How to keep the cost of all geotechnical work as low as possible.

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•  Before ground breaks on any new construction project, the site must be investigated to determine whether the soil has the bearing capacity to support the proposed building. Typically, this investigation is done by a third-party geotechnical engineering consultant, and the results are compiled into the geotechnical soil report.
• The geotech will describe the site and subsurface conditions that will impact design and construction of the project. From that data, they’ll make recommendations about whether ground improvement is necessary, or if deep foundations should be considered to meets the bearing pressure & settlement criteria of the building.
• Since no universal standard governing how much data must be collected during the investigation and written into the final report, the geotechnical engineer and owner are left to negotiate what should be included.
• The geotechnical engineer wants the most thorough investigation possible and the owner wants to keep the cost of all geotechnical work as low as possible.

In our experience as a specialty geotechnical subcontractor, these two requirements are more aligned than they appear on the surface. There are opportunities to save significant sums of money on ground improvement design and construction by getting the clearest data at the report stage.

The subcontractor’s bid will be based on the amount of information provided.

Here are three ways to test a site’s soil strength and settlement parameters:

Standard penetration tests (SPT)

SPTs are a routine soil testing and sampling procedure conducted in the field within a soil boring. A small, hollow tube is hammered into the ground and the number of blows required to advance is measured, giving a rough idea of soil strength. The soil collected within the hollow tube can then be retrieved and used to visually identify the samples or taken to the lab to run further strength tests.

While the data collected from SPT isn’t as robust as that collected from the next two tests, SPTs are an inexpensive way to visually and physically corroborate the information collected in shear strength, consolidation and cone penetrometer testing.

Shelby tubes

Shelby tubes, a cylindrical sampling container, are used to collect undisturbed samples in cohesive soils for visual and lab inspection. Samples are typically collected at five-foot elevation intervals within a boring.

While a myriad of tests can be run on a Shelby tube samples —plasticity, moisture, visual inspections— what we really care about are the results from shear strength testing and consolidation testing. Often times, we see nearly continual Shelby tube samples in a boring; however no testing of these samples has taken place, so we have no strength information of the soil.

Consolidation tests give us the settlement parameters of the soil and shear strength tests give us a more accurate picture of soil strength than SPT blow counts. The data collected from these two tests gives us a good idea of the column strength we can achieve with ground improvement at different points throughout the site.

Undisturbed sampling with Shelby tubes allows for more accurate strength and consolidation testing within the lab compared to samples obtained through traditional SPT methods.

Cone penetrometer testing (CPT)

Both the Shelby tube and SPT testing are done at intervals, testing only one strata at a time within the soil boring. Cone penetrometer testing (CPT) can provide more reliable data than either, with a probe continuously measuring the soil shear strength throughout the elevation profile. It gives you the best information on the actual, in-situ strength of the soil.

CPT is generally more expensive than SPT or Shelby tubes, but it provides better strength data than either. Still, SPTs and Shelby tubes are important to visually corroborate the data gathered from a CPT.

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