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 .