GEOTECHNICAL INVESTIGATION
STUDENT HOUSING BUILDINGS
UCCS SUMMIT VILLAGE
COLORADO SPRINGS, COLORADO
Prepared for:
UNIVERSITY OF COLORADO AT COLORADO SPRINGS
Facilities Services
1420 Austin Bluffs Parkway
Colorado Springs, Colorado 80918
Attention: Mr. Gary Reynolds
CTL|T Project No. CS17805-125
July 29, 2011
5240 Mark Dabling Blvd | Colorado Springs, Colorado 80918 | Telephone: 719-528-8300 Fax: 719-528-5362
TABLE OF CONTENTS
SCOPE..................................................................................................................................... 1
SUMMARY............................................................................................................................... 1
SITE CONDITIONS.................................................................................................................. 2
PROPOSED CONSTRUCTION .............................................................................................. 3
SITE GEOLOGY ...................................................................................................................... 4
PREVIOUS INVESTIGATIONS ............................................................................................... 4
FIELD INVESTIGATION.......................................................................................................... 4
SUBSURFACE CONDITIONS ................................................................................................ 5
Existing Fill ....................................................................................................................... 5
Natural Soils ..................................................................................................................... 6
Bedrock ............................................................................................................................. 6
Ground Water ................................................................................................................... 6
Seismicity.......................................................................................................................... 7
SITE GRADING AND UTILITIES ............................................................................................ 7
FOUNDATIONS ....................................................................................................................... 8
Drilled Piers ...................................................................................................................... 9
Laterally-Loaded Piers .................................................................................................. 10
Closely-Spaced Pier Reduction Factors ..................................................................... 11
FLOOR SYSTEMS ................................................................................................................ 12
BELOW-GRADE CONSTRUCTION ..................................................................................... 15
PAVEMENTS ......................................................................................................................... 15
CONCRETE ........................................................................................................................... 16
SURFACE DRAINAGE.......................................................................................................... 17
CONSTRUCTION OBSERVATIONS .................................................................................... 18
GEOTECHNICAL RISK......................................................................................................... 18
LIMITATIONS ........................................................................................................................ 18
FIG. 1 – LOCATION OF EXPLORATORY BORINGS
FIG. 2 – CONTOURS OF ESTIMATED BEDROCK SURFACE ELEVATION
FIGS. 3 & 4 – SUMMARY LOGS OF EXPLORATORY BORINGS
FIG. 5 – EXTERIOR FOUNDATION WALL DRAIN
APPENDIX A – SWELL CONSOLIDATION TEST RESULTS
TABLE A-1 – SUMMARY OF LABORATORY TESTING
UNIVERSITY OF COLORADO AT COLORADO SPRINGS
SUMMIT VILLAGE STUDENT HOUSING BUILDINGS
CTL|T PROJECT NO. CS17805-125
S:\CS17500-17999\CS17805.000\125\2. Reports\CS17805-125-R1.doc
SCOPE
This report presents the results of our Geotechnical Investigation for two
proposed student housing buildings to be constructed at Summit Village on the campus
of The University of Colorado at Colorado Springs. The purpose of our investigation was
to evaluate subsurface conditions at the site in order to develop geotechnical design
criteria for the proposed structures. This report summarizes the results of our field and
laboratory investigations and presents our design and construction recommendations for
building foundations, floor system alternatives, pavement sections, and below-grade
construction, as well as other details influenced by subsurface conditions. We believe the
investigation was completed in general accordance with our proposal (CTL|T Proposal
No. CS-11-0112) dated June 2, 2011. Evaluation of the project site for the possible
presence of potentially hazardous materials (environmental site assessment) was beyond
the scope of this investigation.
This report was prepared based on conditions disclosed by our exploratory
borings, results of laboratory tests, engineering analyses, and our experience with the
campus and adjacent housing buildings. The design criteria presented in the report were
based on our understanding of the planned construction and site improvements. If
changes occur, we should review the revised plans to determine their effect on our
recommendations. The following section summarizes the report. More detailed
descriptions of subsurface conditions, as well as our design and construction
recommendations, are presented in the report.
SUMMARY
1.
Subsurface conditions encountered in our exploratory borings drilled in
the vicinity of the planned buildings consisted of about 2 to 7 feet of
existing fill (all but one of the borings) underlain predominantly by natural
sands. Sandstone and claystone bedrock were found in each of the
borings underlying the existing fill or natural soils at depths of 2 to 34 feet
below the existing ground surface. The sandstone was generally poorly to
moderately cemented. However, refusal to practical auger drilling occurred
in one boring (TH-1) at a depth of 27.5 feet below the existing ground
surface. Samples of the sandstone and claystone exhibited a variety of
swell-consolidation characteristics, ranging from low to very high
measured swell values when wetted.
UNIVERSITY OF COLORADO AT COLORADO SPRINGS
SUMMIT VILLAGE STUDENT HOUSING BUILDINGS
CTL|T PROJECT NO. CS17805-125
S:\CS17500-17999\CS17805.000\125\2. Reports\CS17805-125-R1.doc
1
2.
At the time of drilling, ground water was encountered in one of the borings
at a depth of 28 feet. When ground water levels were checked again 22
days after the completion of drilling and sampling operations, water was
measured at depths of 31 and 36 feet below the existing ground surface in
two of the borings.
3.
We anticipate the soils encountered at the lower level in each of the
buildings will include grading fill, natural sands, and sandstone and/or
claystone bedrock. To reduce the risk of excessive differential settlement
of shallow foundations underlain by a widely varying combination of soil
and bedrock materials, we recommend the proposed student housing
buildings be constructed with drilled pier foundations bottomed in the
underlying bedrock. Design criteria and construction recommendations for
the drilled pier foundation are presented in the report.
4.
