HomeMy WebLinkAboutRe: sloope stability analysis STS Consultants,Ltd. voice 763-315-6300
10900 73rd Avenue North,Suite 150 fax 763-315-1836
STs CONSULTANTS Maple Grove,Minnesota 55369-5547 web w�ww.stsconsultants.com
June 24, 2003
ij-7S3 ti
Minnetonka Custom Homes
2950 Island View Drive
Mound, MN 55364
Attn: Mr. Bob Pieper
Re: Subsurface Exploration and Slope Stability Analysis for Single Family Dwelling Located at
4753 North Shore Drive in Orono, Minnesota; STS Project 99189
Dear Mr. Pieper:
You have demolished a former house at this address, and are planning to construct a new, larger house.
The City of Orono has requested that you address the stability of the slope adjacent to Lake Minnetonka.
Our services for this project were performed in accordance with the project scope outlined in our proposal
dated June 4, 2003. This work was authorized by Mr. Steven Sexton on June 6, 2003.
The purposes of this exploration are to:
• Perform a subsurface exploration and field testing program consisting of two soil borings to depths of
30 and 50 feet.
• Describe the soil and groundwater conditions encountered in our exploration.
• Perform appropriate soil laboratory tests to determine the shear strength and physical properties of
the soil within the potential sliding zone.
• Perform a slope stability analysis for this particular lot, including consideration of the loads imposed at
the top of the slope by the new house footings.
• Prepare a written report of our findings.
Procedure
STS recommended the testing program necessary, and selected the boring locations and depths. An
STS engineer staked the boring locations in the field, by measuring from the property lines, using the
survey plan provided by Demars-Gabriel Land Surveyors Inc. dated April 11, 2003. The approximate
boring locations are shown on the Soil Boring Location Diagram in the Appendix. The ground surface
elevations at the borings were determined by our drill crew, with reference to the pavement abreast of the
northeast property corner. The elevation of this reference point or benchmark was shown to be 990.9
feet National Geodetic Vertical Datum, on the survey plan.
We drilled the borings on June 11, 2003 with truck-mounted drill rigs operated by a two person crew. We
followed standard soil sampling procedures, and also obtained undisturbed thin-walled tube samples for
laboratory strength testing.
Recovered soil samples were described on field logs, containerized, labeled and transported to our
laboratory for further examination and testing. The field logs also document sample intervals, test data,
observations of drilling resistance, groundwater occurrence and other pertinent conditions.
After the final groundwater level observations, we backfilled the borings with high solids bentonite grout,
to conform to Minnesota Department of Health regulations.
IN -a a az:
Minnetonka Custom Homes
STS Project 99189
June 24, 2003
Page 2
Laboratory Testing Procedures
The penetration test split-spoon samples were visually examined by a geotechnical engineer to estimate
the distribution of grain sizes, plasticity, consistency, moisture condition, color, presence of lenses and
seams, and apparent geologic origin. The engineer classified the soils according to type using the STS
Classification System, which is closely based on the Unified Soil Classification System. A chart describing
the STS Classification System is included in the Appendix. An explanation of typical laboratory
procedures is presented in the Appendix.
We also determined the natural moisture content, density and unconfined compressive strength of
selected soil samples. Some of these data are shown on the boring logs, and some of the data are
appended at the rear of this report.
Exploration Results
This property is on the north shore of Jennings Bay within the West Arm of Lake Minnetonka. The height
of the slope surrounding the lake at this point is about 60 feet, from pavement level of County Road 19 to
average water level in Lake Minnetonka. The major naturally occurring soil type is low plasticity clayey
glacial till deposited by the Des Moines ice lobe of the Wisconsin glaciation. After this soil was formed, it
became desiccated, and is now over-consolidated (that is, it has been subject to stresses in the past
which are greater than the present overburden pressure). We found only minor amounts of surficial soils
including fill and topsoil. The major soil type to a depth of at least 50 feet is sandy clay with traces of
gravel and random thin sand lenses, some of which are water-bearing. Natural moisture contents of the
sandy clay vary from about 32% to 14%, and tend to decrease with increasing depth. Standard
penetration N-values are in the range of 16 to 41 blows per foot, representative of a stiff to hard
consistency. Unconfined compressive strengths were found to be 3400 to 5600 pounds per square foot.
