SS9701-SY 940916reed engineering
PROJECT NO. 1998
8EPTEMEERt 1994
GEOTECHNICAL INVESTIGATION
SLOPE STABILITY ANALYSIS
GRAPEVINE CREEK
BELTLINE AT MOCKINGBIRD
COPPELL, TEXAS
Presented To:
CITY OF COPPELL
GEOTECHNICAL CONSULTANTS
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GEOTECHNICAL CONSULTANTS
September 16, 1994
Project No. 1998
City of Coppell
Engineering Department
P.O. Box 478
Coppell, Texas 75019
ATTN: Mr. Ken Griffin,
PoEo
GEOTECHNICAL INVESTIGATION
SLOPE STABILITY ANALYSIS
GRAPEVINE CREEK
BELTLINE AT MOCKINGBIRD
COPPELL, TEXAS
Gentlemen:
Transmitted herewith are copies of the referenced report. Should
you have any questions concerning our findings or if you desire
additional information, please do not hesitate to call.
Sincerely,
~. Whit] ~
Ronald F. Reed, P.E.
Principal Engineer
FWS/RFR/aap
copies submitted:
(4) city of Coppell/Mr. Ken Griffin, P.E.
(1) Craig Olden, Inc./Mr. Craig Olden
2424STUTZ DRIVE · SUITE 400 · DALLAS, TEXAS 75235 · 214/350-5600 · (FAX) 214/350-0019
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TABLE OF CONTENTS
PAGE
INTRODUCTION ........................................... 1
Project Description ............................... 1
Authorization ..................................... 2
Purpose and Scope ................................. 2
FIELD AND LABORATORY INVESTIGATIONS .................... 2
General ..... ~...] ..........
Laboratory Testing ................................ 4
GENERAL SITE CONDITIONS ................................ 5
Geology and Stratigraphy .......................... 5
Ground Water ...................................... 6
Surface Conditions & Landslide Geomorphology ...... 7
ANALYSIS ............................................... 8
Stability Analysis ................................ 8
Cause(s) of Failure .............................. 11
RECOMMENDATIONS ....................................... 12
General .......................................... 12
Remedial Measures ................................ 18
Earthwork ........................................ 21
Construction Observation ......................... 22
ILLUSTRATIONS
PLATE
PLAN OF BORINGS ........................................ 1
BORING LOGS ........................................... 2-4
KEYS TO TERMS AND SYMBOLS USED ........................ 5&6
LABORATORY TEST RESULTS .............................. 7-10
INTERPRETIVE GEOLOGIC CROSS-SECTION ................... 11
GEOTECHNICAL CONSULTANTS
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ILLUSTRATIONS
(CONTINUED)
3-D SLOPE STABILITY ANALYSES ........................ 12&13
2-D SLOPE STABILITY ANALYSES ........................ 14-16
IDEALIZED REMEDIAL CROSS-SECTION ...................... 17
PROPOSED PIER LOCATIONS ............................... 18
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INTRODUCTION
Project Description
This report presents the results of a geotechnical investigation
of slope instability adjacent Grapevine Creek. The site is
located northwest of the intersection of Beltline Road and
Mockingbird Lane in Coppell, Texas. A relatively large
landslide has occurred within the north slope of the creek.
The failure was initially observed by City of Coppell personnel
in early summer, 1993, prior to construction of a deep storm
drainage tunnel, located well behind and near the base elevation
of the slide. At this time, the upper failure scarp was
relatively minor (on the order of three feet high). At
completion of tunnel construction (early in 1994), the upper
portion of the slide area was filled in, and the slope was
regraded. A few trees were cleared from the lower portion of
the slide area at this time.
The landslide began to reactivate in the spring of 1994,
following heavy rains. Renewed movement reportedly occurred as
a series of slides progressing in the upslope direction. As of
August, 1994 the main head scarp was on the order of eight to
nine feet in height.
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Authorization
This investigation was authorized by Mr.
the city of Coppell on July 19, 1994.
Ken Griffin, P.E., of
Purpose and Scope
The purpose of this investigation
potential cause(s) of the landslide,
has been to evaluate the
and to evaluate alternative
remedial measures to stabilize the slide.
included drilling sample
topographic survey of the
engineering and geologic
report.
The investigation has
borings, field geologic studies,
failure area, laboratory testing,
analyses, and preparation of the
FIELD AND LABORATORY INVESTIGATIONS
General
The field and laboratory investigations have been conducted in
accordance with applicable standards and procedures set forth in
the 1994 Annual Book of ASTM Standards, Volumes 04.08 and 04.09,
"Soil and Rock, Geosynthetics". These volumes should be
consulted for information on specific test procedures.
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Fie14 Investigation
Subsurface conditions were evaluated
drilled to depths of 8.7 to 29.4 feet.
by three sample borings
Borings B-1 and B-2 were
drilled within the slide mass. Boring B-3 was drilled above the
landslide. The approximate location of each boring is shown on
the Plan of Borings, Plate 1 of the report Illustrations.
