geotechnical engineering report - bidnet

5/10/2013
D&S ENGINEERING LABS, LLC. DME - Spencer to Kings Row - Substations 13-0009
TABLE OF CONTENTS
3.0 FIELD AND LABORATORY INVESTIGATION ........................................................................ 2
3.1 Drilling and Sampling ................................................................................................ 2
3.1.1 Field Resistivity Surveys ................................................................................. 3
3.2 Laboratory Testing .................................................................................................... 4
3.2.2 Consolidated-Undrained Triaxial Compression Tests ..................................... 4
3.2.3 Overburden Swell Tests .................................................................................. 5
4.0 SITE CONDITIONS ...................................................................................................................... 5
4.2 Ground Water ........................................................................................................... 9
5.0 ENGINEERING ANALYSIS ......................................................................................................... 9
5.2 Settlement Potential ................................................................................................. 9
6.3 Drilled Shaft Construction Considerations................................................................12
7.0 EARTHWORK RECOMMENDATIONS ................................................................................... 13
7.2 Audra Substation – One half-inch PVM at Equipment Pads .....................................14
7.3 McKinney Substation – One-inch PVM Across the Site ..........................................15
7.4 McKinney Substation – One half-inch PVM at Equipment Pads ...............................15
7.5 Additional Considerations ........................................................................................15
8.1 General ...................................................................................................................16
8.3 Subgrade Strength Characteristics ..........................................................................16
8.4 Rigid Pavement Design and Material Recommendations ........................................17
8.4.1 Rigid Pavement .............................................................................................17
8.5 Subgrade Preparation Recommendations ...............................................................18
10.0 LIMITATIONS ................................................................................................................20
APPENDIX A – BORING LOGS AND SUPPORTING DATA APPENDIX B – GENERAL DESCRIPTION OF PROCEDURES
GEOTECHNICAL INVESTIGATION
DENTON MUNICIPAL ELETRIC NEW ALIGNMENT - SPENCER TO KINGS ROW DENTON, TEXAS
GEOTECHNICAL REPORT – MCKINNEY AND AUDRA SUBSTATIONS
1.0 PROJECT DESCRIPTION
This report presents the results of the geotechnical investigation for a new Denton
Municipal Electric electrical transmission line segment in eastern Denton, Texas
extending from about Spencer Drive in the south to Kings Row in the north. Two new
electrical substations will be located within the new alignment segment; one on the south
side of McKinney Avenue (FM 426) just west of Spring Tree Row, and one on the west
side of Loop 288 just south of Audra Lane. No significant below grade construction or
earth retaining structures are anticipated.
The proposed substation locations are entirely within and/or surrounded by moderately
to well-developed urban areas, though the specific locations for each substation are
currently undeveloped and covered with vegetation and scattered trees at this time. The
northern half of the McKinney Substation property is currently developed for residential
use. Existing homes and related structures will be razed to allow for construction of that
facility. The ground surface within each substation site is relatively level, with estimated
total relief of about two to five feet across each. Photographs showing the current
condition of the sites are included below.
AUDRA SUBSTATION (looking northwest) MCKINNEY SUBSTATION (looking south)
2.0 PURPOSE AND SCOPE
The purpose of this investigation was to:
Identify the subsurface soil and bedrock stratigraphy along the alignment and across
the two proposed substation sites.
Evaluate the physical and engineering properties of the subsurface materials
present.
Provide geotechnical recommendations for use in design and construction of the
proposed transmission line towers and substation facilities and related site work.
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The scope of the Substation investigation included:
Drilling and sampling 12 exploration borings to depths of about 40 feet beneath at
the proposed substation locations. Persistent wet ground conditions at the Audra
substation site delayed drilling operations at this location during the initial subsurface
exploration. As a result of Borings B13, B18, B19, and B20 were completed during
the week of April 22nd with the assistance of a bulldozer
Drilling and sampling three additional shallow borings (A1-A3) just northeast of the
Audra site, but on the same overall tract of land. These borings were advanced to
depths of about 5 feet in order to evaluate suitability of the subsurface soils for use
as borrow materials.
Laboratory testing of selected soil samples obtained during the investigation.
Preparation of a Geotechnical Report that includes:
Recommendations for the design of substation structure foundations.
Evaluation of potential soil heave through Potential Vertical Movement (PVM)
estimates.
one inch or less.
Recommendations for earthwork across the site and beneath structures and
pavements.
Subsequent to Notice-to-Proceed, it was agreed to issue separate geotechnical reports
for the transmission towers and substations.
3.0 FIELD AND LABORATORY INVESTIGATION
3.1 Drilling and Sampling
The borings were advanced utilizing truck-mounted drilling rigs outfitted with
continuous flight augers, hollow stem augers, and wet rotary coring equipment.
Undisturbed samples of cohesive soil were obtained using 3-inch diameter tube
samplers that were advanced into the soils in 1-foot increments by a continuous
thrust of a hydraulic ram on the drilling equipment. A field determination of the
unconfined compressive strength of each cohesive soil sample was obtained using a
calibrated hand penetrometer. The approximate locations of borings explored at the
site are shown on the boring location map included in Appendix A.
Soils and bedrock materials encountered that were not conducive to sampling by
conventional tube sampling techniques were sampled in general accordance with the
Standard Penetration Test (SPT) as described in ASTM D 1586. In this test a
disturbed sample of subsurface material is recovered by advancing a nominal 2-inch
O.D. split-barrel sampler into the formation by percussion means utilizing the energy
from a 140-pound hammer freely falling a distance of 30 inches to drive the sampler.
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The number of blows required to advance the sampler in three consecutive 6-inch
increments is recorded and the number of blows required for the final 12 inches is
noted as the “N”-value. The test is terminated at the first occurrence of: 1) when the
sampler has advanced a total of 18 inches; 2) when the sampler advances less than
one complete 6-inch increment for 50 blows; 3) when the total number of blows
reaches 100; or 4) if there is no advancement of the sampler in any 10 blow interval.
The bedrock shale and limestone strata present were typically drilled and sampled
using a double-tube core barrel fitted with a tungsten-carbide, sawtooth bit. The
lengths of core recovered (REC), expressed as a percentage of the coring interval,
along with respective Rock Quality Designations (RQD), are tabulated at the
appropriate depths on the Log of Boring illustrations. The RQD is the sum of all core
pieces longer than four inches divided by the length of the cored interval. Pieces
shorter than four inches, which were determined to be broken by drilling or by
handling, were fitted together and considered as one piece.
