HomeMy WebLinkAbout1985-01-06 Septic System Design ReportSYSTEM DESIGN
FOR B b N CORPORATT^'1
LOT 4, BLOCK 2, RINGERSWOOD
ORONO, MINNESOTA
1-6-Pq
1. Percolation Rate, Type of System.
Percolation tests and soil rings were taken by others. From informa-
tion supplied, the primary site had an average percolation rate of
107 MPI, and a possible high water table at 1.2 feet. Thus, a pressure
MO add system according to basal width will be needed.
2. Proposed Flow Rate.
pr�posed bedrooms @ 150 GPD is 600 GPD.
3. Soil Treatment Area Needed.
600 GPD x 0.83 factor is 500 Sq. r;.. of rock bed area or i bed 10 feet
wide by 50 feet long.
4. Septic Tanks and Distribution System.
Two septic tanks of 1000 gallons each are recommended. Also, a third
humping tank will be needed to house the pump required for pressure
distribution to the mound area. Distribution to the rock bed is to be
through 3 perforated laterals 40" apart. All construction and materials
should adhere to the provisions of the City of Orono.
5. Mound Dimensions.
For a percolation rate between 61-120 MPI a factor of 5.00 is used
resulting in a basal width rf 5.OU x the rock bed width .f 10 feet
or 50 feet. Since the ground slope is from 0-3% both the downslope
and upslope can be used for this width. Because of the high possible
water table, a 2 foot layer of clean sand will be needed under the
mound, with the total height of the mound being 4 feet. Thus,since
the slopes have to extend out about 20 feet from each side to get
the required basal width, the sideslopes would be about 5:1 which
would be better than the 3:1 required mininum.
Sincerely,
PERCOR, INC.
Mark S. Gronberg, P.E.
#615 Rinyer Si1xli• i-slOn
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F,age
Ringer Subdivision
5
� J Lot 6. Block 1
63 Avera e pert
Peres 1'B61 thru PB66. Borings BB61 thruSloBB 9 -
rates at both sites are acceptable.
roximatel es r2n4efeetm with 1 to 4�.
}' Mottling started at a depth of app Y
.5 to 4.4'. A standard mound system
actual water level at ?
should be used at both sites.
Lot 7, Block 1
Peres PB71 thru PB76. Borings BB71 thru BB73. Slopes average 2-38.
Perc rates were fairly slow, with the secondary site being somewhat
slower than the limit (however, the justification for Lot 5,
Block 1 also applies here). 4ctual water level ranged from 1.7
to 3.5' with mottling generally below 2.5'. A standard mound is
recommended for both sites.
Lot 1, Block 2
Peres PA11 thru PA18. Borings BAll t)lru BA13. Slopes are very
flat. Mottling was indicated below 2.2 ' with actual water
encountered below 2.0'. :wo secondary sites were tasted. All
sites are acceptable for a standard mound system.
Lot 2, Block 2
Peres PA21 thru PA26. Borings DA21 thru DA23. Very flat slopes.
Mottling occurs at 2.0 to 2.4' with actual water encountered
below 2.41. Perc rates on both sites are acceptable. A mo•ind
system is recommended for both sites.
Lot 3, Block 2
Peres PA31 thru PA36. Borings BA31 thru BA33. Slopes averaged
2 to 5%. Mottling starts at a depth of 2.0-3.51. Actual water
luvel was below 2.5'. Perc rates are acceptable. Mound systems
are recommended for both sites.
Lot 4, Block 2
Peres PA41 thru PA46. Borings DA41 !-hru DA43. Slopes are very
flat. Mottling generally was found below 2.0', with actual
water level at about 4.01. Perc rate for primary site is
acceptable; pert rate for se—ndary site is slightly slow but
acceptable for a mount: t--used :)n the justification referred to
for Lot 5, Block 1. 1 recommend a mound system for both sites.
Lot 5, Block 2 =rNRL Lc,r ZPeres PA51 thru PA56. Borings Y5lP'thOru BA53. Slopes are very
flat. Mottling occurred below 1.2 feet with actual water below
4.0 feet. Perc rates are acceptable for the primary site and
slightly olov at the secondary site. (Refer to Lot 5, Bloc% 1)
This lot has the overall. worst set of p^-colation rates of the
di vision. 'However, only two of the Peres .--re taken in the
topsoil, the others, two of which were very slow, were at depth.
