Archives for February 2015

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, correct the cone resistance for pore water pressure effects.

q_t = q_c + u₂(1 – a)

Corrected cone resistance is used in calculating the normalized cone resistance, Q_t, which indicates the cone resistance as a dimensionless ratio while taking into account the in-situ stress:

Q_t = (q_t – σ­_vo)/ σ′­_vo

Some geologic knowledge of the test site – for example soil unit weight and groundwater conditions – is necessary to estimate σ­_vo and σ′­_vo.

Correction of sleeve friction data:

Sleeve friction data is sometimes corrected for the effects of excess pore pressure – that is, pore pressure that is generated in front of the cone as it is pushed into the ground. Excess pore pressures are usually different at the top of the cone (the pore pressure measurement denoted u3) and the bottom of the cone (the pore pressure measurement denoted u2). The corrected sleeve friction, f_t, is calculated from the difference between the two measurements:

f_t = f_s – (u₂ ∙ A_sb – u₃ ∙ A_st)/A_s

This correction is not always possible, since many CPT cones have only one pore pressure sensor, usually located at u₂. When u₃ pore pressure data is available, the corrected sleeve friction is used to calculate the normalized friction ratio, F_r:

F_r = f_t/(q_t – σ­_vo)

If u₃ pore pressure data is not available, the uncorrected sleeve friction f_s is substituted for f_t.

How long is this going to take?

If you’re not looking forward to all this number-crunching, don’t worry – a good CPT data acquisition system will make these corrections for you. Vertek’s HT DataPack is an all-in-one unit for CPT data collection, analysis and plotting. Cone calibration information is stored in nonvolatile memory, and available corrections can be automatically applied to the data. You will be able to quickly analyze your data and even generate report-quality plots in the field. To learn more, download our catalog or check out our website!

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: [Read more…]