Tillet Court is a 3 storey L shaped block of flats of which one wing consisting of 6 units required underpinning.

The underpinning of this section was required due to it being constructed on made ground varying in depth from approximately 3.0Lm to 7.0Lm across the depth of the building, below which lay chalk.

The initial intention was to install a reinforced concrete raft needled into and under all affected external and internal walls supported on precast concrete piles. These were to be installed without causing any vibration due to the risk of further damage to the structure. As this property was in a residential development and the upper two floors of the property were to remain occupied careful consideration was given to ensure the work, including deliveries, was undertaken to ensure minimal disruption to all parties

Following further site investigation, which was carried out by ourselves, it was decided to change the method of piling to Hollow Stem augered piles due to the presence of hard bands in the fill which could prevent the precast pile reaching the chalk founding layer.

The installation of the piles commenced with the initial piles founding in the chalk subsoil as designed. However, as installation continued towards the rear of the block a void was encountered which was initially suspected to be a chalk working. This was confirmed when we lowered a mobile camera down the pile bore.

In order to be able to progress the work it was necessary to survey the working and determine its full extent. This was undertaken using GPS Survey equipment capable of plotting the full extent of the void and relating it back to the existing structure. Following further research, we determined the void extended beyond the area surveyed. This had not been picked up as this further section of the working had collapsed. This section still extended below the building.

Following the discovery of this working it was necessary for the original scheme had to be redesigned. We were confident the base of the working was on undisturbed chalk due to its nature and depth. The redesign was carried out following significant input by ourselves in order to very little delay to our original programme. All variations were costed and agreed prior to being carried out again incurring minimal delay.

We proposed to fill the void with foam concrete, following this the piles would be installed and toed into the top of it ensuring the pile grout filled any remaining void .

The further section of working which had collapsed was treated as fill and ignored in the design of the piles which were then installed into the underlying chalk a sufficient depth to carry the imposed load on them.

Following the installation of the piles the reinforced concrete raft was constructed as our original design with some minor amendments to accommodate the repositioned piles.