Bedrock materials not conducive to sampling by conventional Shelby-tube
techniques were tested in situ using cone penetration tests to examine the resistance
of the bedrock materials to penetration. In this test a 3-inch diameter steel cone is
driven by the energy equivalent of a 170-pound hammer freely falling 24 inches and
striking an anvil at the top of the drill string. Depending on the resistance of the
materials, either the number of blows of the hammer required to provide 12 inches of
penetration(in two increments of 6 inches each), or the inches of penetration of the
cone due to 100 blows of the hammer are recorded (in two increments of 50 blows
each).
All samples were extruded in the field, described by an engineering geologist, placed
in plastic bags to preserve the natural moisture condition, labeled as to appropriate
boring number and depth, and placed in protective cardboard boxes for shipment to
the laboratory. The specific depths, thicknesses and descriptions of the strata
encountered are presented on the individual Boring Log illustrations presented in
Appendix A. Strata boundaries shown on the boring logs are approximate.
3.1.1 Field Resistivity Surveys
Field resistivity surveys were conducted at each of the proposed substation
locations to provide information for consideration for grounding design. A total
of four surveys were performed at each site, one near each of the four corners
of the overall substation footprints. The surveys were conducted using the four
pin Wenner configuration (equal spacing between pins – “A” spacing). The
depth of investigation is approximately equal to the “A” spacing distance.
In the Wenner configuration, a known current is applied between the outer pins
and the resultant electrical potential induced by that applied current is
measured between the inner pins. The resistance, in Ohm-cm, is obtained by
achieving a “null” reading on the readout box, reading the measured resistance
and applying a multiplier factor based on the spacing. The “A” spacing is
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progressively increased until the desired depth of exploration is achieved. For
this investigation, “A” spacings ranged from 1 foot to 20 feet at each survey
location. Current generation and readings were obtained using a Miller 400A
readout. The results of our surveys are included in summary form in Appendix
A.
the subsurface materials encountered and to provide data for developing engineering
design parameters. Field descriptions of the subsurface soil samples obtained
during the field exploration were later refined by a Geotechnical Engineer in the
testing laboratory based on results of the laboratory tests performed.
All recovered soil samples were classified, and described, in part, using the ASTM
and Unified Soil Classification System (USCS) procedures. Bedrock strata were
described using standard geologic nomenclature.
In order to determine soil characteristics and to aid in classifying the soils,
classification testing was completed on selected samples as determined
Geotechnical Engineer in general accordance with the following test procedures.
The results of these tests are presented at the corresponding sample depths on the
appropriate Boring Log illustrations. The classification tests are described in more
detail in Appendix B (General Description of Procedures).
Moisture Content ASTM D 2216
Atterberg Limits ASTM D 4318
Percent Passing No. 200 Sieve ASTM D 1140
Additional tests were performed to aid in evaluating soil strength and volume
change characteristics, including:
Overburden Swell Tests
Unconfined compression tests were performed on selected samples of the
cohesive soils. These tests were performed in general accordance with
ASTM D 2166 In the unconfined compression test, a cylindrical specimen is
subjected to an axial load applied at a constant rate of strain until failure or a
large (greater than 20 percent) strain occurs.
3.2.2 Consolidated-Undrained Triaxial Compression Tests
The shear strength of materials depends on the stresses applied,
consolidation conditions, strain rate or stress application rate, and the stress
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strength measured is determined under undrained conditions, meaning that
excess pore pressures developed during the shearing process are not
allowed to dissipate. Shear strength is expressed in terms of both the total
stress and the effective stress (total stress minus pore pressure) using the
pore pressure measured during the test. The effective stress results are
applied to “long term” field conditions and designs where excess pore
pressures generated by stress distribution changes dissipate over time to
equilibrium conditions.
Three (3) Consolidated Undrained (CU) Triaxial tests were used to evaluate
the soil shear strength for both total stress and effective stress using the
Mohr’s circle relationship. In the CU Triaxial test, a cylindrical specimen is
subjected to triaxial confining pressure. The sample is consolidated, pore
pressure measurements collected and then loaded at a constant rate of strain
until failure occurs.
Selected samples of the near-surface cohesive soils were subjected to
overburden swell tests. In this test, a sample is placed in a consolidometer
and subjected to the estimated overburden pressure. The sample is then
inundated with water and allowed to swell. Moisture contents are determined
both before and after completion of the test. Test results are recorded as the
percent swell, with initial and final moisture content.
4.0 SITE CONDITIONS
4.1 Stratigraphy
Based upon a review of the Geologic Atlas of Texas, Sherman Sheet, this alignment
crosses the contact between overburden and bedrock strata of the Woodbine
Formation and the undivided Grayson Marl / Mainstreet Limestone formations, with
Grayson / Main Street materials to the south and Woodbine materials to the north.
Stratigraphically, the Woodbine overlies the Grayson / Main Street. The contact
between the two geologic units appears to lie between Boring B8 near the McKinney
Substation, and Boring B10 near the Audra Substation (right-of-entry to Boring B9 has
not yet been received).
4.1.1 McKinney Substation
Borings specific to or very near to the McKinney Substation include Borings B4
through B8, and B15 through B17. Descriptions of the various materials
encountered are presented in the following paragraphs, followed by a
stratigraphic tabulation of all locations for this segment of the project at the end
of the section.
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The surficial soils encountered along this portion of the alignment are typically
sands that contain appreciable amounts of clay and are underlain by clay strata
that contain appreciable amounts of sand and/or thin sand seams. The sands
encountered are generally thin, about 1 to 3 feet thick, very loose to loose in
condition, and medium to dark shades of brown in color near the surface and
become shades of gray, brown and orange below depths of about 2 to 6 feet.
The overburden clays encountered extend to the top of bedrock strata at
depths of about 13 feet in Borings B5, B7, and B16, to about 25 feet in Boring
B4,
Shale bedrock strata were encountered beneath the overburden clays. The
upper portions of the shale are differentially weathered, having been leached
by percolating waters over time. The weathered shales are very soft in rock
hardness, light shades of brown and gray in color, and possess a fissile
structure. The weathered shales extend to the top of either unweathered dark
gray shales, soft to medium hard in rock hardness, or to limestone strata.
Limestones were encountered in all borings explored in this segment of the
alignment. Except for Borings B4, B6, and B8, the upper portions of the
limestone are differentially weathered. The weathered limestones range from
soft to medium hard in rock hardness, are light brown and tan in color, and are
highly argillaceous (clay-bearing). The zone of weathering extends to the top
of unweathered limestone at depths of about 27 to 30 feet below current
grades.
Unweathered limestone was encountered in each boring. The unweathered
limestone is medium hard to hard in rock hardness, gray in color, argillaceous
to varying degrees and contains frequent very thin to medium thick shale
seams. These limestones extend to the maximum 42-foot depth explored.
