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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|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, Geotechnical North Carolina, Geotechnical Oklahoma|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|Soil Testing, Geotechnical New Hampshire, Geotechnical New Jersey, Geotechnical New Mexico, Geotechnical New York, Geotechnical North Dakota, Geotechnical Oklahoma, Geotechnical Ohio, Geotechnical Oregon, Geotechnical Pennsylvania, Geotechnical North Carolina, Introductory|0 Comments

CPT Dictionary: Overburden Stress

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. The formula for overburden stress is given by: σvo = overburden stress ɤi = in situ density of soil layer hi = height of soil layer If it’s been a while since you’ve seen summation notation, this means that for each soil layer, you multiply the density of the layer by its height, then add all the resulting weights together until the pressure at the desired depth is known. In practice, the exact height and density of the soil layers at the test site are usually not known, so you may have to determine an average density based on what you do know about the geology of the area. CPT measurements of tip resistance, sleeve friction and pore pressure tend to increase along with increasing depth and increasing overburden stress. This effect can be seen in the graph at right. For this reason, we correct for overburden stress in calculating the normalized friction ratio and normalized tip resistance: to ensure that your data is consistent, it is important to use these parameters in deep tests and in soft, fine-grained soils, as we discussed in an earlier blog. In addition to normalized CPT parameters, overburden pressure allows us to understand and calculate the following engineering parameters: Effective overburden stress: the effective stress on [...]

By smadi66|December 1st, 2019|Geotechnical Nebraska, Geotechnical Massachusetts, Geotechnical Montana, Geotechnical Nevada, Geotechnical New Hampshire, Geotechnical New Jersey, Soil Testing, CPT Dictionary, Geotechnical Michigan, Geotechnical Missouri, Geotechnical Minnesota, Geotechnical Mississippi|0 Comments

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 [...]

By smadi66|November 29th, 2019|Geotechnical Montana, Geotechnical Nevada, Geotechnical New Hampshire, Geotechnical New Jersey, Geotechnical New Mexico, Geotechnical New York, Geotechnical Missouri, Geotechnical Minnesota, Geotechnical Mississippi, Geotechnical Nebraska, DCP|0 Comments