HomeMy WebLinkAbout20101844.tiff • Geotechnical Engineering Report
Seven Cross Hill Guyed Communications Tower
Northeast of WCR 81 and WCR 86
Weld County, Colorado
April 8, 2010
Project No. 21105002C
Prepared for:
Viaero Wireless
1224 West Platte Avenue
Fort Morgan, Colorado 80701
Prepared by:
Terracon Consultants, Inc.
• 301 North Howes Street
Fort Collins, Colorado
2010-1844
Offices Nationwide Established in 1965 lierracon
Employee-Owned terracon.com
Geotechnical Environmental Construction Materials Facilities
Iterracon
April 8, 2010
•
Viaero Wireless
1224 West Platte Avenue
Fort Morgan, Colorado 80701
Attn: Mr. Robert Hirsekorn
Re: Geotechnical Engineering Report
Seven Cross Hill Guyed Communications Tower
Northeast of WCR 81 and WCR 86
Weld County, Colorado
Terracon Project Number: 21105002C
Dear Mr. Hirsekorn:
Terracon Consultants, Inc. (Terracon) has completed the geotechnical engineering exploration
for the above referenced project. This study was performed in general accordance with our
revised master services agreement number D2306380R.2 dated December 5, 2006 and task
order number P20100067D dated March 24, 2010. This report presents the findings of the
subsurface exploration and provides geotechnical recommendations concerning earthwork and
• the design and construction of foundations for the proposed project.
We appreciate the opportunity to be of service to you on this project. If you have any questions
concerning this report, or if we may be of further service, please contact us.
Sincerely,
TERRACON CONSULTANTS, INC.
i
S.100 REGisT h
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tt 73 4/08/2010 vv��rr
Joels C. Malama, P. !rO� ,:'2/ ' D glas . Jobe, P.E.
Department Manager tl�-e/ONALErer enior Vice President
Geotechnical Services ' 'S' Division Manager
Enclosures
cc: 3-Above
1 -File
•
Terracon Consultants, Inc. 301 North Howes Fort Collins, Colorado 80521
P [970] 484 0359 F [970] 484 0454 terracon.com
Geotechnical • Environmental • Construction Materials ■ Facilities
E •SGeotechnical it Guyed
lierracon
Seven Cross Hill Guyed Communications Tower®Weld County, Colorado
April 8, 2010 a Terracon Project No. 21105002C
• TABLE OF CONTENTS
Page
EXECUTIVE SUMMARY iv
1.0 INTRODUCTION 1
2.0 PROJECT INFORMATION 1
2.1 Project Description 1
2.2 Site Location and Description 2
3.0 SUBSURFACE CONDITIONS 2
3.1 Geology 2
3.2 Typical Profile 2
3.3 Groundwater 3
4.0 RECOMMENDATIONS FOR DESIGN AND CONSTRUCTION 4
4.1 Geotechnical Considerations 4
4.1.1 Expansive Bedrock 4
4.1.2 Structural Recommendations 4
4.2 Earthwork 4
4.2.1 Site Preparation 4
4.2.2 Import Material Specifications 5
4.2.3 Compaction Requirements 6
• 4.2.4 Excavation and Trench Construction 6
4.2.5 Grading and Drainage 6
4.2.6 Construction Considerations 7
4.2.7 Corrosion Protection 7
4.3 Foundations 7
4.3.1 Mat Foundation Design Recommendations 7
4.3.2 Mat Foundation Construction Considerations 8
4.3.3 Tower Dead Man Guy Anchorage Design Recommendations 9
4.4 Equipment Building Floor Slab 10
4.4.1 Design Recommendations 10
4.4.2 Construction Considerations 11
4.5 Seismic Considerations 11
5.0 GENERAL COMMENTS 11
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Geotechnical Engineeringeort I
Seven Cross Hill Guyed Communications Tower w Weld County, Colorado 1 erracon
April 8, 2010 • Terracon Project No. 211050020
• TABLE OF CONTENTS (Cont'd)
APPENDIX A— FIELD EXPLORATION
Exhibit A-1 Field Exploration Description
Exhibit A-2 Boring Location Diagram
Exhibit A-3 to A-6 Logs of Borings
APPENDIX B — LABORATORY TESTING
Exhibit B-1 Laboratory Testing
Exhibit B-2 to B-5 Swell Consolidation Testing
Exhibit B-6 Atterberg Limits Results
Exhibit B-7 Grain Size Distribution
Exhibit B-8 Summary of Laboratory Test Results
APPENDIX C — SUPPORTING DOCUMENTS
Exhibit C-1 General Notes
Exhibit C-2 Unified Soil Classification
Exhibit C-3 General Notes — Description of Rock Properties
Exhibit C-4 Laboratory Test Significance and Purpose
