SOIL MECHANICS EXAM - HYDRAULIC CONDUCTIVITY AND FLOW NETS
Hydraulic conductivity depends on what?
It depends on properties of the soil and the liquid
What are the liquid properties in hydraulic conductivity?
-> Density -> Viscosity ->However most practical problems deal with clean water or water contaminated with small amounts of other substances
History of Darcy's Law
-> Henry Darcy performed one of the first quantitative studies of flow of water through sand (1850's) -> He observed that the flow rate of water was directly proportional to the hydraulic gradient for a given sand.
Describe flow lines
-> Is an idealized line along which a water molecule will travel from the upstream to the downstream side in a permeable soil medium -> Flow tubes (or channels) are the strips between any two flow lines -> Nf = Number of flow tubes
What types of institu test exist
-> Slug Test -> Pump Test
Describe an anisotropic soil
-> Soils horizons that have different horizontal and vertical hydraulic conductivities are referred to as anisotropic with respect to hydraulic conductivity -> In stratified soils hydraulic conductivity in some layers is often greater than in others, so ground water flows horizontally much more easily than vertically.
What are the soil properties in hydraulic conductivity?
-> Void Size -> Soil Structure -> Void Continuity -> Particle shape and surface roughness
Describe what flow nets are
-> a pictorial method of studying the path that moving water follows through a soil horizon. -> Flow of water occurs between two points because a difference in energy (energy head). [potential, pressure and velocity heads].
What are the components of flow nets
-> equipotential lines -> flow lines
What are the two types of testing?
-> laboratory -> insitu
Describe equipotential lines
-> lines connecting points of equal energy head -> They always intersect flow lines at right angles. -> The flow elements that are formed are approximate squares (length = width) -> Nd = # of Equipotential Drops
Characteristics of two dimensional flow situation
-> zones of flow or directions of flow are irregular, -> where water enters and escapes from a permeable zone of soil -> where the flow boundaries are not well defined
Explain the equivalent horizontal hydraulic conductivity.
Consider flow parallel to the stratification
Explain the equivalent vertical hydraulic conductivity.
Consider flow perpendicular to the stratification
Darcy's law is valid for what?
Darcy's law is valid for a wide range of soils in which flow is slow enough to be considered laminar, with some exceptions such as clean gravel in which turbulent flow can exist.
What are the characteristics of the Insitu tests?
Describe the characteristics of a much larger volume of soil, which should lead to more reliable results.
Describe the Falling-Head Test
More suitable for soils with low hydraulic conductivities such as clays.
What is Darcy's Law?
Q = [(k∆hA)/l] = kiA Q = flow rate k = hydraulic conductivity (coefficient of permeability) i = ∆h / l = hydraulic gradient A = area perpendicular to the flow direction (includes both solids and voids)
What is the equation for a flow net
Q= k * L * h * (Nf/Nd) Q = Total flow rate through flow net k = Hydraulic conductivity L = Length of aquifer perpendicular to flow h = Head loss through aquifer Nf = Number of flow tubes Nd = Number of equipotential drops
What are the units in hydraulic conductivity?
Units of cm/s, alternate units are ft/min, ft/yr and gallons/day/ft^2
Describe the Constant-Head Test
We determine Q, i and A from the test and compute k. More suitable for coarser grained soils
What is the equation for the Falling-Head Test
k = [(aL)/(At)] * (Ln) * (ho/h1) ho = pressure head at the beginning of the test. h1 = pressure head at the end of the test. a = cross sectional area of burette. t = time A = area perpendicular to the flow direction (includes both solids and voids) k = hydraulic conductivity (coefficient of permeability)
Describe the k values of sand, silt, and clay
k sand > k silt > k clay
What is the equation for the Constant-Head Test
k=Q/(iA) Q = flow rate k = hydraulic conductivity (coefficient of permeability) i = ∆h / l = hydraulic gradient A = area perpendicular to the flow direction (includes both solids and voids)
Whats the equation for the equivalent horizontal hydraulic conductivity?
kH = (sigma (ki * Hi))/(sigma Hi)
Whats the equation for the equivalent vertical hydraulic conductivity?
kV = (sigma Hi)/(sigma (Hi / ki)
Describe what happens in an aquifier of constant thickness
the flow is one dimensional as long as there are no rivers, lakes or wells to affect flow.
