HomeMy WebLinkAboutSuboil Study 11.26.2014Ge"kol 1-f
o9o'f tech
HEPWORTH-PAWLAK GEOTECHNICAL
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT F 7, ASPEN GLEN
GARFIELD COUNTY, COLORADO
JOB NO. 114 437A
NOVEMBER 26, 2014
PREPARED FOR:
WOODBRIDGE MORTGAGE INVESTMENT FUND 2, LLC
ATTN: RICK SALVATO
22 CENTER STREET, FRONT SUITE
FREEHOLD, NEW JERSEY 07728
(risalvQuaol.con
Parker 303-841-7119 0 Colorado Springs 719-6-33-5562 0 Silverthorne 970-468-1989
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY..........................................................................- 1 -
PROPOSED CONSTRUCTION.................................................................................. - 1 -
SITECONDITIONS....................................................................................................- 2-
SUBSIDENCE POTENTIAL.......................................................................................- 2-
FIELDEXPLORATION..............................................................................................- 3-
SUBSURFACE CONDITIONS..............................._....................................................- 3-
DESIGN RECOMMENDATIONS...............................................................................- 4 -
FOUNDATIONS......................................................................................................- 4-
FOUNDATION AND RETAINING WALLS..........................................................: 5-
FLOORSLABS.................................................................................................:.....- 6 -
UNDERDRAINSYSTEM........................................................................................- 6-
SITEGRADING......................................................................................................- 7-
SURFACEDRAINAGE..........................................................................................- 8-
LIMITATIONS...........................................................................................................: 8-
REFERENCES.............................................................................................................- 9-
FIGURE I - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURE 4 - GRADATION TEST RESULTS
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located at
Lot F 7, Aspen Glen Subdivision, 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 proposal for
geotechnical engineering services to Woodbridge Mortgage Investment Fund 2, LLC
dated October 2, 2014. Chen -Northern, Inc. (1991 and 1993) previously conducted
preliminary geotechnical engineering studies for the development and preliminary plat
design.
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 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 presents our conclusions, design recommendations and other
geotechnical engineering considerations based on the proposed construction and the
subsurface conditions encountered.
PROPOSED CONSTRUCTION
Building plans for the proposed residence are conceptual. Typical residences in the area
are one and two story wood frame above a basement or crawlspace with an attached
garage. Ground floors are typically slab -on -grade. Grading for the proposed structure is
assumed to be relatively minor with cut depths between about 3 to 10 feet. We assume
relatively light foundation loadings, typical of the assumed type of construction.
If building loadings, location or grading plans change significantly from those described
above, we should be notified to re-evaluate the recommendations contained in this report.
Job No. 114 437A
Ge Ptech
-2-
SITE CONDITIONS
The vacant property is located at the cul-de-sac of River Park Lane. Vegetation consists
of sparse grass and weeds. Topography at the site consists of relatively flat upper and
lower terraces separated with by a steep slope. The terraces have a slight slope down to
the northwest. Elevation difference between the terraces is about 15 feet. A wetland area
and drainage are located beyond the site to the northwest. The upper terrace appears to
have been graded with minor cuts during subdivision development and gravel and cobbles
are exposed on the ground surface. A golf course fairway is located along the northern
property line.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Aspen Glen
development. These rocks are a sequence of gypsiferous shale, fine-grained
sandstone/siltstone and limestone with some massive beds of gypsum. 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 can produce areas of localized subsidence. During previous
studies in the area, several broad subsidence areas and smaller size sinkhole areas were
observed scattered throughout the Aspen Glen development, predominantly on the east
side of the Roaring Fork River (Chen -Northern, Inc., 1993). These sinkholes appear
similar to others associated with the Eagle Valley Evaporite in areas of the Roaring Fork
River valley.
Lot F 7 is located just outside of one of the broad subsidence areas mapped by Chen -
Northern. Signs of active ground movements have not been observed in the subsidence
area. The nearest sinkhole was mapped about 900 feet to the northeast of Lot F 7.
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
Job No. 114 437A
GeMech
-3-
were relatively shallow, for foundation design only. Based 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 F 7 throughout the service life of
the proposed residence, in our opinion, is low but the site should not be considered totally
risk free. 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 October 7, 2014. Two 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. A previous boring (B-5-93) was
drilled near the front of the lot by Chen -Northern, Inc.
Samples of the subsoils were taken with a 1% inch I.D. spoon sampler. The sampler was
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 encountered consist of silty sandy gravel with cobbles and boulders. Results
of gradation analyses performed on a small diameter drive sample (minus 11/z inch
fraction) of the coarse granular subsoils are shown on Figure 4. Drilling in the dense
Job No. 114 437A
GecPtech
granular soils with auger equipment was difficult due to the cobbles and boulders and
drilling refusal was encountered in the deposit at relatively shallow depths.
No free water was encountered in the borings at the time of drilling and the subsoils were
slightly moist to moist. Chen -Northern, Inc. Boring B-5-93 indicates the granular soils
extend down to about 60 feet and are underlain by Eagle Valley Evaporite Bedrock.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the
nature of the proposed construction, we recommend the building be founded with spread
footings bearing on the natural granular soils.
The design and construction criteria presented below should be observed for a spread
footing foundation system.