Exhibit C-5 to C-6 Report Terminology
•
•
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S ®Geotechnical Engineering Sort • 1�erracon
Seven Cross Hill Guyed Communications Tower Weld County, Colorado
April 8, 2010 ki Terracon Project No. 21105002C
• EXECUTIVE SUMMARY
A geotechnical engineering exploration has been performed for the proposed guyed
communications tower to be located northeast of the intersection of Weld County Road 81 and
Weld County Road 86 in Weld County, Colorado. Based on the information obtained from our
subsurface exploration and the laboratory testing completed, the site appears suitable for the
proposed construction; however, the following geotechnical conditions will need to be
considered:
■ Potentially very highly expansive claystone bedrock was encountered in the borings
drilled on this site. The claystone bedrock materials are not considered suitable for
support of foundations and floor slabs unless modified and/or replaced.
■ Shallow foundations may be considered for the support of the structures provided that
the existing potentially expansive claystone materials are replaced with non- to low
expansive engineered fill. Alternatively, the foundations may be extended to bear on the
sandstone bedrock materials.
■ The 2006 International Building Code, Table 1613.5.2 IBC seismic site classification for
this site is C.
• ■ The amount of movement associated with foundations will be related to the wetting of
underlying supporting claystone bedrock materials. Therefore, it is imperative the
recommendations outlined in the "Grading and Drainage" section of this report be
followed to reduce potential movement. Moisture conditioning and/or replacement of the
bedrock should follow the recommendations outlined in the "Earthwork" section of this
report.
■ 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 this 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
• SEVEN CROSS HILL GUYED COMMUNICATIONS TOWER
NORTHEAST OF WCR 81 AND WCR 86
WELD COUNTY, COLORADO
Project No. 21105002C
April 8, 2010
1.0 INTRODUCTION
A geotechnical engineering report has been completed for the proposed seven cross hill guyed
communications tower to be located northeast of the intersection of Weld County Road (WCR)
81 and WCR 86, in Weld County, Colorado.
As part of our subsurface exploration, a total of four (4) borings were drilled at the site. One (1)
boring (designated as Boring No. 1) was drilled at the location of the proposed tower, to a depth
of about 30 feet below existing site grade and one (1) boring was drilled at each of the proposed
guy support locations to a depth of about 15 feet below existing site grade. The Logs of Borings
and Boring Location Diagram of the current study are included in Appendix A of this report.
The purpose of these services is to provide information and geotechnical engineering
411 recommendations relative to:
■ Subsurface soil and bedrock conditions • Lateral earth pressures
■ Groundwater conditions ■ Seismic considerations
■ Foundation design and construction ■ Grading and Drainage
■ Earthwork
2.0 PROJECT INFORMATION
2.1 Project Description
ITEM DESCRIPTION
330-foot tall guyed communications tower and ancillary support
Anticipated construction
building
Cellular tower: Maximum loads Tower Center: 200 kips (assumed)
Cellular tower:
Tower Center: 1-inch (assumed)
Maximum allowable settlement
Grading Maximum cut and fill of about 1 to 2 feet(assumed)
Cut and fill slopes None anticipated
• Retaining walls None anticipated
Below grade levels None anticipated
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Geotechnical Engineering1iort lierracon
Seven Cross Hill Guyed Communications Tower pl Weld County, Colorado
April 8, 2010 A Terracon Project No. 21105002C
• 2.2 Site Location and Description
ITEM DESCRIPTION
Location The project site is located near the northeast corner of the
intersection of WCR 81 and WCR 86 in Weld County, Colorado.
