Static Loading Test (SLT)
Pile tests are performed to verify or to give confidence regarding the design parameters.
One of the test methods is a static loading test carried out by loading a pile and recording its movement at the pile head level. The obtained pile-head load movement curve is used for analysis of the pile capacity. If the pile is instrumented along the pile shaft (e.g. by means of strain gauges or tell-tales) or at the pile toe level (e.g. by means of load cells or tell-tales) more detailed information can be achieved about the response of the pile to the applied load.
SLT Provides Information About:
- Pile-head load-movement curve
- Pile capacity
- Pile shaft resistance (if the pile is instrumented)
- Pile toe resistance (if the pile is instrumented)
Test Setup Preparations
The test setup consists of a test pile and a reaction system.The pile head of the test pile shall be carefully prepared to allow placement of hydraulic jacks, which apply the load to the pile, and displacement gauge needles, which measure the pile-head movement during the test. The hydraulic jacks should be equipped with load cells to provide the load value accurately. The displacement gauges are placed on a reference frame. Preferably, the reference frame is made of wood due to lower temperature sensitivity than steel. In most cases, the reaction system is a set of steel beams or steel frame anchored to reaction piles. Kentledge (dead loads) placed on a supporting steel frame or platform can also be used as the reaction system.
Measuring Principle of SLT
The SLT can be performed under compressive, tensile or lateral load. The most common test procedure is the maintained load (ML) test in which the load is increased in steps and held constant over a certain specified duration. The load can be applied in one or more loading-unloading cycles. The unloading of the pile is also applied in steps. The pile-head movement should be recorded simultaneously, in order to construct the pile-head load-movement curve. Another test procedure is the constant rate of penetration (CRP) test in which the load is progressively applied to maintain a constant penetration (or extraction) rate of the pile head measured by the displacement gauges. Other types of test procedures (e.g. the quick maintained load test) can be found in the literature or in national recommendations (e.g. American, German or Belgian recommendations)
Analysis and Reporting
The main result of the SLT is the pile-head load-movement curve (an example is presented in Figure 1). Based on the curve the pile capacity can be defined using several methods e.g. the Davisson Offset Limit, the Hansen criterion, the Chin-Kondner Extrapolation etc. If the pile is instrumented, then the information about the pile shaft and toe resistance can be obtained and its development presented as a function of movement. The results from the SLT performed on the instrumented pile can help to optimize the piled foundation design.
Figure 1. An example of the pile-head load-movement curve
References
Standards:
DS/EN 1997-1:2007 Eurocode 7: Geotechnical design – Part 1: General rules. https://webshop.ds.dk/en/standard?Search=DS/EN%201997-1
ISO 22477-1:2018 Geotechnical investigation and testing — Testing of geotechnical structures — Part 1: Testing of piles: static compression load testing. https://www.iso.org/standard/70807.html
(Hand)books:
Handley B. et al (2006). Handbook on Pile Load Testing. Federation of Piling Specialists. https://www.fps.org.uk/content/uploads/2018/12/06-02-27-load-testing-handbook-2006.pdf
Scientific journal publications about Static Loading Tests:
Fellenius, B. H., & Nguyen, B. N. (2019). Common mistakes in static loading-test procedures and result analyses. Geotechnical Engineering Journal of the SEAGS & AGSSEA, 50(3), 20-31.