HomeMy WebLinkAbout8/13/14 Terracon GeoReport South Elementary
Geotechnical Engineering Report
Proposed South Elementary School
Iowa City, Iowa
March 11, 2014
Terracon Project No. 06135652.03
Prepared for:
Iowa City Community School District
Iowa City, Iowa
Prepared by:
Terracon Consultants, Inc.
Iowa City, Iowa
TABLE OF CONTENTS
Page
EXECUTIVE SUMMARY ................................................ i
1.0INTRODUCTION ...................................................
2.0PROJECT INFORMATION ............................................
2.1Project Description ................................................................................................ 2
2.2Site Location and Description ................................................................................ 2
3.0SUBSURFACE CONDITIONS ..........................................
3.1Typical Subsurface Profile ..................................................................................... 3
3.2Groundwater Conditions ........................................................................................ 3
4.0RECOMMENDATIONS FOR DESIGN AND CONSTRUCTION ....................................... 4
4.1Geotechnical Considerations................................................................................. 4
4.2Earthwork .............................................................................................................. 5
4.2.1Site Preparation ......................................................................................... 5
4.2.2Soil Stabilization ........................................................................................ 6
4.2.3Excavation Considerations ........................................................................ 6
4.2.4Fill Material Requirements ......................................................................... 6
4.2.5Compaction Requirements ........................................................................ 8
4.2.6Grading and Drainage ............................................................................... 8
4.3Shallow Foundations ............................................................................................. 9
4.3.1Shallow Foundation Design Recommendations ........................................ 9
4.3.2Shallow Foundation Construction Considerations ................................... 10
4.4Floor Slabs .......................................................................................................... 11
4.4.1Floor Slab Design Recommendations ..................................................... 11
4.4.2Floor Slab Construction Considerations .................................................. 12
4.5Seismic Considerations ....................................................................................... 12
4.6Pavement Recommendations ............................................................................. 12
4.6.1Pavement Subgrade Preparation ............................................................ 12
4.6.2Pavement Design Recommendations ..................................................... 13
4.6.3Pavement Design Considerations ........................................................... 15
4.6.4Pavement Subdrains ............................................................................... 15
4.6.5Pavement Maintenance ........................................................................... 16
4.7Frost Considerations ........................................................................................... 16
5.0GENERAL COMMENTS ...............................................
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TABLE OF CONTENTS (continued)
APPENDIX A Î FIELD EXPLORATION
Exhibit A-1 Site Location Plan
Exhibit A-2 Boring Location Plan
Exhibit A-3 Field Exploration Description
Exhibit A-4 Subsurface Profile
Exhibits A-5 to A-13 Boring Logs B-201 through B-209
APPENDIX B Î SUPPORTING INFORMATION
Exhibit B-1 Laboratory Testing
Exhibit B-2 Consolidation Test
APPENDIX C Î SUPPORTING DOCUMENTS
Exhibit C-1 General Notes
Exhibit C-2 Unified Soil Classification System
Exhibit C-3 Boring Logs B-1 through B-5 (Project 06135652.01)
Exhibit C-4 Boring Logs B-101 through B-105 (Project 06135652.02)
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Geotechnical Engineering Report
Proposed South Elementary School Iowa City, Iowa
March 11, 2014 Terracon Project No. 06135652.03
EXECUTIVE SUMMARY
A geotechnical exploration has been performed for the proposed new elementary school located
west of Moira Avenue SE on Sycamore Street SE in Iowa City, Iowa. Terracon’s geotechnical
scope of service included the advancement of nine borings to approximate depths of 10 to 30
feet below existing site grades. Information from other explorations Terracon performed in the
site area was also used to supplement this exploration. Based on the information obtained from
our subsurface exploration, the following geotechnical considerations were identified:
The proposed South Elementary School building may be supported on shallow foundations
bearing on newly-placed structural fill, stiff to hard native clay, or on compacted crushed
stone fill following a limited overexcavation and backfill procedure.
At the time of the field investigation, frost was encountered in the upper portion of the soil
profile to depths of about 3 feet, and this caused the upper soils to exhibit higher strengths.
When spring thaw occurs, these soils will be susceptible to disturbances and may have
reduced strengths. Should lower strength soils be encountered at foundation bearing
elevation, we recommend overexcavation and backfill with compacted crushed stone to the
design bearing elevation.
Based on the limited cut/fill diagram provided by MMS Consultants, it appears that about 1 to 5
feet of fill will be placed across the school building footprint. Settlement of the underlying soils
from this additional fill weight is anticipated to be approximately ½ to 1 inch. This will be in
addition to settlement caused by the addition of foundations.
Assuming proper site preparation and any necessary foundation bearing soil corrections,
total and differential settlement should be less than about 1 inch and two-thirds (2/3) of total
settlement, respectively.
Near-surface moderate to high plasticity soils (lean clay and fat clay) were encountered in
the borings, and may be subject to significant volume change with variations in moisture
content. For this reason, we recommend a minimum 24-inch thick low plasticity zone be
constructed beneath grade-supported floor slabs and 12 inches below pavements. We
anticipate that this low plasticity zone will be built during fill placement, but construction of
the low plasticity zone may require overexcavation and replacement in portions of the
pavement areas, depending upon final grading plans.
On-site lean clay and silty sand soils appear suitable for use as compacted structural fill.
Fat clay occurring in the upper portion of the soil profile does not meet the low plasticity fill
criteria, and they should not be utilized as fill within 2 feet of finished subgrade elevation in
building areas. Moisture conditioning (e.g. drying of clays) should be anticipated if the on-
site soils are used as fill.
Based on the limited subsurface information obtained during our site exploration, the 2009
International Building Code (IBC), seismic site classification for this site is D.
Earthwork on the project should be observed and evaluated by Terracon. The evaluation of
earthwork should include observation and testing of structural fill, subgrade preparation,
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Geotechnical Engineering Report
Proposed South Elementary School Iowa City, Iowa
March 11, 2014 Terracon Project No. 06135652.03
foundation bearing materials, and other geotechnical conditions exposed during
construction.
