HomeMy WebLinkAboutSubsoil Study for Foundation Design 04.29.21K+rf Kumr & Asmciatæ, lnc.'
Geotechnical and Materials Engineers
and Environmenlal Scientists
An Emplsycc Owncd Compony
5020 Counf Road 154
Glenwood Springs, CO 81601
phone (970) 945-7988
fax: (970) 945-8454
email: kaglanwood@kumarusa.com
www.kurnarusa. com
Offrce Locations: Deriver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit Cormty, Colorado
Aprll29,202l
Dan and Laurel Froedge
3355 Marble Terrace
Colorado Springs, Colorado 80906
dani el. froed ge@.hdrinc. com
Project No. l7-7-462.01
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot74, Filing 7,
Elk Spnngs,1579 Elk Springs Drive, Garfield Country, Colorado
Dan and Laurel:
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 you dated March 30,202I. The data obtained and our recommendations
based on the proposed construction and subsurface conditions encountered are presented in this
report. We previously conducted a subsoil study in the northem part of the building envelope
and reported our findings on June 22,2017.
Proposed Construction: The proposed residence will be a single-story, wood frame structure
with attached garage located on the site as shown on Figure L Ground floors will be a
combination of structural over crawlspace and slab-on-grade. Cut depths are expected to range
between about 2 to 4 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 moderately sloping down to the south-southeast in the building area at about -10 to l5olo
ggtting steeper to the southeast. Vegetation consists of sagebrush, grass and weeds with juniper
trees and basalt rocks exposed on the ground surface south and east of the proposed building
area.
Subsurfnce Conditions: The subsurface conclitions at the site were evaluated by drilling
2 exploratory borings at the approximate locations shou¡n on Figure l. The logs of the borings
aro presonted on Figure 2. The subsoils encountered, belorv about one foot of topsoil, consist nf
about 2 feet of very stiff sandy silt and clay unclerlain by clense basalt gtavel, cobbles and
boulders in acalcareous silt matrix to the maximum boring depth of 8 feet. Drilling in the dense
cobbles and boulders was difficult and practical auger refusal was encorurtered in the borings.
,)
Results of swell-consolidation testing performed on relatively rurdisturbed drive samples of the
sandy silt soils, presented on Figure 3, indicate low compressibility under existing moisture
conditions and light loading and a low collapse potential when wetted. No free water was
encountered in the borings at the timc of drilling and the soils were slightly moist to moist.
f,'oundation Recommendations: Considering the subsoil conditions encountered in the
exploratory borings and the nature of the proposed construction, we recommend spread footings
placed on the undisturbed natural soil designed for an allowable bearing pressure I 500 psf for
support of the proposed residence. There is a risk of foundation movement of around I to
2 inches if the upper finc-grained bearing soils become wetJecl. Placing the spread footings on the
underlying basalt rock soils would reduce the risk of foundation movement. Footings placed
entirely on the underlying dense basalt rock soils can be designed for an allowable bearing
pressure of 2,500 Footings shoulclbe a minimum width of l6 inches for continuous walls
and2 feet for columns. The topsoil 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 soils. Exterior footings should he provided with adequate cover
above their bearing elevations for frost protcction. Placement of footings at lcast 36 inches
below the exterior grade is typically usd in this area. Continuous foundation walls should be
reinforced top and bottom to span local anomalies such as by assuming an unsupported length of
at least 12 feet. Foundation walls acting as retaining structures should be designed to resist a
lateral earth pressure based on an equivalent fluid unit weight of at least 50 pcf for the on-site
soil as backfill, excluding organics and rock larger than 6 inches. A sliding coefficient of 0.40
and equivalent fluid lateral passive earth pressure of375 pcfcan be used to resist lateral loading
on the foundation.
Floor Slabs: Tho natural on site soils, oxclusil'c of topsoil, arc suitoblc to support lightly loadcd
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 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 free-draining gravel should be placed beneath basement level slabs to facilitate drainage.
