HomeMy WebLinkAboutSoils Report 02.08.2016ed
HEPWORTH-PAWLAK GEOTECHNICAL
Flcpworth-PawI.4. G otfhnicil, Inc -
5020 County Rn.ttl 154
Oknwooil Sprint , CoLirtdo 81601
Phone' 970.945.7988
Fax- 970-945 8454
email: hpgea@hpgeotech.com
SUBSOIL STUDY
FOR PRELIMINARY FOUNDATION DESIGN
PROPOSED RESIDENCES
LOTS 22, 30, 39 AND 52 HERON CROSSING AT IRONBRIDGE
GARFIELD COUNTY, COLORADO
JOB NO. 116 014A
FEBRUARY 8, 2016
PREPARED FOR:
BLUE HERON PROPERTIES, LLC
ATTN: JIM LIGHT
430 IRONBRIDGE DRIVE
GLENWOOD SPRINGS, COLORADO 81601
ilight@chaffinlight.com
Parker 303-841-7119 • Colorado Springs 719-633-5562 Silverthorne 970-468.1989
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY - 1 -
PROPOSED CONSTRUCTION - 1 -
SITE CONDITIONS - 2 -
SUBSIDENCE POTENTIAL - 2 -
FIELD EXPLORATION - 3 -
SUBSURFACE CONDITIONS - 4 -
FOUNDATION BEARING CONDITIONS - 4 -
PRELIMINARY DESIGN RECOMMENDATIONS - 5 -
FOUNDATIONS - 5 -
FLOOR SLABS - 6 -
UNDERDRAIN SYSTEM - 7 -
SURFACE DRAINAGE - 7 -
LIMITATIONS - 8 -
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURES 4 THROUGH 7 - SWELL-CONSOLIDATION TEST RESULTS
FIGURE 8 - GRADATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for proposed residences to be located on
Lots 22, 30, 39 and 52, Heron Crossing at Ironbridge, Garfield County, Colorado. The
project site is shown on Figure 1. The purpose of the study was to develop preliminary
recommendations for foundation design. The study was conducted in general accordance
with our proposal for geotechnical engineering services to Blue Heron Properties dated
January 7, 2016 which included a scope of 8 exploratory borings. The proposal was
modified by an email message from Eric Foerster dated January 25, 2016 to only include
an exploratory boring on the four lots listed above. We previously conducted a
preliminary subsoil study in the Heron Crossing at Ironbridge development area which
included 4 exploratory borings and presented our findings in a report dated February 28,
2014, Job No. 113 471A.
A field exploration program consisting of exploratory borings was conducted to 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
preliminary foundation types, depths and allowable pressures of a building on each lot.
This report summarizes the data obtained during this study and presents our conclusions,
recommendations and other geotechnical engineering considerations based on the
proposed construction and the subsurface conditions encountered.
PROPOSED CONSTRUCTION
The proposed residence construction had not been determined at this time and we
understand the findings of this study will be considered in the purchase of the lots. For
the purpose of our study, we assume the residences will be a one or two story, wood
frame structure with or without basement level and located in the building envelope of
each lot as shown on Figure 1. Ground floor could be structural over crawlspace or slab -
Job No. 116 014A setiztech
-2 -
on -grade. Grading for the structures is assumed to be relatively minor with cut depths
between about 3 to 8 feet. We assume relatively light foundation loadings, typical of the
assumed type of construction.
When building loadings; location and grading plans have been determined on a lot by lot
basis, additional geotechnical evaluation, including additional subsurface exploration and
analysis should be performed to develop design level recommendations.
SITE CONDITIONS
The lots were vacant at the time of our field exploration and covered with about 1 to 11/2
feet of snow. The ground surface had generally been rough graded with shallow cuts and
fills up to about 3 to 5 feet deep based on the grading plan for the development. River
Bend Way is asphalt paved and Blue Heron Drive appeared to have been graded to
subgrade level. The natural ground surface generally slopes gently down to the northeast
with on the order of 5 feet or less elevation difference across each building footprint.
Lots 30 and 52 appeared to have been cut up to on the order of 3 to 4 feet below original
ground surface and Lots 22 and 39 appeared to have been filled up to on the order of 3 to
4 feet above original ground surface.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Ironbridge
development. These rocks are a 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 lots. Dissolution of the gypsum under certain conditions can cause
sinkholes to develop and can produce areas of localized subsidence. Several sinkholes
were observed during geologic assessments conducted for the Ironbridge development.
These sinkholes appeared similar to others associated with the Eagle Valley Evaporite in
areas of the Roaring Fork River valley. A sinkhole opened in the cart storage parking lot
Job No. 116 014A Ggistech
-3 -
in January 2005 and irregular bedrock conditions have been identified in the affordable
housing site located roughly 500 to 1,000 feet south of the current development area.
