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How to Read a CPT Soil Behavior Type Chart

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. First of all, it is important to note that, since a traditional CPT test does not involve a soil sample, these charts are not designed to tell you the exact makeup of the soil. Instead, CPT tests indicate the soil’s physical and mechanical properties, or how it behaves. Hence, a CPT soil classification chart is technically referred to as a Soil Behavior Type (SBT) chart. Most CPT soil charts are derived from tip resistance (or normalized tip resistance, Qt) and friction ratio data. The tip resistance is measured in some unit of pressure (Bars, Pa, PSI, etc) and is usually plotted on the vertical axis. This axis is logarithmic, meaning it increases by orders of magnitude rather than linearly as it gets further from the origin. Thus you will see units of 10, 100 and 1000 marked an equal distance apart. The friction ratio is given on the horizontal axis. It is the ratio of the sleeve friction divided by the tip resistance: the two units of pressure cancel, so this unitless ratio is multiplied by 100 and given as a percent. This percentage is generally low: 10% would be considered a high friction ratio, since the CPT cone experiences greater pressure on its tip due to the shear strength of [...]

By smadi66|December 2nd, 2019|Geotechnical North Carolina, Geotechnical Oklahoma, CPT Data, Geotechnical Oregon, Introductory, Cone Penetration, Soil Testing, Geotechnical Nevada, Geotechnical New Hampshire, Geotechnical New Jersey, Geotechnical New Mexico, Geotechnical New York, Geotechnical Ohio, Geotechnical North Dakota|0 Comments

Intro to CPTu: What Can You Learn From Pore Pressure Data?

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. A CPT cone that is equipped with one or more pore pressure sensors is called a piezocone, and a CPT test using a piezocone is often indicated with the abbreviation CPTu. Piezocones may have between one and three pore pressure sensors, located on the cone (denoted u1), directly behind the cone (u2), or at the top of the friction sleeve (u3). Most piezocones for everyday applications use one sensor located at u2 (see image below). The pore pressure sensor consists of a porous filter (usually made of plastic resin), a small cavity of incompressible, low-viscosity fluid, and a pressure transducer. The filter and tubing between the filter and transducer must be fully saturated with fluid, usually glycerin or silicon oil, to ensure fast and accurate readings. The filter must be replaced frequently so that it does not become clogged with soil. The procedure for the CPTu test is slightly different than the [...]

By smadi66|December 2nd, 2019|Cone Penetration, Soil Testing, Geotechnical Rhode Island, Geotechnical South Carolina, Geotechnical South Dakota, Geotechnical Texas, Geotechnical Utah, Geotechnical Tennessee, Geotechnical Vermont, Geotechnical Pennsylvania, Geotechnical Virginia, CPT Data, Geotechnical Washington, Introductory|0 Comments

Beyond the Basics: Contamination Detection and Other Applications of CPT Equipment

Cone Penetration Testing equipment was originally designed – and is still most commonly used – to characterize subsurface soil behavior types. But when you invest in CPT equipment, you are getting the capability to do much more. A variety of sensors and in-situ samplers can be integrated into CPT modules, making CPT equipment a versatile and efficient choice for contamination detection, environmental site assessment, and other field applications. CPT equipment has several advantages over conventional hollow stem auger drilling and percussion drilling based methods, especially in contaminated soils. Specialized CPT tests can identify contaminants and determine the physical extent of the contamination with minimal disturbance of the soil, thus avoiding costly disposal of drill cuttings and minimizing contact between field personnel and potentially hazardous materials. Here’s an overview of some tests and technologies that you can harness via CPT equipment: Temperature: Temperature data is obviously useful in locating zones of different ground temperature, for example frozen soil. However, it can also help to identify soil contaminants that generate heat due to chemical or biological activity. Electrical Resistivity: The electrical properties of soil are changed when the soil is contaminated. For example, soils containing non-aqueous-phase (NAPL) compounds exhibit higher resistivity than normal, while soils containing dissolved organic compounds such as can be found in landfill leachates have significantly lower resistivity. Fluorescence Detection: Most hydrocarbons produce fluorescence when irradiated with certain kinds of light. Thus, hydrocarbon contamination can be efficiently detected by integrating LEDs of a particular wavelength (or sometimes lasers) into CPT cone modules. The detected wavelength of the fluorescent response to the excitation source is graphed in real time and is used to determine the areas of interest and further define contaminants. The integrated camera or video camera module can also be used to visually inspect in-situ characteristics such as [...]

