Pile Load Testing

Introduction
There are commonly two basic types of testing used in conjunction with cast insitu piling works; maintained load testing and integrity testing. These two forms of testing are fundamentally dissimilar and assess different characteristics of the pile and its construction.

Maintained Load Testing
Maintained load testing can be used both as a design aid to understand the settlement characteristics and performance of the pile when under load and as a quality assessment tool.

Carried out on a sacrificial pile in advance of construction of the working piles this form of testing is often referred to as a non-working or preliminary pile load test and is used to help determine the pile-soil interaction and thus maximise the efficiency of the pile design. By carrying out preliminary tests loaded to 2.5 or 3 times the working load of the pile, it is usually permissible to reduce the factor of safety employed in the design to 2.25 or 2.0 and realise potentially significant savings in the overall length of the piles.

When undertaken on a working pile concurrent with the construction of the main piling works the test is referred to as a working pile load test and is used to validate the pile design. In this case the pile is only loaded to a maximum of 1.5 times its safe working load so that the pile is not overstressed and can be incorporated into the works. The test is used in conjunction with a pile design that employs a higher factor of safety, typically 2.5, and is usually carried out at a frequency of 1no. test per 100no. working piles.

In order to perform a maintained load test it is necessary to construct an expendable reinforced concrete cap at the head of the pile. The load is applied through a hydraulic jack and calibrated load cell reacting against a set of steel beams tied to anchor piles. Typically 4no. anchors of similar dimensions to the test pile are used, arranged on a 4m by 2m grid. Alternatively kentledge weights arranged on a steel grillage may be used where ground conditions prevent the use of anchor piles. The pile head displacement is recorded using 4no. dial gauges or electronic transducers bearing directly on to the pile cap.

Testing is normally carried out 7 days after construction of the pile and cap but by using high strength concrete this can be accelerated to 3 or 4 days depending upon the actual loads.

Pile Integrity Testing
Integrity testing is a very quick and relatively inexpensive low strain non-destructive test method which can provide a valuable measure of the integrity of the pile shaft and hence confidence in the quality of the pile construction.

A number of systems are commonly used in the UK including Sonic Echo, TNO, Transient Dynamic Response (TDR) and Relative Impulse Response (RIR) some of which were originally designed for use on driven precast concrete piles whilst others namely the TDR and RIR methods have been developed specifically for use on cast insitu piles.

All of the systems attempt to evaluate the acoustic length of the pile by measuring the time taken for a vibration wave to travel down the pile shaft and be reflected from the pile toe. The vibration wave is generated through striking the pile with a small hammer and the acoustic data is recorded via a transducer placed on the pile head. The tests are differentiated from each other by the nature of the data recorded, the signal processing and the method of evaluation of the data. Interpretation of the data from all of the test methods necessitates an assumption as to the speed of propagation of the vibration wave within the pile concrete and whilst this is dependant, in part at least, upon the age of the concrete it is typically in the range 3500 - 4000m/s. An acoustic anomaly is identified by the reflection, or partial reflection, of the vibration wave from a level above that of the pile toe. Features that can cause a reflection include discontinuities (cracks), a reduction in section (neck), an increase in section (overbreak), a return to the nominal pile diameter from an enlarged section (step) and the interface between layers of relatively stiff and soft soils. It is important to understand that the anomalies thus detected are acoustic anomalies, which may have little or no structural significance. Consequently it must be stressed that anomalous integrity testing results should always be properly evaluated by skilled personnel in conjunction with the detailed pile construction records, site investigation reports, observational evidence and all other reliable data.

All integrity testing systems have finite capabilities that limit the depth of penetration of the test. Their ability to resolve faint reflection signals from depth is a function of the dynamic range of the testing system and the signal to noise ratio of the measurement. Typically though depths of penetration of up to 40-50 pile diameters can be achieved in soft soils eg loose sands and alluvium, 30-40 pile diameters in London Clay and 10-20 pile diameters in stiff soils eg dense sands and gravels and chalks.

Testing should be carried out at least 7 days after the pile has been cast and only after the pile head has been trimmed back to sound concrete but before any pile cap/ground beam reinforcement and/or concrete is placed. The pile head must be free of any standing water and cleared of any loose concrete or debris with unrestricted, safe access available for the testing technician.

An initial analysis of the acoustic data can be made and provisional results provided at site although provision should be made for access to the piles until such time as the full report is available especially in cases where acoustic anomalies have been identified.

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