Permability And 1D Seepage Flashcards
Why are all soils permable
Interconnected voids, water can move through them.
Total head
Ht = hp + hv + he
Pressure, velocity, elevation
Pressure head hp
Pare pressure - hp=u/yw
Elevation head, he
= z distance from datum
Velocity head hv
Darcy velocity hv= v2/2g
Seepage
Difference in head between points needed for seepage to occur. Seepage from a point of high total head to point of low total head.
Characterising seepage, fundamental assumptions
Flow is laminar, incompressible flow (conservation of mass), soil is saturated, flow is steady state (no change with time)
Hydraulic gradient, i
Dimensionless - used to represent what drives the seepage e.g. The changing head.
Coefficient of permability, K
Describes how easily a fluid moves through soil. Affected by: void size, continuity, soil particle shape, soil particle roughness viscosity and density of fluid.
K - fine grained
Smaller voids, less permable
K- coarse grained
Larger voids, more permable
K and soil grade
Poorly graded - more permable.
Isotropic
Soil properties some in all directions
Anisotropic
Soil properties differ by direction
Lab tests to determine k
Constant head test, falling head test
Constant head test
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.
Falling head test
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 =.
Seepage parallel to layers
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.
Seepage normal to layers
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.
Explain why upward seepage is an issue
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
