Preliminary Geotechnical Subsurface Exploration And Limited Geologic Study
Proposed Airpark Village Fort Collins, Colorado
Prepared for:
Airpark Village, LLC
10763 East Mexico Avenue
Aurora, Colorado 80012
Attention: Mr. Lloyd Goff
Job Number 04-3334 October 8, 2004
ENGINEERING CONSULTANTS
41 Inverness Drive East, Englewood, CO 80112-5412
Phone (303) 289-1989
Fax (303) 289-1686
www.groundeng.com
Office Locations:
Englewood
Commerce City
Loveland
Granby
Glenwood Springs
CONCLUSIONS
The subsurface conditions encountered
in the test holes generally consisted of a layer of
topsoil/organics/vegetation or asphalt pavement, approximately 2 to 5 inches
thick, overlying sands, clays, and gravels. These materials were underlain by
claystone bedrock that extended to the test hole termination depths at
approximately 20 to 35 feet, below existing grades. Groundwater was
encountered at the time of drilling at depths of approximately 2 to 7 feet
below existing grades and at depths of approximately 2 to 5 feet below grade
when measured on September 28, 2004. Water levels will fluctuate, however, in
response to annual and longer-term cycles of precipitation, irrigation,
surface drainage, land use, and the development of transient, perched water
conditions.
Shallow groundwater and a historic collapsible soils/sinkhole-type condition
are present at the site. Specific design considerations will be necessary in
order to accommodate development. These may include regional
drainage/diversionary improvements, extensive overexcavation and processing of
the subsurface soils, and raising of site grades by several feet. Site
improvements such as infrastructure may be difficult and time intensive to
install due to potential dewatering, unstable trench slopes, and saturated
soil conditions.
Based on the subsurface conditions encountered in the test holes, the results
of our field exploration and laboratory studies, and common, generally
accepted design procedures utilized in the northern Front Range area, we
anticipate that the majority of the structures at the proposed development
could be founded on shallow foundation systems. Floor systems could consist of
slab-on-grade construction placed on a layer of properly moisture-density
treated, on-site generated or approved import fill materials. This assumes
that proper pad preparation possibly including overexcavation and replacement,
surcharging and settlement monitoring, and/or significant raising of site
grades, as discussed below, is performed as necessary once final
recommendations are developed. Depending on final site grading, site-specific
exploration, and building specific movement tolerances, some of the structures
may require deep foundation/floor systems.
We anticipate pavement sections may consist of a full depth asphalt section of
approximately 5 to 7 inches in private paved areas, such as parking and
internal drives, and approximately 9 to 11 inches in public roadways,
depending upon final roadway classifications. A minimum section of 6 to 7
inches of Portland cement concrete in areas of high truck traffic/concentrated
turning movements such as dumpster pads and loading/unloading areas (private)
may also be necessary. Pavement subgrade materials will need to be properly
moisture-density treated.
These recommendations are presented without the aid of a detailed final site
grading plan as well as a detailed final development plan, which were not
available at the time of this report preparation. To increase efficiency
during the construction process, site grading should be carefully planned to
take into consideration the preliminary foundation, floor system, and pavement
design recommendations as presented in this report. Properly placed fill
materials may significantly reduce the potential of structure, exterior
flatwork, and pavement section movements.
Additional preliminary recommendations with respect to foundations, floor
systems, site grading, water-soluble sulfates, surface drainage, geologic
concerns, and pavement design are contained herein.
PURPOSE AND SCOPE OF STUDY
This report presents the results of a geotechnical evaluation performed by
GROUND Engineering Consultants, Inc. (GROUND) to provide preliminary
geotechnical recommendations for the proposed Airpark Village development to
be located in Fort Collins, Colorado. The project site is located near the
intersection of Airway Avenue and East Lincoln Avenue (See Figure 1). Our
study was conducted in general accordance with GROUND's Proposal No. 0407-1105
(revised), dated August 25, 2004.
Field and office studies provided information regarding surface and subsurface
conditions, including existing site vicinity improvements and depths to
bedrock and groundwater. Material samples retrieved during the subsurface
exploration were tested in our laboratory to assess the engineering
characteristics of the site earth materials, and assist in the development of
our preliminary geotechnical recommendations. Results of the field, office,
and laboratory studies are presented below.
This report has been prepared to summarize the data obtained and to present
our conclusions and preliminary recommendations based on the anticipated
construction and the subsurface conditions encountered. Preliminary design
parameters and a discussion of geotechnical engineering considerations related
to construction considerations for the proposed development are included. When
final structure locations and dimensions are known, lot/parcel-specific final
geotechnical subsurface exploration programs must be performed in order
to confirm the preliminary recommendations provided as well as to provide
additional, detailed, building/parcel-specific design information.
Additionally, detailed subsurface explorations will be necessary in order to
provide pavement design recommendations for the private paved areas upon
completion of near final site grading. This report should not be used for
detailed design purposes.