This material should consist of minus 2-inch oggrcgotc wrth lcss than 50% passing the No. 4
sieve and less than 2%opassingthe No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at leastg5Yo 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 or imported gravel such as road base.
Kumar & Associates, lnc. o Project No.'17 -7 -462.01
-3 -
Underdrain System: Although free water was not encountered during our exploration, it has
been our experience in the areathat 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 (if any), be protected from wetting and hydrostatic pressure buildup by an
underdrain system.
The drains should consist of drainpipe placed in the boftom 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 1 foot below lowest adjacent finish grade and sloped at a minimum 1o/o to
a suitable gravity outlet. Free-draining granular material used in the underdrain system should
contain less than ZYo passing the No. 200 sieve, less than 50olo passing the No. 4 sieve and have a
maximum size of 2 inches. The drain gravel backfill should be at least IYz feet deep.
Surface Drainage: The following drainage precautions should be observed during construction
and maintained at all times after the residence has been completed:
1) Inundation ofthe foundation excavations and underslab areas shouldbe avoided
during construction.
2) Exterior backfill should be adjusted to near optimum moisture and compacted to
at least 95Yo aîthe maximum standard Proctor density in pavement and slab areas
and to at least 90o/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 10 feet in unpaved areas and a minimum slope of
3 inches in the first 10 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
l0 feet from the building. Consideration 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 and practices in this areaat 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 borings drilled at the locations indicated on Figure I
and to the depths shown on Figure 2, the proposed type of construction, and our experience in
Kumar & Associates, lnc. o Project No.'17 -7 -462.01
4-
the area. Our services do not include determining the presence, prevention or possibility of mold
or othff biological contaminants (MOBC) developing in the future. If the client is concerned
about MOBC, then a professional in this speoial field of practice should be consulted. Our
frndings include inlerpolation and extrapolation of the subsurface conditions identifie.d at the
exploratory borings 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 pre'pared for the exclusive use by our elient for design purposes. W'e are not
responsible for technicai 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 implernentation of our recommendations, and to veriry that the recommendations
have been appropriately interyreted. Significant design changes may require add'itional analysis
or modifications to the recommendations presented hetein. We recommend on-site observation
of excavations and foundation bearing strata and testing of structural fìll by a representative of
the geotechnical engineer.
If you have any questions or if we rnay be of further assistance, please let us know.
Respectfu lly Subrnitted,
Kurnar & Associates.
Steven L. Pawlak, P
Reviewed by:
t
Daniel E. Hardin, P.E.
SLP/kac
attachments Figure 1 - Location of Exploratory Borings and Pits
Figure 2 - Logs of Exploratory Borings
Figure 3 - Swell-Consolidation Test Results
Table I - Summary of Laboratory Test Results
cc:Patrick Stuckey - str¡carch(r¿,corncast.net
U'5222
Kumar & Associates, lnc, 6 Project No, 17 -7 -462.01
4;r
ELK SPRINGS DRIVE
PIT 1
qntNc I
ô
LOI 74, FILING 9
15
SCALE-FEET
30
LEGEND: ?T
O eonINIC FOR CURRENT STUDY
T PIT FROM PREVIOUS STUDY REPORT
6/22/17, PRoJEcT No. 17-7-462
Fig. 1LOCATION OF TXPLORATORY BORINGS AND PITS17 -7 - 462.01 Kumar & Associates
BORING 1 BORING 2
0
27 /12
WC=22.9
DD=99
0
50 / 4.5
tNC=26.2
DD=8ü
-200=59l-
L¡J
t¡Jt!
I-Fo-
L¡Jo
5
50/ 4
WC=5.3
-2AO=24
50/1.5
5
5a/ 1
10 10
LEGEND
N
TOPSOIL; ORGANIC SANDY SILT AND CLAY, BROWN
CLAY AND SILT
SLIGHTLY TO HI
(cL-vt-); SANDY, BASALT FRAGMENTS, VERy STtFF, MOtST, MTXED BROWN,
GHLY CALCAREOUS.
