HomeMy WebLinkAboutSubsoils Report for Foundation Designrcn *iffilflffifffinlx'i*"
An Emdoycc Owncd Gompony
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
www.kumarusa.com
Office locations: Denver (HQ), Parker, Colorado Sp.inS, Fort Collins, Glenwood Springs, and Summit Comty, Colorado
October 24,2024
Ryan Williams
267 Rabbit Road
Carbondale, Colorado 81623
929will]'@gmail.com
Project No.24-7-544
Subject:Subsoil Study for Foundation Design, Proposed Residence, Parcel No.
237117200130, TBD Panorama Drive, Garfield County, Colorado
Gentlemen:
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 Ryan Williams dated September 13,2024. The data obtained and our
recommendations based on the proposed construction and subsurface conditions encountered
are presented in this report.
Proposed Construction: The proposed residence will be single-story wood-framed structure
over a walkout basement level with detached garage and ADU located on the site in the area of
Pit I shown on Figure 1. Ground floors will be slab-on-grade or structural over crawl-space.
Cut depths are expected to range between about 3 to 8 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 diflerent from those described
above, we should be notified to re-evaluate the recommendations presented in this report.
Site Conditions: The project site was mostly vacant at the time of our site visit other than a
shipping container. Topography at the site is hillside with moderately sloping terrain down to
the south. Vegetation at the site consists of native grass and weeds, sage brush, and scaffered
pinon trees. Basalt boulders are present at the ground surface across the site.
Subsurface Conditions: The subsurface conditions at the site were evaluated by observing one
exploratory pit at the approximate location shown on Figure l. The log of the pit is presented on
Figure 2. The subsoils encountered, below about I foot of topsoil, consist of basalt gravel,
cobbles, and boulders in a sandy silt matrix. Results of a gradation analysis performed on a
sample of silty, sandy gravel and cobbles (minus S-inch fraction) obtained from the site are
presented on Figure 3. No free water was observed in the pit at the time of excavation and the
soils were slightly moist to moist.
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X'oundation Recommendations: Considering the subsoil conditions encountered in the
exploratory pit and the nature of the proposed construction, we recommend spread footings
placed on the undisturbed natural basalt gravel, cobble and boulder in a sandy silt matrix soil
designed for an allowable soil bearing pressure of 2,000 psf for support of the proposed
residence and garage/ADU. The matrix material of the soils tends to compress after wetting and
there could be some post-construction foundation settlement. Footings should be a minimum
width of 18 inches for continuous walls and2 feet for columns. Loose and disturbed soils and
existing fiIl encountered at the foundation bearing level within the excavation should be removed
and the footing bearing level extended down to the undisturbed natural soils. Exterior footings
should be provided with adequate cover above their bearing elevations for frost protection.
Placement of footings at least 16 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 12 feet.
Foundation and Retaining Walls: Foundation walls and retaining structures which are laterally
supported and can be expected to undergo only a slight amount of deflection should be designed
for a lateral earth pressure computed on the basis of an equivalent fluid unit weight of at least
50 pcf for backfill consisting of the on-site soils. Cantilevered retaining structures which are
separate from the residence and garage/ADu and can be expected to deflect sufficiently to
mobilize the full active earth pressure condition should be designed for a lateral earth pressure
computed on the basis of an equivalent fluid unit weight of at least 40 pcf for backfill consisting
of the on-site soils.
All foundation and retaining structures should be designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings, traffic, construction materials and equipment.
The pressures recorlmended above assume drained conditions behind the walls and a horizontal
backfill surface. The buildup of water behind a wall or an upward sloping backfill surface will
increase the lateral pressure imposed on a foundation wall or retaining structure. An underdrain
should be provided to prevent hydrostatic pressure buildup behind walls.
Bacldill should be placed in uniform lifts and compacted to at least 90% of the maximum
standard Proctor density at a moisture content near optimum. Backfill in pavement and walkway
areas should be compacted to at least 95% of the maximum standard Proctor density. Care
should be taken not to overcompact the backfill or use large equipment near the wall, since this
could cause excessive lateral pressure on the wall. Some settlement of deep foundation wall
backfill should be expected, even if the material is placed correctly, and could result in distress
to facilities constructed on the backfill.
Kumar & Associates, lnc. o Project No. 2h7-5tA
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The lateral resistance of foundation or retaining wall footings will be a combination of the
sliding resistance of the footing on the foundation materials and passive earth pressure against
the side of the footing. Resistance to sliding at the bottoms of the footings can be calculated
based on a coefficient of friction of 0.35. Passive pressure of compacted backfill against the
sides of the footings can be calculated using an equivalent fluid unit weight of 350 pcf. The
coefficient of friction and passive pressure values recommended above assume ultimate soil
strength. Suitable factors of safety should be included in the design to limit the strain which will
occur at the ultimate strength, particularly in the case of passive resistance. Fill placed against
the sides of the footings to resist lateral loads should be compacted to at least 95% of the
maximum standard Proctor density at a moisture content near optimum.
X'loor Slabs: The natural on-site soils, exclusive of topsoil, are suitable to support lightly to
moderately loaded slab-on-grade construction. 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. Floor slab control joints should be used to
reduce damage due to shrinkage cracking. The requirements forjoint spacing and slab
reinforcement should be established by the designer based on experience and the intended slab
use. A minimum 4 inch layer of 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 than2Yo passing the No. 200 sieve.
