Blackberry Farm- PN160428 (3)
GEOTECHNICAL EXPLORATION
on
BLACKBERRY FARMS
NEW BRIDGE AND SLOPE STABILITY STUDY
Off Sandy Lake Road
Coppell, Texas
ALPHA Report No. G121518
Prepared for:
DENTON CREEK LAND COMPANY
225 East Highway 121, Suite 120
Coppell, Texas 75019
Attention: Terry Holmes
November 19, 2012
Prepared By:
ALPHA TESTING, INC.
2209 Wisconsin Street, Suite 100
Dallas, Texas 75229
TABLE OF CONTENTS
On
ALPHA REPORT NO. G121518
1.0 PURPOSE AND SCOPE ....................................................................................................1
2.0 PROJECT CHARACTERISTICS .......................................................................................1
3.0 FIELD EXPLORATION .....................................................................................................2
4.0 LABORATORY TESTS .....................................................................................................2
5.0 GENERAL SUBSURFACE CONDITIONS ......................................................................2
6.0 DESIGN RECOMMENDATIONS .....................................................................................4
6.1 Existing Fill ................................................................................................................4
6.2 Drilled, Straight-Shaft Piers .......................................................................................4
6.3 Spread Footings for The Bridge and Retaining Walls ...............................................6
6.4 Lateral Earth Pressures for Retaining Walls ..............................................................7
6.5 Creek Bank Global Stability – Method of Analysis ...................................................9
6.6 Seismic Considerations ............................................................................................10
6.7 Drainage ...................................................................................................................10
7.0 GENERAL CONSTRUCTION PROCEDURES AND RECOMMENDATIONS ..........10
7.1 Site Preparation and Grading ...................................................................................10
7.2 Foundation Excavations ..........................................................................................12
7.3 Fill Compation .........................................................................................................13
7.4 Groundwater .............................................................................................................14
8.0 LIMITATIONS ..................................................................................................................14
APPENDIX
A-1 Methods of Field Exploration
Boring Location Plan – Figure 1
B-1 Methods of Laboratory Testing
Global Stability Cross-Sections – Figures 2 through 7
Logs of Borings
Key to Soil Symbols and Classifications
ALPHA Report No. G121518
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1.0 PURPOSE AND SCOPE
The purpose of this geotechnical exploration is for ALPHA TESTING, INC. (“ALPHA”) to
evaluate for the “Client” some of the physical and engineering properties of subsurface materials
at selected locations on the subject site with respect to formulation of appropriate geotechnical
design parameters for the proposed bridge. In addition, the global stability of the creek banks at
the site was also evaluated. The field exploration was accomplished by securing subsurface
samples from widely spaced test borings performed across the expanse of the site. Engineering
analyses were performed from results of the field exploration and results of laboratory tests
performed on representative samples.
Also included are general comments pertaining to reasonably anticipated construction problems
and recommendations concerning earthwork and quality control testing during construction.
This information can be used to evaluate subsurface conditions and to aid in ascertaining
construction meets project specifications.
Recommendations provided in this report were developed from information obtained in test
borings depicting subsurface conditions only at the specific boring locations and at the particular
time designated on the logs. Subsurface conditions at other locations may differ from those
observed at the boring locations, and subsurface conditions at boring locations may vary at
different times of the year. The scope of work may not fully define the variability of subsurface
materials and conditions that are present on the site.
The nature and extent of variations between borings may not become evident until construction.
If significant variations then appear evident, our office should be contacted to re-evaluate our
recommendations after performing on-site observations and possibly other tests.
2.0 PROJECT CHARACTERISTICS
A new residential subdivision is proposed on the north side of Sandy Lake Road, about 1,500 ft
east of North McArthur Boulevard in Coppell, Texas. A drawing illustrating the general outline
of the property is provided as Figure 1, the Boring Location Plan, in the Appendix of this report.
The new subdivision will include new residences, streets, retaining walls and a bridge over
Denton Creek. At the time of the field exploration, the site was partially cleared toward the south
and west and heavily treed toward the north and east. Denton Creek runs through the northern,
eastern and southern portions of the property.
This study is limited to recommendations for the bridge, retaining walls and a global stability
evaluation of the creek banks. Foundation recommendations for residential structures are
outside the scope of this report. ALPHA would be pleased to provide geotechnical services for
the residential foundations upon request.
We understand the new bridge will be a double arch type bridge with maximum foundation
loads of up to about 54 kips per ft of bridge width. Based on conversations with the Bridge
Engineer, we understand the bridge foundations could consist of spread footings or drilled piers.
Based on grading plans provided by the project Civil Engineer (Kadkeck & Associates, Project
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No. 10542, dated August, 2012), we understand retaining walls up to about 4 ft in height will be
constructed along the back side of the residential lots between the proposed residences and
Denton Creek.
The referenced grading plans were used in conjunction with information provided by the project
Hydrology Engineer (O’Brien Engineering, Inc.) to develop cross-sections for two-dimensional
analyses of creek bank global stability. The information provided by The Hydrology Engineer
consisted of plans containing spot elevations at various creek locations and was useful in
estimating the depth of the creek bottom.