We expect conventional slab-on-grade floors are considered an attractive
floor system alternative for at-grade areas and we believe a low risk of poor
slab performance will exist for slabs-on-grade underlain by at least 5 feet of
the natural sand and/or new, densely compacted granular fill. We estimate
floor slab movements of about 1 inch or less for this subgrade condition.
Where sandstone and/or claystone bedrock is encountered at floor slab
elevation or within 5 feet of the bottom of the floor slab, we recommend the
bedrock materials be removed to a depth of at least 5 feet. The excavation
should be backfilled with the on-site sand or a similar, non-expansive
material that has been moisture conditioned and densely compacted. Floor
slab movements on the order of about 1 inch can be expected for this
condition. If the owner wishes to further reduce the risk of floor
movements in the lower level of each building, structurally supported
floors should be installed.
5.
The proposed access driveways and parking areas can be paved with 5
inches of asphalt concrete. Alternative pavement sections are included in
the report.
6.
Surface drainage should be designed for rapid runoff of water away from
the proposed buildings. Conservative irrigation practices should be
followed to avoid excessive wetting. Water should not be allowed to pond
adjacent to the structures, or over exterior slabs and pavements.
SITE CONDITIONS
The proposed student housing buildings will be located to the southwest of the
existing Summit Village complex. Columbine Hall is situated to the east of the planned
location for the east proposed building. Parking Lot 7 occupies the ground to the south
of the planned structures. Regent Drive forms the western boundary of the project site.
The location of the site and the surrounding features are presented in Fig. 1.
UNIVERSITY OF COLORADO AT COLORADO SPRINGS
SUMMIT VILLAGE STUDENT HOUSING BUILDINGS
CTL|T PROJECT NO. CS17805-125
S:\CS17500-17999\CS17805.000\125\2. Reports\CS17805-125-R1.doc
2
The ground surface in the area of the east building (borings TH-1 through TH-3) is
comparatively flat in the northern half of the site where an existing basketball slab is
located. The southern half of the anticipated east building footprint slopes downward to
the southwest toward Parking Lot 7 at a grade estimated to be about 15 percent. A small
rock outcrop is present to the south of the southern edge of the basketball slab. The
anticipated area of the west building footprint (borings TH-4 through TH-8) comprises a
shallow storm detention basin with its low point directly west of Parking Lot 7, near
boring TH-8. The ground surface to the north of the bottom of the basin slopes downward
to the south and southwest at grades estimated to be about 8 to 12 percent. No storm
water was present in the basin at the time of our investigation. Vegetation in the area of
the two proposed buildings consists of a thick stand of grasses, weeds, and deciduous
bushes and trees.
PROPOSED CONSTRUCTION
We understand two additional student housing buildings are to be constructed to
the southwest of the existing Summit Village and west of Columbine Hall. Plans for the
buildings are in the very early conceptual stages. It is our understanding the east building
will be similar to the smaller housing buildings that were previously constructed to the
north. The west building may be somewhat larger. We anticipate the new structures will
be similar in style to the existing housing buildings. Current development concepts call
for the proposed buildings to have the first level cut into the existing hillside so that the
northern end of the structure is below-exterior grades and entry will be at the second
floor elevation. The first level of both buildings will “walkout” at the southern end of the
structure. The first level of the west building might be used as a drive-under automobile
parking area. The basement levels of the buildings could also include dorm rooms and
storage areas. Three or possibly four stories are reportedly planned above the basement
level in both buildings. Paved driveways will provide access to the buildings.
We anticipate the proposed buildings will be concrete-frame structures with twin
tee floors and roofs or steel-frame structures with concrete topping slabs over steel
decking. Foundation column loads are expected to be in the range of 400 to 500 kips with
exterior foundation wall loads possibly as high as 3 to 5 kips per lineal foot.
UNIVERSITY OF COLORADO AT COLORADO SPRINGS
SUMMIT VILLAGE STUDENT HOUSING BUILDINGS
CTL|T PROJECT NO. CS17805-125
S:\CS17500-17999\CS17805.000\125\2. Reports\CS17805-125-R1.doc
3
No grading plans were available for our review during the preparation of this
study. Based on our understanding of the planned construction and observations of the
existing site topography, we anticipate maximum cuts on the order of about 12 to 14 feet
will be necessary in the northern end of the site and maximum fills of about 5 to 10 feet
will be needed in the southern end of the building footprints to achieve the desired
building pad elevations.
SITE GEOLOGY
Published geologic mapping (“Geologic Map of the Pikeview Quadrangle, El Paso
County, Colorado,” Jon P. Thorson, Christopher J. Carroll and Mathew L. Morgan,
Colorado Geological Survey, 2001) indicates the site is underlain locally by alluvial
deposits (Qfo). The upper member of the Laramie Formation (Klu) comprises the
underlying bedrock found beneath the near-surface soils. Conditions encountered in our
borings generally confirm the mapping, although our borings suggest some man-made
fill has been placed to adjust site grades.
PREVIOUS INVESTIGATIONS
CTL|Thompson, Inc. (CTL) prepared a Geotechnical Investigation (Job No. CS5371; report dated May 10, 1995) for the Summit Village student housing buildings
(Phases I and II). CTL also prepared a Geotechnical Investigation (Job No. CS-6002;
report dated December 13, 1995) for Columbine Hall. Drilled pier foundations bottomed in
the bedrock were recommended in the 1995 soils investigations for both projects.
Subsurface conditions encountered in the borings drilled during these previous studies
were similar to the conditions found in the borings drilled during this investigation.