PROPOSED CONSTRUCTION
We understand that proposed construction at the
site may ultimately include mixed-usage buildings generally consisting of
residential, retail, and commercial development that may include two-story
residential structures and three-story retail/office buildings. Associated
public roadways, private access drives, and parking areas are also planned. In
addition, we assume that the proposed construction will consist of extensive
overlot grading and installation of underground utilities. As stated, at the
time of this report preparation, detailed development plans, final site
grading plans, and final building layouts and roadway construction details
were not available for our review; however, based on observations conducted in
the field and preliminary site grading information provided by the Client, we
estimate that cuts and fills across the site will generally be on the order of
10 to 20 feet. Greater cuts and fills may be necessary. If the proposed
project, including the anticipated site grading, is significantly different
from that described above, GROUND should be notified to re-evaluate the
preliminary recommendations in this report.
SITE CONDITIONS
At the time of our exploration, the site
generally existed as the Fort Collins Downtown Airport and vacant pasture
land. Vegetation consisted of short to medium grasses and weeds and sparsely
located deciduous trees. The general topography of the site is gently rolling
with slopes up to 3 percent descending towards the southeast. The site varies
in elevation from approximately 4,918 to 4,940 feet msl. The site is bordered
by commercial development to the south and southwest, residential development
and International Boulevard to the north, and commercial development and
Timberline Road to the east. The airport terminal, runway, and hangars, along
with asphalt and concrete
pavement parking are located along the southern
border of the site. Indications of underground utilities were also observed at
the project site. It should be noted that a drainage pond is located near the
eastern perimeter of the site, just east of Test Hole 19. Flowing water was
observed entering this pond through a pipe that appears to extend to the
northwest an unknown distance (see pictures in Appendix). Additionally, Dry
Creek traverses the approximate center of the site.
Based on GROUND's visits to the site, we
observed localized sinkhole development within various areas of the site. In
particular, two significant sinkholes were observed immediately adjacent to
the west of the Valley Air Service maintenance building as well as in numerous
other locations within paved areas surrounding the airport terminal. This
condition is discussed in greater detail in latter sections of this report.
GEOLOGIC SETTING
The subject parcel lies within the Denver Basin
geologic province that consists largely of a sequence of sedimentary rock
formations deposited and preserved in a structural depression in north-central
Colorado. In the general project area, these sedimentary rocks dip eastward at
low angles (less than 10 degrees, typically) and are overlain by a variety of
surficial deposits including alluvial (stream-laid) sediments, eolian
(wind-blown) materials and colluvial (slope-wash) deposits.
The site is mapped as underlain by Middle to
Upper Pleistocene to Holocene alluvial soils associated with the Cache la
Poudre River and its tributaries. The unconsolidated materials are depicted as
underlain by interbedded strata of the upper member of the Cretaceous Pierre
Shale formation. In the Colorado Front Range area, the Pierre Shale consists
largely of clay shales and claystones with local, interbedded sandstones. The
shales and claystones typically are moderately to highly expansive.
GEOLOGIC HAZARDS
Expansive Soils
The shallow earth materials
underlying the site include clay shales and claystones. Swelling clayey soils
and bedrock change volume in response to changes in moisture content that can
occur seasonally, or in response to changes in land use, including
development. Expansion potentials vary with moisture contents, density and
details of the clay chemistry and mineralogy. The swell potential in any
particular area can vary markedly both laterally and vertically due to the
complex interbedding of the site
soil and bedrock materials. Moisture changes also occur erratically, resulting in conditions that cannot
always be predicted.
Swell-consolidation testing of samples of the
Pierre Shale indicated negligible to low potentials for heave (See Table 1).
Although there is a significant amount of risk involved where structures are
placed directly on these types of materials, the depth of non-expansive sands
and gravels overlying the bedrock materials together with the shallow water
table at the site significantly reduces the risk of soil expansion adversely
affecting the site. It is important, however, that the soil conditions be
reviewed on an individual structure basis when site building layouts are
known.
Collapsible Soils Certain surficial deposits in
the Front Range area are known to be susceptible to local hydro-consolidation
or "collapse." Hydro-consolidation consists of a significant volume loss due
to re-structuring of the constituent grains of the soil to a more compact
arrangement upon wetting.
Portions of the site upper surficial soils are
interpreted to have collapsible soil potential. The index parameters for site
soils assessed for this study, did fall into the range typically associated
with collapsible soils (e.g. Naval Facilities Engineering Command, 1986).
Therefore, the likelihood of encountering collapsible soils on the subject
site is considered high. The following observations with regard to collapsible
soils were observed onsite during the site reconnaissance.
• Conical depressions in the native materials were observed across the site as shown in Photographs 3 and 4 contained in Appendix A. The conical depressions are variable in dimension with diameter ranging from 3 to 10 feet with depths ranging from 1 to 3 feet below existing grades.
• Portions of the existing paved runway for the site are experiencing and have experienced over the past 30 years severe pavement distress and pavement failure as a result of piping failure of surficial soils as shown in photographs in 5 and 6 contained in Appendix A.
