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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 Minnesota, Geotechnical Mississippi, Geotechnical Nebraska, CPT Data, Introductory, Geotechnical Maine, Geotechnical Massachusetts, Geotechnical Montana, Geotechnical Michigan, Geotechnical Nevada, Geotechnical Missouri, Geotechnical Maryland|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|Geotechnical Maryland, Geotechnical Iowa, Geotechnical Kansas, CPT Data, Soil Testing, Geotechnical Idaho, Geotechnical Louisiana, Geotechnical Indiana, Geotechnical Maine, Geotechnical Kentucky, Geotechnical Illinois, Geotechnical Georgia|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 Michigan, Geotechnical Maryland, Geotechnical Iowa, Geotechnical Kansas, Geotechnical Louisiana, Geotechnical Maine, Geotechnical Massachusetts, Geotechnical Indiana, Geotechnical Kentucky, Geotechnical Illinois|0 Comments