HomeMy WebLinkAboutSubsoil for Foundation DesignSUBSOIL STUDY
Hqmmrh·Pil11:Llk Oc11rcchnic·il, Inc
5020 Count' RuaJ 1;~
Glcrurnll<l Sprm;?s. Columdu 8160!
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FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 32, HERON CROSSING AT IRONBRIDGE
RIVER BEND \VAY
GARFIELD COUNTY, COLORADO
JO:P N0.116 210A
JUNE9,2016
PREPARED FOR:
RM CONSTRUCTION
ATTN : ERIC LINTJER
5030 COUNTY ROAD 154
GLENWOOD SPRINGS, COLORADO 81601
eric@bulldwithrm.com
Parker 303-841-7119 ° Colorndo Springs 719-633-5562 • Silverthorne 970-468-1989
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ............................................................................ -l -
PROPOSED CONSTRUCTION •..............•.....•...........................•..........................•....... - l -
SITE CONDITIONS ......................................................................................................... - 2 -
SUBSIDENCE POTEITTIAL ...••.•..........•..•.•..............•................•..........•.••............•.•..... -2-
'FIELD EXPLORATION ................................................................................................... 3 ..
SUBSURFACE CONDITIONS ........................................................................................ -3 -
FOUNDATION BEARING CONDITIONS .................................................................. • 4-
DESIGN RE.COMMENDATIONS ........•.........•.•.............••••..............•.•.•.........•.•..•..•...... -4 -
FOUNDATIONS ......................................................................................................... - 4 -
FLOOR SLABS .••...........••...••.•.......•..•••...•.•..•.•..•.••••..••.•••••.......•.•••••••.....•.•.•....••..•.•••... -5 -
UNDERDRAIN SYSTEM .......................................................................................... -6 -
SURFACE DRA.Il'fAGE .............................................................................................. -6 -
L™IT ATIONS ···········-···································································································-7 -
FIGURE 1 -LOCATION OF EXPLORATORY BORINGS
FIGURE 2 -LOOS OF EXPLORATORY BORINGS
FIGURE 3 -LEGEND AND NOTES
FIGURES 4 AND 5 -SWEL~CONSOLIDATION TEST RESULTS
TABLE 1-SUMMARYOFLABORATORYTESTRESULTS
..
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for n proposed residence to be located
on Lot 32, Heron Crossing at lronbridge, River Bend Way, Garfield County, Colorado.
The project site is shown on Figure 1. The purpose of the study was to develop
recommendations for the foundation design. The study was conducted in accordance
with our agreement for geotechnical engineering services to RM Construction dated May
24, 2016. We previously conducted a preliminary subsoil study in the Heron Crossing at
lronbridge development area and presented our findings in n report dated February 28,
2014, Job No. 113 47 lA.
A field exploration program consisting of exploratory borings was conducted lo obtain
information on the subsurface conditions. Samples of the subsoils obtained during the
field exploration were tested in the laboratory to determine their classification,
compressibility or swell and other engineering characteristics. The results of the field
exploration and laboratory testing were analyzed to develop recommendations for
foundation types, depths and allowable pressures for the proposed building foundation.
This report summarizes the data obtained during this study and presenls our conclusions,
design recommendations and other geotechnical engineering considerations based on the
proposed construction and the subsurface conditions encountered.
PROPOSED CONSTRUCTION
Development plnns for the lot were conceptual at the time of our study. In general, the
proposed residence will be a single story, wood frame structure located in the building
envelope shown on Figure 1. Ground floor will be structural over crawlspace in the
living area and slab-on-grade in the garage . Grading for the structure is assumed to be
relatively minor with cut depths between about 2 to 5 feet. We assume relatively light
foundation loadings, typical of lhe proposed type of construction.
Job No 116 210A
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If building loadings. location or grading plans change significantly from those described
nbove, we should be notified to re-eva1uate the recommendations contained in this report.
SITE CONDITIONS
The Jot was vacant and had scattered grass and weeds with old hydroseed in the northern
(front) part at the time of our field exploration. The ground surface had been stripped of
topsoil and the subdivision had been graded. The ground surface slopes gently down to
the north with about 2 feet of elevation difference across the lot. River Bend Way is
asphalt paved and folJows the northern boundary of the lot.
