Speaking in Code – Geotechnical Engineering, Soils & the CODE…All the Dirt.

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The word “geotechnical” is actually pretty new insofar as the building code is concerned and particularly the International (IBC) Building Code. Prior to the issuance of IBC 2009, the word geotechnical was missing from the IBC code. The words “geotechnical report” first popped up in IBC 2009, section 1704.7 when we were advised by the CODE that the approved geotechnical report should be used to determine compliance of the soil fill materials. Previous IBC code cycles had not used the word “geotechnical” but rather referred to this type of data as “soil reports”, “soil investigation”, etc.

Let’s start with what “geotechnical engineering” actually is. Civil engineering verbiage defines “geotechnical engineering” as the analysis, design and construction of foundations, slopes, retaining structures, embankments, roadways, tunnels and other systems that are made of or are supported by soil or rock.

A Little History Break.

Karl Terzaghi (1883-1963) was known as the father of modern soil mechanics and is considered by many as the father of (modern) geotechnical engineering. In 1936 Terzaghi, while a professor at Harvard University, presided over the first conference of the International Society of  Mechanics and Foundation Engineering (ISSMFE). In 1997, the ISSMFE was changed to ISSMGE (International Society of Soil Mechanics and Geotechnical Engineering) to reflect the true scope of the group.

Much earlier, the construction of the pyramids around 2700 B.C. involved many challenges related to soil dynamics, foundations, slope stability, retaining structures, etc. Long before the words geotechnical engineering was uttered, geotechnical construction problems were encountered and solved.

But I Thought This Was a CODE Newsletter…

OK, back to the IBC Code…IBC Chapter 17 (the Special Inspections chapter), section 1705.6 states that “The approved geotechnical report and the construction documents prepared by the registered design professional (RDP) shall be used to determine compliance” regarding soils. Table 1705.6 conveys the requirements for determining soil-bearing capacity and for testing/inspection of soils for controlled fill. Always remember that the special inspection requirements and the requirement for a geotechnical report as set forth in Chapter 17 is a part of the IBC Building Code (often called the general building code).  The purpose of the general building code is to establish minimum requirements during construction unless deleted by exceptions elsewhere in the general building code or the Statewide Building Code.

As important as section 1705.6 of Chapter 17, “Special Inspections and Tests” is regarding soils and geotechnical reports, Chapter 18, “Soils and Foundations” is even more vital when it comes to the geotechnical report, the importance of it and what information should be in the report. Firstly, remember that Chapter 18 is code and is just as enforceable and just as important as Chapter 17 or any other part of the code.  Chapter 18 contains provisions regulating the design and construction of foundations for buildings and other structures and involves geotechnical and structural considerations in the selection and installation of adequate supports for the loads transferred from the structure above (the oil/rock). Section 1803.1 states that geotechnical investigations shall be conducted in accordance with section 1803.2 and reported in accordance with section 1803.6. Section 1803.6 basically stipulates minimum information that must be included in the geotechnical report. Section 1803.2 includes an exception whereby the building official can waive the requirements of a geotechnical investigation under certain conditions.

It should be noted that only the building official can waive the geotechnical investigation.

What, you can’t see that?  Click on chart to enlarge.

 

 

 

Spread Footings

Essentially, there are two parts to the foundation system: the substructure and the soil. The substructure consists of structural components that serve as the medium through which the building loads are transmitted to the supporting earth (soil or rock). The substructure components may consist of shallow foundations, such as basement walls, grade beams, isolated spread footings or combinations of these components. Shallow foundations may also involve the use of mat or raft foundations. As may be required, the substructure can consist of deep foundations involving the use of piles, drilled piers, caissons or other deep foundations. The second part of the foundation system involves the use of soil (including rock) as a structural material to carry the load of the building or any other load transmitted through the substructure.

The substructure and the soil are interdependent elements of the foundation system and must be understood and dealt with as a composite engineering consideration. Indeed the kind of substructure to be used is a DIRECT function of the nature of the soil encountered at the project site. Chapter 18 of the IBC code broadly outlines the conventional systems of foundation construction.

In determining the load-bearing capacity and other values of the soil mass, the CODE provisions address such considerations in terms of “undisturbed” soil. Special provisions are included in Chapter 18 where compacted fill is to be utilized for the foundation support. When compacted fill is utilized for the foundation support, both periodic and continuous special inspections are required in Chapter 17, section 1705.6 and Table 1705.6. These tests and inspections include (but are not limited to) proof rolling the construction site prior to the placement of fill material, dynamic cone penetrometer (DCP) tests, dedicated project inspection, excavate and replace methods, laboratory soil classification tests, field density tests, etc.

The PURPOSE of Chapter 18 of the IBC code is to set forth the MINIMUM requirements for the design and construction of foundation systems for buildings and other structures.