Bi-Directional Static Axial Compressive Load Test For Piling
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Bi-Directional Static Axial Compressive Load Test For Piling


- INTRODUCTION.

Bi-directional Static Load Testing (BDSLT) – Osterberg Cell Equivalent is a method of deep foundation testing to determine the resistance distribution along its length and assess the pile behavior under static loading.The test load will be equivalent to the sum of load applied above and below the bi-directional cell assembly.   

BDSLT is the most innovative static load testing technique in the industry.The BDSLT provides separate, direct measurements of the pile side shear mobilized above an embedded bi-directional cell assembly and the pile end bearing plus any side shear mobilized below the bi-directional cell assembly.The maximum mobilized pile resistance equals two times the maximum load applied by the bi-directional cell assembly.   

The BDSLT will be performed by a qualified testing personnel in accordance with the “ASTM D8169/D8186M-18” Standard Test Method for Deep Foundations Under Bi-Directional Static Axial Compressive Load.

Typical schematic of an embedded bi-directional cell assembly placed within a test pile in preparation for a BDSLT is present below. The resultant line of force of the bi-directional cell assembly shall coincide with the central axis of the foundation element. During initial bi-directional cell pressurization, a fracture plane will form through the concrete surrounding the bi-directional cell assembly, and the pile reinforcement and instrumentation shall not restrain the subsequent expansion of the assembly after the fracture occurs. As indicated below, different types of deep foundations require different methods of bi-directional cell installation.   

  



- EQUIPMENT.

The main equipment to be used by Testing Laboratory for instrumentation and testing are as follows:  

  • Bi-Directional Hydraulic Cell (BD-cell) 
  • Concrete Embedment Strain Gauges 
  • VW Displacement Transducers 
  • VW Pressure Transducer 
  • Pressure Gauge 
  • Automated Digital Level 
  • Aeroquip Hydraulic Hoses and Fittings 
  • Air Driven Pump & Fittings, etc. 
  • Data Taker Geologger 
  • Laptop Computer 
  • Bearing plates prepared in accordance with drawings  
  • Nominal ½” G.I. pipes cut to length schedule  
  • Reinforcement cage of lengths including tremie guide etc. 
  • Welding and lifting equipment etc.  


- TEST PILE PREPARATION.

The test pile preparation is carried out by, or under the supervision of laboratory personnel. A single level bi-directional cell assembly will be installed in the test pile along with the necessary hydraulic system and instrumentation attached to the steel reinforcing cage.   

The final bi-directional cell assembly position is subjected to the approval of the project geotechnical design engineer. For the purpose of fully mobilizing the axial compressive capacity, the engineer will usually locate the bi-directional cell assembly at a location within the pile where the capacity above the assembly equals the capacity below it. The position of bi-directional cell assembly herein is a preliminary estimate based on assessment of the soil investigation provided by the Client. 

- BI-DIRECTIONAL CELL ASSEMBLY.

The assembly will comprise of bi-directional cell(s) and steel bearing plates.  The fabrication of the bi-directional cell assembly will be carried out on site or at the piling contractor’s workshop, where the bi-directional cell(s) will be accurately positioned and welded to steel bearing plates above and below the bi-directional cell(s).  Fabrication drawings for the bi-directional cell assembly for bearing plates are included in the below figure:



Note- 
  • Bottom and top bearing plate welded to the rebar cage. 
  • Tremie pipe should be less than the maximum tremie hole diameter.
  • Tremie guide is constructed to direct the tremie pipe pass to the assembly.

To ensure that no damage occurs to the bi-directional cell(s) during handling or lifting the assembly, temporary reinforcement will be installed across the bi-directional cell assembly.  Two or more pieces of reinforcing steel will be welded between the top and bottom of the bearing plates as to temporarily reinforce the bi-directional cell assembly.   

Temporary reinforcement at the bi-directional cell assembly will be cut once the cage is vertical and is being lowered into the excavated bore, such that no steel crosses the bi-directional cell opening level.  

-REBAR CAGE ASSEMBLY.  

The bi-directional cell assembly will be welded to the reinforcement cage, perpendicular to the long axis of the cage at the approved location.  The attachment to the reinforcing cage will be carried out by fillet-welding the steel bearing plates to the inside of the reinforcing steel cage. The vertical and transverse reinforcing steel bars will be terminated immediately above and below the top surface of the bottom bearing plate (bi-directional cell opening level or pile fracture plane). 
A tremie guide will be constructed on the top bearing plate of the bi-directional cell assembly to guide the tremie pipe through the hole.    

- PILE INSTRUMENTATION.

The preliminary estimate of positioning for the bi-directional cell assembly and instrumentation are also shown as below example for the Schematic Drawing of Test Piles drawing. 