Sinkholes have not been identified in the immediate area of the subject lots. No evidence
of cavities was encountered in the subsurface materials in the boring on each lot;
however, the exploratory borings were relatively shallow, for foundation design only.
Based on our present knowledge of the subsurface conditions at each lot, it cannot be said
for certain that sinkholes will not develop. The risk of future ground subsidence on Lots
22, 30, 39 and 52 throughout the service life of the proposed residences, in our opinion, is
low; however, the owner of each lot should be made aware 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 January 27 and 29, 2016. Four
exploratory borings were drilled at the locations shown on Figure 1 to evaluate 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. The snow had been plowed to
each boring location to allow the truck rig access.
Samples of the subsoils were taken with 1% 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.
Job No, 116 014A
-4 -
SUBSURFACE CONDITIONS
Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2.
The subsoils typically consist of about 6 to 13 feet of sandy silt and 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. Borings 1, 2 and 4 appear to be native soils and Boring 3
appears to be recently placed fill soils. Borings drilled for our previous study in this area
and their location shown on Figure 1 encountered between about 1 to 16 feet of silt and
clay soil above the dense gravel alluvium.
Laboratory testing performed on samples obtained from the borings included natural
moisture content and density and gradation analyses. Results of swell -consolidation
testing performed on relatively undisturbed drive samples of the silt and clay soils,
presented on Figures 4 through 7, indicate low to moderate compressibility under
conditions of light loading and wetting. The samples typically showed a low collapse
when wetted under constant light loading and moderate to high compressibility under
additional Ioading after wetting. The two samples taken from a depth of 10 feet at
Borings 2 and 4 (Lots 30 and 52) showed a minor expansion potential when wetted. The
laboratory testing is summarized in Table 1.
No free water was encountered in the borings at the time of drilling and the subsoils were
slightly moist.
FOUNDATION BEARING CONDITIONS
The upper silt and clay soils have low bearing capacity and low to moderate
compressibility potential mainly when wetted. Shallow spread footings placed on the
natural silt and clay soils can be used with a risk of settlement as described below. The
footing bearing level on Lots 22, 30 and 52 should be deepened below existing ground
surface so there is no more than 6 feet of silt and clay soils below the bearing level as a
Job No. 116 014A
foundation settlement mitigation measure,
-5-
In sub -excavated areas to below design
footing bearing level, and on Lot 39 where existing fill is removed, the onsite soils could
be replaced compacted. Extending footing bearing level down or use of a deep
foundation placed on the underlying dense gravel and cobbles soils could be used to
achieve a low settlement risk. The building settlement and distress can also be Iimited by
use of a structural slab/mat or post tensioned slab foundation.
PRELIMINARY DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the
nature of the proposed construction, the buildings be founded with spread footings
bearing on the natural silt and clay soils or on compacted structural fill with a settlement
risk. If a deep foundation or structural slab/mat 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.
1) Footings placed on a limited depth of the undisturbed natural soils or on
compacted structural fill should be designed for an allowable bearing
pressure of 1,500 psf. Based on experience, we expect initial settlement of
footings designed and constructed as discussed in this section will be about
1/2 to 1 inch or less. Additional differential settlement up to about 1 inch
could occur if the bearing soils are wetted.
2) The footings should have a minimum width of 20 inches for continuous
walls and 2 feet 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.
Job No. 116 014A
-6-
4) Continuous foundation walls should be heavily reinforced top and bottom
to span local anomalies such as by assuming an unsupported length of at
Ieast 14 feet. Foundation walls acting as retaining structures should also
be designed to resist a lateral earth pressure corresponding to an equivalent
fluid unit weight of at least 55 pcf for the onsite silt and clay soil as
backfill.
5) The existing fill, topsoil and loose or disturbed soils should be removed in
the footing areas. The exposed soils in footing area should then be
moistened to near optimum and compacted. Structural fill should extend
laterally beyond the footing edges at least 1/2 the fill depth below the
footing and be compacted to at least 98% of standard Proctor density at
near optimum moisture content. The soils should be protected from frost
and concrete should not be placed on frozen soils.
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. Structural fill
about 2 feet deep consisting of the onsite soils can be used to limit the settlement risk. To
reduce the effects of some differential movement, non-structural 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.
All fill materials for support of floor slabs should be compacted to at least 95% of
maximum standard Proctor density at a moisture content near optimum. Required fill can
consist of the on-site silt and clay soils devoid of vegetation and topsoil.
Job No, 116 014A G ted i
7
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 site retaining walls and
basement areas, be protected from wetting and hydrostatic pressure buildup by an
underdrain system. An underdrain should not be used where there is shallow crawlspace
(usually about 4 feet or less in height) and where slab -on -grade is near surrounding
exterior grade, such as garages because the drain could result in wetting of shallow
footing bearing level.
