HomeMy WebLinkAboutEngineer ReportBob Oddo
Oddo Engineering, Inc .
30:? Eighth Street, Suite 325
P.O. Drawer 160
Glenwood Springs, CO 81602
June 2, 20101
RE: Oddo Residence-Expanded As-Built OWS System, 2425 County Road 137, Garfield County Colorado
SE Job No . 22108.01
Dear Bob:
Pursuant to your request, this letter provides documentation that the existing onsite wastewater system (OWS)
absorption field has adequate capacity for additional usage and installation ofOWS components associated with the
proposed accessory dwelling unit. The OWS system additions recently installed is in general compliance with the
original permined design and performance specifications. The absorption field system constructed on site has the
capacity to serve the needs of an equivalent 5-bedroom home. Sopris Engineering has performed visits to inspect
and document the as built conditions of the previously constructed system and the recent additions. We have
coordinated our efforts with the owner and contractor that are building the ADU. The as built conditions and
installation of the new OWS components are in compliance with the County and State ISDS Rules and regulation s.
The existing system components were installed in accordance with Garfield County Regulations and the design
requirements . The absorption field sizing was based on percolation testing perfonned on May 10. 2002 . The onsite
soils in the area of the absorption field have an average percolation race of 6 mpi. The absorption field \VSS
constructed with 3 parallel chamber rows in trench configuration with 10 chambers per trench utilizing EQ-36
infiltrator gravelless chambers. The trenches were installed in suitable soil consistent across the field area. A 6''
layer of filter sand was installed in the over excavated trench beds to enhance treatment and provide for consistent
absorption. Inspection pons were installed on each end chamber . The field is approximately 80' long by 18' wide
with a minimum of 6 feet ofundisrurbed soil between the sidewalls of each trench. The maximum depth to the
infiltrative surface within the trenches is approximately 36 inches. A 1250 gallon dual companment septic tank
was installed with 4" tee baffles and an effluent filter to accept sewage from the 4-bedroom home. A two-way
cleanout was installed between the house and the tank. A distribution box was installed with 3 individual
distribution lines connected to each trench. The construction was completed utilizing specified materials installed
according to the recommendations and design calculations by Sopris Engineering, LLC. on August 14, 2002 . The
minimum setback distances have been maintained .
A 4" sewer pipe stub out was installed on the influent line to the distribution box for a future bathroom in a
proposed barn structure ._
Recent additions and upgrades to the system were perfonned to provide sewer service to a barn structure with a
small ADU. The Additional OWS components installed include a 1000 gallon, two chamber polyethylene septic
tank installed with a simplex effluent pump and screen vault to transport septic effluent from the new tank to the
distribution box adjacent to the existing absorption field. A 2" PVC transport line was installed from the pump
dosing assembly in the new tank to a 4" stub out pipe connected to a wye fining on the existing gravity inlet pipe
installed into the Distribution box. The existing distribution box was replaced with a new tuf-lite box and all piping
was re installed with a full body wye installed on the 4" inlet pipe to provide the connection from the new pressure
line. The D-box was encased in a concrete box with a access lid near the surface . During construction Sopris
Engineering inspected the existing trenches to assess their performance. The existing absorption trenches are
functioning as expected. The infiltrative surface was moist and evidence of bio mat material was observed . No
I 502 Main Street •Suite A3 • Carbondale, CO 81623 • (970) 704-0311 • Fax (970) 704-0313
SOPRIS ENGINEERING • llC civil consultants
Bob Oddo
SE Job No. 22108.01
June 2, 2010
Page2
evidence of limited capacity, saturation or anaerobic bacteria was observed. Therefore the capacity of the system
appears to be adequate as designed and installed.
The system design capacity is based on the following design flow and field sizing.
The design flow for an equivalent 5 bedroom home, population @ 2 person /bedroom = 10 persons, is
calculated as follows:
Design flow in gallons per day based on per 75 gal/person/day
Qd = 10•15•1.5• =1125 gal/day
A soil absorption trench system. utilizing gravelless chambers was installed.
Based ~pon the design percolation rate, the standard absorption area equation is:
A (SF) = Od •(t) i.: : where
5
A== Area: Qd =Design flow (gal/day)
t = time in minutes
This design calculation results in a minimum absorption area:
A= l 125 *(6) '• = 551 sq. ft.
5
Apply a 50% reduction for utilizing gravelless infiltration chambers in a trench configuration.
551 x 0.5 = 276 sq. ft.
Assume 15.5 square feet per chamber: 276 sq.ft. = 18 chambers
15.5 sq.ft/chrunber(50%)
For (1-10) mpi percolation rates the long term acceptance rate (LT AR). = 1.30 gpd/sf:
A(S.F.)=Qd/LTARx0.5=1125gpd / 1.30 gpd/sf x 0.5= 433 S.F.
A total of 30 chambers were installed resulting in a total absorptive area of (30)(15.5)=460 S.F. The field is
over sized to account for LT AR.
If you have any question or need any additional infonnation, please call.
Sincerely,
SOPRIS ENGINEERING, LLC