This summary should be used in conjunction with the entire report for design purposes. It
should be recognized that details were not included or fully developed in this section, and the
report must be read in its entirety for a comprehensive understanding of the items contained
herein. The section titled GENERAL COMMENTS should be read for an understanding of the
report limitations.
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GEOTECHNICAL ENGINEERING REPORT
PROPOSED SOUTH ELEMENTARY SCHOOL
SYCAMORE STREET SE, WEST OF MOIRA AVENUE
IOWA CITY, IOWA
Terracon Project No. 06135652.03
March 11, 2014
1.0 INTRODUCTION
This report presents the results of our subsurface exploration and geotechnical engineering
services performed for the proposed new elementary school located west of Moira Avenue SE
on Sycamore Street SE in Iowa City, Iowa. The purpose of these services is to provide
information and geotechnical engineering recommendations relative to:
subsurface soil conditions foundation design and construction
groundwater conditions floor slab design and construction
site preparation and earthwork seismic site classification per IBC
excavation considerations pavement design and construction
dewatering considerations frost considerations
The geotechnical engineering scope of service for this project included the advancement of nine
borings to depths ranging from approximately 10 to 30 feet below existing site grades.
A Site Location Plan (Exhibit A-1), a Boring Location Plan (Exhibit A-2), a subsurface soil profile
(Exhibit A-4) and logs of the borings (Exhibits A-5 to A-13) are included in Appendix A of this
report. The results of the laboratory testing performed on soil samples obtained from the site
during the field exploration are included on the boring logs and in Appendix B of this report.
Descriptions of the field exploration and laboratory testing are included in their respective
appendices.
Terracon performed prior subsurface explorations for this project in August 2013 (Terracon
Project No. 06135652.01, Geotechnical Engineering Report dated August 12, 2013) and in
January 2014 (Terracon Project No. 06135652.02, Addendum to Geotechnical Engineering
Report dated January 29, 2014). Some information from these prior explorations was used in
developing the recommendations contained in this report. Boring logs from those explorations
are also included in this report in Appendix C.
A Phase I Environmental Site Assessment (ESA) was prepared for this project and was issued
under separate cover (Terracon Project No. 06137714, dated July 24, 2013). The designer of
any project on this site should be aware of the contents of the ESA.
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Geotechnical Engineering Report
Proposed South Elementary School Iowa City, Iowa
March 11, 2014 Terracon Project No. 06135652.03
2.0 PROJECT INFORMATION
2.1Project Description
Item Description
See Appendix A, Exhibit A-2: Boring Location Plan
Site layout
Elementary school building with proposed footprint of
approximately 55,595 square feet
Driveways
Structures
Passenger vehicle parking
Athletic fields / playground areas
Masonry walls
Building construction
Precast floor sections
Finished floor elevation
669.00 feet
(provided)
Columns: 75 kips
Single-story classroom area walls: 3.5 klf
Maximum loads
Two-story classroom area walls: 6.5 klf
(provided)
Gymnasium walls: 7.5 klf
Slabs: 125 psf max
Cuts of about 1 to 3 feet in parking lot and drive areas
Grading
Fills up to about 5 feet in the school building area
None anticipated
Free-standing retaining walls
None anticipated
Below grade areas
Passenger vehicle parking lots
Pavements
Driveways
2.2Site Location and Description
Item Description
West of Moira Avenue SE on Sycamore Street SE in Iowa
Location
City, Iowa
None – corn field
Existing improvements
Snow
Current ground cover
Farmland with harvested crops
Site grades range from approximately 660 to 670 feet
Existing topography
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Geotechnical Engineering Report
Proposed South Elementary School Iowa City, Iowa
March 11, 2014 Terracon Project No. 06135652.03
3.0 SUBSURFACE CONDITIONS
3.1Typical Subsurface Profile
Based on the results of the borings, subsurface conditions at the boring locations can be
generalized as follows:
Approximate Depth to
Consistency /
Stratum Bottom of Stratum Material Description
Relative Density
(feet)
Surficial 6 to 18 inches Topsoil / Root Zone N/A
Fat clay, trace sand and occasional organic
1 3 to 5 Very soft to hard
zones
Hard to very soft
2 8 to 10 Lean clay, trace sand
with depth
Boring termination of 10 Loose to medium
3Silty sand, with occasional silt seams
to 30 dense
1. Boring B-208 terminated in Stratum 2 soils.
2. Borings B-201 through B-207 and B-209 terminated in Stratum 3 soils.
Conditions encountered at each boring location are indicated on the individual boring logs.
Stratification boundaries on the boring logs represent the approximate location of changes in
native soil types; in situ, the transition between materials may be gradual. Details for each of
the borings can be found on the boring logs in Appendix A of this report.
The subsurface conditions encountered in our exploration were generally similar to those found
during our previous explorations.
3.2Groundwater Conditions
The borings were performed during a period of warmer weather which caused puddles of
standing water from melting snow at the staked locations. Representative groundwater levels
were not able to be observed during drilling and sampling due to this meltwater. The borings
were left open approximately 15 days so delayed water level observations could be made.
Following the delayed observations, the borings were backfilled with on-site soils and/or
bentonite chips. Delayed water levels observations are presented in the table below.
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Geotechnical Engineering Report
Proposed South Elementary School Iowa City, Iowa
March 11, 2014 Terracon Project No. 06135652.03
WATER LEVEL OBSERVATIONS
Feet Below Grade
Boring No. Delayed Reading
B-201 Dry cave-in at 18
B-202 Dry cave-in at 24½
B-203 Dry cave-in at 4½
B-204 Dry cave-in at 18
B-205 Dry cave-in at 15
B-206 Dry cave-in at 4½
B-207 Dry cave-in at 4
B-208 3¼
B-209 Dry cave-in at 3½
These water level observations provide an approximate indication of the groundwater conditions
existing on the site at the time the observations were made. Longer-term observations using
cased holes or piezometers, sealed from the influence of surface water, would be required for a
better evaluation of the groundwater conditions on this site. Considering information obtained
during previous explorations at this project site, we anticipate the typical groundwater level to be
encountered approximately between 15 to 25 feet below existing grades.
