A contractor's guide to geotechnical engineering: knowing how to decipher a soils report and make sure that it reflects field conditions will save everyone time and money

Concrete Construction, May, 2002 by Thomas A. Chapel

Team approach

Regardless of the foundation system recommended, every appropriate member of the design and construction team must review the design criteria and compare them with the plans and specifications prior to construction. It is always easier and cheaper to fix a mistake or clarify a misunderstanding before construction than it is to remove and replace or retrofit a foundation to accommodate a design or construction error. The in formation and guidelines presented in this article will provide you with some practical insight into geotechnical engineering concepts that contractors can apply in a proactive manner to their projects on a daily basis.

Table 1--Standard penetration number (N)

    Consistency of clays                  Relative density of sands

  Standard                             Standard
 penetration                          penetration
 number (N)      Soil consistency      number (N)     Relative density

     0-2             very soft            0-3            very loose
     3-4               soft               4-9              loose
     5-8           medium stiff          10-29          medium dense
    9-16               stiff             30-49             dense
    17-32           very stiff           50-80           very dense
more than 32           hard           more than 80    extremely dense
Table 2--Expected amount of swell for
various reported swell potentials

                      Representative          Representative
                      percent swell           percent swell
Swell potential    (500 psf surcharge)     (1000 psf surcharge)

      Low                 0 to 3                  0 to 2
   Moderate               3 to 5                  2 to 4
     High                 5 to 8                  4 to 6
   Very High               > 8                     > 6

Checking soil consistency

Contractors should always check to see if the soil consistency (and therefore the bearing capacity) that is specified in the soils report is consistent with field conditions. The following rules of thumb can help with that determination:

Clay

* Very soft clay can easily be penetrated several inches by a fist (bearing capacity <700 psf)

* Soft clay can be penetrated several inches by a thumb (500-1500 psf)

* Medium stiff clay can be penetrated several inches by a thumb with moderate effort (1000-2500 psf)

* Stiff clay can be indented readily by a thumb but penetrated with great effort (1500-3000 psf)

* Very stiff clay can be indented readily by a thumbnail (>2000 psf)

* Hard clay can be indented with difficulty by a thumbnail (>3000 psf)

Sand

* Very loose sand can be easily penetrated by a No. 4 (l/2-inch-diameter) rebar pushed by hand (<1000 psf)

* Loose sand can be penetrated with difficulty by a No. 4 rebar pushed by hand (1000-3000 psf)

* Medium sand can be penetrated readily by a No. 4 rebar driven by a 5-pound hammer (>2000 psf)

* Dense sand can be penetrated about 1 foot by a No. 4 rebar driven by a 5-pound hammer (>2000 psf)

* Very dense sand can be penetrated about 3 inches by a No. 4 rebar driven by a 5-pound hammer (>2000 psf)

Thomas A. Chapel is an associate engineer with CTL/Thompson in Fort Collins, Colo. He is a registered professional engineer and a certified professional geologist and bas more than 20 years of experience working on various projects in the western and west-central United States.