3.0 FIELD EXPLORATION
Subsurface conditions on the site were explored by drilling a total of seven (7) test borings.
Borings 1 and 2 were drilled for the bridge to a depth of about 75 ft each. Borings 3 through 7
were drilled for the retaining walls and global stability evaluation to a depth of about 35 ft to
55 ft each. All test borings were performed in general accordance with ASTM D 420 using
standard rotary drilling equipment. The approximate location of each boring is shown on the
Boring Location Plan, Figure 1, enclosed in the Appendix of this report. Details of drilling and
sampling operations are briefly summarized in Methods of Field Exploration, Section A-1 of the
Appendix.
Subsurface types encountered during the field exploration are presented on the Log of Boring
sheets (boring logs) included in the Appendix of this report. The boring logs contain our Field
Technician's and Engineer's interpretation of conditions believed to exist between actual samples
retrieved. Therefore, the boring logs contain both factual and interpretive information. Lines
delineating subsurface strata on the boring logs are approximate and the actual transition between
strata may be gradual.
4.0 LABORATORY TESTS
Selected samples of the subsurface materials were tested in the laboratory to evaluate their
engineering properties as a basis in providing recommendations for foundation design and
earthwork construction. A brief description of testing procedures used in the laboratory can be
found in Methods of Laboratory Testing, Section B-1 of the Appendix. Individual test results are
presented on the Log of Boring sheets enclosed in the Appendix.
5.0 GENERAL SUBSURFACE CONDITIONS
Based on geological maps available from the Bureau of Economic Geology, published by The
University of Texas at Austin, the site lies within Alluvium Deposits associated with the Trinity
River system, underlain by the Eagle Ford Formation. Alluvium deposits typically consist of
sands and clays. The Eagle Ford Formation generally consists of shale bedrock with residual clay
overburden soils.
Subsurface conditions encountered in Borings 1 and 2 performed for the bridge generally
consisted of sandy clay and/or clayey sand to a depth of about 51 ft below the ground surface
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underlain by sand which extended to a depth of about 62 ft and 57 ft, respectively. Clay shale
was then encountered in Boring 1 to a depth of about 68 ft underlain by shale which extended to
the 75 ft boring termination depth. Shale was encountered below the sand in Boring 2 and
extended to the 75 ft boring termination depth.
Clay, sandy clay and/or clayey sand was encountered in Borings 3 through 6, extending to the
termination depth of about 35 ft. At Boring 7, clay, sandy clay and clayey sand extended to a
depth of about 46 ft, underlain by sand to a depth of about 51 ft over gravel which extended to
the 55 ft termination depth of the boring. The upper 8 ft and 7 ft of sandy clay soils encountered
in Boring 6 and 7, respectively, were visually classified as fill material or possible fill. More
detailed stratigraphic information is presented on the Log of Boring sheets attached to this report.
The granular materials (clayey sand, sand and gravel) encountered in the borings are considered
relatively permeable and are expected to have a relatively rapid response to water movement.
However, the clay, sandy clay, clay shale and shale materials encountered in the borings are
considered relatively impermeable and are expected to have a relatively slow response to water
movement. Therefore, several days of observation would be required to evaluate actual
groundwater levels within the depths explored. Also, the groundwater level at the site is
anticipated to fluctuate seasonally depending on the amount of rainfall, prevailing weather
conditions, the water level in nearby Denton Creek, and subsurface drainage characteristics.
Free groundwater was encountered during drilling and immediately upon completion of drilling
in most of the boreholes. Depths of groundwater encountered on drilling tools during drilling and
in open boreholes immediately upon completion of drilling, as measured from existing grade at
the time of drilling, are tabulated below.
Table A
Depth of Observed Groundwater Below Existing Grade
Boring No. Depth of Groundwater
On Drilling Tools (ft)
Depth of Groundwater
Immediately Upon
Completion of Drilling (ft)
1 13 21
2 13 13
3 16 16
4 28 31
5 12 12
6 NONE DRY
7 20 21
It is common to detect seasonal groundwater in granular soils and fill material, in natural
fractures within the clayey matrix, at the soil/rock (shale) interface of from fractures in the
rock, particularly during or after periods of precipitation. If more detailed groundwater
information is required, monitoring wells or piezometers can be installed.
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Further details concerning subsurface materials and conditions encountered can be obtained from
the boring logs provided in the Appendix of this report.
6.0 DESIGN RECOMMENDATIONS
The following design recommendations were developed on the basis of the previously described
Project Characteristics (Section 2.0) and conditions encountered at the boring locations
If project criteria should change, including the location of the bridge, the location of the retaining
walls or the proposed grades, our office should conduct a review to determine if modifications to
the recommendations are required. Further, it is recommended our office be provided with a
copy of the final plans and specifications for review prior to construction.
