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EGB373 (completed)

Seepage

QuestionAnswer
Flow Net • A graphical representation of the flow field (flow paths and energy or head variation) • simple, flexible and economic • Can solve simple problems or for making preliminary decisions
Flow Net (1) - Flow line: Flow path of a particle of water
Flow line close to impervious boundaries flow parallel... to impervious boundaries (impermeable walls or “cut-offs”)
Flow Channel / Flow Tube: area between two consecutive flow lines
Flow Net (2) - Equipotential line: A line representing constant head
Equipotential lines (info) • Are lines of equal total head • The total head loss between consecutive equipotential lines is constant • Equipotential lines can be derived from boundary conditions and flow lines
Characteristics of a flow net • Drawn to a scale (isotropic: same scale) • Flow lines run parallel to impervious boundaries • Flow lines and equipotential lines intersect at 90⁰ – Makes curvilinear squares
Characteristics of a flow net (2) • Total head loss between consecutive equipotential lines is constant • Flow can not occur across flow lines – Flow rate through a flow channel is constant
A dam is a barrier that... impounds water. Dams generally serve the primary purpose of retaining water
A cofferdam is a temporary structure designed to keep water and/or soil out of the... excavation in which a bridge pier or other structure is built
Seepage through Impermeable Structures - The flow under impermeable structure, such as sheet piles, concrete dams, masonry dams is... confined
Seepage through Earth Dams(1) - The seepage taking place through permeable structure (earth dams) is unconfined One boundary of the flow region being a phreatic surface on which the pressure is atmospheric. The phreatic surface constitutes... the top flow line and its position must be estimated before the flow net can be drawn
Seepage through Earth Dams (2) - Once the phreatic surface through earth dam is constructed with upstream and downstream corrections, number of equipotential... drops can be marked and the flow net can be constructed
Flow lines P= variable, H = variable Note: impermeable surface is flow line
Line of zero pressure/ top line of seepage/free ground water surface P=0, H = variable
Equipotential lines P= variable, H = constant Note: permeable surface is equipotential line
Causes of Earth Dam Failure (1） Seepage Problems
Seepage Problems - Seepage Through Pervious Foundation Uplift pressure acts vertically on downstream side, so that... counterweight fill or relief well are recommended to prevent heaving and local slide
Causes of Earth Dam Failure (2） Boiling & Piping
Boiling & Piping - Hydraulic Fracture Concentration of flow lines at downstream toe leads to the increase in hydraulic gradient Upward seepage force causes reduction in effective stresses in foundation, and... quick sand and piping take place when counterweight loading is not enough
Causes of Earth Dam Failure (3) - When the dam rests on an impermeable base the discharge must... occur on the surface of the downstream slope. This can cause erosion which may result in catastrophic consequences
Causes of Earth Dam Failure (3) <= This problem is an example of unconfined seepage, one boundary of the flow region being a... phreatic surface on which the pressure is atmospheric
Seepage control in earth dams (1) - Seepage under and /or through dam can cause: Loss of stored water Dam failure – erosion, piping
A dam with central core of low permeability will reduce... seepage through dam
Seepage control in earth dams (2) - Impermeable cut-off such as grout curtain, concrete diaphragm wall will eliminate... underseepage – foundation soil is more permeable than the dam
Any measure designed to lengthen the seepage path (impermeable upstream blanket) will result in a... partial reduction in under seepage – foundation soil is more permeable than the dam
Seepage through anisotropic soils (1) - Most soils are anisotropic with regards to... permeability due to stratification
A maximum value of permeability is in the direction of... stratification
A minimum value of permeability is in the direction... normal to that of stratification
Transfer condition – (Flow across a soil interface) Where the anisotropic conditions include a boundary (interface) between soils of different permeability which is inclined to the direction of flow • The flow lines will be refracted (at the interface flow direction changes abruptly) • If the flow takes place from high permeability into less permeability soil, the flow lines are refracted towards the normal at the interface
Seepage through earth dams (3) - A parabola is a curve such that any point along it is equidistant from both a fixed point, called... focus, and fixed straight line, called the directrix
Seepage through earth dams (4) - The phreatic surface in the earth dam is defined by a basic parabola with... some corrections at the entry and exit (downstream) – Casagrade (1937)
Created by: Asher - S
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