Table 1. Subsurface Stratigraphy – McKinney Substation
Boring No.
B5 2.8 13.0 NE 20.0 29.5 40.0 Dry prior to
coring
B7 1.2 13.0 NE 20.6 27.1 40.0 Dry prior to
coring
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B15 1.6 18.0 NE 25.0 28.6 41.0 Dry prior to
coring
B16 1.1 13.5 NE 27.0 29.8 41.0 Dry prior to
coring
Note: NE = Not Encountered
4.1.2 Audra Substation
Borings near to and within the Audra Substation footprint include Borings B12
through B14, and B18 through B20. Persistent soft and wet ground conditions
delayed drilling access to Borings B13, B18, B19 and B20 until the week of April
22nd. Descriptions of the various materials encountered within this segment are
presented in the following paragraphs, followed by a stratigraphic tabulation at
the end of the section.
Similar to the borings advanced for the McKinney Substattion, the surficial soils
encountered within and near the Audra Substation are typically sands that
contain appreciable amounts of clay. However, in contrast to the soils
encountered in the vicinity of McKinney Substation, the surface sands at the
Audra site are underlain by clay strata that are variable shades of brown, red
and orange in color and contain small amounts of sandstone fragments. The
overburden clays encountered at the Audra Substation site extend to the top of
bedrock strata at depths ranging from about 12 to 14 feet,
Beneath the overburden clays, very weakly to weakly cemented sandstone
bedrock strata were encountered. These materials are differentially weathered,
having been leached by percolating waters over time. The weathered
sandstones are typically soft to medium hard in rock hardness, shades of brown
and gray in color, and are argillaceous in composition. The weathered
sandstones extend to the top of shale strata at depths of about 20 to 34 feet.
Unweathered dark gray shale strata were encountered in all borings explored in
the vicinity of the Audra Substation. Only in Boring B14, were the upper
portions of the shale differentially weathered. The unweathered dark gray shale
was found to be very soft to soft in rock hardness. These shales extend to the
top of limestone strata at depths of about 31 in Boring B14, and to the
termination depths of about 27 feet in Borings B13, B18, B19 and B20, and 40
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feet in Borings B11 and B12. The unweathered limestones encountered are
moderately hard in rock hardness, gray in color, argillaceous to varying degrees
and contain frequent very thin to medium thick shale seams.
Table 2. Subsurface Stratigraphy – Audra Substation
Boring No.
Top of Dark Gray
B12 13.0 NE 25.0 NE 40.0 Dry prior to
coring
Note: NE = Not Encountered
Audra Potential Borrow Area 4.1.3
Three borings were advanced to depths of about 5 feet below existing grade in
the northeast quadrant of the Audra tract to evaluate suitability of the
subsurface soils for use as fill (Borings A1-A3). In general, suitable fill
materials were found in each boring as described below.
At Boring A1 to the south, nearest to the Audra Substation footprint, low to
moderate plasticity clays were found to a depth of 5 feet. At Boring A2 near the
middle of the investigation area, low to moderate plasticity clays were
encountered to a depth of about 2 feet, with clayey sands below and extending
to 5-foot depth; however, the clay content of these sands was found to be
rather low and are considered marginal as borrow fill for the Audra Substation.
At Boring A3, farthest north away from the substation site, clayey sands were
encountered from the ground surface to 5-foot depth. The uppermost
approximately 2.5 feet is considered to be rather “clean” fill and included gravel
and crushed limestone fragments. These sands are low plasticity “select fill-
type” materials with clay contents higher than those of Boring A2.
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4.2 Ground Water
Groundwater was encountered in several borings during drilling, prior to the
introduction of water used for coring purposes. Groundwater levels should be
anticipated to fluctuate with seasonal and annual variations in rainfall when
considering below grade foundation and certain utility excavations.
5.0 ENGINEERING ANALYSIS
5.1 Estimated Potential Vertical Movement (PVM)
Potential Vertical Movement (PVM) was evaluated utilizing a variety of different
methods for predicting movement as described in Appendix B, and augmented by our
experience and professional opinion. Depending on location, the plasticity of the near
surface soils range from low to high. Near the ground surface the soils were found to
be average to wet in moisture condition at the time of each portion of our field
investigation. The soils generally become dry of average below depths of about 3
feet.
Specific information regarding final site elevations for the McKinney site was not
provided to this office by the time of this report. Assuming that final grades will be
near existing grades and based upon the results of our analysis, the McKinney
substation site is estimated to possess a Potential Vertical Movement of about 1-1/2
to 2 inches.
The site grading plan for the Audra site indicates about three (3) feet of fill at the
south end and about a foot at the north end. The PVM at Audra is estimated to be
about 1-1/2 inches or less at the soil moisture conditions existing at the time of the
field investigation. However, if the near surface soils are allowed to dry appreciably
prior to or during construction, the resultant PVM could approach 3-1/2 inches at
McKinney and 2 to 3 inches at Audra.
5.2 Settlement Potential
Settlement of the existing soils under the anticipated loading is estimated to be on the
order of one half inch or less assuming the soil is prepared in accordance with the
earthwork recommendations included in this report.
6.0 FOUNDATION RECOMMENDATIONS
The soils present at the site have the potential for vertical movement with changes in soil
moisture content. If some movement can be tolerated after earthwork preparation of
native soils has been completed, we anticipate that either a footing / mat foundation, or
a pier-supported foundation will perform satisfactorily for structures and equipment pads.
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If post-construction vertical movements on the order of those described cannot be
tolerated, consideration may be given to a drilled shaft foundation system with
structurally-supported floor slabs / equipment pads.
Recommendations for subgrade preparation to reduce potential post-construction
movement are described in the Earthwork Section of this report. Note that a soil-
supported foundation / floor system may experience some movement with changes in
soil moisture content. Non-load bearing walls, partitions, and other elements bearing on
the floor slab will reflect these movements should they occur. However, with appropriate
design, adherence to good construction practices and appropriate post-construction
maintenance, these movements can be minimized and controlled.
6.1 Shallow Foundations (Mats)
Structures supported on shallow foundations may be subject to differential vertical
movements as described in the previous section of this report. If some movement
can be tolerated, structures and equipment pads may be supported on shallow
foundations. For large equipment pad shallow foundations, we recommend that
structural loads be supported on reinforced concrete, monolithic shallow mats
founded in properly prepared subgrade soils at a minimum depth of 36 inches below
final exterior grades. Mat foundations should be designed using a maximum
allowable bearing pressure of 2,200 pounds per square foot when placed on
prepared natural subgrade as described in the Earthwork section of this report.