Where provided, 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 1 foot below lowest adjacent
finish grade and sloped at a minimum 1% to a suitable gravity outlet or sump. Free -
draining granular material used in the underdrain system should 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 Ph feet deep. An
impervious membrane such as a 20 mil PVC liner should be placed beneath the drain
gravel in a trough shape and attached to the foundation wall with mastic to prevent
wetting of the bearing soils.
SURFACE DRAINAGE
Providing proper surface grading and 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 each residence has been completed:
1) Inundation of the foundation excavations and underslab areas should be
avoided during construction.
Job No. 116 014A Giggled,
-8-
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 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 paved areas.
Soil lined drainage swales should have a minimum slope of 3 to 4%.
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 10 feet from foundations. Consideration should be given to
use of xeriscape to reduce the potential for wetting of soils below the
building caused by irrigation.
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 borings drilled at the Iocations
indicated on Figure 1, 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 extrapolation of the subsurface
conditions identified 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 so that re-evaluation of the recommendations may be made.
Job Na 116 0I4A Geztech
This report has been prepared for the exclusive use by our client for planning and
preliminary design purposes. We are not responsible for technical interpretations by
others of our information. As the project evolves, we should conduct additional
subsurface exploration and 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
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.
Respectfully Submitted,
HEPWORTH - PAWLAK GE r . NICAL, INC.
4*. eta
Steven L. Pawlak, P.E.
Reviewed by:
4
trY
'PcR S.222
Daniel E. Hardin, P.E.
SLP/ksw
ii
Job No. 11 b 014A GalgbEGh
APPROXIMATE SACLE:
1" = 120'
• BORING FOR CURRENT
STUDY
O BORING FOR PREVIOUS
STUDY (JOB NO. 113 471A,
FEB. 28, 2014)
(8) DEPTH IN FEET TO TOP
OF GRAVEL ALLUVIUM
NOTE: CONTOURS SHOWN ARE
PRE -DEVELOPMENT
LOCATION OF EXPLORATORY BORINGS
Figure 1
1
m
cuu
0
5
10
15
20
25
BORING 1
LOT 22
15/12
WC=9.1
DD=101
-200=87
12/12
WC=5.9
DD=95
-200=71
13/12
WC=3 2
+4=3
-200=22
BORING 2
LOT 30
://1'
14/12
/J WC=5.3
D0=94
20/12
WC=6.6
DD=110
-200=88
21/12
WC=8.8
DD=103
9/6,50/3
BORING 3
LOT 39
19/12
WC=83
DD=102
-200=76
20/12
WC=3 5
DD=115
-200=47
50/3
Note: Explanation of symbols is shown on Figure 3.
116 014A
BORING 4
LOT 52
12/12
WC =8.3
DD= 107
11/12
WC=7-2
DD=103
-200=85
13/12
WC=8.7
D0=109
50/5
LOGS OF EXPLORATORY BORINGS
5
10
t
a.
0
15
20 ~
25
Figure 2
LEGEND:
El
R a�
111
111
15/12
T
SILT AND CLAY (ML -CL); sandy, scattered gravel (possible fill) in Boring 3 (Lot 39), very stiff, slightly moist,
brown, low plasticity, slightly porous.
SAND, GRAVEL AND COBBLES (GM -GP); silty to very silty, boulders, sand layer at Boring 1 (Lot 22), dense,
slightly moist, brown, rounded rock.
Relatively undisturbed drive sample; 2 -inch I.D. California liner sample.
Drive sample; standard penetration test (SPT), 1 3/8 inch I.D. split spoon sample, ASTM D-1586.
Drive sample blow count; indicates that 15 blows of a 140 pound hammer falling 30 inches were
required to drive the California or SPT sampler 12 inches.
Practical drilling refusal.
NOTES:
1. Exploratory borings were drilled on January 27 and 29, 2016 with 4 -inch diameter continuous flight power auger.
2. Locations of exploratory borings were measured approximately by pacing from features shown on the site plan
provided.
3. Elevations of exploratory borings were not measured and the logs of exploratory borings are drawn to depth.
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 transitions may be gradual.
