PileDriver magazine feature Lloyd Acoustics
The N7 National Primary Route is the main link between Dublin and Limerick. Irelandâ€™s National Development Plan 2000-2006 identified that urgent improvements to this section of road infrastructure were needed. Limerick County Council and North Tipperary County Council, in partnership with the National Roads Authority (NRA), planned to develop a new 38km â€˜high quality dual carriagewayâ€™ between Nenagh and Limerick (Western Limerick).
The 38km project consists of 28km motorway standard cross section, on a green field site, it will traverse two peat bogs (marshes) â€“ at Annaholty and Drominboy â€“ and connect to a 10km section of the existing Nenagh Bypass, which will be widened to dual carriageway standard.
In November 2006 Limerick County Counci
l awarded the contract to design and build the N7 Nenagh to Limerick High Quality Dual Carriageway to Bothar Hibernian N7 JV
. Bothar Hibernian is a consortium comprising contractors Mota-Engil (Portugal), Michael McNamara Company and Coffey Construction Ltd. Mota-Engil Engenharia e Construcao S.A
. is a Portuguese public quoted company with turnover in excess of â‚¬1.3 billion. They have 60 years of engineering and construction experience in 20 countries over 3 continents.Michael McNamara and Company
is a wholly Irish owned company with almost 60 years experience. It has a turnover in excess of â‚¬ 500 million and employs over 500 people placing it in the top three building contractors in Ireland.Coffey Construction Limited
. Coffey Group is also Irish owned and with 33 years experience is one of the market leaders in the Building, Environmental and Civil Engineering Industry, with a turnover in excess â‚¬120 million.Lloyd Acoustics Limited
are an independent pile foundation testing company, providing re-assurance to piling and main contractors of their foundations using modern testing techniques.
In April 2008 Lloyd Acoustics (LA) signed a contract with Bothar Hibernian N7 JV to become the specialist independent pile testing company on the project. The agreement nominated LA to carry out dynamic pile testing, CAPWAP analysis and static load testing on the piles which form the foundation for the new road as it crosses the two peat bogs at Annaholty and Drominboy.
Bother Hibernian, along with their nominated engineers, Hyder Consulting considered various methods for the piling of the two bogs. The use of Continuous Flight Auger, bored cast in situ piles and AuGeo piles was discounted due to the extreamly weak soils. This left the driven pile options, steel piles were deemed too expensive and wood would be susceptable to the organic deterioration of material in the peat soils, therefore pre-cast concrete piles were considered the most appropriate solution. The mechanical method of jointing the piles was chosen, due to tensile and lateral load capacity and cost.
The driving criteria for the hydraulic piling was also designed by the engineers Hyder Consulting, Junttan PM 20 piling rigs were chosen rather than crawler cranes because of their low centre of gravity and ability to pitch 13 and 14 single meter length piles and drive them with the correct, efficient, hydraulic impact hammer.
Hyder also had overall responsibility for the pile design, the design of the temporary piling platform, the spacing of the piles, the load application at each pile, the overall pile driving, review and â€˜setâ€™ at final driving. This was aimed at providing external foundation adjudication.
FK Lowry Piling were the nominated piling contractors, they are an Irish based company with over 30 years experience in the industry, they also own one of the largest pre-cast piling facilities in the country. FK Lowry encountered several obstacles during the contract, not least of which was actually getting equipment and plant to site.
It was decided because of size of the project and number of piles involved, that casting in yards in Northern Ireland and transporting piles to site was too expensive. FK Lowry set up a casting facility to make piles close to site and haul them a few miles to the two bogs. This was not an easy task, piles are made in a very controlled environment and it was difficult to start producing with unfamiliar concrete and contractors unfamiliar with how to work the casting beds. However, this was deemed necessary as a stockpile of 30,000 linear metres of 12m piles
was needed at any one time and the total estimated contract was for 200,000 linear metres
The Junttan piling rigs weighed in excess of 60 tonnes and the temporary platform was so weak that it was necessary to have a distance of 60m between each piling rig. This meant the driving of the piles had to be done in a very organised manner and the program had to be scheduled to accommodate this. Trying to keep the verticality of piles also proved very difficult, the bog was constantly moving due to the vehicles tracking and trying to lift lengths of concrete 30cm square by 12m long and keep them vertical was challenging.
When it came to driving the preliminary piles there were a few unexpected incidents. The bog was so weak that after the initial blows to punch the pile through the temporary stone platform, the pile fell 12m in 2.5 seconds, which was an alarming experience for the rig drivers.
The piles were then jointed and although the estimated lengths were 18-21m, critical preliminary piles showed the drivability was much greater than had been predetermined in the engineers design. A commercial decision was made, that the test piles should be jointed again at 21m and driven until the 'set' criteria was reached, incredibly the piles reached depths of 36m in some areas of the bogs. The piles were now much deeper than the pre-determined contract dictated and the design engineers were adamant that pile capacity must be achieved to avoid a re-design of the piling layout.