Fluctuations of the groundwater levels will likely occur due to seasonal variations in the amount
of rainfall, runoff and other factors not evident at the time the borings were performed.
Therefore, groundwater levels during construction or at other times in the life of the structure
may be different than the levels indicated on the boring logs. Also, trapped or “perched” water
could be present within the sand or silt seams within native clay soils. Significant quantities of
perched water may be present in the topsoil, particularly in plow zones and in the near-surface
soils that have been loosened by freeze-thaw action, during wetter/cooler climatic conditions.
The possibility of groundwater level fluctuations and perched water should be considered when
developing the design and construction plans for the project.
4.0 RECOMMENDATIONS FOR DESIGN AND CONSTRUCTION
4.1Geotechnical Considerations
Based on the results of the subsurface exploration, laboratory testing, and our analyses, it is our
opinion that the proposed elementary school can be supported on shallow foundations bearing
on new compacted structural fill placed during the site grading portion of the project, on suitable
native soils, or if overexcavation of lower strength soils is needed, on compacted crushed stone
fill extending to suitable native soils.
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Geotechnical Engineering Report
Proposed South Elementary School Iowa City, Iowa
March 11, 2014 Terracon Project No. 06135652.03
A primary geotechnical consideration for development of the South Elementary School is the
presence of a zone of lower strength lean clay soil combined with the effects of raising the entire
site grade. Based on the boring elevations and the proposed finished grades, up to about 5 feet
of new fill will be required across the school footprint. Settlement within the underlying clay soils
from the weight of the new fill is estimated to be on the order of ½ to 1 inch. This settlement will
be in addition to settlement attributed to the loadings of the new shallow foundations. We
anticipate that the addition of new shallow foundations bearing on/in the new fill will cause up to
an additional 1 inch of settlement within the clay soils. To avoid excessive settlements, the new
fill should be allowed to consolidate the underlying soils. We estimate that 4 to 6 weeks will be
required for 90 percent of primary consolidation to occur.
We anticipate that stiff to hard native clay soils will be encountered directly below the newly-
placed site grading fill and that most foundations will bear on these stiff native soils. However,
frost was present in the soils on this site to depths of about 3 feet below grade, obscuring in-situ
strengths in the uppermost portion of the site. Once these soils are allowed to thaw, they may
lose their apparent higher strengths and some foundation locations may require overexcavation
of lower strength soils and replacement with properly-compacted crushed stone backfill. Where
required, we recommend a crushed stone thickness of ½ the width of column footings or the
entire width of wall footings.
4.2Earthwork
4.2.1 Site Preparation
Topsoil, vegetation, and any otherwise unsuitable materials should be removed from the
construction areas. Approximately 6 to 18 inches of topsoil were encountered across the 19
borings drilled at this site. Excessively wet or dry material should either be removed or moisture
conditioned and recompacted. Soft and/or low-density soil should be removed or compacted in
place prior to placing new fill.
Due to the presence of fat clay soils in the upper portion of the soil profile, we recommend that
at least 24 inches of low plasticity material be present below floor slabs. We anticipate that this
will be satisfied during the site grade raising process, but a limited overexcavation may still be
needed in areas of smaller fills. The proposed pavement area subgrades should also be
stripped of native moderate to high plasticity clay to accommodate at least 12 inches of low
plasticity structural fill soils back to the planned bottom of pavement. This low plasticity fill zone
includes the base course layer recommended in section 4.6 Pavement Recommendations.
Following stripping of topsoil, the exposed subgrade should be scarified and recompacted by
the contractor and test probed by Terracon. This scarification and recompaction process will
help to identify soft or loose areas below the subgrade level. Soft or loose areas should be
undercut, moisture conditioned, and recompacted or replaced with approved structural fill.
Subgrade conditions should be observed by Terracon during construction.
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Proposed South Elementary School Iowa City, Iowa
March 11, 2014 Terracon Project No. 06135652.03
Dewatering during construction is not anticipated, but could be required depending on seasonal
water level fluctuations. We expect that sump pits and pumps would generally be adequate for
dewatering excavations in clay soils. More extensive dewatering measures may be required for
excavations extending into water bearing sand soils.
Upon completion of filling and grading, care should be taken to maintain the subgrade moisture
content prior to construction of floor slabs and pavements. Construction traffic over the
completed subgrade should be avoided to the extent practical. The site should also be graded
to prevent ponding of surface water on the prepared subgrades or in excavations. If the
subgrade should become frozen, desiccated, saturated, or disturbed, the affected material
should be removed or these materials should be scarified, moisture conditioned, and
recompacted prior to floor slab and pavement construction.
4.2.2 Soil Stabilization
A Terracon representative should observe subgrade preparation and could assist in developing
appropriate stabilization procedures based on conditions encountered during construction.
Methods of subgrade improvement could include scarification, moisture conditioning, and
recompaction, removal of unstable materials and replacement with granular fill (with or without
geosynthetics) and chemical stabilization. The appropriate method of improvement, if required,
would be dependent on factors such as schedule, weather, the size of area to be stabilized, and
the nature of the instability. More detailed recommendations can be provided during
construction as the need for subgrade stabilization occurs. Performing site grading operations
during warm seasons and dry periods would help reduce the amount of subgrade stabilization
required.
4.2.3 Excavation Considerations
As a minimum, all temporary excavations should be sloped or braced as required by
Occupational Safety and Health Administration (OSHA) regulations to provide stability and safe
working conditions. Temporary excavations will probably be required during grading operations
and installation of utilities. Contractors are usually responsible for designing and constructing
stable, temporary excavations and should shore, slope or bench the sides of the excavations as
required, to maintain stability of both the excavation sides and bottom. All excavations should
comply with applicable local, state and federal safety regulations, including the current OSHA
Excavation and Trench Safety Standards.