Borings B-1 and B-2 were advanced by means of hand-operated
drilling equipment. Samples of cohesive soils were obtained
with thin-walled Shelby tube samplers on maximum two-foot
intervals. Boring B-3 was drilled with a truck-mounted drilling
rig using wet rotary techniques. Samples of cohesive soils and
weathered shale were obtained with three-inch diameter, thin-
walled Shelby tube samplers. The underlying unweathered shale
was continuously cored using a double-tube, NX-size core barrel
equipped with a carbide-tooth cutting bit. Water was used as a
drilling fluid. The boring was bailed of the drilling fluid at
completion, and ground water observations made in all boreholes.
Sample depth, description of materials, and soil classification
[Unified Soil Classification System (USCS)] are presented on the
Boring Logs, Plates 2 through 4. Keys to terms and symbols used
on the boring logs are included as Plates 5 and 6. Results of
water level observations in the open bore holes are reported on
the individual boring logs.
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Field reconnaissance was performed in order to evaluate surface
geologic and geomorphic conditions in the general area of the
failure. A topographic survey of the failure area was provided
by the city of Coppell in August, 1994. Cross-sections and
topography presented in this report are based on the City of
Coppell survey.
Laboratory Investigation
Upon return to the laboratory,
by an engineering geologist
all samples were logged in detail
in accordance with the USCS and
standard geologic nomenclature. Samples of cohesive soils were
evaluated for consistency by use of a pocket penetrometer.
Pocket penetrometer test results are shown on the boring logs.
Selected samples of the upper soils and weathered shale were
subjected to Atterberg Limits, moisture content, unit weight and
partial gradation determinations. Unconfined compression tests
were performed on selected samples of the unweathered shale.
Results of these tests are summarized on Plate 7.
Selected samples of the upper soils and weathered shale were
subjected to consolidated-drained (CD) direct shear tests in
order to evaluate parameters for slope stability analysis. Both
peak and residual shear were evaluated. Results of the direct
shear tests are shown graphically on Plates 8 through 10.
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GENERAL SITE CONDITIONS
Geology and stratigraphy
Site geology consists of terraced alluvial soils overlying
weathered and unweathered shale of the Cretaceous Eagle Ford
Formation. The terrace soils are associated Pleistocene
deposition within the floodplain of Grapevine Creek and its
tributaries.
consists of dark gray,
fissile clay shale with
seams. The Eagle Ford
In its unweathered state, the Eagle Ford Formation
soft (rock classification), slightly
occasional thin, very weak bentonitic
weathers to produce highly plastic
residual deposits of low shear strength.
In general, subsurface conditions encountered in the borings
consist of fill and alluvial deposits overlying weathered and
unweathered shale. The fill is attributed to site grading
during original construction. Interpretive Geologic Cross-
section A - A' has been prepared for visual reference and is
presented on Plate 11. The plan location of the cross-section
is shown on Plate 1.
Fill encountered in the borings typically consists of dark gray,
gray, olive-gray and reddish-brown, low plasticity sandy clay.
Approximately three feet of dark olive-gray, high plasticity
clay fill with thin sand seams was encountered within the lower
portion of the fill in Boring B-3.
GSOTECHNICAL CONSULTANTS
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Alluvial soils underlying the fill consist of reddish-brown,
dark gray and tan, low plasticity sandy clay. The fill and
alluvial soils are underlain by olive-gray, light gray and
reddish-tan, very soft to soft (rock classification) completely
weathered to slightly weathered shale with iron-stained joints.
Completely weathered shale consists of highly plastic residual
clay. The degree of weathering generally decreases with depth.
The weathered shale is underlain by dark gray, soft, unweathered
shale. The unweathered shale extends to the termination depths
of the borings. A six-inch thick weathered bentonite seam was
encountered at approximate Elev. 443 in Boring B-1. The
remaining borings did not extend deep enough to intercept this
seam. However, based on experience with the Eagle Ford
Formation, the bentonite is anticipated to be relatively
persistent in lateral extent.
Ground Water
Ground water was encountered at depths of 0.8 to 16.6 feet
(approx. Elev. 456 to 448) at the time of the field
investigation (late July, 1994). The ground water was
particularly shallow within the landslide mass. The ground
water is perched above the unweathered shale within the
overlying weathered shale and sandy clay soils. The ground
water gradient is toward Grapevine Creek. The depth to ground
water will fluctuate with variations in seasonal and yearly
rainfall and creek flow level.
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Surface Conditions and Landslide Geomorpholog¥
The overall slope is approximately 30 feet high from
to the thalweg of the creek. Slopes outside of the
are generally on the
(3.5H:lV). The lower
the order of 2H:lV.
the crest
landslide
order of 3.5 horizontal to one vertical
portion of the slope, near the toe, is on
The site is located on the outside of a relatively sharp meander
bend in Grapevine Creek. The outside of a meander bend is
termed the "cut bank" and is subject to constant erosion
associated with stream action.
deposits appears to be present
channel.
