Rare information has emerged about the work that’s now being carried out to strengthen the Wellington Town Hall.
The Lattey Group, which is installing new piles under the floor of the 110-year-old Town Hall, has published four pictures of the work.
— Lattey Group (@LatteyGroup) July 10, 2020 
The screw piles will be resting on base isolaters – the same system, the company notes, that was used at Te Papa.
The Lattey Group’s website  describes the screw piling system in considerable detail:
Screw piles are one of the deep foundation systems used by Lattey Group over a wide range of building projects. Due to their design, screw piles are quick to install and produce a minimal amount of noise and disturbance. This makes them a pile foundation system especially well suited to urban building sites in New Zealand.
Screw piling technology offers some clear advantages over other foundation techniques. It produces no spoil, which means that there is no debris to remove from the site. This is a significant factor in cost reduction as well as an enormous time saver.
Compared to other foundation systems, screw piling is also very quiet, only producing 85Db of noise, which is equivalent to using a handsaw or sitting in a busy restaurant. Since screw piling requires a minimal impact and produces very little vibration, there is no unhappy neighbours or councils to worry about. It is ideal for seismic strengthening of existing buildings with low headroom, which is often the case in dense urban environment. The result will be an increased NBS ratings.
Other advantages of screw piling include speed of installation and reduced cost in comparison to conventional driven or bored piles. Lattey can provide pile capacities in excess of 250 tons in compression and tension to depths of up to 40 metres.
Screw piles are made of round hollow steel and equipped with helices. They are installed by wounding them into the ground, pretty much like you would a screw into a piece of wood – hence their name. Lattey Group works with engineers to design and build their screw piles so that they fit ground conditions exactly and perform optimally throughout the building’s lifetime.
Screw piles work by dispersing load, a property that makes them versatile and well-suited to a wide range of terrains, including earthquake zones and areas which are at risk of liquefaction.
Once the screw piles have been installed to the appropriate depth, they are filled with concrete, thereby increasing their load-bearing capacity.
Screw piling requires specialised equipment and skills and cannot be performed properly by just any company. Over the years, Lattey has invested in the right sort of plant and developed a specialised skillset as well as very close working relationships in order to provide excellence in screw pile design and installation in New Zealand.
Geotechnical and structural engineering input is required from the onset of a screw piling design project or any other pile foundation system, in order to get optimal results for the building. Prior to installation, many factors have to be taken into account like site plan, access limitations, terrain and soil specific requirements and properties, neighbouring sites, etc.
Lattey regularly successfully design and instal screw piling as an effective deep foundation system for our building projects.
Lattey installed screw piling foundations on a massive scale at the Sebel Hotel in Lower Hutt where 30 screw piles a day were completed down to a 7-meter depth. The entire project consisted of 190 screw piles. As it offered a significant reduction in cost, the choice of screw piling on this high-profile site allowed the much awaited project to go ahead after many months of delay.
Screw Piling was also successfully used on the Wakefield Hospital in Wellington. The screw piling on this site was tested to 260 tons in July 2019. 80 Screw piles in total were installed to about 12m depth over a ten-day period late August, with pile demands varying widely. An interesting aspect of this particular foundation system is that it rests on base sheer isolators, a series of rubber blocks designed to extend and move up to 600mm in the event of an earthquake. This is a similar system that has been used and can be observed under the Te Papa Museum.
Installing Screw Piles inside the Queensgate Shopping Mall in Lower Hutt. pic.twitter.com/y4XlxCMyoI 
— Lattey Group (@LatteyGroup) September 12, 2019