4.2.4 Fill Material Requirements
The near-surface fat clay soils will have Atterberg limits greater than those recommended
below, and should not be used as fill within 2 feet of finished floor slab or within 12 inches of
bottom of pavement elevation. If excavations extend deeper into the soil profile, the lean clay
soils generally appear suitable for reuse as structural fill. However, moisture conditioning of
these soils should be expected if they are to be reused as structural fill.
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Geotechnical Engineering Report
Proposed South Elementary School Iowa City, Iowa
March 11, 2014 Terracon Project No. 06135652.03
Fill placed in building and pavement areas should ideally be low plasticity cohesive soil or
granular soil. Fill placed in confined excavations, utility trenches crossing pavements, and
structure foundation overexcavations should consist of relatively clean and well-graded granular
material. This should provide for greater ease of placement and compaction in confined areas
where larger compaction equipment cannot be operated. The use of granular fill in these
isolated and potentially deeper excavations would reduce the potential for differential settlement
for building components.
Compacted structural fill should meet the following material property requirements:
1
Fill Type USCS Classification Acceptable Location for Placement
Low Plasticity
CL-ML, CL General site grading fill
2
Cohesive
Green (non-structural) locations
High Plasticity
CL/CH, CH
3
Cohesive
General site grading fill
General site grading fill
GW, GP, GM, GC
Granular
SW, SP, SM, SC
Below foundations, pavements
Unsuitable MH, OL, OH, PT Green (non-structural) locations
The on-site soils encountered in the borings
On-Site Soils CH, CL, SM, SP generally appear suitable for reuse as structural
3
fill.
1. Structural fill should consist of approved materials that are free of organic matter and debris.
Frozen material should not be used, and fill should not be placed on a frozen subgrade. A sample
of each material type should be submitted to the geotechnical engineer for evaluation prior to use
on this site.
2. Low plasticity cohesive soil would have a liquid limit less than 45 and a plasticity index of less than
23.
3. CH and CL/CH soils should not be used for structural fill within 2 feet of finished subgrade elevation
in building areas.
Appropriate laboratory tests, including Atterberg Limits for cohesive soils, organic content tests
for dark colored soils, and standard Proctor (ASTM D698) moisture-density relationship tests
should be performed on proposed fill materials prior to their use as structural fill. Further
evaluation of any on-site soils or off-site fill materials should be performed by Terracon prior to
their use in compacted fill sections.
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Geotechnical Engineering Report
Proposed South Elementary School Iowa City, Iowa
March 11, 2014 Terracon Project No. 06135652.03
4.2.5 Compaction Requirements
Item Description
9 inches or less in loose thickness when heavy,
self-propelled compaction equipment is used
Maximum Fill Lift Thickness
4 inches in loose thickness when hand-guided
equipment (i.e. jumping jack or plate compactor) is
used
98% beneath foundations and within 1 foot of
finished pavement subgrade
Minimum Compaction
1,2,3
Requirements
95% above foundations and more than 1 foot below
pavement subgrade
Low plasticity cohesive: -2% to +3%
1
Moisture Content Range High plasticity cohesive: 0 to +4%
Granular: -3% to +3%
1. As determined by the standard Proctor test (ASTM D698).
2. Lean to fat clay and fat clay should not be compacted to more than 100 percent of standard Proctor
maximum dry density.
3. If the granular material is a coarse sand or gravel, or of a uniform size, or has a low fines content,
compaction comparison to relative density may be more appropriate. In this case, granular
materials should be compacted to at least 70% relative density (ASTM D 4253-00 and D 4254-00).
4.2.6 Grading and Drainage
Final surrounding grades should be sloped to provide effective drainage away from the school
building and pavements during and after construction. In addition, roof drainage should be
collected by a system of gutters and downspouts and transmitted by pipe to the storm water
drainage system or discharged a minimum of 5 feet away from the structure. As an alternative,
splash blocks may be used as long as the ground surface is paved and slopes away from the
structure. Grades around the structure should also be periodically inspected and adjusted as
necessary, as part of the structure’s maintenance program.
Water permitted to pond next to the school building or on pavements can result in greater soil
movements than those discussed in this report. These greater movements can result in
unacceptable differential floor slab and pavement movements, cracked slabs and walls, and
roof leaks. Estimated movements described in this report are based on effective drainage for
the life of the structure and pavement and cannot be relied upon if effective drainage is not
maintained.
Trees or other vegetation whose root systems have the ability to remove excessive moisture
from the subgrade and foundation soils should not be planted next to the structure. Trees and
shrubbery should be kept away from the exterior edges of the foundation element a distance at
least equal to 1½ times their expected mature height.
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Geotechnical Engineering Report
Proposed South Elementary School Iowa City, Iowa
March 11, 2014 Terracon Project No. 06135652.03
4.3Shallow Foundations
Based upon the subsurface conditions encountered in the borings, and the proposed
construction, the proposed new elementary school can be supported on conventional spread
footing foundations. As previously discussed, there is a layer of softer clay soils present at this
site. We anticipate that the foundations in the southern portion of the school building will bear in
or on the new fill material. However, some foundations, particularly in the northern portion of
the building, may extend through the new fill, and we recommend these foundations bear on stiff
to hard native clay. If lower strength soils are encountered at the foundation bearing elevation,
we recommend overexcavation of these materials to depths of ½ the width of column footings or
the full width of wall footings. The excavations should then be backfilled with properly
compacted crushed stone backfill to the design bearing elevation.
4.3.1 Shallow Foundation Design Recommendations
Description Value
Newly placed and compacted structural fill, or
Native, stiff to hard clay soil, or
Suitable bearing materials
Crushed stone backfill following overexcavation
procedure discussed above
1
2,000 psf
Net allowable bearing pressure
Columns: 30 inches
Minimum dimensions
Walls: 18 inches
42 inches: perimeter footings and other footings in
Minimum embedment below
unheated areas
2
finished grade
3
20 inches – interior footings in heated areas
Approximate total settlement from
Less than about 1 inch
4, 5
foundation loads
Estimated differential settlement
About two-thirds (2/3) or total settlement
from foundation loads
1. The recommended net allowable bearing pressure is the pressure in excess of the minimum
surrounding overburden pressure at the footing base elevation. The allowable foundation bearing
pressures apply to dead loads plus design live load conditions. The design bearing pressure may
be increased by one-third when considering total loads that include wind or seismic conditions.