A thin veneer of recent alluvial
above the shale within the creek
The upper limit of the landslide is defined by an approximate
nine-foot high head scarp. Soils exposed in the head scarp are
slickensided as a result of movement of the slide. A prominent
bulge is present at the toe of the slide. The toe appears to
have been subject to recent erosion associated with stream
action.
The slide is approximately 60 feet long from head scarp to toe,
and approximately 90 feet wide from flank to flank. As
illustrated on Plate 11, two secondary slides were observed
within the main landslide mass. Several minor scarps are
associated with each failure zone. Numerous tension cracks are
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present within the toe bulge. Based on geologic constraints and
landslide geomorphology, the deepest portion of the main slide
mass is anticipated to occur at the interface between the
bentonite seam and the underlying unweathered shale.
A deep erosional gully has developed along the south flank of
the slide. The gully has formed as a result of surface runoff
between the slope and an adjacent berm associated with the
railroad line located downstream of the site. A relatively
shallow slide has developed immediately downstream of the gully.
The shallow slide was not defined by the survey. However, it
has been located approximately on Plate 1.
ANALYSIS
Stability Analyses
Slope stability analyses were performed in order
to evaluate the
probable cause(s) of the landslide, and the shear strengths
governing at failure. Slope stability analyses were aided by
use of a computer program, CLARA 2.31. The program utilizes
both two- and three-dimensional algorithms based on Bishop's
Method of Slices. For the failure analysis, three-dimensional
analysis was chosen over the more simplified two-dimensional
technique in order to evaluate the influence of variations in
topographic conditions, particularly the deep erosional gully
located adjacent to the south flank of the slide.
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In order to evaluate the overall stability of the slope,
approximate topographic conditions existing prior to failure
were reconstructed. Subsurface conditions were estimated based
on information from the borings, field geologic mapping, and
geologic interpretation. A ground surface isometric plot of the
reconstructed topographic conditions existing prior to failure
is included on Plate 12.
The shape and location of the failure surface considered for the
analysis was based on landslide geomorphology, and on the
relative strength of subsurface materials. Only the main
landslide mass was considered in the analysis. Previous
landslide activity is anticipated to have significantly weakened
the soils within the slide mass. Also, satisfaction of negative
pore water pressures associated with rainfall infiltration and
ground water tends to reduce effective shear strength. The
bentonite seam is particularly susceptible to rapid strength
loss associated with relatively small displacements.
Initial values of cohesion and friction angle, as well as other
pertinent properties, were estimated from the results of the
laboratory testing. Residual strength was utilized for the
bentonite seam based on the discussion above. A friction angle
of 90 degrees is utilized in the program for the unweathered
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shale in order to model a "hard" layer below which the failure
is not anticipated to extend. The program automatically
utilizes the shear strength parameters of the layer immediately
above the "hard" layer.
The critical shear
back-calculated using iterative solutions for the
safety. The process was continued until a factor of
one (conditions of incipient failure) was obtained.
illustrates the sliding surface isometry, along
critical
strength parameters governing at failure were
factor of
safety of
Plate 12
with the
shear strength parameters calculated from the analyses.
Subsequent analyses were performed to evaluate the influence of
the deep erosional gully on initial stability. To accomplish
this, topographic conditions existing prior to erosion of the
gully were estimated and the factor of safety was calculated
using the critical strength parameters from the above analyses.
Ground surface and failure surface plots are presented along
with the results on Plate 13. As seen on Plate 13, a factor of
safety of 1.02 was calculated for conditions existing prior to
erosion of the gully, suggesting that it did not have a
significant influence on initial failure of the main slide.
Based on these results, subsequent analyses have been carried
out in two dimensions rather than three. It should be noted
that, although the gully does not appear to have significantly
influence initial failure, it does have bearing on the stability
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of the slide
translated to
of safety at,
this balance,
result
in its current state. That is, the slide has
a position of limiting equilibrium, with a factor
or only slightly above, one. Minor disturbance of
by either gully or stream erosion, will likely
in further slide movement.
Translation of the slide to its current position has resulted in
further reduction in shear strength along the slide plane. A
two-dimensional analysis was carried out to evaluate the shear
strength parameters required to maintain limiting equilibrium
(factor of safety of one). The results are presented on Plate
14. Disturbance of the existing equilibrium, by stream or gully
erosion, or a rise in ground water level, will result in
reactivation of the slide.
Cause(s) of Failure
Initial failure of the slide is attributed to a combination of
excess pore pressures associated with heavy rainfalls and high
ground water, and removal of support at the toe of the slope by
erosion along the cut bank. Cut bank erosion likely caused a
relatively shallow slide to develop at the base of the slope.