Current Development The project site is vacant and undeveloped.
Existing improvements The project site is bounded to the north, east and west by farm
land, primarily ranching land and to the site by WCR 86.
The site is predominantly covered with a sparse to moderate
Current ground cover growth of weeds and grass with some bare land.
Existing topography The site is relatively flat with an estimated elevation difference
of about 2 to 5 feet.
3.0 SUBSURFACE CONDITIONS
3.1 Geology
Surficial geologic conditions at the site, as mapped by the U.S. Geological Survey (USGS) ('Tweto,
1979), consists of the Laramie Formation of the Upper Cretaceous Age. This formation within
this area has been reported to include shale, claystone, sandstone and major coal beds.
Due to the relatively flat nature of the site, geologic hazards at the site are anticipated to be low.
Seismic activity in the area is anticipated to be low; and from a structural standpoint, the
property should be relatively stable. With proper site grading around the proposed structure,
erosional problems at the site should be reduced.
3.2 Typical Profile
Based on the results of the borings, subsurface conditions encountered on the project site can
be generalized as follows:
Material Description Approximate Depth lft j Bottom of Stratum Consistency/Density
Topsoil About 1/2 foot below existing site grade. N/A
Claystone bedrock About 6-1/2 to 7 feet below existing site Firm to medium hard
grade.
About 18 feet below existing site grade in
Sandstone bedrock Boring No. 1 and full depth of exploration of Firm to medium hard
about 15 feet below existing site grade in
Boring Nos. 2 through 4.
• 1Tweto, Ogden, 1979, Geologic Map of Colorado, United States Geological Survey.
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eotechnical Engineering nic • l�erracon
Seven Cross Hill Guyed Communications Tower n Weld County, Colorado
April S, 2010 ■ Terracon Project No. 21105002C
• Material Description Approximate Depth to Bottom of Stratum
P (ft) Consistency/Density
Hite Full depth of exploration of about 30 feet
Clay g stone with lignite lenses Weathered to hard
below existing site grade in Boring No. 1.
Subsurface conditions encountered at the boring locations are indicated on the individual Logs
of Borings. Stratification boundaries on the Logs of Borings represent the approximate depths of
changes in soil and bedrock type, the transition between materials may be gradual.
Laboratory testing indicates the claystone bedrock materials are highly plastic. Swell/expansion
testing performed on the claystone bedrock indicates these materials exhibited low to very high
expansive potential at the samples in-situ moisture content and density. The sandstone bedrock
is non-expansive.
Laboratory testing for water soluble sulfates indicate moderate to severe sulfate exposure levels
in the range of 300 to 7,900 mg/I. A summary of the laboratory test results is attached in
Appendix B.
3.3 Groundwater
Groundwater was not encountered at the time of field exploration. The borings were backfilled
•
with auger cuttings prior to leaving the site; therefore subsequent groundwater levels were not
measured. These observations represent groundwater conditions at the time of the field
exploration, and may not be indicative of other times or at other locations. Groundwater levels
can be expected to fluctuate with varying seasonal and weather conditions.
Due to the low permeability of the bedrock encountered in the borings, a relatively long period of
time may be necessary for a groundwater level to develop and stabilize in a borehole in these
materials. Long term observations in piezometers or observation wells sealed from the influence
of surface water are often required to define groundwater levels in materials of this type.
Zones of perched and/or trapped groundwater may also occur at times in the subsurface soils
overlying bedrock, on top of the bedrock surface or within permeable fractures in the bedrock
materials. The location and amount of perched water is dependent upon several factors,
including hydrologic conditions, type of site development, irrigation demands on or adjacent to
the site, fluctuations in water features, seasonal and weather conditions.
