Farnham Mill Nursing Home

The site for this inspirational nursing home project was essentially a marsh, with more flooded areas than land, meaning the bearing capacity of the soil was very low. It was so low, in fact, that everything that was going to be constructed on the site was to be piled; the buildings, the roads, parking areas and even the manholes.

The BIG issues and challenges 1. Getting the piling rig onto site 2. Making massive savings in the large suspended concrete areas

So how do you support a 72-tonne piling rig on a boggy marsh?

A ‘normal’ piling platform design to support the piling rig would never be feasible with the soil subgrade for this site – the rig would simply sink. We knew a different approach was needed. We knew the piling platform had to be able to take loads of up to 200 kN/m2 at the surface of the supporting layer; so we developed a solution where the area was filled with layers of imported material to give a minimum thickness of approximately 700mm of crushed material.

This was made up of; 300mm of 80-40mm no-fines crushed hardcore installed on a geotextile fabric at the bottom; 400mm of 6F2 material; and two layers of Tensar TX 160 triaxial geogrid. This formed a relatively rigid layer of compacted material that allowed any water to pass through it as it was compacted.

We were also concerned about the possibility of ‘punching’ through the granular piling platform layer whilst the rig was in operation, as this is when there are the highest pressures exerted on the support layers.

The solution to this was to use TuffTrack Heavy Duty Road Mats that are mats manufactured from high density polyethylene and are each approximately 3m long x 2.5m wide and 38mm thick. These mats have been tested for pressures in excess of 297 kN/m2, so they would assist in preventing the punching through of the granular support layer.

This ‘rigid’ granular support layer was classed as temporary works and was installed for the installation of the piles only; it was not to be relied upon for any other purpose and not to support any elements on a permanent basis.

This solution proved to be a big success and all the piles were successfully installed as required in a safe and practical way to meet the required specifications and loads

RBA The Instituition of Structural Enginners Trada member ACE