Frequently Asked Questions

The following series of frequently asked questions often helps our clients to decide on the best method of investigating their site. Each site is of course unique, but the problems described below are in general typical and it is useful to see how our past clients have dealt with these issues.

Often a synthesis of different methods is required to adequately explore the site in concern.

Question 1

I wish to develop a “brown field site” which has been filled in the past. What is the best approach to provide me with information prior to my housing or commercial development?

Answer 1

To develop a “brown field site” the following items should form the basis of your investigation:-

What is the composition of the filled ground and to what depth does it extend?
What is the composition of the underlying native soils?
What is the ultimate bearing capacity of the fill material and the underlying soils?
If the underlying soils are not capable of supporting the loads from your structure using traditional strip, pad or raft foundations, to what depth must I drive piles?
Is the site contaminated? If so what are the major contaminants and in what concentrations are they present?
Do utilities cross the site? Often service drawings are available, but often their accuracy is questionable.

A ground investigation consisting of the following parts will be necessary to adequately investigate the site:-

Cable percussion boreholes to determine composition of fill and native soils, their thicknessess, ultimate bearing capacities and locate suitable bearing strata. Laboratory testing should be carried out soil samples.
Trail pitting or window sampling within the surface layer to carefully remove samples for contamination testing. Generally an ICRCL or CLEA testing regime prevails.
Geophysics, utilising ground radar and / or Electromagnetic surveying can be used to determine location of utilities, buried tanks, foundations, shafts or basements.

Question 2

I need to construct a water / sewage / gas etc. pipeline or cable route in a rural environment. Typically what type of investigation should I undertake?

Answer 2

To plan a linear pipeline/ cable route the following features should be considered:-

Will I need to excavate rock at any point along the proposed pipeline route?
What is the composition and ultimate bearing capacity of the underlying native soils at installations / buildings along the route?
Are the native soils likely to produce high rates of corrosion in steel pipelines?
Will native soils cause “hot spots” in the cable network?
Is the site contaminated? If so what are the major contaminants and in what concentrations are they present?

A ground investigation consisting of the following parts will be necessary to adequately investigate the site:-

Cable percussion boreholes to determine composition of fill and native soils, their thicknessess, ultimate bearing capacities and locate suitable bearing strata. Laboratory testing should be carried out soil samples. These should be conducted at proposed building and installation locations. Along the route at regular intervals shallow probes can be carried out using a rotary rig or small probing rig.
Geophysics in the form of seismic refraction surveying can be used to map soil thicknessess and rock strength along the route. This can be used to extrapolate borehole information between boreholes.
Geophysics in the form of electromagnetic or electrical resistivity surveying can be used to determine the location of less resistivity / high conductivity soils likely to cause corrosion in pipe work.
Geophysics in the form of thermal resistivity surveying can be applied to determine the rate of heat dissipation from soils in contact with electricity cables.

Question 3

I intend to construct a wind farm in a mountainous region. What type of ground investigation do I need to carry out?

Answer 3

To successfully plan and construct a wind farm the following issues will need to be addressed:-

On what type of material will I need to found my turbine bases, sub stations and meteorological masts?
How thick and of what composition are the surface soils beneath the route of my proposed road network? What thickness of road base will I need to use?
How will the composition of the ground affect measures will I need to take to earth my personnel and plant from lightning strikes or electrical surges?
What cable rating should I specify for the cable network?

A ground investigation consisting of the following parts should be carried out to satisfy the above items:-

Trial pit analysis should be carried out to assess ground conditions and establish depth to a suitable bearing stratum. In-situ testing should also be carried out to determine field values for allowable bearing capacity. Samples can also be tested for pH to provide coarse screening for contamination. One trial pit should be carried out at the location of each structure.
In-situ CBR testing should be carried out at intervals along the route of the proposed road network. This will enable road base thicknessess to be determined.
Electrical resistivity should be carried out at each turbine base, sub station and met mast to determine the ability of the ground to dissipate charge. From these parameters a model of the subsurface conductivity can be produced from which an earthing strategy can be designed. Rotary drilling can then be used to install electrodes as required.
Geophysics in the form of thermal resistivity surveying can be applied to determine the rate of heat dissipation from soils in contact with electricity cables. From this information a cable rating can be determined.

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