Dynamic pile load testing is a technique used to assess the load carrying characteristics of foundation piles. Martyn Ellis, operations director, Infrastructure Services, ESG discusses the growth of foundation piling in the UK and the methodology behind the tests.
Britain is constructing record numbers of tall structures, with well over 100 towers of 20 stories or more planned for London alone in the coming years. Further to this, construction started in late 2016 on the world’s biggest wind farm, which is set to be built at Hornsea, off the Yorkshire coast; 174 turbines, each 190m in height, will be erected in the first phase of the project alone.
These types of structures are built on deep foundations, and an essential characteristic of this method of construction is the presence of foundation piles. Piles are driven or bored deep into the ground at the construction site to facilitate the transfer of the lateral load of the structure to the earth, firmly anchoring it in the process. Piles therefore allow for bigger and taller structures to be built, but it is of the highest importance that their load bearing capabilities are thoroughly tested to ensure that they provide adequate levels of support.
Bearing the load
Dynamic pile load testing is a technique employed to assess the bearing capacity or load carrying characteristics of a single installed foundation pile. The use of piles is not restricted to tall buildings, and because of this, any building with piles as part of its foundation is subject to testing. These can include apartment complexes, high rise developments, industrial plants, large retail outlets, and transport infrastructure such as bridges. All forms of piles can be tested, but the two most commonly deployed in the UK are driven piles and cast in situ bored piles.
The tests are carried out by attaching extremely sensitive electronic measuring equipment to the pile wall and striking the pile head with a heavy falling weight – usually the piling installation hammer. The generated stress wave is then recorded using a battery operated Pile Driving Analyzer (PDA) from Pile Dynamics Inc.
From this data, it is possible to work out the soil resistance characteristics and integrity of the installed pile by using one-dimensional wave mechanics formulas. The resulting data can be further analysed with the CAPWAP method to better understand soil behaviour and to plot a load versus deflection curve, which can be used to see if the pile is suitable to bare its intended load.
The most common scenario for using this technique is with driven piles of differing varieties, such as precast concrete, tubular steel and steel “H” piles. Pre-formed driven piles of this nature have set dimensions and are made from a uniform material with known parameters. This makes it easier to perform a successful analysis of the recorded stress wave data. Many piles may be assessed during a one-day site visit, while for larger projects a number of tests can be carried out over a longer period of time. Piles can be tested as they are driven, with clients benefitting from real time analysis so further driving can be carried out if required.
For offshore and coastal projects, such as the construction of wind turbines, oil platforms and jetties, ESG commonly records data during the whole of the pile installation process. The data captured can help clients better understand the seabed conditions and the efficiency of the drive system used for the installation of the piles. Analysis of this data can help find a more efficient piling method, resulting in significant savings in terms of both cost and time. An offshore pile may be reassessed up to seven days after the original installation, once the water pressure on the soil has reached its equilibrium state, giving a more accurate long-term reflection of its load bearing capabilities.
Testing bored concrete piles requires more forward preparation and planning than driven piles. Firstly, the client will need to nominate the piles on the site that need to be investigated. This is to allow the piling crew to form a pile cap above ground using the same pile material and dimensions of the main body of the pile. As these piles are bored and subsequently cast from concrete, as opposed to being driven, there is unlikely to be suitable equipment on site to create the impact necessary to create a stress wave, meaning machinery will need to be brought on site for this purpose.
ESG has two different hydraulically operated systems primarily used for testing bored piles, one with a 1.8 tonne drop weight and the other a larger, more robust guide frame with the capability of a 5 tonne or 8 tonne drop weight. The smaller system has recently been commissioned, and has the advantage of being easily transported, allowing a greater number of piles to be tested in a shorter space of time.
ESG is one of the few providers of dynamic pile load testing in the UK with full UKAS accreditation (No. 0001), and has a wealth of experience in the application of stress wave techniques. Our team has been involved a number of ground breaking schemes, including the PISA project in collaboration with DONG Energy and Oxford University. Get in touch to find out how ESG's team can help you with deep foundation testing.