I recommend that a mound system be used -)n this sicc. If peres
were re -taken in the topsoil, it is likely they would ljass as
(lid nearly all others done in this manner.
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EXHIBIT
November 27, 1981
P.O. Box 209
Wayzata, Minnesota 55391
Mr. Judd Ringer
405 South Willow Drive
Long Lake, Minnesota 55356
Dear Mr. Ringer:
We have completed a site evaluation for on -site sewage treatment facilities
of 15 proposed lots located East of Willow Road and North of Trunk Highway
No. 12 in Orono, Minnesota. A location map has been enclosed showing the
location of the soil borings and percolation tests. Soil boring logs and
percolation test data is also enclosed.
The soil boring show a uniformly high water table throughout the area tested.
This condition prohibits the installation of conventional soil absorption
systems.
The gentle slopes and percolation rates encountered indicate that Mound
Systems should be satisfactory on these sites. We recomend that the future
homes be provided with Mound Systems of soil absorption.
Please call if you have any questions.
Thank you.
Yours very truly,
CARL W. BORG, P.E. e
CNB:vl
cc: Mr. Michael Gaffron
City of Orono
Ene. 3 copies - Location `lap
Sheets 1 - 3 - Soil Borings
1 Sheet Percolation Test Data
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#615 Ringer Subdivision
• Page 7
installed in these limitin
have evolved over the last
are not allowed: 1) where
minutes per inch of water
and 2) where the highest k
soil layer is less than 3'
In most cases, these same
which is usually "loamier"
amount of effluent.
g soils, a set of governing criteria
50 years. Specifically, trench systems
the percolation rate is slower than 60
level drop in a percolation test hole;
nown water table or indicated saturated
below the bottom of the trench.
soils have a varying amount of topsoil
and will accept and treat a limited
The pressure -mound system is designed specifically to use this
loamy topsoil layer for effluent treatment and disposal. In
brief, a 12-24" layer of sand (placed over the natural roughened
topsoil) is fed effluent from a pressure distribution system in
an overlying rock bed. The biomat "valve" forms at the rock/
sand interface which is 12-24" above the natural soil. Because
of the pressure distribution, an unsaturated air/effluent contact
is maintained as treatment occurs in the sand bed. This treated
effluent is then dispersed into the topsoil and allowed to move
laterally through the topsoil,possibly even past the boundaries
of the mound system (yet still below the surface) until it
eventually seeps downward or evaporates over a wide area.
It would be poor planning for Orono to allow installation of
standard trench drainfields in soils which have high-water table
characteristics, since many systems likely would eventually fail,
as was the case in Medina-Morningside. Since Orono has shown
a strict desire to limit the extension of sanitary sewers, it
appears reasonable to allow (and, in fact, promote) the use of
alternate on -site sewage treatment methods where development
pressures persist. Mound systems have been installed over the
last 10 to 15 years around the country as an alternative to
trenches. In Orono, pressure distribution mounds were first
installed in 1978. To date, the City'has 13 pressure mounds
in service, with no recorded failures.
In my opinion, under specified site conditions, mound systems
are a safe, sanitary, reliable method of sewage treatment.
#615 Ringer Subdivision
Page 6
Lot 6, Block 2
Peres PA61 thru PA66. Borings BA61 thru BA63. Slopes are nearly
level. Mottling occurred below 1.5', with actual water
encountered below 5.7'. (Note that soil appears saturated
below 2.5') Perc rates were acceptable, although � of 6 taken
at depth very very slow. Both primary and secondary sites should
use mound systems.
Lot 1, Block 3
Peres PCll thru PC14. Boring BC11 and BC12. This lot contains
an existing house, garage and barn. The existing septic system
is unknown and was not located during my routine inspection in
July 1981, although no evidence of failure was noted. A future
drainfield site has been tested to the south of the house. Average
percolation rate is 21 minutes per inch; soil borings indicate
soils suited for a shallow trench or mound system, with mottling
beginning at a depth of 51. This is the highest elevated point
in the entire subdivision and appears to be the only lot possibly
suited for a shallow trench system. An attempt should be made
to locate the existing system to ensure it is within the lot
boundaries, although there is only a•remote possibility that it
isn't.
