HomeMy WebLinkAboutSubsoils Study for Foundation Designt (tll åiffiifi'ffiïEHnilÍå *"'
An Employrc Ormcd Compony
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970)945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
www.kumarusa.com
Ofüce Locations: Denver (HQ), Parker, Colorado Springs, Foñ Collins, Glenwood Spdngs, and Summit County, Colorado
September 1,2021
RA Nelson,LLC
Attn: Matt Gwost
P.O. Drawer 5400
Avon, Colorado 81620
mgwost@ranelson.com
Project No. 2l-7-606
Subject: Subsoil Study for Foundation Design, Proposed Richter Residence, Lot 17, Filing
8, Elk Springs, 2l2l Elk Springs Drive, Garfield County, Colorado
Dear Matt:
As requested, Kumar & Associates, Inc. performed a subsoil study for design of foundations at
the subject site. The study was conducted in accordance with our agreement for geotechnical
engineering services to RA Nelson, LLC dated July 14, 2021. The data obtained and our
recommendations based on the proposed construction and subsurface conditions encountered are
presented in this report.
Proposed Construction: Plans or the proposed residence were conceptual at the time of our
study. The proposed residence is assumed to be a one- or two-story wood-frame structure with
attached garage located on the site in the area of the building envelope shown on Figure 1.
Ground floors could be a combination of structural over crawlspace and slab-on-grade with cut
depths between about 2 to 5 feet. Foundation loadings for this type of construction are assumed
to be relatively light and typical of the proposed type of construction.
If building conditions or foundation loadings are significantly different from those described
above, we should be notified to re-evaluate the recommendations presented in this report.
Site Conditions: The subject site was vacant at the time of our field exploration. The ground
surface is gently sloping down to the east at a grade of about 5 percent. There is an estimated
3-foot elevation difference across the building envelope. Vegetation consists of grass and weeds
with cactus.
Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating
three exploratory pits at the approximate locations shown on Figure l. The logs of the pits are
presented on Figure 2. The subsoils encountered, below about I foot of topsoil, consist of very
,,,
stiff, blocky sandy clay, transitioning to calcareous silty sandy clay between 1/z and 3 feet deep
down to the maximum explored depth of 9 feet deep. Results of swell-consolidation testing
performed on a relatively undisturbed sample of the blocky sandy clay, presented on Figure 3,
indicate low compressibility under existing moisture conditions and light loading and a low
expansion potential when wetted. Results of swell-consolidation testing performed on a
relatively undisturbed sample of the calcareous sandy silty clay, presented on Figure 3, indicate
low to moderate compressibility under conditions of loading and wetting. No free water was
observed in the pits at the time of excavation and the soils were slightly moist.
Foundation Bearing Conditions: The blocky clay subsoils encountered in the upper portion of
the pits possess an expansion potential when wetted. The underlying calcareous sandy silty clay
soils possess a low bearing capacity and low settlement potential when wetted. 'We recommend
removing the upper blocky brown clay soils from below foundation and slab areas and placing
spreadfootingsandslabs.on-gradeentirelyontheunderlyingca1careousc@
structural fill. The exposed soils should be observed and further evaluated for swell potential at---.the time of excavation prior to forming footings.
Foundation Recommendations: Considering the subsoil conditions encountered in the
exploratory pits and the nature of the proposed construction, we recommend the upper blocky
clay soils be removed from foundation areas and placing spread footings on the undisturbed
natural calcareous clay soil designed for an allowable soil bearing pressure of 1,500 psf for
support of the proposed residence. The soils tend to compress after wetting under load and there
could be some post-construction foundation settlement. Footings should be a minimum width of
l8 inches for continuous walls ard2 feet for columns. Topsoil, brown blocky clay and loose
disturbed soils encountered at the foundation bearing level within the excavation should be
removed and the footing bearing level extended down to the undisturbed natural calcareous clay
soils. S lish design bearing level should consist of road base
to at least 98% of standard Proctor density atnear optimum moisture content.
