Drilled Shaft Construction Issues in Caving Ground and Rock Sockets

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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets
Case Histories on Drilled Shaft Projects Designed
for Seismic and Lateral Loads.  A Contractors
Perspective Solutions Offered for Improvements
on Design and Means and Methods to Produce a More
Constructible Project PRESENTED BY TERRY
TUCKER President, Malcolm Drilling Company,
Inc. www.malcolmdrilling.com
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• Drilled Shaft Construction Issues in Caving
  Ground and Rock Sockets
• Methods to control and prevent caving
• Designs with constructability issues

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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets
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Caving Soil and Rock Sockets

• Solutions
• Slurry Head
• Drill Casing
• Temporary casing withdrawn during concrete
  placement
• Permanent casing left in place during concrete
  placement

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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets

• Slurry Head
• Economical Solution
• Can be used with Conventional Drill Equipment
• Higher Production
• Fast drill rates possible
• Pouring shafts could also be completed quickly
• Care must be taken to ensure integrity of shaft
• Choosing the right product
• Must monitor properties of slurry Density, sand
  content, etc.
• Fluid Exchange is recommended
• Replace dirty slurry with slurry that is free of
  suspended solids

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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets
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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets
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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets
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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets

• Slurry Head
• Disposal
• Calculated Risk
• Factors such as soil conditions, groundwater
  level, surcharges from equipment and roadways
  must be considered when using a drill slurry
• Effectiveness of slurry diminishes as hole
  diameter increases
• Stability of shaft may be compromised by small
  changes to slurry head
• Potential Problems
• Increased concrete take
• Increased time cleaning out shaft
• Soil and rock intrusions into shaft / caving
  soils
• Sinkholes resulting in unsafe conditions

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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets
Use of a drill slurry is typically the fastest,
most economic solution to caving ground. However,
as conditions become more difficult there is a
higher chance of shaft defects. Some soils and
rock can be impossible to keep open with a drill
slurry (i.e. sloped, fractured rock with lenses
of rock decomposed to a silty clay)
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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets

• Temporary/Permanent Casing
• Benefits and Drawbacks
• Installation Methods of casing
• Telescoping / Freefall Method
• Vibratory method
• Rotational (vibration free) method

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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets

•  
• Advantages of Temporary Casing
• Better chance of achieving a dry hole
• Generally only a water head is required with full
  depth casing (no mineral or synthetic slurry
  required)
• Mechanical control of substrata materials,
  removing the potential for caving during shaft
  construction, specifically during concrete
  placement
• Shaft can remain open for extended construction
  periods during the removal if obstructions are
  encountered or during drilling rock sockets.
• Positive control to ensure no loss of ground if
  contractor is allowed to advance casing to any
  depth he deems necessary to control caving.

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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets

•  
• Disadvantages of Temporary Casing
• Slower Drill Rates
• Can easily get into multiple shifts to construct
  a single shaft
• Extended Concrete Pours
• Pouring concrete while removing casing
• Casing can get stuck

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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets

• Installation Methods
• Telescoping / Freefall casing
• Does not require specialized equipment
• Quick installation
• May require multiple shaft diameters
  (telescoping)
• In some situations casing is installed after hole
  is drilled
• Doesnt eliminate risk of ground loss
• This method has the highest risk of resulting in
  a stuck casing

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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets

• Installation Methods
• Vibratory Methods
• Very Quick installation
• Care must be taken to keep casing straight
• Difficulty going through hard rock, cobbles and
  boulders

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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets
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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets

• Installation Methods
• Rotational Methods
• Best method for going through varying ground
  conditions
• Sands, gravels, cobbles, boulders, hard rock,
  etc.
• Provides best chance for avoiding a shaft defect
• Requires specialized equipment
• Top drive drills
• Rotator / Oscillator
• Slower than conventional methods

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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets
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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets
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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets
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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets
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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets

• Temporary Vs. Permanent Casing
• Temporary Casing Flexible
• Case as much or as little as needed
• Permanent Casing Rigid
• Becomes an obstacle if there is caving below its
  tip
• Care should be taken if a Permanent Casing is
  specified to prevent caving, an alternative to
  use temporary casing should also be specified.

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Installation of Permanent Casing
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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets

•  
• A. Problem Zone Transition from Drilled Shaft to
  above Grade Column
• Design Approaches Used By Various Designers
• One size column/drilled shaft with pour joint at
  grade.
• Embedded column steel with lap length greater
  than 15 feet, with pour joint allowed at bottom
  of column steel or at grade.
• Reduced column size transitioned over 10 feet in
  top of drilled shaft.
• Pour Joint allowed at bottom of column steel or
  at grade.
• Alternate Design Approaches by Designers
• Transition with a pile cap or cap beam.
• Transition with a pin connection at top of
  drilled shaft

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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets

•  Constructability issues with these design
  approaches
• Single size solution (commonly used by Caltrans
  and other DOTs)
• Typically requires full length column steel or a
  minimum of 20 feet above grade due to no splice
  zone requirements
• Makes it difficult to remove temporary casing(s).
• Difficult to maintain cage alignment.
• Difficult to tremie pour concrete
• Difficult to set long rebar cages

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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets

• Constructability issues with these design
  approaches (continued)
•  Column embedded steel with long lap length
  (greater than 15 feet).
• Requires either to set a deep permanent casing to
  allow for a low construction joint (lap length
  with column steel), or double cage (shaft and
  column) to be set and poured to grade.
• If low cut off is provided with permanent casing,
  additional issues with high groundwater table/cut
  off below slurry head.
• Need for telescoping casing or other means to
  allow temporary casing construction below
  permanent casing (oversized permanent casing)

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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets

•  Column embedded steel with long lap length
  (greater than 15 feet) Cont.
• Difficulty of concrete cover between shaft and
  column cage within the embed length, specifically
  if pour is brought to grade without the use of
  permanent casing if not allowed.
• Flow of concrete impeded by additional rebar and
  inspection pipes.
• When Gamma-Gamma testing is used by Caltrans,
  they specify an inspection tube (2 diameter)
  located as close as 2 from nearest vertical
  reinforcement.
• Very tight window for 1/2 or 3/8 aggregate
  considering horizontal reinforcement is typically
  very tight in these embed zones.
• Contractor required to repair defects associated
  with this issue
• Overpouring to achieve sound concrete at cold
  joint is problematic.
• Lots of chipping!
• In the absence of a construction joint,
  remediation of any defective shaft concrete is
  extremely difficult in this double cage zone.

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Removing Contaminated Concrete from Top of Pile
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Embedded Column Cage
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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets
In summary the above described common Caltrans
solutions present the Contractor with many issues
to overcome in order to provide the Owner with an
adequate product. There is a high
potential for many disputes.
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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets

• Approach using a reduced column size with short
  transition (10-12)
• Commonly used by WSDOT
• Reduced column size transitioned over 10 feet in
  top of drilled shaft. Pour Joint allowed.
• Requires an approximately 12 foot permanent
  transition casing to remain in ground from grade
  to concrete cut off.
• This solution works very well for work below the
  groundwater table whereby this transition casing
  is extended above water levels to act as a
  cofferdam
• Transition casing can be either oversized to
  allow for specified shaft diameter installation
  through the inside of the transition casing or,
  if sufficient cover is specified (6 inches) then
  transition casing could be installed in this
  annulus (between the shaft and the potential
  temporary casing.

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Plastic Hinging Zone
Casing Shoring
Column to Shaft Connection
Permanent Casing
Stepped Shaft 6 oversize for shafts larger than
5 Dia. 12 oversize for shafts 5 Dia.
smaller.
Temporary Casing
Shaft Terminology
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• Preliminary selection of column size.
• Select minimum shaft size based on column size.

MAXIMUM SHAFT SHORING DIAMETER  
    Max. Col. Dia. (Shaft Dia.) - 2(Conc.
Cvr.) - 2(Constr. Tol.) - 4(Thickness
Cage)   Max. Shaft Shoring Dia. Based on 1-0
min. clr. to column for forming. Therefore, Shaft
Shoring Dia. (Shaft Dia.) 2(1-0 clr.) -
2(Conc. Cvr.)   2.1 Cage Thickness assumes a
11 vert. 4 spiral 2.6 Cage Thickness assumes
a 14 vert. 5 spiral 3.3 Cage Thickness
assumes a 18 vert. 16 spiral   Construction
tolerances and concrete cover for shafts are per
WSDOT Special Provisions.
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Slip Casing with Oscillator Method
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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets
In summary, the above described (WSDOT) solutions
have been successfully used over the last 10
years with excellent results yielding
significantly lower remediation or concrete
defects in the embed zone.
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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets

• Alternate Design Approach by Various Designers
• Transition with a pile cap or cap beam.
• Easier solution from the perspective of
  installation of drilled shafts.
• However, this requires special design
  considerations and/or potential need for
  installation of cofferdams and/or temporary
  shoring to install pile caps.
• This method is particularly useful for bents
  where loads require multiple piers.
• Transition with a pin connection at top of
  drilled shaft
• Requires different design approach generally
  columns are pinned at the top of the drilled
  shaft and rigidly connected to the bridge
  structure.
• Typically limited to columns that are of average
  length less than 30 feet.
• Setting of pin connection can be problematic

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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets
In summary the above described design
approaches may work well, but require additional
design consideration in the event of seismic
loading, and may not work well where site
constraints limit the use of cofferdams and
temporary shoring.
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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets

• Construction Considerations for Drilled Shafts
  with Rock Sockets
• Problem Zone -Transition from overburden to
  rock socket.
• Is overlying material prone to caving?
• Where does the rock socket start? Is there
  potential for boulders overlaying rock and/or
  inclined rock surface (sloped rock). What is
  bedrock?
• Log of test borings
• What do the test borings indicate for materials
  to be encountered in the transition between
  overburden and the rock socket.

