Permability And 1D Seepage

Question 1

Why are all soils permable

Answer 1

Interconnected voids, water can move through them.

Question 2

Total head

Answer 2

Ht = hp + hv + he

Pressure, velocity, elevation

Question 3

Pressure head hp

Answer 3

Pare pressure - hp=u/yw

Question 4

Elevation head, he

Answer 4

= z distance from datum

Question 5

Velocity head hv

Answer 5

Darcy velocity hv= v2/2g

Question 6

Seepage

Answer 6

Difference in head between points needed for seepage to occur. Seepage from a point of high total head to point of low total head.

Question 7

Characterising seepage, fundamental assumptions

Answer 7

Flow is laminar, incompressible flow (conservation of mass), soil is saturated, flow is steady state (no change with time)

Question 8

Hydraulic gradient, i

Answer 8

Dimensionless - used to represent what drives the seepage e.g. The changing head.

Question 9

Coefficient of permability, K

Answer 9

Describes how easily a fluid moves through soil. Affected by: void size, continuity, soil particle shape, soil particle roughness viscosity and density of fluid.

Question 10

K - fine grained

Answer 10

Smaller voids, less permable

Question 11

K- coarse grained

Answer 11

Larger voids, more permable

Question 12

K and soil grade

Answer 12

Poorly graded - more permable.

Question 13

Isotropic

Answer 13

Soil properties some in all directions

Question 14

Anisotropic

Answer 14

Soil properties differ by direction

Question 15

Lab tests to determine k

Answer 15

Constant head test, falling head test

Question 16

Constant head test

Answer 16

Primarily for coarse grained soils. Has a porous stone or filter on either end to prevent washing away of sample grains. Head difference between top and bottom of sample is constant.

Question 17

Falling head test

Answer 17

For course and fire grained soils. Also has porous store on either end. Head difference between top and bottom of sample is changing throughout test. Water in stand life drops with time), no water added to toe of sample. Good for testing silt and Clay as they have a lower permability. Continuity - flow rate in and out are =.

Question 18

Seepage parallel to layers

Answer 18

Head loss through each layer is the same. Flow rate is split across layers. All hydraulic gradients are equal. Equivalent characteristics controlled by the most permeable layer.

Question 19

Seepage normal to layers

Answer 19

Head loss through each layer will vary. Flow rate through each layer is the same. Total head loss = sum of head drops. Equivalent characteristics controlled by the most internal layer.

Question 20

Explain why upward seepage is an issue

Answer 20

Upward seepage reduces effective stress, at some point seepage could reduce effective stress to zero or negative. Pore pressure exceeded, no interparticle forces, material may wash away → instability → quick condition or piping