Groundwater level fluctuations 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 higher or lower
than the levels indicated on the boring logs. The possibility of groundwater level fluctuations
should be considered when developing the design and construction plans for the project.
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Geotechnical Engineeringort • l�erracon
Seven Cross Hill Guyed Communications Tower®Weld County, Colorado
April 8, 2010 n Terracon Project No. 21105002C
• 4.0 RECOMMENDATIONS FOR DESIGN AND CONSTRUCTION
4.1 Geotechnical Considerations
4.1.1 Expansive Bedrock
Laboratory testing indicates the claystone bedrock encountered at this site has low to very high
expansive potential at the samples in-situ moisture content and dry-densities. This report
provides recommendations to help mitigate the effects of soil and bedrock shrinkage and
expansion. However, even if these procedures are followed, some movement and cracking in
the structures should be anticipated. Eliminating the risk of movement and distress may not be
feasible, but it may be possible to further reduce the risk of movement if significantly more
expensive measures are used during construction.
4.1.2 Structural Recommendations
Based upon the field and laboratory testing completed, the proposed structures may be
supported on shallow foundations such as mat foundations provided the expansive claystone
bedrock materials are removed to a depth of about 6 feet below foundation elevation and
replaced with on-site or imported non- to low expansive engineered fill. Alternatively, shallow
foundations may be extended to bear on sandstone bedrock materials. The recommended
depth of overexcavation could be terminated shallower than 6 feet if non- to low expansive
• granular materials are encountered.
Design and construction recommendations for the foundation system and other earth connected
phases of the project are outlined below.
4.2 Earthwork
The following presents recommendations for site preparation, excavation, subgrade preparation
and placement of engineered fills on the project. All earthwork on the project should be
observed and evaluated by Terracon. The evaluation of earthwork should include observation of
overexcavation operations, testing of engineered fills, subgrade preparation, foundation bearing
soils and other geotechnical conditions exposed during the construction of the project.
4.2.1 Site Preparation
Strip and remove existing vegetation, unsuitable fills and other deleterious materials from
proposed construction areas. All exposed surfaces should be free of mounds and depressions
which could prevent uniform compaction.
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S Geotechnical Engineering"Port1 Terracon
Seven Cross Hill Guyed Communications Tower IA Weld County, Colorado
April 8, 2010 w Terracon Project No. 21105002C
• Stripped materials consisting of vegetation and organic materials should be wasted from the site
or used to revegetate landscaped areas or exposed slopes (if any) after completion of grading
operations.
All exposed areas which will receive fill, once properly cleared, should be scarified to a minimum
depth of 8 inches, moisture conditioned to the recommended moisture content and properly
compacted. Prepared subgrade soils should be protected from moisture content loss during fill
placement, after grading operations and through construction.
Evidence of soft, loose or unsuitable fills or underground facilities such as septic tanks,
cesspools, basements and utilities was not observed during the site reconnaissance, such
features could be encountered during construction. If unexpected soft, loose or unsuitable fills or
underground facilities are encountered, such features should be removed and the excavation
thoroughly cleaned prior to backfill placement and/or construction.
The stability of subgrade soils may be affected by precipitation, repetitive construction traffic or
other factors. If unstable conditions are encountered or develop during construction, workability
may be improved by overexcavating wet zones and mixing these soils with crushed gravel or
recycled concrete and recompacting. Use of lime, fly ash, kiln dust, cement or geotextiles could
also be considered as a stabilization technique. Laboratory evaluation is recommended to
• determine the effect of chemical stabilization on subgrade soils prior to construction.
Lightweight excavation equipment may be required to reduce subgrade pumping. Terracon
should be notified if such conditions are encountered to provide appropriate recommendations.