SUBSIDENCE POTENTIAL
Bedro_ck of the Pennsylvanian age Eagle Vnlley Evaporite underlies the Ironbridge
development. These rocks are n sequence of gypsiferous shale, fine~grained sandstone
and siltstone with some massive beds of gypsum and limestone. There is a possibility
that massive gypsum deposits associated with the Eagle Valley Evaporite underlie
portions of the lot. Dissolution of the gypsum under certain conditions can cause
sinkholes to develop and cnn produce areas of localized subsidence. Several sinkholes
were observed during geologic assessments conducted for the Ironbridge development.
These sinkholes appe~d similar to others associated with the Eagle Valley Evapc>ritc in
areas of the Roaring Fork River valley. A sink.bole opened in the cart storage parking lot
in January 2005 and irregular bedrock conditions have been identified in the affordnble
housing site located roughly 500 to 1,000 feet south of the current development area.
Sinkholes were not observed in the immediate area of the subject lot. No evidence of
cavities was encountered in the subsurface materials; however. the exploratory borings
were relatively shallow, for foundation design only. Bns~d on our present knowledge of
the subsurface conditions at the site, it cannot be said for certain that sinkholes will not
develop. The risk of future ground subsidence on Lot 32 throughout the service life of
the proposed residence, in our opinion, is low; however, the owner sho1:1ld be made aware
Joh No. 116 2IOA
-3 -
of the potential for sinkhole development. If further investigation of possible cavities in
the bedrock below the site is desired, we should be contacted.
FIELD EXPLORATION
The field exploration for the project was conducted on May 25, 2016. Two exploratory
borings were drilled at the locations shown on Figure 1 to evalu:ite the subsurface
conditions. The borings were advanced with 4 -inch diameter continuous flight augers
powered by a truck-mounted CME-45B drill rig. The borings were logged by a
representative of Hepworth-Pawlak Geotechnical, Inc.
Samples of the subsoils were taken with P/a inch and 2 inch I.D. spoon samplers. The
samplers were driven into the subsoils at various depths with blows from a 140 pound
hammer falling 30 inches. This test is similar to the standard penetration test described
by ASTM Method D-1586. The penetration resistance values are an indication of the
relative density or consistency of the subsoils. Depths at which the samples were taken
and the penetration resistance values are shown on the Logs of Exploratory Borings.
Figure 2. The samples were returned to our laboratory for review by the project engineer
and testing.
SUBSURFACE CONDITIONS
Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2.
The subsoils consist of about 8 to 1 OV7 feet of stiff, slightly moist, sandy. silty clay
overlying dense, slightly silty, sandy gravel and cobbles with boulders. Drilling in the
coarse granular soils with auger equipment was difficult due to the cobbles and boulders
and drilling refusal was encountered in the deposit.
Laboratory testing performed on samples obtained from the borings included natural
moisture content and density and finer than sand size gradation analyses. Results of
sweU-consolidation testing performed on relatively undisturbed drive samples of the clay
Job No . 116 '.?IOA
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and silt soils, presented on Figure 4 and 5, indicate low to moderate compressibility under
conditions of loading and wetting in the sample from Boring 1 al 2¥1 feet. The sample
from Boring 2 at S feet showed a low collapse potential (settlement under constant
loading) when wetted and high compressibility under additional loading. The laboratory
testing is summarized in Table I.
No free water was encountered in the borings nt the time of drilling and the subsoils were
slightly moist.
FOUNDATION BEARING CONDITIONS
The upper snndy, silty clay soils have low bearing capacity and low to moderate
compressibility mainly when wetted. Shallow spread footing5 placed on the natural clay
soils can be used with a risk of settlement as described below. The footing bearing level
should be at least 3 feet below existing ground surface so there is no more than 7 feet of
compressible soils below the bearing level. Use of n deep foundation placed on the
underlying dense gravel nnd cobbles soils could be used to achieve a low settlement risk.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the
nature of the proposed construction, the building can be founded with spread footings
bearing on the natural clay and silt soils with a settlement risk. If n deep foundation is
desired, we should be contacted for supplemental recommendations.
