Ground Investigation

Ground Investigation and Trial Holes

Domestic Extensions

If you are just doing a simple extension then often a ground investigation is not necessary but particularly in a clay sub soil area savings in the foundation design can be made if the subsoil type is known.  With clay subsoils the clay can be categorised as High, Medium or Low Shrinkable Potential and if the amount of stone and larger fragments  in the sample is enough even Non-Shrinkable.

In a high Shrinkable soil if there is a Leyland Cyprus hedge say 3m away from the excavation the foundations would need to be taken to a depth of about 2.8m but anything more than 2.5m requires a site specific assessment by an engineer and normally a piled foundation is opted for.  If the subsoil is of Medium Shrinkability the foundation depth needs to be 2.48m and for Low Shrinkability the depth required is 2.06m.  If Non-shrinkable the depth needs to be only 1m (or in fact less but normally the 1m is the minimum Building Inspectors accept).

To show the importance of knowing the shrinkability of a clay soil if we take a situation where we have a 6m length of foundation near to the Leyland Cypress as above in a non-shrinkable soil we have a base cost of the foundations.  Increasing the depth for a Low Shrinkable soil results in 2.87m3 of extra excavation and concrete.  Assuming concrete at £70/m3, excavation at £40/m3 and muck away at £150/skip with 50% bulking this costs an extra £467.00.  The Medium Shrinkable case is an extra £665.00 and the High Shrinkable is an extra £832.00 or significantly more if piled.  A trial hole and a soils test to confirm the shrinkability costs a fraction of this sum.  If the subsoil is not tested and has a clay content normally the Building Inspectors expect you to take as a minimum Medium Shrinkability and often High Shrinkability.

David French Partnership have our own in-house soils testing laboratory and can do Shrinkability testing, Sulphate and pH testing and sieve analysis as well as other more specialised tests.  The in-house laboratory enables the soils to be tested quickly and economically.

New Build Housing and Industrial Projects

On larger projects a more elaborate investigation is required which will vary depending on the size of the project and any planning conditions that are applied.

Depending on the project size it is normally required that a number of trial holes are excavated using a JCB digger or similar typically to at least 3m deep.  These enable a good assessment of the soils to be made and bulk samples taken for laboratory testing.  The testing needs to categorise the subsoil type and check for pH and sulphates to enable the correct concrete to be specified.  With most projects it is also necessary to check for contamination and this is done by sending samples to specialist testing laboratories and dependent on their findings a remediation strategy is prepared.

On projects where soakaways are required the Local Authority often require a site permeability test to be carried out to enable the soakaway size to be designed.  This is not a very easy test to carry out.  The normal requirement is for the test to be done to BRE 365.  This involves digging a trial hole and fitting a slotted pipe into the hole and backfilling with graded shingle.  A water bowser is used to fill the hole following a specified sequence and then measuring the time for the water level to drop a specified amount.  The test can take a long time in poorly permeable soils and hence can be costly to do.

On small domestic projects the cost of the BRE 365 test is hard to justify and so often these are designed by empirical means.  
On new developments it is often a planning condition that a Phase I, Phase II or Phase III Environmental Assessment is carried out.

The Phase I Environmental Site Assessment typically covers

  • A desktop study to cover the past and current land use of the land and consideration if there may be areas of contamination on the site requiring investigation.
  • Reference to old maps and other available data.
  • A walkover of the site is generally required.
  • Reviewing environmental files maintained by the site owner and regulatory agencies such as consideration of mineral extraction reports where there may have been coal mines, extraction of gravel, limestone, clay or a wide range of other minerals.
  • Local knowledge.

This information is then used to develop a site specific conceptual model and risk assessment. This involves looking at any potential source of contamination and assessing the possibility of this reaching and affecting either people or the wider environment. This determines the need for further site investigation, i.e., the need for any type of intrusive sampling and analysis or may indicate that further investigation is not warranted.

The Phase II Environmental Site Assessment focuses on gathering specific information as required about the property and can include the following tasks:

  • Surface and subsurface soil sampling, groundwater and surface water sampling, soil vapour sampling (along with laboratory analysis), sediment sampling, collection of plant or aquatic species samples
  • Above/underground storage tank content and tightness testing, asbestos containing material (ACM) sampling, contamination sampling and testing, PCB sampling and identification, geomagnetic or geophysical surveys
  • Directly measuring conditions such as noise levels or radiation
  • Using environmental models to evaluate the potential migration of the contamination

The results from the Phase II ESA are used to determine the need for a remediation and to plan how this is to be carried out, and whether there is a need to notify the appropriate regulatory authority.

The Phase III Environmental Site Assessment focuses on remediating the site.

  • Removal, and disposal of existing contaminated area(s) through a variety of methods.
  • On site treatment of contaminated soils, groundwater, and waste streams.
  • Validation of the site after remediation.
  • Implementation of waste reduction plans, environmental management systems, and other at source remedial measures.
  • Toxicological and Ecological Risk Assessments on contaminated properties where standard remediation methods are not practically feasible.
  • A written report is prepared with evidence that the site has been remediated to the satisfaction of the Contaminated Land Officer and Environment Agency.

A Phase III remediation is usually based on findings and conclusions outlined in a Phase II investigation. The Phase III remediation is often the final step in transforming a property with environmental liabilities back into an appreciable asset.

Where a site is contaminated further testing is sometimes required to determine the Waste Acceptance Criteria (WAC) to determine whether it can go to a normal landfill site or has to be treated as special waste and to be taken to a landfill that accepts contaminated material.

David French Partnership off all the above services.