Movement of the lower portion of the slope led to a reduction in
support of the slope above. The force imbalance created by the
removal of toe support led to development of
surfaces. This process continued to propagate
upslope, ultimately leading to major landsliding.
new sliding
progressively
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Based on discussions with city of Coppell personnel, it appears
that the old sliding surface observed in the summer of 1993 was
reactivated in the spring of 1994. As discussed previously,
prior slide activity would have significantly reduced the shear
strength of the materials within the slide. Infiltration
associated with heavy rainfall in the months preceding failure
would have reduced negative pore water pressures within the
slope and also caused a rise in the ground water level.
Satisfaction of negative pore pressures reduces effective shear
strength. Elevated ground water reduces the normal stresses,
and thus the resisting forces in the lower part of the slope.
RECOMMENDATIONS
General
Remedial
existing
future erosion.
evaluated to
reinforcement,
measures must address both stabilization of the
landslide and protection of the toe of the slope from
Several alternatives have been considered and
stabilize the landslide including soil
internal slope drainage, concrete or gabion
retaining walls, reinforced concrete piers for shear resistance
along the slide plane, and various combinations of each. The
evaluation involved extensive calculations and slope stability
analyses of the various alternatives considered. Only those
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alternatives which are considered to be most
an engineering and economic point of view,
detail. Others will be discussed briefly
paragraphs.
feasible, both from
were evaluated in
in the following
Primarily
considered was
"fibergrids".
polypropylene
because of economy, one of the first alternatives
use of a new soil reinforcement product called
This product consists of fibrillated
fibers which are blended into the soil. The
addition of the fibers results in an increase in effective shear
strength of the reinforced mass. Test results indicate the
potential for significant increases in effective cohesion even
in highly plastic clays. In order to be effective, the fibers
must be intimately and uniformly mixed into the soil mass. The
borings revealed that a significant portion of the slide mass is
composed of shale in various states of weathering. The shale,
even upon weathering to significant degrees, retains much of its
parent structure. In order to make effective use of the
fibergrids, the shale structure would have to be completely
destroyed. Because
with processing the
considered to be the
of the anticipated difficulty associated
shale, the use of fibergrids is not
most feasible alternative to stabilize the
existing landslide.
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Consideration was given to stabilizing the landslide by means of
a retaining wall at the base of the slope. The retaining wall
would also serve the function of providing erosion protection at
the toe of the slope. Any structure located at the toe of the
slope must meet several criteria: 1) the height/configuration
must be such that the cross-sectional area of the creek is not
significantly altered; 2) the height/configuration should be
such that turbulent flow is not created leading to accelerated
erosion; 3) the design must be sufficient to restrain further
movement of the existing landslide; and 4) the height must be
sufficient to provide an adequate factor of safety against
development of a new slide which could override the wall. In
order to meet criteria 1) and 2), the wall height would have to
be limited to about six or seven feet. A vertical concrete
retaining wall would not be well suited to limiting creation of
erosional "knick points". A gabion wall would be more suitable
for this function. Because of the loads imposed by the existing
landslide, a substantial (and costly) retaining wall, founded on
piers and supported by prestressed ground anchors, is
anticipated to be required. Also, slope stability analyses
indicate that the potential exists for development of an
overriding landslide above the wall. Therefore, additional
remedial measures would be required in order to stabilize the
landslide. If these measures are correctly designed and
implemented, a retaining wall at the toe of the slide is not
necessary.
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Based on this discussion, it is recommended that the main
landslide be stabilized by means other than a retaining wall,
and that erosion protection at the toe of the slope be provided
by means of gabion mattresses placed on an appropriate slope
angle. Details associated with protection of the toe are
provided in the Remedial Measures section.
The head scarp at the top of the landslide must be accommodated
by use of either a retaining wall system, or regrading of the
slope to pre-slide conditions. Regardless of the means utilized
to rectify existing grades, some means of stabilization will be
required to increase the factor of safety of the landslide to an
acceptable level.
Consideration has been given to stabilizing the existing
landslide by means of reinforced concrete piers within the
slide, coupled with minimal regrading of the slide surface.
This would require construction of a retaining wall along the
head scarp of the slide. A wall height of approximately ten to
twelve feet would be required in this case. A gravity structure
would impose additional surcharge loads on the head of the
slide, adding to the driving forces. Therefore, a retaining
wall a% %he head of the ~lide would have to be founded on
relatively deep piers, or else additional piers would have to be
added to stabilize the slide. Additional lateral support of a
retaining wall by means of ground anchors is also anticipated.
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Since piers will be required to stabilize the landslide
regardless of final site grades, use of a retaining wall at the
head of the slide does not appear to be the most economical
solution.
Based on the discussions above, regrading of the landslide to
pre-slide topographic conditions, coupled with use of reinforced
concrete piers within the slide mass appears to be the most
economically feasible solution to stabilization of the
landslide. Therefore, this alternative was analyzed in further
detail.
