DId you know that our CPTSND Data Acquisition program can average repeat seismic strikes?
DCP (Dynamic Cone Penetration) Testing is a simple, reliable and cost-effective method to evaluate the in-situ stiffness profile of soil to a depth of about three feet. Its extreme portability, minimal disturbance of the subgrade, and ability to produce a continuous depth profile make it an ideal system for testing the mechanical properties of a pavement system during any stage of construction.
The Vertek Lightweight CPT Push System is the most portable hydraulic CPT push system on the market. Offering 10 tons of push force, yet compact enough to be transported and operated by a two-person team, this system is ideal for testing locations that would be inaccessible to a rig-based or truck-mounted system.
Weighing only 480 pounds, the hydraulic load frame is can be transported to the job site via truck or small trailer, then unloaded and rolled to hard-to-access test locations by hand. The system is designed so that the handle weight is less than 25 lbs when tilted on its wheels for travel, and large tires make the system easy to roll on uneven ground.
DCP (Dynamic Cone Penetrometer) testing is a highly portable, lightweight soil testing method. It is ideal for shallow tests and can be carried by hand from one location to the next, making it a good choice for applications such as road bed construction and maintenance. However, traditional DCP testing has drawbacks: though the equipment is lightweight, the test requires two people—one to operate the hammer and the other to measure the displacement with each blow. This manual process makes the test quite labor-intensive, and human errors in measurement and recording can make it difficult to obtain consistent results.
The Vertek Smart DCP system takes this portable, low-cost testing method into the 21st century with laser measurement and real-time data acquisition.
In our last blog, we discussed using the CPT to estimate the shear strength of soil, which helps gauge how soil will behave during changes in stress. One important application of this capability is the estimation of soil liquefaction potential, meaning the potential of soil to dramatically lose strength when subjected to changes in stress.
Liquefaction is of particular concern in sandy, saturated soils. Shaking due to an earthquake or other sudden force causes the grains of loosely packed, sandy soils to settle into a denser configuration. If the soil is saturated and the loading is rapid, pore water does not have time to move out of the way of settling soil: pore water pressure rises, effectively pushing the soil grains apart and allowing them to move more freely relative to each other. At this point, the soil can shift and flow like a liquid—hence the name liquefaction.
Shear strength is the ability of a material to resist shear forces—that is, forces that produce a sliding failure in the material parallel to the direction of the force. The diagram at right demonstrates shear stress, along with tensional and compressional stress. (What's the difference between a stress and a force? Stress is defined as force per area.)
How is this relevant to soil testing? Well, consider a sliding failure in soil, such as occurs along a fault plane in an earthquake. Shear strength tells us a great deal about how the soil will behave under shear forces and during changes in stress, for example due to an earthquake or excavation.
Overburden stress, also called vertical stress or overburden pressure, is the pressure imposed on a layer of soil by the weight of the layers on top of it. Overburden stress can cause errors or drift in CPT measurements, creating the need for correction factors in deeper tests depths and soft or fine-grained soils. However, overburden stress is also useful in determining the soil’s mechanical properties. In this blog, we’ll give an overview of the effect of overburden stress on CPT testing and what we can learn from it.
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.
CPT cones are available in multiple sizes, but the 10 cm2 cone is the industry standard. Other sizes, the most common of which is the 15 cm2 cone, are essentially scale models of the 10 cm2 cone, having the same proportions as specified by the ASTM Standard for CPT testing.
What factors determine what cone size you should use?
Most CPT cones range from 5 cm2 to 15 cm2 in cross-sectional area, though smaller cones (down to 1 cm2) are used in specialized lab or research applications. Different cone sizes have different advantages depending on the testing situation:
In a previous blog, we talked about how pore pressure data is used to correct and adjust soil behavior type characterizations – but this is only one application of this important and revealing information. Pore pressure data can also be used to estimate the depth of the water table and the direction and rate of groundwater flow. This information is useful both for site characterization and for geo-environmental and remediation applications.
What is a Pore Pressure Dissipation Test?
As a CPT cone is pushed into saturated subsurface soil, it creates a localized increase in pore pressure (denoted excess pore pressure, ui) as groundwater is pushed out of the way of the cone. In a pore pressure dissipation test, the downward movement of the cone is paused and the time it takes for the pore pressure to stabilize is measured. This stable pore pressure is called equilibrium pore pressure, uo. This information allows the user to identify important hydrogeologic features: