Defining CPT parameters

CPeT-IT uses established empirical correlations to estimate geotechnical parameters. Many of them have constants that have a range of values depending on soil type, geologic origin and other factors. The software uses default values that have been selected to provide, in general, conservatively low estimates of the various geotechnical parameters. These default values are assigned to every new CPTU data you import automatically.

The following empirical correlations have been used to calculate parameters:

Input:

•	Units for display (Imperial or metric) (atm. pressure, pa = 0.96 tsf or 0.1 MPa)

•	Elevation of ground surface (ft or m)

•	Depth to water table, zw (ft or m) – input required for each CPT sounding. Equilibrium water pressures are assumed hydrostatic relative to the input GWL

•	Net area ratio for cone, a (default to 0.80)

•	Relative Density constant, CDr (default to 350)

•	Undrained shear strength cone factor for clays, Nkt (default to 14)

•	Over Consolidation ratio number, kocr (default to 0.33)

•	Unit weight of water, (default to gw = 62.4 lb/ft³ or 9.81 kN/m³)

•	Probe radius (default to 0.0183 m)

Output:

Total cone resistance, qt (tsf or MPa)	qt = qc + u x (1-a)
Friction Ratio, Rf (%)	Rf = (fs/qt) x 100%
Soil Behavior Type (non-normalized), SBT	see note
Unit weight, g (pcf or kN/m³)	based on SBT, see note
Total overburden stress, sv (tsf)	svo = g x z
Insitu pore pressure, uo (tsf)	uo = gw x (z - zw)
Effective overburden stress, s'vo (tsf )	s'vo = svo - uo
Normalized cone resistance, Qt1	Qt1 = (qt - svo) / s'vo
Normalized friction ratio, Fr (%)	Fr = fs / (qt - svo) x 100%
Normalized Pore Pressure ratio, Bq	Bq = u – uo / (qt - svo)
Soil Behavior Type (normalized), SBTn	see note
SBTn Index, Ic	see note
Normalized Cone resistance, Qtn (n varies with Ic)	see note
Estimated permeability, kSBT (cm/sec or ft/sec)	see note
Equivalent SPT N60, (blows/ft or blows/30cm)	see note
Estimated Constrained Modulus, M	see note
Estimated Relative Density, Dr, (%)	see note
Estimated Friction Angle, f', (degrees)	see note
Estimated Young’s modulus, Es (tsf or MPa)	see note
Estimated small strain Shear modulus, Go (tsf or MPa)	see note
Estimated Undrained shear strength, su (tsf or kPa)	see note
Estimated Undrained strength ratio	su/sv'
Estimated Over Consolidation ratio, OCR	see note

In order to view or edit these constants select a CPTU sounding and from the menu CPT data click on the CPT Properties command (alternatively you may click on the button located on the CPT File Manger toolbar). The following dialog will appear:

CPT calculation properties dialog

The calculation properties are:

•	Average interval: Available selection is 1, 3 or 5. The number denotes how many values will be used in order to produce averaged results. If 1 is selected there is no averaging of data. When either 3 or 5 is selected, the data are averaged every 3 or 5 intervals and presented as a rolling average.

•	Ground elevation: Elevation of the free ground surface. This value can be entered for each CPT file or applied to all files.

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GWT: Ground water table below the free ground surface starting from zero value at the surface. For over-water projects, a negative value should be provided to identify that the water level is above the ground surface. For over-water CPT it is often common that zero load CPT base line measurements are taken at the mudline and in these cases the GWT should be input as zero, since the total weight of the water above the mudline has been removed in the baseline measurements. For over-land projects, this is typically the depth below ground surface.

•	Probe radius: Radius of the cone used for the calculation of ch in the dissipation module.

•	Cone area ratio: The net area ratio for the cone (default value to 0.85).

•	Dr constant: Relative density constant, CDr (default value to 350).

•	Undrained strength factor: Undrained shear strength cone factor for clays, Nkt (default value to 14).

•	OCR number: Over consolidation ratio number, kocr (default value to 0.33).

•	Unit weight of water: Unit weight of water (default value to 9.81 kN/m³ or 62.40 lb/ft³).

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Auto transition layer detection: When checked the software will try to detect data that are in transition from either clay to sand or vise-versa. Data belonging to transition zones will not be plotted in the estimations plots and will not contribute to average values used in the Geotechnical Section module. Click on the Tune button to fine tune the detection algorithm.

In order to apply any changes made to the selected CPTU just click on the OK button. If you wish the changes made to be the default values for every CPTU that you will create click on the Save as default button. If you click on the Apply to all button, all values entered in this dialog will be applied to every CPTU sounding in the current project.

Closing the above dialog will not cause any recalculation of the estimated properties. To do so you will either need to recalculate all CPTU soundings or just the selected one. Click on the Calculation menu and select the appropriate command. Alternatively you may click on the button in order to recalculate all CPTU soundings.