HomeMy WebLinkAbout1982-01-05 Septic System Design ReportEVDO
..iI,•'I.O L,T ._
TO: Jeanne A. Mabusth, Zoning Administrator
FROM: Michael P. Gaffron, Septic 0 stem Inspector
DATE: January 5, 1982
SUBJECT: Ringer Subdivision, North Willow Drive #615
su-mary: All proposed lots were found to have Nigh 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 tiv, entire
site are fairly level and suited for mound systems.
This report will consist of: a) a short review of each lot of the
prcposed 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 rianage.,,ent plan. The Ringer subdivision is
perhaps the most extensive development in Orono to da that will
require all mound systems.
A. Ringer Subdivision
A preliminary review of the Hennepin County Soil Survey indicated
Pxtersnive areas of Cordova silty clay loam and Shorewood silty
clay loam as well as Leseur lo,m would be found at this site.
Thesc soile all would be expected to have a seasonal water table
or saturation zone between 1 and 3 i,:et below the surfar:.
During the course of the soils investigation, Imet with the site
evaluator, Carl Borg, and realtor Graydon McCulley on the
site. We made a Loring which definitely indicated these high
water table soils were present. I Ater confirmed with Dr. James
L. Anderson of the University of ;-;innesota Soils Extension
Department that mound systems would be.appropriate for these soil
types. These are much the same soils as round in the McAina-
florningside area, which experience conventic,.,al trench septic
system failures duff to the so4l conditions. In cjoneral, the
loamy topsoil characteristics for the Ringer property are suitable
for proper functioning of ftandard preaszure-mound se,)t`_c systems
as per Orono's Code and Dsr_sign Manual.
For cl.arity's sake I nave la`3eled the northern m, st 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 PS16. Borings BB11 thru BB13. -,-erage perc
rates for both primary and alternate sites are acceptable. Slopes
are quite level. Saturation indications (mottling) started at
1.7 ti 2.5 feet. Actual water rible at 5-7' depth. A mound
system should be used due to th,: presence of shallow saturation
zones.
DATA
CuMPILATION
BY LOT
Average perc.
# of
Average
Depth to
Reco±ri-*ended
�
rate
holes
slope
mottling
GysLem
m
1
A
Lot
1,
Block
1
Primary
3
mpi
4
0-3 %
2.5
ft.
Hound
Secondary
52
mpi
2
0-3 %
1.7
ft.
Hound**
c
Lot
2,
Block
1
eximary
9
mpi*
3
0-? %
2.2
ft.
Mound
Secondary
59
mpi
2
0-2 %
2.0
ft.
Mound
�'•
m
Lot
3,
Block
1
Primary
18
m; i*
3
4-6 %
1.5
ft.
Mound**
0
o
Secondary
11
mpi
2
8-10 %
1.5
ft.
Hound**
-•
Lot
4,
Block
1
Primary
12
mpi
a
6-8 %
2.8
ft.
Mound
Secondary
28
mpi
t
5-7 %
2.0
ft.
Mound
Lot
5,
Block
1
Primary
14
mpi
4
6 %
2.7
ft.
Mound
Secondary
148
mpi
2
2-3 %
2.0
ft.
Mound /1
Lot
6,
Block
1
Primary
39
mpi*
3
2-4 %
2.4
ft.
Mound
Secondary
20
mpi*
1
2 %
2.5
ft.
Mound
Iot
7,
Block
1
Primary
87
mpi*
3
2-3 %
2.5
ft.
Mound
Secondary
135
mpi
2
2 %
2.5
ft.
Mound /1
Lot
1, Block 2
Primary
73 mpi* 3
0-2 %
2.7 ft.
hound
f1 V
d,er&
SL.:ondary (a)
54 mpi* 1
0-2 %
2.2 ft.
Hound
ub Z
Secondary (b)
84 mpi 2
0-2 %
---
Mound 4
Lot
2, Block 2
Primary
63 mpi 4
0-2 %
2.3 ft.
Mound
Secondary
84 mpi* 1
0-2 %
2.0 ft.
Mound
*
Average perc
rate is based
on one additional test
hole which
did not per -,.late
fast enough to be measurable, hence average may be somewhat hirjher.
**
Mound system
should have at
least 24" sand base.
Due to high
water tables,
all
other mounds
noted on Cris
page ahould have a'- least
18" sand
base.
/1
Average percolation rate is
higher than normally
allowed for
a mound system
by code.
well OPI-1--
t
1
0615 Ringer Subdivision
Page 5
T.ot 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 use4 at both sites.
Lot 7, Block 1
Peres PB71 thru PB76. Boring3 BB71 thru BB73. Slopes average 2-3i.
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). Actual water level ranged from 1.7
to 3.5' with mottling generally below 2.51. A standard mound is
recommended for both sites.
