Groundwater Depth for Standard Onshore Testing

The ground water depth is critical to many derived results, and can be defined in a number of ways. In order of priority:

CPT_GENERAL table, Groundwater_Depth field
CPT_POINT_PARAMETERS table, Groundwater_Depth field
CPT_PROJECT_PARAMETERS table, Groundwater_Depth field
CPT_PROJECT_PARAMETERS table, Groundwater_Elevation field

In the CPT_PROJECT_PARAMETERS table, the Water_Density_Field field is also required, and should be set appropriately for fresh or salt water.

On the CPT_GENERAL table define:

CPT_GENERAL.Offshore_Water_Depth: Nothing
CPT_GENERAL.Zero_Location: S – Surface or Nothing
CPT_GENERAL.Pre_drilled_Depth: Nothing or value as required, doesn't influence calculation
CPT_GENERAL. Groundwater_Depth: Distance from ground level (0 depth) to the groundwater surface, or use Groundwater_Depth fields on other tables listed above

In situ pore pressure is calculated relative to the atmospheric pressure using the following formula:

$\begin{array}{l}If\quad z \quad < \quad z_w \quad \text{then}\quad u_o = 0 \\ \ \\ If \quad z \quad \geq z_w \quad \text{then}\quad u_o = (z-z_w) \cdot \gamma_w\end{array}$

Where:

$\begin{array}{l}z\end{array}$ is the depth below the reference level
$\begin{array}{l}z_w\end{array}$ is the groundwater depth below the reference level
$\begin{array}{l}\gamma_w\end{array}$ is the unit weight of water (defined as density in CPT_PROJECT_PARAMETERS. Water_Density_Field)

Overwater Testing

The critical fields related to pore pressure calculation for over water testing are:

CPT_GENERAL.Offshore_Water_Depth or CPT_POINT_PARAMETERS.Offshore_Water_Depth
CPT_GENERAL.Zero_Location
CPT_GENERAL.Pre_Drilled_Depth
CPT_GENERAL. Groundwater_Depth

If CPT_GENERAL.Offshore_Water_Depth or CPT_POINT_PARAMETERS. Offshore_Water_Depth fields have data then only CPT_GENERAL. Groundwater_Depth will be used, and the Groundwater fields on CPT_PROJECT_PARAMETERS are ignored. This is only the case for over water testing.

In the CPT_PROJECT_PARAMETERS table, the Water_Density_Field field is also required, and should be set appropriately for fresh or salt water.

The following sections describe how to handle the various overwater scenarios. The aim of corrections is to make the derived data in all strokes within the one PointID comparable.

Derived parameters for tests using scenarios Zero at Seabed and Zero at Bottom of Borehole are comparable after the zero corrections have been applied, and the results are relative to the seabed.

Zero on Deck / Surface

In this case the transducers, including depth, are zeroed on deck (at atmospheric pressure). Measured parameters don't require zero corrections.

Define:

CPT_GENERAL.Offshore_Water_Depth or CPT_POINT_PARAMETERS.Offshore_Water_Depth: Distance from seawater surface to seabed surface (required for Total Stress)
CPT_GENERAL.Zero_Location: S - Surface
CPT_GENERAL.Pre_drilled_Depth: Nothing or value as required, doesn't influence calculation
CPT_GENERAL. Groundwater_Depth: Distance from deck level (0 depth) to the seawater surface

In situ pore pressure is calculated relative to the atmospheric pressure using the following formula:

$\begin{array}{l}If\quad z \quad < \quad z_w \quad \text{then} \quad u_o = 0 \\ \ \\ If \quad z \quad \geq z_w \quad \text{then} \quad u_o = (z-z_w) \cdot \gamma_w\end{array}$

Where:

$\begin{array}{l}z\end{array}$ is the depth below the reference level
$\begin{array}{l}z_w\end{array}$ is the groundwater depth below the reference level
$\begin{array}{l}\gamma_w\end{array}$ is the unit weight of water (defined as density in CPT_PROJECT_PARAMETERS. Water_Density_Field)

Total Stress and Effective Stress are calculated relative to deck/ground surface.