Exterior footings should be provided with adequate cover above their bearing elevations for frost
protection. Placement of footings at least 36 inches below the exterior grade is typically used in
this area. Continuous foundation walls should be heavily reinforced top and bottom to span local
anomalies such as by assuming an unsupported length of at least 14 feet. Foundation walls
acting as retaining structures should be designed to resist alateral earth pressure based on an
equivalent fluid unit weight of at least 55 pcf for the on-site soil as backfill.
Floor Slabs: The upper blocky clay soils possess an expansion potential and slab heave could
occur if the subgrade soils were to become wet. Slab-on-grade construction can be used
Kumar & Associates, lnc. 6 Project No. 21-7-606
-3^
provided the upper-blocky clay is removed and replaced with compacted structural fill and
precautions are taken to limit potential movement. A positive way to reduce the risk of slab
movement, which is commonly used in the area, is to construct structurally supported floors over
crawlspace.
To reduce the effects of some differential movement, floor slabs should be separated from all
bearing walls and columns with expansion joints which allow unrestrained vertical movement.
Interior non-bearing partitions resting on floor slabs should be provided with a slip joint at the
bottom of the wall so that, if the slab moves, the movement cannot be transmitted to the upper
structure. This detail is also important for wallboards, stairways and door frames. Slip joints
which will allow at least Ilz-inches of vertical movement are recommended. Floor slab control
joints should be used to reduce damage due to shrinkage cracking. Slab reinforcement and
control joints should be established by the designer based on experience and the intended slab
use. Floor slab control joints should be used to reduce damage due to shrinkage cracking. The
requirements for joint spacing and slab reinforcement should be established by the designer
based on experience and the intended slab use.
A minimum 4-inch layer of relatively free draining gravel should be placed beneath basement
level slabs to facilitate drainage. This material should consist of minus 2-inch aggregate with
less than 50% passing the No. 4 sieve and less than2Vo passing the No. 200 sieve.
Required fill beneath slabs can consist of the on-site calcareous soils or a suitable imported
granular material, excluding topsoil and oversized rocks. The fill should be spread in thin
horizontal lifts, adjusted to at or above optimum moisture content, and compacted to at least
95o/o of the maximum standard Proctor density. All vegetation, topsoil and loose or disturbed
soil should be removed prior to fill placement.
The above recommendations will not prevent slab heave if the expansive soils underlying slabs-
on-grade become wet. However, the recommendations will reduce the effects if slab heave
occurs. All plumbing lines should be pressure tested before backfilling to help reduce the
potential for wetting.
Underdrain System: Although free water was not encountered during our exploration, it has
been our experience in the area and where clay soils are present that local perched groundwater
can develop during times of heavy precipitation or seasonal runoff. Frozen ground during spring
runoff can create a perched condition. We recommend below-grade construction, such as
retaining walls, crawlspace and basement ateas, be protected from wetting and hydrostatic
pressure buildup by an underdrain system.
Kumar & Associates, lnc. o Project No. 21-7-606
-4-
The drains should consist of drainpipe placed in the bottom of the wall backfill surrounded above
the invert level with free-draining granular material. The drain should be placed at each level of
excavation and at least I foot below lowest adjacent finish grade and sloped at a minimum lo/o to
a suitable gravity outlet. Free-draining granular material used in the underdrain system should
contain less than 2Yopassing the No. 200 sieve, less than 50% passing the No. 4 sieve and have a
maximum size of 2 inches. The drain gravel backfill should be at least llz feet deep. An
impervious liner such as 20 mil PVC should be placed below the drain gravel in a trough shape
and attached to the foundation with mastic to keep drain water from flowing beneath the wall and
into other areas of the building.
Surface Drainage: Providing and maintaining proper surface drainage will be critical to the
long-term satisfactory performance of the residence. The following drainage precautions should
be observed during construction and maintained at all times after the residence has been
completed:
l) Inundation ofthe foundation excavations and underslab areas should be avoided
during construction. Drying could increase the expansion potential of the clay
soils.
2) Exterior backfill should be adjusted to near optimum moisture and compacted to
at least 95o/o of the maximum standard Proctor density in pavement and slab areas
and to at least 90%o of the maximum standard Proctor density in landscape areas.
Free-draining wall backfill should be covered with filter fabric and capped with
about 2 feet of the on-site, finer graded soils to reduce surface water infiltration.
