HomeMy WebLinkAboutEngineer's Design Report 03.07.2019March 7, 2019
Helffenstein, Dennis A & Diana M
18425 Sunburst Drive
Monument CO 80132
email: helffen3@gmail.com
RE: Design Report, On-site Wastewater Treatment System (OWTS), Helffenstein Residence, 1475 C.R.
129, Glenwood Springs, Sec: 2 Township: 6 Range: 89 TR in Lot 2, No Name, CO
SE Job No. 19033, Parcel No: 2185-021-00-008.
Dear Dennis & Diana:
This letter report presents the applicable findings in regard to the design and construction of an Onsite
Wastewater Treatment System (OWTS) to replace the existing system at the above referenced Site, in
support of your OWTS Construction permit application to Garfield County. The proposed site
improvements are limited to the OWTS replacement only to serve the existing 3 -bedroom residence shown
on the Site Plan prepared by SE.
The OWTS design is based on our evaluation of the site conditions with information provided by others for
use in obtaining a Garfield County OWTS construction permit. We have evaluated the existing 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. Based on the physical site features,
soils, and existing building structure, we have prepared an OWTS design plan with construction. The
OWTS design and construction recommendations with specifications are delineated on the civil design plans, C1-
OWTS SITE PLAN and Details, dated 3-6-2019, by Sopris Engineering.
Existing & Proposed Site Conditions
The site is located on 1475 County Road 129, in the community of No Name, Garfield County, Colorado. The
existing site includes: a single family home with a detached shop/shed, driveway improvements, and site landscape
features. The 1.0 acre site is situated on a moderate sloping terrain with slope grades of 3% to 10% adjacent to a
steeply incised embankment on the left bank of No Name Creek. The existing driveway is accessed of County
Road 129 on the East side of the property. The site vegetation consists of grasses, and scattered shrubs, mature
evergreen trees. The location of the proposed OWTS field is along the eastern portion of the lot.
OWTS Design Criteria and Sizing
Based on our findings the design and installation of an engineered OWTS with enhanced treatment is feasible in
accordance with applicable County and the State of Colorado Regulation 43. The existing residential has three
bedrooms currently served by an existing OWTS system in need of replacement due to failing steel tank lid. 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 primary ASTM -33 filter sand per a minimum equivalent
3 -bedroom design flow capacity.
502 Main Street • Suite A3 • Carbondale, CO 81623 • (970) 704-0311 • Fax (970) 704-0313
SOPRIS ENGINEEHINC • LLC
civil consultants
OWTS Design and Construction
Helffenstein Residence
SE Job No. 19033.01
March 7, 2019
Page 2
Design recommendations include the installation of a 1,000 gallon 2 -compartment reinforced septic tank and a 30 -
inch ribbed PVC pump basin that will be equipped with an Orenco Systems simplex high head pump in a Biotube
vault with associated piping and controls. We recommend the use of a full pressure dosed Sand Filter (SF) system
to provide good secondary treatment performance in the relatively rocky soils. The SF field will consist of two beds
installed to provide for equal pressure distribution of effluent across a 450 S.F. treatment/absorption SF field with
36 -inches of filter sand below gravelless chamber spray shield units. The effluent will be dosed across the three
distribution rows via an individual custom perforated pressure distribution pipe running the length of each 19 feet
by 12 feet sand filter beds.
Design Calculations OWTS
Installation of an OWTS with a minimum 3 -bedroom capacity.
From County adopted State Regulation 43:
Minimum population based on 3 bedrooms = 6 persons.
First 3 bedrooms (2 persons/bedroom) Over 3 bedrooms (1 person per bedroom)
Gallons per day per person =75 gal/person/day, No peaking factor
Max. Design flow (Qd)gallons/day = (# of people) x (avg. flow)gal/person/day.
Design flow Qd = 6*75=450 gpd
Septic Tank Design: Use a 1000 gallon, two compartment septic tank with an external bio -tube dosing assembly in a
30 -inch diameter pump basin assembly.
Sub Surface Conditions and Testing
Subsurface soil investigations and USDA soil texture method tests were performed by Sopris Engineering. The soils
in the area of the proposed absorption field were sampled and characterized by application of the USDA soil texture
method test. The soils were tested to determine the native soils hydraulic loading rate and to analyze soil/rock
content for installation of an intermittent sand filter treatment system. The soils below 3 to 8 inches of topsoil
consist of medium dense gravelly sandy loam material to 1.5 to 5 feet below the surface with 25% rock content and
scattered boulders . No limiting soil conditions were encountered. The soils are assumed to be consistent across the
site.
The native soils sampled from 2-3 feet below the surface are characterized as a soil type 2 consisting of sandy loam
texture with moderate granular structure. 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. Due to limited area and to provide optimal treatment
an intermittent sand filter is recommended to provide for level 3 treatment. No free water was encountered in
excavations on site. Groundwater levels are expected to be below 10 feet from the existing surface grades.
The sand filter treatment bed system that will be fully pressure dosed.
An additional soil evaluation will be performed during construction to verify the soils in the proposed field
excavation and to confirm that appropriate sand filter media is installed.
OWTS Design and Construction
Helffenstein Residence
SE Job No. 19033.01
March 7, 2019
Page 3
Intermittent Sand Filter 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 and gravelless chamber
units as spray shields providing level 3 treatment prior to infiltration into the native soils. The new treatment system
will consist of a fully pressure dosed sand filter Treatment bed System with a 36 -inch layer of primary filter sand
(ASTM -33, "washed ASTM -36 Concrete sand").