2. Finished grade is defined as the lowest adjacent grade within 5 feet of the foundation for perimeter
(or exterior) footings and finished floor level for interior footings.
3. Interior footings should be constructed to a minimum embedment of 42 inches if they will be
subjected to frost conditions during construction.
4. The above settlement estimates from foundation loads have assumed that the maximum footing
width is 6.5 feet for column footings and 4 feet for continuous footings.
5. The above settlement estimates also consider that where 5 feet or more of new fill is placed,
adequate time is allowed for consolidation and monitoring settlements of the fill and underlying
native soils prior to foundation construction.
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Geotechnical Engineering Report
Proposed South Elementary School Iowa City, Iowa
March 11, 2014 Terracon Project No. 06135652.03
Footings, foundation walls, and masonry walls should be reinforced as necessary to reduce the
potential for distress caused by differential foundation movement. The use of joints at openings
or other discontinuities in masonry walls is recommended.
Foundation excavations should be observed by Terracon. If the soil conditions encountered
differ significantly from those presented in this report, supplemental recommendations will be
required.
4.3.2 Shallow Foundation Construction Considerations
If unsuitable bearing soils are encountered in footing excavations, the excavations should be
extended deeper to suitable soils and the footings could bear on properly compacted crushed
stone backfill extending down to the suitable soils. The footings could also bear directly on
these soils at the lower level. Overexcavation for compacted backfill placement below footings
should extend laterally beyond all edges of the footings at least 8 inches per foot of
overexcavation depth below footing base elevation. The overexcavation should then be
backfilled up to the footing base elevation with well-graded granular material placed in lifts of 9
inches or less in loose thickness and compacted to at least 98 percent of the material's
maximum standard Proctor dry density (ASTM D698). The overexcavation and backfill
procedures are shown in the figure below.
The base of all foundation excavations should be free of water and loose or soft soils prior to
placement of reinforcing steel and concrete. If groundwater is encountered at the time of
construction, it should be lowered and controlled to a minimum depth of 2 feet below the
excavation elevation. Should the soils at the bearing level become disturbed, the affected soil
should be stabilized or removed prior to placement of concrete. Concrete should be placed as
soon as possible after excavating to minimize disturbance of bearing soils.
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Geotechnical Engineering Report
Proposed South Elementary School Iowa City, Iowa
March 11, 2014 Terracon Project No. 06135652.03
4.4Floor Slabs
Based on the provided fill diagram, we anticipate that the entire footprint of the school building
will receive at least 1 foot of new structural fill above the existing site grade. When combined
with stripping about 12 inches of topsoil, this fill is expected to create the recommended 2-foot
low plasticity fill zone between the bottom of the floor slab and the top of the in-situ fat clay soils.
If any areas exist where fat clay soils are still present at the finished slab elevation, we
recommend further removal of these fat clay soils so that at least 24 inches of low plasticity
structural fill is present below floor slabs.
4.4.1 Floor Slab Design Recommendations
Item Description
Interior floor system Slab-on-grade portland cement concrete
Minimum 6 inches of free-draining (less than 6%
passing the U.S. No. 200 sieve) crushed
aggregate compacted to at least 95% of ASTM D
698
At least 24 inches of low plasticity cohesive or
Floor slab support
granular soils with at least 18% passing the U.S.
No. 200 sieve material should be present below
floor slabs where fat clay soils are present directly
below slabs
100 pounds per square inch per inch (psi/in) for
Estimated modulus of subgrade reaction
point loads
1. To reduce contamination of the recommended 6-inch thick drainage layer of free-draining material
(e.g. IDOT Gradation 12a, Section 4121, or other aggregate with less than 6 percent passing the U.S.
No. 200 sieve), we recommend that all footing construction be completed before the drainage
aggregate is placed.
The use of a vapor retarder should be considered beneath concrete slabs on grade that will be
covered with wood, tile, carpet or other moisture sensitive or impervious coverings, or when the
slab will support equipment sensitive to moisture. When conditions warrant the use of a vapor
retarder, the slab designer should refer to ACI 360 for procedures and cautions regarding the
use and placement of a vapor retarder.
Where floor slabs are tied to perimeter walls or turn-down slabs to meet structural or other
construction objectives, our experience indicates that any differential movement between the
walls and slabs will likely be observed in adjacent slab expansion joints or floor slab cracks that
occur beyond the length of the structural dowels. The structural engineer should account for
this potential differential settlement through use of sufficient control joints, appropriate
reinforcing or other means.
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Geotechnical Engineering Report
Proposed South Elementary School Iowa City, Iowa
March 11, 2014 Terracon Project No. 06135652.03
4.4.2 Floor Slab Construction Considerations
On most project sites, the site grading is generally accomplished early in the construction
phase. However as construction proceeds, the subgrade will likely be disturbed due to utility
excavations, construction traffic, desiccation, rainfall, etc. Correction to subgrades prior to
placement of base course crushed stone and concrete should be anticipated, particularly where
subgrades consist of and/or are underlain by high moisture content clay soils or loose sands.
Terracon should review the condition of the floor slab subgrade immediately prior to slab
construction. Particular attention should be given to high traffic areas that were rutted and/or
disturbed earlier and to areas where backfilled trenches are located. Areas where unsuitable
conditions are located should be repaired by scarification, moisture conditioning, and
recompaction or by removing the affected material and replacing it with structural fill.