Slope stability analyses, considering proposed finished slope
grade, were performed in order to evaluate the additional shear
resistance required along the sliding plane to provide a factor
of safety of at least 1.3. Two conditions were analyzed. The
first considered that minimal regrading of the existing ground
would be performed prior to placement of fill required to
achieve finished grade. The second condition considered that
the entire landslide mass would be overexcavated and recompacted
prior to placement of additional fill. The analyses indicate
that the additional shear resistance required for Condition One
would exceed that required for Condition Two by appro×ima%ely 70
pounds per square foot (psf). This translates to an additional
total shear resistance of approximately 300 kips for Condition
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One over Condition Two. More piers, or larger diameter piers,
are required if the existing slide mass is not overexcavated and
recompacted. Therefore, overexcavation and recompaction of the
existing slide area is recommended.
The next step in the analysis involved determining the number
and size of piers required to achieve the necessary increase in
shear resistance along the existing slide plane to provide a
factor of safety of at least 1.3. Based on this analysis, a
total of 21 piers, each 33 inches in diameter, are required.
Further slope stability analyses were performed to evaluate the
minimum top elevation of the piers required to provide an
adequate factor of safety against an overriding slide. Based on
this analysis, a minimum top elevation of 450 will provide a
factor of safety of at least 1.3 against an overriding slide.
However, a factor of safety of 1.1 was calculated for a shallow
slide below the piers. The results are presented on Plate 15.
In order to increase the factor of safety against a shallow
slide to at least 1.3, a shear key, excavated into the
unweathered shale, was modeled. The results of this analysis
are presented on Plate 16 and indicate that a minimum eight-foot
wide shear key, extending at least four feet into unweathered
shale, will provide a factor of safety of 1.3.
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Remedial Measures
Based on the analyses and discussions above,
that the existing landslide be stabilized by
it is recommended
overexcavation and
recompaction of the entire failed area, coupled with
installation of 21 reinforced concrete piers to provide
additional shear resistance, and a drainage system to lower the
phreatic surface. An idealized remedial cross-section is
presented on Plate 17.
The piers should be a minimum of 33 inches in diameter and
should penetrate a minimum of six feet into dark gray,
unweathered shale below the excavated slide mass. The top
elevation of the piers should be 450 or higher. The piers
should be spaced in three rows of seven piers each, with a
center-to-center spacing of 12 feet between piers. Each row
should be spaced eight feet apart, and offset six feet from
adjacent rows. A proposed plan layout of the piers is presented
on Plate 18. The piers should be reinforced with 1-1/2 percent
steel throughout their full depth.
Prior to installation of the piers, the entire slide mass should
be excavated. The excavation should extend at least one foot
into undisturbed soil along the back of the slide and at least
one foot into dark gray, unweathered shale below the bentonite
seam along the base of the slide. The excavated backslope
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should be no steeper than iH:iV. A shear key should be
excavated at the toe of the slope. The shear key should be at
least eight feet wide and should extend a minimum of four feet
into dark gray, unweathered shale.
Excavated soils should be blended as much as possible to provide
a uniform backfill. The excavated soils should be replaced and
compacted in maximum eight-inch loose lifts in accordance with
the Earthwork section. All imported fill required to achieve
final surface grades should consist of a uniformly blended sandy
clay as outlined in the Earthwork section.
A drainage blanket should be constructed along the excavated
backslope between approximate Elev. 450 and 460. The drainage
blanket should be a minimum of two feet in width and should
consist of a durable crushed stone such as ASTM C-33, Size 67 or
coarser. The crushed stone should be separated from the
surrounding soils by a filter fabric such as ADS 600, or
equivalent. A minimum six-inch diameter perforated drainage
pipe (ADS N-12, or equivalent) should be installed at the base
of the drainage blanket. The perforated drainage pipe should be
drainage pipes (ADS N-12, or equivalent) on
horizontal centers. The solid drainage pipes
to drain (minimum 5 percent slope) at the toe
of the slope. The discharge points of the solid pipes should be
approximately one foot above the "normal" creek flow level.
teed into solid
maximum 12-foot
should be sloped
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The toe of the slope should be protected by means of a nine-inch
thick gabion mattress. The mattress should extend upslope on an
approximate angle of 1.75H:lV up to a minimum elevation of 450.
The mattress should also be extended out into the creek channel
for a minimum distance of six feet. The mattress should be
extended at least ten feet beyond the horizontal limits of the
slide in both directions. Consideration should be given to
turning the ends of the mattress down at least two feet below
ground surface to account for future erosion which may occur at
the ends. The mattress should be anchored, as necessary to
prevent uplift, particularly on the upstream end.
Consideration should be given to installing a drainage pipe of
sufficient size to handle surface runoff in the existing
erosional gully and backfilling the gully. If this cannot be
accomplished, then the gabion mattress should be wrapped around
this end of the slope and a mattress installed in the base of
the gully to prevent further erosional downcutting.
The slope should be revegetated as soon as possible after
completion of grading operations. Consideration should be given
to covering the slope with an erosion control fabric, such as
BonTerra S2, during the revegetation process. Particular
attention should be paid to the upstream and downstream slopes
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beyond the gabion mattress. These slopes should be covered with
an erosion control fabric, such as BonTerra C2, and revegetated
as soon as possible after construction. All BonTerra fabrics
can be pre-seeded with any mix.
