As we noted in a previous post about Soil Quality, there are a wide range of reasons that soil needs to be tested. For some applications, it is important to get data about soil that is sub-surface, and in many cases getting data from deep under a site can be useful or essential. When most people imagine how you would gather data from soil that is deep underground, they imagine using a drilling rig of some kind. Sure enough, there are special kinds of boring tools that will let you drill deep into the ground and extract a sample of the soil at depth for analysis.
Advantages & Disadvantages of Geotechnical Boring
Using Geotechnical Boring, whether it be small-diameter or large-diameter equipment allows users to see the solid that is extracted. This can be useful for gaining an understanding of the sub-surface topology if a goal is to create a multi-dimensional map of the subsurface Geological conditions. There are significant disadvantages however to using Geotechnical Boring to obtain soil samples for testing. One disadvantage is that the operation of boring is for obtaining samples only, you can’t gather data from the boring activity itself and therefore all of this investment in equipment, labor and time provides value only in that it presents a sample for testing.
Another disadvantage is that the soil being sampled then needs to be tested using some type of laboratory equipment. This often means removing a large number of samples from the site, getting them safely in an organized way to a lab facility somewhere, hopefully nearby, and waiting for the lab results to come back. If there are apparent conflicts in data, or a particular part of the site needs more evaluation, the entire process needs to be started up from scratch again.
Perhaps the biggest disadvantage with Geotechnical Boring is that the act of boring for the sample and extracting it can substantially change the physical characteristics of the sample itself. The sample can be ‘disturbed’ in that at a boundary layer between two soil types, soils can become intermixed and a clear boundary condition made harder to identify. The sample is likely loosened and soil that is really quite solid can test as less dense or cohesive than it actually is. The act of boring can affect the grain size of the sample being extracted.
An Introduction into Cone Penetration Testing (CPT)
CPT (Cone Penetration Testing) is gaining rapid acceptance among the engineering community because it addresses each of these disadvantages and more. The act of the ‘push’ as the CPT tool is progressed into the ground is what generates the data for evaluation. There is no wasted time, energy and resources digging holes (and possibly having to fill them back up or seal them off) just to get a sample of what’s way down deep. CPT gives you the data that you need as you are getting to the depth you want data on.
CPT is an In Situ test, providing advantages that are especially beneficial if you do not have the time, expense and risk associated with sending samples off to a lab. Samples can’t get disorganized or lost because CPT uses a data-logger to ensure that all of the root data taken is recorded and saved in a well-organized format. Engineers can gather data in real-time, and as a result move along much faster (which can provide immense savings to the overall project costs). Site owners and engineers can call for additional soundings ‘on the spot’ to corroborate data or to fill in gaps in information between spots that are identified for sounding on the site plan. CPT measures the soil properties in the native conditions that the soil is in, whether that be under extreme pressure, subject to sub-surface water or conditions exist at a specific boundary layer. CPT data then, in addition to being lower cost and faster, tends to be a more accurate representation of the actual conditions underground.
Geotechnical Boring can give users an advantage in the field because it can add value to a set of site CPT soundings. By drilling a hole (or more than one if conditions seem to change dramatically over a site being evaluated), your CPT data can be evaluated against a benchmark of data, including a visual review of site borings. This helps site owners and engineers to be able to place the CPT sounding data into context and is a nice quality check that the CPT data is being correctly interpreted for the site.
Geotechnical Boring Lacks the Accuracy and Efficiency that CPT Offers
Although Geotechnical Boring may seem like a sufficient option for site subsurface investigation, Geotechnical Boring doesn’t provide the accuracy and efficiency that Cone Penetration Testing (CPT) can offer. Geotechnical Boring has the advantage that it uses many of the skills of conventional well drilling. Because the Boring operations and technical analysis, such as laboratory tests, are separate, Geotechnical Boring can require less skilled operators on site than CPT. However, a price is paid in efficiency as often the site samples are sent to a separate lab for testing. This costs additional time and money, while increasing earthen waste.
Though both Geotechnical Boring and CPT provide suitable options for obtaining useful information, there are many more benefits to using CPT over Geotechnical drilling. CPT is not only a cost effective option but it also provides immediate electronic data for review. You’ll find that as you embark on CPT, that your customers value the immediate results you can provide them, as well as your ability to sample more locations more quickly than Geotechnical Boring rigs can, often at an increase of 3 : 1. Not only is CPT a better option for proficiency, but it’s usually a lower cost solution while delivering more data.