PISA Stay Rod Installation
Once Alexander Mitchell had invented the helical screw pile the development of high torque installation equipment and research by AB Chance ensured a growing confidence in the system. ABC Anchors was established to distribute AB Chance screw pile products for the utility industries and continues to do so.
The Chance power installed screw anchors (PISA) system is installed using the auger drive on Pole Erection Units (PEU) the length and breadth of the United Kingdom. Torque monitoring equipment allows operators to verify that minimum design capacities are achieved with a range of helix configurations to cover soils from very hard to extremely soft. PEU operators have the great advantage of seeing the soil profile as they drill holes using the auger and can then select the optimum helix to hold the load.
Research by AB Chance established the torque to capacity equation – Qu = Kt x T where Qu = uplift (or compression) capacity in kN, Kt = torque factor m-1 and T = torque in kNm. This equation has been proven to be a very reliable predictor of pile/anchor capacity, especially for piles with a shaft diameter less than 150mm. A simple test can establish a site specific value for Kt, generally starting at 30 for small diameter piles and reducing as the shaft gets bigger.
AB Chance has pioneered the use of screw piles both for tension and compression loads, for utility poles and pylons as well. Pylons are often installed in poor soils or even waterlogged ground only accessible once it is frozen. Whereas it is tricky to use concrete in freezing conditions, screw piles work really well and can accept load immediately. In these applications weight saving and consequent reduction in plant mobilisation costs will make projects viable where no alternative exists.