Earthwork
Ail vegetation and topsoil containing
removed at the start of earthwork
organic material should be
construction. The entire
landslide mass should then be excavated. The excavation should
extend at least one foot into undisturbed soil along the back of
the slide. Excavated benches should be created so that backfill
can be placed on level, continuous surfaces. The excavation
should be extended at least one foot into dark gray, unweathered
shale below the bentonite seam along the base of the slide. The
key trench should be excavated as described in prior paragraphs.
Excavated soils should be blended to provide as uniform a mix as
possible prior to backfilling. Backfill using site-excavated
soils should be placed in maximum loose lifts of eight inches
and compacted to between 92 and 98 percent of the maximum
density as determined by ASTM D-698, "Standard Proctor". The
moisture content should range from -1 to +4 percentage points
above optimum.
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Imported fill required to achieve finished grade should consist
of a uniformly blended sandy clay with a Plasticity Index (PI)
of between 10 and 20. Imported fill should be placed and
compacted in accordance with the guidelines provided above.
Crushed stone for use in the drainage system should be placed in
maximum loose lifts of $ inches and compacted to a minimum of 60
percent of the relative density as determined by ASTM D-4254.
Construction observation
It is recommended that a representative of this office be
present to observe all construction activities in order to
confirm a proper bearing stratum and construction procedures.
Field density tests should be performed at a minimum rate of one
test per lift, per 2,000 square feet in all compacted fills.
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reed engineering
Slope Stability Analysis
Project No. 1998 Grapevine Creek
Coppell, Texas
Date: 07-25-94 Location: See Plate I
uJ CORE Pock(et Penet¢ometer Readings z
> ~n Tons Per Sq. Ft.
::~ ~ ~- ~ ~ Standard Penetration Tests
~~ ~ ~ (j o DESCRIPTION OF STRATA Blows per Foot -
LU ~ OC
~ ~ ~ 4. 10 20 30 40 50 O0
O- 452.7-
,,-L SANDY CLAY, medium gray & brown,
stiff, w/trace gravel (fill) (CL) /
/
450.2-
,-J- CLAY, olive-gray G yellowish-tan,
medium stiff, w/pockets of
5- reddish-brown sandy clay, very ~ .~l,r I:wlo~Or-17 g' 447.7-
CLAY, mottled might gray, reddish-tan '"-......
=~==-I- -~ g brown, soft to stiff, w/weathered "-- 444.4-
~-_-~__i shale fragments S some voids
~o- ~=r.--- (completely weathered shale) (CH) ~ 44~,~=
SHALE, dark gray ~ reddish-brown,
very soft, jointed, weathered to
slightly weathered
~5- weathered bentonite seam @
9.8'-10.3'
SHALE, dark gray, soft
Total Depth = 10.5'
20-
Water at 9,5' at end of day. Water at 5.0'
and blocked at I0.0' on 07-27-94,
25-
30-
35-
BORING LOG B-1 PLATE 2
6EOTE[~I~IIC,N. CONSULTANTS
Project No, 1998
Date; 07-25-94
BORING LOG B-2
Slope Stability Analysis
Grapevine Creek
Coppell, Texas
DESCRIPTION OF STRATA
SAND, light brown, fine- to
medium-grained (fill) (SP)
SANDY CLAY, reddish-brown ~ dark
gray, soft, very moist (fill) (CL)
CLAY, light gray,reddish-tan E;
brown, soft, moist, w/weathered
shale fragments (fill) (CH)
SHALE, light reddish-brown E; medium
gray, very soft, jointed, severely
weathered
SHALE, dark gray, reddish-brown 6
tan, soft, jointed, weathered
SHALE, dark gray, soft, w/occasional
iron-stained joints, slightly
weathered
Total Depth = 8.7'
Water at 0.4' at end of day. Water at 0.6'
and blocked at 8,3' on 07-27-94,
reed engineering
GROUP
Location: See Plate $
Pocket Penetrometer Readings
Tons Per Sq. Ft. -~
Standard Penetration Tests
Blows per Foot - ·
{ I 2 3 4 45+
PLATE 3
~OTECHNIC~L CONSULT~TS --
Project No, 1998
Date: 07-25-94
BORING LOG B-3
Slope Stability Analysis
Grapevine Creek
Coppell, Texas
DESCRIPTION OF STRATA
