Foundations of success
Martyn Ellis explores the latest developments in foundation testing, and explains what construction professionals need to know to ensure their buildings comply with guidelines and are capable of standing the test of time
The UK construction industry is enjoying a tall structure building boom at the moment. In London alone, more than 100 towers of 20 storeys or more are expected to be built over the next few years. While cities such as Manchester, Liverpool and Newcastle all also have multiple skyscraper projects under way. In addition, construction started just last year on the world’s largest offshore windfarm near Hornsea, East Yorkshire. More than 174 turbines, each 190m in height, will be built in the first phase of the project alone.
What all these projects have in common is a requirement for deep foundations and piles to transfer the forces of the structure to the strata and firmly anchor it in place. However, it is not enough to simply bore and construct these foundations – there is always the possibility of a geotechnical design failure or structural failure. There could be inclusions in the concrete from a partially collapsed bore wall, or fractures induced by ground heave coupled with unusual occurrences or delays during concreting. There is even the possibility of mechanically induced flaws from excavations and plant movements across the site. All of these can affect the ability of the foundations to withstand their anticipated loads.
With this in mind, it is crucial that an appropriate level of testing is carried out on all foundations on site. This is a necessity both to pass Building Control inspections, and to ensure the eventual buyers of a development are able to obtain warranties on the build quality of their new property. It is vital that these quality assurance procedures are undertaken in line with the latest guidance, and with the most advanced techniques, to ensure the foundations are fit for purpose, and also to demonstrate due diligence to future property owners.
Earlier this year, the Institute of Civil Engineers (ICE) introduced the third edition of its Specification for Piling and Embedded Retaining – the standard in the UK for highquality piling. The guidance is an evolution of the previous edition, but has been updated to reflect evolving EU and UK legislation, as well as changes in technology.
The principal difference in the updated specification that engineers need to be aware of is the requirement for maintained load testing to be carried out automatically, rather than manually, from a remote location with online access to ‘live’ data. This is crucial to ensure optimum accuracy, and minimise the risk of errors affecting the results.
Engaging foundation testing specialists can help engineers ensure that their quality assurance processes are as accurate as possible. There are UKAS accredited foundation testing teams available now, ESG being the only team that hold accreditation for Maintained Load Testing, that can help provide high-precision measuring, and ensure compliance with the latest ICE guidelines.
Under the updated ICE guidelines, it is still the case that, when carrying out quality assurance on deep foundations, the most appropriate technique must be chosen. One such method that is being widely adopted in the UK as a means of meeting this requirement is a technique known as Thermal Integrity Profiling (TIP).
This method uses the heat generated by curing cement (hydration energy) to assess the quality of cast-in-place concrete, such as bored piles, diaphragm walls, or barrettes. The thermal profile of the material as it cures can be used to evaluate the homogeneity and integrity of the pile within the deep foundation element.
When implementing TIP on a site, it is important for the location plan to provide a cable for every 0.25m to 0.35m of diameter. Ideally, an even number of cables should be used, containing a thermal sensor every 0.3m to aid analysis of questionable areas and evaluate cage eccentricity.
The expected temperature at any point on a pile is dependent on the diameter of the shaft, mix design, the time of measurement and the distance from the centre of the shaft. With all this in mind, TIP measurements can be used to estimate the actual shape of the pile, and compared with concreting logs to assess the overall quality of the shaft. Data is acquired by thermal sensor cables tied to the rebar cage, installed prior to or soon after concreting.
TIP measurements that are colder than normal at a particular point on the shaft indicate necks, inclusions, or poor quality concrete, while warmer than normal measurements are indicative of bulges. Variations of temperatures between diagonally opposite pairs of cables reveal cage eccentricity.
Another advantage of using TIP to measure pile quality for engineers is the speed with which results can be available after placement. It also eliminates concerns regarding the connection of sonic tubes used in the crosshole logging method.
Laying the foundations
With the publication of the new edition of the ICE guidelines, engineers need to ensure not only that the pile designs they use meet best practice guidance, but that the testing techniques they employ are fully compliant.
Working with UKAS accredited foundation testing experts can go a long way towards achieving this goal. Such consultants have a comprehensive understanding of the latest standards, as well as innovative new analytical practices, which can help engineers ensure that the piles they install are of the highest possible standard. In doing so, they can ensure the foundations offer the stability required to guarantee the long-term performance of the structures they are supporting.
Martyn Ellis is operations director, Infrastructure Services at ESG. ESG is the UK’s leading provider of testing, inspection and compliance services, offering comprehensive solutions for the Infrastructure, Built Environment and Energy & Waste sectors. The company, which employs more than 1400 people, delivers in excess of seven million tests a year to over 5000 customers.
ESG is committed to continuous improvement and the development of new, innovative methodologies, processes and products. The business invests heavily in both employees and the latest technology to ensure it remains at the forefront of innovation
For more information, please see www.esg.co.uk