BS-Theory: Water pressure -What is a potential?
In subsurface hydraulics the potential (h) is composed of:
the elevation head (y) of the point under consideration (in metres) and
the water pressure (u) at the point under consideration (in metres).
If p is the water pressure in kN/m² at a particular point, then water pressure, u (in metres) = p/γw
(γw = unit weight of water ≅ 10.0 kN/m³).
h = p/γw + y
h = potential [m]
p = water pressure [kN/m²]
y = elevation head [m]
An example:
In this example, two permeable soil layers are separated by a low-permeability layer (clay). In the excavation, groundwater is kept at the base level. The groundwater in the gravel layer below the clay is under pressure and rises to 3.5 m below the top of the wall (represented in the diagram by the water levels in the two standpipes on the right). The lowest point in the right-hand standpipe is at a higher water pressure than that in the left-hand standpipe, but both have the same potential of 3.5 m below the top of the wall. Because potential is a function of the elevation head and the water pressure [m], the left-hand standpipe makes up for its lower water pressure by its greater elevation head.
If you want to calculate such a system using flow conduits, it is sufficient to enter a potential of 3.5 m below the top of the wall for any point within the layer of gravel. The potential above the layer of clay is defined by groundwater levels to the left and right of the wall. This boundary condition is used automatically by the program. Together with the permeabilities, the program calculates the potentials along the flow conduit resulting from these boundary conditions. From the calculated potentials (h), water pressure (p) and also the gradient (i) can now be determined. Applying the above formula for p:
p = γw · (h - y)
For the gradient (i) we have:
i = Δh / ΔL
Δh = potential difference between two points
ΔL = distance between the two points
Using flow conduits, the load-decreasing effect of the upwardly directed hydraulic gradient on passive earth pressure can now be taken precisely into consideration.