SANDY CLAY, dark gray ~;
reddish-brown, hard, w/some fine
gravel (fill) (CL)
SANDY CLAY, reddish-brown,
olive-gray ~ tan, hard (fill) (CL)
CLAY, dark olive-gray, very stiff,
w/occasional thin layers of
reddish-tan sand (fill) (CL)
SANDY CLAY, reddish-brown, dark
gray ~ tan, hard (fill) (CL)
SHALE, olive-gray ~ reddish-tan,
soft, jointed, weathered
SHALE, dark gray ~; reddish-brown,
soft, jointed, weathered to slightly
weathered
SHALE, dark gray, soft
high angle joint I~ 21.7'
high angle joint 9 22.5 '- 22.8'
high angle joint @ 23.4' - 23.6'
high angle joint @ 24.9' - 25.4'
high angle joint ~ 25.8' - 26J'
Total Depth = 29.4'
Bailed to 20.0' at completion. Water at 21.5'
after 5 minutes. Water at 16.6' and blocked
at 25,3' on 07-27-94,
reed engineering
GROUP
Location: See Plate
PLATE 4
6EOTECHNICAL CONSULTANTS
reed enoI
GROUP
reed engineeri~ng
P Slope Stability Analysis GROUP
roiect No. 1998 Grapevine Creek I
Date: 07-25-94 Coppell, Texas Location: See Plate 1~ I~ TypeofFill
> m m COR ~ Pocket Penetrometer Readngs z
%' ~ ~ ~ be Tons Per Sq Ft -~ o~ -- CLAY (CL)
~ ~ ~ ~ ~- ~5 ~ DESCR]PTION OF SIRATA 8lows ef Foot + <~ ~ (LL<50)
0 ,~ ..... 4fz. f~ CLAY (CH)
SANDY CLAY, dark gray ~ ILL>50)
_ ~/~j]j J~ reddish-brown, hard, w/some fine Fl I I I I I J I I ,FI I I 1
-- SILT (NL)
~ ~_z'_.dJJ/ / SANDY CLAY, reddish-brown, | I i I I I I I ILl I I I i 46821 (LL<50)
~'/J I/ / w/occasional thru myers of I I I I I I k I t I I I I I / SILT (NH)
I I I I I II 1 t I I 11464"1
~o- ×...¢1/ / SANDY C,^Y~.da~ I-I I I I t I I ~ I I I It /
' 1/I ~,) (CL) t11111t111111t /
15- ;~ SILTY(sM) SAND
?0 4520
CLAYEY GRAVEL
(GC)
~5 ::-- ~ ',~ ~ GRAVEL
' ~-~--~ t [weathered)
tO .... 4433 SHALE
(unueathered)
(ueathered)
(unweathered)
~5
(weathered)
BORING LOG B-3 PLATE 4! SANDSTONE
{unweathered)
UNEISTURBEO ~ STANDARD
(Shelby Tube & PENETRATION
NX-Core) TEST .~ = Water Ieve~ at time of drilling.
~ TH[] CONE ~ = Subsequent water revel and date.
DISTURBEDV PENETRONETER
__ TEST
KEYS TO SYMBOLS USED ON BORfNG LOGS PLATE 5
engineering
GROUP
SO]'L PROPERTfES
SPT
N-Values Relative
(blows/foot) Density
o - 4 ......................... Very Loose
4 -tO ......................... Loose
10-30 ........................ Nedium Dense
30-50 ....................... Dense
50 + ......................... Very Dense
Pocket
Penetrometer
(T.S.F.) Consistency
<0.25 ..................... Very Soft
0.25-0.50 .............. Soft
0,50-t. OO ............... Hedium Stiff
I.OO-2.00 ............... Stiff
2.00-4.00 ............. Very Stiff
4.00 + ................... Hard
ROCK PROPERTIES
~ DIAGNOSTIC FEATURES
Very Soft .......................... can be dented with moderate finger pressure.
Soft .................................... Can be scratched easily with fingernail,
Hoderately Hard ............. Can be scratched easily with knife but not with fingernail.
Hard ................................... Can be scratched with knife with some difficulty; can be broken by light to moderate
hammer blow.
Very Hard ......................... Cannot be scratched with knife; can be broken by repeated heavy hammer blows.
DIAGNOSTIC FEATURES
Slightly Wealhered ..............Slight discoloration inwards from open fractures.
Weathered ............................. Discoloration throughout; weaker minerals decomposed; strength somewhat less
than fresh rock; structure preserved.
Severely Weathered ........... Host minerals somewhat decomposes; much softer than fresh rock; texture becoming
indistinct but fabric and structure preserved.
Completely Weathered...,... Hinerals decomposed to soil; rock fabric and structure destroyed (residual soil).
KEY TO DESCRIPTIVE TERHS ON BORING LOGS
PLATE 6
6EOTECI-NICN. CONSULTANTS
reed engineering
0 I I I I
· I I I
o I I I I
,~ I I I I
o 0
GEOTECHNiCAL CONSULTANTS
PLATE 7
4OOO
3000
2000
1000
0
1000 2000 3000 4000 5000 600{
NORMAL LOAD (psf)
4000
Boring No.