FIELD INVESTIGATION
The field investigation included drilling eight exploratory borings within the
anticipated building footprints. Borings TH-1 through TH-3 were located in the general
vicinity of the proposed east building. Borings TH-4 through TH-8 were intended to
evaluate the subsurface conditions in the area of the west building. The borings were
advanced to depths of 25 to 40 feet using 4-inch diameter, continuous-flight auger and a
UNIVERSITY OF COLORADO AT COLORADO SPRINGS
SUMMIT VILLAGE STUDENT HOUSING BUILDINGS
CTL|T PROJECT NO. CS17805-125
S:\CS17500-17999\CS17805.000\125\2. Reports\CS17805-125-R1.doc
4
truck-mounted drill rig. Drilling was observed by our field representative who logged the
conditions found in the borings and obtained samples. Graphical logs of the conditions
encountered in the borings, the results of field penetration resistance tests, and
laboratory test data are presented in Figs. 3 and 4. Swell-consolidation test results are
presented in Appendix A. Laboratory test data is summarized in Table A-1.
Soil and bedrock samples obtained during drilling were returned to our laboratory
and visually classified, and laboratory testing was assigned to representative samples.
Testing included moisture content and dry density, swell-consolidation, sieve analysis,
and water-soluble sulfate content tests.
SUBSURFACE CONDITIONS
Subsurface conditions encountered in our exploratory borings drilled in the
vicinity of the planned buildings consisted of a layer of existing fill (all but one of the
borings) underlain predominantly by natural sands. Sandstone and claystone bedrock
were found in each of the borings underlying the existing fill or natural soils. The
pertinent engineering characteristics of the soils and bedrock encountered are discussed
in the following paragraphs.
Existing Fill
About 2 to 7 feet of silty to clayey sand fill was encountered at the existing ground
surface in all but one of our borings. Results of field penetration resistance tests
indicated the fill was loose to dense. The variable consistency of the fill suggests
portions of the material may have been placed under uncontrolled conditions. We must
therefore consider the existing fill to be of suspect quality and unsuitable to underlie the
planned structures, in its current condition. If free from deleterious substances, we
anticipate the fill can probably be excavated, moisture conditioned, and densely
compacted as new fill within the planned development.
UNIVERSITY OF COLORADO AT COLORADO SPRINGS
SUMMIT VILLAGE STUDENT HOUSING BUILDINGS
CTL|T PROJECT NO. CS17805-125
S:\CS17500-17999\CS17805.000\125\2. Reports\CS17805-125-R1.doc
5
Natural Soils
Natural, silty to clayey sand was encountered at the ground surface or beneath the
existing fill in five of the eight borings and extended to depths of 6 to 34 feet. The sand
was loose to medium dense based on the results of field penetration resistance tests. Six
samples of the natural sand contained 14 to 35 percent clay and silt-size particles
(passing the No. 200 sieve). Our experience indicates the sands are typically nonexpansive or exhibit low measured sells when wetted.
A thin layer of sandy clay, about 3 feet thick, was encountered in one boring (TH-3)
with the predominant sands, at a depth of 6 feet. This material typically exhibits low to
moderate measured swells.
Bedrock
Sandy to very sandy claystone and slightly silty to clayey sandstone were found in
each of the borings underlying the existing fill or natural soils at depths of 2 to 34 feet
below the existing ground surface. Contours of the estimated bedrock surface elevation
are presented in Fig. 2. Field penetration resistance test results indicated the bedrock
was medium hard to very hard. The sandstone encountered in our borings was generally
poorly to moderately cemented. However, refusal to practical auger drilling occurred in
one boring (TH-1) at a depth of 27.5 feet indicating cemented sandstone layers are
present. Five samples of the claystone exhibited low to very high measured swells of 1.2
to 7.4 percent when wetted under a vertical pressure of 1,000 psf. One sample of the
clayey sandstone swelled 1.5 percent when wetted. Three samples of the sandstone
tested in our laboratory contained 10 to 21 percent clay and silt-size particles (passing
the No. 200 sieve).
Ground Water
At the time of drilling, ground water was encountered in one of the borings at a
depth of 28 feet. When ground water levels were checked again 22 days after the
completion of drilling and sampling operations, water was measured at depths of 31 and
36 feet below the existing ground surface in two of the borings. Ground water elevations
UNIVERSITY OF COLORADO AT COLORADO SPRINGS
SUMMIT VILLAGE STUDENT HOUSING BUILDINGS
CTL|T PROJECT NO. CS17805-125
S:\CS17500-17999\CS17805.000\125\2. Reports\CS17805-125-R1.doc
6
will fluctuate in response to seasonal precipitation variations and landscaping irrigation.
Although ground water is anticipated to be encountered during installation of drilled pier
foundations, we do not expect ground water to significantly impact construction of the
proposed buildings.
Seismicity
This area, like most of central Colorado, is subject to a degree of seismic activity.
We believe the soils and bedrock on the site classify as Site Class C (dense soil and soft
rock) according to the 2009 International Building Code (2009 IBC).
SITE GRADING AND UTILITIES
No grading plans were available for our review during the preparation of this
study. We anticipate maximum cuts on the order of about 12 to 14 feet will be necessary
in the northern end of the site and maximum fills of about 5 to 10 feet will be needed in
the southern end of the building footprints to achieve the desired building pad elevations.
Prior to grading fill placement, vegetation and other deleterious materials such as
stockpiled soils and debris, and concrete flatwork should be removed from the site.
Organic topsoil can be stockpiled and placed in landscaped areas. Existing fill should be
excavated to expose the underlying natural soils. If free of deleterious materials, the
existing fill soils can probably be incorporated into the new grading fill. Fill materials
should consist of the on-site sands and sandstone that have been moisture conditioned
to within 2 percent of optimum moisture content and compacted in thin lifts to at least 95
percent of maximum standard Proctor dry density (ASTM D 698). Sandstone placed as
grading fill should be mechanically broken down into particles of less than 2 inches in
diameter. Claystone bedrock should be placed as fill outside of the planned building
footprints or be removed from the site. The placement and compaction of the grading fill
should be observed and tested by a representative of our office during construction.
