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How Long Does Geotechnical Testing Really Take?

A brief discussion of the various types of geotechnical testing, their purpose, and the time frame based on their guiding standards.

If you are beginning construction of a building, road, or other structure, you may be wondering how long geotechnical testing will take and if it will cause delays to your initial project timeline. Some projects are straightforward, and an entire geotechnical investigation could be complete in as little as three weeks. While more complex structures or demanding designs may require months to complete an investigation. We will discuss some of the most common geotechnical testing performed and their timelines. This discussion should give you an idea of how long some of these tests should be expected to take and why.

Many Tests are Comprised of Multiple Steps

Geotechnical testing is often made up of a combination of numerous simple procedures, referred to here as building block procedures, combined to quantify the mechanical or chemical characteristics of a soil. Each of these procedures, though simple, are precise and, per their respective standard, have a designated minimum time frame.

Common “Building Blocks” and Tests with Timelines

Below is a list of some of the most common procedures and a brief description of their purpose.

Preparation (Trimming/molding): Trimming or molding a specimen is performed for two reasons. Firstly, the test requires a specific size/shaped specimen. This is often some type of cylinder or prism which helps simplify and standardize calculations. Secondly, a specimen may require remolding after addition of an additive (lime, fly ash, etc.), or we need a sample to have a particular moisture content and/or density. Trimming or molding a sample can take anywhere from a few minutes to a few hours depending on the type of test to be performed with the given specimen.

Moisture Content: Determination of moisture content can be a stand-alone test, or a routine “building block” for more complex tests.  The moisture content of a specimen is often performed at the beginning and end of a test procedure. This is because saturation, water absorption, and dry and wet density are all values dependent on the moisture content and important for geotechnical design. The time required to dry each specimen is dependent on the soil type. Cohesive soils take the longest amount of time to determine moisture content, where drying can range from 12-24 hours.

Saturation/moisture adjustment: Saturation and moisture adjustment is performed several ways depending on the test being performed. This is the process of adding or removing water from a sample. Certain tests require a soil to be at a specific moisture content. Depending on the procedure being used, saturation or moisture adjustment can take anywhere from a couple hours to a couple days. However, in some extreme cases, such as highly over-consolidated clays, saturation could take weeks to complete!

Consolidation (Triaxial): Triaxial consolidation, or consolidation of a sample within a triaxial cell is specific to consolidated-undrained (CU) and consolidated-drained (CD) tests. The procedure of applying pressure to the exterior of the specimen to consolidate the sample takes no less than 24 hours but can take up to 10 days depending on the specimen and how the consolidation data is analyzed.

Consolidation (1D): Consolidation tests are used to determine the amount of vertical strain (movement) a soil undergoes when subjected to various loading conditions. This test is performed by applying a series of increasing loads in succession, followed by the removal of these loads in stages.  In all, the number of loads typically range from 10 to 14. The reason this test is one of the longest performed in a geotechnical laboratory is that a single load in a one-dimensional consolidation test can take up to 24 hours. Thus, consolidation tests can take up to several weeks to complete depending upon the type of soil being tested.    

Additional Test Timeframes:

The following table provides timeframes for completion of other common tests.  It is important to note that this data is based on an ideal time frame and tests could take longer to run depending upon the type of soil being tested and/or test variations specific to a project.

It is crucial to consider the length of time to complete laboratory testing when defining project performing tests, as the procedures and timelines for each test are dictated by the appropriate standard (ASTM, DOT, USACE, HCFCD, etc.). Differences in timeframe between laboratories is likely due to work backlog or possible inefficiencies present within the laboratories.

Braun Intertec offers a wide range of resources including extensive professional experience on rock and soil mechanics for any type of project. If you have any questions on the tests mentioned above, or other tests you are considering, please reach out to us.

 

Casey Harmeier Staff Engineer

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