As you know, Cone Penetration Testing is not the only method for determining the mechanical properties of soil. Another method is the Standard Penetration Test, or SPT: in this test, a borehole is drilled to a desired depth, then a hollow sampler is inserted and driven downwards with a hammer. The hammer blows are counted until the sampler travels the desired depth (usually 18”) – this number, denoted NSPT, indicates the mechanical properties of the soil. As with CPT data, a handful of corrections are commonly applied: for example, the N60 value indicates NSPT data corrected for the mechanical efficiency of a manual hammer, estimated at 60% at shallow overburden conditions.
In a previous blog, we discussed the pore pressure sensor that is common to most modern CPT cones and briefly introduced why this reading is helpful in soil profiling. Today we’ll take a closer look at how pore pressure data is used to correct and analyze CPT data.
Pore pressure data is used to correct or “normalize” sleeve friction and cone resistance readings in the presence of in-situ moisture and overburden stress. This is especially important in soft, fine-grained soils where in-situ moisture takes longest to dissipate, and in tests at depths greater than 100 feet. Corrections based on pore pressure data also help standardize soil behavior type characterizations when CPT cones of different shapes and sizes are used.
How are these corrections calculated, and how do they work?
The most basic CPT tests classify soil based on tip resistance and sleeve friction measurements. In coarse soils and shallow testing depths, this data may be sufficient to accurately characterize the soil behavior. However, most modern CPT cones incorporate a third measurement: pore water pressure. What does this measurement mean and how can it add to our understanding of soil behavior?
Pore pressure is simply a measure of the in-situ groundwater pressure, i.e. the water pressure in the “pores” between soil grains. This data is used to determine the compressibility and permeability of the soil, as well as indicating groundwater conditions. It is used to correct or “normalize” the sleeve friction and tip resistance readings in the presence of in-situ moisture and overburden stress. This is especially important in soft, fine-grained soils where in-situ moisture takes longest to dissipate, and in tests at depths greater than 100 feet.
As you analyze your CPT data, you are likely to come across several different charts designed to classify soil type based on CPT results.If you are new to the field, these charts can be a bit confusing, so here’s a brief overview of one of the more common chart types.
Soil behavior classification via CPT is fast, efficient, and frequently automated via software. Still, understanding the classification method is important, as it will help you to recognize and determine the cause of any errors or irregularities in the data.
Cone Penetration Testing allows the tester to identify the nature and sequence of subsurface soil types and to learn the physical and mechanical characteristics of the soil – without necessarily taking a soil sample.
How does it work?
During a CPT test, a hardened cone is driven vertically into the ground at a fixed rate, while electrical sensors on the cone measure the forces exerted on it. The zone behavior type of the subsurface layers can be extrapolated from two basic readings: cone or tip resistance and sleeve friction.
Even if you already have a solid grasp of what Cone Penetration Testing is and how CPT rigs test soils, understanding soil test results is a bigger task. You likely already know that CPT rigs are equipped with automated interpretation programs, but that doesn't mean test results are easily readable right away. Fortunately, even if you aren't a technician, it is possible to gain some understanding into soil test results. Read on to find out how.
The basics of soil test results
At the most basic level, the results of CPT testing are based on the relationship between cone bearing, sleeve friction and pore water pressure. With these three measurements, you can learn quite a bit about soil composition and conditions. For example, friction ratio measured by the sleeve is used to determine soil type. Soil is then classified according to the Unified Soil Classification System (USCS).
It could be that you've learned everything there is to know about Cone Penetration Testing, but if you don't know about geotechnical reporting, you're missing out on a big step in the process. A geotechnical report is a tool used to communicate site conditions, as well as design and construction recommendations to be relayed to personnel. In other words, you're taking the results of your CPT testing and putting them into an easy-to-understand report along with relevant conclusions. Sound simple? There's more to it than you might think.
As we've noted in other posts, CPT provides a number of benefits over traditional methods of subsurface soil characterization. These benefits include:
Reports from a specific sounding are easily traced back to the source data, and because CPT is a continuous process, data points in between those reported can be evaluated post-test. This is in contrast to geotechnical boring where individual samples need to be tracked and accounted for from the busy worksite to a remote lab and through to reports and documentation. This can be cumbersome and prone to errors.