Below Grade

Many times, in the construction of a new building, the foundation is the first thing to get constructed and then it is immediately covered up.  This can create lots of questions as to what is actually happening below grade.

First, let’s talk about scope and responsibilities.  Typically, a geotechnical engineer will do soil borings.  This includes mobilizing a drill rig and drilling down a certain distance to get soil samples.  With the help of the structural engineer, the geotechnical engineer will determine points of interest or points that capture the most comprehensive snapshot of the building footprint.  The geotechnical engineer then takes these samples back to their testing lab, tests them and puts together a soils report with design information.  This report includes allowable soil bearing capacity of the site soil, swell/expansive properties and expectations, overexcavation recommendations, lateral soil pressure values, and recommended foundation types.  Depending on the soil and building construction, these foundation types can range from concrete drilled piers down to bedrock, continuous spread footing or mid-depth foundation systems such as helical piers or rammed aggregate piers.

The depth to bedrock can vary drastically.  In the mountains, bedrock is typically right at the soil surface.  In other areas, bedrock can be down up to 60 feet or more.  The elevation of bedrock can influence the type of foundation recommended.  Also the type of soil has a direct impact on the recommended foundation type.  Some native soils (clay) can actually increase in volume (swell) once saturated with water.  If not properly addressed, this can create extreme foundation issues.

The geotechnical engineer puts together all of these recommendations and soil design values into a geotechnical/soils report.  This report goes to the design team and the structural engineer takes the information presented in this report to design the foundation system.

The structural engineer’s work begins at the top of the structure and works down, deriving building forces and directions (vectors) and working them down to the soil.  All superstructure loads eventually have to be support by the soil below.  Once all of these loads have been determined, the structural engineer designs the foundations based on these loads and the soil design parameters derived by the geotechnical engineer.

So there; now you have it.  Some clarity to what is happening below your building.  Just like a tree needs a strong root system to stand tall, a building needs a strong foundation to do the same!  The taller the building, the deeper/wider the foundation elements.