The design and construction criteria presented below should be observed for a spread
footing foundation system.
l) Footings placed on the undisturbed natural soils should be designed for an
allowable bearing pressure of 1,000 psf. Based on experience. we expect
Job No . l 16 llOA
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inilial settlement of footings designed and constructed as discussed in this
section will be about Vl to I inch or less. Additional differential settlement
up to about I inch could occur if the bearing soils are wetted.
2) The footings should have a minimum width of 20 inches for continuous
walJs and 2 feel for isolated pads.
3) Exterior footings and footings beneath unheated areas should be provided
with adequate soil cover above their bearing elevation for frost protection.
Placement of foundations at least 36 inches below exterior grade is
typically used in this area.
4) Continuous foundation walls should be heavily reinforced top and bottom
to span local anomalies such as by assuming an unsupporled length of at
least 14 feet. Foundation walls acting as retaining structures should also
be designed to resist a lateral eanh pressure corresponding to an equivalent
fluid unit weight of at least 55 pcf for the onsite clay and silt soil as
backfill.
5) Any existing fill, topsoil and loose or disturbed soils should be removed in
the footing nreas. The exposed soils in footing area should then be
moistened and compacted.
6) A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
FLOOR SLABS
The natural on-site soils, exclusive of topsoil, can be used to support lightly loaded slab-
on-grade construction with a settlement risk if the bearing soils are wetted. 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.
Aoor 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.
Job No. l 16 210A
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All fill materials for support of floor slabs should be compacted to nt least 95% of
maximum standard Proctor density at a moisture content near optimum. Required fill can
consist of the on-site clay nnd silt soils devoid of vegetation and topsoil.
UNDERDRAIN SYSTEM
It is our understand ing that the finished floor elevation at the lowest level will be al or
above the surrounding grade. Therefore, a foundation drain system is not required. It has
been our experience in the area that local perched groundwa ter 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
and basement areas, be protected from wetting nnd hydrostatic pressure buildup by an
underdrain and wall drain system. An undcrdrain should not be provided around the
crawlspace to help prevent surface water infiltration down to the bearing soils.
If the finished floor elevation of the proposed structure has a basement level , we should
be contacted to provide recommendations for an underdrain system. All earth retaining
structures should be properly drained .
SURFACE DRAINAGE
Providing proper surface grading nnd drainage is very important to the satisfactory
performance of the building. The following drainage precautions should be observed
during construction and maintained at all times after the ~sidence has 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% 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.
3) The ground sudnce surrounding the exterior of the building should be
sloped to drain away from the foundation in all directions. We
Job No. ll6 210A
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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 IO feet in paved areas .
Drainage swales should have a minimum slope of 3%.
4) Roof gutters should be provided with downspouts and drains that
discharge well beyond the limits of all backfill.
5) Landscaping which requires regular heavy irrigation such as sod should be
located at least IO feet from foundations. Consideration should be given to
use of xerisc<ipe lo reduce the potential for wetting of soils below the
building caused by irrigation .
LIMITATIONS
This study has been conducted in accordance with generJlly 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 borings drilled at the locations
indicated on Figure I, 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 chis special field of practice should be
consulted. Our findings include interpolation and extrapolation of the subsurface
conditions identified at the exploratory borings and variations in the subsurface
conditions may not become evident until excavation is perfonned. If conditions
encountered during construction appear different from those described in this report, we
should be notified so that re -evaluation of the recommendations may be made.
This report has been prepared for the exclus ive use by our client for design purposes. We
are not responsible for technical interpretations by others of our infonnation. 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
verify that the recommendations have been appropriately interpreted. Significant design
Jcb No 116 210A.
-8·
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 n representative of the geotechnicnl
engineer.
Respectfully Submitted,
SLP/ksw
cc:
Job No . 116 2lOA
116 210A
RIVER BEND WAY
APPROXIMATE SCALE
1· = 30'
------~
LOT33
LOCATION OF EXPLORATORY BORINGS Figure 1
0
5
~
I
..c:. a.