3) The ground surface surrounding the exterior of the building should be sloped to
drain away from the foundation in all directions. We recommend a minimum
slope of 12 inches in the first l0 feet in unpaved areas and a minimum slope of
3 inches in the first 10 feet in pavement and walkway areas. A swale may be
needed uphill to direct surface runoff around the residence.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill.
5) Landscaping which requires regular heavy irrigation should be located at least
l0 feet from the building. Consicleration should be given to the use of xeriscape
to limit potential wetting of soils below the foundation caused by irrigation.
Limitations: This study has been conducted in accordance with generally accepted geotechnical
engineering principles ancl practi<;es irr this area aL this tirne. We rttakc rto w¿rranty eiLher
express or implied. The conclusions and recommendations submitted in this report are based
Kumar & Associafes, lnc. o Project No. 21-7-606
5
upon the data obtained from the exploratory pits excavated at the locations indicated on Figure I
and to the depths shown on Figure 2,the proposed type of construction, and our experience in
tlte arca. Our services do not include determining the presence, prevention or possibility of mold
or otherbiological contaminants (MOBC) developing in the future. If the client is concerned
about MOBC, then a professional in this special field of practice should be consulted. Our
findings include interpolation and extrapolation of the subsurface conditions identified at the
exploratory pits and variations in the subsurface conditions may not become evident until
excavation is performed. If conditions encountered during construction appear different from
those described in this report, we should be notified at once so re-evaluation of the
recommendations may be made.
This report has been prepared for the exclusive use by our client for design purposes. We are not
responsible for technical interpretations by others of our information. As the project evolves, we
should provide continued consultation and field services during construction to review and
monitor the implementation of our recoûrmendations, and to veriry that the recommendations
have been appropriately interpreted. Significant design changes may require additional analysis
or modifications to the recommendations presented herein. 'We recommend on-site observation
of excavations and foundation bearing strata and testing of structural fill by a representative of
the geotechnical engineer.
If you have any questions or if we may be of further assistance, please let us know.
Respectfu lly Submitted,
Kumar & Associates,
James H. P
Reviewed by:
fr*-/,
Steven L. Pawlak, P.E.
JHPlkac
attachments Figure I - Location of Exploratory Pits
Figure 2 - Logs of Exploratory Pits
Figure 3 - Swell-Consolidation Test Results
Table I - Summary of Laboratory Test Results
tt
I/t
H.
586d¡
Kumar & Associates, lnc. o Project No. 21'7-606
J
$Ìåt'
Ltfiury ËÅgË#[,
T TÍ
r.
LOT I5
۾l?l,6ll sq- ft'
f.792 ac.*-
\una'aa'ro"r*
*fæ'=EË ff"F**$,þ mt ?
¿qÞt
'ül
rfl
?¡-
Ì
t
t¡)Is
1\¡ 14.ı Th¡çÉ+
HTR'IîÈ.
\
I
Ìt
I
"/-(
IfJ
t
I
¡
0fi
t
I
1
r
i
mS'
e,/
$-
,t
4þ
7s
LTT 78
l
@fJå808 sq. ft'
J.t?Ê at.+*
"--'--r-Þ //
fr
Tfl
Lff 16I
¡t
¡
APPROXIMATE SCALE-FEET
Þ
ûÈ
r\þ
ÞlJt
I
0
X
H
=q
e
ü\}+\-
"Et
ütJ
t
50
6t.14'
&-tt'
¡^t
*JË\BRl*
\"\Ilr
-d--.1..\\
fo
PIT 3
B
rd
Srtrl r#'#þ
38t.77
5'!Qt'
#lt¿ffr
T
P)T 2
Ëffi'- ûìqtq
¡d *i,ggru
''''f rry,i.#
Ltr 17
ffiîþ
î 21, ñî sq. ft,
Í.795 nc'+*
21 -7 -606 Kumar & Associates LOCATION OF EXPLORATORY PITS Fig. 1
I
E
P
I
PIT 1 PIT 2 PIT 3
0 0
WC=7.5
DD= I 02
t--t¡l
LrJtL
I-t-fL
t¡Jo
5
tNC=12.4
DD=9 1 5
FtJLI
LL
I-l-fL
L¡lo
f
J WC=9.3
-2OO=18
10 10
LEGEND
TOPSOIL; CLAY, SANDY, SILTY, ORGANICS, FIRM, SLIGHTLY MOIST, BROWN
CLAY (CL); SANDY, SILTY, BLOCKY, SCATTERED GRAVEL AND COBBLES, VERY STIFF,
SLIGHTLY MOIST, CALCAREOUS BELOW 3 FEET, BROWN TO TAN.