This pressure may be increased to 4,000 psf if placed on at least 24 inches of
compacted aggregate base. We recommend that mat foundations be a minimum
of 16 inches thick.
Mat excavations should not be left open overnight. Concrete or engineered fill
should be placed the same day that footings are excavated. We recommend that a
representative of D&S observe all footing excavations prior to placing concrete to
verify the excavation depth, cleanliness, and integrity of the mat bearing surface.
Any mat excavations left open overnight should be observed by D&S prior to
placing concrete to evaluate the depth of additional excavation required. In the
event that reinforcement and concrete cannot be placed on the day final excavation
grades are achieved, the base of the excavation may be deepened slightly and
covered by a thin seal slab of lean concrete or flowable fill to protect the integrity of
the foundation bearing material.
The bottom of all mat excavations should be free of any loose or soft material prior
to the placement of concrete. All equipment pads should be adequately reinforced
to minimize cracking as noted movements may occur in the foundation soils.
6.2 Drilled Shaft Foundations
We understand that building structures at the substation sites will consist of
prefabricated metal buildings that will be erected on pier-supported steel frames that
are suspended above the ground surface. For these structures, we recommend a
minimum clear space of 6 inches be provided between the bottoms of the steel
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frames and the final ground surface. Any appurtenances connected to the buildings
should be pier-supported and should also be isolated from the ground surface by
means of a void space.
Structural cardboard forms may be used to provide the required voids beneath the
appurtenances for building structures. If carton forms are used, care should be
taken to assure that the void boxes are not allowed to become wet or crushed prior
to or during concrete placement and finishing operations. We recommend that
masonite (1/4” thick) or other protective material be placed on top of the carton
forms to reduce the risk of crushing the cardboard forms during concrete placement
and finishing operations. We recommend using side retainers to prevent soil from
infiltrating the void space.
6.2.1 Straight-sided Drilled Shafts
We recommend that prefabricated metal buildings, and/or conduit racks, be
supported on reinforced concrete, straight-shaft drilled piers bearing into light
brown and orange-brown clay (Audra) or orange-brown and gray clay
(McKinney)at a minimum depth of 12 feet below final exterior grade. We
recommend the piers be a minimum of 18 inches in diameter, or larger if
needed to accommodate anchor bolts, embed plates, or other considerations.
Piers bearing in the light brown and orange-brown clay for an allowable end
bearing of 3,500 pounds per square foot (psf), and an allowable downward
skin friction of 750 psf for that portion of the shaft below a depth of 6 feet
below final exterior grade.
The uplift tension forces caused by expansive near surface clays will be
resisted by the structural load on the shaft plus the uplift side resistance
developed around that portion of the shaft below a depth of 10 feet below
final exterior grade. These pressures are approximated to be an average of
about 800 pounds per square foot of shaft area in contact with overburden
soils above a depth of 8 feet. If additional uplift resistance is required to
counterbalance potential uplift pressures, consideration should be given to
increasing the shaft length or using underreamed (belled) piers. The shafts
should be provided with sufficient steel reinforcement throughout their length
to resist the uplift pressures that will be exerted by the near surface soils.
Typically ½ percent of steel by cross-sectional area is sufficient for this
purpose (ACI 318).
We anticipate that a straight-sided drilled pier foundation system designed
and constructed in accordance with the information provided in this report
should limit potential settlement to less than one inch.
6.2.2 Underreamed Shafts (Belled Piers)
Prefabricated metal buildings or conduit racks may be supported on auger-
excavated, reinforced concrete, underreamed (belled) piers bearing in the
overburden clay at a minimum depth of 14 feet below final exterior grades.
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Groundwater was encountered during our field investigation, and care should
be exercised to maintain the bottom of underreamed shafts above the level of
groundwater seepage at the time of shaft installation. If groundwater is
encountered at or above the design bearing elevation, this office should be
contacted to evaluate those conditions during construction.
Drilled-and-underreamed piers supporting structural loads may be designed
using an allowable end bearing pressure of 3,500 pounds per square foot
(psf) in the light brown and orange-brown or gray and orange-brown clays.
Such piers should have a shaft diameter of at least 16 inches, or larger if
needed to accommodate anchor bolts, embed plates, or other considerations.
The bell diameter should not exceed 2.75 times the shaft diameter and the
minimum clearance between the edges of bells should be 5 feet. If the
location of piers requires less clearance between bells than 5 feet, this office
should be contacted for recommendations.
The piers should contain sufficient steel reinforcement to resist the uplift
pressures that will be exerted by the near surface soils. These pressures are
approximated to be on the order of 1,000 psf of shaft area over the upper 10
feet of any shaft in contact with near surface overburden soils and weathered
shale. Typically ½ percent of steel by cross-sectional area is sufficient for this
purpose (ACI 318). Uplift forces acting on individual shafts will be resisted by
the vertical shaft load plus the weight of a conical wedge of soil above the
under-ream. This weight of soil should be taken as a wedge extending
upward from the base of the under-ream at an angle of 40 degrees from
vertical.
Underreamed drilled pier foundations designed and constructed in
accordance with the information provided in this report will have a Factor of
Safety of at least 3 against shear failure, and will experience settlement of
less than one inch.
Groundwater was encountered during the field portion of this investigation during
drilling at depths of about 9 to 15 feet below current ground surface. If the rate of
groundwater seepage precludes use of conventional pumps, temporary casing may
be required. If needed due to excessive groundwater seepage or if sloughing of
overburden soils is observed, temporary casing should be installed to a sufficient
depth into sandstone to obtain an adequate seal against sloughing or groundwater.
After the satisfactory installation of the temporary casing, the required penetration
into the bearing material may be excavated by conventional means through the
casing.
The installation of all drilled piers should be observed by experienced geotechnical
personnel during construction to verify compliance with design assumptions
including: 1) verticality of the shaft excavation, 2) identification of the bearing
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stratum, 3) minimum pier diameter and depth, 4) correct reinforcement is placed,
and 5) proper removal loose. D&S would be pleased to provide these services in
support of this project.
During construction of the drilled shafts, care should be taken to avoid creating an
oversized cap ("mushroom") in excess of the shaft diameter, particularly near the
ground surface, that could allow expansive soils to heave against. If near surface
soils are prone to sloughing and “mushroom” formation, the tops of the shafts
should be formed above the depth of sloughing using cardboard or other circular
forms equal to the diameter of the shaft.