All fillmaterials for support of floor slabs should be compacted to at least95o/o of maximum
standard Proctor density at a moisture content near optimum. Required fill can consist of the
on-site soils devoid of vegetation, topsoil and oversized rock.
Underdrain System: Although free water was not encountered during our exploration, it has
been our experience in the area 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 areas, be protected from wetting and hydrostatic pressure buildup by an underdrain
system.
The drains should consist of rigid PVC 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 Yzo/oto a suitable gravity outlet. Free-draining granular material used in the
underdrain system shoutd contain less than 2% passing 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 lVzfeet deep and covered with filter fabric such as Mirafi l40N or 160N.
Kumar & Associates, lnc. @ Project No. 24-7-StA
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Surface Drainage: The following drainage precautions should be observed during construction
and maintained at all times after the residence and garage/ADU have been completed:
1) Inundation of the foundation excavations and underslab areas should be avoided
during construction.
2) Exterior backfill should be adjusted to near optimum moisture and compacted to
at least 95%o of the maximum standard Proctor density in pavement and slab areas
and to at least 90% of the maximum standard Proctor density in landscape areas.
Free-draining wall backfill should be 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 6 inches in the first 10 feet in unpaved areas and a minimum slope of
3 inches in the first l0 feet in pavement and walkway areas.
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
10 feet from the building.
Limitations: This study has been conducted in accordance with generally accepted geotechnical
engineering principles and practices in this area at this time. We make no warranty either
express or implied. The conclusions and recommendations submitted in this report are based
upon the data obtained from the exploratory pit excavated at the location indicated on Figure 1
and to the depth shown on Figure Z,the proposed type of construction, and our experience in the
area. Our services do not include determining the presence, prevention or possibility of mold or
other biological 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 exhapolation of the subsurface conditions identified at the exploratory
pits and variations in the subsur ce 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 pu{poses. 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 recommendations, and to verifr that the recoiltmendations
have been appropriately interpreted. Significant design changes may require additional analysis
Kumar & Associates, lnc. @ Projec't No. 2&7-5'14
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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 rqnesentative 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, lnc,
Robert L. Duran, P.E.
Reviewed by:
Daniel E. Hardin, P.E.
RLD/kac
attachments Figure I - Location of Exploratory Pit
Figure 2 - Logs of Exploratory Pit
Figure 3 - Gradation Test Results
t
Kumar & Associates, lnc.6 Project No. 24-7-544
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APPROXIMATE SCALE*FEET
24-7-544 Kumar & Associates LOCATION OF EXPLORATORY PIT Fig. 1
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I
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PIT 1 LEOEND
o
TOPSOIL; SILTY CLAY WITH SCATTERED GRAVEL
AND COBBLES, FIRM, SLIGHTLY MOIST, DARK
BROWN, ORGANIC.
coBBLEs AND BOULDERS (GM); lN A SANDY
SILT MATRIX, DENSE, SLIGHTLY MOIST, L]GHT
TAN, BASALT ROCKS.
t DISTURBED BUTI( SATTPI,E.
5
10
-l WC=7.4
-i +4=47
-2OO=25
NOTEg
1. THE EXPLORATORY PIT TVAS EXCAVATED WTH A BACKHOE ON
SEPTEIIBER 17,2021.
2. THE TXPLORATORY PIT WAS LOCATED BY THE CUENT.
3. THE EI,"EVATION OF IHE EXPLORATORY PIT WAS NOT TIEASURED
AND THE LOG OF THE EXPLORATORY PIT IS PLONED TO DEPTH.
15
1. THE EXPLORATORY PIT LOCATION SHOU1O 8E CONSIDERTD
ACCURATE ONLY TO TI{E DEGREE IMPUED BY TI{E TETHOD USED.
5. THE UNES BENVEEN IIATERIAIS SI{OIYN 01{ fl{T EXPLORATORY
PIT LOGS REPRESENT THE APPROXIHATE EOUNDARIES BENYEEN
MATERIAL TYPES AND THE TRANSITIONS ITAY BE GRADUAL
6. GROUNDWATER WAS NOT ENCOUNTTRED IN THE PIT AT THE TIME
OF EXCAVATION. PIT WAS BACKFII.I.ED SUBSEOUENT TO
SATIPUNG.
7. I.ABORATORY TEST RESULTS:
WC = WAIER CONTENT (X) (ASTIT D 2216);
+/+ = PERCENTAGE REIAINED ON NO. 4 SIEVE (TSTU O TZZ);
-2fi1 = PERCENTAGE PASSING N0. 200 SIEVE (ASIU D 1140).
24-7-544 Kumar & Associates LOGS OF EXPLORATORY PITS Fig. 2
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20
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70
lo
90
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CI.AY TO SILT COSBLES
Cobblag tS X GRAVEL 2A X
UQUID UIIIT
Sllty Sondy Grovd ond Cobbbr
sAilD 2A X
PI.ASIICITY INDEX
SILT AI{D CLAY 25
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SAYPLE OF:FROII: Pll t O E-9'
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Sl.ya onoltalr taallng b pdom.d lno@dom r0t ASII 0C915, ASII D7g2E,A$I Cr!6 and,/d AS|X Dll,ro.
}ryDROYETER ANALYSIS stEvE AxalYsls
rff rt^llrog
fsi.s/n2r Hta 7 Ht!rt !E rr uI tu
u.3. traD m llREl 0[.E n 30r tE oPt'l0tcs
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SAND GRAVEL
FINE MEDIUM FINE COARSE
24-7-544 Kumar & Associates GRADATION TEST RESULTS Fig. 5