Drilled Shaft Construction Issues in

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26 Νοε 2013 (πριν από 3 χρόνια και 11 μήνες)

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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 Contractor’s 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





2

Drilled Shaft Construction Issues in
Caving Ground and Rock Sockets



Methods to control and prevent caving



Designs with constructability issues

3

Drilled Shaft Construction Issues in
Caving Ground and Rock Sockets

4

Caving Soil and Rock
Sockets


Solutions



Slurry Head




Drill Casing


Temporary casing withdrawn during concrete placement


Permanent casing left in place during concrete placement


5


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


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

9



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

Drilled Shaft Construction Issues in
Caving Ground and Rock Sockets

10

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)



Drilled Shaft Construction Issues
in Caving Ground and Rock
Sockets

11


Temporary/Permanent Casing


Benefits and Drawbacks


Installation Methods of casing


Telescoping / Freefall Method


Vibratory method



Rotational (vibration free) method


Drilled Shaft Construction Issues
in Caving Ground and Rock
Sockets

12




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.


Drilled Shaft Construction Issues
in Caving Ground and Rock
Sockets

13




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



Drilled Shaft Construction Issues
in Caving Ground and Rock
Sockets

14


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


Doesn’t eliminate risk of ground loss



This method has the highest risk of resulting in a stuck casing


Drilled Shaft Construction Issues
in Caving Ground and Rock
Sockets

15


Installation Methods


Vibratory Methods


Very Quick installation


Care must be taken to keep casing straight


Difficulty going through hard rock, cobbles and boulders


Drilled Shaft Construction Issues
in Caving Ground and Rock
Sockets

16


Drilled Shaft Construction Issues in
Caving Ground and Rock Sockets

17


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



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

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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 it’s tip


Care should be taken if a Permanent Casing is specified to
prevent caving, an alternative to use temporary casing should
also be specified.

Drilled Shaft Construction Issues in
Caving Ground and Rock Sockets

23

Installation of Permanent Casing

24



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


Drilled Shaft Construction Issues in
Caving Ground and Rock Sockets

25



Constructability issues with these design approaches:


Single size solution (commonly used by Caltrans and other DOT’s)


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

Drilled Shaft Construction Issues in
Caving Ground and Rock Sockets

26


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)


Drilled Shaft Construction Issues in
Caving Ground and Rock Sockets

27



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.

Drilled Shaft Construction Issues in
Caving Ground and Rock Sockets

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Removing Contaminated Concrete
from Top of Pile

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6"
¢ COLUMN
¢ NEW LINE
¢ PIER
¢ NEW LINE
¢ PIER
#5
#5
#5
#5
#10
#10
#11 BUNDLED
#11 BUNDLED
8'-0"ø SHAFT
8'-0"ø SHAFT
Embedded Column Cage

Perfect

Scenario

Allowable Construction
Tolerances

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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.


Drilled Shaft Construction Issues in
Caving Ground and Rock Sockets

31

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.

Drilled Shaft Construction Issues in
Caving Ground and Rock Sockets

32

¢ SHAFT & COLUMN
PLASTIC
HINGE ZONE
SHAFT-COLUMN
CONNECTION
PERMANENT CASING
TEMPORARY
CASING
APPROX. GROUND
CASING SHORING
Shaft Terminology

Plastic Hinging Zone

Casing Shoring

Column to Shaft Connection

Permanent Casing

Temporary Casing

Stepped Shaft

6” oversize for
shafts larger
than 5’ Dia. &
12” oversize for
shafts 5’ Dia. &
smaller.

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MAXIMUM SHAFT SHORING DIAMETER




SHAFT DIA.


CONSTR.
TOL.


CONC. COVER
SHAFT


THICKNESS
REIN. CAGE


COLUMN
DIA. MAX.


SHAFT
SHORING DIA.





















