MODULE 4 Flashcards
groundwater table
surface at which pore pressure, u, is zero
phreatic surface
another name for the surface along which pore pressure is zero (term commonly used when analysing 2-D ground water flow using flownets)
phreatic zone
zone of soil below groundwater table in which the soil is fully saturated and the pore pressures positive
capillary zone
depending on soil grain size distribution, there may be a capillary zone of fully saturated soil above the groundwater table in which the pore pressure is negative.
unsaturated zone
zone of partially saturated soil (above capillary zone) in which voids are filled with both air and water
aquifer
stratum that can transmit large quantities of groundwater (typically gravels and sands or fractured rock)
aquiclude
a stratum that is virtually impermeable (typically clays and very fine silts)
aquitard
stratum that is somewhat impermeable and delays water seepage (typically sandy silts or similar)
unconfined aquifer
water-bearing stratum with an impermeable bottom flow boundary but upper flow boundary is free to reach its own level
- water flows with ease => if more water arrives groundwater table rises accordingly
confined aquifer
both upper and lower boundaries are impermeable, but water flows with ease through the aquifer
- artesian then water would overflow in a well from ground level due to high pore pressure
- sub artesian then piezometric level is below ground level
total head key concept
it is a difference in TOTAL HEAD that causes water to flow from one location to another, not a difference in pore pressure
total head
h(tot) = h + z
- z measured from arbitrary datum
soil permeability
k = K * γ(f) / n(f)
- K is intrinsic permeability of soil alone (m^2)
- γ(f) is unit weight of fluid or permeant (kN/m^2)
- n(f) is dynamic viscosity of fluid or permeant (kNs/m^2)
permeability factors
permeability depends on both soil matrix or structure (via intrinsic permeability K) and fluid properties (via γ(f) and n(f))
true seepage velocity
v(true) = v(D) / n = V(d) * (1 + e) / e
Lab Based Methods
Permeameter Testing
- Constant Head Permeameter: coarse grained soils
- Falling Head Permeameter: fine grained soils
Field Based Methods
Well - Pumping
- Confined well-pumping test
- Unconfined well-pumping test
k Flow parallel to laminations
k = ( d(1)k(1) + … + d(n)k(n) ) / ( d(1) + … + d(n) )
k Flow perpendicular to laminations
k = ( d(1) + … + d(n) ) / ( d(1)/k(1) + … + d(n)/k(n) )
soil fluidisation
upward seepage of water (if flow strong enough) causing pore pressure to be greater than the stress due to the weight of the soil
- critical value of hydraulic gradient, i(crit), for fluidisation to occur
i(crit)
i(crit) = ∆h(crit) / z = ( γ - γ(w) ) / γ(w)
equipotential line
line representing constant head
- no velocity along an equipotential
flowline
flow path of a particle of water - parallel to direction of flow at every location along their length
flow net
graphical representation of a flow field
- flowlines and equipotentials must cross at right angles
seepage stress
stress imposed on a soil as water flows through it
analysing flow paths
take a 2D slice or section
- adequate to analyse in 2D in situations where geometry of construction is long compared to its width such as dams and embankments, excavations, slopes and cuttings
SUMMARY of flownet rules
- boundary conditions must be satisfied
- flowlines must intersect equipotential lines at right angles
- cells between flowlines and equipotentials must be curvilinear squares
- quantity of flow through each flow channel is constant
- head loss between each consecutive equipotential is positive
- flowline cannot intersect another flowline
- equipotential cannot intersect another equipotential
impermeable boundaries
flowlines
permeable boundaries
equipotential lines
Unconfined Flownet
only one position is correct as a condition of the top flowline is that equipotentials are forced to start at equal intervals of vertical height
Steps for Unconfined Flownet
- Draw top flowline
- Mark equipotential intersections
- Sketch flowlines and equipotentials
4, Adjust to obtain curvilinear squares - Label equipotentials with their heads
(top flow line h = z as u = 0 )
How to Draw Flownets for Anisotropic Soil
- Draw structure and soil mass to suitable scale
2, Redraw system by scaling the horizontal x-distance by: α = sqrt( k(z) / k(x) ) - Use transformed system to identify impermeable/permeable boundaries as usual
- Proceed as usual to construct flow net
- for calculating flow must use permeability of transformed system: k(t) = sqrt ( k(x) * k(z) )
q(t) = k(t) H N(f) / N(h)
Factor of Safety
F = i(crit) / i
