Everything You Need To Know About Geotechnical Reports

A geotechnical report is filled with crucial information acquired from a site-specific soil study, and is used to properly design your building’s foundation. The requirements, content, and composition of the geotechnical report are governed by the building code adopted by most municipalities in Texas – the International Building Code (IBC).

Why should you get a geotechnical report?

O3 Structural Studio insists on working with a geotechnical report because of the prevalence of expansive soils in Texas. Expansive soils, if not properly countered with an adequate foundation design, can wreak havoc on a building and lead to problems like heaving, settlement, and other undesirable cosmetic issues. While the soil study and resulting geotechnical report is just one element of getting an appropriate foundation design for your project, it is a crucial one for every party involved in the construction of any foundation.

When is a soil study required?

The IBC points out that geotechnical reports might be required by the building official in the jurisdiction you’re building in. This is typically the case in regions with expansive soils, like in the Bryan-College Station area, Houston, and DFW. You can find out if a geotechnical report is required for your project by checking with the city in which you plan to build.

How is a geotechnical report made?

A geotechnical report is created by sampling the soil and testing its properties. The sampling is performed by a field crew that takes borings at locations around where your foundation will be located. When the crew drills the borings, they bag and name the samples and take them back to a testing lab to run a myriad of classification tests.

What are they sampling and testing for?

The soil investigation tries, first and foremost, to classify what kind of soil you have. For buildings, the geotechnical engineer will probably use the USCS classification system to develop a soil profile. This is particularly important because certain building foundation types are conducive to certain soil types, and you don’t want to put the wrong building on the wrong support material.

The soil investigation also identifies if there is groundwater at your site, and if so, what elevation the groundwater is located at. Groundwater close to the surface of the building pad can impact the design and construction of the foundation.

USCS Classification System - Source: California Department of Transportation https://dot.ca.gov/-/media/dot-media/programs/maintenance/documents/office-of-concrete-pavement/pavement-foundations/uscs-a11y.pdf

OK so they classified the soil – now what?

If the soil is adequate enough to support your structure, the geotechnical engineer overseeing the study will then recommend foundation types and design values for that soil to the structural engineer in the report. These values will include the allowable bearing pressure (amount of weight that the soil can take per square inch). The geotechnical engineer also recommends a and the target soil stratum for the building foundation.

What types of foundations are typically recommended?

There are two main types of foundations – shallow foundations and deep foundations. Shallow foundations are typically less than 5’ depth from the top of grade. They include grade beams, footings, slabs on grades, etc.

Shallow Foundations

If your report recommends shallow foundations on compacted fill, the report will include the necessary compaction density and moisture level when the building pad is installed. It will also include how thick each layer of soil can be before compaction. Not following these rules can lead to differential settlement – your building will settle unevenly. Differential settlement can lead to cosmetic cracking, plumbing issues, or worse. For more information on compacted fill - see IBC section 1803.5.8.

Deep foundations are foundations that reach a deeper layer in the ground, typically more than 5’ depth from the top of grade. They include piers, piles, helical piers, etc. Deep foundations are also a common recommendation when groundwater is located close to the surface.

Drilled Pier Foundations

If your report recommends deep foundations, the report will include the capacity of the foundation when properly installed, and the  minimum spacing requirements for those deep foundation elements. Deep foundation recommendations must also include installation procedures, field inspection and reporting procedures, and the target soil layer to be used for bearing. For more information on deep foundations – see IBC section 1803.5.5.

What should be in your geotechnical report?

Summarizing, a geotechnical report must include the following items:

1.     A plot showing the location of the soil investigation

2.     Soil boring logs

3.     A record of the soil profile

4.     Elevation of the water table

5.     Foundation recommendations and design criteria, including bearing capacity and expansive soil mitigation provisions

6.     Expected total and differential settlement

7.     Deep foundation information

8.     Special design and construction provisions especially for expansive soils

9.     Compacted fill material properties and testing

Need help on a project? Call 713-909-0458 or email info@o3structural.com