6. No free water was encountered in the borings at the time of drilling. Fluctuation in water level may occur with time.
7. Laboratory Testing Results:
WC = Water Content (%)
DD = Dry Density (pcf)
+4 = Percent retained on the No. 4 sieve
-200 = Percent passing No, 200 sieve
l' 116 014A
LEGEND AND NOTES
Figure 3
Compression %
Compression - Expansion
0
1
2
3
1
0
1
2
0.1
1.0 10
APPLIED PRESSURE - ksf
100
Moisture Content = 8.7 percent
Dry Density = 109 pcf
Sample of: Sandy Silty Clay
From: Boring 4 at 10 Feet
Expansion
upon
wetting
0.1
1.0 10
APPLIED PRESSURE - ksf
100
Moisture Content = 8.3 percent
Dry Density = 107 pci
Sample of: Sandy Silt and Clay
From: Boring 4 at 2 Y Feet
Compression
.upon
wetting
\>
L
0.1
1.0 10
APPLIED PRESSURE - ksf
100
Moisture Content = 8.7 percent
Dry Density = 109 pcf
Sample of: Sandy Silty Clay
From: Boring 4 at 10 Feet
Expansion
upon
wetting
0.1
1.0 10
APPLIED PRESSURE - ksf
100
3/445
Compression
upon
wetting
Moisture Content = 5.9 percent
Dry Density = 95 pcf
Sample of: Sandy Silt and Clay
From: Boring 1 at 4 Feet
Compression
0
1
2
3
4
5
0.1
116014A
1.0
APPLIED PRESSURE - ksf
10 100
H
Hepworth--Pardak Gootechnical
SWELL -CONSOLIDATION TEST RESULTS
Figure 4
1
0
0
SWELL -CONSOLIDATION TEST RESULTS
m
co
C
(D
� I - 3Hf1S98d 0311ddd
O
0
1
CO
CO
Compression %
f
C
0
ru
W CD
0 4
0 G)
r0 N
Da a
CD CO
CO Q
n
0
cp0
N
co
= Iualua3 3Jnlsioy�
v -O
C) 0
a
1
Compression - Expansion %
Compression %
1
0
1
2
0
1
2
3
4
5
6
Moisture Content = 8.8 percent
Dry Density = 103 pcf
Sample of: Sandy Silty Clay
From: Boring 2 at 10 Feet
Expansion
upon
wetting
0.1
1.0 10
APPLIED PRESSURE - ksf
100
i
Moisture Content = 8.3 percent
Dry Density = 102 pcf
Sample of: Sandy Silt and Clay
From: Boring 3 at 2 y2 Feet
Compression
_upon
wetting
0.1
1.0 10
APPLIED PRESSURE - ksf
100
RCENT ; : ►
HYDROMETER ANALYSIS SIEVE ANALYSIS
FIR TIME READINGS U.S. STANDARD SERIES CLEAR SQUARE. OPENINGS if
0 45 MIN 15 7 MIN 60MIN19MIN 4 MIN 1 MIN. #200 #100 #50 #30 #16 #8 #4 3/8' 3/4' 1 1/2' 3' 5' 6' 8' 100
10
20
30
40
50
60
70
80
90
100
1
rr -
i
r
1.
4
001 .002 005 009 019 037 .074 .150 -300 600 1.18 2.36 4.75 9 5 19.0 37.5 76.2 152 203
12 5 127
CLAY 10 Stt
GRAVEL 3 %
DIAMETER OF PARTICLES IN MILLIMETERS
NUE
SAN i)
NCF.45,11MI LC OA
Flt CZ:AFSE
COBBLE
SAND 75 % SILT AND CLAY 22 %
LIQUID LIMIT % PLASTICITY INDEX %
SAMPLE OF: Silty Sand with Gravel FROM: Boring 1 at 9 Feet
90
80
70
60
50
40
30
20
10
0
RCENT ► t
116 014A
GRADATION TEST RESULTS
Figure 8
HEPWORTH-PAWLA, .cOTECHNICAL, INC.
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Job No. 116 014A
SAMPLE LOCATION
MOISTURE
CONTENT
(%)
NATURAL
DRY DENSITY
(POO)
GRADATION
PERCENT
PASSING NO.
200 SIEVE
ATTERBERG LIMITS
UNCONFINED
COMPRE551VE
STRENGTH
(PSF)
SOIL OR
BEDROCK TYPE
BORING/LOT
DEPTH
(ft)
GRAVEL
PO
SAND
(%}
LIQUID UMIT
(0)
PLASTY
INDEX
(%)
1/22
21/2
9.1
101
87
Sandy Silt and Clay
4
5.9
95
71
Sandy Silt and Clay
9
3.2
3
75
22
Silty Sand with Gravel
2/30
21/2
5.3
94
Sandy Silt and Clay
5
6.6
110
88
Sandy Silt and Clay
10
8.8
103
Sandy Silty Clay
3/39
21/2
8.3
102
76
Sandy Silt and CIay
4 /z
3.5
115
47
Very Sandy Silt and Clay
with Gravel
4/52
21/2
8.3
107
Sandy Silt and Clay
5
7.2
103
85
Sandy Silt and Clay
10
8.7
109
Sandy Silty Clay