At this point Lloyd Acoustics began the testing program. We performed re-strike dynamic testing of the preliminary piles, at a period of time after installation, to establish that the pile was still intact, had itâ€™s integrity and that the joints were not damaged. This test also verified the capacity at the end of drive and that the pile was calibrated to the â€˜setâ€™ criteria. Capacities were then verified by CAPWAP testing, this yielded that the distribution of pile load was a 70/30% split, this being a 70% bottom to a 30% shaft resistance, this on piles as deep as 36m!
Lloyds were also to carry out static load testing; this application is commonly used to verify the applied load to the pile head with hydraulic jacks. The original specification had called for Kentledge load testing, but it had become apparent that this method was not suitable. The footprint of any Kentledge system, regardless of frame size, is so highly concentrated that the pile head movement would be influenced as the support blocks were on unstable ground. It also wasn't possible to guarantee that the platform would hold, considering Kentledge loads of up to 300 tonnes were needed.
LA were asked to come up with an alternative design to statically load the test piles in accordance with ICE procedures (the standard UK code of practice). We recommended the use of sacrificial ground anchors and had to research the market quite thoroughly at this point, as the loadings on these tests needed to be as high as 300 tonnes. This proved difficult as in a self drilled bar application the anchors have several different wall thicknesses and therefore, to relate this directly to capacity, you must have sufficient rock sockets.
At this stage the pre-cast piles had found rock at much deeper levels than expected, so this was deemed as an excellent solution, because at each of the nominated static load test positions, the rock depth would be verified by the installation of the sacrificial anchors. This was accepted by Bothar as being an ideal opportunity to calibrate the piles and recalibrate the rock levels of the site, which originally had been shown at much less depth.
The installation of these anchors was then put out to tender with LA winning the contract. The works were carried out by a specialist drilling contractor, drilling on behalf of and supervised by LA, the selection of piles for the static load tests was done by Hyder. Over the extent of the two bogs we drilled over 7000 linear meters of two types of self drilled bar, the typical lengths of these bars ranged from 29m to 38m. The rock socket on each bar was between 5 and 6m and they were installed in a cement grout application.
To cope with the demand we had full time representation on site and site office was set up, giving our engineers a facility to analyse data and to monitor the static load tests, it also allowed us to oversee the installation of ground anchors.
The static load tests were then carried out using specialist automated beam systems, designed specifically for the bog and able to operate with just two anchors. This, in combination with a high yield anchor bars, contributed to a significant cost saving, as it was originally assumed that four anchors would be used.
Using our automated, hydraulic load testing systems in tandem with our reaction beams was an efficient method of carrying out the static load tests required. Lloyd Acoustics automated load testing system allowed several tests to be monitored remotely by an engineer at the site office, this allows the on site technicians to continue with other testing or preparation for subsequent tests. At the peak of testing we had three load test systems running simultaneously, with test information sent directly to our dedicated website for review. The automated systems apply incremental loading, via hydraulic jacks, directly to the pile head, without the need for manual application. This application is carried by data loggers controlling low to high pressure pumps.
One of the main obstacles we faced was the volume of site traffic passing in heavy earthmoving vehicles, the weight of these machines and the uneven surface of the temporary platform caused a significant amount of vibration across the site. During load tests very sensitive gauges are measuring pile displacement, the vibration created by site traffic was disturbing the frame to which the gauges were attached, therefore leading to poor data quality and false readings. This led us to the decision to perform the static load tests at weekends when there would be no vehicular movement and therefore no vibration. On more productive weekends we were able to carry out seven tests using three systems.
In addition to our remote monitoring, we were able to display data from the testing live on our website. All parties involved in the piling welcomed this technological advantage, as it gave up to the minute data on how the piles were performing. Along with the current data, our website also provided access to an archive service in which all previous tests reports could be downloaded by our clients.
The static load testing began in May 2008 and continued until Christmas, at which point testing was abandoned for the winter. This was due to poor weather and other commercial issues.
Testing recommenced in April 2009 and was fully completed by August 2009. Lloyd Acoustics successfully carried out 28 preliminary static load tests in Annaholty Bog and 28 in Drominboy. These piles were also subjected to 100% dynamic tests and CAPWAP analysis. Lloyds dynamically testing approximately 10% of the overall contract piles, of which 7000 - 8000 were installed, this equated to nearly 800 dynamic tests carried out by Lloyd Acoustics drop hammer, a specifically designed re-strike hammer mounted on a JCB handler for viability and handling. Contract testing also included approximately 185 static pile load tests across both bogs with sacrificial anchors installed at each test location to provide tension reaction.
The first 7km section of the Nenagh to Limerick N7, the area including the new Thurles link road, opened on 17th December 2009. The remaining sections are expected to carry traffic from April 2010. Pat Furlong, BÃ³thar Hibernianâ€™s project manager for the Nenagh-Limerick scheme, has expressed confidence that the new road will open by the revised deadline of April. He said â€˜The vast majority of outstanding works are now rapidly nearing completion and that the project is effectively at its â€œfinishing offâ€ stage. Construction of the road through bog lands near Annaholty and Drominboy proved more difficult than anticipated and attributed to the delay but the problem has been successfully resolved through modern day engineering.â€™