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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets

• Problem Zone -Transition from overburden to rock
  socket (continued)
• Rock and soil classifications
• If the rock behaves or has the consistency of
  soil it should not only be classified as a soil,
  but also treated as a soil from a
  constructability standpoint.
• If the potential for caving in the overburden and
  weathered rock exists, consider specifying the
  use of temporary casing

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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets

• Construction Considerations for Drilled Shafts
  with Rock Sockets
• Problem Zone - Transition from overburden to rock
  socket. (continued)
• Example of unconstructable specification(s)
• Provide permanent casing (in intimate contact
  with ground)
• Do not disturb surrounding soil
• Install permanent casing to a specified bedrock
  tip elevation regardless of material encountered
  at tip of casing, either too soft (caving) or too
  hard (continuation of casing through hard rock)
• Specifying minimum casing diameter that does not
  provide clearance to step down casing if
  required.
• Requirement that permanent casing be installed
  prior to construction of rock socket
• Installation of seal concrete at the bottom of
  the casing for dewatering purposes

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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets

• Construction Considerations for Drilled Shafts
  with Rock Sockets (continued)
• Rock Socket Construction
• Provide sufficient rock data immediately adjacent
  to the shaft, or at the location of the drilled
  shaft
• Consider the implementation of a full size test
  shaft. (ADOT)
• Can be done in conjunction with a full scale load
  test
• Test shafts with various agencies and owners has
  resulted in a reduction of the frictional
  transfer area of approximately 25
• Continuous coring of rock samples
• Proper classification of rock, including the
  transition from overburden to the rock socket.
  Note no ambiguous geologic descriptors in lieu
  of proper geotechnical terms, but accurate
  descriptions as to how the rock will behave
  during construction.

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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets

• Construction Considerations for Drilled Shafts
  with Rock Sockets
• Rock Socket Construction (continued)
• Rock Strength as a minimum, one compressive
  break every 5 feet of core length.
• Unconfined Compressive strength test only.
• Provide clear and concise reference as to what
  logging methods were used.
• Consider and evaluate potential of slip surfaces
  as causing mass caving (block failures for large
  diameter shafts 5 foot diameter and greater).
• Provide onsite geotechnical engineer to determine
  actual rock contact may not be required if
  sufficient data is provided for each shaft
  location
• Drilling equipment should not be used a guideline
  to determine the quality or strength of the rock
  (refusal or too soft rock issues)

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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets

• Construction Considerations for Drilled Shafts
  with Rock Sockets
• Rock Socket Construction ( continued)
• Provide an in-depth constructability review
  taking into consideration the actual conditions
  encountered during the site investigation.
• Blanket disclaimers as to
• The potential of caving or need of temporary
  casing.
• High water inflow rates, if not verified
• However it should be evaluated that 6 or 8 foot
  shaft will behave differently as compared to a
  2.5 diameter cored hole
• Provide mechanism under the contract to shorten
  or extend pile length if certain defined strength
  parameters are not encountered or encountered at
  an elevation other than anticipated

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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets

• Construction Considerations for Drilled Shafts
  with Rock Sockets
• Rock Socket Construction (continued)
• Consider the use of end bearing capacity in rock
  sockets, by specifying more strict slurry clean
  out methods or values, e.g. maximum sand content
  0.5, full replacement of drill slurry prior to
  concrete placement, use of S.I.D. (shaft
  inspection device camera)

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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets
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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets

•  Construction Considerations for Drilled Shafts
  with Rock Sockets (continued)
• In Summary
• It is absolutely imperative to perform a state of
  the art soils investigation to provide the best
  information to the Engineer and the Contractor to
  successfully construct the deep foundations as
  designed.
• Unfortunately, soils are made by nature and not
  by man, and the products of nature are always
  complex Natural soil is never uniform. Its
  properties change from point to point while our
  knowledge of its properties are limited to those
  few spots at which the samples have been
  collected (Karl Terzaghi)
• However, regardless of the extensiveness of the
  geotechnical investigation. a differing site
  condition still may be encountered. A thorough
  subsurface investigation will provide the best
  opportunity to minimize the risk of cost and time
  overruns to both the Contractor and the Owner.

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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets
 Closure Details of shaft / column transitions
and construction problems related to caving
ground and rock sockets have been presented for
consideration to achieve a successful completion
of drilled shafts in this very difficult
application. Big steps have been made in the
construction of deep foundations over the past 10
years. A new generation of very powerful top
drive hydraulic drill units is now in use
throughout the United States. Additionally, big
vibratory hammers, rotators and oscillators have
proven their effectiveness in caving and
difficult ground conditions.
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Drilled Shaft Construction Issues in Caving
Ground and Rock Sockets

• It benefits all members involved in the design
  and construction of these specialty deep
  foundations to become familiar with these new
  types of equipment.
• Often, a poorly written specification is the
  biggest hurdle to the successful completion of a
  deep foundation drilling project.