4’


4”


4”


2.1”


2’
-
0”



5’
-
6”



5’


5”


5”


2.6”


2’
-
6”



6’
-
3”



6’


6”


6”


2.6”


3’
-
0”



7’
-
0”



7’


6”


6”


2.6”


4’
-
0”



8’
-
0”



8’


6”


6”


3.3”


4’
-
6”



9’
-
0”



9’


6”


6”


3.3”


5’
-
6”



10’
-
0”



10’


6”


6”


3.3”


6’
-
6”



11’
-
0”






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.



Preliminary selection of column size.


Select minimum shaft size based on column size.

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CASING SHORING*
PERMANENT CASING
TEMPORARY OR PERMANENT
CASING IF NEEDED
2'-0" MIN. OVERLAP
EXISTING GROUNDLINE
OR HIGH WATER
2'-0" MIN. OVERLAP
1'-6" PLUS IS DESIRABLE
CONTRACTOR MAY INCREASE INSIDE
DIAMETER OF SHAFT CASING BY 6"
OVER WHAT IS SPECIFIED. DESIGNER
SHOULD CONSIDER THIS IN SHAFT DESIGN.
SEAL
3'-0" PLUS IS DESIRABLE
RIVER OR CHANNEL BOTTOM
CONSTR. JOINT WITH
ROUGHENED SURFACE
CONSTR. JOINT WITH
ROUGHENED SURFACE
EXISTING GROUNDLINE
OR HIGH WATER
(SEAL VENT ELEVATION)
CASING SHORING
PERMANENT
CASING
TEMPORARY OR PERMANENT
CASING IF NEEDED
CONTRACTOR MAY INCREASE INSIDE
DIAMETER OF SHAFT CASING BY 6"
OVER WHAT IS SPECIFIED. DESIGNER
SHOULD CONSIDER THIS IN SHAFT DESIGN.
SHAFT CASING DETAILS IN SHALLOW
EXCAVATIONS AND LOW WATER
* WHEN THIS DIMENSION EXCEEDS 3'-0" USE "SHAFT CASING
DETAILS IN DEEP EXCAVATIONS OR HIGH WATER".
SHAFT CASING DETAILS IN DEEP
EXCAVATIONS OR HIGH WATER
35

¢ SHAFT
¢ SHAFT
ƒ 4 Ð" VOID
APPROX. GROUND
APPROX. GROUND
INSTALL SLI P CASING
BACKFILL VOI D
BACKFILL w/CDF OR APPROVED
NATI VE MATERI AL
10'ø SHAFT CONSTRUCTED
WITH THE OSCILLATOR METHOD
8.435'
9.06 8'
9.06 8'
9'-10"
OSCI LLATOR CASI NG
PERMANENT CASI NG
( SLIP CASI NG)
REINFORCING CAGE
PERMANENT CASI NG
( SLIP CASI NG)
Slip Casing with Oscillator Method

36

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.

Drilled Shaft Construction Issues
in Caving Ground and Rock
Sockets

37


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

Drilled Shaft Construction Issues in
Caving Ground and Rock Sockets

38

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.

Drilled Shaft Construction Issues in
Caving Ground and Rock Sockets

39


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.

Drilled Shaft Construction Issues in
Caving Ground and Rock Sockets

40



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

Drilled Shaft Construction Issues in
Caving Ground and Rock Sockets

41

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

Drilled Shaft Construction Issues in
Caving Ground and Rock Sockets

42

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.

Drilled Shaft Construction Issues in
Caving Ground and Rock Sockets

43

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)


Drilled Shaft Construction Issues in
Caving Ground and Rock Sockets

44

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

Drilled Shaft Construction Issues in
Caving Ground and Rock Sockets

45

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)

Drilled Shaft Construction Issues in
Caving Ground and Rock Sockets

46

Drilled Shaft Construction Issues in
Caving Ground and Rock Sockets

47


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.

Drilled Shaft Construction Issues in
Caving Ground and Rock Sockets

48


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.


Drilled Shaft Construction Issues in
Caving Ground and Rock Sockets

49


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.

Drilled Shaft Construction Issues in
Caving Ground and Rock Sockets