Lot 1, Block 2 fyUAL j pT & &LOCK Z
Peres PA11 thrs PA18. Borings BA11 thr BA13. Slopes are very
flat. Mottling was indicated beio Z with actual water
encountered below 2.01. Two secB° y 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.
1ottling occurs at 2.0 to 2.4 with actual water encountered
below 2.4'. Perc rates on both s'_,ss are acceptable. A mound
system is recommended for both sites.
Lot 3L Block 2
Peres PA31 thru PA36. Borings BA31 thru DA33. Slopes averaged
2 to 5%. Mottling starts at a depth of 2.0-3.5'. Actual water
level was below 2.51. Perc: rates are'bcceptable. Mound systems
are recommended for both sites.
Lot 4, Block 2
Peres PA41 thru PA46. ^.orings BA41 thru BA43. Slopes are very
flat. Mottling generally was found below 2.01, with actual
water level at about 4.01. 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, nloek 1. I recommend a mound system for both sites.
Let 5, 'dock 2
Peres PA51 thru PA56. Borings BA51 thru DA53. Slopes ara very
flat. Mottling occurred below 1.2 foot with actual water below
4.0 feet. Perc rates are acceptable for the primary site and
-lightly slow at the secondary site. (Rofer to Lot 5, Block 1)
TLis lot has the overall worst .yet of percolation sates of the
division. 'However, only two of the peres were taker in the
topsoil, the others, two of which were very slow, wire at depth.
1 reconunend 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 runner.
1615 Ringer
. page 6
Lot 6, Block 2
j?eres PA61 thru PA66. Borings BA61 thru BA63. Slopes are nearly
level. Mottling occurred below 1.51, with actual water
encountered below 5.71. (Note that soil appears saturated
below 2.5 ) Perc rates were acceptable, although t of 6 taken
at depth very very slow. Both primary and secondary sites should
use mound systems.
Lot 1, Block 3
Peres PC11 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 Vie entire subdivision and appears to be the only lot possibly
suited for a shallow trench system. Hn attempt should be made
to locate the existing system to ens, -ire it is within the lot
boundaries, although there is only a r^_mote possibility that it
isn't.
Lot 2, Block 3
Perce 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. do
evidence of system failure is noted. While the existing system
undoubtedly consists of trenches, soils data provided for a future
d.rainfield site indicates mottling at a 3.5' dopt;s. 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 he advisable to locate the existing system t.i ensure
it is within tl c new lot boundaries.
B. Discussion of Mound Systems
Since the proposed Ginger subdivision appears to need mound systems
throughout, it might be worthwhile to review the conditions which
favor use of mound sy9tem3 over conventional trench systems.
As you are awarn, a bi.omat forma at the rock-sr-1 interface in a
standard tre�ich syjtem. This biomat zncts as a "valve" to slow
the downward percolation of septic tank effluent to a rate wire
the flow is unonturated, i.e. air :n the soil pores is in contact
with the percolating effluent. Under those essential conditions,
adequate sewage treatment will occur. Howe e r, in many coils,
either a seasonal z turated condition or extrLmely f+ne, impermeable
soil texture (or both, tends to oliminate tho ;-Ar/effluent contact
and also severely liraits th- soils capacity to aLsorb large amou:its
of effluent. To ensure that a standard tren.;h system is not
*615 Ringer Subdivision
Page 7
installed in these limiting soils, a set of governing criteria
have evolved over the last 50 years. Specifically, trench systems
are not allowed: 1) where the percolation rate is Plower than 60
minutes per inch of water level drop in a percolation test hole;
and 2) where the highest known water table or indicated saturated
soil layer is less than 3' below the bottom of the trench.
In most cases, these same soils have a varying amount of topsoil
which is usually "loamier" and will accept and treat a limited
amount of effluent.
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 throng:, the topsoil,possibly even past the boundaries
of the mound system (•tit 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 liAely 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 %,'-ere 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 .nethod of sewage treatment.
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mr,_ .lug titNCER
EXP!AIT
November 27, 1981
P.J. Box 209
Wayzata. Minnesota 55391
Mr. Judd Ringet
405 South Willow Drive
Long Lake, Minnesota 55356
Dear Mr. Ringer*
We have completed a site evaluation for on -Ate sewage treatment facilities
of 15 proposed lots located fast of Willow Road and North of Trunk Highway
No. 12 in Orono, Minr,:sota. A location nap has been enclosed showing the
location of the soil borinCe and percolation tests. Soil boring logs and
percolation test data is also enclo3ed.
The soil boring show a untfornly hi6l. wa,.er 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 recommend that the future
homes be provided with Mound Systems of soil absorption.
Please call if you have any questions.
Thank you.
Yours very truly.
QZ,V,Z �-i V1,
CARL W. BORG, P.E. el
CWB:vl
cc: Mr. Hichael r.Affron
City of Orono
Enc. 3 copies - Location `tap
Sheets 1 - 3 - Soil Borings
1 Sheet Percolation Test Data