Design criteria and global stability evaluations provided in this report were developed assuming
grading at the site will conform to the proposed grades on the referenced grading plan. Cutting
and filling on the site other than apparent on the referenced grading plan can alter the
recommended foundation design and global stability parameters. Therefore, it is recommended
our office be contacted before performing other cutting and filling on site to verify appropriate
design parameters are utilized for final foundation design.
6.1 Existing Fill
As discussed in Section 5.0 existing fill and possible fill soils were observed to depths of
about 8 ft and 7 ft in Borings 6 and 7, respectively. If compaction records verifying the
relative compaction of these fill soils cannot be obtained, we recommend any existing fill
soils encountered below the bearing depth of bridge or retaining wall footings be removed
to its entire depth and replaced with properly placed and compacted fill soil as described
in Section 7.3 of this report. Although not encountered at the borings, fill materials can
contain organics, boulders, rubble, and other debris which could be encountered during
site grading and general excavation. The earthwork and excavation contracts should
contain provision for removal of unsuitable materials in the existing fill. Test pit
excavations performed prior to construction can be used to evaluate the depth, extent and
composition of uncontrolled fill at this site. ALPHA TESTING would be pleased to
provide this service if desired.
6.2 Drilled, Straight-Shaft Piers for Bridge Structure
Our findings indicate the proposed bridge could be supported using a system of drilled,
straight-shaft piers bearing at least 3 ft into the gray shale which was encountered at a
depth of about 68 ft and 57 ft below the ground surface in Borings 1 and 2, respectively.
Deeper penetrations will be required to develop skin friction and/or uplift resistance.
Gray clay shale was encountered overlying the gray shale bearing stratum in Boring 1.
This gray clay shale has relatively softer strength characteristics compared to the deeper
gray shale and is not suitable for support of foundations utilizing the recommendations in
this report. The gray clay shale is visually similar to the gray shale, and care should be
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taken to verify the pier shafts extend through the relatively softer gray clay shale to bear
in the gray shale. Pier shaft excavations should be monitored by experienced geotechnical
personnel to verify penetration into the gray shale stratum.
Piers bearing at least 3 ft into the gray shale can be dimensioned using a net allowable
end-bearing pressure of 18 kips per sq ft. An allowable skin friction of 300psf can be
used along the pier shaft in contact with overburden soils (clay, sandy clay, clayey sand
and and) below a depth of at least 15 ft below final grade and against the upper 3 ft of
gray shale. An allowable skin friction of 2.7 kips per sq ft can be used for the portion of
the shaft located in the gray shale (neglecting the upper 3 ft of gray shale and the portion
of gray shale above any temporary casing). The minimum clear spacing between piers
should be at least two (2) pier shaft diameters, based on the larger pier, to develop the full
load carrying capacity from skin friction. The above bearing pressure contains a factor of
safety of at least three (3) considering a general bearing capacity failure and the skin
friction value has a factor of safety of at least two (2). Normal elastic settlement of piers
under loading is estimated at less than about ½ inch.
Each pier should be designed with sufficient length and contain full length steel
reinforcing to resist the uplift pressure (soil-to-pier adhesion) due to potential soil swell
along the shaft from post construction heave and other uplift forces applied by structural
loadings. The magnitude of uplift adhesion due to soil swell along the pier shaft cannot
be defined accurately and can vary according to the actual in-place moisture content of
the soils during construction. It is estimated this uplift adhesion will not exceed about
1.5 kips per sq ft. This soil adhesion is approximated to act uniformly over the upper
12 ft of the pier shaft in contact with clayey type soils. Uplift forces from soil adhesion
can be neglected over any portion of the pier shaft in contact with any clayey sand.
The uplift resistance of each pier can be computed using an allowable skin friction value
300 psf along the portion of the pier shaft in overburden soils at least 15 ft below the
ground surface and in the top 3 ft of gray shale. An allowable skin friction of uplift
resistance for the portion of the pier shaft at least 3 ft into the gray shale can be 2.3 kips
per sq ft. Also, the portion of the shaft in gray shale above the bottom of any temporary
casing should be designed considering an allowable value of skin friction for uplift
resistance of 200 psf. These uplift resistance values have a factor of safety of at least
two (2).
All grade beams connecting piers should be formed and not cast in earthen trenches.
Grade beams should be formed with a nominal 6-inch void at the bottom. Commercially
available cardboard box forms (cartons) are made for this purpose. The cardboard
cartons should extend the full length and width of the grade beams. Prior to concrete
placement, the cartons should be inspected to verify they are firm, properly placed, and
capable of supporting wet concrete. Some type of permanent soil retainer, such as
pre-cast concrete panels, must be provided to prevent soils adjacent to grade beams or
wall panels from sloughing into the void space at the bottom of the grade beams.
Additionally, backfill soils placed adjacent to grade beams must be compacted as outlined
in Section 7.3 of this report.
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6.3 Spread Footing for the Bridge and Retaining Walls
Spread footings could be considered for support of the bridge and retaining walls
provided some movement in the foundation is acceptable. If foundation movements in
the bridge and/or retaining walls are not acceptable, it will be necessary to support these
structures on drilled piers as discussed in Section 6.2.