4.2.2 Import Material Specifications
Approved on-site soils or approved imported materials may be used as fill material. Imported
soils (if required) should meet the following material property requirements:
Percent finer by weight
Gradation (ASTM C136)
3" 100
No. 4 Sieve 50 -100
No. 200 Sieve 15 - 50
Liquid Limit 30 (max)
Plasticity Index 15 (max)
Maximum expansive potential (%)* 1.0
*Measured on a sample compacted to approximately 95 percent of the ASTM D698
maximum dry density at about 3 percent below optimum water content. The sample is
• confined under a 150 psf surcharge and submerged.
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G ESeven
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gmiort �` lierracan
Seven Cross Hill Guyed Communications Tower■Weld County, Colorado
April 8, 2010 r Terracon Project No. 21105002C
. 4.2.3 Compaction Requirements
Engineered fill should be placed and compacted in horizontal lifts, using equipment and
procedures that will produce recommended moisture contents and densities throughout the lift.
ITEM DESCRIPTION
Fill Lift Thickness 8 to 12-inches or less in loose thickness
Compaction Requirements 95% of the materials standard Proctor maximum dry
density (ASTM D 698)
Moisture Content Cohesive Soil +0 to +3 % of the optimum moisture content
(clay)
Moisture Content Non-Cohesive Soil -3 to +3 % of the optimum moisture content
(sand and gravel)
1. We recommend engineered fill be tested for moisture content and compaction during placement.
Should the results of the in-place density tests indicate the specified moisture or compaction limits
have not been met, the area represented by the test should be reworked and retested as required
until the specified moisture and compaction requirements are achieved.
2. Specifically, moisture levels should be maintained low enough to allow for satisfactory compaction
to be achieved without the fill material pumping when proofrolled.
3. Moisture conditioned clay materials should not be allowed to dry out. Processed clayey soils
should be protected from moisture content loss throughout construction. A loss of moisture within
these materials could result in an increase in the materials expansive potential. Subsequent
• wetting of these materials could result in undesirable movement.
4.2.4 Excavation and Trench Construction
Excavations into the on-site soils and bedrock materials will encounter a variety of conditions.
The individual contractor(s) should be made responsible for designing and constructing stable,
temporary excavations as required to maintain stability of both the excavation sides and bottom.
All excavations should be sloped or shored in the interest of safety following local and federal
regulations, including current OSHA excavation and trench safety standards.
Soils penetrated by the proposed excavations may vary significantly across the site. The
contractor should verify that similar conditions exist throughout the proposed area of excavation.
If different subsurface conditions are encountered at the time of construction, the actual
conditions should be evaluated to determine any excavation modifications necessary to
maintain safe conditions.
As a safety measure, it is recommended that all vehicles and soil piles be kept to a minimum
lateral distance from the crest of the slope equal to no less than the slope height. The exposed
slope face should be protected against the elements.
4.2.5 Grading and Drainage
All grades must be adjusted to provide positive drainage away from the structure during
construction and maintained throughout the life of the proposed project. Infiltration of water into
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Geotechnical Engineeringgport • l��rr�con
Seven Cross HO Guyed Communications Tower•Weld County, Colorado
April 8, 2010 ® Terracon Project No. 21105002C
• utility or foundation excavations must be prevented during construction. Water permitted to pond
near or adjacent to the perimeter of the structure (either during or post-construction) can result
in significantly higher soil movements than those discussed in this report. As a result, any
estimations of potential movement described in this report cannot be relied upon if positive
drainage is not obtained and maintained, and water is allowed to infiltrate the fill and/or
subgrade.
Exposed ground should be sloped at a minimum of 10 percent grade for at least 10 feet beyond
the perimeter of the structure. If incorporated, paved surfaces should be sloped at least 2
percent away from improvements.
4.2.6 Construction Considerations
Construction traffic over prepared subgrade should be avoided to the extent practical. Excessive
traffic could increase compaction and reduce moisture content within the subgrade. Subsequent
wetting of potentially expansive clayey soils could result in undesirable movement if the clayey
soils undergo some moisture content loss.
