Flashcards
Groundwater
Cards in this deck (26)
- 1What is meant by the phreatic surface; the phreatic zone and the vadose zone?The phreatic surface is when the GWL is not horizontal. Water below the phreatic surface is in the phreatic zone and have positive pore pressure. Water above the phreatic surface is in the vadoes zone and has zero or negativ pore pressure.
- 2What is an unconfined aquifer?A water bearing stratum: bottom flow boundary is virtually impermeable but the upper boundary is free to reach its own level. Water flows with ease in this layer. Typically gravels, sands and fractured rock.
- 3What is a confined aquifer?Both the upper and lower boundaries are impermeable but water flows with ease in this layer. Typically gravels and sands that lie beneath clays and silts.
- 4What is the difference between artesian and sub-artesian?In an artesian aquifer, water would overflow the ground surface due to the high pore pressure if a well was drilled into them. In a sub-artesian aquifer, the water is still under pressure but the piezometric level is below ground level.
- 5What is an aquiclude?A stratum that is virutally impermeable (may sit above a confined aquifer). Typically clays and very fine silts.
- 6What is an aquitard?A stratum that is somewhat impermeable and delays water seepage. Typically sandy silts or silts.
- 7Desrice what is meant by negative pore pressures.Negative pore pressures are the result of suction stresses that pull particles together, increasing the effective stress. Suctions are the results of meniscuses of water between soil grains formed due to surface tension effects. Can happen when the soil is just saturated or is unsaturated.
- 8How are negative pore pressures limited?The limiting pore pressure is known as the air entry value Ue and is limited by the vapor pressure of water Vp. When pressure in the water becomes too negative, the water will vaporise or cavitate and will boil. Therefore, u>(-)Ue and u>Uc.
- 9How is Ue related to the particle?Ue is inversely proportinal to pore size d* or particle size d. This means that finer grained soils can maintain higher capillary pressure and therefore gigher capillary rise than coarser grained soils.
- 10Give two examples of negative pore pressure.1. Steeo clay slopes. Can stand for a long period of time due to suction forces. During rainfall, slopes fail and the suction forces are reduced by the infiltration of water.
2. Sand castles. Moist silty sand develops suction. Gravels or dry/wet sand doesnt develop suction. - 11What is a standpipe piezometer?A field instrument that measures total head at a point. Open only at base for 200mm so that any perched GWLs or influences from water flow above the point of interest does not effect the reading. Useful in heterogeneous/layered soils as well as homogeneous.
- 12For a standpipe piezometer in ground below hydrostatic GWL, what causes the water to flow?Hydrostatic so Ua does not equal Ub but ha=hb so there is no flow.
- 13For a standpipe piezometer in ground where groundwater is not hydrostatic, what causes the water to flow?Ua does not equal Ub. ha does not equal hb so the flow is caused by a difference in head.
- 14What is the difference bewteen the average velocity and true velocity in a soil?Average VD is used in Darcy's law but flow actually occurs through interconnected voids so the path is tortuous, not straight. Therefore the actual path is longer than a straight line, so the true velocity must be faster than the superficial seepage velocity.
- 15How does the soil type, particle size and particle shape effect permeability in Darcy's law?The larger the grains, the higher the permeability. The more rounded/smoother the grains the higher the permeability.
- 16How does the void ratio effect permeability in Darcy's law?Higher the void ratio, higher the permeability. K is approximately xe^2
- 17How does the homogeneity, layering and fissuring effect permeability in Darcy's law?Fissures(this gaps/cuts): water will take the path of least resistance through any gaps in the soil. Layers: a thin impermeable layer can act as a barrier to flow.
- 18How do saturated/unsaturated zones influence Darcy's Law?The presence of trapped air bubble significantly reduces permeability due to surface tension effects. hence a phreatic surface in coarse soils is usually regarded as an impermeable flow boundary. Flow can occur below/along the boundary but not above or across it.
- 19When is the constant head test used?For coarse grained soils.
- 20When is the falling head test used?For fine grained soils.
- 21What are the issues with measuring permeability in the lab?Can easily lose fine particles when sampling resulting in an overestimation of K. The soil structure may be disturbed. Clay smearing in soils can lead to an underestimation of K. Large scale features of inhomogeneity may not be picked up.
- 22How is K determined in the field?The pumping test: Water is pumped from a well. the fall in GWL recorded at points away from the well. Flow towards the well is assumed to be horizontal.
- 23Difference between using an observation well or a standpipe piezometer to find K?Well: open throughout the depth via a slotted pipe and screen. Measure head over full depth
Pipe: Open only at base. measure head at a point so is more useful in heterogeneous soils than wells. - 24Describe well pumping in an uncinfined aquifer.During pumping, the GWL is drawn down into the aquifer. We assume heights h1 and h2 at r1 and r2 from the well. Assume flow towards the well is radial and horizontal (so drawdown must be samll compared to the aguifer thickness) Analytically, flow thickness is reduced towards the well.
- 25Describe well pumping in a confined aquifer.A confined aquifer has impermeable top and bottom strata and a water-filled higher permeability stratum. Assumed GWL does not draw down into aquifer. Well penetrates to bottom of aquifer where aquiclude starts. Flow is radial and horizontal towards the well.
- 26Why are some soils anisotropic?They have different properties in different directions. Soil is often assumed to be homogeneous but anisotropic with respect to permeability. Most have a coefficient of permeability maximum in the direction of stratification and minimum normal to the stratification.
Information
- Description
- Geotechnical Engineering Flashcards on Groundwater, created by Billie Santos on 04/03/2017.
- Created by
- Billie Santos
- Views
- 3
- Rating