Concrete used for the shafts should have a slump of 8 inches ± 1. Individual shafts
should be excavated in a continuous operation and concrete placed as soon as
practical after completion of the drilling. All pier holes should be filled with concrete
within 8 hours after completion of drilling. In the event of equipment breakdown,
any uncompleted open shaft should be backfilled with soil to be redrilled at a later
date. Backfilled shafts that have reached the target depth prior to the delay should
be extended a minimum of 5 feet deeper than the original target depth
7.0 EARTHWORK RECOMMENDATIONS
In order to reduce Potential Vertical Movements to less than one-inch across the site
and to less than one half-inch for soil-supported equipment pads, we have the following
recommendations for subgrade preparation for the substations.
7.1 Audra Substation – One-inch PVM Across the SIte
Strip the site of all vegetation and remove any remaining organic or
deleterious material. We expect that a minimum of 8 inches stripping depth
will be required.
Excavate to a depth of two (2) feet beneath the exposed stripped grades
across the site and stockpile the material for reuse. The excavation should
extend a minimum of five (5) feet beyond the outer edge of the perimeter
pavement.
Scarify, rework, and recompact the upper 12 inches of the exposed subgrade
soils. The soils should be compacted to between 93 and 98 percent of the
maximum density as determined by ASTM D 698 (Standard Proctor), and to
at least plus three (+3) percentage points above its optimum moisture
content.
Within 24 hours of recompacting the reworked excavated exposed subgrade,
begin fill operations using the stockpiled excavated on-site soil. The fill
should be placed in maximum 8-inch compacted lifts and compacted to
between 93 and 98 percent of the maximum Standard Proctor density and to
at least plus three (+3) percentage points of its optimum moisture content
until depleted.
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Shape the compacted fill to provide positive slope toward the north prior to
placing additional grade-raise fill to the required subgrade elevations.
Grade-raise fill should have a Plasticity Index of not less than 6 nor more than
25, should be essentially free of organic materials and particles in excess of 4
inches their maximum direction, and should have not less than 30 percent
material passing a No. 200 mesh sieve. This fill should be placed in
maximum 8-inch compacted lifts and compacted to at least 95 percent of the
maximum Standard Proctor density, and to between minus 3 and plus three (-
3 to +3) percentage points of its optimum moisture content.
Each lift of rework or fill should be tested for moisture content and
compaction by a testing laboratory at the rate of one test per 5,000 square
feet, but with a minimum of 3 tests per lift.
7.2 Audra Substation – One half-inch PVM at Equipment Pads
Excavate to a minimum depth of 24 inches below the bottom of mat
foundations. Excavations should extend at least to the exterior mat
dimensions and then extend up to the ground surface at a slope no steeper
than 1: horizontal to 1: vertical.
Scarify, rework, and recompact the upper 12 inches of the exposed subgrade
soils. The soils should be compacted to between 93 and 98 percent of the
maximum density as determined by ASTM D 698 (Standard Proctor), and to
at least plus three (+3) percentage points above its optimum moisture
content.
Place geogrid across bottom and sides of the pad excavations to at least the
bottom of mat elevation. Geogrid may be either Tensar BX-1100 or Triax
160, or approved equivalent.
Place select fill or aggregate base in maximum 6-inch compacted lifts to
bottom of mat elevation. Select fill should have a liquid limit less than 35 and
a plasticity index between 6 and 18, should be essentially free of organic
materials and particles in excess of 4 inches their maximum direction, and
should have not less than 30 percent material passing a No. 200 mesh sieve.
The select fill should be placed in maximum 6-inch compacted lifts and
compacted to at least 95 percent of the maximum Standard Proctor density
and within three (-3 to +3) percentage points of its optimum moisture content.
Aggregate base meeting the gradation, plasticity, and durability requirements
of TxDOT Standard Specification Item 247, Type A, Grade 1 or grade 2 may
be used in lieu of select fill and should be compacted to at least 95 percent of
the maximum Standard Proctor density.
Backfill around the equipment pad containment walls above the select fill or
aggregate base should be clay soils with a Plasticity index greater than 25.
15
Backfill should be placed in maximum 8-inch compacted lifts and should be
compacted to a minimum of 95 percent of the maximum density as
determined by ASTM D 698 (Standard Proctor), and to its optimum moisture
content or above.
Each lift of fill or backfill should be tested for moisture content and
compaction by a testing laboratory with a minimum of 3 tests per lift.
7.3 McKinney Substation – One-inch PVM Across the Site
Same preparation as for Audra site, except: Excavate to a depth of three (3) feet
beneath the exposed stripped grades across the site and stockpile the material for
reuse. The excavation should extend a minimum of five (5) feet beyond the outer
edge of the perimeter pavement
7.4 McKinney Substation – One half-inch PVM at Equipment Pads
Same as preparation as for Audra equipment pads.
7.5 Additional Considerations
In order to minimize the potential for post-construction vertical movement, consideration should be given to the following:
Final subgrade should slope away from the foundations a minimum of 5 percent
in the first 5 feet.
Water should not be allowed to pond next to foundations.
If this project is delayed more than two months from the date of this report, we recommend that the moisture contents of the soils be checked to verify if any additional soil treatment is necessary.
8.0 PAVEMENTS AND DRAINAGE
We understand that final sitework will consist of a concrete paved “loop road” around the
outer perimeter of each substation, with a center roadway connecting each long side of
the loop. Other surface areas not covered with structures, equipment, or pavement will
receive a covering of free-draining gravel / crushed stone approximately 6 to 8 inches in
thickness. We further understand that the final subgrade will be shaped to provide a
positive slope from south to north to minimize potential ponding of rainfall and runoff on
top of the subgrade soils. Total fall will be about 5 to 6 feet. We also understand that
requirements to limit post-construction runoff to current levels have been waived.
Considering the existing subsurface conditions, the earthwork recommendations
presented previously, and the foregoing discussion, our recommendations for
pavements are presented in subsequent paragraphs.
16
8.1 General
The pavement designs given in this report are based upon the geotechnical
information developed during this study and design criteria assumptions based on
conversations with Denton Municipal Electric personnel and the design team. The
pavement designs shown below were produced considering the pavement design
practices for rigid pavements, the guidelines and recommendations of the American
Concrete Pavement Association (ACPA) as well as our experience and professional
opinion. However, the Civil Engineer-of-Record should produce the final pavement
design and all associated specifications for the project.