The site should also be graded to prevent ponding of surface water on prepared subgrade or in
excavations. Excessive wetting of the on-site clayey soils will reduce the soils ability to support
fills and/or structures. If the subgrade becomes significantly wetted, the affected area should be
• removed, allowed to dry and should be properly compacted at the recommended moisture
content. 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 foundation, floor slab and pavement construction.
Terracon should be retained during the construction phase of the project to observe earthwork
and to perform necessary tests and observations during overexcavation operations,
excavations, subgrade preparation; proof-rolling; placement and compaction of controlled
compacted fills; backfilling of excavations into the completed subgrade, and just prior to
construction of foundations.
4.2.7 Corrosion Protection
Results of soluble sulfate testing indicate that ASTM Type V Portland cement should be
specified for all project concrete on and below grade. Foundation concrete should be designed
for moderate to severe sulfate exposure in accordance with the provisions of Section 318,
Chapter 4, of the AO Design Manual.
4.3 Foundations
4.3.1 Mat Foundations Design Recommendations
• As discussed, provided the potentially expansive claystone bedrock materials are replaced with
approved non- to low expansive engineered fill materials, a mat foundation may be considered
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Geotechnical Engineering ort • l��rr�con
Seven Cross Hill Guyed Communications Tower m Weld County, Colorado
April 8, 2010 a Terracon Project No. 21105002C
• suitable for support of the proposed tower and the ancillary building. Design recommendations
are presented below:
DESCRIPTION VALUE
Net allowable bearing pressure ' 2,000 psf
Overexcavation Requirements 2 6 feet below mat foundation bearing elevation
Modulus of subgrade reaction 60 pci
Minimum embedment below finished grade
for frost protection' 30 inches
Approximate total movement° 1 inch
Estimated differential movement 1/2 to 3/4 inch
1. The recommended net allowable bearing pressure is the pressure in excess of the minimum
surrounding overburden pressure at the footing base elevation.
2. Overexcavation may be terminated if granular materials are encountered prior to the 6 feet below
the mat foundation elevation.
3. In addition, to reduce the effects of seasonal moisture variations in the subgrade soils. For exterior
foundations, depth below the lowest adjacent exterior grade within 5 horizontal feet of the structure.
4. The foundation movement will depend upon the variations within the subsurface soil profile, the
structural loading conditions, the embedment depth of the footings, the thickness of compacted fill,
• the quality of the earthwork operations, and maintaining uniform soil water content throughout the
lite of the structure. The estimated movements are based on maintaining uniform soil water content
during the life of the structure. Additional foundation movements could occur if water from any
source infiltrates the foundation soils; therefore, proper drainage and irrigation practices should be
incorporated into the design and operation of the facility. Failure to maintain soil water content and
positive drainage will nullify the movement estimates provided above.
The base of all foundation excavations should be free of water and loose soil prior to placing
concrete. Concrete should be placed soon after excavating to reduce bearing soil disturbance.
Should the soils at bearing level become excessively dry, disturbed or saturated, or frozen, the
affected soil should be removed prior to placing concrete. Place a lean concrete mud-mat over
the bearing soils if the excavations must remain open over night or for an extended period of
time. It is recommended that the geotechnical engineer be retained to observe and test the soil
foundation bearing materials.
4.3.2 Mat Foundation Construction Considerations
The base of all foundation excavations should be free of water and loose soil and rock prior to
placing concrete. If unsuitable soils are encountered at the base of the over-excavation,
supplemental recommendations will be required, such as additional removal and replacement.
Over-excavation for compacted backfill placement below footings should extend laterally
beyond all edges of the footings at least 8 inches per foot of over-excavation depth below
• footing base elevation.
