The procedure for undertaking a pile tension test typically involves several steps, including preparation, instrumentation, loading, and data analysis. A general outline of the process follows:

Preparation:

• Select the pile(s) to be tested based on project specifications and requirements
• Clear the area around the pile(s) to ensure safe and unobstructed access
• Install any necessary equipment, such as reaction frames or anchors, to apply and measure the tensile load
• Ensure that the pile is properly installed and secured in the ground according to design specifications

Instrumentation:

• Install instrumentation to measure parameters such as load, displacement, and strain.
• Common instruments used include load cells, displacement transducers, and strain gauges.
• Calibrate the instrumentation to ensure accurate measurement of the applied load and pile response.

Loading:

• Apply the tensile load to the pile using hydraulic jacks, winches, or other appropriate equipment
• Gradually increase the load in increments while monitoring the pile’s response as per the Client’s requirements
• Record data at regular intervals, including load applied, pile displacement as per the Client’s requirements

Data Collection:

• Continuously monitor and record data throughout the loading process
• Measure and record any settlement or movement of the pile and surrounding soil
• Ensure that data collection is performed accurately and consistently to capture relevant information

Unload and Analysis:

• After reaching the desired load level or observing specific pile behavior, gradually unload the pile

Reporting:

• Present the collected data for assessment by Engineer

Throughout the entire process, safety precautions should be strictly followed to prevent accidents or damage to equipment and personnel. This includes setting up a working area at least 20m around the testing site ensuring personel who are not actively involved in the test cannot enter the area.

The jack applies force simultaneously upward against the shaft resistance above its location and downward against the combined resistance below the jack assembly (including the toe or base resistance).

The cage is equipped with instrumentation to measure shaft and toe movements, as well as soil resistance from the applied load. Load-cells can be installed in various types of deep foundation elements, including bore holes and continuous flight auger (CFA).

Bi-Directional Static Load Test Benefits include

• High-capacity static load test method for deep foundation elements.
• Separates soil/rock resistance and movement data for shaft and toe.
• Determines magnitude of the mobilised shaft and toe resistances.
• Embedded strain gauges within the foundation element determine the soil/rock resistance distribution along the foundation length for optimizing foundation design.
• Not restricted by structural or geotechnical limit of load frame, reaction beams, or reaction piles.
• Multiple Load-Cells can be used at a single jack assembly location or at multiple jack assembly locations to apply larger loads.

Generalized plan and elevation views of single Load-Cell jack assemblies

Generalized plan and elevation views of multiple Load-Cell jack assemblies

The test consists of attaching one or two accelerometers to a pile and using a hand-held hammer to impact the pile top. The PIT collects the acceleration data and displays curves that reveal any significant changes in a cross section that may exist along the shaft. The impact of the hammer generates a stress wave that propagates down the foundation and reflects back up. The accelerometer collects data that reveals the pattern of wave propagation and reflection.

PIT tests may also help estimate the depth of the pile toe (the pile length) if the pile is intact with embedment less than 30 times the pile diameter. Initial PIT data is evaluated in the field and later transferred to a computer for further analysis.

During a lateral load test, a horizontal force is applied to the structure at a specified height above the ground level. This force is can be applied by pulling piles together or pushing them apart. 

Typically applied gradually using hydraulic jacks or other load-applying devices. Instruments such as load cells, strain gauges, or displacement transducers are used to measure the applied force and the resulting lateral displacement or deflection of the structure.

Laterally loaded screw piles using hollow cylinder and load cell arrangement with 4 displacement sensors

These screw piles are being pulled together laterally. The method uses a hollow hydraulic cylinder and load cell.

The test is performed on a prepared ground surface. If the prepared soil surface has any hollows or voids, they must be filled with dry sand or crusher dust to prepare a level surface on which to bed the plate. Our plate load test equipment is designed around a hydraulic cylinder and load cell to mobilise 250kN.

We have 3 plate sizes:

• 645mm diameter with a 765kPa max. bearing capacity
• 500mm diameter with a 1270kPa max. bearing capacity
• 300mm diameter with a 3500kPa max. bearing capacity

Displacement is measured by four 50mm travel transducers mounted to the rods in the timber beams.

Timber is used to minimise thermal distortions.

By measuring the strain at different points along the pile, engineers can assess the structural integrity and behaviour of the pile under load. This information is crucial for evaluating the performance of the pile foundation, identifying potential issues such as overloading or structural weaknesses, and ensuring the safety and stability of the overall structure.

The expected temperature at any location is dependent on the shaft diameter, mix design, time of measurement and distance to the centre of the shaft. TIP evaluates the concrete quality of the entire cross-section, including outside the reinforcing cage, and along the entire length, without maximum length limitations.

Static tests are performed slowly which results in large displacements being recorded. This is because the shear strength of soil is loading rate dependent. Because of this mechanism, dynamic tests do not take static behaviour like creep and consolidation into account. Creep is an important consideration for all soil types and any form of rapidly loaded test will always distort ultimate load values.

Static pile load testing is the most definitive method for determining pile capacity and should be used to establish a correlation with dynamic test methods. If dynamic testing completely replaces static testing on a project, no correlation is established and signal matching relies on experience alone. This extra risk comes with an increased factor of safety over static testing plus correlated dynamic testing.

This image shows the 4 displacement sensors monitoring the travel of the pile head during the load cycle.

A target load and a time to reach the target is entered. Load is then controlled by the data acquisition computer using a solenoid valve control unit and electric pump. The software will use feedback from the load cell to control the hydraulic valve unit. The trace depicted is the output from the load cell against time.

The same data acquisition and control system is used for static compression tests, tension tests and lateral tests.

A CFA pile being loaded in compression. The pile also has strain gauges installed on the rebar cage. This will provide a stress profile down the length of the pile.

A precast pile that has been installed by jacking in. The rig used to install the piles is used as kentledge for the sustained load test. The pile has strain gauges embedded at intervals down the length of the pile.

Our PDA testing and CAPWAP® Analysis services are carried out by our experienced technicians.

Pile Driving Analyser systems also evaluate shaft integrity, driving stresses, and hammer energy when monitoring installation. 

PDA tests are performed during driving on sacrificial or production piles, taking advantage of the high strain impact provided by the driving hammer. Re-strike PDA tests are often recommended; if the driving hammer is no longer on location, a drop weight may be used to impact the pile.

CMR Tech utilised the latest PDA-8G system, where the engineer has the option of being in the field for the pile driving test or offf-site using SiteLink® Technology. SiteLink technology transmits PDA test data via Internet to an engineer located elsewhere who follows the test in real time. Test data displays and results are identical in both modes.

The PDA-8G is furnished with the PDA Software Suite, CAPWAP and GRLWEAP. CAPWAP analysis of PDA data is essential for a Dynamic Load Test. It provides the soil resistance distribution along the foundation and simulates a static load test. Extensive correlations between CAPWAP simulated and actual static load tests have proven the reliability of this method of determining pile capacity.