We believe the on-site sand soils can be excavated using conventional, heavyduty equipment. Very hard and cemented layers of sandstone, that require rock teeth or
jack-hammering, could be encountered in utility trench excavations. The on-site, natural
UNIVERSITY OF COLORADO AT COLORADO SPRINGS
SUMMIT VILLAGE STUDENT HOUSING BUILDINGS
CTL|T PROJECT NO. CS17805-125
S:\CS17500-17999\CS17805.000\125\2. Reports\CS17805-125-R1.doc
7
sand soils and granular grading fill materials will likely cave into unsupported, steep
utility trench excavations. Based on the Occupational Safety and Health Administration
(OSHA) criteria governing excavations, the on-site, natural sands and granular grading
fills will probably classify as Type C soils. The bedrock will probably classify as Type B
soil. Temporary excavations in Type B and Type C materials require a maximum slope
inclination of 1:1 and 1.5:1 (horizontal to vertical), respectively, unless the excavation is
shored or braced. Should ground water seepage occur, flatter slopes may be necessary.
Water and sewer lines are often constructed beneath paved areas. Compaction of
utility trench backfill will have a significant effect on the life and serviceability of
pavements. We recommend utility trench backfill be placed in thin, loose lifts, moisture
conditioned to within 2 percent of optimum moisture content, and compacted to at least
95 percent of maximum standard Proctor dry density (ASTM D 698). Personnel from our
firm should periodically observe utility trench backfill placement and test the density of
the fill materials during construction.
FOUNDATIONS
Our investigation indicates granular grading fill, natural sands, sandstone, and
possibly claystone bedrock will be encountered at shallow foundation levels within the
basement levels of the two proposed buildings. Spread footing foundations could be
designed for these types of materials. However, comparatively high foundation loads are
anticipated for the multi-story structures, resulting in the need for large footings to
accommodate the lower allowable soil pressure applicable to the sand soils. The soil
materials and bedrock will behave differently under the foundation loads which would
likely result in excessive differential settlements between the footings. We believe the
more reliable foundation approach for the two proposed buildings that will reduce the
risk of foundation movements and the associated damage is a drilled pier system
bottomed in the underlying bedrock. The piers will transfer the foundation loads to the
comparatively strong bedrock formation which will reduce the risk of excessive
differential movements between the different portions of structures. We recommend the
drilled pier foundations for the proposed buildings be designed and constructed in
accordance with the criteria presented in the following sections.
UNIVERSITY OF COLORADO AT COLORADO SPRINGS
SUMMIT VILLAGE STUDENT HOUSING BUILDINGS
CTL|T PROJECT NO. CS17805-125
S:\CS17500-17999\CS17805.000\125\2. Reports\CS17805-125-R1.doc
8
Drilled Piers
1.
Piers should be designed for a maximum allowable end pressure of 40,000
psf. We recommend an allowable skin friction of 4,000 psf for the portion of
pier in comparatively unweathered bedrock. Any bedrock penetration
within the top 5 feet of the pier should be neglected from the load capacity
calculations.
2.
Piers should be designed for a minimum deadload pressure of 15,000 psf
based on pier cross-sectional area only. If this deadload cannot be
achieved through the weight of the structure, the pier length and bedrock
penetration should be increased beyond the minimum values specified in
the next paragraph. The bedrock should be assigned a skin friction value
of 4,000 psf for resistance to expansive soil uplift and tensile structural
loads.
3.
Piers should penetrate at least 8 feet into relatively unweathered bedrock
and have a total drilled length of at least 20 feet. Pier lengths of up to 40 to
50 feet are possible in the southern end of the anticipated western building
footprint to achieve the required bedrock penetration.
4.
The quantity and size of column reinforcement, or the size of base plates,
may dictate the most convenient size of drilled piers. Economy can be
achieved by varying the depth of penetration and limiting the number of
pier diameters. The minimum pier diameter will depend on the length to
diameter ratio (L/D), among other considerations. We recommend the L/D
ratio be kept in the range of 25 to 30. In addition, a minimum diameter of 18
inches is recommended for all piers to facilitate the use of temporary
casing, if needed.
5.
We anticipate a comparatively large, commercial-grade pier drilling rig will
be needed to achieve the expected pier lengths and bedrock penetration.
Rock augers and rock teeth may be needed to cut through cemented
sandstone layers that could be encountered during drilled pier installation.
6.
Pier drilling should produce shafts with relatively undisturbed bedrock
exposed. Excessive remolding and caking of bedrock on pier walls should
be removed by roughening.
7.
Piers should be reinforced their full length. The reinforcement should
extend into grade beams or foundation walls. A minimum steel/pier area
ratio of 0.01 using Grade 60 steel (420 Mpa) is recommended. More
reinforcement will probably be required for structural considerations. The
design of pier reinforcement should consider concrete placement and the
possibility of tremie placement for dewatering of shafts.
8.
There should be at least a 4-inch thick, continuous void beneath the grade
beams, between the piers, to concentrate the deadload of the structure
onto the piers.
UNIVERSITY OF COLORADO AT COLORADO SPRINGS
SUMMIT VILLAGE STUDENT HOUSING BUILDINGS
CTL|T PROJECT NO. CS17805-125
S:\CS17500-17999\CS17805.000\125\2. Reports\CS17805-125-R1.doc
9
9.
Piers should have a center-to-center spacing of at least three pier
diameters when designing for vertical loading conditions, or they should
be designed as a group. Piers aligned in the direction of lateral forces
should have a center-to-center spacing of at least six pier diameters.
Reduction factors for closely-spaced piers are discussed in the following
section.
10.