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Geotechnical EngineeringSort • l�c'rracon
Seven Cross Hill Guyed Communications Tower a Weld County, Colorado
April 8, 2010 a Terracon Project No. 21105002C
• Fill should be placed in lifts of 8
inches or less in loose thickness
and compacted to at least 95
percent of the material's a,_yit�tra�}1� �t�lic-(sr�tq
maximum dry density (ASTM xr�ti�tir tst
ttt� a
D698). Compactive effort should
be in accordance with 213O v3° >Ids
recommendations provided in the DurFo"1nn°"°' Y~ %_
EARTHWORK section of this tS
envied FR
report. Concrete should be
placed soon after excavating to ty-•
reduce bearing soil disturbance. /il ttc ;Alit=,i
Should the soils at bearing level
become excessively dry, Note:Excavation in sketch is shown vertical for convenience.
disturbed or saturated, or frozen, Excavations should be sloped as necessary for safety.
the affected soil should be
removed prior to placing concrete. It is recommended that the geotechnical engineer be
retained to observe and test the soil foundation bearing materials.
4.3.3 Tower Dead Man Guy Anchorage Design Recommendations
For the design of the guy anchors, the critical foundation parameters will be those related to
•
resisting uplift and horizontal forces on each guy anchor. The soil mass providing uplift
resistance for the foundation should be calculated as the zone contained within planes that
extend up and out from the edges of the top of the foundation to the ground surface at an angle
of approximately 30 degrees from vertical. The base of the inverted cone may be assumed to
extend from the top of the foundation if the footing sides are vertical. The ultimate uplift capacity
should then be taken as the sum of the weight of soil in this zone plus the weight of the concrete
footings. Effective unit weights of 110 pcf for compacted on-site soils and 145 pcf for reinforced
concrete can be used for these calculations. On-site soils should be compacted as outlined in
the "Fill Materials and Placement" section of this report. The ultimate combined uplift capacity
should then be divided by a factor of safety of at least 3.0 to obtain the allowable uplift capacity.
The horizontal component of the resultant force on the anchors will be resisted by the passive
pressure developed at the face of the anchor and the base friction present at the bottom of the
anchor. The passive pressure and coefficient of base friction for the anticipated soils to be
encountered at the guy locations are presented below:
Material Type Passive Pressure Coefficient of Base Friction
Engineered fill 250 psf/ft 0.35
Sandstone Bedrock 400 psf/ft 0.40
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Seven Cross Hill Guyed Communications Tower®Weld County, Colorado
April 8, 2010 r, Terracon Project No. 21105002C
The passive pressures are expressed in pounds per square foot (psf) per foot of embedment
below the minimum embedment depth of 36 inches. The maximum allowable passive pressure
is 2,000 psf.
Foundation and guy anchor excavations should be observed by the geotechnical engineer or his
authorized representative. If the subsurface conditions encountered differ from those presented
in this report, supplemental recommendations will be required.
4.4 Equipment Building Floor Slab
4.4.1 Design Recommendations
To reduce the potential for movement due to the presence of potentially expansive claystone
bedrock at or near slab elevation, structural floors are recommended. Provided the existing
expansive claystone bedrock materials are removed to a depth of 6 feet below floor slab
elevation and replaced with non- to low expansive fill materials and the owner is willing to
accept the risk of movement, slab-on-grade can be utilized for this project. If slab-on-grade
floors are utilized, the following should be followed.
ITEM DESCRIPTION
Overexcavation Requirementsw 6 feet below slab bearing elevation
100 pounds per square inch per inch per inch (psi/in)for
Modulus of subgrade reaction point loading conditions bearing on approved imported
engineered fill.
Estimated total movement' 1 inch
1. Floor slabs should be structurally independent of any building footings or walls to reduce the
possibility of floor slab cracking caused by differential movements between the slab and
foundation.
2. Overexcavation may be terminated prior to the 6 feet below slab bearing elevation if granular
materials are encountered prior to the 6 feet below the slab bearing elevation.
3. We recommend floor subgrades be maintained in a relatively moist condition until floor slabs are
constructed. If the subgrade should become desiccated prior to construction of floor slabs, the
affected material should be removed or the materials scarified, water conditioned, and
recompacted. Upon completion of grading operations in the building areas, care should be taken to
maintain the recommended subgrade water content and density prior to construction of the building
floor slabs.
