It bought the hydraulically operated rig, with its extensive mast manoeuvrability, to modernise and replace its 30-year-old crane-mounted leader rigs.
There were some initial concerns that the new 60t rig would be too large, but these have proved unfounded following the completion of a variety of different contracts.
"We were initially sceptical and thought the PM23LC might prove to be too big for our type of work," says Stent Foundations operations manager Malcolm O'Sullivan. "But over the past 10 months the rig has shown to be extremely versatile and far better than expected."
The company used the rig to install 872 precast concrete piles while working for Barnwood Construction on a warehouse development in Avonmouth for client St. Modwen Properties. It installed roughly equal numbers of 235mm2 and 275mm2 pointed toe piles, all 15m long and jointed in two equal length sections, at a rate of about 40 per day. Rig operators drove the piles through made ground and soft alluvium to penetrate 2m into underlying mudstone.
Stent's standard PM23LC rig, fitted with a 4t Junttan HHK 4A impact hammer, is capable of pitching and driving piles up to a maximum single length of 15m, and can be ready for work within a short time of arriving on site.
Despite the rig's complex leader mast geometry, rigging and derigging can be achieved in less than an hour. This includes the time it takes to install the hammer, which is carried on and under the mast during transportation. The long undercarriage and extendable tracks, coupled with a low centre of gravity, make the rig very stable.
The mast is extremely manoeuvrable, which allows the rig to work on level ground as well as on embankments and in shallow cuttings. In addition to the standard side-to-side and backward and forward rake, the leader mast can be moved 4.7m above, and 4.3m below, ground level.
The leader can also be positioned up to 6.65m in front of the tracks. This reach and overall working envelope allows easy and fast spotting of the piles during pitching, without having to constantly move the rig.
"The PM23LC is more sophisticated and versatile than any other piling rig in the UK. To achieve that kind of reach you would have to revert to using a crane-mounted rig with hanging leaders," says O'Sullivan.
"We have made use of this feature on projects where we have stood the rig on top of an embankment and driven piles into the slope below, and, similarly, in a cutting to drive piles into level ground above. It is very versatile and can handle all our precast concrete pile sections from 187mm2 up to 400mm2, as well as steel tubes up to 760mm in diameter, steel sheets, steel bearing piles and timber piles."
The HHK 4A hammer has a single piece drop weight that can also operate a 5t impact hammer by adding a 1t block to the drop weight. The impact hammer, operated from the host rig's hydraulic system, accelerates the drop weight during its fall. Stent claims this boosts the impact energy and increases the efficiency at full stroke over a conventional free-fall drop hammer of the same weight, by up to 20%.
"The hammer we are using on this rig is another key to the machine's overall success," adds Stent works manager Andrew Parker. "We can adjust blow rate up to 100blows/min and drop height from 50mm up to 1.2m. During independent monitoring it has shown to achieve impact energy 25% above the theoretical maximum."
The 4A hammer produces maximum impact energy of 47kNm at full drop height of 1.2m. Impact energy, drop height and blow rate can be adjusted by the operator to suit ground conditions and pile type. All relevant data is displayed on the rig's computer screen in the operator's cab. One of 11 hammers in the new generation Junttan A series, it is suitable for driving all types of piles, such as steel tubes, H sections, steel sheets, precast concrete and timber piles.
Finally, the rig's hydraulic system is filled with biodegradable oil. This means that with its reach, it can be used to work near water, in environmentally sensitive areas.