Pore Water Pressure Calculator

1. What is Pore Water Pressure Calculator?

Definition: This calculator computes the pore water pressure (\( PW_p \)) at a given depth below the water table, based on the uplift or pressure head (\( h_w \)) and the unit weight of water (\( \gamma_{water} \)).

Purpose: It helps geotechnical engineers and hydrologists determine the pressure exerted by water within the pores of soil or rock, critical for analyzing slope stability, foundation design, and groundwater effects.

2. How Does the Calculator Work?

The calculator uses the following equation:

\( PW_p = h_w \cdot \gamma_{water} \)

Where:

\( h_w \): Uplift or pressure head (depth below the water table, in m, ft, or cm);
\( \gamma_{water} \): Unit weight of water (in N/m³ or lb/ft³, default 9810 N/m³ for water);
\( PW_p \): Pore water pressure (in Pa, kPa, psi, or bar);
Results are displayed with 3 decimal places (or scientific notation if less than 0.001).

Steps:

Enter the uplift or pressure head (\( h_w \)) and select the unit (m, ft, cm).
Enter the unit weight of water (\( \gamma_{water} \)) and select the unit (N/m³, lb/ft³).
Click "Calculate" to compute the pore water pressure.
Change the result unit dropdown to convert the pressure to a different unit (Pa, kPa, psi, bar).

3. Importance of Pore Water Pressure Calculation

Calculating pore water pressure is crucial for:

Geotechnical Engineering: Assessing the stability of slopes, embankments, and foundations by understanding water pressure effects.
Groundwater Management: Evaluating the impact of groundwater on soil strength and permeability in construction projects.
Seismic Analysis: Determining the potential for liquefaction in saturated soils during earthquakes.

4. Using the Calculator

Example 1: Calculate the pore water pressure with \( h_w = 1 \, \text{m} \), \( \gamma_{water} = 9810 \, \text{N/m³} \), result in Pa:

Uplift Head: 1 m;
Unit Weight: 9810 N/m³;
\( PW_p = 1 \cdot 9810 = 9810 \, \text{Pa} \);
Result: Pore Water Pressure = 9810.000 Pa.

Example 2: Calculate the pore water pressure with \( h_w = 5 \, \text{ft} \), \( \gamma_{water} = 62.428 \, \text{lb/ft³} \), result in psi:

Uplift Head: 5 ft = \( 5 \times 0.3048 = 1.524 \, \text{m} \);
Unit Weight: 62.428 lb/ft³ = \( 62.428 \times 157.087 \approx 9809.989 \, \text{N/m³} \);
\( PW_p = 1.524 \cdot 9809.989 \approx 14950.425 \, \text{Pa} \);
Convert to psi: \( 14950.425 \times 0.000145038 \approx 2.168 \, \text{psi} \);
Result: Pore Water Pressure = 2.168 psi.

5. Frequently Asked Questions (FAQ)

Q: What is Pore Water Pressure?
A: Pore water pressure is the pressure exerted by water within the pore spaces of soil or rock, influenced by the depth below the water table and the water’s unit weight.

Q: Why is the unit weight of water adjustable?
A: The unit weight can vary slightly depending on temperature, salinity, or impurities in the water, so the calculator allows for customization.

Q: How does pore water pressure affect soil stability?
A: High pore water pressure reduces the effective stress in soil, decreasing its shear strength and potentially leading to failures like landslides or foundation settlement.