic tank. The filtered tank effluent will be gravity discharged to the STU via a distribution box.
The effluent will be equally distributed from the distribution box to the end caps at the first leaching
chamber in each trench. 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-3
plan.
I 502 Main Street• Suite A3 ·Carbondale, CO 81623 • (970) 704-0311• Fax (970) 704-0313
SOPRIS ENGINEERING • llC civil consultants
Existing Site Location and Conditions
Andre J. Ulrych
SE Job No. 15178.01
March 10, 2016
Page2
The subject site is located at 9183 State Highway 82, Carbondale in Lots 1-6, Block 3, Section: 20
Township: 7 Range: 88 of the 6th P.M. on Tract ofland in Government Lot 1, Garfield County, Colorado.
The site is located on a relatively flat dry land area with a slight slope downward toward the West
Southwest. Vegetation consists of sagebrush, shrubs and grasses. The Site comprises approximately 3 .4
acres. An irrigation ditch traverses through the property along the western property boundary. The
existing ground surface in the proposed absorption field area has average slope of less than I 0%. Domestic
water is supplied by an existing community water system.
The site has an existing gravel surfaced driveway that traverses through the property from the primary
access, near the northeast comer of the property, off the frontage Road that parallels the highway on the
on the west.
Proposed Site Conditions
The proposed site improvements include the construction of a 1864 S .F. footprint residential structure with
a attached car port. A new OWTS will be constructed that is designed for a 3-bedroom residential capacity.
The proposed site improvements will include appropriate site grading around the residence, OWTS, and
driveway improvements within the development area. Buried Electric and phone utility service lines have
been extended to the structure from the existing pedestals off site. A water service pipe has been installed
from the existing domestic well (CDWR Permit #82372) on site to the residence. A new buried propane
tank and service pipe will be installed to serve the structure.
Drainage
All historic drainage paths will be maintained and best management practices will be utilized to manage
onsite drainage. The existing site has broad moderately sloped vegetated areas, over porous terrain. The
undisturbed vegetated areas down gradient of the residence, driveway and OWTS will effectively manage
the storm runoff. The existing conditions provide for natural storm water treatment and attenuation with
infiltration. The proposed site will be graded to maintain and improve the existing vegetation conditions to
provide continued attenuation and management of onsite drainage. The majority of area on this site is
native, landscape and gravel areas so the developed runoff rate for this site is the same as the historic rate.
The runoff rates will be naturally attenuated as the storm water flows over the native and improved
landscape or gravel surface areas. Appropriate best management practices will be utilized during
construction and maintained after construction.
Construction Erosion control
Temporary erosion control will be installed and used until revegetation and other permanent erosion
control measures are in place. Silt fencing will be installed at the toe of the downstream edge of
disturbance areas for all construction areas onsite. Straw erosion/sediment control waddles will be
installed as needed at the toe of graded slope areas and in the drainage swales, until permanent vegetation
is in place.
Subsurface Conditions
Andre J. Ulrych
SE Job No. 15178.01
March 10, 2016
Page3
Subsurface soil investigations and USDA soil texture method tests were perfonned by Sopris Engineering
on August 21, 2015. 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 the appropriate absorptive surface depth in the native soils. A shallow and deep excavated
pit was observed in the west central portion of the site. The soils below 6 to 12 inches of topsoil consist of
medium dense silt loam with moderate granular to fine grained massive structure overlying cobbles and
gravel in a sandy silty clay loam matrix to the 7 foot depth explored. These soils appear to consist of glacial
outwash materials consisting of sand, silt, clay, gravels, cobbles and small boulders. The soil appears to be
consistent across the site.
The native soils sampled from 1-3 feet below the surface are characterized as a soil type 2A consisting of
silt loam texture with moderate granular to fine grained massive structure. This soil has an effective
loading rate for conventional soil treatment of 0 .50 gal/S.F./day for a level 1 conventional treatment
system. The equivalent percolation rate is approximately 26-40 minutes per inch. Increased quantities of
cobbles and gravel are encountered at depths below 4 feet. No free water was encountered in excavations
on site. Groundwater levels are expected to be well below 10 feet from the existing surface grades along
down gradient western portion of the site. The soils appear suitable for a conventional shallow absorption
field consisting of gravelless infiltration chamber units. Due to the presence of cobbles and gravels, the
design recommendations include the placement of a minimum of 4" of concrete sand in the bottom of the
excavated trenches to provide a leveling course and to promote unifonn distribution across the
treatment/absorption surface.
OWTS Design
The new system was designed based the current County design criteria and the design criteria approved in
the adopted Regulation 43 . The design is in compliance with the current County regulations with area
calculations based on the soil type and texture with an appropriate long term acceptance rate (LT AR).
Design recommendations include the installation of a three compartment 1000-gallon two-compartment
concrete septic tank with concrete access riser and lid installed to the surface. The tank will be equipped
with an outlet effluent filter sanitary tee. The soil treatment unit (STU) will consist of three leaching
chamber trenches installed to provide for equal gravity distribution of effluent across a 630 S.F.
treatment/absorption STU field installed in the native soils. The leaching chambers will be installed over a
4" filter sand levelling course overlying the native soils in the trench excavations if cobbles are
encountered. The effluent will be distributed through a concrete distribution box with individual
distribution pipes connected to the head of each trench.