4.5Seismic Considerations
Description Value
12
2009 International Building Code Site Classification (IBC) D
Site Latitude N 41° 37’ 18.89”
Site Longitude W 91° 30’ 47.01”
SSpectral Acceleration for a Short Period 0.108g
DS
SSpectral Acceleration for a 1-Second Period 0.084g
D1
1
2009 International Building Code,
Note: In general accordance with the Table 1613.5.2. IBC Site Class is based
on the average characteristics of the upper 100 feet of the subsurface profile.
2
Note: The 2009 International Building Code (IBC) uses a site soil profile determination extending to a depth of
100 feet for seismic site classification. The current scope does not include a 100 foot soil profile determination.
Borings extended to a maximum depth of 75 feet under previous explorations, and this seismic site class definition
considers that medium dense sand and/or stiff to hard fat clay continues below the maximum depth of the subsurface
exploration. Additional exploration to deeper depths, or seismic velocity testing would be required to confirm the
conditions below the current depth of exploration.
4.6Pavement Recommendations
4.6.1 Pavement Subgrade Preparation
The subgrade for pavements should be prepared in accordance with section 4.2 Earthwork. As
previously noted, we recommend that at least 12 inches of low plasticity material be present
below pavements where fat clays exist. In addition to the scarification and compaction
recommended, we recommend the exposed subgrade be proofrolled. Unstable and/or organic
material encountered below subgrade level should be further undercut and replaced with
structural fill. The upper 1 foot of subgrade material should be compacted to at least 98 percent
of the material’s maximum dry density as determined by ASTM D698.
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Geotechnical Engineering Report
Proposed South Elementary School Iowa City, Iowa
March 11, 2014 Terracon Project No. 06135652.03
If there is a delay between subgrade preparation and paving, the pavement subgrades should
be carefully re-evaluated as the time for pavement construction approaches. Within a few days
of the scheduled paving, we recommend the pavement areas be proofrolled again with a loaded
tandem axle dump truck (gross weight of about 25 tons) in the presence of Terracon personnel.
Particular attention should be given to the areas that were rutted and disturbed earlier during
construction operations and frequent movement of construction equipment. Areas where
unsuitable conditions exist should be repaired by removing and replacing the materials with
properly compacted fill.
4.6.2 Pavement Design Recommendations
Traffic load information was not available at the time of this report; therefore, a formal pavement
design is not provided. Some typical pavement sections are provided below. Asphaltic cement
concrete pavement thicknesses are based on the Asphalt Paving Association of Iowa (APAI)
Asphalt Paving Design Guide and local design practice. Portland cement concrete thicknesses
are based on the American Concrete Institute (ACI) ACI 330R-08 – Guide for the Design and
Construction of Concrete Parking Lots. Thickness recommendations for Passenger Vehicle
Parking sections are based on light passenger vehicle (gross weight less than 4 tons) traffic
only, and only occasional truck traffic such as snow removal trucks (APAI Class II, ACI Traffic
Category A). As part of the layout design of the project we recommend the designer use signs
and preventive structures to restrict heavy truck traffic from entering these areas. The School
Bus Lanes and Driveways sections are based on less than 25 trucks per day (APAI Traffic
Class III, ACI Traffic Category B).
As a minimum, we suggest the following typical pavement sections be considered.
1
Recommended Pavement Section Thickness (inches)
Asphaltic Portland Aggregate
Traffic Area Alternative
Cement Cement Base Total
3
Concrete Concrete Course
A --- 5 6 11
Passenger Vehicle
Parking
B 6 --- 6 12
A --- 6 6 12
School Bus Lanes and
2
Driveways
B 8 --- 6 14
1. All materials should meet the current Iowa Department of Transportation (IDOT) Standard
Specifications for Highway and Bridge Construction.
Asphaltic Surface - IDOT Type A Asphaltic Cement Concrete: Section 2303
Asphaltic Base - IDOT Type B Asphaltic Cement Concrete, Class I: Section 2303
Concrete Pavement - IDOT Portland Cement Concrete Type C: Section 2301
2. In areas of anticipated heavy traffic, fire trucks, delivery trucks, or concentrated loads (e.g.
dumpster pads), and areas with repeated turning or maneuvering of heavy vehicles, a minimum
Responsive Resourceful Reliable 13
Geotechnical Engineering Report
Proposed South Elementary School Iowa City, Iowa
March 11, 2014 Terracon Project No. 06135652.03
concrete thickness of 7 inches is recommended but should be evaluated further when loading
conditions are known.
3. A minimum 1.5-inch surface course should be used on ACC pavements.
The estimated pavement sections provided in this report are minimums for the assumed design
criteria, and as such, periodic maintenance should be expected. Areas for parking of heavy
vehicles, concentrated turn areas, and start/stop maneuvers could require thicker pavement
sections. Edge restraints (i.e. concrete curbs or aggregate shoulders) should be planned along
curves and areas of maneuvering vehicles. A maintenance program that includes surface
sealing, joint cleaning and sealing, and timely repair of cracks and deteriorated areas will
increase the pavement’s service life. As an option, thicker sections could be constructed to
decrease future maintenance.
All concrete for rigid pavements should have a minimum 28-day compressive strength of 4,000
psi, and be placed with a maximum slump of 4 inches. Although not required for structural
support, a minimum 6-inch thick freely-draining granular base course layer is recommended to
help reduce potential for slab curl, shrinkage cracking, and subgrade “pumping” through joints.
Proper joint spacing will also be required to prevent excessive slab curling and shrinkage
cracking. All joints should be sealed to prevent entry of foreign material and dowelled where
necessary for load transfer.
Where practical, we recommend “early-entry” cutting of crack-control joints in Portland cement
concrete pavements. Cutting of the concrete in its “green” state typically reduces the potential
for micro-cracking of the pavements prior to the crack control joints being formed, compared to
cutting the joints after the concrete has fully set. Micro-cracking of pavements may lead to
crack formation in locations other than the sawed joints, and/or reduction of fatigue life of the
pavement.
