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 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, ft, is calculated from the difference between the two measurements:
ft = fs – (u2 ∙ Asb – u3 ∙ Ast)/As
fs = sleeve friction
Ast = cross-sectional area of the top of the cone
Asb = cross-sectional area of the bottom of the cone (often the same as Ast)
As = surface area of the friction sleeve
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, Fr:
Fr = ft/(qt – σvo)
qt = corrected cone resistance
σvo = total vertical stress
If u3 pore pressure data is not available, the uncorrected sleeve friction fs is substituted for ft.
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!