CEB 3103 GEOTECHNICAL ENGINEERING I SOIL WATER AND

CEB 3103 GEOTECHNICAL ENGINEERING I SOIL WATER AND WATER FLOW Prepared by R. Elakya, Assistant Professor Department of Civil Engineering

SOIL WATER Water present in the void spaces of a soil mass is called ‘Soil Ø Water’ The sub-surface water which occupies the voids in the soil Ø above the ground water table. Ø Movement of water into soil - Infiltration Ø Downward movement of water within the soil - Percolation, Permeability or Hydraulic conductivity 2

SOIL WATER FORMS OF SOIL WATER There are mainly two forms of soil water. Ø Gravitational water ü Free water ü Ground water ü Capillary water Ø Held water ü Adsorbed water ü Capillary water ü Structural water Department of Civil Engineering Fig. 1 Soil water 3 Source: Fig. 1 - https: //www. tutorvista. com/biology/types-of-soil-conservation

SOIL WATER Gravitational water ü The water in the soil due to the movement of water under gravitational forces. Free water : ü Similar properties as that of liquid water ü Moves under the influence of gravity, or due to difference in hydrostatic pressure head. ü Sources - precipitation, run-off, floodwater, melting snow, water from certain hydraulic operations. Department of Civil Engineering 4

SOIL WATER Ground water : ü Fills up the voids in the soil up to the ground water table and translocates through them. ü Fills coherently and completely all voids which makes the soil completely saturated. ü Ground water subjected to atmospheric pressure - Ground water table ü Elevation of the ground water table at a given point - Ground water level Department of Civil Engineering 5

SOIL WATER Capillary water : Water in a suspended condition, held by the forces of surface ü tension within the interstices and pores of capillary size in the soil. Retained as minute bodies of water filling part of the pore ü space between particles. Department of Civil Engineering 6

SOIL WATER Held water ü Water held in soil pores or void spaces because of certain forces of attraction. Adsorbed water : ü Strongly attracted to soil mineral surfaces by electrostatic forces especially clays. ü Dry soil mass adsorb water from atmosphere even at low relative humidity known as hygroscopic water content. ü Water lost from an air-dry soil when heated to 105ºC. ü Neither affected by gravity nor by capillary forces and would not move in the liquid form. Department of Civil Engineering 7

SOIL WATER Structural water : ü Chemically combined as a part of the crystal structure of the mineral of the soil grains ü Cannot be separated/removed when subjected to loading conditions or oven drying to 105ºC - 110ºC Department of Civil Engineering 8

STRESSES IN SOIL Ø Stresses (Total Stress) within a soil mass caused by external loads applied to the soil and also self-weight of the soil. Ø Total stress increases with depth (Z) and with unit weight of soil (ɣ). Ø At any point inside a soil mass, resisted by the soil grains and water present in the pores or voids (saturated soil). Vertical total stress at depth Z, σv = ɣ. Z Fig. 2 Stress in soil mass Department of Civil Engineering Source: Fig. 2 http: //environment. uwe. ac. uk/geocal/Soil. Mech/stresses. htm 9

STRESSES IN SOIL Ø Below a water body, the total stress is the sum of the weight of the soil up to the surface and the weight of water above this. σv = ɣ. Z + ɣw. Zw Fig. 3 Stress in submerged soil mass Department of Civil Engineering Source: Fig. 3 http: //environment. uwe. ac. uk/geocal/Soil. Mech/stresses. htm 10

STRESSES IN SOIL Pore Pressure/Neutral stress Ø Pore water pressure (u) - Pressure of groundwater held within a soil or rock, in gaps between particles (pores). Ø Pore water pressures below the phreatic level of the groundwater are measured with piezometers. Ø Magnitude of the pore water pressure at water table - zero. Ø Below the water table, pore water pressure - positive. u = Ɣw. h Ɣw – Unit weight of water Fig 4. Pore water pressure in soil mass Department of Civil Engineering Source: Fig. 4 http: //environment. uwe. ac. uk/geocal/Soil. Mech/stresses. htm 11

STRESSES IN SOIL Effective Stress / Inter-granular Pressure Effective stress - Pressure transmitted through grain to grain at the contact points through a soil mass causing displacements. Compression and Shear strength of the soil depends on effective stress. Effective stress (σ') acting on a soil is calculated from two parameters, total stress (σ) and pore water pressure (u) according to: σ‘ = σ – u Fig. 5 Total stress, Effective stress and Pore water pressure Department of Civil Engineering Source: Fig. 5 – Schofield and Wroth, “Critical State Soil Mechanics” 12

STRESSES IN SOIL Fig. 6 Schematic representation of Total stress, Effective stress and Pore water pressure Department of Civil Engineering Source: Fig. 6 http: //environment. uwe. ac. uk/geocal/Soil. Mech/stresses. htm 13

STRESSES IN SOIL Example 1 For the soil deposit shown below, draw the total stress, pore water pressure and effective stress diagrams. The water table is at ground level. Department of Civil Engineering 14

STRESSES IN SOIL Solution: Total stress At - 4 m, σ = 1. 92 x 4 = 7. 68 T/m 2 At -11 m, σ = 7. 68 + 2. 1 x 7 = 22. 38 T/m 2 Pore water pressure At - 4 m, u = 1 x 4 = 4 T/m 2 At -11 m, u = 1 x 11 = 11 T/m 2 Effective stress At - 4 m , σ‘ = 7. 68 - 4 = 3. 68 T/m 2 At -11 m , σ‘ = 22. 38 - 11 = 11. 38 T/m 2 Department of Civil Engineering 15