Groundwater Conditions
Boring 1 extended to a tip elevation of about 960.7 feet at 30 foot depth, and was dry. In boring 2, we
observed groundwater at 25.6 foot depth while drilling corresponding to elevation 952.5 feet. We believe
that the groundwater in boring 2 emanated from a sand seam within the clay till profile.
The average long-term water elevation in Lake Minnetonka is 930 feet. The Lake Minnetonka water level
is controlled within a relatively small range of fluctuation by the Grays Bay dam.
Analysis and Recommendations
The clay soil at this site is competent. The house can be supported on conventional footings bearing
directly on the native sandy clay, or on compacted sandy clay fill. A conventional footing design may be
used. We recommend that the footing stresses on the clay subsoil should not exceed 2,000 pounds per
square foot. Continuous footings under bearing walls should be at least 22 inches wide for practical
reasons, regardless of the theoretical bearing pressure on the subsoil. Footings in unheated areas such
as under an entry canopy of under the overhead garage door should be based at least 5 feet below
outside finished grade for frost protection. Similarly, footings supporting an attached, elevated deck
should be based at least 5 feet below grade.
We recommend that a perimeter drain system be installed around the outside of the house, in accordance
with FHA Special Bulletin 87-1. The backfill around and over the drain pipe should be a free-draining
granular material such as pea gravel or 1-1/2 inch crushed rock. We strongly recommend that the on-site
clay soils not be used as wall backfill, except for the uppermost 12 to 18 inches. Clayey backfill against
Minnetonka Custom Homes
STS Project 99189
June 24, 2003
Page 3
the entire height of the basement walls would slow or stop the flow of infiltrating water to the drain pipes.
This could lead to a build-up of hydrostatic pressures against the walls, and contribute to seepage
through the walls.
The subgrade masonry walls should be thoroughly braced internally prior to backfilling. The soil used as
exterior basement backfill for a distance of at lest 18 inches from the walls should be a free draining sand
or sand and gravel mixture with less than 7% passing the No. 200 sieve. On-site clay soil should only be
used to cap the backfill at the surface. Wall backfill should be compacted sufficiently so that it does not
settle after construction, but it should not be over-compacted or it could create excessive lateral stresses
against the walls. Roof drainage should be carefully controlled.
Basement walls backfilled with a thin layer of sand surrounded by sandy clay should be designed to resist
the lateral pressure exerted by an equivalent fluid having a density of 52 pounds per cubic foot.
Slope Stability Analysis
We performed a slope stability analysis, using a limit equilibrium procedure. In this analysis, we also
factored in an allowance for a footing load on the soil of 2,000 psf. Based on our analysis, the minimum
factor of safety is about 3.1. Usually, for a slope of this type, a minimum of factor of safety against failure
of 2 is considered adequate.
Thus, in our opinion, this slope is stable. We wish to caution, however, that care should be taken to avoid
excessive erosion of the slope. Thus surface runoff water that drains over the top of the slope should be
channeled or controlled in such a manner as to avoid erosion of the soil.
A graphical depiction of our slope stability analysis is appended.
Construction Safety
All excavations must comply with the requirements of OSHA 29 CFR, Part 1926, Subpart P "Excavations
and Trenches". This document states that excavation safety is the responsibility of the contractor.
Reference to this OSHA requirement should be included in the job specifications.
The responsibility to provide safe working conditions on this site, for earthwork, building construction, or
any associated operations is solely that of the contractor. This responsibility is not borne in any manner
by STS Consultants, Ltd.
Field Observation and Testing
We recommend that the earthwork and footing installations for this project be observed and tested by a
geotechnical engineer or qualified engineering technician to determine if the soil and groundwater
conditions encountered are consistent with those anticipated based on our exploration. Foundation
subgrades should be tested to check for adequate bearing conditions. Subgrades for slabs, pavement
and new structural fill should be test rolled and unsuitable areas improved. Fill placement and
compaction should be monitored and tested to determine that the resulting fill conforms to specified
density, strength or compressibility requirements. Structural materials should also be tested for
conformance to specifications. STS would be pleased to provide the necessary field observation,
monitoring and testing services during construction.
Minnetonka Custom Homes
STS Project 99189
June 24, 2003
Page 4
General Qualifications
This report has been prepared to aid in the evaluation of the single family dwelling at this property and the
slope behind the house, and to assist the building contractor and/or engineer in the design and
assessment of this project. The scope is limited to the specific house building project and location
described herein, and our description of the project represents our understanding of the significant
aspects relevant to soils and foundations.