The proposed retaining walls can be supported using spread footings bearing at a depth of
at least 2 ft below final grade on native clay soils. We understand spread footings for the
bridge will bear at a depth of about 2 ft of the creek bottom elevation. The creek bottom
is estimated to be about 8 ft deep near the proposed bridge location.
Where existing fill soils are encountered at the planned foundation bearing level, the fill
should be removed to expose suitable firm native soils. The resulting excavation should
be backfilled to the design foundation bearing level with controlled engineered fill, lean
concrete (2,000 psi at 28 days), or structural concrete. Backfill under retaining wall
foundations should be compacted to at least 97 percent of standard Proctor maximum dry
density (ASTM D 698) and within the range of 1 percentage point below to 3 percentage
points above the material's optimum moisture content.
Careful monitoring during construction is necessary to locate any pockets or seams of soft
clays, existing fill, or other unsuitable materials that might be encountered in excavations
for spread footings. Unsuitable materials and existing fill encountered at the foundation
bearing level should be removed to expose suitable firm native soils. The foundation can
be formed at the undercut grade, or the undercut can be backfilled to the design bearing
level with lean concrete (2,000 psi at 28 days) or flexible base backfill (see Section 7.3).
All footings should be excavated and concrete placed within 48 hours.
To protect the bottom of large spread footing excavations for the bridge, a lean concrete
mudmat (2,000 psi at 28 days) about 3 to 5 inches thick should be placed in the base of
the footing excavation.
A net allowable bearing pressure of 2 kips per sq ft can be considered for bridge or wall
footings bearing on native soil or on new fill soil placed as recommended above.
Continuous footings should have a least dimension of 18 inches and spot footings should
have a least dimension of 24 inches for bearing capacity considerations. All footings
should be proportioned such that the resultant force on the foundation acts downward
within the middle one-third of the footing.
Resistance to sliding will be developed by friction along the base of the footing and
passive earth pressure acting on the vertical face of the footing and a possible key
installed in the base of the footing. It is recommended a coefficient of friction of 0.3 be
used along the bottom of the footing. The available passive earth resistance on the
vertical face of a possible key installed in the base of the footing may be calculated using
a uniform allowable passive earth pressure of 500 psf for footings bearing against
native clayey soils or engineered fill clay soils placed and compacted as discussed in
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Section 7.3. Passive resistance on the vertical face of the footing within 2 ft of the final
site grade should be neglected.
Bridge footings at this site could experience potential movements on the order of about
1 inch and wall footings at this site could experience potential seasonal movements on the
order of about 1 to 3 inches. These potential seasonal movements were estimated in
general accordance with methods outlined by Texas Department of Transportation
(TxDOT) Test Method Tex-124-E and engineering judgment and experience. Estimated
movements were calculated assuming the moisture content of the in-situ soil within the
normal zone of seasonal moisture content change varies between a "dry" condition and a
"wet" condition as defined by Tex-124-E. Movements exceeding those predicted above
could occur if positive drainage of surface water is not maintained or if soils are subject
to an outside water source, such as leakage from a utility line or subsurface moisture
migration from off-site locations.
6.4 Lateral Earth Pressures for Retaining Walls
Retaining walls should be designed to resist the expected lateral earth pressures. The
magnitude of lateral earth pressure against retaining walls is dependent on the method of
backfill placement, type of backfill soil, drainage provisions, and type of wall (rigid or
yielding) after placement of the backfill. Experience demonstrates when a wall is held
rigidly against horizontal movement (restrained at the top), the lateral pressure (at-rest
lateral earth pressure) against the wall is greater than the normally assumed active
pressure. Yielding walls (rotation at the top of at least 0.1 percent of the wall height) and
walls not sensitive to some movements can be designed for active earth pressures (ka).
Rigid walls should be designed using the higher at-rest lateral earth pressures (ko). Walls
should be designed using the equivalent fluid pressures provided in the tables below,
considering a triangular distribution and assuming a horizontal ground surface extending
backward from the top of the wall (Table B) or assuming a ground surface extending
from the top of the wall is slope not steeper than 1 vertical to 4 horizontal (Table C).
The equivalent fluid pressures provided do not include a factor of safety.