Long Term Acceptance Rate (LTAR)
Considering the application of the new state regulation 43:
Receiving Wastewater Treatment Level 1; loading rate for primary sand filter material is (1.0 gal./S.F./day) for the
minimum 36" depth of treatment media; Loading factors; (Table 10-2, pressure dosed application bed = 1.0 )
A(sf) = Qd .: A = Area;
LTAR
A(sf) = 450 gpd x 1.0 = 450 S.F.
1.0 Gal/S.F./day
LTAR = 1.0 Gal/S.F./day - primary ASTM -33, " Washed Concrete sand"
Qd = flow (gal/day) = 450 gpd
L.F.1=1.0 pressure dosing
Install two 19' by 12' sand filter beds. Excavate the bed to a maximum 4.5 depth below finish grade. Scarify surface
and backfill with 36 -inches of ASTM -33, "Washed Concrete sand". Typical for each bed, install 3 runs of custom
perforated distribution laterals on top of sand layer and cover with 36 Quick -4 chambers in 3rows with 4 chamber
units per row for use as pressure dose spray shields, as delineated on the plan. The beds will be dosed from the
simplex high head distribution pump system installed in the dosing pump basin.
The standard Quick 4 chamber spray shields shall be installed over the top of the sand on 4 foot centers and
backfilled with a minimum 12 -inch depth of cover. Filter fabric shall be placed over the chambers and the entire
extents of the sand filter bed. The slightly mounded field shall be constructed with final surface grading at a
minimum 6 -inches above the up gradient surface grades with a minimum 3 -inch topsoil layer placed over the field
surface.
The chamber rows shall be completed with standard end caps installed at both ends. A piezometer monitoring well
pipe riser with threaded cap will be installed near a corner of the field and be extended down to the excavated
surface depth at the bottom of the placed sand.
Effluent Distribution System
The OWTS field shall be fully pressure dosed along the entire length of the distribution rows. An Orenco Systems
Bio Tube pump vault system will be installed with an effluent screen in the pump basin adjacent to the septic tank.
The pump system shall incorporate a single high head pump. The simplex pump assembly will demand dose
effluent through a single 1.25 -inch diameter PVC transport pipe from the pump assembly and connect to manifold
pipes that connects to the three custom perforated pipes at the head of each bed. The 1.25" discharge transport
pipe shall be installed with a minimum 2.5% continuous slope to allow for drainage of the pipe.
OWTS Design and Construction
Helffenstein Residence
SE Job No. 19033.01
March 7, 2019
Page 4
The effluent will be equally pressure dosed through each 1.25 -inch custom perforated distribution lateral running
the length of the treatment beds installed on top of the sand filter layer. The pipe will have 3/16 -inch orifices drilled
every 2.0 feet oriented upward to spray against the interior chamber unit that will broadcast the effluent onto the
sand surface. The first and last orifice will be drilled and oriented downward with an orifice shield to allow
drainage of the line. Individual 1.25" ball valves will be installed at the end of each perforated distribution lateral
centered below one of the pair of end cap vertical inspection port penetrations at the terminal end of the field. Two
vertical 4 -inch inspection ports will be installed from the pair of end cap inspection port penetrations at the terminal
end of the field and one 4 -inch inspection port will be installed from either end cap penetration at the head of the
field. All inspection ports shall extend a minimum 3 -inches above the finish grade and be topped with a treaded cap.
Pump operations shall be controlled by a wall mounted exterior control panel in direct line of site with the tank.
Automatic float switches for on, off and high water with alarm activation shall be installed on a dedicated PVC float
tree. Two separate circuits from the electrical circuit panel and/or junction box shall be routed to the control panel
for separate power to the controls and pump.
Construction and Inspections
Prior to construction of the permitted system, the engineer should be contacted by the contractor and construction
project manager 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 and calls are essential and required prior to installation of the OWTS.
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 location of the absorption treatment field to
confirm design assumptions made and existing conditions. The engineer will provide construction recommendations
and make adjustments to the field layout and sizing if needed with discussion of construction procedures.
As necessary a revised Construction document drawing will be submitted to the County as needed if changes are
required. All installations shall be inspected, surveyed, documented and reported/delineated in the Certification
letter and As -built drawing.
County Regulations require that the Design Engineer 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. Photo documentation during construction and site system inspections is required to be performed by
the contractor and Engineer. System component installation inspection and review of photo documentation is
required of the exposed system components prior to backfilling. Photo documentation will be included with the As -
Built Record drawing submittal package to finalize the OWTS permit. Therefore the Engineer should be contacted
as needed and notified of the construction progress and scheduling to allow time to schedule specific inspection
times.
OWTS Operation and Maintenance
All components of the engineered OWTS shall be inspected on a regular basis and be properly maintained. The
responsibility for repair and maintenance of the system will remain with the property 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
OWTS Design and Construction
Helffenstein Residence
SE Job No. 19033.01
March 7, 2019
Page 5
requirements. The operating components of the OWTS shall be inspected by the engineer within 30 days of being
placed into operation and thereafter every six months by the owner or qualified service provider. 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.
If you have any question or need any additional information, please call.
Sincerely,
SOPRIS ENGINEERING, LLC
Paul E. Rutledge
Design Engineer
Yancy Nichol, PE
Principal