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What is Triaxial Testing and is it the Best Method for Testing Soil?

Those familiar with soil testing probably already know that there are a number of ways to test soil. One of the most common methods is the Standard Penetration Test, which is best known for its simplicity and versatility, but is held back by its lack of accuracy compared to more advanced options. More advanced methods include, of course, Cone Penetration Testing and Mud Rotary Drilling, both of which are common. Another common method is Triaxial Testing. What is Triaxial Testing? In order to conduct Triaxial Testing, you need a Triaxial Apparatus, which is made up of a Triaxial cell, universal testing machine and pressure control panel. For testing soil and other loose granular materials like sand and gravel, the material is placed in a cylindrical latex sleeve and submerged into a bath of water, or another liquid, which puts pressure on the sides of the cylinder. A circular metal plate at the top of the cylinder, called a platen, then squeezes the material. The distance the platen travels is measured, along with the net change in volume of the material. Like Cone Penetration Testing, Triaxial Testing is used to measure the properties of soils, but can also be used on more solid materials like rock. Typically, Triaxial Testing is used to solve problems of stability by: Determining the shear strength and stiffness of soil when retaining reservoirs of water Measuring stress/strain behavior Monitoring the internal response of the particulate medium It is also used for pore water pressure measurement and determining contractive behavior, which is common in sandy soil. As such, this soil testing method is well-suited to helping engineers improve their building designs while limiting structural/build failures by imparting a proper understanding of material behavior and an assessment of the characteristics of a build site. Primary benefits of Triaxial [...]

By smadi66|December 3rd, 2019|Geotechnical Maryland, Geotechnical Minnesota, Geotechnical Iowa, Geotechnical Kansas, Introductory, Cone Penetration, Soil Testing, Geotechnical Louisiana, Geotechnical Indiana, Geotechnical Maine, Geotechnical Kentucky, Geotechnical Massachusetts, Geotechnical Michigan|0 Comments

How to Interpret Soil Test Results from CPT Testing

Even if you already have a solid grasp of what Cone Penetration Testing is and how CPT rigs test soils, understanding soil test results is a bigger task. You likely already know that CPT rigs are equipped with automated interpretation programs, but that doesn't mean test results are easily readable right away. Fortunately, even if you aren't a technician, it is possible to gain some understanding into soil test results. Read on to find out how. The basics of soil test results At the most basic level, the results of CPT testing are based on the relationship between cone bearing, sleeve friction and pore water pressure. With these three measurements, you can learn quite a bit about soil composition and conditions. For example, friction ratio measured by the sleeve is used to determine soil type. Soil is then classified according to the Unified Soil Classification System (USCS). CPT can also measure: Soil parameters Computer calculations of interpreted soil behavior types (SBT) Additional geotechnical parameters It's also possible to determine temperature shifts and zero load offset through the use of baseline readings. This essentially means comparing test results to those generated from initial testing before work begins on a site. With careful observation, it's possible to determine even more about the soil tested. Some examples include noting trends in water content to determine the type of soil (ie, sand does not retain water as well as clay) and knowing that larger values of cone resistance and sleeve friction usually indicate coarser soils, while lower values tend to indicate fine-grained soils. Although they won't put you on the level of a trained technician, these basics should make soil test results much easier to understand. More importantly, with this information in mind, you should have a much greater understanding of CPT testing as [...]

By smadi66|December 3rd, 2019|Geotechnical Mississippi, CPT Data, Introductory, Soil Testing, Geotechnical Louisiana, Geotechnical Kentucky, Geotechnical Maine, Geotechnical Michigan, Geotechnical Massachusetts, Geotechnical Missouri, Geotechnical Maryland, Geotechnical Minnesota, Geotechnical Kansas|0 Comments

CPT 101: Determining Soil Profiles from CPT Data

Cone Penetration Testing allows the tester to identify the nature and sequence of subsurface soil types and to learn the physical and mechanical characteristics of the soil – without necessarily taking a soil sample. How does it work? During a CPT test, a hardened cone is driven vertically into the ground at a fixed rate, while electrical sensors on the cone measure the forces exerted on it. The zone behavior type of the subsurface layers can be extrapolated from two basic readings: cone or tip resistance and sleeve friction. Cone Resistance, denoted qc, represents the ratio of the measured force on the cone tip and the area of the normal projection of the cone tip. The cone resistance indicates the undrained (i.e., including in-situ moisture) shear strength of the soil. Sleeve Friction, denoted fs, is the friction force acting on the sleeve divided by its surface area. The relationship between these two measurements is expressed in the Friction Ratio, denoted Rf and given as a percent. It is the ratio of the sleeve friction to the cone resistance. High friction ratios (high friction, low cone resistance) indicate clayey soils, while low friction ratios indicate sandy soils. The relationship between friction ratio and cone resistance is the simplest method of identifying soil strata with a CPT system, and is especially convenient because the soil behavior type can be extrapolated immediately as the data is collected. An example soil classification chart is given below (though this example uses the corrected cone ratio qt, which we’ll discuss in another blog). As you can imagine, several factors can affect the accuracy of these predictions, for example: Overburden Stress: the pressure exerted on a substrate by the weight of the overlying material Pore Water Pressure: the pressure of the groundwater in the gaps between soil [...]

By smadi66|December 2nd, 2019|Cone Penetration, Geotechnical Louisiana, Geotechnical Maine, Geotechnical Michigan, Geotechnical Massachusetts, Geotechnical Missouri, Geotechnical Montana, Geotechnical Maryland, Geotechnical Minnesota, Geotechnical Mississippi, Geotechnical Nebraska, CPT Data, Introductory|0 Comments

CPT 102: Common Corrections in CPT Data Analysis

In a previous blog, we discussed the pore pressure sensor that is common to most modern CPT cones and briefly introduced why this reading is helpful in soil profiling. Today we’ll take a closer look at how pore pressure data is used to correct and analyze CPT data. Pore pressure data is used to correct or “normalize” sleeve friction and cone 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. Corrections based on pore pressure data also help standardize soil behavior type characterizations when CPT cones of different shapes and sizes are used. How are these corrections calculated, and how do they work? Correction of cone resistance data: The corrected cone resistance, qt, corrects the cone resistance for pore water pressure effects. qt = qc + u2(1 - a) qc = cone resistance u2 = pore pressure measured directly behind the cone a = cone area ratio (this value is dependent on the design and geometry of the cone, and is determined via lab calibration) Corrected cone resistance is used in calculating the normalized cone resistance, Qt, which indicates the cone resistance as a dimensionless ratio while taking into account the in-situ stress: Qt = (qt – σvo)/ σ′vo σvo = total vertical stress σ′vo = effective vertical stress (the stress in the solid portion of the soil – in other words, the total vertical stress minus the stress due to in-situ water and air) Some geologic knowledge of the test site – for example soil unit weight and groundwater conditions – is necessary to estimate σvo and σ′vo. Correction of sleeve friction data: Sleeve friction data is sometimes corrected for the effects of [...]

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

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]

By smadi66|November 29th, 2019|Geotechnical Indiana, Geotechnical Kentucky, Geotechnical Illinois, Geotechnical Michigan, Geotechnical Maryland, Geotechnical Iowa, Geotechnical Kansas, Geotechnical Louisiana, Geotechnical Maine, Geotechnical Massachusetts|0 Comments