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1 Drilled Shaft Construction Issues in Caving Ground and Rock Sockets Case Histories on Drilled Shaft Projects Designed for Seismic and Lateral Loads.

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Presentation on theme: "1 Drilled Shaft Construction Issues in Caving Ground and Rock Sockets Case Histories on Drilled Shaft Projects Designed for Seismic and Lateral Loads."— Presentation transcript:

1 1 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.

2 2 Drilled Shaft Construction Issues in Caving Ground and Rock Sockets Methods to control and prevent caving Designs with constructability issues

3 3 Drilled Shaft Construction Issues in Caving Ground and Rock Sockets

4 4 Caving Soil and Rock Sockets Solutions Slurry Head Drill Casing Temporary casing withdrawn during concrete placement Permanent casing left in place during concrete placement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 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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9 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 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 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 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 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 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 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 16 Drilled Shaft Construction Issues in Caving Ground and Rock Sockets

17 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

18 18 Drilled Shaft Construction Issues in Caving Ground and Rock Sockets

19 19 Drilled Shaft Construction Issues in Caving Ground and Rock Sockets

20 20 Drilled Shaft Construction Issues in Caving Ground and Rock Sockets

21 21 Drilled Shaft Construction Issues in Caving Ground and Rock Sockets

22 22 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 23 Installation of Permanent Casing

24 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 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 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 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

28 28 Removing Contaminated Concrete from Top of Pile

29 29 Embedded Column Cage Perfect Scenario Allowable Construction Tolerances

30 30 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 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 32 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.

33 33 MAXIMUM SHAFT SHORING DIAMETER SHAFT DIA.CONSTR. TOL. CONC. COVER SHAFT THICKNESS REIN. CAGE COLUMN DIA. MAX. SHAFT SHORING DIA. 4’4” 2.1”2’-0”5’-6” 5’5” 2.6”2’-6”6’-3” 6’6” 2.6”3’-0”7’-0” 7’6” 2.6”4’-0”8’-0” 8’6” 3.3”4’-6”9’-0” 9’6” 3.3”5’-6”10’-0” 10’6” 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.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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35 35 Slip Casing with Oscillator Method

36 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 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 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 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 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 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 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 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 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 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 46 Drilled Shaft Construction Issues in Caving Ground and Rock Sockets

47 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 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 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


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