Subsurface Pressure Head Calculator
Our soil & sediment mechanics calculator computes subsurface pressure head accurately. Enter measurements for results with formulas and error analysis.
Formula
Pressure Head (h) = u / (rho x g)
Pressure head equals the pore water pressure (u) divided by the unit weight of water (rho x g). Total hydraulic head (H) is the sum of pressure head and elevation head: H = h + z. The difference between measured pressure head and hydrostatic pressure head reveals excess pore pressure or artesian conditions.
Worked Examples
Example 1: Piezometer Reading Below Water Table
Problem: A piezometer at 8m depth records pore pressure of 29,430 Pa. Water table is at 5m depth. Elevation of measurement point is 2m above datum.
Solution: Pressure Head = 29430 / (1000 x 9.81) = 3.0 m\nElevation Head = 2.0 m\nTotal Head = 3.0 + 2.0 = 5.0 m\nHydrostatic head at this depth = 8 - 5 = 3.0 m\nExcess pressure head = 3.0 - 3.0 = 0.0 m (hydrostatic equilibrium)
Result: Pressure head: 3.0 m | Total head: 5.0 m | Hydrostatic conditions
Example 2: Artesian Condition Detection
Problem: At 15m depth, pore pressure is 147,150 Pa. Water table is at 5m. Elevation of point is 0m datum.
Solution: Pressure Head = 147150 / (1000 x 9.81) = 15.0 m\nHydrostatic head = 15 - 5 = 10.0 m\nExcess pressure head = 15.0 - 10.0 = 5.0 m\nThis excess indicates upward artesian pressure.
Result: Pressure head: 15.0 m | Excess: 5.0 m | Artesian conditions present
Frequently Asked Questions
What is subsurface pressure head?
Subsurface pressure head is the height of a water column that corresponds to the pore water pressure at a given point below the ground surface. It is calculated by dividing the pore water pressure by the product of fluid density and gravitational acceleration (h = u / (rho * g)). Positive pressure head indicates saturated conditions below the water table, while negative pressure head (suction or matric potential) occurs in the unsaturated zone above the water table.
How is total hydraulic head calculated?
Total hydraulic head is the sum of pressure head and elevation head at any given point in a groundwater system. Mathematically it is expressed as H = h + z, where h is the pressure head and z is the elevation above a chosen datum. Groundwater always flows from higher total head to lower total head. In a piezometer, the total head equals the water level elevation in the standpipe, making it directly measurable in the field.
What does negative pressure head mean?
A negative pressure head indicates that the pore water pressure is below atmospheric pressure, which occurs in the unsaturated (vadose) zone above the water table. This negative pressure is called matric suction or tension, and it results from capillary forces holding water in soil pores against gravity. The more negative the pressure head, the drier the soil and the more strongly water is held. Sandy soils typically have pressure heads near zero just above the water table, while clay soils can maintain large negative pressure heads.
Why is pressure head important in geotechnical engineering?
Pressure head directly determines effective stress in soil, which controls its strength and compressibility. When pore water pressure rises (positive pressure head increases), effective stress decreases, potentially leading to slope failures, liquefaction, or foundation problems. Engineers install piezometers to monitor pressure head changes during construction, behind retaining walls, and beneath dams. Accurate pressure head measurements are essential for slope stability analysis and the design of dewatering systems.
How is atmospheric pressure measured and what does it indicate?
Atmospheric pressure is measured in millibars (hPa) or inches of mercury (inHg) using barometers. Standard sea-level pressure is 1013.25 hPa or 29.92 inHg. Falling pressure indicates approaching storms, while rising pressure suggests fair weather. Pressure decreases approximately 12 hPa per 100 meters of altitude gain.
How do I get the most accurate result?
Enter values as precisely as possible using the correct units for each field. Check that you have selected the right unit (e.g. kilograms vs pounds, meters vs feet) before calculating. Rounding inputs early can reduce output precision.