TABLE B
LATERAL EARTH PRESSURE
Horizontal Ground Surface Extending Back from the Top of the Wall
Material Condition
Equivalent Fluid Pressure,
pcf
Drained
Undrained
including
Hydrostatic
Pressure
Free Draining Granular Soil
Ф=30˚, ϒт =125 pcf
At-Rest, ko=0.50
Active, ka=0.33
63 94
41 83
On-site Clayey Soil,
Ф=12˚, ϒт =125 pcf
At-Rest, ko=0.8
Active, ka=0.7
-- 113
-- 107
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TABLE C
LATERAL EARTH PRESSURE
Ground Surface Extending Back from the Top of the Wall is Sloped Upw ard
at 1 Vertical to 4 Horizontal or Flatter
Material Condition
Equivalent Fluid Pressure,
pcf
Drained
Undrained
including
Hydrostatic
Pressure
Free Draining Granular Soil
Ф=30˚, ϒт =125 pcf
At-Rest, ko=0.65
Active, ka=0.43
82 103
54 90
On-site Clayey Soil,
Ф=12˚, ϒт =125 pcf
At-Rest, ko=0.9
Active, ka=0.8
-- 119
-- 113
Free draining granular backfill should consist of a clean, non-plastic, relatively
well-graded granular soil consisting of sand, gravel or a sand and gravel mixture, with
less than 5 percent finer than the No. 200 sieve size. To reduce surface water seepage
into the free draining backfill, the top 1-ft of the backfill should consist of on-site clay
soil with a plasticity index of at least 25.
The free draining granular backfill (if used) should extend outward at least 2 ft from the
base of the wall and then extend upward on a 1 (horizontal) to 2 (vertical) slope. The free
draining granular backfill should be separated from the adjacent native soils using a filter
fabric (Mirafi 140N, or equivalent) to prevent intrusion of native soils into the free
draining granular backfill.
Complete drainage of the free draining granular backfill could be provided to prevent the
development of hydrostatic pressures behind the wall. A typical drainage system could
consist of perforated plastic PVC pipes placed in filter trenches excavated parallel to the
base of the walls for their entire length. The drain pipes should be positioned at a depth
lower than the bottom elevation of the wall and should also be wrapped with filter fabric
(Mirafi 140N, or equivalent). A drainage system is beneficial regardless of the type of
backfill used behind the wall. As a minimum, weep holes should be provided for
freestanding walls, although weep holes alone will not be sufficient to prevent occasional
buildup of hydrostatic pressure behind the walls.
Lightweight, hand-controlled vibrating plate compactors are recommended for
compaction of backfill adjacent to walls to reduce the possibility of increases in lateral
pressures due to over-compaction. Heavy compaction equipment should not be operated
near the walls. Also, compaction of backfill soils behind walls should not exceed
100 percent standard Proctor maximum dry density (ASTM D 698) to further limit lateral
earth pressures against walls.
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The lateral earth pressures above do not include the effects of surcharge loading on the
wall due to sloping backfill except as noted in Table C, or from other loads near the
walls. Surcharge loads should be multiplied by the appropriate lateral earth pressure
coefficient from Table B above and applied as a uniform lateral load over the full height
of the wall.
6.5 Creek Bank Global Stability – Method of Analysis
Stability analyses of selected sections of the creek bank were performed for this study
using the GSTABL7 with STEDwin computer program, which is distributed by Gregory
Geotechnical Software. The modified Bishop method of analysis was used. The
GSTABL7 program generates numerous trial failure surfaces (within specified geometric
limits), computes a factor of safety for each trial surface, and reports the lowest safety
factors for stability. The factor of safety against global movement is defined as the ratio
of resisting forces (or moments) to driving forces. It is customary to accept a minimum
factor of safety against global failure of about 1.3 for rapid draw down conditions and
about 1.5 for all other conditions considering water retaining earth structures and
embankments. The critical conditions evaluated for the creek banks included rapid draw
down and maximum pool water elevation.
Residual effective stress conditions (long term, drained conditions) were considered for
this analysis. Effective stress parameters for in-situ clay soils were based on compressive
soil strength tests (ASTM D 2166), historical testing by ALPHA on similar soils, and our
experience. The residual drained shear strength of the in-situ soils were conservatively
estimated based on previous experience. We estimate the drained shear strength of the
in-situ materials is at least 250 psf and the residual drained friction angle is about
18 degrees.
Results of our stability analyses indicate the existing creek banks will have a factor of
safety of at least 1.3 for rapid draw down conditions and at least 1.8 for maximum pool
conditions. The results of our global stability analyses are presented on Figures 2 through
7 in the Appendix of this report.
The computed factors of safety are based on three (3) selected cross sections of the creek
bank that were identified to be critical cross-sections based on the overall creek bank and
site geometry. These cross sections were labeled as CS-1, CS-2, and CS-3, and are
shown on the enclosed Boring Location Plan (Figure 1). It was assumed a surcharge load
would be transmitted by the proposed residential structures. Therefore, any changes in the
development configuration, including building pad locations would require a review to
determine suitability for global stability. In addition, any scouring and erosion of the
creek bank could change the geometry of the creek bank and also affect global stability.
Therefore, the creek banks should be protected from scouring and erosion such that the
existing surface geometry of the creek banks remains relatively unchanged.
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6.6 Seismic Considerations
The Site Class for seismic design is based on several factors that include soil profil e
(soil or rock), shear wave velocity, and strength, averaged over a depth of 100 ft. Since
our boring did not extend to 100-foot depths, we based our determinations on the
assumption that the subsurface materials below the bottom of the boring were similar to
those encountered at the termination depth. Based on Section 1613.3.2 of the 2012
International Building Code and Table 20.3-1 in the 2010 ASCE-7, we recommend using
Site Class C (very dense soil and soft rock) for seismic design at this site.