Standard of Care
The recommendations and opinions contained in this report are based on our professional judgment. The
soil testing and geotechnical engineering services performed for this project have been conducted in a
manner consistent with that level of skill and care ordinarily exercised by other members of the profession
currently practicing in this area under similar budgetary and time constraints. No other warranty, express
or implied, is made.
Sincerely,
SH CONSULTANTA LTD.v eviewed by:
Mervyn Mind s, P.E. James H. Overtoom, P.E.
Senior Project Engineer Principal EngineerNice President
MM/dn
Encs.: Soil Boring Location Diagram
Boring Logs
Unconfined Compression Strength Test Results
Slope Stability Analysis
STS Soil Classification System
C699189-1.doc
OWNER LOG OF BORING NUMBER
Minnetonka Custom Homes
1 PROJECT NAME ARCHITECT-ENGINEER
srs r.ft,„ffi u, Slope Stability Analysis for House
SITE LOCATION UNCONFINED COMPRESSIVE STRENGTH
4753 North Shore Dr., Orono, MN TONS/FT.2 3 4 5
LU PLASTIC WATER LIQUID
LIMIT% CONTENT% LIMIT%
° a DESCRIPTION OF MATERIAL X-----a----�
o Wa t" Ei 10 20 30 40 50
a 0. a. 0 STANDARD
SURFACE ELEVATION +990.7 NGVD z m ® PENETRATION BLOWS/FT.
10 20 30 40 50
1 AS Sandy clay,trace gravel,random thin sand seams or lenses at
21 and 29 ft.depths-gray-brown to brown-very stiff-(CL,
lenses SP)
HS
2 SS
HS /
3
3 SS
l �
HS
I
4 ST j
Qu=3400 PSF 121
I I
HS I
I I
I 12
5 SS • � '
I
HS I I
I
I
HS Qu=4600 PSF 112 j
7 SS 30.0
End of boring at 30.0 ft.
Boring backfilled with high solids bentonite grout.
0
O
!7
y
F-
a
The stratification lines represent the approximate boundary lines between soil types: in situ,the transition may be gradual.
rn WL BORING STARTED STS OFFICE Minneapolis Area-06
°o D 6/11/03
�i WL BORING COMPLETED ENTERED BY SHEET NO. OF
z 6/11/03 DN 1 1
Ft WL RIG/FOREMAN APP'D BY STS JOB NO.
m Diedrich D-120/BM MM 99189
OWNER LOG OF BORING NUMBER 2
Minnetonka Custom Homes
1 PROJECT NAME ARCHITECT-ENGINEER
srsco.imtnsLuL Slope Stability Analysis for House
SITE LOCATION <)-UNCONFINED COMPRESSIVE STRENGTH
4753 North Shore Dr., Orono, MN TONS/Fr2 3 4 5
W PLASTIC WATER LIQUID
U LIMIT% CONTENT% LIMIT
p w DESCRIPTION OF MATERIAL x____11----�
I- to
Z o 10 20 30 40 50
LU w
o w a nom. a 0 STANDARD
U z ® PENETRATION BLOWS/FT.
U) CUD) W.L SURFACE ELEVATION +978.1 NGVD m 1011 20 30 40 50
1 SS Probably FILL: Organic clayey silt-black-stiff-(OL)
HS 22
4.0
2 SS Sandy clay,trace gravel,sand lenses at 30.3,36 and 41 ft. /
depths-brown to gray-stiff to hard-(CL,lenses SP)
HS /
/ 133
3 SS �
j
HS
4 ST
Qu=5600 PSF 125 I I
HS I
13
5 SS
I
HS I I
I i
6 ST t ♦ I
Qu=4400 PSF 120
HS
I !41
16
7 SS I
I i
HS I
I
I
8 SS 30
*
I j
HS
I !
9 SS �2
I
HS I
I /
10 SS
I
HS I
I j
m 11 SS 50.5 8
50.5 o End of boring at 50.5 ft. *Cal brated Pene mete
Boring backfilled with high solids bentonite grout.
a
m
The stratification lines represent the approximate boundary lines between soil types: in situ,the transition may be gradual.
rn WL BORING STARTED STS OFFICE Minneapolis Area-06
°0 25.6 ft BCR 6/11/03
WL BORING COMPLETED ENTERED BY SHEET NO. OF
z 6111/03 DN 1 1
R WL RIG/FOREMAN APP'D BY STS JOB NO.