Piers should be carefully cleaned and dewatered prior to placement of
concrete. Ground water was encountered during this investigation. The
sands that overlie the bedrock may cave into large diameter, unsupported
pier excavations. We anticipate temporary casing and possibly drilling
slurry will be needed to properly clean and dewater some of the drilled pier
shafts during installation. Concrete should be ready on-site and placed in
the pier holes immediately after the holes are drilled, cleaned, and
observed by our representative, to avoid collecting water and possible
contamination of open pier holes. Temporary casing should be available
on-site in the event it is needed to install the drilled piers. Concrete should
not be placed by free fall if there is more than 3 inches of water at the
bottom of the hole. If ground water is encountered in the bedrock
formation and cannot be controlled through the use of temporary casing, it
may be necessary to pump the concrete to the bottom of the pier holes and
displace accumulated water with the rising column of concrete.
11.
Concrete placed in pier holes should have sufficient slump to fill the pier
hole and not hang on the reinforcement or the sides of the casing (if
needed) during extraction. We recommend a slump in the range of 5 to 7
inches.
12.
Some movement of the drilled pier foundations is anticipated to mobilize
the strength of the bedrock. We estimate this movement to be on the order
of 1/4 to 1/2-inch to mobilize skin friction. Differential movement may be
equal to the total movement. Designs should consider these potential
movements and accommodate them as much as practical.
13.
Formation of mushrooms or enlargements at the top of piers should be
avoided during pier drilling and subsequent construction operations.
14.
The installation of the drilled piers should be observed by a representative
of our firm to confirm the piers are bottomed in the proper bearing strata
and to observe the contractor’s installation procedures.
Laterally-Loaded Piers
Lateral load analysis of piers can be performed with the software analysis package
LPILE by Ensoft, Inc. We believe this method of analysis is appropriate for piers with a
pier length to diameter ratio of seven or greater. Suggested criteria for LPILE analysis are
presented in the following Table A.
UNIVERSITY OF COLORADO AT COLORADO SPRINGS
SUMMIT VILLAGE STUDENT HOUSING BUILDINGS
CTL|T PROJECT NO. CS17805-125
S:\CS17500-17999\CS17805.000\125\2. Reports\CS17805-125-R1.doc
10
The
50
value represents the strain corresponding to 50 percent of the maximum
principle stress difference. The moduli of subgrade reaction for static (ks) and cyclical (kc)
conditions are used by the program to generate the slope of the initial portion of the “p-y
Curves”.
TABLE A
SOIL INPUT DATA FOR “LPILE”
Soil Type
Natural Sand and
Sand Fill
Bedrock
0.07
0.08
-
60
30
-
-
0.004
90
2,000
90
1,000
Density
(pci)
Cohesion, c
(psi)
Friction Angle
(degrees)
50
(in/in)
ks
(pci)
kc
(pci)
Other analysis procedures require input of a horizontal modulus of subgrade
reaction (Kh). We believe the following formulas listed in Table B are appropriate for
calculating horizontal modulus of subgrade reaction (Kh) values.
TABLE B
MODULUS OF SUBGRADE REACTION
Soil Type
Natural Sand and
Sand Fill
Bedrock
Modulus of Subgrade Reaction, Kh
(tcf)
Kh = 20 x z
d
Kh = 300
d
Where z = depth (ft); d = pier diameter (ft).
Closely-Spaced Pier Reduction Factors
For axial loading, a minimum spacing of three diameters (center-to-center) is
recommended. At one diameter (piers touching), the skin friction load reduction factor
UNIVERSITY OF COLORADO AT COLORADO SPRINGS
SUMMIT VILLAGE STUDENT HOUSING BUILDINGS
CTL|T PROJECT NO. CS17805-125
S:\CS17500-17999\CS17805.000\125\2. Reports\CS17805-125-R1.doc
11
for both piers would be 0.5. End bearing values need not be reduced, provided the bases
of the piers are at similar elevations. Interpolation can be used between one and three
diameters.
Piers in-line with the direction of lateral loads should have a minimum spacing of
six diameters (center-to-center) based upon the larger pier. If a closer spacing is required,
the modulus of subgrade reaction for initial and trailing piers should be reduced. At a
spacing of three diameters, the effective modulus of subgrade reaction of the first pier
can be estimated by multiplying the given modulus by 0.6. For trailing piers in a line at
three diameter spacing, the factor is 0.4. Linear interpolation can be used for spacing
between three and six diameters.
Reductions to the modulus of subgrade reaction can be accomplished in LPILE by
inputting the appropriate modification factors for the p-y curves. Reducing the modulus
of subgrade reaction in trailing piers will result in greater computed deflections on these
piers. In practice, the grade beam can force deflections of all piers to be equal. Loaddeflection graphs can be generated for each pier using the appropriate p-multiplier
values. The sum of the pier lateral load resistance at selected deflections can be used to
develop a total lateral load versus deflection graph for the system of piers.
For lateral loads perpendicular to the line of piers, a minimum spacing of three
diameters (center-to-center) can be used with no capacity reduction. At one diameter
(piers touching), the piers can be analyzed as one unit. Interpolation can be used for
intermediate conditions.
FLOOR SYSTEMS
Based on our understanding of the proposed construction, subsurface conditions
encountered in our borings, and the results of laboratory testing, we anticipate existing
fill, natural sands, sandstone, and possibly claystone may be encountered at the finished
floor elevation in the basement level of both proposed buildings. Existing fill should be
excavated from floor slab areas to expose the underlying natural soils. The excavation
should be backfilled with granular soils placed and compacted in accordance with the
recommendations presented in SITE GRADING AND UTILITIES.