Pavement design methods are intended to provide structural sections with adequate thickness
over a particular subgrade such that wheel loads are reduced to a level the subgrade can
support. The support characteristics of the subgrade for pavement design do not account for
shrink/swell movements of a potentially expansive clay subgrade such as the soils encountered
in Borings B-207 through B-209. Thus, the pavement may be adequate from a structural
standpoint, yet still experience cracking and deformation due to shrink/swell related movement
of the subgrade. It is, therefore, important to minimize moisture changes in the subgrade to
reduce shrink/swell movements.
Openings in pavements, such as decorative landscaped areas, are sources for water infiltration
into surrounding pavement systems. Water can collect in the islands and migrate into the
surrounding subgrade soils thereby degrading support of the pavement. This is especially
applicable for islands with raised concrete curbs, irrigated foliage, and low permeability near-
surface soils. The civil design for the pavements with these conditions should include features to
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Geotechnical Engineering Report
Proposed South Elementary School Iowa City, Iowa
March 11, 2014 Terracon Project No. 06135652.03
restrict or to collect and discharge excess water from the islands. Examples of features are
edge drains connected to the storm water collection system, longitudinal subdrains, or other
suitable outlet and impermeable barriers preventing lateral migration of water such as a cutoff
wall installed to a depth below the pavement structure.
Terracon has observed dishing in some parking lots surfaced with ACC. Dishing is usually
observed in frequently-used parking stalls (such as near the front of buildings), and occurs
under the wheel footprint in these stalls. The use of higher-grade asphaltic cement, or surfacing
these areas with PCC, should be considered. The dishing is exacerbated by factors such as
irrigated islands or planter areas, sheet surface drainage to the front of structures, and placing
the ACC directly on a compacted clay subgrade.
4.6.3 Pavement Design Considerations
Long term pavement performance will be dependent upon several factors, including pavement
and subgrade thicknesses, maintaining subgrade moisture levels and providing for preventive
maintenance. The following recommendations should be considered the minimum:
Final grade adjacent to paved areas should slope down from the edges at a minimum 2%;
The subgrade and pavement surface should have a minimum 2% slope to promote proper
surface drainage;
Install below pavement drainage systems surrounding areas anticipated for frequent wetting;
Install joint sealant and seal cracks immediately;
Seal all landscaped areas in or adjacent to pavements to reduce moisture migration to
subgrade soils;
Place compacted, low permeability backfill against the exterior side of curb and gutter; and,
Place curb, gutter and/or sidewalk directly on clay subgrade soils rather than on unbound
granular base course materials.
4.6.4 Pavement Subdrains
Based on the presence of high plasticity fat clays near the finished pavement subgrade
elevation, we recommend installing a pavement subdrain system to control moisture, improve
stability, and improve long term pavement performance.
We recommend that at least 6 inches of free-draining granular material should be placed
beneath the pavements. The use of a free draining granular base will also reduce the potential
for frost action. We recommend that pavement subgrades be crowned at least 2 percent to
promote the flow of water towards the subdrains, and to reduce the potential for ponding of
water on the subgrade. The design recommendations for the subdrains are provided in the
following table:
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Geotechnical Engineering Report
Proposed South Elementary School Iowa City, Iowa
March 11, 2014 Terracon Project No. 06135652.03
Subdrain Design Recommendations
Value
Item
Free draining granular base thickness below 6 inches of material meeting IDOT
Specification 4121 or 4123
pavement
Minimum drain pipe diameter 4 inches
16 inches or greater to provide minimum 6
Drain trench width inch annulus of drainage aggregate around
drain pipe.
3½ feet
Invert depth below subgrade elevation
Maximum drain pipe spacing 50 feet
IDOT Section 4131 (porousbackfill)
Subdrain trench backfill material
The subdrains should be hydraulically connected to the free-draining granular base layer.
Subdrains should be sloped to provide positive gravity drainage to reliable discharge points
such as the storm water detention basin. Periodic maintenance of subdrains is required for
long-term proper performance.
The pavement surfacing and adjacent sidewalks should be sloped to provide rapid drainage of
surface water. Water should not be allowed to pond on or adjacent to these grade supported
slabs, since this could saturate the subgrade and contribute to premature pavement or slab
deterioration.
4.6.5 Pavement Maintenance
The pavement sections provided in this report represent minimum recommended thicknesses
and, as such, periodic maintenance should be anticipated. Therefore preventive maintenance
should be planned and provided for through an on-going pavement management program.
Maintenance activities are intended to slow the rate of pavement deterioration and to preserve
the pavement investment. Maintenance consists of both localized maintenance (e.g. crack and
joint sealing and patching) and global maintenance (e.g. surface sealing). Preventive
maintenance is usually the first priority when implementing a pavement maintenance program.
Additional engineering observation is recommended to determine the type and extent of a cost
effective program. Even with periodic maintenance, some movements and related cracking
may still occur and repairs may be required.
4.7Frost Considerations
The soils on this site are frost susceptible, and small amounts of water can affect the
performance of the slabs on-grade, sidewalks and pavements. Exterior slabs should be
anticipated to heave during winter months. If frost action needs to be eliminated in critical
areas, we recommend the use of non-frost susceptible fill or structural slabs (e.g., structural
stoops in front of building doors).
Responsive Resourceful Reliable 16
APPENDIX A
FIELD EXPLORATION
Geotechnical Engineering Report
Proposed South Elementary School Iowa City, Iowa
March 11, 2014 Terracon Project No. 06135652.03
Field Exploration Description
Our field exploration consisted of performing nine soil borings at the project site. The borings
were extended to depths of about 10 to 30 feet below the existing grades. The boring locations
were selected by M2B Engineers and MMS Consultants and laid out in the field by MMS
Consultants personnel. The approximate boring locations are indicated on the attached Boring
Location Plan. The ground surface elevations indicated on the boring logs and Boring Location
Plan are also approximate (rounded to the nearest ½ foot), and were provided by MMS
personnel. The locations and elevations of the borings should be considered accurate only to
the degree implied by the means and methods used to define them.