Lot 2, Block 3
Peres PC21 thru PC24. Borings BC21 and BC22. This lot contains
an existing house and small shed. The existing septic system is
unknown and was not located during my inspection last July. No
evidence of system failure is noted. While the existing system
undoubtedly consists of trenches, soils data provided for a future
drainfield site indicates mottling at a 3.5' depth. This indicates
that the replacement system, should it ever be necessary, would
probably have to be a mound. Percolation rates averaged 56 mpi.
It would alto be advisable to locate the existing system to ensure
it is within the new lot boundaries.
B. Discussion of Mound Systems
Since the proposed Ringer subdivision appears to need mound systems
throughout, it might be worthwhile to review the conditions which
favor use of mound systems over conventional trench systems.
As you are aware, a biomat forms at the rock -soil interface in a
standard trench system. This biomat acts as a "valve" to slow
the downward percolation of septic tank effluent to a rate where
the flow is unsaturated, i.e. air in the soil pores is in contact
with the percolating effluent. Under these essential conditions,
adequate sewage treatment will occur. However, in many soils,
either a seasonal saturated condition or extremely fine, impermeable
soil texture (or both) tends to eliminate the air/effluent contact
and also severely limits the soils capacity to absorb large amounts
of effluent. To ensure that a standard trench system is not
_ #615 Ringer Subdivision
Page 5
Lot 6, Block 1
Peres PB61 thru PB66. Borings BB61 thru BB63. Average perc
rates at both sites are acceptable. Slopes range from 2 to 4%.
Mottling started at a depth of approximately 2.4 feet, with
actual water level at 2.5 to 4.41. A standard mound system
should be used at both sites.
Lot 7, Block 1
Peres PB71 thru PB76. Borings BB71 thru BB73. Slopes average 2-3%.
Pere rates were fairly slow, with the secondary site being somewhat
slower than the limit (however, the justification for Lot 5,
Block 1 also applies here). Actual water level ranged from 1.7
to 3.5' with mottling generally below 2.5'. A standard mound is
recommended for both sites.
Lot 1, Block 2
Peres PAll thru PA18. Borings BAll thru BA13. Slopes are very
flat. Mottling was indicated below 2.2 ' with actual water
encountered below 2.0'. Two secondary sites were tested. All
sites are acceptable for a standard mound system.
Lot 2, Block 2
Peres PA21 thru PA26. Borings BA21 thru BA23. Very flat slopes.
Mottling occurs at 2.0 to 2.4' with actual water encountered
below 2.4'. Perc rates on both sites are acceptable. A mound
system is recommended for both sites.
Lot 3, Block 2
Peres PA31 thru PA36. Borings BA31 thru BA33. Slopes averaged
2 to 5%. Mottling starts at a depth of 2.0-3.5'. Actual water
level was below 2.5'. Perc rates are*acceptable. Mound systems
are recommended for both sites.
Lot 4, Block 2
Peres PA41 thru PA46. Borings BA41 thru BA43. Slopes are very
flat. Mottling generally was found below 2.0', with actual
water level at about 4.0'. Perc rate for primary site is
acceptable; perc rate for secondary site is slightly slow but
acceptable for a mound based on the justification referred to
for Lot 5, Block 1. I recommend a mound system for both sites.
Lot 5, Block 2 F7A/*L `Or y JeLO� Z
Peres PA51 thru PA56. Borings BA51 thru BA53. Slopes are very
flat. Mottling occurred below 1.2 feet with actual water below
4.0 feet. Perc rates are acceptable for the primary site and
slightly slow at the secondary site. (Refer to Lot 5, Block 1)
This lot has the overall worst set of percolation rates of the
division. 'However, only two of the peres were taken in the
topsoil, the others, two of which were very slow, were at depth.
I recommend that a mound system be used on this site. If peres
were re -taken in the topsoil, it is likely they would pass as
did nearly all others done in this manner.
#615 Ringer Subdivision
Page 4
Lot 2, Block 1
Peres PB21 thru PB26. Borings BB21 thru BB23. Average perc rates
for bDth primary and alternate sites are acceptable. Slopes are
nearly level at the tested sites, ranging from 0 to 3%. Mottling
indicated below 2', with actual wa-_er table at 3.2 to 4.21. A
standard mound system should be used due to the presence of shallow
saturation zones.