UNIVERSITY OF COLORADO AT COLORADO SPRINGS
SUMMIT VILLAGE STUDENT HOUSING BUILDINGS
CTL|T PROJECT NO. CS17805-125
S:\CS17500-17999\CS17805.000\125\2. Reports\CS17805-125-R1.doc
12
We expect conventional slab-on-grade floors are considered to be an attractive
floor system alternative. We anticipate a low risk of poor slab performance will exist for
slabs-on-grade underlain by at least 5 feet of the natural sand and/or new, densely
compacted granular fill. We estimate floor slab movements of about 1 inch or less for this
subgrade condition. Where sandstone and/or claystone bedrock is encountered at floor
slab elevation or within 5 feet of the bottom of the floor slab, and a slab-on-grade floor is
the desired floor system alternative, we recommend the bedrock materials be removed to
a depth of at least 5 feet. The excavation should be backfilled with the on-site sand or a
similar, non-expansive material that has been moisture conditioned to within 2 percent of
optimum moisture content and compacted in thin lifts to at least 95 percent of maximum
standard Proctor dry density (ASTM D 698). Floor slab movements on the order of about 1
inch can be expected for this condition. If the risk of possible movement of the floor slabs
is unacceptable, the owner should consider construction of structurally supported floors
in the basement level of the proposed buildings.
If slab-on-grade floors are to be installed in the basement level of either or both of
the proposed buildings, and the owner accepts the risk of movement and associated
damage, we recommend the following precautions for slab-on-grade construction at this
site. All parties must realize that even small movements of the floor slab (less than 1-inch)
can damage comparatively brittle floor treatments, such as ceramic tile, that might be
used in a restroom area. The following precautions can help reduce, but not eliminate
damage or distress due to slab movement.
1.
New fill placed below the basement floor slabs should consist of the onsite sand or a similar, non-expansive material moisture conditioned to
within 2 percent of optimum moisture content and compacted in thin lifts
to at least 95 percent of maximum standard Proctor dry density (ASTM D
698). The placement and compaction of the below-slab backfill should be
observed and tested by a representative of our office during construction.
2.
While, in our opinion, there is no need from a geotechnical standpoint for a
vapor retarder at this site, the 2009 International Building Code (IBC)
requires a vapor retarder be placed between base course or the subgrade
soils and the concrete slab-on-grade floor, unless the designer of the floor
(structural engineer) waives this requirement. The merits of installation of a
vapor retarder below floor slabs depend on the sensitivity of floor
coverings and building use to moisture. A properly installed vapor retarder
(10 mil minimum) is more beneficial below concrete slab-on-grade floors
where floor coverings, painted floor surfaces or products stored on the
UNIVERSITY OF COLORADO AT COLORADO SPRINGS
SUMMIT VILLAGE STUDENT HOUSING BUILDINGS
CTL|T PROJECT NO. CS17805-125
S:\CS17500-17999\CS17805.000\125\2. Reports\CS17805-125-R1.doc
13
floor will be sensitive to moisture. The vapor retarder is most effective
when concrete is placed directly on top of it, rather than placing a sand or
gravel leveling course between the vapor retarder and the floor slab. The
placement of concrete on the vapor retarder may increase the risk of
shrinkage cracking and curling. Use of concrete with reduced shrinkage
characteristics including minimized water content, maximized coarse
aggregate content, and reasonably low slump will reduce the risk of
shrinkage cracking and curling. Considerations and recommendations for
the installation of vapor retarders below concrete slabs are outlined in
Section 3.2.3 of the 2006 report of the American Concrete Institute (ACI)
Committee 302, “Guide for Concrete Floor and Slab Construction (ACI
302.R-96)”.
3.
We recommend slab-on-grade floors be separated from exterior walls and
interior bearing members with joints that allow for independent vertical
movements of the slab relative to the foundation. Provision of a 1-1/2 inch
thick slip joint in slab-bearing partitions can reduce the risk of cracking of
drywall resulting from slab movements. If the “float” is provided at the tops
of partitions, the connection between interior, slab-supported partitions
and exterior, foundation-supported walls should be detailed to allow
differential movement. These architectural connections are critical to help
reduce cosmetic damage should foundations and floor slabs move relative
to each other. We have seen instances where these architectural
connections were not designed and constructed properly and resulted in
moderate cosmetic damage, even though the movement experienced was
well within the anticipated range. The architect should pay special
attention to these issues and detail the connections accordingly. Masonry
block partitions (load bearing or not) should be constructed on their own
independent foundations and not a thickened floor slab.
4.
Underslab plumbing should be avoided as much as possible. If underslab
plumbing is necessary, service lines should be pressure tested for leaks
during construction. Any utility line that penetrates a slab should be
isolated from the slab with a joint to allow for free vertical movement.
5.
Slab-supported mechanical systems should have flexible connections to
allow for vertical movement without rupturing supply lines.
6.
Frequent control joints should be provided in the slabs to reduce the
effects of curling and help reduce shrinkage cracking. Our experience
indicates a joint spacing of not greater than 30 times the slab thickness is
effective in this area.
7.
Exterior flatwork and sidewalks should be separated from the buildings.
These slabs should be designed to function as independent units.
Movement of these slabs should not be transmitted directly to the
foundation of the structure.
UNIVERSITY OF COLORADO AT COLORADO SPRINGS
SUMMIT VILLAGE STUDENT HOUSING BUILDINGS
CTL|T PROJECT NO. CS17805-125
S:\CS17500-17999\CS17805.000\125\2. Reports\CS17805-125-R1.doc
14
BELOW-GRADE CONSTRUCTION
We understand portions of the first level of each building will be constructed
below exterior grades. The foundation walls will be subjected to lateral earth loads that
are dependent on the height of the wall, soil type, and backfill configuration. For backfill
materials that consist of the on-site silty to clayey sands and walls that are not free to
rotate, such as foundation walls, we recommend the walls be designed to resist “at-rest”
earth pressures. We recommend design for the “at-rest” earth pressure condition using
an equivalent fluid density of at least 60 pcf.