Depth (ft)
Unit Dry Weight (pcf)
B-1
5.0-6.0
90.5
Normal Load (psf):
Moisture Content (%)
0EFOHE: 31.1
AFTER: 33.6
.POINT NUMOER
® ® ®
1293 25~7 5174
30,6 26.8
29.3 30.7
3000
2000
1000
Cohesion (pst):
Friction Angle {o):
PEA~K RESIDUAL
300 150
33.7 30.7
0.1 0.2
SHEAR DISPLACEMENT {inches)
0.3
CONSOLIDATED-DRAINED DIRECT SHEAR TEST PLATE 8
GEOTECHNICAL CONSULTANTS
reed engineering
4OD0
3000
2000
1000
0 1000
2000 3000 4000 5000 6000
NORMAL LOAD (psf)
4000
Boring No. B-1
Depth (ft) 10.0-10.5
Unit Dry Weight (pcf) 71.4
POINT NUMBER
® ® ®
Normal Load (psf): 1293 258? 5174
Moisture Content (%)
BEFORE: 42.6 54.3 46.7
AFTER: 42.6 55.2 89.7
3OO0
2000
1000
PEAK RESIDUAL
Cohesion (psf): 400 360
Friction Angle (°'): 26.2 13.9
0.1 0.2
SHEAR DISPLACEMENT (inches)
0.3
CONSOLIDATED-DRAINED DIRECT SHEAR TEST PLATE 9
GEOTECHNICAL CONSULTANTS
reed engineerin~g
4000
3000
2000
1000
0 1000 2000
Boring No. B-3
Depth (ft) 14.0-15.0
Unit Dry Weight (pcf) 95.4
Normal Load (psf): 1293
Moisture Content {%)
BEFORE: 27.3
AFTER: 33.6
3000 4000 5000 6000
NORMAL LOAD {psf)
4000
3000
POIN] NUMBER
2857 5174
26.0 25.0
29.8 28.7
2000
1000
PEAK RESIDUAL
Cohesion (psf): 580 250
Friction Angle (o): 20.0 12.6
0 0.1 0.2 0.3
SHEAR DISPLACEMENT {inches)
CONSOLIDATED-DRAINED DIRECT SHEAR TEST PLATE 10
GEOTECHN~CAL CONSULTANTS
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reed engineering
GROUND SURFACEISOMETRY
SLIDING SURFACE ISOMETRY (F.S.=I.00)
CRITICAL SHEAR STRENGTH PARAMETERS
LAYER UNIT COHESION FRICTION
NO. DESCRIPTION WEIGHT (pcf) (psf) ANGLE (.)
G SHALE, UNWEATtlERED 130 G,000 90
(~ [~ENTONIrE 108 0 14
(~ SHALE, WEATt IERED 120 207 13
Q SANDY CLAY 126 0 24
3-D SLOPE STABILITY ANALYSIS-INITIAL SLIDE PLATE 12
GEOTECHNICAL CONSULTANTS '
reed engineering
GROUND SURFACEISOMETR¥-WITHOUT GUL~
SLIDING SURFACE ISOMETR¥ (F.S.=1,02)
CRITICAL SHEAR STRENGTH PARAMETERS
LAYER UNIT COHESION FRICTION
NO, OESCRIPTION WEIGHT (pcf) (psf) ANGLE (°)
(~ SHALE, UNWEATHERED 130 6,000 90
(~ BENTONITE 106 0 14
Q SHALE, WEATHERED 120 207 13
(~) SANDY CLAY 126 0 24
3-D SLOPE STABILITY ANALYSIS-INITIAL SLIDE PLATE 13
GEOTECHNICAL CONSULTANTS ·
reed engineering
ONE DIVISION = 10~
CRITICAL SHEAR STRENGTH PARAMETERS
LAYER UNIT COHESION FRICTION
NO. DESCRIPTION WEIGHT (pcf) (psf) ANGLE (o)
(~ SHALE, UNWEATHERED 130 6,000 90
(~ BENTONITE 106 0 14
(~ SHALE, WEATHERED 120 52 13
(~ SANDY CLAY 126 0 24
2-D SLOPE STABILITY ANALYSIS-EXISTING CONDITIONS PLATE 14
GEOTECHNICAL CONSULTANTS
reed engineering
540 ~
520 -
,500 -
~ ~CONCRETE PIERS
O~E D[V[S[O~ = 20'
LAYER UNIT COHESION FRICTION
NO. DESCRIPTION WEIGHT (pcf) (psf) ANGLE
~ SHALE UNWEATHERED 130 6,000 90
~ SHALE WEATHERED 120 207 13
FiLL
2O7
2-D SLOPE STABILITY ANALYSIS-WITH PIERS PLATE 15
GEOTECHNICAL CONSULTANTS
re~d engineering
520
440 ~HEAR K
420 -
ONE DIVISION = 20'
LAYER UNIT COHESION FRICTION
NO. DESCRIPTION WEIGHT (pcf) (psf) ANGLE (~)
(~ SHALE UNWEATHERED 130 6.000 90
Q SHALE WEATHERED 120 13
207
G SANDY CLAY 126 100 24
Q FILL 125 207
20
2-D SLOPE STABILITY ANALYSIS-WITH PIERS AND SHEAR KEY PLATE 16
GEOTECHNICAL CONSULTANTS
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