INSIGHT
Geotechnical investigations
By Eamonn Ryan
Civil engineering contractors regularly complain of the flawed nature of the geotechnical reports on which they base their work.
Trevor Green, technical director and head of the Geotechnical Department at Jones & Wagener, suggests that more widespread use of SAICE’ s Site Investigation Code of Practice would be a positive step in the right direction.
“ The importance of the geotechnical investigation is often underestimated. It often seems clients and developers are reluctant to spend money on something they cannot see, and would rather spend it on tiles, carpets, or other finishes within the structure, where they can more readily appreciate the value. Geotechnical is all about what’ s below the ground, and the only time many people even think about this aspect of a project is when something goes wrong,” says Green.
What frequently occurs, he explains, is that contractors are handed a geotechnical report ahead of construction and they assume all is in order without questioning the quality of the research.“ No one wants to spend money they don’ t have to, and this is particularly the case with commercial developments in a tight economy. This is aggravated by the fact that geotechnical reports have to be done right at the beginning, at the conceptual stage, often before the finances are organised or units sold, and developers want to spend the least
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Eamonn Ryan amount of money because everything is at risk. They may consequently and often unwittingly commission a relatively superficial report that does not embrace all the risks of the site— but which creates the impression of a job properly done. There’ s an expression that you always pay for the geotech, whether up front or by the end of the job,” adds Green.
You get what you pay for
Up front, that cost may not even be exorbitant, as low as R100 000 to R150 000 for a geotechnical investigation in straightforward geology for a large multi-level office development in Sandton or Rosebank— and it saves money in the long run. A lot of piling or lateral support work is done on a‘ design-andconstruct basis’. This involves a client commissioning a geotechnical report and a structural engineer providing some schematic drawings. These go to the various specialist geotechnical contractors who cost the work according to their own design.“ If you have poor information, you are going to price the most conservative design because you have all these risks that are unknown. If you know exactly what is in the ground, or at least have a good idea, you don’ t have to price in as much of the risk.” This is where a good geotechnical investigation can save you money. For this reason, you often get the same specialist geotechnical contractor working on adjacent sites because of their familiarity with the local geology, which mitigates the risk.
A shallow water table is often problematic for projects. Again, using
Trevor Green, technical director and head of the Geotechnical Department at Jones & Wagener.