Survey - Investigation - Testing

Pile Load Testing

General

Pile load testing is an important factor for the piling works that should be properly planned and factored into the project schedule and project costs at an early stage in order to validate the design safety factors, pile design & lengths, to establish the pile installation criteria and to quality control the piling works. gbc - geotechnical engineering provides pile load testing for your project by applying static and dynamic test methods as follows: 
  • Static pile load testing (axial and lateral)
  • Dynamic compression load testing by pile driving analyzer (PDA)  
  • Pile integrity testing (PIT)

Static pile load testing

Static pile load testing is one of the most common methods for testing the actual in-situ capacity of a pile. 

The test procedure is based on the direct measurement of pile head displacement in response to a physically applied load. The test pile is loaded using a calibrated hydraulic jack that applies the test load to the pile by pushing against a beam placed directly over the test pile. The test beam is restrained by an anchorage system consisting of reaction piles installed in the ground to provide tension resistance. Once the load test frame is installed and fully instrumented, the hydraulic jack applies the test load in a series of increments according to the testing requirements as per project specifications. Each load is held for a certain amount of time until either twice the design load or pile failure is reached, whichever comes first. Pile movement is recorded with each incremental load.

Piles can be tested for compression, tension or lateral loads. By providing actual capacity and deflection values, the test results can be used to confirm that the pile design load can be adequately supported. Depending on the test pile’s performance, the results may also allow for project cost savings by permitting an increase in the pile design load, a reduction in the overall pile length, or a quantification of capacity in difficult or unknown soil conditions.

gbc - geotechnical engineering provides the following detailed services:
  • Establishing testing programme
  • Pile testing planning
  • Pile Installation and pile head preparation
  • Reaction system
  • Load application system and monitoring
  • Displacement measurement
  • Testing Procedure
  • Documentation and reporting (evaluation of test results / conclusions and recommendations)

Establishing pile testing programme
gbc - geotechnical engineering provides the pile testing programme for your project with respect to the applied standard, foundation design and soil investigation / soil conditions by specifying the number, locations, installation depths of test piles and test piles & procedures concerning direction of forces (axial - compression, tension / lateral).

Pile testing planning
We review the relevant documents (project master schedule, specification, pile testing programme etc.) and establish the detailed work programme, method statements and construction drawings for the site implementation.

Pile Installation and pile head preparation
The piles (test piles and anchor piles) will be installed according to pile testing programme and project specifications. We monitor the pile driving process and all relevant pile installation data will be recorded. After installation of the test pile, the pile head will be carefully prepared to allow the load and displacement monitoring instruments to be placed centrally, flush and perpendicular to the pile head.

Reaction system
The elements of an anchored reaction system are the steel girder or load frame, anchoring elements and connections. The elements of a non-anchored reaction system are load distribution beams, support structures and surcharge masses (Kentledge). The reaction system will be designed and executed for the applied test load, project specifications (standards etc.) and environmental conditions (soil, weather / climate etc.) and installed after pile installation and pile head preparation works are completed.

Load application system and monitoring
The load will be applied with one or more hydraulic jacks, having a capacity of at least four times the pile design load. Jacks will be used with a minimum travel of 6 in. (150 mm), but not less than 25 percent of the test pile's maximum cross-section dimension. The hydraulic jacks are equipped with following main components:
  • automatic load-maintaining pump with manual supplement to control load application
  • pressure gage for the jack so that the pressure reading corresponding to the pile design load  is between one-fourth and one-third of maximum gage pressure. 
  • placing a load cell (either electric or hydraulic) to measure strains for load monitoring during the load test

Displacement measurement
The vertical displacement of the pile head will be measured using redundant systems: mechanically using precision gauges or electrical displacement transducers, and in addition optically, e.g. by means of precise levelling. A reference system is used for mechanical measurement. It consists of reference beams, arranged on either one or both sides of the test pile and supported under statically defined conditions approximately 2,5 · D, but at least 2,5 m, from the pile. During the test, the reference system shall be protected from actions resulting from the test load, vibrations and temperature differentials. We apply 3 dial gauges or 3 linear variable differential transformers (LVDTs) and a readout unit to be attached to reference beams.

Testing Procedure
The load should be applied in at least two cycles (unless specified in project specifications / contract documents). The load will be applied in increments as a percentage of the maximum test load. Each load  increment and decrement will be held for the specified interval of time. Readings of gross settlement, load and time will be taken and recorded immediately before and after the application of each load increment.  