1) Footings placed on the undisturbed natural granular soils should be
designed for an allowable bearing pressure of 2,500 psf. Based on
experience, we expect settlement of footings designed and constructed as
discussed in this section will be about 1 inch or less.
2) The footings should have a minimum width of 16 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.
4) Continuous foundation walls should be reinforced top and bottom to span
local anomalies such as by assuming an unsupported length of at least 10
feet. Foundation walls acting as retaining structures should also be
designed to resist lateral earth pressures as discussed in the "Foundation
and Retaining Walls" section of this report.
5) All topsoil and any loose or disturbed soils should be removed and the
footing bearing level extended down to the relatively dense natural
Job No. 114 437A C,CPtE--Ch
-5-
granular soils. The exposed soils in footing area should then be moistened
and compacted. If water seepage is encountered, the footing areas should
be dewatered before concrete placement.
6) A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
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 45 pcf
for backfill consisting of the on -site soils. Cantilevered retaining structures which are
separate from the residence 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 recommended 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.
Backfill 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.
Job No. 114 437A
G'(�0'1'teCh
I Me
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 0.50. Passive pressure of compacted
backfill against the sides of the footings can be calculated using an equivalent fluid unit
weight of 400 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.
FLOOR SLABS
The natural on -site soils, exclusive of topsoil, are suitable to support lightly 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 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
aggregate with at least 50% retained on the No. 4 sieve and less than 2% passing the No.
200 sieve.
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 granular 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
Job No. 114 437A
Gc5&' ech
-7-
precipitation or seasonal runoff. Frozen ground during spring runoff can also 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 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. 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 1 %2 feet deep.
SITE GRADING
The risk of construction -induced slope instability at the site appears low provided cut and
fill depths are limited. We assume the cut depths for the basement level (if any) will not
exceed one level, about 10 to 12 feet. Fills should be limited to about 8 to 10 feet deep at
the downhill side of the residence where the slope steepens. Embankment fills should be
compacted to at least 95% of the maximum standard Proctor density near optimum
moisture content. Prior to fill placement, the subgrade should be carefully prepared by
removing all vegetation and topsoil and compacting to at least 95% of the maximum
standard Proctor density. The fill should be benched into the portions of the hillside
exceeding 20% grade.
Permanent unretained cut and fill slopes should be graded at 2 horizontal to 1 vertical or
flatter and protected against erosion by revegetation or other means. The risk of slope
instability will be increased if seepage is encountered in cuts and flatter slopes may be
necessary. If seepage is encountered in permanent cuts, an investigation should be
conducted to determine if the seepage will adversely affect the cut stability. This office
should review site grading plans for the project prior to construction.
Job No. 114 437A
GeH<Pitech
SURFACE DRAINAGE
The following drainage precautions should be observed during construction and
maintained at all times after the residence 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 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.
Free -draining wall backfill should be capped with about 2 feet of the on -
site soils to reduce surface water infiltration.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill.
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 locations
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
Job No. 114 437A
Mech
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 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 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 GEOTECHNICAL, INC.
Louis E. Eller
Reviewed by:
Daniel E. Hardin, P.E.
LEE/ksw
REFERENCES
Chen -Northern, Inc., 1991, Preliminary Geotechnical Engineering Study, Proposed
Aspen Glen Development, Garfield County, Colorado, prepared for Aspen Glen
Company, dated December 20, 1991, Job No. 4 112 92.
Chen -Northern, Inc., 1993, Geotechnical Engineering Study for Preliminary Plat Design,
Aspen Glen Development, Garfield County, Colorado, prepared for Aspen Glen
Company, dated May 28, 1993, Job No. 4 112 92.
Job No. 114 437A CagUPb�
WETLANDS
APPROXIMATE SCALE
1"=50'
114 437A
1
1
LOT F7
000*
'00
` Q ok
BORING 2
1
BENCH MARK: GROUND AT PROPERTY 1
CORNER; ELEV. = 100.0', ASSUMED.
BORING
1
LOT F 6 1
i
i
1,
B-5-93 O
RIVER
PARK LANE
LOT F 8
LOCATION OF EXPLORATORY BORINGS I Figure 1
BORING 1
BORING 2
ELEV.= 102.3
ELEV.= 99.5'
105
105
o�:
100
100
b 21,50/3
a)
LL
+4=54
LL
-200-11
'
o
g
a'
roe
50/6
co
w
95
95
w
90
90
Note: Explanation of symbols is shown on Figure 3.
1—I
114
437A
LOGS OF EXPLORATORY BORINGS
Figure
2
He worth—Pawlak Geatechnical
LEGEND:
0o GRAVEL, COBBLES AND BOULDERS (GP -GM); sandy, silty, dense, slightly moist, brown, subrounded rocks.
e
Drive sample; standard penetration test (SPT), 1 3/8 inch I.D. split spoon sample, ASTM D-1586.
39/12 Drive sample blow count; indicates that 39 blows of a 140 pound hammer falling 30 inches were
required to drive the SPT sampler 12 inches.
TPractical drilling refusal.
NOTES
1. Exploratory borings were drilled on October 7, 2014 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 measured by instrument level and refer to the Bench Mark shown on Figure 1.
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 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:
+4 = Percent retained on the No. 4 sieve
-200 = Percent passing No. 200 sieve
114 437A I I LEGEND AND NOTES Figure 3
Hepworth—Pawlak Geotechnical