The details of the embedded instrumentation are as follows: 
  
Two numbers of traditional telltale casing and rod extensometers extending from the top of 
bottom of plate of the bi-directional cell assembly to top of the pile will be used to allow for 
measurement of the downward bottom plate displacement, with respect to the top of the pile.  

Two numbers of traditional telltale casing and rod extensometers extending from the top of 
the bi-directional cell assembly to top of the pile will be used to allow for measurement of the upward top plate displacement, with respect to the top of the pile.  

Concrete embedment strain gauges will be used to assess the load distribution along the pile above and below the bi-directional cell assembly. Four strain gauges at 90 degrees around the perimeter at five levels in the pile. 


- PILE EXCAVATION & INSTALLATION.

The excavation of the deep foundation will proceed under the Piling Contractor approved work plan.
 
The base of the excavation should be cleaned and approved by the engineer prior of reinforcement cage installation and concrete placement.  

Once the assembly has been welded to the reinforcing cage and instrumented according to plan, the cages may be prepared for lifting and installation into the pile excavation.

The lifting of each cage section to vertical will be carried out using a lifting table, beam, or several pick points to ensure the cage remains straight during lifting. 

All reinforcing cages will be fabricated by the piling contractor with sufficient cage stiffeners or welded mild steel hoops at regular spacing, lifting points and lifting beams as required for their preferred lifting technique.
 
For more than one cage sections is used, the first cage will be lowered into the excavation and supported on the temporary casing while the subsequent cage section is lifted and spliced. 

All BDSLT instrumentation will be connected during the splicing process and instrumentation cables extended to the top of the next cage section. 

After splicing all cage sections, the fully assembled cage will be lowered into the excavation and supported from the casing so that the bi-directional cell assembly is positioned at the correct elevation.

The reinforcing cage should not be allowed to rest on the base of the excavation.  


- CASTING OF CONCRETE.

After the reinforcement cage installed on the excavation, the tremie pipes of sufficient length will then be assembled and installed.The tremie guide constructed between the opening in the top plate of the bi-directional cell assembly and the main vertical rebar will guide the tremie pipe through the bi-directional cell assembly.

Cut-outs will be provided in the bi-directional cell assembly steel bearing plates through which to pass the tremie pipe and allow for concrete to flow through the bi-directional cell assembly. The length of tremie pipe below the bi-directional cell assembly must be free of joints or other protrusions that will exceed the noted diameter.
  
Concrete will then be cast in a continuous operation as per standard/approved concreting procedures under the responsibility of the piling contractor.

The concrete will be placed approximately 1.0 meter above the design cut-off level.
 
The approved concrete mix must allow for slump and workability to enable the concrete to pass through and around the bi-directional cell assembly. 

The mix should also contain sufficient retarding agent to maintain workability for the entire duration of the pile concreting procedure.

 A minimum concrete slump of 200 mm and sufficient retarder to provide at least 6 hours of workability are recommended for the test piles on this project. 

In addition to concrete samples (cubes or cylinders) those taken for quality assurance procedures, the contractor require to take sufficient additional samples for determining compressive strength and modulus of the concrete at the following times. 

  • Prior to testing to determine if the pile concrete is of sufficient compressive strength to start the BDSLT. 
  • On the date of the test.   
  • Sufficient spares should be provided for in case strengths are lower than expected and additional samples must be tested after further curing.  


- LOAD TESTING.

BDSLT will commence after the pile concrete reached a minimum strength as required by the project specification.The applied loads will be increased in fixed increments of 10% of the anticipated maximum test load and in decrements of 50% of the maximum applied load.
 
Each load increment will be maintained for a minimum period of 15 minutes and until the specified creep rate has decreased to less than 0.06 mm over a 15-minute period but for no more than 1 hour per increment.The creep criteria will be followed for plate displacements of up to 25 mm (either top or bottom plate). Instrument readings will be automatically recorded every 30 or 60 seconds during the entire test. Example for proposed loading schedule as per below figure.    

Testing will begin by pressurizing the bi-directional cell in order to break the tack welds that hold it closed and to form the fracture plane in the concrete surrounding the base of the bi-directional cell assembly.  

After the break occurred, the pressure will be immediately release from zero pressure. 

BDSLT will proceed as per approved loading schedule. 
 
During loading, the bi-directional cell will be internally pressurized, creating an upward force on the pile in upper side shear and an equal, but downward force in combined lower skin friction and/or end bearing.  

The bi-directional cell load is determined by relating the applied hydraulic pressure to the bi-
directional cell load calibration factor.
  
A calibrated, high-pressure bourdon gauge will be used to read the pressure on the pump line and a calibrated pressure transducer will read the pressure on the return line.
   