BASALT COBBLES AND BOULDER (CM); CALCAREOUS SANDY SILT MATRIX, DENSE, SLIGHTLY
MOIST, GREY,
DRIVE SAMPLE, 2_INCH I.D. CALIFORNIA LINER SAMPLE.
i DRIVE SAMPLE, 1 3/8-|NCH t.D. SPLTT SPOON STANDARD PENETRATTON TEST.
t-7 /12 DRIVE SAMPLE BLOW COUNT. INDICATES THAI 27 BLOWS 0F A 14O-POUND HAMMER-,1'' FALLING 50 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
1 PRACTICAL AUGER REFUSAL.
NOTES
THE EXPLOR^IORY BORn'IGS WERE DRTLLED 0N
^PR|L
8,2021 W|TH ^ 4-ll'tcH D|^METER
CONTINUOUS_FLIGHT POWER AUGER.
2. THE LOCATIONS OF THE EXPLORATORY BORINGS WERE MEASURED APPROXIMATELY BY PACING
FROM FEATURES SHOWN ON THE SITE PLAN PROVIDED.
3. THE ELEVAT¡ONS OF THE EXPLORATORY BORINGS WERE OBTAINED BY INTERPOLATION BETWEEN
CONTOURS ON THE SITE PLAN PROVIDED.
4. THE EXPLORATORY BORING LOCATIONS AND ELEVATIONS SHOULD BE CONSIDERED ACCURATE
ONLY TO THE DEGREE IMPLIED BY THE METHOD USED.
5. THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY BORING LOGS REPRESENT THE
APPROXIMATE BOUNDARIES BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE GRADUAL.
6. GROUNDWAÏER WAS NOT ENCOUNTERED IN THE BORINGS AT THE TIME OF DRILLING
7. LABORATORY TEST RESULTS:
wc = WATER CONTENT (%) (ASTM D2216);
DD = DRY DENSITY (pcf) (ASTM D2216);
-200= PERCENTAGE PASSING NO. 200 SIEVE (ASTM D1140);
17 -7 - 462.01 Kumar & Associates LOGS OF EXPLORATORY BORINGS Fig. 2!
t
-:
1
0
-1
J
JU
=an
I
zIF
ô
-Jo
U)zoO
-2
z
4
R
-6
-7
-8 1.0 APPLIED PR 100
JJ
LJ
=U)
I
z.otr
Õ
=o(nzoO
1
U
4
-l
2
-3
I.O APPLIED PRESSURE - KSF 10 100
SAMPLE OF: Scndy Sill wi'lh Gravel
FROM: Boring 1 @ 1'
tNC = 26.2 %, DD = 80 pcf
-2QO = 59 %.
ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
SAMPLE OF: Scndy Silt with Grcvel
FROM:Boring2@0.5'
WC = 22.9 %, DÐ = 99 pcf
NO MOVEMENÏ UPON
WETTING
D-4546.
not b€ Eproducsd, orc€pt ìn
17 -7 - 462.01 Kumar & Associates SWTLL-CONSOLIDAIION TTST RESULTS Fig. 3
I Grt iffi'ffi¡"fffii*'YËü**'TABLE ISUMMARY OF LABORATORY TEST RESULTSNo.l7-7-.162.01SOIL TYPESandy Silt with Grar.elSilty Sandy GravelSandy Silt with GravelUNCONFINEDCOMPRESSIVESTRENGTHLIQUID LÍUIITPLASTICINDÐ(PERCENTPASSING NO200 stEVE592022.9('/,)SAND%tGRAVEL8099I4v,I226.25.3SAMPLE LOCATIONDEPTHBORINGNATURALDRYDENSITYNATURALMOISTURECONTENT