Subsurface Soil Investigation Letter 10-25-2014Interstate Geotechnicai Engineering, Inc
Patrick 3 Hines, PE
8167 100'h st s
Cottage Grove MN 55016
(612) 414 -5770
25 October 2014
Mr James Rudos
14846 Timber Hill Road
Minnetonka MN 55345
Re: Subsurface Soil Investigation
Proposed Single Family Residence
W of 2061 W 65th St, Chanhassen, Minn
In accordance with your authorization, I have completed the above referenced investigation
to determine site suitability for the proposed construction. A summary of my findings
together with boring logs and my conclusions and recommendations are presented in the
enclosed report.
I found, as you already have been briefed, more than usual problems with the site. There is
a total of 5'/4'& 61/4' at the locations of Boring 1 & Probe 1A, respectively, of unsuitable soil
in the form of topsoil fill, general and varying till, and buried original humus. Below at first
is a low sand content lean clay, generally dark in color, saturated, very soft to soft (w/ depth).
It is "normally consolidated" (consolidated enough to only support its own weight, not
additional weight of fill soils or structures above). The overburdening fill has not been
inplace long enough to force further consolidation. The moisture content test in this material
indicated nearly 42 %, high for a lean clay. This material extends only from 53/4 to 7' in
Boring 1, but from 61/4' to 83/2 in Probe IA. The first 9" of this material in Probe lA is very
soft and is a borderline marl material. All of the aforementioned materials are unsuitable for
structure support should be removed. I do acknowledge that excavators are often reluctant
to excavate further once the buried organic soil is cleared, so understandings should be
inplace at the time of gathering bids.
Base soil below at first is a lean clay, a little sandy, dark gray, saturated, rather soft to
medium. It has inclusions of a poorly graded (biased finer grained) sand with a few fines,
also dark gray, saturated, loose to firm. Deepest soil, commencing at 1 Pin Boring 1, is a lean
clay, rather sandy, dark gray, low gravel content, medium. Theses materials are glacial till.
The sand inclusions in the upper portion are glacial outwash. All of these base mineral soils
are suitable for residential construction, with conditions. Refusal to auger advancement was
not encountered by boring termination depth of 23', indicating lack of bedrock to this level.
Very importantly, groundwater was found in the main Boring at 1.8' (elev 961/4±). It is
aquifer groundwater, but marginally so. It appears to be contained primarily in the sand
inclusions at 7' +. But it also is coming from the upper soft and very soft clay, possibly from
Mr James Rudos
25 October 2014
Page 2
the lower organic soil. Proper construction on the site (removing the softer lean clay,
promoting good site drainage) will mitigate some of this. Thus, water found presently can
be regarded as a maximum level. Slab grade of the proposed residence should be set in
accordance with City requirements utilizing this level.
I recommend complete removal_ of all unsuitable soil which is likely to be the fill and buried
organic soil as encountered plus the overly soft clay found in both the boring and probe.
While this amounts only to another 11/4 in the vicinity of Boring 1, it is another 2'+ in the
vicinity of Probe 1A. As indicated, excavators may be reluctant to excavate below organic
soil level, but it is necessary (see longer discussion in the report). It is so important that full
time inspection of the excavation by a soils engineer may be necessary. Of course, other
zones of unsuitable soil not represented by the boring and probe could still be encountered
due to the nature of small volume random sampling. Any other depths or zones of unsuitable
soil should be removed as well. You could lower footings somewhat so that you have to
provide less oversizing and not stray onto the neighbor's property. But footings here should
not be lowered so much as to rest directly upon the suitable base soil as it is only marginally
suitable. You need a little fill (2' is recommended) to spread footing loads out with depth.
Footings maybe generally designed allowing the usual 2000 psf foundation bearing capacity,
which will likely result in normal strip or pad footings.
Special attention is again drawn to the advisory that excavations be examined by the Soils
Engineer to verify soil boring results and to document adequacy of site preparation. Again
this is very important as how much softivery soft clay must be removed will require a high
degree of field judgment. Fill, as used, should be tested for compaction adequacy. Soil
correction should not be abated below slabs in any manner (ie, do not let the excavator try
to "trench around" slab areas).
Thank you for the opportunity to have been of service. If you have any questions on this, or
if I can be of assistance in any additional capacity, do not hesitate to contact me at your
convenience.
Sincerely,
INTERSTATE GEOTECHNICAL ENGINEERING, Inc
atrick J Hines,
President
enclosures
pH/SC