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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 Jersey, 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|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|Geotechnical Oklahoma, Geotechnical Oregon, Geotechnical Rhode Island, Uncategorized, Geotechnical South Carolina, Geotechnical Ohio, CPT Dictionary, Geotechnical Pennsylvania, Geotechnical North Carolina, Soil Testing, Geotechnical New Mexico, Geotechnical New York, Geotechnical North Dakota|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 New Mexico, Geotechnical New York, Geotechnical Missouri, Geotechnical Minnesota, Geotechnical Mississippi, Geotechnical Nebraska, DCP, Geotechnical Montana, Geotechnical Nevada, Geotechnical New Hampshire, Geotechnical New Jersey|0 Comments

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