MHB was recently commissioned by BAM Nuttall to carry out the bulk earthworks design for the new 275 kV Alyth Substation in Perthshire which is being developed by SSEN Transmission.

The works involve raising the site levels to form a new “platform” to accommodate switchgear and buildings for the substation. The area is underlain by up to 25m of soft soils and the main technical issue was to determine the extent and rate of settlement of all parts of the finished site. Any areas where settlement was likely to be excessive was required to be built on piled foundations or alternative ground treatment methods were to be used.

A comprehensive site investigation had been carried out by SSEN Transmission and this was supplemented by the BAM Nuttall/MHB team to incorporate a trial embankment to measure the actual site behaviour under load. The trial embankment was 50m by 30m in plan size and 3m in height. The trial included remote settlement monitoring equipment and groundwater pressure monitoring to give real time data to the design team as the embankment settlement took place.

The results of this trial demonstrated that settlement took place more quickly than anticipated and allowed us to remove the requirement for ground treatment and minimize the number of piled foundations on the site. This resulted in savings in construction cost as well as programme.

As part of the validation of the works, extensive settlement monitoring was put in place over the full site area to demonstrate that the client’s settlement requirements had been met.

Other MHB services on the project included advice on earthworks acceptability, detailed topographical surveys and drainage design incorporating SUDS systems to minimise runoff from the site.