The Importance of an Initial Desk Top Study

15 December 2019

The importance of an initial Desk Top Study and Preliminary Risk Assessment was highlighted during a recent site investigation project, where Ridge was commissioned to evaluate any risks associated with the potential acquisition of a rural site for development.

For the purposes of a Stage 1: Risk Assessment, a desk study and site walkover should be undertaken. Following the collation of available desk study information, specifically historical maps, there was evidence to suggest a small water body or pond, associated with agricultural purposes, was located on the site.

Nevertheless, it was clear from the site walkover that this pond was no longer present, it was suggested that it had likely been infilled. There was little evidence, other than the historical maps, to suggest a pond had ever been within this area of the site and therefore discussions were had with the landowner and the Local Authority to determine how this pond was infilled; and more importantly, with what? Unfortunately, neither of these parties were aware of a former pond.

Following the production of a Preliminary Conceptual Site Model (CSM) and Contaminated Land Risk Assessment it was determined that a Tier 2: Generic Quantitative Risk Assessment (GQRA) or ‘Site Investigation’ would be required to refine the contaminated land risks. The pertinent risk being the unknown composition of the infill material.

The Site Investigation involved targeting the former pond with machine excavated trial pits to determine the composition and extent of the infill material. To the surprise of the Engineer and Technicians on site the arisings from the location of the former pond were returned as scrap metal, rusted boilers, glass, bricks, damaged fire extinguishers, ceramics and concrete (generally Made Ground). Unfortunately, the exact vertical extent of the fill could not be determined as the trial pits flooded. However, we were able to use a dynamic sampling rig (or windowless sampling rig) to drill through this material into the underlying natural soils – at no extra cost to the client, as the rig was already on site. The infilled Made Ground extended throughout the former pond to a depth ranging 3.00-5.00m.

The Geo-Environmental Engineer obtained several different soil samples of the Made Ground/ fill material, which were then submitted to an environmental laboratory for analysis. This material was identified as Hazardous owing to the presence of asbestos and other contaminants, including zinc. The client was updated throughout the works to inform them on progress and highlight any snagging issues for planning and construction programme.

It was decided by the Client, with input from the Engineer, not to proceed with the purchase of the site owing to the complications involved with removing the Made Ground as well as engineering a foundation solution to account for this.

The Ridge Geo-Environmental team also offer a range of comprehensive surveys, including winter groundwater monitoring, geotechnical services, soakage testing (BRE 365) and due diligence surveys.