• Existing buildings at the site area
have experienced foundation distress, and pavement failure as a result of
piping failure of surficial soils as shown in photographs in 7 and 8 contained
in Appendix A.
Additional geotechnical evaluations of the subject site must address the
potential of consolidation in the foundation soils, so that appropriate,
remedial design and construction can be implemented, if necessary.
Radon Testing for the
possible presence of radon gas prior to project development does not yield
useful results regarding the potential accumulation of radon in completed
structures. Radon accumulations most typically are found in basements or other
enclosed portions of buildings built in areas underlain at relatively shallow
depths by granitic crystalline rock. The likelihood of encountering radon in
concentrations exceeding applicable health standards on the subject site,
underlain by relatively deep soils and sedimentary bedrock, is significantly
lower.
GROUND recommends that radon testing be performed in each building on-site, after construction is completed. Proper ventilation usually is sufficient to mitigate potential radon accumulations. Building designs should accommodate such ventilation for all building areas.
Seismic ActivitylFaulting Neither site reconnaissance nor review of available geologic maps indicated the trace of an active or potentially active fault traversing or immediately adjacent to the site. Therefore, the likelihood of surface fault rupture at the site is considered to be low.
The closest documented active fault to the site is the Rocky Mountain Arsenal Fault, which is located approximately 40 miles to the south (Kirkham and Rogers, 1981). This fault is approximately 15 miles in length, trends generally northwest/southeast and is considered to be a right-lateral, strike-slip fault. The most recent significant seismic movements associated with the fault occurred in the 1960's, generating earthquakes up to magnitude 5.5. Research performed by the U.S. Geological Survey concluded that a strong correlation existed between the seismic activity of this fault and pressure injection of liquid waste into a disposal well located at the nearby Rocky Mountain Arsenal. Pressure injection in the disposal well was discontinued in 1966 and only minor seismic activity along the fault has been recorded since. The risk of this fault giving rise to damaging, earthquake-induced ground motions at the site is considered to be relatively low given the low previously recorded seismic magnitudes.
Other faults mapped as `potentially active' are located in the Rocky Mountains to the west and northwest of the site. As `potentially active' faults, they constitute at lower risk in comparison to active faults.
The project area falls within Seismic Performance Category A based on AASHTO guidelines, and is considered to have a low probability for large, damaging earthquakes. The site is in the Uniform Building Code Seismic Zone 1, although the Colorado Geological Survey recommends that building design following the guidelines for Zone 2 be considered. Compared with other regions of Colorado, recorded earthquake frequency in the project area is low.
Slope Stability and Erosion As noted in the "Site Conditions" section of this report, the site is relatively flat to gently sloping. During our preliminary reconnaissance of the site area, no evidence was noted of mass-wasting processes associated with steep slopes, such as landslides, slumps or unusual soil creep. Therefore, the likelihood of project developments being affected by large scale, unanticipated slope instabilities is considered low.
Preliminarily, we recommend that un-retained, permanent slope cuts be less than 10 feet in height and maintain a maximum 3:1 (horizontal : vertical) slope angle or less with proper erosion control measures implemented. Proper surface drainage controls to reduce the potential for erosional slope damage need to be implemented in the grading design to control runoff, which may be increased due to proposed pavement surfaces, structures and landscape irrigation. Re-vegetation or other means of protection should be used on graded slopes.
Flooding The subject property lies near Dry Creek about 1,500 feet from the Cache La Poudre River. The groundwater table is quite shallow. Significant fluctuation in the flow of these drainages can be expected seasonally. In addition, local, surface saturation may result during episodes of heavy rainfall and associated temporary ponding of run-off in areas of relatively slow surface drainage.
Wetlands Potential No surface water, typical phreatophytes or other indications of conditions similar to jurisdictional wetlands were apparent on the site during GROUND's site reconnaissance with the exception of the drainage pond located near the eastern perimeter of the site. However, during site development all regulations concerning
wetland protection, as well as any other areas designated as wetlands by the Federal Wetlands Protection Act should be adhered to. Explicit designation of wetlands was not included as part of the scope of this study, and relatively shallow groundwater exists site wide,
Mining Activity and Subsidence Review of U.S. Geological Survey topographic maps covering the site (e.g., U.S.G.S. 1960, revised 1984) and Jones, and others (1978) and other available, published maps depicting areas of coal extraction, did not indicate past mining activities on or adjacent to the subject parcel. No indications of mining activities were apparent on the site during the site reconnaissance. Therefore, there appears to be little potential for surface subsidence associated with consolidation of former mine workings at depth.
Published geologic maps do not indicate formations underlying the site at shallow depths that include evaporite (salt, gypsum, etc.) deposits, limestones or other materials vulnerable to subsurface dissolution. Therefore, the likelihood of subsidence or other mining-related hazards appears to be low.
Based on the published information reviewed for the site and the findings of this preliminary assessment, the site generally appears to be feasible for development with respect to potential geologic hazards, provided appropriate geotechnical and civil design are implemented to reduce the potential of ongoing, continued collapse of surficial soils on the site to an acceptable level.
