1 drilled shaft construction issues in caving ground and rock sockets case histories on drilled...

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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

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•Methods to control and prevent caving

•Designs with constructability issues

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•Drill Casing•Temporary casing withdrawn during concrete placement•Permanent casing left in place during concrete placement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

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• 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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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)


Sockets

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•Temporary/Permanent Casing•Benefits and Drawbacks

•Installation Methods of casing

•Telescoping / Freefall Method

•Vibratory method

• Rotational (vibration free) method


Sockets

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•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.


Sockets

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Sockets

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Sockets

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Sockets

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Sockets

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Installation of Permanent Casing

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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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• 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


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•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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• 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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6"

¢ COLUMN

¢ N

EW

LIN

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¢ PIER

¢ N

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¢ PIER

#5 #5

#5 #5

#10 #10

#11 BUNDLED #11 BUNDLED

8'-0"ø SHAFT 8'-0"ø SHAFT

Embedded Column Cage 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.


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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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¢ SHAFT & COLUMN

PLA

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N

PE

RM

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APPROX. GROUND

CASING SHORING

Shaft Terminology

Plastic Hinging ZoneCasing 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 spiral2.6” Cage Thickness assumes a #14 vert. & #5 spiral3.3” 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.Preliminary selection of column size.

•Select minimum shaft size based on column size.Select minimum shaft size based on column size.

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CA S ING S HORING *

P ERMA NENT CA S ING

TEMP ORA RY OR P ERMA NENTCA S ING IF NEEDED

2'-0

" M

IN. O

VERL

AP

EXIS T ING G ROUNDLINEOR HIG H WA TER

2'-0

" M

IN. O

VERL

AP

1'-6 " P LUS IS DES IRA BLE

CONTRA CTOR MA Y INCREA S E INS IDEDIA METER OF S HA FT CA S ING BY 6 "OVER WHA T IS S P ECIF IED. DES IG NERS HOULD CONS IDER THIS IN S HA FT DES IG N.

S EA L

3 '-0 " P LUS IS DES IRA BLE

RIVER OR CHA NNEL B OTTOM

CONS TR. J OINT WITHROUGHENED S URFA CE

CONS TR. J OINT WITHROUGHENED S URFA CE

EXIS TING G ROUNDLINEOR HIG H WA TER( S EA L VENT ELEVA TION)

CA S ING S HORING

P ERMA NENTCA S ING

TEMP ORA RY OR P ERMA NENTCA S ING IF NEEDED

CONTRA CTOR MA Y INCREA S E INS IDEDIA METER OF S HA FT CA S ING BY 6 "OVER WHA T IS S P ECIF IED. DES IG NERS HOULD CONS IDER THIS IN S HA FT DES IG N.

S HA FT CA S ING DETA ILS IN S HA LLOWEXCA VA TIONS A ND LOW WA TER

* WHEN THIS DIMENS ION EXCEEDS 3 '-0 " US E "S HA FT CA S INGDETA ILS IN DEEP EXCA VA TIONS OR HIGH WA TER".

S HA FT CA S ING DETA ILS IN DEEPEXCA VA TIONS OR HIG H WA TER

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¢ SHAFT

¢ SHAFT

ƒ 4Ð" VOID

APPROX. GROUND APPROX. GROUND

INSTALL SLIP CASING BACKFILL VOID

BACKFILL w/CDF OR APPROVEDNATIVE MATERIAL

10'ø SHAFT CONSTRUCTEDWITH THE OSCILLATOR METHOD

8.435'

9.068' 9.068'

9'-10" OSCILLATOR CASING

PERMANENT CASING(SLIP CASING)

REINFORCING CAGE

PERMANENT CASING(SLIP CASING)

Slip Casing with Oscillator Method

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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.


Sockets

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•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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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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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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•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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•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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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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•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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•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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•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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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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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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•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.


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