8.2 Behavior Characteristics of Expansive Soils Beneath Pavement
Soils for this site are considered to be slightly to moderately expansive and have the
potential for volume change with changes in soil moisture content. The moisture
content can be maintained to some degree in these soils by covering them with an
impermeable surface such as pavement areas. However, if moisture is introduced to
the subgrade soils by surface or subsurface water, poor drainage, addition of
excessive rainfall after periods of no moisture, or removed by desiccation, the soils
can swell or shrink significantly, resulting in distress to pavements in contact with the
soil in the form of cracks and displacements. The edges of pavements are
particularly prone to moisture variations, and these areas often experience the most
distress (cracking).
In order to minimize the negative impacts of expansive soil on pavement areas and
improve the long term performance of the pavement, we have the following
recommendations:
If possible, provide an elevated pavement which provides the maximum practical
drainage away from the pavement (a minimum of 5% slope for the first 5 feet
and preferably 10 feet away from the pavement is suggested)
Avoid long areas of low slope roadway. Adjust slopes to account for the Potential
Vertical Movement.
8.3 Subgrade Strength Characteristics
Based on the testing from the investigation and support characteristics after
performing the recommended subgrade soil preparation, we recommend using a
California Bearing Ratio (CBR) value of 4 for the pavement section design. A
corresponding resilient modulus of 5,500 psi may also be used. We also
recommend a Modulus of Subgrade Reaction (k) of 150 pounds per cubic inch (pci)
for the subgrade soils (300 pci if pavement is placed over aggregate base).
17
8.4 Rigid Pavement Design and Material Recommendations
We assume that the client will choose to use a Jointed Reinforced Concrete
Pavement (JRCP) as it is commonly used in this area. The typical types of rigid
pavement constructed in this area for this type of project are as follows:
1. Jointed Reinforced Concrete Pavement (JRCP):
This is the most common type of pavement in the North Texas.
It is reinforced for temperature and shrinkage cracking, and for resistance
due to expansive soil movement
Joint placement and sawcut placement is critical for performance
Generally used for low volume roadways and parking lots
2. Continuously Reinforced Concrete Pavement (CRCP):
This type of pavement typically has the lowest maintenance
It is heavily reinforced to control cracking
Typically utilized for high volume traffic areas
3. Jointed Plain Concrete:
Basic unreinforced pavement and due to expansive soils in this area is not
recommended for roadways and parking lots.
8.4.1 Rigid Pavement
With the understanding that heavy equipment may periodically access the
substation sites, we recommend that Portland Cement Concrete Pavement
for this site have a minimum thickness of 6 inches. We have the following
concrete mix design recommendations:
Minimum design compressive strength: 3,500 psi
Minimum design tensile strength: 525 psi
Use of well graded aggregate meeting the requirements of ASTM C-33
with nominal aggregate size no greater than one and one half (1 ½”) inch
Portland cement content limited to between 520 and 600 lbs per cubic
yard.
Use of Air Entraining Agent to achieve an concrete Air Content of 4% to
6% by volume
15 to 20% flyash may be used with the approval of the Civil Engineer of
record
Curing compound should be used and placed within one hour of finishing
operations
8.4.2 Pavement Reinforcing Steel
Due to the absence of specific traffic loading and design life parameters, but
understanding that heavy equipment will be accessing we recommend that a
18
minimum of 0.2% of steel be used for all concrete pavement sections. This is
approximately the equivalent of #4 bars at 16” on center each way for a 6-
inch thick concrete pavement. Areas with less severe loading may perform
adequately with less reinforcement. Please contact this office once specific
traffic loading data is available if additional pavement analyses are desired.
Reinforcement chairs should be used beneath all pavement such that the
reinforcement is placed one-third (T/3) of the pavement thickness from the
top of the pavement using metal or plastic chairs.
8.4.3 Pavement Joints and Cutting
The performance of concrete pavement depends to a large degree on the
design, construction, and long term maintenance of concrete joints. The
following recommendations and observations are offered for consideration by
the Civil Engineer and/or pavement Designer-of-Record:
Contraction joints (sawcuts) should have a spacing of about 30 times the
pavement thickness each way, with a maximum spacing of about 15 to 20
feet. Note that tighter sawcut spacing will control contraction cracking
better than a wider spacing, and a spacing of about 12 feet is considered
very satisfactory.
Sawcuts should be completed as soon as practicable after surface
finishing, typically within a few hours after concrete placement, preferably
within a maximum of 10 to 12 hours after placement.
Joints should be cleaned and sealed as soon as possible after concrete
placement to avoid infiltration of water, sediment, etc. into the open joint
and possibly negatively impacting the subgrade. To be most effective,
joint sealing should be performed preferably within a day or two.
8.5 Subgrade Preparation Recommendations
following:
After the site has been brought to grade in accordance with the Earthwork
Section of this report, cut as needed to final roadway subgrade elevation.
Proofroll the exposed pavement subgrade with a fully loaded tandem axle
dump truck or similar rubber-tired equipment weighing at least 15 tons.
Any areas which rut excessively or are observed to pump should be
undercut and replaced with compacted fill. Proofroll observations should
be performed by qualified personnel under the direction of a licensed
geotechnical engineer. D&S Engineering would welcome the opportunity
to perform those services for this project.
19
Undercut areas to receive fill should have fill placed in maximum 8- inch
loose lifts and should be compacted to between 95 and 100% of Standard
Proctor at a moisture content at or above the material’s optimum moisture
content as determined by that same test. Fill materials may be on-site or
imported materials essentially free of organic materials and particles in
excess of 4 inches their maximum direction. Imported fill material should
have not less than 30 percent material passing a No. 200 mesh sieve and
a Plasticity Index not more than 35.
Place a minimum of 6-inches of aggregate base beneath the pavement
areas. The aggregate base should extend a minimum of three (3) feet
beyond the edges of the pavement. Aggregate base should meet the
gradation, plasticity, and durability requirements of TxDOT Standard
Specification Item 247, Type A, Grade 1, and should be compacted to at
least 95 percent of the maximum Standard Proctor density.
To improve the long-term performance of the pavement, a geogrid such
as Tensar BX-1100 or Triax 160, or approved equivalent, may be placed
between the subgrade soil and the aggregate base, and should extend to
the edges of the aggregate base.
Field density and moisture content testing for the roadway aggregate
base should be performed at the rate of one test per 300 linear feet.
Place crushed stone around the paved areas as shown on the plans.
Non-Pavement Areas 8.5.2
We understand that no-paved areas within the pavement loop at each
substation will receive about 6 to 8 inches of crushed stone over the prepared
subgrade. For these areas, we recommend the following:
After the site has been brought to grade in accordance with the Earthwork
Section of this report, place a geotextile “filer fabric” between the
subgrade soil and the crushed stone to prevent soil migration into the
stone
Place crushed stone around the paved areas as shown on the plans.