SUBSURFACE EXPLORATION
The subsurface exploration for the project was
conducted September 20 through September 22, 2004. A total of twenty-three
(23) test holes were drilled with a track-mounted, continuous flight power
auger rig to evaluate the subsurface conditions as well as to retrieve soil
and bedrock samples for laboratory testing and analysis. The test holes were
drilled at the approximate locations as indicated in Figure 1. Temporary
piezometers were installed to a depth of approximately 10 feet below the
existing grade in eleven (11) of the test holes to measure water levels. A
representative of GROUND directed the subsurface exploration, logged the test
holes in the field, and prepared the soil samples for transport to our
laboratory.
Samples of the subsurface materials were taken with a 2-inch I.D. California
liner sampler. The sampler was driven into the substrata with blows from a
140-pound hammer falling 30 inches. This procedure is similar to the Standard
Penetration Test described by ASTM Method D1586. Penetration resistance values
when properly evaluated indicate the relative density or consistency of the
soils and bedrock. In addition, large disturbed samples were obtained from the
auger cuttings for laboratory compaction testing (the "standard Proctor").
Depths at which the samples were obtained and associated penetration
resistance values are shown on the test hole logs.
The approximate locations of the test holes are shown in Figure 1. Logs of the
exploratory test holes are presented in Figures 2 through 6. Explanatory notes
and a legend are provided in Figure 7.
LABORATORY TESTING
Samples retrieved from our test holes were examined and visually classified in
the laboratory by the project engineer. Laboratory testing of soil and bedrock
samples obtained from the subject site included standard property tests, such
as natural moisture contents, dry unit weights, grain size analyses, and
liquid and plastic limits. Swell-consolidation potential and water-soluble
sulfate contents were determined for selected samples, as well. Compaction
tests were performed on the representative composite bulk samples. Laboratory
tests were performed in general accordance with applicable ASTM and AASHTO
protocols.
Results of the laboratory testing program are summarized on Table 1
e
SUBSURFACE CONDITIONS
The subsurface conditions encountered in the test holes generally consisted of
a layer of topsoil/organics/vegetation or asphalt pavement, approximately 2 to
5 inches thick, overlying sands, clays, and gravels. These materials were
underlain by claystone bedrock that extended to the test hole termination
depths at approximately 20 to 35 feet, below existing grades. Groundwater was
encountered at the time of drilling at depths of approximately 2 to 7 feet
below existing grades and at depths of approximately 2 to 5 feet below grade
when measured on September 28, 2004. Water levels will fluctuate, however, in
response to annual and longer-term cycles of precipitation, irrigation,
surface drainage, land use, and the development of transient, perched water
conditions.
Sands andlor clays
were observed in each of the test holes. The sands and clays were often silty,
fine to coarse grained with occasional gravels, non-plastic to highly plastic,
very soft to very stiff (very loose to medium dense), moist to wet, brown in
color, and occasionally calcareous.
Sands and gravels
were also observed in each of the test holes. The sands and gravels were
medium to coarse grained, non-plastic to low plastic, medium dense to dense,
moist to wet, and brown in color.
Claystone bedrock
was also observed in each of the test holes. The bedrock was fine to medium
grained, medium to highly plastic, medium hard to very hard, slightly moist to
moist, and gray to dark gray in color.
Swell-consolidation testing suggested a negligible to low potential for heave
and consolidation in the on-site soils and bedrock. Swells ranging from 0.3 to
0.9 percent and consolidations ranging from 0.1 to 0.3 percent were measured
upon wetting under a 1,000 psf surcharge pressure. Potential for consolidation
related to the sinkhole condition is high without significant grading or other
mitigative efforts. Higher swell potentials may be present at the project
site.
COLLASPSIBLE SOILS CONSIDERATIONS
As referenced in the Collapsible Soils section of this report, the sand and
clay soils encountered in the upper 5 to 10 feet of surficial overburden
materials across the site have a potential for collapse. These materials
typically have a low in-place density and high moisture content.
Recommendations to reduce the potential of collapsing soil affecting new
development are addressed in the Site Grading portion of this report.
GROUNDWATER CONSIDERATIONS
As mentioned throughout this report, shallow groundwater is present at the
project site and was encountered at the time of drilling and when checked soon
after, at depths of approximately 2 to 7 feet below existing grades. It has
been GROUND's experience that for many smaller sized developments, where
localized shallow groundwater is encountered, water infiltration can often be
effectively reduced through proper drainage design including raising structure
grades, the use of perimeter drain and underdrain systems, pumps, detention
ponds, channels, etc. However, within the project site, groundwater does not
appear to be localized; rather it is GROUND's opinion that this condition is
most likely regional, i.e., extends beyond the perimeter of the site for a
significant distance. Therefore, the groundwater condition may require a
substantial diversionary system rather than commonly utilized surface and
shallow subsurface drainage methods. Often, these extensive regional drainage
improvements may be detrimental to upgradient and downgradient areas and may
not be financially feasible for projects of this size. Therefore, it is common
practice to raise site grades creating a separation between structural
elements and the groundwater. We would estimate the site to be raised
approximately 5 to 8 feet or more from existing grades in order to obtain an
adequate separation. Specific site evaluations will be necessary to further
address this option.
