The foundations of a building are arguably the most important part of the design. After all, they support all the structural elements which are built above them. It is important to ensure that the constructed foundations are free from defects and able to support the design loading. This article will dive into the types of pile testing and why they are needed on building projects.
The different types of pile testing available to Structural and Geotechnical Engineers to check for strength, serviceability and geotechnical parameters are as follows:
- Static Testing: (Compression test, Tension Test and Lateral Load test)
- High-Strain dynamic testing
- Rapid Load Testing and
- Integrity Testing
Click on the different types of pile testing above to jump straight to the relevant section of this article…
Pile testing is especially important as the majority of the pile is imbedded into the founding soil making visual inspection impossible. Therefore different types of pile testing is employed to verify a piles integrity and strength where visual inspections cannot be performed. First lets define what a pile exactly is…
A pile foundation is a deep foundation type with structural form resembling that of a column. Pile foundations penetrate the support soil and use friction forces between the side of the pile and the soil and/or end bearing between the soil and the pile toe to support the required design load. Piles can be constructed from cast in-place concrete, precast concrete, timber or steel.
For a detailed look into the different pile types and the benefits of each for the support of buildings, take a look at THIS article.
Why are Different Types of Pile Testing Needed on Building Projects?
Different types of pile testing are performed to achieve the following outcomes:
- Assess the piles serviceability
- Assess the design geotechnical strength
- Assess the ultimate geotechnical strength
- Testing to confirm adequacy of construction methods
- Confirm the pile shaft integrity
Increases Load Capacity
Depending on the types of pile testing used to verify the pile strength, a higher design capacity can be determined. Many piling design codes allow for reduced safety factors on the piles capacity if the strength of the pile is verified through appropriate methods of pile testing. This can mean that the cost of the site-testing could be more than covered by costs saved on pile length, diameter and reinforcing bar.
The Australian Piling Design standard for example requires a strength reduction factor of 0.4 or less to be adopted if no load testing is proposed for the piles.
Verify Geotechnical Assumptions
Different types of pile testing can be adopted to supplement the site specific geotechnical investigation.
For all building projects, a geotechnical site investigation and report are produced. The site investigation may include:
- Installation of standpipes to determine water table levels over time
- Sampling of the soil via extraction of bore holes (a circa 100mm diameter core taken from the soil with depths ranging anywhere from 1m to 40m+ depending on the structure to be built and the type of soil conditions)
- Test pits to ether verify soil conditions or investigate the founding depth and extent of existing footings which are required to remain and may have new footings propose to be constructed close to them.
The site sampling is then taken to a laboratory for testing of the soil specimens to determine soil bearing capacities which then inform the pile design.
Sometimes however it may not be feasible to obtain enough site samples to produce a thorough geotechnical report. Instances which may cause this include:
- An existing building may be located on the site which is operational until construction comments (at which time it will be demolished). This may prevent access for core samples to be taken
- The site may be located adjacent to live infrastructure which cannot be shut down such as rail or roads.
- The site may be located in a sensitive environment which prevents frequent access to the public such as a defence site or highly classified facility.
In these instances, different types of pile testing can then be used to make up for the lack of soil investigation which has been performed on the site. I’ve worked on a number of rail grade separation projects in Victoria Australia where reduced soil sampling was taken due to the rail needing to be live for as long as possible. The piles associated with the new construction were then adequately tested while they were built to make up for the lack of test coverage of the site.
Verify Site Workmanship
The majority of construction for a pile occurs away from sight (deep under ground). The pile construction is not as readily viewable compared to an inspection of a structural column, beam, wall or slab.
Different types of pile testing can be adopted to make up for this lack of site visibility, specifically for piles. Proof testing and integrity testing of the pile can help ensure that the piles have been constructed as expected and free from:
- Honeycombing in concrete for cast in place bored piers
- Free from cracks and damage for pre-cast driven piles caused during the installation process
- Free from other miscellaneous defects which may have been generated during the construction process.
What are the Types of Pile Testing used in Buildings?
Now lets take a look at the different types of pile testing which are commonly used for building structures…
Static Testing of Piles
Static load tests of piles are used to measure a piles behaviour under a known loading.
A static load test involves application of a heavy constant dead load (kentledge) to the top of the pile. The load is gradually increased incrementally. The pile is generally not tested to its full design capacity but rather a fraction of the design load. The resultant displacement is measured and compared against the applied load verify the geotechnical performance of the pile including:
- Skin friction
- End bearing
The same principle is used for a field plate test which Geotechnical Engineers use to determine a soils ultimate bearing capacity and spring stiffness for shallow foundations. To learn more, take a look at THIS article.
The arrangement and sequencing for a standard static load test of piles is as follows:
- The pile is constructed
- A hydraulic jack is set up on the pile head
- A grillage of primary and secondary steel beams is set up above the jacks in preparation to support the dead load
- A datum bar is set-up adjacent to the pile and hydraulic jack which provides a reference point for the future settlement measurements of the pile after it is loaded.
- Sufficient dead load is placed upon the grillage system. The load is pre-determined by the Structural Engineer and Geotechnical Engineer. The dead load can be provided by anything from bricks, concrete blocks or sand bags.
- The hydraulic jack bears against the pile head and underside of main primary beam of the grillage. The hydraulic jack is progressively expanded which results in an incrementally increasing load being applied from the dead load grillage to the pile head.