9.0 SEISMIC CONSIDERATIONS
North central Texas is generally regarded as an area of low seismic activity. Based on
the boring log data and general geologic information gathered, we recommend that Soil
Site Class “C” be used at this site. There does not appear to be a significant hazard from
slope instability, liquefaction or subsurface rupture due to faulting or lateral spreading
that would occur during earthquake motion.
20
accordance with currently accepted geotechnical engineering principles and practices.
Variations in the subsurface conditions are noted at the specific boring locations for this
study. As such, all users of this report should be aware that differences in depths and
thicknesses of strata encountered can vary between the boring locations. Statements in
the report as to subsurface conditions across the site are extrapolated from the data
obtained at specific boring locations. The number and spacing of the exploration borings
were chosen to obtain adequate geotechnical information for the design and
construction of a industrial structure foundations. If there are any conditions differing
significantly from those described herein, D&S should be notified to re-evaluate the
recommendations contained in this report.
Recommendations contained herein are not considered applicable for an indefinite
period of time. Our office must be contacted to re-evaluate the contents of this report if
construction does not begin within a one year period after completion of this report.
The scope of services provided herein does not include an environmental assessment of
the site or investigation for the presence or absence of hazardous materials in the soil,
surface water, or groundwater.
All contractors referring to this geotechnical report should draw their own conclusions
regarding excavations, construction, etc. for bidding purposes. D&S is not responsible
for conclusions, opinions or recommendations made by others based on these data.
The report is intended to guide preparation of project specifications and should not be
used as a substitute for the project specifications.
Recommendations provided in this report are based on our understanding of information
provided by the Client to us regarding the scope of work for this project. If the Client
notes any differences, our office should be contacted immediately since this may
materially alter the recommendations.
mgthomas
Oval
mgthomas
CONSISTENCY: FINE GRAINED SOILS
SPT (# blows/ft)
> 50
Description No visible sign of weathering Penetrative weathering on open discontinuity surfaces, but only slight weathering of rock material Weathering extends throughout rock mass, but the rock material is not friable Weathering extends throughout rock mass, and the rock material is partly friable Rock is wholly decomposed and in a friable condition but the rock texture and structure are preserved A soil material with the original texture, structure, and mineralogy of the rock completely destroyed
Designation Fresh Slightly weathered
Moderately weathered
Highly weathered
Completely weathered
Residual Soil
Description Can be carved with a knife. Can be excavated readily with point of pick. Pieces 1" or more in thickness can be broken by finger pressure. Readily scratched with fingernail. Can be gouged or grooved readily with knife or pick point. Can be excavated in chips to pieces several inches in size by moderate blows with the pick point. Small, thin pieces can be broken by finger pressure. Can be grooved or gouged 1/4" deep by firm pressure on knife or pick point. Can be excavated in small chips to pieces about 1" maximum size by hard blows with the point of a pick. Can be scratched with knife or pick. Gouges or grooves 1/4" deep can be excavated by hard blow of the point of a pick. Hand specimens can be detached by a moderate blow. Can be scratched with knife or pick only with difficulty. Hard blow of hammer required to detach a hand specimen. Cannot be scratched with knife or sharp pick. Breaking of hand specimens requires several hard blows from a hammer or pick.
Trace Few Little Some With
Designation Very Soft
Very Hard
< 5% of sample 5% to 10% 10% to 25% 25% to 35% > 35%
Condition
Final Moisture Content, %
Content, %
CLIENT: SGS
DRILLED BY: Stacy Cromeans (D&S)
START DATE: 1/29/2013 DRILL METHOD: Cont. Flight Auger
LOGGED BY: Ser Thao (D&S)
FINISH DATE: 1/29/2013
GPS COORDINATES: N33.21460, W97.10284
APPENDIX B - GENERAL DESCRIPTION OF PROCEDURES
ANALYTICAL METHODS TO PREDICT MOVEMENT
CLASSIFICATION TESTS
Classification testing is perhaps the most basic, yet fundamental tool available for
predicting potential movements of clay soils. Classification testing typically consists of
moisture content, Atterberg Limits, and Grain-size distribution determinations. From
these results a general assessment of a soil’s propensity for volume change with
changes in soil moisture content can be made.
Moisture Content
By studying the moisture content of the soils at varying depths and comparing them with
the results of Atterberg Limits, one can estimate a rough order of magnitude of potential
soil movement at various moisture contents, as well as movements with moisture
changes. These tests are typically performed in accordance with ASTM D 2216.
Atterberg Limits
Atterberg limits determine the liquid limit (LL), plastic limit (PL), and plasticity index (PI)
of a soil. The liquid limit is the moisture content at which a soil begins to behave as a
viscous fluid. The plastic limit is the moisture content at which a soil becomes workable
like putty, and at which a clay soil begins to crumble when rolled into a thin thread (1/8”
diameter). The PI is the numerical difference between the moisture constants at the
liquid limit and the plastic limit. This test is typically performed in accordance with ASTM
D 4318.
Clay mineralogy and the particle size influence the Atterberg Limits values, with certain
minerals (e.g., montmorillonite) and smaller particle sizes having higher PI values, and
therefore higher movement potential.
A soil with a PI below about 15 to 18 is considered to be generally stable and should not
experience significant movement with changes in moisture content. Soils with a PI
above about 30 to 35 are considered to be highly active and may exhibit considerable
movement with changes in moisture content.
Fat clays with high very liquid limits, weakly cemented sandy clays, or silty clays are
examples of soils in which it can be difficult to predict movement from classification
testing alone.
Grain-size Distribution
The simplest grain-size distribution test involves washing a soil specimen over the No.
200 mesh sieve with an opening size of 0.075 mm (ASTM D 1140)). This particle size
has been defined by the engineering community as the demarcation between coarse-
grained and fine-grained soils. Particles smaller than this size can be further
distinguished between silt-size and clay-size particles by use of a Hydrometer test
(ASTM D 422).
Once the characteristics of the soil are determined through classification testing, a number of
movement prediction techniques are available to predict the potential movement of the soils.
Some of these are discussed in general below.
TEXAS DEPARTMENT OF TRANSPORTATION METHOD 124-E
The Texas Department of Transportation (TxDOT) has developed a generally simplistic
method to predict movements for highways based on the plasticity index of the soil. The
TxDOT method is empirical and is based on the Atterberg limits and moisture content of
the subsurface soil. This method generally assumes three different initial moisture
conditions: dry, “as-is”, and wet. Computation of each over an assumed depth of
seasonal moisture variation (usually about 15 feet or less) provides an estimate of
potential movement at each initial condition. This method requires a number of
additional assumptions to develop a potential movement estimate. As such, the
predicted movements generally possess large uncertainties when applied to the analysis
of conditions under building slabs and foundations. In our opinion, estimates derived by
this method should not be used alone in determination of potential movement.
