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Why Are There So Many Kinds of CPT Rigs?

- Posted by VertekCPT


cpt-rigsAs you look through our website, you'll see that there are a number of different types of CPT Rigs. As you can imagine, they each have their purpose, or 'application'. In order to understand why different types of CPT Rigs exist, it's helpful to remember how CPT works in the first place.

The goal of Cone Penetration Testing is to drive a hardened cone vertically into the the ground and to keep it moving at a specific rate of progress. The force that it takes to maintain the cone moving downward at a defined rate is an indicator of how hard the soil is at a given depth. The friction the cone sleeve encounters along the way gives us an indication of the make up of the soil.

As you can imagine, as the cone progresses further downward and encounters different types of soils, sands, clays and rocks it can take a substantial amount of pressure to keep it moving! One of the first factors that influences the design of CPT Rigs is the maximum amount of pressure that will be required to perform a specific test, to a specific depth in a given geological area. As much as 20 tons of downward force may be required to perform a broad enough range of tests to make a given rig viable for a market. 

For every ton of downward pressure on the test cone through the rod system, you have to have a reaction force equal to this to keep the Rig from lifting up. This means that either the Rig has to be heavier than the maximum push force, plus a safety margin, or it needs to be anchored down in some way as to reliably resist the upward force generated by the test (or 'sounding').

Depending upon the types of surface conditions that you may face in your area, you may or may not be able to effectively anchor a Rig down. This means that you may need a larger, heavier Rig to perform some tests. If an anchor system is practical, then it may make more sense to look at CPT Rigs that are lighter in weight but that are built in a sturdy fashion to enable an anchoring system to work.

The other primary factor that goes into shaping the design of CPT Rigs is the terrain that the Rig will have to traverse. Muddy, swampy or sandy ground, or ground that is sensitive to weight bearing loads, may require a tracked CPT Rig to work effectively. CPT soundings that take place in a wooded environment, or around structures such as bridge abutments may require a small Rig that is more maneuverable. 

In some cases, very small CPT Rigs, portable hand-operated CPT Rigs or even CPT Rigs that are mounted on the back of a drilling vehicle might be a good option.

Topics: Cone Penetration, Introductory