Waiting for the hammer to fall: load cells and pile load testing
Pile driving is an intrinsic part of many major construction projects, so it is important that they are up to the job. Testing these piles to ensure they can take the load and forces required, both at THE start of a project, and ongoing as required. As CMWGeosciences say:
“Testing of piles is routinely carried out in order to verify that the design and construction of the piles are in line with expectations or to assist in the design of piles ahead of the main piling construction works.”
Load cells are there to measure the force applied, safely relay the data, and help engineers ensure that the piles meet national standards and the contract specifications.
Types of pile testing
Types of pile testing include:
Static load testing
A pile is tested by adding load onto it at specific increments, with the load applied measured by a load cell until the pile fails or reaches its required capacity.
Dynamic load testing
The pile is loaded over a much shorter time frame using a dynamic load application such as a pile driving hammer (usually the one used to drive the piles in the first place).
Osterberg cell (O-ring) testing
In this test, the load cell is installed in the pile itself, at a pre-set position. In effect, the pile becomes both the tested section and the gauge for its reaction to the load
Rapid load testing aka hybrid testing
The load is only applied for a very short time, around 100 milliseconds.
Lateral load tests may also need to be done to the side of piles, which is usually performed using a hydraulic jack fitted with a load cell. Pull out tests may also need to be performed. Testing may also be involved in checking the suitability of the ground for piles, which involves initial load testing up to 2.5 times the safe carrying capacity of the pile.
Taking your time
One of the main issues of static pile testing is that it is very time-consuming and labour-intensive, with extra loads physically added at every increment. Another issue is that the piles may not be easily accessible (at least, not individually). Plus, pile driving and testing often requires a gap of a couple of days to let things settle before testing can start.
Sacrificial elements
In O-ring testing, the O-cell itself is ’sacrificed’ during the process, as it is attached to the reinforcing steel cage or other supporting structure. According to LoadTest:
“As the load is applied to the O-cell, it begins working in two directions; upward against upper skin friction and downward against base resistance and lower frictional capacity (if applicable), the O-cell test requires no kentledge, reaction beam or anchor piles.”
In 2020, a testing specialist company brought out a solution that solved two issues at once; a bidirectional pile testing method that measured both the force at the end of the shaft, and the shaft itself. This consist of:
“An extendable element within the structure that allows for separate movement in both the shaft and the base, providing individual data on the end bearing and shaft resistance and fully mobilising the base.”
There is an added bonus in that this new method of testing can be used as soon as the concrete has reached the required strength. It also ensures consistency of testing data across all piles on site including barettes, bored piles, preformed steel and pre-cast concrete. As the developers say:
“The load cell is incorporated within the steel reinforcement cage off site; when used in conjunction with thermal integrity profiling (TIP), it can be loaded as soon as the concrete has reached the required strength. In addition, all loading takes place underground, eliminating the need for energy to be stored in loaded beams and thread bars, which takes time and space on site.”
Need specialist load cell advice? Call us
If you’ve got an innovative solution to a construction or other testing situation involving load cells, we can help. Contact us to discuss your specific requirements, and our team can help design and manufacture load cells to your exact specifications, right here in the UK.
