SUBJECT:- GEOTECHNICS & APPLIED GEOLOGY SEMINAR TOPIC:- SOIL WATER, PERMEABILITY & SEEPAGE PREPARED BY:- MODI AYUSHI A. PATEL SAMIRA PATEL MOSINA SAYANIYA KAJAL J. PATEL HITESH G.

INDEX  Permeability and seepage  Some definitions  Factors affecting 0n permeability of soil  Darcy's law of permeability  Assumptions of Darcy's law  Determination of coefficient of permeability  Field permeability tests  Flow net  Soil water  Free water  Held water  Structural water  Adsorbed water  Capillary water

PERMEABILITY AND SEEPAGE & SOIL WATER

PERMEABILITY :- - Permeability is defined as the property of soil which permits flow of water through it. - A soil is highly previous when water can flow through it easily. ( e.g. gravel ) - In an impervious soil, the permeability is very low and water cannot easily flow through it. (e.g. clays) SOME DEFINITIONS:- (1) Total head :- - The total head at any point in a flowing fluid is equal to, Total head = Datum pressure + Pressure head + Velocity head.: H = z + p ̲ / Yw + V2 / 2g (2) Hydraulic head (h) : - - It is equal to the difference in the elevations of water levels at the entry and exit point of soil mass. - Obviously, its equal to the loss of head through the soil. The Hydraulic head also known as the Effective Head.

(3 ) Hydraulic Gradient(i) :- - The loss of head per unit length of flow through the soil is known as Hydraulic Gradient..: i = h/l where, h=hydraulic gradient l=length of soil specimen (4) Percolation (Seepage) :- - The rainwater after falling on the earth surface, some part of it seeps through the soil and meet the ground water. This process is called Percolation. Seepage is the flow of water under gravitational forces in a permeable medium. (5) Discharge Velocity (v) :- - The discharge of water per unit of total cross-sectional area(A) of soil, is known as discharge velocity..: v=q/A where, q=discharge A=total c/s area of soil mass

( 6) Seepage velocity (Vs) : - The rate discharge per unit of cross-sectional area of voids (Av) perpendicular to the direction of flow is known as seepage velocity. Vs = q/Av or porosity = e/1 + e Since, Av < A, seepage velocity (Vs) will be more than the discharge velocity (v). Seepage velocity is also called true velocity or actual velocity. (7) Piping :- - Hydraulic structures, such as weirs and dams, built on pervious foundations sometimes fail by formation of a pipe-shaped channel in its foundation, known as piping failure. (8) Coefficient of permeability (K) :- - The average velocity of flow through total cross-sectional area of soil mass under unit hydraulic gradient is called coefficient of permeability (k)..: v = k - The unit of k is cm/sec or m/day.

FACTORS AFFECTING PERMEABILITY OF SOILS The following factors affect the permeability of soils :- (1) Particle size. (2) Properties of pore fluid. (3) Void ratio of soil. (4) Shape of particles. (5) Structure of soil mass. (6) Degree of saturation. (7) Adsorbed water. (8) Impurities in water. (1) Particle size :- permeability varies approximately as the square of the grain size. ( Allen hazen’s formula).

(2) Properties of pore fluid :- - The permeability is directly proportional to the unit weight of water and inversely proportional to its viscosity ( ƞ )..: K1/K2 = ƞ 1/ ƞ 2 - It is usual practice to report the coefficient of permeability at 27°c. the following equation can be used for conversion of the permeability to 27°c. K27 = k. ƞ / ƞ 27 where, K27 = coefficient of permeability at 27°c. ƞ 27 = viscosity at 27°c. K = permeability at test temperature. ƞ = viscosity at test temperature. (3) Void ratio of soil :- - The coefficient of permeability varies as e3 / (1+e). For a given soil, greater the void ratio, the higher is the value of the coefficient of permeability. (4) Shape of soil particles :- - Permeability of a soil depends upon the shape of particles. - Angular particles have greater specific surface area as compared with rounded particles.

(5) Structure of soil mass :- - Stratified soil deposits have greater permeability parallel to the plan of stratification than that perpendicular to this plane. - For the same void ratio the permeability is more in case of flocculated structure as compared to that in the dispersed structure. (6) Degree of saturation :- - If the soil is not fully saturated it contains air pockets formed due to entrapped air. The permeability of partially saturated soil is considerably smaller than that of fully saturated soil. (7) Adsorbed water :- - The fine grained soils have a layer of adsorbed water strongly attached to their surface. This adsorbed water layer is not free to move under gravity. It cause an obstruction to flow of water in the pores and hence reduces the permeability of soils. (8) Impurities in water :- - Any foreign matter in water has a tendency to plug the flow passage and reduce the permeability of soil.

DARCY'S LAW OF PERMEABILITY “For Laminar flow in saturated soil, Discharge(q) is proportional to the Hydraulic Gradient(i).”.:q=kiA where, q=Discharge i=hydraulic gradient k=coefficient A=total c/s area of soil mass.:q/A=ki - But we know that q/A=v,.:v=ki where, v=velocity

ASSUMPTION OF DARCY'S LAW  Soil is saturated.  flow is Laminar.  flow is Steady and Continues.  Temperature at a time of Testing.

Determination coefficient of permeability Methods of determine coefficient of permeability Laboratory methods 1.Constant head permeability test 2.Falling head permeability test Field methods 1.Pumping out tests 2.Pumping in tests Indirect methods 1.Computation from the particle size 2.Computation from consolidation test

Field Permeability Tests FIELD PERMEABILITY TESTS Pumping out tests 1.For unconfined aquifer 2.For confined aquifer Plumbing in tests 1.Open end test 2.Single packer test 3.Double packer test

Flownet stream line flow lines  The general solution of the Laplace equation yields two set of curves orthogonal to each other. One set of curves represents the path along which the individual particles of water seep through the soil are called stream line or flow lines. equipotential lines.  Flow lines indicate the direction of flow. The other set of curves represents lines of equal head and are termed equipotential lines. potential drop.  The head loss caused by water crossing two adjacent equipotential lines is termed as the potential drop.

SOIL WATER  The water present in the voids of soil mass is called soil water. Soil water Free Water (Gravitational Water) Structural Water Adsorbed Water Capillary Water Held Water

Free water  ‘ free water’ is the water in excess of the moisture that can be retained by the soil. Free water moves in the pores of the soil under the influence of gravity. It is also known as gravitational water.  Free water flows from one point to other where there is the difference of head. The rate at which the head is lost along the flow passage is equal to the hydraulic gradient. The flow of free water in soils is just like laminar flow in pipes.

Held water:  ‘ held water’ is that water which is held in the pores of the soil mass because of certain forces of attraction. It is not free to move under the influence of gravity.  Held water is further divided in two types: 1.Structural water 2.Adsorbed water 3.Capillary water

Structural water:  The water chemically combined in the crystal structure of the mineral of the soil is called ‘structural water’.  This water can not be removed without the breaking the structure of the mineral.  A temperature of more than 300C is require for removing the structural water.  In soil engineering, the structural water consider as an integral part of the soil solids.

Adsorbed water:  Water held by electrochemical forces existing on the soil surface is know as ‘adsorbed water.  Properties of adsorbed water are: -It has more viscosity. -Greater surface tension. -It is heavier than the normal water. -The boiling point is higher and freezing point is lower than the normal water.

Capillary water  The water held in the interstices of soil due to capillary forces (surface tension) is called ‘capillary water’.  This water is in suspended condition within the interstices and pores of the capillary rise of the soil.  Capillary water exists in soil so long as there is an air-water interface.  The capillary water are is always under tension (negative pressure).

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