CPT Dictionary: Soil Shear Strength

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. The in-situ shear strength of soil is difficult to measure, and many methodologies for doing so have been proposed. In general, estimating undrained shear strength--that is, the shear strength of the soil with in-situ moisture--using the CPT is accomplished via the relationship between overburden stress and cone resistance, as shown in the equation below. su = (qc – σvo)/Nk Where: su = undrained shear strength (unitless) qc = cone resistance (psi) σvo = overburden stress (psi) Nk = empirical cone factor (a unitless constant) Nk is determined in the lab, for example via triaxial compression tests. The exact value varies based on the type of reference test used, so it is important to be consistent in this regard. Most test methods return values between 10 and 30, varying with factors such as OCR (over-consolidation ratio), pore pressure, and soil plasticity. Several alternative methods may be used to estimate undrained shear strength via CPT, depending on the test conditions and available data. One such method uses pore pressure at u2 (directly behind the cone) in place of overburden stress: su = (qc – u2)/Nk The disadvantage of this method is [...]

CPT Dictionary: Soil Liquefaction

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. This dramatic reduction of soil stiffness and strength causes soil to shift under pre-existing forces—say, the pressure of a building’s foundation or the pull of gravity on a slope. The increased pore pressure also increases the force of the soil on in-ground structures such as retaining walls, dams, and bridge abutments. How can the potential for these effects be evaluated using the CPT? The subject is complex, as the wealth of research on the subject over several decades shows! Many approaches for determining cyclic liquefaction potential rely on the cyclic stress ratio (CSR), which requires a seismic analysis of the site. It expresses the ratio of the average cyclic shear stress in an earthquake of a given magnitude and the effective vertical overburden stress at the test site. CSR = 0.65(MWF)(amax/g)(σvo/σ′vo)rd Where: MWF = Magnitude Weighting Factor = (Magnitude)2.56/173 amax = maximum ground surface acceleration g = acceleration of gravity, 9.81m/s2 σvo [...]

Smart DCP – Get the app for instant data logging and laser accuracy!

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. Rather than relying on by-hand measurement to track the displacement of the cone, our laser measurement system makes this process both instantaneous and accurate, eliminating the need for a two-person test. Displacement data is transmitted wirelessly and collected via our convenient smartphone app. Where a traditional DCP system would require two people, Smart DCP requires only one person and a phone--yet it allows the test to be completed much faster. Time and labor savings don't stop when the in-situ test is completed: rather than having to manually enter and plot your data on your laptop or computer, our smartphone app lets you log data in real time and gives you access to powerful presentation and analysis capabilities in the field. The raw and processed data can be transmitted by text or email in multiple formats. Like any other app, it can simply be downloaded from the app store on your phone and is available for iOS and Android operating systems. See the Smart DCP system in action in the video below, and check out our website for more information about [...]

Human-Portable Hydraulic Power: The Vertek Lightweight CPT Push System

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. The hydraulic power pack and cylinders, weighing 430 lbs and 335 lbs respectively, are independent of the frame for ease of transportation. The system is easy to assemble and disassemble via hydraulic quick disconnects. The twin cylinders are coupled by a platen that can push or pull digital electronic or mechanical cones and water or soil samplers. The anchoring system includes four sturdy augers, a drive unit and all necessary tools. Watch the easy set-up and see the system at work in the video below. At Vertek CPT, we love to develop innovative yet practical CPT solutions with real ROI. Our Lightweight CPT Push System offers ultra-mobile yet robust hydraulic push power to bring your CPT business wherever you need to go. From lab applications to remote locations on rough terrain, this system is highly portable, economical, and provides enough depth and power for many types of soil tests. [/fusion_youtube]

Data Analysis With DCP

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 following simple equation is traditionally used to express the stiffness of a material from DCP test values: PR = Depth of Penetration / Number of Blows If you are new to DCP testing, you may be wondering whether the PR value can be used to calculate to other, more familiar geotechnical parameters, and whether DCP test results correlate well with those from other testing systems. Much has been researched and written on this subject, and the short answer is yes —DCP testing can easily and repeatably measure the same parameters as other in-situ and lab-based soil testing methods. For example, the California Bearing Ratio (CBR) test is another penetration test commonly used to measure the load bearing capacity of road beds. Perhaps you want to know the CBR values for a test site, but you have opted for a DCP system instead, due to its simplicity and lower cost. No problem! PR values can be converted to CBR values by applying a simple equation. This widely used conversion was developed by the U.S. Army Corps of Engineers and is used by many state DOTs and federal agencies: Log (CBR) = 2.465 - 1.12 Log (PR) This calculation and many others can be performed automatically by a state-of-the-art DCP setup. The Vertek SmartDCP kit can be operated and transported by a single user by hand, and provides instantaneous data collection and graphing capabilities via smartphone app. Data can [...]

Seismic Averaging in SCPTu testing

Did you know that our CPTSND Data Acquisition program can average repeat seismic strikes? Once you have a strike on the screen, simply accept (retain) the strike and then add another strike of the same type ( A strike for example). This second strike will display below the first strike and when the second strike is accepted (retained) it will be averaged with the first strike and only the averaged strike will remain on the screen. If a third strike is added and then accepted (retained) it will be averaged with the result of the first two. (NOTE: Our software does not retain all the individual strikes- once they are averaged only the average is on file) Averaging strikes is recommended by some of the top GeoTechs in the nation!

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