The load will be removed, and testing completed once either one of the following situations occur:  
  • The maximum required test load has been applied. 
  • The bi-directional cell(s) approaches its maximum nominal stroke of 150 mm. 
  • Plunging failure* of the pile section above or below the bi-directional cell location are 
  • achieved.  

- DATA COLLECTION & MEASUREMENTS.

All embedded instruments will be connected directly to a DataTaker Geologger for automated data collection and recording.This arrangement allows LVWDTs and strain gauge readings to be recorded simultaneously and stored automatically at 30 to 60 second intervals during the test (depending on the number of instruments being read).The use of a data logger will allow for greater accuracy than using a manual readout box.

Laboratory will perform the BDSLT with the following instruments:

  • Top of pile displacement is measured using direct measurement using automated optical level or using two LVWDT reference to the steel beam.  
  • Top of bi-directional cell displacement is measured using two LVWDT attached to the top of pile and set over the traditional telltale rods extending from top of bi-directional cell(s) to top of pile. 
  • Bottom of bi-directional cell displacement is measured using pair of LVWDTs attached to the top of the pile and set over the traditional telltale rods extending from bottom of bi-directional cell to top of pile.   
  • Base of pile displacement will be measured using two LVWDT attached to the top of pile and set over the traditional telltale rods extending from base of the pile to top of pile (if applicable). 
  • The strain distribution along the pile will be measured using concrete embedded vibrating wire strain gauges.    

The measured displacement and strain data provide for the following: 

  • Top of Pile Displacement (direct measurement using automated optical level or LVWDT). 
  • Upper Pile Compression above the bi-directional cell (measured by traditional telltale rods referenced to top of pile). 
  • Upward Top of Bi-directional Cell Displacement (calculated as the sum of measured Upper Pile Compression above the bi-directional cell and measured Top of Pile Displacement). 
  • Bi-directional Cell Expansion (calculated as Downward Bottom of Displacement plus Upper Pile 
  • Compression above bi-directional cell). 
  • Downward Bottom of bi-directional cell Displacement (measured by traditional telltale rods). 
  • Downward Tip of Pile Displacement (calculated as tip of pile displacement), if applicable. 
  • Lower Pile Compression (calculated as Downward Bottom of bi-directional cell Displacement minus Downward Tip of Pile Displacement). 
  • Load distribution along the pile (derived from the direct measurement of strain by vibrating wire strain gauges and estimated pile stiffness values).  Note pile resistance above the bi-directional cell is generated from pile side shear whereas the pile section below the bi-directional cell generates resistance from pile side shear and end bearing.These various components of resistance may be assessed using the strain gauge measurements.   

- DATA REPORTING.

In general, the report will compromise the following:

  • Project identification and location. 
  • Test site location. 
  • Date and type of test. 
  • Brief description of embedded bi-directional cell assembly and pressure measurements including capacity. 
  • Description of instrumentation used to measure pile movement.  
Tabulation of readings for each test instrument: 

  • Time of readings (nearest 1 second). 
  • Bi-directional cell pressure measured by gauge (typically nearest 100 psi or less). 
  • Bi-directional cell pressure measured by sensor, (typically nearest 10 psi or less). 
  • Load from pressure (nearest 5 kN or less). 
  • Pile top movement (nearest 0.2 mm or less). 
  • Bi-directional cell assembly expansion movement (nearest 0.2 mm or less). 
  • Pile compression movement (nearest 0.01 mm or less). 
  • Pile strain measurement (nearest 1 microstrain). 
  • Reference beam and wireline movement when measured.  
  • Plots of load versus plate movement above and below the bi-directional cell assembly.
  • Relevant field notes and leak checks. 
  • Calibration reports for the embedded pile instrumentation. 
  • Owner, engineer, all other qualified engineers if applicable, pile contractor, boring contractor.
  • Nearest test boring(s) or sounding(s), and their location with reference to test location. 
  • Identification and location of test pile. 
  • Date test pile installed.
  • Installation equipment and procedures. 
  • Design load of test pile. 
  • Type and dimensions of test pile. 
  • Test pile material including basic specifications. 
  • Tested top and bottom elevations of the pile, and ground elevation referenced to a datum. 
  • Embedded length of test pile. 
  • Tested length of test pile. 
  • As-built sketch of the pile cross-section. 
  • Description of internal steel reinforcement used in test pile (size, length, number longitudinal bars, arrangement, spiral, or tie steel). 
  • Approximate groundwater surface elevation (near the time of test if possible, especially if 
  • expected to be time dependent).
  • Observed test pile quality and defects. 
  • Cast date for concrete piles. 
  • Concrete and/or grout sample strengths and date of test.  












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