The lateral pressure described above does not include allowances for surcharge
loads such as hydrostatic pressure. Drains are typically recommended surrounding
habitable below-grade areas. We recommend a drain system similar to the detail shown
in Fig. 5 be installed adjacent to the basement walls. The drain should include a vertical
drainage column consisting of at least 12 inches of washed gravel or an approved drain
board.
We recommend foundation wall backfill be compacted to limit settlement. Backfill
should be moisture conditioned to within 2 percent of optimum moisture content and
compacted in thin lifts to at least 95 percent of maximum standard Proctor dry density
(ASTM D 698). The placement and compaction of foundation wall backfill should be
observed and tested by a representative of our office during construction.
Even properly compacted backfill will consolidate somewhat under its own
weight. Typically, consolidation on the order of about 1 percent of the backfill thickness
can be expected. Utility service lines that penetrate the basement walls should be
designed and installed with enough flexibility to accommodate backfill settlement of this
magnitude. Rigid utility line penetrations are not recommended.
PAVEMENTS
We understand paved access driveways and possibly some small, automobile
parking areas will be constructed in association with the proposed buildings. Our
exploratory borings suggest the subgrade soils within the anticipated driveways and
UNIVERSITY OF COLORADO AT COLORADO SPRINGS
SUMMIT VILLAGE STUDENT HOUSING BUILDINGS
CTL|T PROJECT NO. CS17805-125
S:\CS17500-17999\CS17805.000\125\2. Reports\CS17805-125-R1.doc
15
parking areas will consist predominantly of natural sand soils and granular grading fill.
This type of material generally exhibits good pavement support characteristics. Based on
our experience, a Hveem stabilometer (“R”) value of 45 was assigned to the subgrade
materials for design purposes.
We anticipate the access driveways and parking areas will be subjected to
automobile traffic and an occasional heavy vehicle load such as a trash truck. We
considered a daily traffic number (DTN) of 2 for the access driveways which corresponds
to 18-kip Equivalent Single-Axle Loads (ESAL) of 14,600, for a 20-year pavement design
life. We believe the driveways and parking areas can be paved with 5 inches of asphalt
concrete or 3 inches of asphalt concrete over 6 inches of aggregate base course.
Any trash dumpster sites should include a concrete pavement that is at least 6
inches thick and large enough to support the entire length of the trash truck and
dumpster during the emptying process. Joints between concrete and asphalt pavements
should be sealed with a flexible compound.
Our design considers pavement construction will be completed in accordance
with the City of Colorado Springs “Standard Specifications” and the Pikes Peak Regional
Asphalt Paving Specifications. The specifications contain requirements for the pavement
materials (asphalt, base course, and concrete) as well as the construction practices used
(compaction, materials sampling, and proof-rolling). Of particular importance are those
recommendations directed toward subgrade and base course compaction and proofrolling. During proof-rolling, particular attention should be directed toward the areas of
confined backfill compaction. Areas that pump excessively should be stabilized prior to
pavement construction. A representative of our office should be present at the site during
placement of fill and construction of pavements to perform density testing.
CONCRETE
Concrete in contact with soils can be subject to sulfate attack. We measured the
soluble sulfate concentration in two samples from this site at less than 0.1 percent.
Sulfate concentrations less than 0.1 percent indicate Class 0 exposure to sulfate attack
for concrete in contact with the subsoils, according to ACI 201.2R-01, as published in the
UNIVERSITY OF COLORADO AT COLORADO SPRINGS
SUMMIT VILLAGE STUDENT HOUSING BUILDINGS
CTL|T PROJECT NO. CS17805-125
S:\CS17500-17999\CS17805.000\125\2. Reports\CS17805-125-R1.doc
16
2008 American Concrete Institute (ACI) Manual of Concrete Practice. For this level of
sulfate concentration, the ACI indicates Type I/II cement can be used for concrete in
contact with the subsoils. In our experience, superficial damage may occur to the
exposed surfaces of highly permeable concrete, even though sulfate levels are relatively
low. To control this risk and to resist freeze-thaw deterioration, the water-to-cementitious
material ratio should not exceed 0.50 for concrete in contact with soils that are likely to
stay moist due to surface drainage or high water tables. Concrete subjected to freezethaw cycles should be air entrained.
SURFACE DRAINAGE
Performance of the foundation systems, floor slabs, concrete flatwork, and
pavements to be constructed at this site will be significantly influenced by the moisture
conditions existing within the near-surface soils. Overall surface drainage patterns must
be planned to provide for the rapid removal of storm runoff. Water should not be allowed
to pond adjacent to foundations or over pavements or concrete flatwork. We recommend
the following precautions be observed during construction and maintained at all times
after the buildings are completed.
1.
Excessive wetting or drying of the open foundation excavations should be
avoided.
2.
Foundation wall backfill should be graded to provide for the rapid removal
of runoff. We recommend a slope equivalent to at least 6 inches in the first
10 feet. In pavement and flatwork areas adjacent to the buildings, the slope
may be reduced to 2 inches in the first 5 feet.
3.
Exterior wall backfill should be placed in thin, loose lifts, moisture
conditioned to within 2 percent of optimum moisture content and
compacted to at least 95 percent of maximum standard Proctor dry density
(ASTM D 698).
4.
Roof downspouts and drains should discharge well away from the
structures. Downspout extensions and/or splash blocks should be
provided to help reduce infiltration into the backfill adjacent to the
buildings.
5.
Landscaping concepts should concentrate on use of plantings that require
little or no supplemental irrigation after the vegetation is established.