Zero at Seabed

In this scenario the transducers are zeroed at the seabed, with the hydrostatic pressure at that water depth.

Define:

CPT_GENERAL.Offshore_Water_Depth or CPT_POINT_PARAMETERS.Offshore_Water_Depth: Distance from seawater surface to seabed, doesn't influence calculation
CPT_GENERAL.Zero_Location: SB - Seabed
CPT_GENERAL.Pre_drilled_Depth: Nothing or value as required, doesn't influence calculation
CPT_GENERAL. Groundwater_Depth: Nothing

The in situ pore pressure, total stress and effective stress are calculated relative to the seabed, hence:

$\begin{array}{l}u_0 = z \cdot \gamma_w\\ \ \\ \sigma_{v0} = z \cdot \gamma_{sat}\\ \ \\ \sigma'_{v0} = \sigma_{v0} - u_0\\\end{array}$

Where:

$\begin{array}{l}z\end{array}$ is the depth below the seabed
$\begin{array}{l}\gamma_w\end{array}$ is the unit weight of water (defined as density in CPT_PROJECT_PARAMETERS. Water_Density_Field)
$\begin{array}{l}\gamma_w\end{array}$ is the unit weight of saturated soil

Zero at Bottom of Borehole

This scenario applies when testing overwater and the transducers are zeroed the bottom of a predrilled borehole. It is common for multiple 3m strokes to be done down a borehole, possibly alternated with other in situ tests and sampling. The corrections allow the multiple pushes to be comparable, by correcting the readings relative to the seabed surface.

Define:

CPT_GENERAL.Offshore_Water_Depth or CPT_POINT_PARAMETERS.Offshore_Water_Depth: Distance from seawater surface to seabed, doesn't influence calculation
CPT_GENERAL.Zero_Location: BB – Bottom Borehole
CPT_GENERAL.Pre_drilled_Depth: Distance from seabed to bottom of borehole (where CPT started)
CPT_GENERAL. Groundwater_Depth: Nothing

In this scenario the $\begin{array}{l}q_c\end{array}$ and $\begin{array}{l}u_2\end{array}$ will be corrected using the following formulas before they are used by the calculation code:

$\begin{array}{l}q_c = q_c^* + d \cdot a \cdot \gamma_w \\ \ \\ u_2 = u_2^* + \gamma_w \cdot d\end{array}$

The in situ pore pressure, total stress and effective stress are calculated relative to the seabed, hence:

$\begin{array}{l}u_0 = (d + z) \cdot \gamma_w \\ \ \\ \sigma_{v0} = (z + d) \cdot \gamma_{sat}\\ \ \\ \sigma'_{v0} = \sigma_{v0} - u_0\\\end{array}$

The CPT Tool calculation assumes data in the Depth field = $\begin{array}{l}d + z\end{array}$

Lunne et al. (1997) and Fugro (1995)

Where:

$\begin{array}{l}q_c\end{array}$ is the cone resistance corrected
$\begin{array}{l}q_c^*\end{array}$ is the cone resistance measured
$\begin{array}{l}u_2\end{array}$ is the pore pressure corrected
$\begin{array}{l}u_2^*\end{array}$ is the pore pressure measured
$\begin{array}{l}a\end{array}$ is the cone area ratio
$\begin{array}{l}\gamma_w\end{array}$ is the unit weight of water (defined as density in CPT_PROJECT_PARAMETERS. Water_Density_Field)
$\begin{array}{l}d\end{array}$ is the pre-drilled depth, depth from the seabed to bottom of the borehole where the CPT started
$\begin{array}{l}z\end{array}$ is the depth below the bottom of the borehole
$\begin{array}{l}\gamma_{sat}\end{array}$ is the unit weight of saturated soil

1. The depth reference level for all strokes in a PointID must be the same. Hence, when importing pushes with the reference depth at the bottom of the borehole the correspondence file must include the calculation Depth = $\begin{array}{l}d + z\end{array}$ .
2. Dissipation tests measured pore pressure and in situ pore pressure are corrected as described above. And again Depth of the Dissipation test must = $\begin{array}{l}d + z\end{array}$ .

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