sewage treatment in cork and munster
 

SITE IMPROVEMENT WORKS

 
In certain circumstances a site, which is intended for a single house development will present particular difficulties arising out of the site assessment. Some sites may have a high water table, may have insufficient subsoil depth, or may have unsuitable subsoil for the purposes of treatment and percolation of the pre-treated wastewater from a treatment system. It may be possible in some such cases to render the site suitable for development after the carrying out of specific engineering works on the site known as ‘site improvements’. Site improvement works should only be attempted under the supervision of a chartered engineer or other suitably qualified professional as such works are technically difficult to carry out correctly. A constructed soil filter system (raised mound) is not considered to be site improvement works as it is itself a treatment system.

The option to carry out site improvements might be considered in circumstances where a high water table is a problem (i.e., within 300mm of the ground surface). The conditions that give rise to a high water table are site specific; these include topography, nature of soils, bedrock and outfalls. Detailed design procedures are available in drainage manuals.

In other cases such as where the site is overlain by insufficient depth of subsoil (i.e. less than 300mm) or unsuitable subsoil the site may be improved by the placement of suitable soil in lifts across the whole site rather than just infilling in the area around a proposed mound system. It is necessary to perform testing of each 300mm layer as the process of emplacing lifts of soil progresses. After each lift is placed, percolation tests should be carried out. A 150 mm square hole is excavated to a depth of 150 mm in the placed soil. After pre-soaking to completely wet the soil, 0.5 litres of water is poured into the hole and the time in minutes for the water to soak away is recorded. This time should be between 10 minutes and 2 hours.
However, in many cases site improvements works will not be acceptable and in such cases the site is unsuitable for discharge to ground and may be deemed unsuitable.

Examples of sites where site improvement works will not be successful are:

  • Sites where the slope exceeds 1:8.
  • Sites where T is greater than 90, indicating a high risk of ponding.
  • Site where the separation distances cannot be satisfied.

Having carried out the required site improvement works the appropriate parts of the site characterisation form should be re-completed and an assessment of the overall suitability of the site can be made. A site cannot be deemed to have passed the onsite assessment if the recommendations include significant site improvement works.

The site characterisation form and details of the site improvement works including additional testing results should be submitted to the planning authority.