Saw-cut control joints should be placed in the slab to help control the location and extent of
cracking. For additional recommendations refer to the ACI Design Manual. Joints or any cracks
that develop should be sealed with a water-proof, non-extruding compressible compound
specifically recommended for heavy duty concrete pavement and wet environments.
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Seven
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gmport lierracon
Seven Cross Hill Guyed Communications Tower ra Weld County, Colorado
April 8, 2010 n Terracon Project No. 21105002C
• Should structural loads be concentrated on the perimeter of the slab, a monolithic turn down
edge should be considered. Thickened interior slab sections can be used to support
concentrated or continuous slab loading provided that:
loads do not exceed 900 plf.
e thickened sections have a minimum width of 12 inches.
ii thickness and reinforcement are consistent with structural requirements.
Foundations and slabs should be reinforced as necessary to reduce the potential for distress
caused by differential foundation movement. Foundation excavations should be observed by the
geotechnical engineer. If the soil conditions encountered differ significantly from those
presented in this report, supplemental recommendations will be required.
4.4.2 Construction Considerations
On most project sites, the site grading is generally accomplished early in the construction phase.
However as construction proceeds, the subgrade may be disturbed due to utility excavations,
construction traffic, desiccation, rainfall, etc. As a result, the floor slab subgrade may not be
suitable for placement of concrete and corrective action will be required.
Particular attention should be paid to high traffic areas that were rutted and disturbed earlier and
to areas where backfilled trenches are located. Areas where unsuitable conditions are located
• should be repaired by removing and replacing the affected material with properly compacted fill.
All floor slab subgrade areas should be water conditioned and properly compacted to the
recommendations in this report immediately prior to placement of the base rock and concrete.
4.5 Seismic Considerations
Code Used Site Classification
2006 International Building Code(IBC) I C
1. In general accordance with the 2006 International Building Code, Table 1613.5.2.
2. The 2006 International Building Code (IBC) requires a site soil profile determination extending a
depth of 100 feet for seismic site classification. The current scope requested does not include the
required 100 foot soil profile determination. Borings for the building extended to a maximum depth
of approximately 30 feet and this seismic site class definition considers that similar soil conditions
exist below the maximum depth of the subsurface exploration. Additional exploration to deeper
depths could be performed to confirm the conditions below the current depth of exploration.
Alternatively, a geophysical exploration could be utilized in order to attempt to justify a higher seismic
site class.
5.0 GENERAL COMMENTS
• Terracon should be retained to review the final design plans and specifications so comments
can be made regarding interpretation and implementation of our geotechnical recommendations
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Geotechnical Engineering.ort • l��rr�con
Seven Cross Hill Guyed Communications Tower®Weld County, Colorado
April 8, 2010 m Terracon Project No. 21105002C
in the design and specifications. Terracon should also be retained to provide testing and
observation during the over excavation (if performed), grading, foundation and construction
phases of the project.
The analysis and recommendations presented in this report are based upon the data obtained
from the borings performed at the indicated locations and from other information discussed in
this report. This report does not reflect variations that may occur between borings, across the
site, or due to the modifying effects of weather. The nature and extent of such variations may
not become evident until during or after construction. If variations appear, we should be
immediately notified so that further evaluation and supplemental recommendations can be
provided.
The scope of services for this project does not include, either specifically or by implication, any
environmental or biological (e.g., mold, fungi, bacteria) assessment of the site or identification or
prevention of pollutants, hazardous materials or conditions. If the owner is concerned about the
potential for such contamination or pollution, other studies should be undertaken.
This report has been prepared for the exclusive use of our client for specific application to the
project discussed and has been prepared in accordance with generally accepted geotechnical
engineering practices. No warranties, either express or implied, are intended or made. Site
N safety, excavation support, and dewatering requirements are the responsibility of others. In the
event that changes are planned in the nature, design, or location of the project as outlined in
this report, the conclusions and recommendations contained in this report shall not be
considered valid unless Terracon reviews the changes, and either verifies or modifies the
conclusions of this report in writing.
•
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