F
t
HAND DRIVEN 2-INCH DIAMETER LINER SAMPLE
DISTURBED BULK SAMPLE.
NOTES
1. THE EXPLORATORY PITS WERE EXCAVATED WITH A RUBBER TRACKED EXCAVATOR ON AUGUST 6,
2021.
2. THE LOCATIONS OF THE EXPLORATORY PITS WERE MEASURED APPROXIMATELY BY PACING FROM
FEATURES SHOWN ON THE SITE PLAN PROVIDED.
3. THE ELEVATIONS OF THE EXPLORATORY PITS WERE NOT MEASURED AND THE LOGS OF THE
EXPLORATORY PITS ARE PLOTTED TO DEPTH.
4. THE EXPLORATORY PIT LOCATIONS SHOULD BE CONSIDERED ACCURATE ONLY TO THE DEGREE
IMPLIED BY THE METHOD USED.
5. THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY PIT LOGS REPRESENT THE
APPROXIMATE BOUNDARIES BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE GRADUAL.
6. GROUNDWATER WAS NOT ENCOUNTERED IN THE PITS AT THE TIME OF EXCAVATION. PITS WERE
BACKFILLED SUBSEQUENT TO SAMPLING.
7. LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D 2216);
DD = DRY DENSITY (PCt) (ASTU D 2216);
-2OO= PERCENTAGE PASSING NO. 200 SIEVE (ASTM D 1 1 40).
21 -7 -606 Kumar & Associates LOGS OF EXPLORATORY PITS Fig. 2
SAMPLE OF: Blocky Silty Sondy Cloy
FROM¡Pit1E^2'
Y,lC = 7.3 %, DD = 1O2 pc'f
EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING(ì -*
\
\
àq
-)J
Ld
=a/1
I
zotr
o
JoU'z.o(J
1
0
-1
2
-3
-4
t.0 APPLIED PRESSURE _ KSF 10
àq
JJ
l¡J
=a,r1
I
z.otr
o
=o
1J''z.o()
1
0
-1
2
-3
-4
1.0 APPLIED PRESSURE - KSF 10
SAMPLE OF: Colcoreous Silly Sondy Cloy
FROM:Pit2E^4'
WC = 12.4 %, DD = 91 pcf
EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
I
\
\
\
I )
Iha!ô tod lêrult3 oÞply onty þ rna
romplca t6tcd. Thô têlt¡ñg rcporl
rholl ñot b. Fprcducod, êxcê9t ln
full, rlthout tho writroñ opprovol ol
(umor oñd Assoclotæ, lnc, Sw.ll
:oNo!¡dotlon ldtlng portormêd lñ
¡ccodonc. with ASTV D-4546.
21 -7 -606 Kumar & Associates SWELL_CONSOLIDATION TEST RESULTS Fig. 3
rcÂiffi,ffi:ffiii'iiå*"
:t
TABLE I
SUMMARY OF LABORATORY TEST RESULTS
No.21-7-606
GRÂN flaìN AT]LIM¡TSSAMPLE LOCATION
lnsfl
UNCONFINED
COMPRESSIVE
STRENGTHPITDEPTH
to/^\
NATURAL
MOISTURE
CONTENT
locfl
NAÏURAL
DRY
DENSITY
GRAVEL
(/"1
SAND
(%l
PERCENT
PASSING NO,
200 stEVE
LIQUID LIMIT
(%l l"/o)
PLASTIC
INDEX SOIL TYPE
102 Blocky Silty Sandy ClayI2I.-)
2 4 12.4 9l Calcareous Silty Sandy
Clav
J 8-8%9.3 18 Calcareous Sand, Silt and
Clav