SUCTION
Suction measurements may be used along with other movement prediction methods to
predict soil movement. Suction is a measure of the ability of a soil to attract or lose
moisture between the soil particles. Since changes in soil moisture result in volume
changes within the soil mass of fine-grained soils (clays and to some degree silts), a
knowledge of the suction potential of a soil mass at a given point in time may be used to
estimate potential future volume changes with changes in soil moisture content. For this
analysis, a series of suction measurements versus depth is typically performed on a
number of soil samples recovered from a boring in order to develop a suction profile.
SWELL TESTS
Swell tests can lead to more accurate site specific predictions of potential vertical
movement by measuring actual swell volumes at in situ initial moisture contents. One-
dimensional swell tests are almost always performed for this measurement. Though
swell is a three-dimensional process, the one-dimensional test provides greatly improved
potential vertical movement estimates than other methods alone, particularly when the
results are “weighted” with respect to depth, putting more emphasis on the swell
characteristics closer to the surface and less on values at depth.
WIRE REINFORCEMENT INSTITUTE
The Wire Reinforcement Institute (WRI) has developed a design methodology using a
weighted plasticity index. This index is modified for ground slope and the strength of the
soil. These values are also used as input into the movement potential.
POTENTIAL VERTICAL MOVEMENT
A general index for movement is known as the Potential Vertical Rise (PVR). The actual
term PVR refers to the TxDOT Method 124-E mentioned above. For the purpose of this
report the term Potential Vertical Movement (PVM) will be used since PVM estimates are
derived using multiple analytical techniques, not just TxDOT methods.
It should be noted that all slabs and foundations constructed on clay or clayey soils have
at least some risk of potential vertical movement due to changes in soil moisture
contents. To eliminate that risk, slabs and foundation elements (e.g., grade beams)
should be designed as structural elements physically separated by some distance from
the subgrade soils (usually 6 to 12 inches).
In some cases, a floor slab with movements as little as 1/4 of an inch may result in
damage to interior walls, such as cracking in sheet rock or masonry walls, or separation
of floor tiles. However, these cracks are often minor and most people consider them
'liveable'. In other cases, movement of one inch may cause significant damage,
inconvenience, or even create a hazard (trip hazard or others).
Vertical movement of clay soils under slab on grade foundations due to soil moisture
changes can result from a variety causes, including poor site grading and drainage,
improperly prepared subgrade, trees and large shrubbery located too close to structures,
utility leaks or breaks, poor subgrade maintenance such as inadequate or excessive
irrigation, or other causes. A sampling of more common moisture control procedures to
reduce the potential for movement due to these causes is presented in Appendix C.
PVM is generally considered to be a measurement of the change in height of a
foundation from the elevation it was originally placed. Experience and generally
accepted practice suggests that if the PVM of a site is less than one inch, the associated
differential movement will be minor and acceptable to most people.
SETTLEMENT
Settlement is a measure of a downward movement due to consolidation of soil. This can
occur from improperly placed fill (uncompacted or under-compacted), loose native soil,
or from large amounts of unconfined sandy material. Properly compacted fill may settle
approximately 1 percent of its depth, particularly when fill depths exceed 10 feet.
EDGE AND CENTER LIFT MOVEMENT (ym)
The Post-Tensioning Institute (PTI) has developed a parameter of movement defined as
the differential movement (ym) estimated using the change in soil surface elevation in
two locations separated by a distance em within which the differential movement will
occur; em being measured from the exterior of a building to some distance toward the
interior. All calculations for this report are based on the modified PTI procedure in
addition to our judgment as necessary for specific site conditions. The minimum
movements given in the PTI are for climatic conditions only and have been modified
somewhat to account for site conditions which may increase the actual parameters.
“Center lift” occurs when the center, or some portion of the center of the building, is
higher than the exterior. This can occur when the soil around the exterior shrinks, or the
soil under the center of the building swells, or a combination of both occurs.
“Edge lift” occurs when the edge, or some portion of the exterior of the building, is higher
than the center. This can occur when the soil around the exterior swells. It is not
uncommon to have both the center lift and the edge lift phenomena occurring on the
same building, in different areas.
SPECIAL COMMENTARY ON CONCRETE AND EARTHWORK
RESTRAINT TO SHRINKAGE CRACKS
One of the characteristics of concrete is that during the curing process shrinkage occurs
and if there are any restraints to prevent the concrete from shrinking, cracks can form.
In a typical slab on grade or structurally suspended foundation there will be cracks due
to interior beams and piers that restrict shrinkage. This restriction is called Restraint to
Shrinkage (RTS). In post tensioned slabs, the post tensioning strands are slack when
installed and must be stressed at a later time. The best procedure is to stress the cables
approximately 30% within one to two days of placing the concrete. Then the cables are
stressed fully when the concrete reaches greater strength, usually in 7 days. During this
time before the cables are stressed fully, the concrete may crack more than
conventionally reinforced slabs. When the cables are stressed, some of the cracks will
pull together. These RTS cracks do not normally adversely affect the overall
performance of the foundation. It should be noted that for exposed floors, especially
those that will be painted, stained or stamped, these cracks may be aesthetically
unacceptable. Any tile which is applied directly to concrete or over a mortar bed over
concrete has a high probability of minor cracks occurring in the tile due to RTS. It is
recommended if tile is used to install expansion joints in appropriate locations to
minimize these cracks.
UTILITY TRENCH EXCAVATION
Trench excavation for utilities should be sloped or braced in the interest of safety.
Attention is drawn to OSHA Safety and Health Standards (29 CFR 1926/1910), Subpart
P, regarding trench excavations greater than 5 feet in depth.
FIELD SUPERVISION AND DENSITY TESTING
Field density and moisture content determinations should be made on each lift of fill with
a minimum of 3 tests per lift in equipment pad areas, 1 test per lift per 5,000 sf in other
fill areas, and 1 test lift per 100 linear feet of utility trench backfill. Supervision by the
field technician and the project engineer is required. Some adjustments in the test
frequencies may be required based upon the general fill types and soil conditions at the
time of fill placement.
It is recommended that all site and subgrade preparation, proofrolling, and pavement
construction be monitored by a qualified engineering firm. Density tests should be
performed to verify proper compaction and moisture content of any earthwork.
Inspection should be performed prior to and during concrete placement operations. D&S
would be pleased to assist you on this project.
Cooper Creek-McKinney DME Geotech Final Report
McKinney Substation Geotech

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