~ 10
15
116 210A
BORING 1
ELEV.= 5931 .5'
9/12
WC=7.3
DO::i:98
11/12
WC=72
00=94
·200•76
1516.31/6
1916.50/3
BORING2
ELEV.= 5930 5'
111'2
WCcs66
00•100
·200=83
10/12
WC •7.0
00.,.90
47112
501'2
Note: Explanation of symbols is shown on Figure 3.
LOGS OF EXPLORATORY BORINGS
0
5
~ •
£ a.
Cl) c
10
15
Figure 2
LEGEND: Q CLAY (CL); silty, sandy, stilf, slightly moist, brown, slightly porous
~ GRAVEL AND COBBLES (GM·GP); silty, sandy, boulders . dense, slightly moist, brown, rounded rock
p Relatively undisturbed drive sample; 2-inch 1.0. California liner sample.
~ Drive sample; standard penetration test (SPl), 1 3/8inch1 .0. split spoon sample, ASTM D-1586.
10112
Drive sample blow count; indicates that 10 blows of a 140 pound hammer falling 30 inches were
required to drive the California or SPT sampler 12 inches.
T Practical drilling rerusel.
NOTES:
1. Exploratory borings were drilled on May 25, 2016 with 4-inch diameter continuous flight power auger.
2. Locations or exploratory borings were measured approximately by pacing from features shown on the site plan
provided.
3 . Elevations or exploratory borings were approximated from contours shown on Figure 1.
4. The exploratory boring locations and elevations should be considered accurate only lo lhe degree implied by the
method used .
5. The tines between materials shown on the exploratory boring logs represent the approximate boundaries between
material types and transitions may be gradual.
6. No free water was encountered in the borings at the ti me of drilling . Fluctuation in water level may occur with lime.
7. Laboratory Testing Results :
WC = Water Content (%)
DD = Dry Density (pcl)
-200 = Percent passing No . 200 sieve
116 210A ~ Heoworth-Pawlak Geotec:hnlcal
LEGEND AND NOTES Figure 3
Moisture Content = 7.3 percent
Dty Density ... 98 pcf
Sample of: Sandy Silty Clay
From: Boring 1 at 2 ~ Feel
0
~ -i,...-.... -r--1--~
i"'o-( 1 n"
" ,--i"'o ... ,. ,.. _No movement
!'\. upon
~ 2 welling
c ~
.Q \ Cl) en 3 Q)
0.
E \ 8
4
1
5 1[>
.
.
01 t.O 10 100
APPLIED PRESSURE-ksf
116 210A catech SWELL-CONSOLIDATION TEST RESULTS Figure 4
HSJllWO!nl+PAWLAK G£0nECHNIC:.U.
Moisture Content -7.0 percent
Dry Density = 90 pcf
Sample of: Sandy Silty Clay
From: Boring 2 al 5 Feet
0 -----... :·o
1 .----~ Compression v ..... i--~'"'upon .c:::. i--i.-wetting '" 2 "' I c:; \ 0 'iii
tJI 3 CJ ...... a. \ E
0
C)
4
5
\
6 \
7 \
\
a I
9 \
\
10
11 \
I~
12
OT 10 10 100
APPLIED PRESSURE • ksf
116210A ~ch SWELL-CONSOLIDATION TEST RESULTS Figure 5
HEJ>WORTH-PA~K G£0Tl'.C:HNIC:AL
HEPWORTH-PAWLAK GEOTECHNICAL, INC.
TABLE1 · Job No. 116 210A
SUMMARY OF LABORATORY TEST RESULTS
SAMPLE LOCATION NATURAL NATURAL GRADATION ATTERBERG UMTTS UNCONFINED PERCENT MOISTURE ORV GRAVEL SAND PASStNG LIQUID PLASTIC COMPRESSIVE SOIL OR BORING DEPTH CONTENT DENSITY (%) N0.200 LIMIT INDEX STRENGTH BEDROCK TYPE (%)
uu "'' I Den SIEVE
(%) '"'' lPSs:\
1 2Vi 7.3 98 Sandy, Silty Clay
5 7.2 94 76 Sandy, Silty Clay
2 21h 6.6 100 83 Sandy, Silty Clay
5 7.0 90 Sandy, Silty Clay