The borings were drilled with an ATV-mounted, rotary drilling rig using continuous flight, solid-
stemmed augers to advance the boreholes. Samples were obtained using either thin-walled
tube or split-barrel sampling procedures. In the thin-walled tube sampling procedure, a thin-
walled tube or seamless steel tube with a sharp cutting edge is pushed hydraulically into the
ground to obtain relatively undisturbed samples of cohesive or moderately cohesive soils. In the
split-barrel sampling procedure, a standard 2-inch O.D. split-barrel sampling spoon is driven into
the ground with a 140-pound hammer falling a distance of 30 inches.
A CME automatic SPT hammer was used to advance the split-barrel sampler in the borings
performed for this project. A significantly greater efficiency is achieved with the automatic
hammer compared to the conventional safety hammer operated with a cathead and rope. This
higher efficiency has an appreciable effect on the SPT-N value. The effect of the automatic
hammer's efficiency has been considered in the interpretation and analysis of the subsurface
information for this report. The number of blows required to advance the sampling spoon the
last 12 inches of a normal 18-inch penetration is recorded as the standard penetration
resistance value. These values are indicated on the boring logs at the corresponding depths of
occurrence. The samples were sealed and returned to the laboratory for testing and
classification.
Field logs of the borings were prepared by the drill crew. Each log included visual classification
of the materials encountered during drilling as well as the driller's interpretation of the
subsurface conditions between samples. The boring logs included with this report represent an
interpretation of the field logs by a geotechnical engineer and include modifications based on
laboratory observation and tests on select samples.
Responsive Resourceful ReliableExhibit A-3
APPENDIX B
SUPPORTING INFORMATION
Geotechnical Engineering Report
Proposed South Elementary School Iowa City, Iowa
March 11, 2014 Terracon Project No. 06135652.03
Laboratory Testing
Soil samples were tested in the laboratory to measure their natural water contents. Dry unit
weight measurements were performed on portions of intact thin-walled tube samples. The
unconfined compressive strength of some thin-walled tube samples was also measured. A
hand penetrometer was used to estimate the unconfined compressive strength of some
cohesive samples. The hand penetrometer provides a better estimate of soil consistency than
visual examination alone. The following index tests were performed to aid in classifying the
soils and evaluating their engineering properties:
Three (3) Atterberg limits (liquid and plastic) tests;
One (1) organic content (loss on ignition) determination;
One (1) one-dimensional consolidation test.
The results of the laboratory tests are shown on the boring logs, adjacent to the soil profiles, at
their corresponding sample depths and as Exhibit B-2.
As a part of the laboratory testing program, the soil samples were classified in the laboratory
based on visual observation, texture, plasticity, and the limited laboratory testing described
above. Additional testing could be performed to more accurately classify the samples. Portions
of the recovered samples were placed in sealed containers, and the samples will be retained for
at least 1 month in case additional testing is requested. The soil descriptions presented on the
boring logs for native soils are in accordance with our enclosed General Notes and Unified
Soil Classification System (USCS). The estimated group symbol for the USCS is also shown
on the boring logs, and a brief description of the Unified System is attached to this report.
Responsive Resourceful ReliableExhibit B-1
APPENDIX C
SUPPORTING DOCUMENTS
UNIFIED SOIL CLASSIFICATION SYSTEM
Soil Classification
A
Criteria for Assigning Group Symbols and Group Names Using Laboratory Tests
Group
B
Group Name
Symbol
E F
GW Well-graded gravel
Cu 4 and 1 Cc 3
Clean Gravels:
Gravels:
C
Less than 5% fines
E F
GP Poorly graded gravel
Cu 4 and/or 1 Cc 3
More than 50% of
coarse fraction retained
F,G,H
Fines classify as ML or MH GM Silty gravel
Gravels with Fines:
Coarse Grained Soils:
on No. 4 sieve
C
More than 12% fines
F,G,H
Fines classify as CL or CH GC Clayey gravel
More than 50% retained
E I
SW Well-graded sand
Cu 6 and 1 Cc 3
Clean Sands:
Sands:
on No. 200 sieve
D
Less than 5% fines
E I
SP Poorly graded sand
Cu 6 and/or 1 Cc 3
50% or more of coarse
fraction passes No. 4
G,H,I
Fines classify as ML or MH SM Silty sand
Sands with Fines:
sieve
D
More than 12% fines
G,H,I
Fines classify as CL or CH SC Clayey sand
J K,L,M
CL Lean clay
PI
Inorganic:
J K,L,M
ML Silt
PI
Silts and Clays:
Liquid limit less than 50
K,L,M,N
Liquid limit - oven dried
Organic clay
Organic: OL
0.75
Fine-Grained Soils:
K,L,M,O
Liquid limit - not dried
Organic silt
50% or more passes the
K,L,M
CH Fat clay
No. 200 sieve
Inorganic:
K,L,M
MH Elastic Silt
Silts and Clays:
Liquid limit 50 or more
K,L,M,P
Liquid limit - oven dried Organic clay
Organic: OH
0.75
K,L,M,Q
Liquid limit - not dried Organic silt
Highly organic soils:
Primarily organic matter, dark in color, and organic odor PT Peat
AH
Based on the material passing the 3-inch (75-mm) sieve
B
I
If field sample
If soil contains
J
If Atterberg limits plot in shaded area, soil is a CL-ML, silty clay.
C
K
Gravels with 5 to 12% fines require dual symbols: GW-GM well-graded
gravel with silt, GW-GC well-graded gravel with clay, GP-GM poorly
whichever is predominant.
graded gravel with silt, GP-GC poorly graded gravel with clay.
L
If soil contains
D
Sands with 5 to 12% fines require dual symbols: SW-SM well-graded
group name.
sand with silt, SW-SC well-graded sand with clay, SP-SM poorly graded
M
If soil contains 30% plus No. 200, predominantly gravel, add
sand with silt, SP-SC poorly graded sand with clay
N
PI
2
(D)
30O
E PI
Cu = D/D Cc =
6010
P
DxDPI plots on or above
1060
Q
F
If soil contains
G
If fines classify as CL-ML, use dual symbol GC-GM, or SC-SM.
Exhibit C-2