MODULE 4

Question 1

groundwater table

Answer 1

surface at which pore pressure, u, is zero

Question 2

phreatic surface

Answer 2

another name for the surface along which pore pressure is zero (term commonly used when analysing 2-D ground water flow using flownets)

Question 3

phreatic zone

Answer 3

zone of soil below groundwater table in which the soil is fully saturated and the pore pressures positive

Question 4

capillary zone

Answer 4

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.

Question 5

unsaturated zone

Answer 5

zone of partially saturated soil (above capillary zone) in which voids are filled with both air and water

Question 6

aquifer

Answer 6

stratum that can transmit large quantities of groundwater (typically gravels and sands or fractured rock)

Question 7

aquiclude

Answer 7

a stratum that is virtually impermeable (typically clays and very fine silts)

Question 8

aquitard

Answer 8

stratum that is somewhat impermeable and delays water seepage (typically sandy silts or similar)

Question 9

unconfined aquifer

Answer 9

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

Question 10

confined aquifer

Answer 10

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

Question 11

total head key concept

Answer 11

it is a difference in TOTAL HEAD that causes water to flow from one location to another, not a difference in pore pressure

Question 12

total head

Answer 12

h(tot) = h + z

- z measured from arbitrary datum

Question 13

soil permeability

Answer 13

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)

Question 14

permeability factors

Answer 14

permeability depends on both soil matrix or structure (via intrinsic permeability K) and fluid properties (via γ(f) and n(f))

Question 15

true seepage velocity

Answer 15

v(true) = v(D) / n = V(d) * (1 + e) / e

Question 16

Lab Based Methods

Answer 16

Permeameter Testing

1. Constant Head Permeameter: coarse grained soils
2. Falling Head Permeameter: fine grained soils

Question 17

Field Based Methods

Answer 17

Well - Pumping

1. Confined well-pumping test
2. Unconfined well-pumping test

Question 18

k Flow parallel to laminations

Answer 18

k = ( d(1)k(1) + … + d(n)k(n) ) / ( d(1) + … + d(n) )

Question 19

k Flow perpendicular to laminations

Answer 19

k = ( d(1) + … + d(n) ) / ( d(1)/k(1) + … + d(n)/k(n) )

Question 20

soil fluidisation

Answer 20

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

Question 21

i(crit)

Answer 21

i(crit) = ∆h(crit) / z = ( γ - γ(w) ) / γ(w)

Question 22

equipotential line

Answer 22

line representing constant head

- no velocity along an equipotential

Question 23

flowline

Answer 23

flow path of a particle of water - parallel to direction of flow at every location along their length

Question 24

flow net

Answer 24

graphical representation of a flow field

- flowlines and equipotentials must cross at right angles

Question 25

seepage stress

Answer 25

stress imposed on a soil as water flows through it

Question 26

analysing flow paths

Answer 26

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

Question 27

SUMMARY of flownet rules

Answer 27

- 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

Question 28

impermeable boundaries

Answer 28

flowlines

Question 29

permeable boundaries

Answer 29

equipotential lines

Question 30

Unconfined Flownet

Answer 30

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

Question 31

Steps for Unconfined Flownet

Answer 31

1. Draw top flowline 2. Mark equipotential intersections 3. Sketch flowlines and equipotentials 4, Adjust to obtain curvilinear squares 5. Label equipotentials with their heads (top flow line h = z as u = 0 )

Question 32

How to Draw Flownets for Anisotropic Soil

Answer 32

1. Draw structure and soil mass to suitable scale 2, Redraw system by scaling the horizontal x-distance by: α = sqrt( k(z) / k(x) ) 3. Use transformed system to identify impermeable/permeable boundaries as usual 4. Proceed as usual to construct flow net 5. 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)

Question 33

Factor of Safety

Answer 33

F = i(crit) / i