As a result of the groundwater condition at the site, the construction of
basements or other below-grade levels may also not be feasible. In the event
roadways and other at-grade elements are not raised from the existing grades,
overexcavation of subgrade materials and dewatering during construction will
likely be required in order to obtain satisfactory compaction.
Site soils excavated in their current state may not meet the moisture-density
requirements specified in latter sections of this report due to moisture
contents above optimum as well as saturation. In order to use these materials
with high to very high moisture contents, extensive processing to "dry" and
mix them, subsequently creating a homogenous and consistent material, will be
required. In the event these materials cannot be processed correctly, import
fill materials may be necessary.
In addition, utility installation may be difficult due to the presence of
shallow groundwater. Properly designed and installed dewatering systems may be
required during construction within trenches. The risk of slope instability
will be significantly increased in areas of seepage along excavation slopes.
Should site constraints prohibit the use of recommended slope angles,
temporary shoring should be used.
GROUND would encourage the Client to consult with a Civil Engineer
specializing in water modeling and detailed drainage design to further
evaluate the surface and subsurface water conditions and its impact on the
desired development.
SITE GRADING
A detailed final site grading plan was not available at the time of this
report preparation. Based on observations conducted in the field and
preliminary site grading information provided by the Client, we estimate that
cuts and fills across the site will generally be on the order of 10 to 20
feet. Greater cuts and fills may be necessary. Due to the collapsible nature
of the surficial soils and shallow groundwater conditions at the project site,
detailed site grading recommendations will need to be developed.
Based on the previous site history of collapsing soils and groundwater
occurring at or above existing site grades, it appears that an import of
offsite granular materials placed in a properly moisture-density treated state
will be required to raise overall grades of the site to allow for future
development. The final finished grade for the site-specific proposed
structures and flatwork areas will be dependent upon a drainage study by a
civil engineer.
Site grading options for consideration to achieve this goal are as follows.
1. Partial removal and replacement or mixing of the underlying surficial
collapsible soils with properly compacted imported granular fill materials.
2. Densification of collapsible soils inplace by methods such as surcharge or
dynamic compaction with subsequent placement of overlying properly compacted
imported granular fill materials.
GROUND recommends consultation with other Project Team members with regard to
site grading options once a review of proposed drainage improvements has been
performed. Therefore, the information presented in this section should be
utilized solely as a guideline for preliminary design purposes.
Site grading should be carefully planned to take into consideration the
preliminary recommendations regarding foundation and floor systems as
presented in this report. Site grading should be planned carefully to provide
positive surface drainage away from buildings, and all pavements, utility
alignments, and flatwork. Surface diversion features should be provided around
paved areas to prevent surface runoff from flowing across the paved surfaces.
Site soils free of deleterious materials are generally suitable for placement
as compacted fill. Claystone fragments larger than 2 inches and cobbles larger
than 6 inches should not be incorporated into project fills. Care should be
taken, however, with regard to achieving and maintaining proper moisture
contents during placement and compaction. We anticipate that a high percentage
of the on-site soils may exhibit significant pumping, rutting, and deflection
at moisture contents near optimum and above.
Following the development of site-specific grading and additional
site-specific subsurface exploration, the placement of on-site fill materials
should be monitored by a representative of GROUND. Quality control testing
including nuclear density methods should be conducted in order to ensure
proper placement.
Prior to earthwork construction, existing vegetation, topsoil, and other
deleterious materials should be removed and disposed of off-site. Relic
underground utilities, if encountered should be abandoned in accordance with
applicable regulations, removed as necessary, and capped at the margins of the
property. Based on conversations with the airport staff, various
deleterious/uncontrolled materials have been placed in the locations of
previous "sinkholes" including but not limited to concrete block, asphalt,
gravel, and sand. Any deleterious materials should be removed in their
entirety and replaced with moisture-density treated onsite-generated or
approved import till materials. Remnant foundation elements and all
construction debris generated as a result of the demolition of the on-site
structures should be entirely removed and the resultant excavations properly
backfilled. The Geotechnical Engineer should test the excavation backfill
during placement.
Topsoil should not be incorporated into common fill placed on the site.
Instead, topsoil should be stockpiled during initial grading operations for
placement in areas to be landscaped or for other approved uses. If surfaces to
receive fill expose loose, wet, soft or otherwise deleterious material,
additional material should be excavated or other measures taken, to establish
a firm platform for filling.
During planning for site grading, the Owner should consider utilizing granular
or overburden materials in structure areas and placing claystone materials in
lower fill zones in non-building areas if possible.
