The risk of unknown ground conditions is a major cause of cost over runs on construction projects. A poor knowledge of ground conditions can not only lead to delays in completion but it can result in significant revisions to the proposed design. A comprehensive ground investigation should provide sufficient information to inform the Contractor and provide the most cost effective price for construction.
The exact content of the Ground Investigation package will depend on a host of items such as the size of the development, the number of structures, the types of structures, the footprint of the infrastructure (roads, underground utilities etc), the type of landforms present (flat field, "brownfield" site, slopes, cliffs, etc.), habitat (agricultural lands, marsh, peat, rock, etc.).
As a rule of thumb it is important to determine ground information (soil and rock types, layer thicknesses, level of water table and strength data) for the site of each structure which requires a foundation. The ideal separation between the points for which we have information will depend on the variability of ground conditions, the vulnerability of the intervening infrastructure and ancillary structures and any specific design differences.
Normally exploratory holes are used to gather this information, either by using boreholes or by assessment of excavations (trial pitting). The depth of such intrusive works will depend on size and significance of the foundations structures required and the loads to be imposed on them.
Where the development site is significant in size geophysical methods can be employed to extrapolate the information gained by the intrusive geotechnical investigations. Alternatively, where the land is sensitive and the project would benefit from minimal intrusion geophysical techniques can be used on their own to collect the relevant information. The choice of geophysical technique will depend on the type of ground information required.
Supplementary forms of ground investigation such as in-situ testing, gas and water monitoring etc are very site specific and their will depend on preliminary results from the ground investigation campaign.
Geophysics can assist in understanding the sub-surface in the following ways:-
Mapping of the thickness of soils overlying rock and determining their relevant strength.
Identifying of buried underground utilities and culverts.
Locating and mapping of anomalies (e.g. buried mineshafts, cavities, archaeological features, metal objects).
Non-destructive assessment of infrastructure, such as roads to determine as-built construction detail.
The following list indicates the most common geophysical survey methods and their relevance to geotechnical engineering:-
The type of contamination present of sites varies greatly. It can generally be split into three different classes, as follows:-