- With each incremental increase in loading, the settlement of pile is measured using the datum bar as a reference. This can be later extrapolated to verify the full design load on the pile.
The image below shows a cross-section of this typical arrangement…
There is another variation to the static load test of piles which adopts a structural steel “crown” and ground anchors in lieu of a steel grillage and dead load system. The arrangement for the crown pile load test is as follows:
- Pile is constructed
- Hydraulic jacks are set-up on the pile head
- A structural steel “crown” is placed on the jacks and temporarily propped to remain stable
- Diagonal ground anchors are installed from the pile head to the soil adjacent to the test pile.
- The hydraulic jacks are progressively extended between the pile head and base of the crown thus generating downward force on the pile.
- With each incremental in crease in load, the pile movement is measured in reference to the datum bar. This can be later extrapolated to verify the full design load on the pile.
Advantages of Static Load Pile Testing
- Due to the controlled nature of the process, the static load test gives the most accurate results compared to other types of pile testing.
- While it is usually not feasible to test a pile to its full design load (particularly for very tall buildings) the constant static nature by which the load is applied closely resembles the majority of the piles in-service behaviour.
- The settlement data vs. applied load can then be extrapolated to the full design load behaviour to verify the results.
- Static load testing can be performed on all soil types and also include upwards, downwards and horizontal load components on the pile.
Disadvantages of Static Load Pile Testing
- The process can be quite time consuming
- There is a large space requirement to perform the testing. This may be problematic on constrained sites
- The testing process can be costly
High-Strain Dynamic Pile Testing
High strain dynamic testing is used in two scenarios:
- While the pile is being installed (for the case of a driven pile in this instance it is often called dynamic pile monitoring)
- Testing applied to a pile after installation
The process involves striking the pile head with a large hammer or drop weight. The impact then produces a compressive wave which travels down he length of the pile. Accelerometers are fixed to the pile to then measure the piles resultant movement.
The results are then analysed and the expected pile capacity is produced.
Advantages of High-Strain Dynamic Pile Testing
- Testing can be carried out quicker compared to static load testing
- Less site real-estate is required for setting up the test
- Large cranes are not required compared to static load test where loading blocks need to be delivered and lifted into place.
Disadvantages of High-Strain Dynamic Pile Testing
- The test itself doesn’t apply a loading which truly resembles the static nature of the load which the pile may experience in service conditions.
- Due to the impact, the testing can be loud which may be disturbing to neighbouring properties
- High vibrations are produced during the test which again may be disturbing to neighbouring properties.
- Results are not as accurate as other types of pile testing such as the static load test.
Rapid Load Testing of Piles
Rapid load testing (RLT) of piles is one of the types of pile testing developed only recently. The rapid load test was developed and subsequently refined in the early 1990’s.
The RLT, in terms of its application, sits between the dynamic and static types of pile testing. The load duration applied to the pile head is not as short as a dynamic test, nor is it as long as the static load test. A dynamic load test through application of a hammer or drop weight results in a circa 5 to 10ms load duration while the rapid load test results in a 50 to 100ms load duration.
Rapid load testing is performed by a specialised unit which houses the loading device and all the necessary monitoring equipment to measure the results. These include accelerometers and strain gauges.
The RLT units achieve a longer load duration compared to dynamic impact testing through one of two ways:
- Combustion: A cylinder is placed on the pile head. Above the cylinder lies a large mass. Within the cylinder is a combustible gas which is detonated. The pressure caused by the denotation accelerates the mass upwards and presses the pile head downwards.
- Drop Weight: A drop weight is housed within the RLT unit. It is simply dropped onto the pile head, however the difference with dynamic load testing is that a cushion is provided at the base of the weight. The cushion lengthens the load duration of the applied weight.
The application of this quasi-static load better simulates the piles eventual in-service load conditions. Although the static load test better represents the true loading behaviour on the pile, the rapid load test provides an alternative which can still produce very accurate results in a fraction of the time.
Advantages of Rapid Load Pile Testing
- The specialised RLT units are quick and simple to set up with all the necessary equipment provided within the unit
- Produces accurate results
Disadvantages of Rapid Load Pile Testing
- The approach is relatively specialised meaning that not all piling contractors have the necessary knowledge or equipment to perform the test
- Noise and vibration production, although less than a dynamic load test, is still relatively high. This may cause disturbance to neighbouring properties.
Integrity Testing of Piles
Integrity testing of piles is the one test type which does not verify the geotechnical strength of a pile. Rather, this type of testing checks for the workmanship of the pile itself.
Integrity testing is commonly used for cast in place concrete piles to check for concrete defects such as honeycombing and cracking.
Similarly with a dynamic load test, the integrity test uses an impact on the pile head. However the magnitude of the applied for is much smaller by comparison (can be applied by a person wielding a hammer rather than a drop weight).
Sensitive monitoring equipment is placed on the pile head. The compression wave of the hammer blow travels down the length of the pile. With a pile free from defects, this compression wave travels to the base of the pile then back up to the pile head.
For a known pile length and diameter, the time required for this compression wave to travel from pile head to pile to and back to pile head can be calculated. If the monitoring equipment on the pile head picks up that the travel time for the compression wave is less than that expected, this can be an indication that defects exist in the pile shaft and further investigation and repair is required.