Settlements will occur in filled ground, typically on the order of 1 to 2
percent of the fill depth. If fill placement is performed properly and is
tightly controlled, in GROUND's experience approximately 70 percent of that
settlement will take place during earthwork construction. The remaining
potential settlement likely will take 1 to 3 years, or longer, to be realized.
No fill materials should be placed, worked, rolled while they are frozen,
thawing, or during poor/inclement weather conditions. Care should be taken to
place all fills in a controlled state. In the event that proper moisture and
compaction is not achieved, the potential for structural movement, related to
differential settlement, greatly increases. The Geotechnical Engineer should
observe the exposed excavation surface prior to placement of fill, and observe
earthwork operations and test the soils.
Potential earthwork contractors should be made aware that significant
processing and reprocessing of the on-site materials may be required. The
placement of on-site fill materials should be monitored on a full-time basis
by a representative of the Geotechnical Engineer. Quality control testing
should be conducted at an increased frequency in order to ensure proper
placement.
Permanent site slopes supported by on-site soils up to 10 feet in height
should be constructed no steeper than 3:1 (horizontal : vertical). Minor
raveling or surficial sloughing should be anticipated on slopes cut at this
angle until vegetation is well re-established. Surface drainage should be
designed to direct water away from slope faces.
ANTICIPATED FOUNDATION/FLOOR SYSTEMS
Based on the subsurface conditions encountered in the test holes, the results
of our field and laboratory studies, and the nature of the proposed
construction, we believe the proposed structures could be founded on a shallow
foundation system, constructed on either properly moisture-density treated,
on-site materials, exclusive of claystone bedrock or approved import fill
materials. This assumes that proper pad preparation possibly including
overexcavation and replacement, surcharging and settlement monitoring, and/or
significant raising of site grades, if so required, is performed once final
recommendations are developed. GROUND anticipates that shallow foundation
systems may be designed for an allowable soil bearing pressure of 1,500 psf to
3,000 psf, based on final site grades and man-made fill depths. Additionally,
due to the presence of relatively shallow groundwater across the site, the
potential for water infiltration beneath structural elements should be
addressed during preliminary developmental layout.
Depending on final site grading, site-specific exploration, and building
specific movement tolerances, some of the structures may require deep
foundation systems. Potential foundation systems may include drilled piers or
driven piles. Driven piles may prove to more cost effective due to the
granular nature and caving potential of the overburden materials, along with
the shallow groundwater depths encountered at the site.
The on-site soils, exclusive of topsoil and any deleterious materials, may be
suitable to support lightly to moderately loaded slab-on-grade construction
following additional exploration, evaluation, and development of site specific
recommendations. As stated above, overexcavation and replacement with properly
processed on-site materials or import structural fill materials may also be
deemed necessary. Proposed building structures having tight tolerances for
post-construction movements may require the use of structural floor systems.
Site specific conditions will need to be identified in order to provide final
recommendations.
WATER-SOLUBLE SULFATES AND CORROSIVITY
The concentrations of water-soluble sulfates measured in selected samples
obtained from the test holes ranged from 0.70 percent up to 1.00 percent by
weight. Such concentrations of water-soluble sulfates represent a severe
environment for sulfate attack on concrete exposed to these materials. Degrees
of attack are based on the scale of 'negligible,' 'moderate,' 'severe' and
'very severe' as described in the "Design and Control of Concrete Mixtures,"
published by the Portland Cement Association (PCA).
Based on these data and PCA and Colorado Department of Transportation (CDOT)
guidelines, GROUND preliminarily recommends the use of sulfate-resistant
cement in all concrete exposed to site soils, conforming to one of the
following requirements:
1) Type V, as specified by ASTM C150.
2) Type II with a maximum CM content of 5 percent and a maximum content of
(C4AF + 2[C3A]) of 25 percent.
3) Type II or Type I/ll and 15 to 20 percent of the cement shall be replaced
with an approved Type F fly ash.
4) A blended cement conforming to Type HS, as specified by ASTM C1157.
Other cement types or blends may be acceptable, however, if type-specific test
data demonstrate equal or superior sulfate-resistance to Type V cement. Test
data should be provided to the Geotechnical Engineer for review, and the
cement approved, prior to use. All concrete used should have a maximum
water/cement ratio of 0.45 by weight. All concrete exposed to site soils
should have a minimum compressive strength of 4,250 psi. Concrete mixes should
be relatively rich and should be air entrained. Additional sampling and
testing should be performed in order to confirm these preliminary findings.
Data was collected to determine the potential corrosive environment for metal
placed beneath the ground surface. The testing included pH determination and
electrical resistivity measurements. Test results are summarized on Table 1.
The acidity of the subsurface materials was assessed by conducting pH tests on
samples of the on-site soils. The pH test results indicated values ranging
from approximately 7.4 to 8.0, which indicate that the materials are slightly
basic. These characteristics will not contribute greatly to corrosion.
Electrical resistivity measurements conducted on samples of the overburden
soils indicate resistivity values ranging from 427 to 1,358 ohm-centimeters.