Lot 3, Block 1
Peres PB31 thru PB26. Borings BB31 thru BB33. Slopes in the
primary site are approximately 6-8%, on a slight flattening
Of the 10-12% slopes surrounding it. Borings in the primary site
indicate 1 to 2 ' of loamy topsoil over a 6" layer of organic clay
topsoil over a plastic sandy clay. Actual water table at the
primary site was below 5'. However, the high plasticity indicates
a heavy clay that is probably saturated. The secondary site was
mottled below 1.5 feet with an actual saturated zone at 1' depth.
Coupled with a slope of 10% the secondary site is marginal for a
mound system and would require at least 2' of sand base as part
of the design. I suggest that a standardpressure mound system
be required for both sites with added sand depth for the secondary
system. Please note that this lot is the most tenuous in the
subdivision in terms of slope and soils consideration for using
a mound system. Should a proposed house location encroach on
either of the tested sites, careful review and possible additional
testing will be needed to ensure that a mound can still be used.
The site does not appear capable of supporting any kind of a trench
system.
Lot 4, Block 1
Peres PB41 thru PB46. Borings '13B41 thru BB43. Slopes average 5% -
7% overall. Borings indicate actual'water table at 4' with mottling
at a depth of 3-4' for the primary and 21 at the secondary site.
Perc rates were fairly uniform throughout these tested sites. A
depression/drainageway to the immediate south of the tested sites
must be avoided during construction of the systems. I would
recommend a standard pressure mound for both sites.
Lot 5, Block 1
Peres PB51 thru PB56. Borings BB51 ,:hru BB53. Slopes are 38 to
6% overall. Mottling at the primar}- site started at 2.7 to 3.5'
with actual water table at 3.7 to 5.0'. This site is acceptable
for a standard mound system. The alternate site has a perc rate
considered too slow for any system by code. However, close
scrutiny shows that the Peres were taken at depth in the sandy
clay subsoil, not in the topsoil. In general, throughout the entire
subdivision the testing done at a depth of 12", in the topsoil,
indicates acceptable percolation rates, with testing at 24" or more
indicating slower, marginal perc rates. In my opinion, this site
is no different than most of the others, and would have an acceptable
perc rate if tested at the 12" depth in the sandy loam topsoil.
No further'testing should be required; a mound system is recommended.
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EXh1t31T �N
TO: Jeanne A. Mabusth, Zoning Administrator
FROM: Michael P. Gaffron, Septic System Inspector
DATE: January 5, 1982
SUBJECT: Ringer Subdivision, North Willow Drive #615
Summary: All proposed lots were found to have high water tables,
necessitating the use of mound systems. Percolation rates in the
upper soil profile generally were suitable for mound systems,
with slower rates found in deeper test holes. Slopes on thn entire
site are fairly level and suited for mound systems.
This report will consist of: a) a short review'of each lot of the
proposed division and b) a discussion of mound systems,including
how and why they work and their current and future use as part of
Orono's land use management plan. The Ringer subdivision is
perhaps the most extensive development in Orono to date that will
require all mound systems.
A. Ringer Subdivision
A preliminary review of the Hennepin County Soil Survey indicated
extensive areas of Cordova silty clay loam and Shorewood silty
clay loam as well as Leseur loam would be found at this site.
These soils 1-11 would be expected to have a seasonal water table
or saturation zone between 1 and 3 feet below the surface.
During the course of the soils investigation, I met with the site
evaluator, Carl Borg, and realtor.Graydon McCulley on the
site. We made a boring which definitely indicated these high
water table soils were present. I later confirmed with Dr. James
L. Anderson of the Unijersity of Minnesota Soils Extension
Department that mound systems would be. appropriate for these soil
types. These are much the same soils as found in the Medina-
I4orningside area, which experience conventional trench septic
system failures due to the soil conditions. In general, the
loamy topsoil characteristics for the Ringer property are suitable
for proper functioning of standard pressure -mound septic systems
as per Orono's Code and Design Manual.
For clarity's sake I have labeled the northern most tier cf seven
lots as Block 1, the western group of six lots as Block 2, and
the two lots to the southeast as Block 3.
Lot 1, Block 1
Peres PBll thru PB16. Borings BB11 thru BB13. Average perc
rates for both primary and alternate sites are acceptable. Slopes
are quite level. Saturation indications (mottling) started at
1.7 to 2.5 feet. Actual water table at 5-7' depth. A mound
system should be used due to the presence of shallow saturation
zones.