Cyclic testing is commonly used in offshore site investigations. Foundations of offshore structures must be able to withstand wind and wave action and cyclic testing is essential in determining how the ground will support the foundations under repeated dynamic loading. But increasingly, engineers are specifying sophisticated tests such as resonant column, cyclic triaxial and simple shear for onshore projects as well.
Geotechnical contractor Fugro already has experience in this type of testing through its worldwide offshore operations. At the company's UK headquarters in Hemel Hempstead, laboratory technical manager Nick Ramsey says it is not unusual for American engineers to specify cyclic tests for onshore projects because of the need for foundations to be able to withstand earthquake loading and associated phenomena such as soil liquefaction.
And although earthquake damage is very rare in the UK, some structures must be designed to withstand such loading, however low the risk. Fugro recently carried out testing for earthquake loading for a Royal Navy dockyard. But Ramsey adds that the tests are also important when accurate data is needed for soil-structure interaction work, especially small strain effects.
Fugro's recent reorganisation of its Hemel Hempstead laboratory facilities aims to meet the 'significant' increase in demand for this sort of testing. The advanced laboratory boasts some of the most sophisticated testing equipment in the country. For a commercial operation 'this is a unique facility in the UK', says Ramsey.
He estimates that £1M has been invested in the advanced laboratory, including new resonant column and simple shear test rigs. While the firm does carry out conventional testing, it is not set up to undertake it on the scale typically required by engineers. 'We are not competing with other contractors because we cannot handle the bulk work,' Ramsey explains. In fact, much of the advanced laboratory work is carried out for other site investigation companies like Soil Mechanics and Foundation and Exploration Services. Ramsey says that Fugro offers a full range of testing, analysis and advice, with a 50/50 split between in-house and external clients.
Much of the advanced testing is tailored specifically to client needs, which means that while the rest of the laboratory is Namas accreditation, the cyclic tests are not. Any changes have to be agreed with the client, says Ramsey. 'We discuss what the client needs to get from the data. It is a more involved process than conventional testing.'
There is also generally no set time frame for the advanced tests. 'It is more flexible,' says Ramsey, 'schedules are often not specified.' Part of the reason is that testing takes a lot longer than conventional tests. For example, a resonant column test can take up to two weeks.
Obviously, this makes such testing expensive. Ramsey concedes that the costs are high compared to conventional testing but says the large amounts of high quality data obtained justifies the financial outlay.
Continual tailoring of the tests means that cyclic testing equipment is constantly advancing, says Ramsey. This has been matched (and helped) by the rapid development of sophisticated and affordable computing power.
One major advance has been the switch from pneumatic to hydraulic systems. Ramsey explains that hydraulic systems are easier to control and give a more accurate reflection of wave forms. Wind and wave loading usually has a frequency around 1Hz but other tests need frequencies of around 10Hz to 20Hz, which is much easier with an hydraulic system.
On the computing side Fugro has developed its own in-house testing software, that records data and allows technicians and engineers instant access to the test under way and also real time data. Monitoring of most of the tests is now automatic and although the computers do not control the testing, Ramsey says that it is not far off.
The next stage, soon to be implemented, is to give the computers more control so that work can be done overnight and at weekends. Engineers will be able to log on through the company's intranet from remote stations and the computer will be able to contact the engineers, by pager for example, if anything goes wrong.
The downside is that the specialised nature of testing means workload is not steady. Ramsey says that when the laboratory is very busy it is sometimes difficult to achieve reliable turnaround time. Work can be farmed out to Fugro's other laboratories in Houston and the Netherlands, 'but there are obvious cost implications'. Houston, he says, is probably the closest to the Hemel Hempstead laboratory in terms of capability.
The company is hoping to achieve 'University quality in a commercial environment', says Ramsey.