Documentation and reporting (evaluation of test results / conclusions and recommendations)
  • Time, load, pile head movements, settlements and reference beam movements and anchor piles
  • Load-settlement curve
  • Time-settlement curve
  • Time-load curve
  • Report and recommendations on the ultimate pile capacity

Dynamic compression load testing by Pile Driving Analyser (PDA)

Dynamic load testing of piles by Pile Driving Analyser (PDA) is a fast and effective method of assessing foundation bearing capacity that requires instrumenting a deep foundation with accelerometers and strain transducers (sensor) and analysing data collected by these sensors. PDA testing can be undertaken during initial driving or during re-strike drives after allowing soil setup on the pile for a suitable period of time. in order to verify pile resistance, CAPWAP analysis is performed on field data from a selected representative hammer blow from the PDA test record and the following information on the pile resistance can be derived:
  • Pile load bearing capacity 
  • Skin friction distribution over the pile length and the end bearing capacity 
  • Prediction of load-settlement chart 
  • Evaluation of pile integrity
  • Hammer performance
  • Maximum driving stresses

PDA testing is being executed in conformance with ASTM D4945 - 12: Standard Test Method for High-Strain Dynamic Testing of Deep Foundations. PDA Testing can be used on all types of piles, such as concrete piles, bored piles, steel piles and so on. In order to carry out this method of testing an impact hammer is required. The hammer should ideally be sufficiently large to fully mobilise and therefore characterise the dynamic pile capacity without damaging the pile, and in the case of driven piling will usually be the same hammer as used to install the pile. Dynamic load testing of bored cast-in-place will generally require the use of a separate hammer or drop weight.
Dynamic load testing is carried out with two (2) accelerometers and strain transducers (sensor) attached to a section of pile. For large diameter bored piles, four (4) accelerometers and strain transducers (sensor) are used. On concrete pile, the sensors are connected to the pile with anchor bolts. On steel pile, the sensors are bolted to the pile using threaded holes or welded mounting block. The generated compressive stress wave travels down the piles and reflects from the pile toe upward. The stress waves, which are picked up by the transducers, are processed and automatically stored in the computer for further analysis and reporting. The analysis is carried out using the signal matching program CAPWAP. Pile and soil data are modelled and a response is calculated based on one dimensional wave equation theory. The signal matching process utilizes an iterative method in which the results of each analysis are compared to the actual measured pile behaviour. Appropriate dynamic soil parameters are refined until a satisfactory match is achieved. As result, the above-mentioned information on the pile resistance can be derived.

PDA testing is a quick, low cost and very reliable tool. You save time. You save money. Therefore, we can establish an optimized & comprehensive pile testing programme for your project in order to minimize the number of static pile load tests and to maximize the number of dynamic pile load tests (PDA testing) under the condition to calibrate PDA testing against static pile loading test results.

Our experienced and qualified personnel applies modern testing & analysing equipment and conduct the dynamic load pile testing by providing the following detailed services: 
  • Establishing pile testing programme (with respect to the project specification)
  • Installation of accelerometers and strain transducers (sensor)
  • Monitoring of pile driving process
  • Measurement of stress waves
  • CAPWAP analysis and deriving information on pile resistance
  • Documentation and reporting

Pile integrity testing (PIT)

Low Strain Pile Integrity Testing (PIT) is a quick and cost effective method to evaluate the shaft integrity of concrete piles. The method is used to examine pile quality in cases of doubt, e.g. irregularities during pile  installation, non-conformities with requirements of quality assurance, suspected deficiencies, uncertainties of existing pile foundations, etc.

The testing is able to provide information on: 
  • ƒPile Toe Level ƒ  
  • Shaft restraints 
  • Cracks ƒ  
  • Reductions in section ƒ  
  • Zones of poor quality concrete
 
The test method is performed with a hand held hammer, a sensitive accelerometer and the Pile Integrity Tester. The accelerometer is attached to the top of the pile, and then a compressive wave is generated by tapping the pile head with a hammer. When the downward compression wave encounters a change in cross section or in concrete quality, it generates an upward tension wave that is later observed at the pile top. The velocity recorded along with the subsequent reflections from the pile toe or pile discontinuities are graphically displayed. The effectiveness of the system is limited to a pile length not exceeding (30 to 60) * pile diameters depending on soil condition and concrete quality. 

Our experienced and qualified personnel applies modern testing equipment (TECO pile integrity tester) and conduct the pile integrity testing by providing the following detailed services:
  • Establishing testing programme
  • Pile head preparation works
  • Testing & measurement
  • Evaluation of measurements and conclusion on pile integrity
  • Documentation and reporting