M. Diedrich D-60/TM MM 99189
Unconfined Compression Test
Project: 99189 Date:
Boring No. B-1 STS Project No:
Sample No. S-4 Prepared by:
Depth (ft): 15.0-16.0
Ww+T 368.20
Dw+T 314.80
Dia. 1.661n Tare 33.20
Height: 56in Wt. Of Water 53.40
ini. Area 5.64 int Wt.Of Dry Soil 281.60
.................
Ini. Vol. 31.36 in :
i Water Content= . 891j
............. ..............
Wt. of Specimen Wet 1185>18 gms
Yd Wet Yd
143.9 121.0
Classification:
Load Dial Load Strain Dial Unit Corrected Stress Stress Stress
Reading Cell Reading Strain Area
(x0.0001 in) (lbs x0.01 in) (% (in2) (Ib/int) (tsf) (ps
I;: 0
<:..�r.: 8.5 1.: 0.180 5.65 1.51 0.109 217
i . ....;:;. 16.5 .;;: 0.360 5.66 2.92 0.210 421
..........
24.5 :;: 0.540 5.67 4.33 0.312 623
1l ..... 32.6 :. 0.719 5.68 5.73 0.413 825
.:.:::..: 48.6 1.259 5.71 8.50 0.612 1225
6 6 9 8 5 5 3 0.808 1617
4. 1. 7 .7 11.2
.............................. .......
96 3 058 5 8 6 6 96 2392
7 »»>'1 <»» 2 1 1 1.1
....................
. :
4 ....:
128.7 ... 3.957 5.87 21.91 1.578 3155
_..... ..
144.1 38;> ..;:.:> 6.835 6.05 23.80 1.713 3427
Qu = 1.71 tsf
Qu = 3,400 psf
H/W Ratio Correction = 3,425 psf
Corrected Qu = 3,400 psf
Unconfined Compression Test
Project: 99189 Date:
Boring No. B-1 STS Project No:
Sample No. S-6 Prepared by:
Depth (ft): 25.0-26.0
Ww+T 321.50
DW+T 278.60
Dia. 1-68 in Tare 34.90
Height: 5:75::in Wt. Of Water 42.90
Ini. Area 5.64 in Wt.of Dry soil 243.70
...............
Ini. Vol. 32.44 in Water Content IT60
... ..... ........
Wt. of Specimen Wet 118> E gms
Yd Wet Yd
131.4 111.7
Classification:
Load Dial Load Strain Dial Unit Corrected Stress Stress Stress
Reading Cell Reading Strain Area
x0.0001 in (lbs) x0.01 in) % (in) (Ib/int) (ts psf)
.:! ..: 0 0
28.5 # ' 0.261 5.66 1.51 0.108 217
t;: 16.5 0.522 5.67 2.92 0.210 420
24.5 4.a:; 0.783 5.69 4.32 0.311 622
1# 1 32.6 ��..: 0.870 5.69 5.72 0.412 824
1�1� 48.6 8.5; :.<;: 1.652 5.74 8.47 0.610 1220
..............................
64.6 1 ,;.; 3.130 5.82 11.09 0.799 1598
33 ,,;..;., 96.7 ;....;;;; 5.913 6.00 16.12 1.161 2321
l 128.7 ; 7.478 6.10 21.11 1.520 3040
44�. )1, ;;;;,. 189.0 ;:......; 10.783 6.32 29.89 2.152 4304
X15......;::: 200.8 # ..:...::.;:: 11.826 6.40 31.38 2.259 4519
_.. ...................._..._..
Qu = 2.26 tsf
Qu = 4,500 psf
H/W Ratio Correction = 4,565 psf
Corrected Qu = 4,600 psf
Unconfined Compression Test
1
Project: 99189 Date:
Boring No. B-2 STS Project No:
Sample No. S-4 Prepared by:
Depth (ft): 13.5-15.5
Ww+T 357.30
Dw+T 307.00
Dia. 2ft8 in Tare 34.70
Height: &56 in Wt. Of Water 50.30
Ini. Area 5.64 in Wt.Of DrySoil 272.30
. ...............
