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Geotechnical Engineering & Soil Testing: The Ultimate Guide

I bet you rarely give a second thought to the soil under any building you walk into. What if I told you there are entire teams of people who are just dedicated to analyzing the dirt any building will be built on? These people are geotechnical engineers and the technicians that work with them. They are dedicated to testing and analyzing the soil before any structure is built.

A geotechnical investigation is the first step in any construction project and is a necessary part of any soils report along with a proctor test. Without these two pieces of information an engineer will not give you pad certification. Nor will a building department approve your plans to build.

So you understand that you need these things, but you have no idea what these things are. So we are going to dive into all the different kinds of soil tests with SPEC's Ultimate Guide to Geotechnical & Soil Tests.

Geotechnical Investigations

Since geotechnical investigations are the first step in any construction project, we will review these kinds of tests first.

Geotechnical investigations must be done first in order to gain an understanding of the conditions of the soil below the topsoil. What is found will affect the kind of foundation that a proposed structure will be built on. The different kinds of tests can included:

  • Standard Penetration Tests (SPTs)

  • Post- Hole Tests

  • Percolation Tests

  • Penetrometer Tests

Standard Penetration Tests (SPTs) are very common low cost testing procedure used in geotechnical engineering. The tests are easily conducted by simply drilling a hole to a desired depth then a weight is dropped repeatedly until the sampler reaches 6 inches. This is then repeated two more times until the sampler has reached a depth of 18 inches. These kinds of test can provide

  • Determine relative density of the soil

  • Angle of shearing for cohesionless soils

  • Soil Strength for cohesive soils

Post- Hole Tests (Test Pits)

Post-Hole tests are manually dug shallow pits that reveal the subsurface conditions. These are dug to the desired depth to test and are typically used for shallow foundations.

Percolation tests are a fairly simple tests to conduct. They are performed by digging holes and observing how long it takes for the water to be absorbed by the soil. More importantly, this test determines how long it takes for the soil to drain moisture that is added to the ground.

Penetrometer tests are a diagnostic tool used to measure he extent and depth of subsurface compaction. Soils tend to compact both at the surface and below it, however, determining how to deal with below surface compaction is much more difficult than surface compaction. The most common type of penetrometer used in during soil testing is a cone penetrometer. This has cone at the end which is hammered or pushed into the soil. The cone penetrometer can be static or dynamic. Geotechnical engineering most commonly uses a dynamic penetrometer which is driven into the ground with hammer blows at a constant rate.

Soil Testing

Once you have completed your geotechnical investigation, your fun with dirt is not over. You can now begin to prepare the soil you have tested and once you do that you're going to need a whole different set of tests at this stage. Some of the tests you may need may include:

  • Density Testing & Proctors

Field Density Test & Proctors

At this stage ensuring that the soil is compacted correctly and that is has the desirable moisture content is key. That's where density tests come in. Density tests, regardless of method, will tell you the density of soil. I know it's very obvious, however, there are several different methods of obtaining soil density. The most common density test include:

  • Sand Cone Test

  • Rubber Balloon Test

  • Nuclear Density Test

At this point, a soil sample is also taken in order to perform a proctor test. A proctor takes the soil sample dries it out and then adds water to it. The soil is then compacted in a mold with a proctor hammer. The test produces a graph looking much like this:

This curve shows the relationship between how much water is in the soil and how dense the soil can become. The top of the curve represents the optimum moisture content and optimum density of the soil.

Specialty Engineering has been a leader in the construction materials testing and inspection industry for over 20 years. Our engineers have molded the skyline of South Florida and it all starts with our experience and leadership in areas such as geotechnical investigations and soil testing. To learn more about our process and our engineers click here.

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