A small Yorkshire practice has developed a new static foundation load testing system for restricted access sites.
Terrain Geotechnical Consultants, a small Yorkshire-based practice of geotechnical and structural engineers, rose to a challenge outside its normal expertise last autumn when it was asked to load test piles - and came up with a new system for doing so.
'Our market has traditionally been the subcontract design of foundation systems, typically for piling subcontractors, ' explains Terrain managing director Stuart Bradshaw.
'Branlow Piling Solutions asked if we wanted to be involved with the load testing of its piles. The difficulty for us, being primarily office based, was gearing up for this type of work, as we don't have suitable premises, lifting equipment or vehicles that could cater for the handling of the large steel testing beams that are normally required.
'Not wanting to decline, but with an eye on costs, we took on the challenge of coming up with a system we could handle using manpower and a light vehicle. Our own logistical limitations meant the steel sections had to be small enough to be manhandled, with the obvious corollary being that the beams could be installed in areas with restricted access or low headroom.' Piles installed where there is dif' cult access can usually only be tested using dynamic methods because of the size of the static load testing equipment.
The Terrain system uses beams that are universal column sections, suitably stiffened, which bolt together on site. Each piece is small enough to be slid into position using a sack trolley, Bradshaw says.
'At each pile position the beams are slid off the sack trolley onto timber baulks and bolted together.
Only the individual sections require manhandling, which can be easily undertaken by two men.' The result is a set of two parallel test beams that cater for a maximum test load of 1200kN over a span of 1.8m.
'The beams are ideally designed for piles of 350mm diameter and smaller, however, the system is scalable for larger loads by installing one or two other parallel beams, ' Bradshaw explains.
'For the majority of mini piles, 1200kN is 1.5 times a safe working load of 800kN, [which is] a fairly typical maximum load for piles of up to 350 mm diameter.' The system has been used successfully on two Branlow contracts so far. The first involved testing a pile to 1200kN in a basement with no crane access. Two adjacent working piles were used as anchors, with the beam sections manoeuvred into position after being lowered into the basement in an excavator bucket.
The second contract, for redevelopment of a former British Aerospace site at South Gyle in Edinburgh, involved testing the original pad foundation to 1000kN. The plan was to reuse some of the pad foundation for the new development.
Engineer for the scheme, PWP Consulting, specified a loading test to four times working load.
The secondary beams were modified slightly to accommodate four anchor piles - in this case self-drilling thread bars, each comprising a Dywidag R38N MAI bar with a 76mm sacrificial drill bit installed 9m into the underlying coal measures which were described as 'broken black shale'.
Although there were no specific access restrictions at this site it did present an excellent opportunity to test the system, Bradshaw says.
'The combination of the use of the modular beam with the self-drillers is seen as advantageous for carrying out a static load test in low headroom at relatively low cost, ' he explains.
'Each pile was designed to take 250kN. On completion of the compression test on the pad foundation we did a pullout test to 250kN on one of the anchors and all of the extension measured was elastic.' He says the combination of the modular test beam arrangement with self-drilling anchors enables static load tests to be carried out almost anywhere.
'Additionally, the use of the self driller reduces the span of the beam needed, as normally anchor piles are spaced three pile diameters from the test pile. Potentially, the maximum test load could therefore be increased to more than 1200kN without the need for additional beams.' Terrain has applied for a patent for its test beam.
Bradshaw says the system could reduce the specified factor of safety for piles installed in basements or on other sites with low headroom or restricted access and therefore reduce pile lengths. It will also allow validation of piles using a maintained or continuous rate of penetration test, which can only be carried out by applying a static load.
'With the push towards sustainability and reuse of foundations, we see the technique as being of benefit for redevelopment of buildings where facades are to be retained and in basements, next to railway tracks or in retail premises that are being modified with new mezzanine floors, for example.'