6.7 Drainage
Adequate drainage should be provided to reduce seasonal variations in moisture content
of foundation soils. All pavement and sidewalks within 5 ft of the structures should be
sloped away from the structures to prevent ponding of water around the structures. Final
grades within 5 ft of the structure should be adjusted to slope away from the structures at
a minimum slope of 2 percent. Maintaining positive surface drainage throughout the
life of the structures is essential.
Trench backfill for utilities should be properly placed and compacted as outlined in
Section 7.3 of this report and in accordance with requirements of local City standards.
Since granular bedding backfill is used for most utility lines, the backfilled trench should
not become a conduit and allow access for surface or subsurface water to travel toward
the new structures. Concrete cut-off collars or clay plugs should be provided where
utility lines cross building lines to prevent water from traveling in the trench backfill and
entering beneath the structures.
7.0 GENERAL CONSTRUCTION PROCEDURES AND RECOMMENDATIONS
Variations in subsurface conditions could be encountered during construction. To permit
correlation between test boring data and actual subsurface conditions encountered during
construction, it is recommended a registered Professional Engineering firm be retained to observe
construction procedures and materials.
Some construction problems, particularly degree or magnitude, cannot be anticipated until the
course of construction. The recommendations offered in the following paragraphs are intended
not to limit or preclude other conceivable solutions, but rather to provide our observations based
on our experience and understanding of the project characteristics and subsurface conditions
encountered in the boring.
7.1 Site Preparation and Grading
Although not encountered at the borings, existing fill materials can contain organics,
boulders, rubble, and other debris which could be encountered during site grading and
general excavation. The earthwork and excavation contracts should contain provision for
removal of unsuitable materials in the existing fill. Test pit excavations performed prior
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to construction can be used to evaluate the depth, extent and composition of uncontrolled
fill at this site. ALPHA would be pleased to provide this service if desired.
All areas supporting foundations, flatwork and areas to receive new fill should be
properly prepared.
After completion of the necessary stripping, clearing, and excavating, and prior to
placing any required fill, the exposed soil subgrade should be carefully evaluated
by probing and testing.
Any undesirable material (organic material, wet, soft, or loose soil) still in place
should be removed. Prior to placement of any fill, the exposed soil subgrade
should then be scarified to a minimum depth of 6 inches and recompacted as
outlined in Section 7.3.
The exposed soil subgrade should be further evaluated by proof-rolling with a
heavy pneumatic-tired roller, loaded dump truck or similar equipment weighing
approximately 10 tons to check for pockets of soft or loose material hidden
beneath a thin crust of possibly better soil.
Proof-rolling procedures should be observed routinely by a Professional Engineer
or his designated representative.
Any undesirable material (organic material, wet, soft, or loose soil) exposed
during proofrolling should be removed and replaced with well-compacted material
as outlined in Section 7.3.
Prior to placement of any fill, the exposed soil subgrade should then be scarified
to a minimum depth of 6 inches and recompacted as outlined in Section 7.3.
If fill is to be placed on existing slopes (natural or constructed) steeper than six horizontal
to one vertical (6:1), the fill materials should be benched into the existing slopes in such a
manner as to provide a minimum bench width of five (5) feet. This should provide a
good contact between the existing soils and new fill materials, reduce potential sliding
planes and allow relatively horizontal lift placements.
The contractor is responsible for designing any excavation slopes, temporary sheeting or
shoring. Design of these structures should include any imposed surface surcharges.
Construction site safety is the sole responsibility of the contractor, who shall also be
solely responsible for the means, methods and sequencing of construction operations.
The contractor should also be aware that slope height, slope inclination or excavation
depths (including utility trench excavations) should in no case exceed those specified in
local, state and/or federal safety regulations, such as OSHA Health and Safety Standard
for Excavations, 29 CFR Part 1926, or successor regulations. Stockpiles should be
placed well away from the edge of the excavation and their heights should be controlled
so they do not surcharge the sides of the excavation. Surface drainage should be carefully
ALPHA Report No. G121518
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controlled to prevent flow of water over the slopes and/or into the excavations.
Construction slopes should be closely observed for signs of mass movement, including
tension cracks near the crest or bulging at the toe. If potential stability problems are
observed, a geotechnical engineer should be contacted immediately. Shoring, bracing or
underpinning required for the project (if any) should be designed by a professional
engineer registered in the State of Texas.
Due to the nature of the clayey and sandy soils found near the surface at the borings,
traffic of heavy equipment (including heavy compaction equipment) may create pumping
and general deterioration of shallow soils. Therefore, some construction difficulties
should be anticipated during periods when these soils are saturated.
7.2 Foundation Excavations
All foundation excavations should be monitored to verify foundations bear on suitable
material. The bearing stratum exposed in the base of all foundation excavations should be
protected against any detrimental change in conditions. Surface runoff water should be
drained away from excavations and not allowed to collect. All concrete for foundations
should be placed as soon as practical after the excavation is made. Drilled piers should be
excavated and concrete placed the same day.
