Seepage is the flow of fluid through soil pores. After measuring or estimating the intrinsic permeability (κi), one can calculate the hydraulic conductivity (K) of the soil, and the flow rate can be estimated. K has unit m/s and is the average velocity of water moving through a porous medium under a unit hydraulic gradient. It is the ratio constant between the mean velocity and the hydraulic gradient in Darcy’s law. In most natural and engineering situations, the hydraulic gradient is less than 1, so the value of K for the soil usually represents the maximum potential velocity. A typical value of hydraulic conductivity for natural sand is about 13 1023 m/s, while K for clay is similar to that of concrete. The amount of seepage below dams and sheet piles can be estimated using a graphical construction called a flownet.When the seepage velocity is great enough, erosion can occur because of the frictional drag exerted on the soil particles. Vertically upwards seepage is a source of danger on the downstream side of sheet piling and beneath the toe of a dam or levee. Erosion of the soil, known as ‘piping’, can lead to failure of the structure and to sinkhole formation. Seeping water removes soil, starting from the exit point of the seepage, and erosion advances upgradient. The term sand boil is used to describe the appearance of the discharging end of an active soil pipe.
Seepage in an upward direction reduces the effective stress within the soil. In cases where the hydraulic gradient is equal to or greater than the critical gradient (i.e., when the water pressure in the soil is equal to the total vertical stress at a point), effective stress is reduced to zero. When this occurs in a non-cohesive soil, a ‘quick’ condition is reached and the soil becomes a heavy fluid (i.e., liquefaction has occurred). Quicksand was so named because the soil particles move around and appear to be ‘alive’. (Note that it is not possible to be ‘sucked down’ into quicksand.) In geotechnical engineering, soils are considered a three-phase material composed of rock or mineral particles, water and air. The voids of a soil, the spaces in between mineral particles, contain water and air.
The engineering properties of soils are affected by four main factors: the predominant size of the mineral particles, the type of mineral particles, the grain size distribution and the relative quantities of mineral, water and air present in the soil matrix. Fine particles (fines) are defined as particles less than 0.075 mm in diameter.
Leave a Reply