Posted by Johnny Flores, P.E (TX). I recently received a proposal request from an out of state organization.  The proposal request contained a site layout plan, a description of the scope of work, suggested boring locations, and suggested boring depths of 50 feet.  It was put together very well – even had a cover letter.  I was impressed.   However, the number and suggested depth of the borings really had no relation to the site geology or physiography or the likely structural loads, since this was a single story building.  A quick check of geologic maps and nearby geotechnical reports confirmed my suspicion.  The project site was located in an area with expansive clay underlain by limestone.  I contacted the organization and explained the above points, suggested they confer with their structural engineer and if the loads were indeed relatively light, they should increase the number of borings and decrease the boring depth.

I am telling this true story,  because it illustrates the importance of using available information, such as geologic maps, nearby geotechnical reports, combined with information about the proposed structural loads and site physiography to develop a savvy field investigation plan.   Geologic maps and their accompanying booklets are written by geologists for geologists.  However, good geotechnical engineers use these regularly in planning investigations and identifying formations.  Excellent geotechnical engineers obsess over them.  With regard to loads, I already knew that similar single story structures in the area had been designed with drilled shafts bottomed in the limestone at depths of about 15 to 20 feet, so more likely than not, a 30 ft deep boring would suffice.

In addition, a quick check from Google Earth Pro indicated the site was flat and level with no trees.  But, it had been previously improved with buildings and parking which were destined to be razed for the new project.   It was obvious from Google Earth Pro images the existing parking lots were showing signs of distress (consistent with expansive soils).  These observations presented additional uncertainty for the site subsurface soil regime and underscored the need for additional borings and perhaps test pits.

The above checks were completed in a matter of minutes at no upfront cost to the client and really helped eliminate the guesswork for the field investigation.