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33 Cards in this Set
- Front
- Back
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Bernoulli's equation with soils |
velocity head = 0 |
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Head Losses in soils |
Water loses energy as it moves around soil particles |
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Differences in head... |
are required for flow |
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Hydrostatic Pore Pressure |
Water pressure in soil voids due to pressure head or energy |
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No flow |
Water in soil is static or quasi-static "hydrostatic conditions" |
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Steady flow |
Flow conditions assumed |
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Transient flow |
Flow conditions change over time |
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Darcy's Law describes the relationship between |
Hydraulic conductivity (permeability) Head loss and gradient Flow geometry (size of flow path) Flow rate/velocity |
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Permeability (hydraulic conductivity) |
-Soil/fluid property -Depends on: nature of the soil grain size distribution fluid properties |
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Hydraulic gradient |
Fluids flow down gradient (from higher head to lower head) |
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Velocity |
Darcy velocity (flow velocity) Seepage velocity (velocity of water through pores) |
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Factors Influencing Permeability |
-Viscocity of fluid -Size and continuity of pore spaces or joints -Size, surface roughness, and shape of soil particles -Soil void ratio -Soil fabric -Presence of discontinuities -Degree of saturation |
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The smallest particles in a soil mass... |
control permeability |
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Fines content... |
has a greater influence on permeability than density |
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Indirect methods to determine k |
-no measurement of k -assume soil is isotropic |
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Hazen's Equation details |
-k is in [cm/s] -C = 100 if D10 is in cm -C = 1 if D10 is in mm |
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Use Hazen's Equation |
-for clean, saturated, poorly graded fine sands (SP) - Cu < 5 - 0.1 mm < D10 < 0.3 mm |
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Kozeny-Carman Equation details |
- Specific surface assumes that the soil is spherical - Ss = 6/D10 - Equation must be dimensionally correct |
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Direct Methods to determine k |
- Constant Head Permeability Test - Falling Head Permeability Test |
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Constant Head Permeability Test |
- Inlet and outlet that keeps the head constant while water flows through the soil |
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Falling Head Permeability Test |
- There is a standpipe where you can measure the change in head as water flows through the soil |
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Temperature Correction |
- Accounts for variation in viscosity - standard temperature is 20 degrees celcius - corrected by ratio of viscosities |
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Potential problems with direct measurement methods |
-lab samples may not be fully saturated -stratification of the soil mass - anisotropy -soil fabric in the lab sample is not the same as the field due to sampling disturbance -relative density of the soil mass -the small lab sample may not be representative -we do not simulate the density, stress conditions or soil fabric of the in-situ soil -we measure vertical permeability when flow in the ground is horizontal -flow could occur along the boundary of the permeability cell |
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Direct methods to determine k (field tests) |
Advantage: Testing a large volume of soil - Borehole Piezometer Test - Pump Test |
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Borehold Piezometer Test |
- Measure the natural water level in the borehole - Either raise the water level (fill a standpipe), or bail out the hole - Determine the time it takes for the water to return to the original level |
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Pump Test |
Most accurate and most expensive - Pump water out of a large discharge well - Measure the water level in observation wells spaced at distance from the pumping well - At steady state, measure water drawdown in discharge well |
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Flow through stratified soil |
Find system equivalent k based on 2 or more k's |
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Aquifer |
Layer of water bearing soil (e.g. sand, silt) or rock Permeable enough for water flow and extraction by pumping from a well |
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Confined Aquifer |
Impermeable layer above and below permeable layer |
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Confined Aquifer - Artesian Condition |
Impermeable layer above and below permeable surface Piezometric level above ground surface -Total head is above ground water surface -Water flows out of wells and boreholes |
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Perched Aquifer |
A isolated aquifer that rests on a small impermeable layer (aquitard) |
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Capillarity |
Rise of free surface due to surface tension between the fluid (water) and the soil grains Primarily a fine-grained soil phenomenon |
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Capillarity Equation details |
hc = (3 x 10^-5) /d [m] d = 20% (D10) [m] |