Irrigated sod, if it is included in the landscaping plan, should not be located
within 6 feet of the foundation walls. Irrigation should be limited to the
UNIVERSITY OF COLORADO AT COLORADO SPRINGS
SUMMIT VILLAGE STUDENT HOUSING BUILDINGS
CTL|T PROJECT NO. CS17805-125
S:\CS17500-17999\CS17805.000\125\2. Reports\CS17805-125-R1.doc
17
minimum amount sufficient to maintain vegetation. Application of more
water will increase likelihood of slab and foundation movements.
CONSTRUCTION OBSERVATIONS
We recommend that CTL | Thompson, Inc. provide observation and testing
services during construction to allow us the opportunity to verify whether soil conditions
are consistent with those found during this investigation. If others perform these
observations, they must accept responsibility to judge whether the recommendations in
this report remain appropriate.
GEOTECHNICAL RISK
The concept of risk is an important aspect with any geotechnical evaluation
primarily because the methods used to develop geotechnical recommendations do not
comprise an exact science. We never have complete knowledge of subsurface
conditions. Our analysis must be tempered with engineering judgment and experience.
Therefore, the recommendations presented in any geotechnical evaluation should not be
considered risk-free. Our recommendations represent our judgment of those measures
that are necessary to increase the chances that the structure will perform satisfactorily. It
is critical that all recommendations in this report are followed during design and
construction.
LIMITATIONS
Our borings were located to obtain a reasonably accurate indication of subsurface
foundation conditions and as physical access constraints would allow. The borings are
representative of conditions encountered at the exact boring location only. Variations in
subsurface conditions not indicated by the borings are possible. We recommend a
representative of our office observe the completed foundation excavations.
Representatives of our firm should be present during construction to perform
construction observation and materials testing services.
UNIVERSITY OF COLORADO AT COLORADO SPRINGS
SUMMIT VILLAGE STUDENT HOUSING BUILDINGS
CTL|T PROJECT NO. CS17805-125
S:\CS17500-17999\CS17805.000\125\2. Reports\CS17805-125-R1.doc
18
TH - 1
EL. 6386
EAST BUILDING
TH - 2
EL. 6382
TH - 3
EL. 6371
6,390
6,390
6,385
6,385
6,380
22/12
WC=8.0
DD=93
-200=30
50/2
6,380
50/6
WC=4.3
-200=21
6,375
6,375
50/1
6,370
6,370
50/2
50/3
21/12
WC=4.7
DD=101
SS=<0.1
6,365
6,360
42/12
WC=20.4
DD=107
SW=7.4
6,360
6,355
50/9
WC=21.7
DD=102
SW=4.9
6,355
6,365
50/2
50/6
WC=4.9
DD=106
-200=10
50/6
WC=14.1
DD=114
SW=2.1
6,350
6,350
50/6
6,345
UNIVERSITY OF COLORADO AT COLORADO SPRINGS
SUMMIT VILLAGE STUDENT HOUSING BUILDINGS
CTL|T PROJECT NO. CS17805-125
S:\CS17500-17999\CS17805.000\125\2. REPORTS\CS17805-125_GINT.GPJ
6,345
Summary Logs of
Exploratory
Borings
FIG. 3
ELEVATION - FEET
ELEVATION - FEET
50/2
TABLE A-1
SUMMARY OF LABORATORY TESTING
CTL|T PROJECT NO. CS17805-125
BORING
TH-1
TH-2
TH-2
TH-2
TH-3
TH-3
TH-3
TH-4
TH-4
TH-4
TH-4
TH-4
TH-5
TH-5
TH-5
TH-6
TH-6
TH-6
TH-7
TH-7
TH-7
TH-7
TH-8
TH-8
TH-8
TH-8
DEPTH
(FEET)
4
4
19
29
4
9
14
4
9
14
19
29
4
24
34
4
14
29
4
9
19
34
4
14
24
34
MOISTURE
DRY
CONTENT DENSITY
(%)
(PCF)
8.0
93
4.3
4.9
106
14.1
114
4.7
101
20.4
107
21.7
102
5.5
98
5.0
102
7.3
98
6.8
110
18.9
109
10.8
109
18.4
112
18.2
105
6.0
97
5.4
100
14.6
115
11.2
117
8.4
99
6.3
108
10.9
110
11.4
113
15.3
100
9.1
112
21.1
106
ATTERBERG LIMITS
LIQUID
PLASTICITY
LIMIT
INDEX
(%)
(%)
* SWELL MEASURED WITH 1000 PSF APPLIED PRESSURE.
NEGATIVE VALUE INDICATES COMPRESSION.
SWELL TEST RESULTS*
SWELL
SWELL
PRESSURE
(%)
(PSF)
2.1
5000
7.4
4.9
22000
12000
3.5
19000
1.2
1.5
PASSING WATER
NO. 200 SOLUBLE
SIEVE SULFATES
(%)
(%)
DESCRIPTION
30
FILL, SAND, CLAYEY
21
SANDSTONE, SILTY
10
SANDSTONE, SLIGHTLY SILTY
CLAYSTONE, SANDY
<0.1
SAND, CLAYEY (SC)
CLAYSTONE, SANDY
CLAYSTONE, SANDY
FILL, SAND, SILTY
<0.1
SAND, SILTY (SM)
23
SAND, SILTY (SM)
18
SANDSTONE, SILTY
CLAYSTONE, SANDY
FILL, SAND, CLAYEY
CLAYSTONE, SANDY
SANDSTONE, CLAYEY
22
SAND, SILTY (SM)
SAND, SILTY (SM)
SANDSTONE, CLAYEY
FILL, SAND, CLAYEY
17
SAND, SILTY (SM)
14
SAND, SILTY (SM)
SANDSTONE, SILTY
FILL, SAND, CLAYEY
35
SAND, CLAYEY (SC)
25
SAND, CLAYEY (SC)
SANDSTONE, CLAYEY
Page 1 of 1