Electrical resistance is related to moisture content and the quantity of
dissolved salts and other deteriorative compounds in the soil. Soil
resistivities between 0 and 2,000 ohm-centimeters are classified as having a
`bad' corrosive resistance, as presented in the "Handbook of Steel Drainage
and Highway Construction Products" published by the American Iron and Steel
Institute. We recommend a qualified corrosion engineer review the information
to design an appropriate level of corrosion protection for buried metal at the
project site.
UTILITY AND EXCAVATION RECOMMENDATIONS
On-site soils and bedrock excavated from trenches may be suitable for use as
trench backfill, provided they are processed properly. Additional subsurface
exploration should be performed within utility alignments to further evaluate
potentially encountered subsurface conditions.
The test holes for the subsurface exploration were excavated to the depths
indicated by means of track-mounted, flight auger drilling equipment. We
anticipate no significant excavation difficulties in the majority of the site
with conventional heavy-duty excavation equipment in good working condition.
Depending on final site grading plans, equipment with large ripper teeth may
be required in order to adequately remove and assist in properly placing fill
materials.
We preliminarily recommend temporary excavation slopes up to 5 feet deep not
steeper than a ratio of 1.5 (H) to 1 (V) in the on-site soils and 3/
(H) to 1 (V) in the native soils. Some surface sloughing may occur on
the slope faces at these angles.
Should site constraints prohibit the use of the recommended slope angles,
temporary shoring should be used. The shoring should be designed to resist the
lateral earth pressure exerted by building, traffic, equipment, and
stockpiles. GROUND can provide shoring design upon request.
Groundwater was encountered in the test holes at depths of approximately 2 to
7 below existing grades during our subsurface exploration and at depths of
approximately 2 to 5 feet below grade when measured on September 28, 2004.
However, local perched water conditions may develop at shallower depths as
stated in various sections of this report resulting in significant seepage in
excavations.
Good surface drainage should be provided around temporary excavation slopes to
direct surface runoff away from the slope faces. A properly designed swale
should be provided at the top of the excavations. In no case should water be
allowed to pond at the site. Slopes should be protected against erosion.
Erosion along the slopes will result in sloughing and could lead to a slope
failure. Any excavations in which personnel will be working must comply with
all OSHA Standards and Regulations (CFR 29 Part 1926). The Contractor's
"responsible person" should evaluate the soil exposed in the excavations as
part of the contractor's safety procedures. GROUND is providing this
information solely as a service to the Client and is not assuming
responsibility for construction site safety or the contractor's activities.
SURFACE DRAINAGE RECOMMENDATIONS
The following drainage precautions should be taken into consideration during
design of the proposed development,
1) Wetting or drying of the foundation excavations and underslab areas should
be avoided during and after construction as well as throughout the life of the
facilities. Permitting increases/variations in moisture to the supporting
soils may result in a decrease in bearing capacity and an increase in
settlement, heave, and/or differential movement.
2) Positive surface drainage measures should be provided and maintained to
reduce water infiltration into foundation soils. The ground surface
surrounding proposed building exteriors 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 the areas not covered with pavement or concrete slabs, or
a minimum 3 percent in the first 10 feet in the areas covered with pavement or
concrete slabs. In no case should water be allowed to pond near or adjacent to
foundation elements. Drainage measures also should be included in project
design to direct water away from sidewalks and other hardscaping as well as
utility trench alignments which are likely to be adversely affected by
moisture-volume changes in the underlying soils or flow of infiltrating water.
3) Roof downspouts and drains should discharge well beyond the perimeters of
the structure foundations, or be provided with positive conveyance off-site
for collected waters.
4) Landscaping which requires watering should be located 10 or more feet from
building perimeters. Irrigation sprinkler heads should be deployed so that
applied water is not introduced into foundation soils. Landscape irrigation
should be limited to the minimum quantities necessary to sustain healthy plant
growth. Use of drip irrigation systems can be beneficial for reducing
over-spray beyond planters. Drip irrigation can also be beneficial for
reducing the amounts of water introduced to building foundation soils, but
only if the total volumes of applied water are controlled with regard to
limiting that introduction. Controlling rates of moisture increase beneath the
foundations and floors should take higher priority than minimizing landscape
plant losses. Where plantings are deemed absolutely necessary within 10 feet
of a building, GROUND recommends that the plants be placed in water-tight
planters, constructed either in-ground or above-grade, to reduce moisture
infiltration in the surrounding subgrade soils. Planters should be provided
with positive drainage and landscape underdrains.
5) Plastic membranes should not be used to cover the ground surface adjacent
to foundation walls. Perforated "weed barrier" membranes that allow ready
evaporation from the underlying soils may be used.
PRELIMINARY PAVEMENT SECTIONS
A pavement section is a layered system designed to distribute concentrated
traffic loads to the subgrade. Performance of the pavement structure is
directly related to the physical properties of the subgrade soils and traffic
loadings. The standard care of practice in pavement design describes the
recommended flexible pavement section as a "20-year" design pavement. However,
most flexible pavements will not remain in satisfactory condition without
routine maintenance and rehabilitation procedures performed throughout the
life of the pavement.