Prolonged exposure of the bearing surface to air or water will result in changes in strength
and compressibility of the bearing stratum. Therefore, if delays occur, drilled pier
excavations should be slightly widened and deepened to provide a fresh penetration
surface, or a new (deeper) full penetration should be provided for drilled piers. Spread
footing foundations should be slightly deepened and cleaned to provide a fresh bearing
surface.
All pier shafts should be at least 1.5 ft in diameter to facilitate clean-out of the base
and proper monitoring. Concrete placed in pier holes should be directed through a
tremie, hopper, or equivalent. Placement of concrete should be vertical through the
center of the shaft without hitting the sides of the pier or reinforcement to reduce the
possibility of segregation of aggregates. Concrete placed in piers should have a minimum
slump of 5 inches (but not greater than 7 inches) to avoid potential honey-combing.
Observations during pier drilling should include, but not necessarily be limited to, the
following items:
Verification of proper bearing strata and consistency of subsurface stratification
with regard to boring logs,
Confirmation the minimum required penetration into the bearing strata is
achieved,
Complete removal of cuttings from bottom of pier holes,
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Proper handling of any observed water seepage and sloughing of subsurface
materials,
No more than 2 inches of standing water should be permitted in the bottom of pier
holes prior to placing concrete, and
Verification of pier diameter and steel reinforcement.
Groundwater was encountered as shallow as 12 ft below existing grade at the boring
locations. In addition, clayey sand and sand was encountered in the borings. From our
experience, groundwater and/or caving granular soils should be expected during pier
installation. Temporary casing will be necessary to prevent sloughing of granular soils
during pier drilling operations and to control water seepage as encountered in the boring.
Casing should be seated in the gray shale or clay shale below the depth of seepage, and all
water and loosened material should be removed from the cased excavation before starting
the design penetration. As casing is extracted, care should be taken to maintain a positive
head of plastic concrete and minimize the potential for intrusion of water seepage or
sloughing of sandy soils. In addition, it may be necessary to use drilling fluids and/or
process the pier shaft excavations through the wet granular soils. Vibratory methods to
install casing may also be beneficial for this project.
From our experience with conditions as encountered in the borings, sometimes
groundwater cannot be controlled by the use of casing, and underwater placement of pier
concrete may be required. Special mix designs are usually required for tremied or
pumped concrete. Proper concreting procedures should include placement of concrete
from the bottom to the top of the pier using a sealed tremie or pumped concrete. The
tremie should be maintained at least 5 ft into the wet concrete during placement. It is
recommended a separate bid item be provided for casing and underwater concrete
placement on the contractor’s bid schedule. Pier drilling contractors experienced in
similar soil and groundwater conditions should be utilized for this project.
Although not encountered at the borings, existing fill can contain boulders, concrete, and
other rubble that can cause obstruction to pier installation. The pier installation contract
should contain provision for penetration or removal of obstructions.
7.3 Fill Compaction
Clay and sandy clay soils with a plasticity index equal to or greater than 25 should be
compacted to a dry density between 93 and 98 percent of standard Proctor maximum dry
density (ASTM D 698). The compacted moisture content of the clays during placement
should be within the range of 2 to 6 percentage points above optimum.
Sandy clay and clayey sand materials with a plasticity index below 25 should be
compacted to a dry density of at least 95 percent of standard Proctor maximum dry
density (ASTM D 698) and within the range of 1 percentage point below to 3 percentage
points above the material's optimum moisture content.
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Clayey material used as fill should be processed and the largest particle or clod should be
less than 6 inches prior to compaction.
In cases where either mass fills or utility lines are more than 10 ft deep, the fill/backfill
below 10 ft should be compacted to at least 100 percent of standard Proctor maximum
dry density (ASTM D-698) and within 2 percentage points of the material's optimum
moisture content. The portion of the fill/backfill shallower than 10 ft should be
compacted as outlined above.
Compaction should be accomplished by placing fill in about 8-inch thick loose lifts
and compacting each lift to at least the specified minimum dry density. Field density
and moisture content tests should be performed on each lift.
7.4 Groundwater
Groundwater was encountered as shallow as 12 ft below the ground surface at the
borings. However from our experience in similar conditions, shallower groundwater
seepage may be encountered in excavations for foundations, utilities and other general
excavations at this site. The risk of seepage increases with depth of excavation and
during or after periods of precipitation. Standard sump pits and pumping may be
adequate to control seepage on a local basis. In some areas, sump pits may not be
adequate to control seepage and supplemental dewatering measures may be necessary to
control groundwater seepage. Supplemental dewatering measures include (but are not
limited to) submersible pumps in slotted casings and well points.
In any areas where cuts are made to establish final grades, attention should be given to
possible seasonal water seepage that could occur through natural cracks and fissures in
the newly exposed stratigraphy. In these areas, subsurface drains may be required to
intercept seasonal groundwater seepage. The need for these or other de-watering devices
on the site should be carefully addressed during construction. Our office could be
contacted to visually observe the final grades to evaluate the need for such drains.