In!. Vol. 31.36 in3 Water Content 18.47
Wt. of Specimen Wet 19> #Dgms
Yd Wet Yd
148.1 125.0
Classification:
Load Dial Load Strain Dial Unit Corrected Stress Stress Stress
Reading Cell Reading Strain Area
4.0001 in (Ibs) x0.01 in) (% (in2) (Ib/inz) (tsf) psf)
8.5 <<' 0.000 5.64 1.51 0.109 218
(f 16.5 0.000 5.64 2.93 0.211 422
.:......:: .....:;; 24.5 .::.........j..;;;...:::.»:: 0.180 5.65 4.34 0.313 625
1# 32.6 ;; 0.360 5.66 5.75 0.414 828
1a? 48.6 .::.....:..:. 0.719 5.68 8.55 0.616 1231
64.6 1.079 5.70 11.33 0.816 1631
3tJ 96.7 $,;;.....:. 1.619 5.73 16.86 1.214 2427
128.7 I ',. 2.698 5.80 22.20 1.598 3197
189.
0 5.036 5.94 31.82 2.291 4582
«:. :.:.
......... <<
80
561
3
2
36 0
6 835
6 05
38 98 2.807
............................
:..:..:....::::::::
Qu = 2.81 tsf
Qu = 5,600 psf
H/W Ratio Correction = 5,642 psf
Corrected Qu = 5,600 psf
Unconfined Compression Test F'Pl,wv-61
Project: 99189.0 Date:
Boring No. B-2 STS Project No:
Sample No. S-6 Prepared by:
Depth (ft): 23.5-25.5
Ww+T 309.90
Dw+T 267.70
Dia. ::2:-7S- :in Tare 33.80
Height: 56 in Wt. Of Water 42.20
Ini. Area 5.94 int Wt.Of Dry Soil 233.90
18 64Ini. Vol. 33.02 in' Water content
,..
M. of Specimen Wet 1p { gms
Yd
141.9 Wet 120Yd.2
Classification: SILTY CLAY,trace to some gravel,trace sand -gray-(CL)
Load Dial Load Strain Dial Unit Corrected Stress Stress Stress
Reading Cell Reading Strain Area
x0.0001 in lbs (x0.01 in) (% (in2) (Ib/int) (tsf) sf)
:.....::::: :.....;;:;:.: ............... ...... 0
......................... ...........
8.5 0.180 5 95 3 0.103 206
... 1 4
....:;;.���3...:::::::.. 16.5 . :::....:....: 0.360 5.96 2.77 0.200 399
:: 24.5 ::i . 0.540 5.97 4.11 0.296 592
1#> 32.6 ? .., 0.719 5.98 5.44 0.392 784
4:5 48.6 <:; 0.989 6.00 8.10 0.583 1166
....::. #]:::..::.... 64.6 :; 1.259 6.02 10.74 0.773 1547
3iI 96.7 9t7:
1.799 6.05 15.98 1.151 2301
# I 128.7 1 2.338 6.08 21.16 1.524 3047
» '. i t>»>. 189.0 }< >< 5.396 6.28 30.10 2.167 4335
5
7.55 6.42 30.88 2.223 4447
198.4
4
Qu = 2.22 tsf
Qu = 4,400 psf
H/W Ratio Correction = 4,410 psf
Corrected Qu = 4,400 psf
STS Consultants,Ltd.
4753 North Shore Drive
Slope Stability Analysis
Orono, Minnesota
STS Project 99189
99189 6-19—** 9:40
4753 North Shore Dr.
990 10 most critical surfaces, MINIMUM JANBU FOS = 3.108
Proposed Footing Surcharge
2000 psf
970 ym= 120 pcf
c=800 psf
n
m 950 �=00
%i Ym= 145 pcf
N yw= 149 pcf
X c=2250 psf
Q 930 -----w 1
----------------------- ---
r Lake Minnetonka
910
890
0 20 40 60 80 100 120 140 160
X—AXIS (feet)
r
z iloi
04
� o
Z
^ p RESIDENCE
y) C
O r
O
`) D
0
y z
D I I I I m I I c_n I v
A I O
m I I = I o f
IIII n m
I A
m
rn III
D
- m
rr
m �
0
00
v
D
r
m
y
Z O h
1
m �
N
O
I�
O DEKED
DATE: DATE: REVISIONS BY.
> V) SOIL BORING LOCATION DIAGRAM
D T- � N 6/18/2003 DATE TEXT ---_—. BY:
`� �' 00 A 2 =" SLOPE STABILITY ANALYSIS CDATEZCO CO -OU' o m TnSINGLE FAMILY DWELLING
_ -ni m c -----
Eo APPROVED BY: DATE:
o cO o ORONO, MINNESOTA MM -
Z � m
c� N a FOR: MINNETONKA CUSTOM HOMES `"°""
%:\99189\0689001.OWG