We anticipate pavement sections may consist of a full depth asphalt section of
approximately 5 to 7 inches in private paved areas such as parking and
internal drives and approximately 9 to 11 inches in public roadways, depending
upon final roadway classification. A minimum section of 8 to 7 inches of
Portland cement concrete in areas of high truck traffic/concentrated turning
movements such as dumpster pads and load ingJunloading areas (private) may
also be necessary. Pavement subgrade materials will need to be properly
moisture-density treated.
The collection and diversion of surface drainage away from paved areas is
extremely important to satisfactory performance of the pavements. The
subsurface and surface drainage systems should be carefully designed to ensure
removal of the water from paved areas and subgrade soils. Allowing surface
waters to pond on pavements will cause premature pavement deterioration. Where
topography, site constraints, or other factors limit or preclude adequate
surface drainage, pavements should be provided with edge drains to reduce loss
of subgrade support. The long-term performance of the pavement also can be
improved greatly by proper backfilling and compaction behind curbs, gutters,
and sidewalks so that ponding is not permitted and water infiltration is
reduced.
ADDITIONAL EXPLORATION REQUIREMENTS
The above data and recommendations are based on a limited preliminary
subsurface exploration only. Additional geotechnical studies must be
performed to further evaluate the site for site-specific foundation and floor
system recommendations, final site grading and pavement recommendations.
CLOSURE
Geotechnical Review
The poor performance of many pavements, foundations, and subsurface
structures has been directly attributed to inadequate geotechnical review and
earthwork quality control. Therefore, project plans and specifications should
be reviewed by the Geotechnical Engineer to evaluate whether they comply with
the intent of the recommendations in this report. This review should be
reported in writing.
In addition, building-specific geotechnical exploration(s) must be completed
for the project prior to final design and construction. The preliminary
geotechnical recommendations presented in this report are highly contingent
upon the completion of final geotechnical studies as well as construction
observation and materials testing of project earthworks by representatives of
GROUND. If another geotechnical consultant is selected to provide these
services, then that consultant must assume all responsibility for the
geotechnical aspects of the project by concurring in writing with the
recommendations in this report, or by providing alternative recommendations.
Limitations
This report has been prepared for
Airpark Village, LLC as it pertains to design of the proposed multi-use
development as described herein. It may not contain sufficient information for
other parties or other purposes. In addition, GROUND has assumed that the
final geotechnical subsurface exploration will commence by Fall of 2006 or
before. Changes in project plan development or schedule should be brought to
the attention of the Geotechnical Engineer, in order that the preliminary
geotechnical recommendations may be re-evaluated and, as necessary, modified.
The preliminary geotechnical conclusions and recommendations in this report
relied upon subsurface exploration at a limited number of exploration points
as shown on Figure 1. Subsurface conditions were interpolated between and
extrapolated beyond these locations. Findings were dependent on the limited
amount of direct evidence obtained at the time of this geotechnical
evaluation. Our recommendations were developed for site conditions as
described above. Actual conditions exposed during construction may be
anticipated to differ, somewhat, from those encountered during site
exploration.
The materials present on-site are stable at their natural moisture content,
but may shrink, swell, or lose bearing capacity with changes in moisture
content. It is the responsibility of the design team (the architect(s), civil
engineer(s), landscape architect(s), structural engineer(s), and ownership) as
well as the construction and maintenance contractor(s) to ensure that moisture
is directed away from site improvements prior to infiltrating the underlying
soils.
Performance of the proposed foundation and pavement will depend on
implementation of the recommendations in this report and on proper maintenance
after construction is completed. Because water is the principal cause of
volume change in expansive soils and rock, the design, construction, and
maintenance of the improvements must eliminate changes in moisture content of
the site soils.
This report was prepared in accordance with generally accepted soil and
foundation engineering practice in the Fort Collins, Colorado, area, at the
date of preparation. GROUND makes no other warranties, either express or
implied, as to the professional data, opinions or recommendations contained
herein. Because of numerous considerations, which are beyond GROUND's control,
the economic or technical performance of the project cannot be guaranteed in
any respect.
We appreciate the opportunity to complete this portion of the proposed Airpark
Village development project. GROUND welcomes the opportunity to provide the
Owner with proposals for final subsurface exploration programs as well as
construction observations and materials testing services prior to the
commencement of construction activities.
Please contact us with any questions or if you require additional information.
Sincerely,
GROUND Engineering Consultants, Inc.
Andrea Hornig, EIT
Reviewed by James Kowalsky, P.E.
Graphics
Area Aerial
Photograph
Area Aerial Photograph
Area Aerial Photograph
Drainage pipe in swampy area east of test hole 19
Conical depression area in native materials
Severe pavement distress associated with existing runway
Active piping failure adjacent to existing structure due to increase surface runoff
Summary of lab test results (page1)
Summary of lab test results (page2)
Summary of lab test results (page3)
