Environmental Engineering Reference
In-Depth Information
with proper procedures. Underconsolidation results if artesian pressures prevent a soil
stratum from draining and consolidating, causing it to remain in a very loose or very soft
state. The condition is found in river valleys and other lowlands where buried aquifers
extend continuously into nearby hills ( Figure 8.27) .
8.3.2
Subsurface Flow
The Hydraulic Gradient
General Conditions
Hydrostatic conditions refers to pressures in fluids when there is no flow. The pressure P at
depth h in water equals the unit weight of water
γ w times the depth, plus atmospheric
pressure P a expressed as
P
γ w h P a
(8.1)
Hydrostatic pressure is equal in all directions, i.e., P a
P h .
Groundwater flow occurs when there is an imbalance of pressure from gravitational
forces acting on the water and the water seeks to balance the pressure. Water movement
in the ground occurs very slowly in most materials, creating a time lag in the leveling-out
process. Typical velocities range from 6 ft/day to 6 ft/year; therefore, the water table usu-
ally follows the ground surface, but at a subdued contour. In dry climates or free-draining
materials, however, the groundwater level is approximately horizontal.
Hydraulic gradient and permeability are the two factors upon which groundwater move-
ment is dependent. The hydraulic gradient between two points on the water table is the
ratio between the difference in elevation of the two points and the distance between them.
It reflects the friction loss as the water flows between the two points.
Flow-Condition Nomenclature
Flow-condition nomenclature is illustrated in Figure 8.22.
Static condition refers to no flow, and in Figure 8.22a water will rise to the same
piezometric level in any tube extending from the inclined sand-filled glass tube.
Pressure surface is created when flow is allowed to occur. The levels in the tubes
drop as shown in Figure 8.22b.
Hydraulic head h is the difference in water-level elevation between the two tubes,
or the head lost during flow.
Hydraulic gradient i is the ratio of the hydraulic head h to the length of flow path
L , expressed as
i
h / l
(8.2)
Pressure head h p is the height to which water will rise in the vertical tube from the point of
interest or reference (also referred to as piezometric head).
Elevation head h e is the height of the point of interest or reference with respect to some
arbitrary datum.
Tailwater elevation , such as a lake or pool where the elevation is constant, is the reference
datum selected for most seepage problems.
Total head h t equals the pressure head plus the elevation head.
Steady-state condition usually means a state of constant flow, with no acceleration or
deceleration or changes in piezometric levels.
 
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