The SDR-26 or schedule 40 PVC sewer pipes will be installed with respect to existing grades. The gravity
sewer pipes routed to the 2-compartment septic tank or STU will be bedded and backfilled with Class-6
base material and encased in blue board insulation if depths are less than 3 feet within any drive area. Any
additional installation techniques not shown will be documented and included with the AS-Built
certification drawing to be submitted after construction is completed. The system will meet all required
Andre J. Ulrych
SE Job No. 15178.01
March 10, 2016
Page4
setbacks and be installed within the general location indicated on the plan. Our design is outlined below
and delineated on the attached design and construction plan.
Design Flow Criteria
The design flow is calculated as follows:
The new OWTS shall be based on a minimum 3-bedroom capacity for single family residence.
From the Garfield County OWTS Regulation 43;
Minimum population based on 3 bedrooms @ 2 person /bedroom = 6 persons.
Gallons per day per person =75 gal/person/day, No peaking factor
Max. Design flow (Qd) gallons/day =(# of people) x (avg. tlow)gal/person/day.
Design flow Qd = 6*75=450 gpd
The septic tank capacity required for a minimum 48 hour detention time is calculated as follows:
V = 450_gal/day * 2 =900 gallons.
Use a I 000 gallon 2-compartment concrete septic tank . The tank shall have inlet and outlet sanitary tees
with an effluent filter on the outlet. The tank access shall be maintained with riser and lid to the surface
grade.
Soil Treatment Unit/ Absorption Svstem Design
The sizing of the STU system is per the guidelines by the manufacturer and as approved by the County
based on Regulation 43, for sizing of absorption fields utilizing gravelless leaching chambers. The STU
trenches will be excavated to the native soil level specified allowing placement of the chamber at relatively
shallow depths over a 4" filter sand levelling course. The chamber will be covered with filter fabric and
shall be carefully backfilled with excavated material to specified depths.
The treatment/absorption field is designed based on utilizing the effective Long Term Acceptance Rate
(LT AR) for the native soils and the application of appropriate loading factors for a soil treatment unit
system utilizing gravity distribution to gravelless chamber trenches and use of water saving fixtures. 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 new state regulation 43:
Receiving Wastewater Treatment Level 1; loading rate for silt loam is (0.50 gal./S.F./day) for soil type
2A.; Loading factors; (Table I 0-2, Gravity application trenches = 1.0 ) and
A(sf) = Od x L.F.:
LTAR
(Table 10-3, gravelless chambers = 0.7)
A= Area;
LT AR= 0.50 Gal/S.F./day for silt loam
Qd = flow (gal/day) = 450 gpd
L.F.1=1.0 Gravity Distribution
L.F.2=0.7 Chambers
A(sf) = 450 gpd x 1.0 x 0.7 = 630 S.F.
0.50 Gal/S .F./day
Number of quick-4 chambers:
630S.F = 50 chambers Use 54 chambers
12 S.F./Chamber
Andre J. Ulrych
SE Job No. 15178.01
March 10, 2016
Page5
Use 54 Quick-4 chambers in three trenches 74' long by 3' wide with 18 chambers units per trench plus two
end caps as delineated on the plan. The septic tank effluent will be gravity distributed to the head of each
chamber row via individual distribution pipes from a distribution box. Additional soil evaluation will be
performed during construction to verify the soils in the excavation.
The trenches need to be shallow with a maximum excavation depth of 24 inches and approximately 6-
inches of final grading topsoil mounded over the trenches placed to allow positive drainage away from the
trench surface. Each trench will be constructed level with successive trenches installed following natural
contour grades accordingly. A minimum separation distance of 6 feet will be maintained between the
individual trench sidewalls.
An inspection port should be installed on the top cutout at each head and end chamber. This will allow for
location of the field and for checking the performance of the system over time.
A 4" discharge line will discharge effluent to a distribution box installed up gradient and adjacent to the
field. The 4" PVC effluent discharge line will be installed with a minimum I% slope. The distribution box
will incorporate a riser section and access lid at the surface. The effluent will be equally distributed from
the distribution box through individual 4" solid distribution pipes to the head of each trench. The invert
out of the distribution box shall be at a level above or equal to the top of the chambers end cap.
The engineer, prior to installation shall inspect the trench excavation to confirm suitable soil conditions.
All septic system components and trench installations are to be approved by the Engineer and Gunnison
County prior to backfilling.
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 OWS.
The engineer, prior to excavation shall inspect the staked location, of the proposed treatment/absorption
bed and tank . 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 installation will be
inspected , surveyed, reported and delineated in the Certification letter and As-built drawing.
Andre J. Ulrych
SE Job No. 15178.01
March 10, 2016
Page6
County Regulations require that the Design 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.
OWS Operation and Maintenance
All components of the engineered OWTS shall be inspected on a regular basis and be properly maintained.
The system and responsibility for repair and maintenance of the system will remain with the Lot Owner.
The owner shall retain the services of qualified personnel to inspect the OWTS and to perform all
maintenance and repairs necessary to ensure that the system is in good operating condition and is in
compliance with the manufacturer's performance requirements. The components of the OWTS system will
be inspected within 30 days of being placed into operation and thereafter annually. The tank, absorption
field and other system components should be visually inspected annually for debris, damage, leaks, or other
potential problems. In general, for a properly utilized system, septic tanks should be pumped every 3 -5
years. 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.
Our design and recommendations are based our site assessment, soils evaluation and upon data supplied to
us by others. If subsurface or site conditions are found to be different from those presented in this report,
we should be notified to evaluate the effect it may have on the proposed OWTS. If the County
Environmental Health Department requests changes or modifications to this design, we should be
contacted to evaluate the effect on the OWTS.
Sincerely,
SOPRIS ENGINEERING, LLC .. )
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