8.0 LIMITATIONS
Professional services provided in this geotechnical exploration were performed, findings
obtained, and recommendations prepared in accordance with generally accepted geotechnical
engineering principles and practices. 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. ALPHA, upon written request, can be
retained to provide these services.
ALPHA TESTING, INC. is not responsible for conclusions, opinions or recommendations made
by others based on this data. Information contained in this report is intended for the exclusive
use of the Client (and their designated design representatives), and is related solely to design of
the specific structures outlined in Section 2.0. No party other than the Client (and their
designated design representatives) shall use or rely upon this report in any manner whatsoever
ALPHA Report No. G121518
15
unless such party shall have obtained ALPHA’s written acceptance of such intended use. Any
such third party using this report after obtaining ALPHA’s written acceptance shall be bound by
the limitations and limitations of liability contained herein, including ALPHA’s liability being
limited to the fee paid to it for this report. Recommendations presented in this report should not
be used for design of any other structures except those specifically described in this report. In all
areas of this report in which ALPHA may provide additional services if requested to do so in
writing, it is presumed that such requests have not been made if not evidenced by a written
document accepted by ALPHA. Further, subsurface conditions can change with passage of time.
Recommendations contained herein are not considered applicable for an extended period of time
after the completion date of this report. It is recommended our office be contacted for a review
of the contents of this report for construction commencing more than one (1) year after
completion of this report. Non-compliance with any of these requirements by the Client or
anyone else shall release ALPHA from any liability resulting from the use of, or reliance
upon, this report.
Recommendations provided in this report are based on our understanding of information
provided by the Client about characteristics of the project. If the Client notes any deviation from
the facts about project characteristics, our office should be contacted immediately since this may
materially alter the recommendations. Further, ALPHA TESTING, INC. is not responsible for
damages resulting from workmanship of designers or contractors. It is recommended the Owner
retain qualified personnel, such as a Geotechnical Engineering firm, to verify construction is
performed in accordance with plans and specifications.
APPENDIX
ALPHA Report No. G121518
A-1 METHODS OF FIELD EXPLORATION
Using standard rotary drilling equipment, a total of seven (7) test borings were performed for
this geotechnical exploration at the approximate locations shown on the Boring Location
Plan, Figure 1. The test boring locations were staked by either pacing or taping and estimating
right angles from landmarks which could be identified in the field and as shown on the site plan
provided during this study. The location of test borings shown on the Boring Location Plan is
considered accurate only to the degree implied by the methods used to define them.
Relatively undisturbed samples of the cohesive subsurface materials were obtained by
hydraulically pressing 3-inch O.D. thin-wall sampling tubes into the underlying soils at selected
depths (ASTM D 1587). These samples were removed from the sampling tubes in the field and
examined visually. One representative portion of each sample was sealed in a plastic bag for use
in future visual examinations and possible testing in the laboratory.
Some soil samples were obtained using split-spoon sampling procedures in accordance with
ASTM Standard D 1586. Disturbed samples were obtained at selected depths in the borings by
driving a standard 2-inch O.D. split-spoon sampler 18 inches into the subsurface material using a
140-pound hammer falling 30 inches. The number of blows required to drive the split-spoon
sampler the final 12 inches of penetration (N-value) is recorded in the appropriate column on the
Log of Boring sheets.
Texas Department of Transportation Texas Cone Penetration (TCP) tests were completed in
the field to determine the apparent in-place strength characteristics of the rock type materials.
A 3-inch diameter steel cone driven by a 170-pound hammer dropped 24 inches is the basis for
TxDOT strength correlations. Depending on the resistance (strength) of the materials, either the
number of blows of the hammer required to provide 12 inches of penetration, or the inches of
penetration of the cone due to 100 blows of the hammer are recorded on the field logs and are
shown on the Log of Boring sheets as “TX Cone” (reference: TxDOT Test Method TEX 132-E).
Logs of the borings are included in the Appendix of this report. The logs show a visual
description of subsurface strata encountered in the borings using the Unified Soil Classification
System. Sampling information, pertinent field data, and field observations are also included. The
subsurface samples will be retained in the laboratory for at least 30 days and then discarded
unless the Client requests otherwise.
ALPHA Report No. G121518
B-1 METHODS OF LABORATORY TESTING
Representative samples were evaluated and classified by a qualified member of the Geotechnical
Division and the boring logs were edited as necessary. To aid in classifying the subsurface
materials and to determine the general engineering characteristics, natural moisture content
tests (ASTM D 2216), Atterberg-limit tests (ASTM D 4318), percent material finer than the
No. 200 sieve tests (ASTM D 1140) and dry unit weight determinations were performed on
selected samples. In addition, unconfined compressive strength tests (ASTM D 2166) and
pocket-penetrometer tests were conducted on selected soil samples to evaluate the soil shear
strength. Results of all laboratory tests described above are provided on either the accompanying
Log of Boring sheets as noted.