By Frederick Levy
I am an Engineering Doctorate researcher at the University of Southampton (www.southampton.ac.uk/idtc). I also work at National Grid and as part of my postgraduate doctorate studies, I have been working on the lattice pylon foundations. Over a period of six months, I have dedicated some time to the T-pylon development process. I’ve focused principally on the conceptual foundation option development for the structure, while aiding other members of the development team on civil engineering related areas.
The T-pylon foundation is one of the most critical structural components of the pylon. The design of the T-pylon means an end to the traditional pad/pyramid and chimney foundations used for lattice pylons. Any overhead line foundation solution developed must be robust enough to weather the 80- year design life of the T-pylon, yet be capable of quick installation with varying degrees of access, and minimise land-take post-construction.
Pylons are designed to withstand varying magnitudes of wind and ice, environmental loads (actions), and security load cases, arising from potential wire (phase) breakage. The resultant loads try to overturn the pylon since the actions are perpendicular to the structure. For lattice pylon foundations, this overturning moment translates into large vertical loads in uplift or compression with a small horizontal component due to the arrangement of the pylon legs. Conversely, with the T-pylon monopole, the single point of contact with the ground means that any foundation solution must directly accommodate the large overturning moment, which is in excess of both the horizontal or vertical loads.
The foundations of wind turbines also experience large overturning moments, arising from wave and wind actions. A monopile solution is often used for wind turbines in shallow waters, up to depths of 30m. A monopile foundation is essentially a hollow steel tubular section which is hammered into the ground, often providing a full strength foundation solution immediately post-installation. Applications of this foundation type include the London Wind Turbine Array, where 175 were installed, and more recently, for the Kasso-Tjele electricity transmission overhead line in Denmark, where over 400 were installed for the Eagle pylon, a design by Bystrup (www.bystrup.dk).
The monopile is the preferred foundation solution for the T-pylon, but not all ground conditions in the UK will be suitable for it. The ultimate capacity of a monopile designed to resist lateral loads is assessed using Brinch-Hansen’s (www.geoteknisk.dk/media/4903/geo.dgi.bulletin.no.12.pdf) static equilibrium approach, supported with more refined analysis conducted using the subgrade reaction method (p-y method) and other numerical analysis tools (finite element modelling). Preliminary designs of the T-pylon monopiles indicate that they may have design lengths that are up to half the height of the T-pylon itself, with lengths of 18m, compared to a 35m pylon height), diameters of approximately 2m and thicknesses of roughly 30mm. The monopile dimensions are constrained between length (L) and diameter (D) ratios between L/D<4, where the foundation ceases to act like an elastic beam in the p-y method at and L/D>10, where the monopile becomes prone to buckling.
For installation, a steel monopile is delivered to the chosen site where a large crane uprights and places it in the driving position on the ground’s surface. Traditionally, a scaffolding assembly has been used to support the pile laterally. Aarsleff DK (www.aarsleff.dk) developed a ‘pile gripper’, which can be mounted on a JCB bulldozer performing the same task as the scaffolding, but significantly reduce setup time while improving safety. While the pile gripper holds the monopile in place, the large crane lifts the driving hammer onto the top of the monopile. The hammer then drives the monopile into the soil with a series of blows from a hydraulically accelerated drop weight. The hammer can achieve full foundation installation in under an hour, depending on ground conditions,, with reported horizontal tolerances of +/- 20mm/m with a total of +/- 50mm. Using this methodology, one can set-up and install the monopole in a single day.
A key component of the monopile foundation is its interface with the monopole of the T-pylon. The T-pylon’s foundation and super-structure are connected together by bolting together two flanges: one on the base of the T-pylon monopole and one on the top of the monopile. The flange on the bottom of the monopole is relatively simplistic to fabricate compared to the monopile flange, and can be fabricated during manufacture offsite. The additional complexity of the monopile flange arises due to the high stresses the monopile experiences during driving, resulting in damage to the steel at the top of the hollow section.(‘embrittlement’).
During the T-pylon foundation development process, we investigated several options for attaching the flange to the monopole. It is possible to ram a flange, however expensive modifications to the pile hammer are required, in addition to thickening the steel at the top of the monopile. This solution also does not allow for verticality corrections, unlike a grouted or welded solution. The grouted solution uses a cement based grout to create a stiff interface between shear keys on the monopile and an extended flange section. We can ensure verticality by manipulating the extended flange section before the cement grout has fully cured. The final and preferred solution is to weld the flange directly to the monopile after having removed the brittle section, and in the process, creating a horizontal plane suitable for verticality tolerances. A fully designed welding procedure (‘qualified weld’), in conjunction with non-destructive testing onsite ,would ensure that the resulting connection is fit for purpose. We would fabricate the weld using an automated device attached to the head of the monopile. A weld inspector onsite would then conduct non destructive testing to ensure we had produced an accurate and uniform weld.
Using the welded solution, total installation time including set-up and post weld heat treatment, of a full strength monopile foundation is approximately two to three days.. This compares favourably to the 28 days it would take for the grouted solution to reach the requisite strength for T-pylon erection.
Throughout the T-pylon development process, we considered the environmental impact and sustainability of different options, and did not exempt the foundations solutions. Because the monopile mobilises soil strength to resist loads, the surface area of land the monopole takes up is significantly less than the alternative pile group solutions (see below). The environmental impact (e.g. noise, possible site contamination and temporary works) of construction with a monopile is also mitigated because of the short installation time and associated low volumes of traffic. And, the steel of the monopile can contain up to 15% recycled material. Finally, the preferred foundation solution with a welded flange would yield a cement free option.
Other foundation types
Using a desk study of borehole logs, we looked at a proposed overhead line route. Our research showed that the monopile could be deployed at 115 locations on a route with 141 pylons. The remaining pylon locations were in stiffer ground, increasing the risk that a monopile could not be driven into the ground,(rejection), or where the ground was weak, requiring excessive monopile lengths. Numerical software would allow us to perform a monopile driveability analysis in advance of installation, reducing the likelihood of rejection onsite. In cases where the ground is unsuitable for a monopile, alternative solutions must be sought. National Grid has developed several alternatives in conjunction with LS Transmission Consultancy (www.lstc.co.uk).
The alternative foundation designs involve the use of a reinforced concrete cap with an array of bored minipiles or driven piles. Bored minipiles involve the augering of small (approximately 300mm) diameter holes, backfilled with concrete with a single steel bar through the centre. A driven pile solution can take the form of driven precast reinforced concrete piles or a series of steel casings with shear keys. The driven piles of both foundation types are tied to a reinforced concrete cap, which is a time consuming process.
Although not as conspicuous as the T-pylon superstructure, its foundation is nevertheless an important feature. We considered different foundation options using the same criteria as the main structure — namely that any preferred solution must be sustainable, cost efficient, and capable of coping with the demands of the overhead line network. As a result of the process, we envision installing a monopile where possible to support the T-pylon. The short installation time, thanks to associated efficiencies (e.g. low concrete usage and rapid installation), is the primary motivation behind this preference. Other foundation solutions are available to support the monopile solution for T-pylon where ground conditions are not suitable.