By smadi66|December 1st, 2019|Geotechnical Kansas, Soil Testing, Geotechnical Services, Geotechnical Delaware, Geotechnical Idaho, Geotechnical Indiana, Geotechnical Louisiana, Geotechnical Kentucky, Geotechnical Illinois, Geotechnical Georgia, Geotechnical Florida, Geotechnical Iowa|0 Comments

Intro to Seismic CPT

What is Seismic Cone Penetration Testing? Seismic CPT or SCPT is a method of calculating the small strain shear modulus of the soil by measuring shear wave velocity through the soil. The small strain modulus is an important quantity for determining the dynamic response of soil during earthquakes, explosive detonations, vibrations from machinery, and during wave loading for offshore structures. The wave speeds and moduli derived from seismic CPT measurements aid in the determination of soil liquefaction potential and improve the interpretation of surface seismic surveys by providing wave speed profiles as a function of depth. Seismic waves from SCPT tests have been detected at depths of up to 300 feet. How does it work? SCPT testing is performed as part of a normal CPT or CPTU test. Equipment consists of a CPT rig, push system, and: SCPT Cone: The SCPT cone is a CPT or CPTU cone that is equipped with one or more geophone sensors. These sensors measure the magnitude and arrival time of seismic shear and compression waves. Wave Generator: Seismic shear waves are generated at the soil surface in one of two ways: The simplest method is to press a steel bar onto the ground lengthwise using the weight of the CPT rig, then strike the end of the bar with a large hammer. An electronic trigger attached either to the hammer or the bar records the exact time of the strike. Another method uses an electronic wave generator attached to the CPT rig. This method increases repeatability and reduces physical strain and testing time for the field team. The CPT test must be paused briefly at the desired intervals to perform the wave generation and data collection. These pauses may be used to conduct a pore pressure dissipation test as well. Data Acquisition System: As [...]

By smadi66|December 1st, 2019|Geotechnical New Mexico, Geotechnical New York, Geotechnical North Dakota, Geotechnical Oklahoma, Geotechnical Ohio, Geotechnical Oregon, Geotechnical Pennsylvania, Geotechnical North Carolina, Introductory, Soil Testing, Geotechnical New Hampshire, Geotechnical New Jersey|0 Comments

Understanding the Relationship between SPT Data and CPT Data

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. Since SPT is one of the most common in-situ soil testing methods, you may find it necessary to compare information from both SPT and CPT tests, or convert from one set of parameters to the other, for example from SPT N60 values to CPT tip resistance values. Several methods have been proposed for calculating this relationship. Below are two of the most frequently used: Robertson and Campanella: This method for correlating SPT and CPT data uses the following relationship between SPT N60 data and CPT tip resistance: (qc/pa)/N60 qc = tip resistance (psi) pa = atmospheric pressure (psi) Soil behavior type can be determined from this equation based on the following table: This is perhaps the simplest method for relating the results of these two tests, but it can cause some confusion when the results fall on the border of two soil behavior type zones, or in situations where the ratio of CPT to SPT data could indicate one of several different soil types. Jefferies and Davies: This is a more robust method for determining SPT N-values based on CPT data, or vice versa. It avoids the discontinuities of [...]

By smadi66|December 1st, 2019|Soil Testing, Geotechnical Idaho, Geotechnical Louisiana, Geotechnical Indiana, Geotechnical Maine, Geotechnical Kentucky, Geotechnical Illinois, Geotechnical Georgia, Geotechnical Maryland, Geotechnical Iowa, Geotechnical Kansas, CPT Data|0 Comments

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 [...]

By smadi66|November 29th, 2019|Uncategorized, Geotechnical South Carolina, Geotechnical Ohio, CPT Dictionary, Geotechnical Pennsylvania, Geotechnical North Carolina, Soil Testing, Geotechnical New Mexico, Geotechnical New York, Geotechnical North Dakota, Geotechnical Oklahoma, Geotechnical Oregon, Geotechnical Rhode Island|0 Comments

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 [...]

By smadi66|November 29th, 2019|Cone Penetration, Soil Testing, Experienced, Geotechnical South Dakota, Geotechnical Texas, Geotechnical Utah, Geotechnical Vermont, Geotechnical Virginia, Geotechnical Tennessee, Geotechnical Washington, Geotechnical Wisconsin, Geotechnical Wyoming, Geotechnical West Virginia, CPT Dictionary|0 Comments