What Owners & Developers Should Know:
Notice that the title of this discussion doesn’t start with, “When is it Necessary”? I would also take it a step further and say that the title of this discussion should have an exclamation mark at the end of the title in lieu of a question mark. Truly, a wise man will build his house upon a rock.
The stark reality of the matter is that no one knows what is underneath the ground until you drill down and analyze the soil condition. For example, below the site, there could be an old mine shaft, large amounts of pyrite, global instabilities due to topography, etc.
A common misconception shared by many is that structural engineers analyze and determine the soil properties on a given site for their foundation design. A structural engineer’s scope of work encompasses the design of a structure down to the bottom of the foundation. The soil conditions are to be determined by a licensed professional geotechnical engineer. Therefore, in order to design a foundation for a structure, there must be soil recommendations for that particular site.
A geotechnical report for a nearby site should not be accepted, as conditions can vary widely.
Otherwise, the foundation design will have to assume worst conditions. The cost of this overdesign will far outweigh the cost of the initial investigation.
Allowable soil bearing pressure is not the only useful information provided in a geotechnical report. For instance, the seismic soil site class can have a huge impact on the design of the structure as a whole.
According to the International Building Code (IBC), if you do not have a geotechnical report, then you have to assume a “D” site class. The magnitude of the seismic loading of a site class “D” compared to a site class “C” is significantly more.
Other useful information includes:
- Soil subgrade modulus
- Active and passive soil pressure coefficients, etc.
This type of information is needed for slab on grade and retaining wall designs.
In closing, a geotechnical report is necessary for the design and construction process of a structure.
I would also recommend having a geotechnical report done before deciding to develop any given site or better yet, BEFORE the land is purchased. Many projects have been canceled due to unforeseen soil conditions. As they say, “hindsight is 20/20;” therefore, not only having a geotechnical report but also having it early is paramount for every project.
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Thanks for reading,
Jason works as a Senior Structural Engineer at Allegheny Design Services. His responsibilities include but are not limited to project management, production of construction documents with CAD and structural engineering design. He attended West Virginia University where he received his bachelor’s degree in civil engineering. He has extensive engineering experience as both a field engineer and a structural engineer. He has also acquired skills in the Autodesk Revit 3D modeling system for the production of construction documents.