Fichas
Ch. 7 - Permeability
Fichas del conjunto (23)
- 1HeadFlow of water through interconnected voids from the points of high energy to low
energy. - 2pore water pressure.The pressure in the water within the soil voids
- 3pressure headif pore water pressure is known then the pressure can also
be expressed in terms of equivalent height of water which will exert this pressure. - 4Elevation head (z)vertical elevation from the datum
- 5PiezometerA open standpipe for measuring pressure head.
- 6Total headPiezometric head
- 7i=hydraulic gradient
- 8k=hydraulic conductivity or coefficient of permeability.
- 9v=discharge velocity
- 10Darcy's Lawv=ki
- 11the quantity of water flowing through the soil in unit timeq = vA= kiA
where: A=c/s area - 12gamma(w) =9.81 kN/m^3
- 13Factors affect permeability of soil1) Soil type
2) The properties of pore fluid (viscosity)
3) The void ratio of the soil (will be discussed later)
4) The shape and arrangement of pore ‐ very difficult to describe mathematically
5) Degree of saturation‐ increase in degree of saturation increases the
permeability - 14Constant head permeability test isused for coarse‐grained soils
- 15Falling head permeability test isused for fine‐grained
soils - 16Phreatic zone:Portion below groundwater table.
- 17Aquifer:Some soils, such as sands and gravel, can transmit large quantity of groundwater.
- 18Aquicludes:Other soils such as clay transmit water very slowly,
- 19Aquitards:Intermediate soils, such as silty sand, can pass water slow‐to‐moderate rate
- 20Unconfined aquifer:Upper aquifer. Bottom flow boundary is defined by an aquiclude while the
upper flow boundary (groundwater table) is free to reach its natural state. - 21Confined aquifer:Lower aquifer(s). Both upper and lower flow boundaries are defined by
aquiclude. Most confined aquifers are artesian, which means water at the top of the aquifer is
under pressure. - 22Field Instrumentations1. Open standpipe piezometer
2.Observation Well - 23Essential Points- Flow of water through soils is governed by Darcy’s law, which states that
the velocity is proportional to the hydraulic gradient (v=ki).
- The proportionality constant is the hydraulic conductivity.
- The hydraulic conductivity depends on soil type, particle size, pore fluid
properties, void ratio, pore size, homogeneity, layering and fissuring,
and entrapped gases.
- In coarse-grained soils, the hydraulic conductivity is determined using a
constant-head test while for fine-grained soils a falling-head test is used.
- In the field, a pumping test is used to determine the hydraulic
conductivity.
- Wellpoints are used at a construction site to lower the groundwater level.
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- Bachelor Geotech Fichas sobre Ch. 7 - Permeability, creado por pdm800 el 09/07/2013.
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