STRESSES IN SOIL Example 2 Determine the neutral and effective stress at a depth of 16 m below the ground level for the following conditions: Water table is 3 m below ground level ; G = 2. 68; e = 0. 72; average water content of the soil above water table is 8%. Solution: Department of Civil Engineering 16

STRESSES IN SOIL Department of Civil Engineering 17

STRESSES IN SOIL Department of Civil Engineering 18

SOIL PERMEABILITY PERMEA BILITY OF SOIL ü Darcy's law states that there is a linear relationship between flow velocity (v) and hydraulic gradient (i) for any given saturated soil under steady laminar flow conditions. ü If the rate of flow is q (volume/time) through cross-sectional area (A) of the soil mass, Darcy's Law can be expressed as v=q/A=k. i where k – permeability of soil (cm/sec) i – hydraulic gradient (Δh/L) Δh - difference in total heads L – Length of soil mass Department of Civil Engineering Fig. 7 Flow of water in soil Source: Fig. 7 - NPTEL 19

SOIL PERMEABILITY What is permeability of soil? ü Permeability is defined as the property of a porous material which permits the passage or seepage of water through its interconnecting voids. Rate of permeability varies based on void spaces between the grains (irregular shape of the individual particles) ü Fig. 8 Comparison of Permeability of different soil Department of Civil Engineering Source: Fig. 8 - https: //www. pinterest. com/jvonstorch/muro-contenci/ 20

SOIL PERMEABILITY FOR DIFFERENT SOILS For different soil types as per grain size, the orders of magnitude for permeability are as follows: Department of Civil Engineering 21

SOIL PERMEABILITY FACTORS AFFECTING SOIL PERMEABILITY Department of Civil Engineering 22

SOIL PERMEABILITY DETERMINATION OF CO-EFFICIENT OF PERMEABILITY Department of Civil Engineering 23

SOIL PERMEABILITY CONSTANT HEAD PERMEABILITY TEST Ø Ø Quantity of water (Q) that flows under a given hydraulic gradient through a soil sample of known length & cross sectional area in a given time (t). Water is allowed to flow through the cylindrical sample of soil under a constant head. For testing of pervious, coarse grained soils k = Coefficient of permeability Q = total quantity of water t = time L = Length of the coarse soil Department of Civil Engineering 24

SOIL PERMEABILITY CONSTANT HEAD PERMEABILITY TEST SETUP Fig. 9 Constant Head Permeability test setup Department of Civil Engineering Source: Fig. 9 - Venkatramaiah, C. , “Geotechnical Engineering” 25

SOIL PERMEABILITY FALLING HEAD PERMEABILITY TEST Ø Relatively for less permeable soils Ø Water flows through the sample from a standpipe attached to the top of the cylinder. Ø The head of water (h) changes with time as flow occurs through the soil. At different times the head of water is recorded. t = time L = Length of the fine soil A = cross section area of soil a= cross section area of tube k = Coefficient of permeability Department of Civil Engineering 26

SOIL PERMEABILITY FALLING HEAD PERMEABILITY TEST SETUP Fig. 10 Falling Head Permeability test setup Department of Civil Engineering Source: Fig. 10 - Venkatramaiah, C. , “Geotechnical Engineering” 27

SOIL PERMEABILITY Example 3 A sample in a variable head permeameter is 8 cm in diameter and 10 cm high. The permeability of the sample is estimated to be 10 × 10– 4 cm/s. If it is desired that the head in the stand pipe should fall from 24 cm to 12 cm in 3 min. , determine the size of the standpipe which should be used? Solution: Department of Civil Engineering 28

SOIL PERMEABILITY Department of Civil Engineering 29

SOIL PERMEABILITY Example 4 The discharge of water collected from a constant head permeameter in a period of 15 minutes is 500 ml. The internal diameter of the permeameter is 5 cm and the measured difference in head between two gauging points 15 cm vertically apart is 40 cm. Calculate the coefficient of permeability. Solution: Department of Civil Engineering 30

SOIL PERMEABILITY – STRATIFIED SOIL DEPOSITS Ø Ø Soil deposit consists of a number of horizontal layers having different permeabilities, the average value of permeability can be obtained separately for both vertical flow and horizontal flow, as k. V and k. H respectively. Consider a stratified soil having horizontal layers of thickness H 1, H 2, H 3, etc. with coefficients of permeability k 1, k 2, k 3, etc. Fig. 11 Permeability of stratified soil deposits Department of Civil Engineering Source: Fig. 11 - NPTEL 31

SOIL PERMEABILITY For vertical flow For horizontal flow Department of Civil Engineering 32

SOIL PERMEABILITY Example 5 A horizontal stratified soil deposit consists of three layers each uniform in itself. The permeabilities of these layers are 8 × 10– 4 cm/s, 52 × 10– 4 cm/s, and 6 × 10– 4 cm/s, and their thicknesses are 7, 3 and 10 m respectively. Find the effective average permeability of the deposit in the horizontal and vertical directions. Solution: Department of Civil Engineering 33

SOIL PERMEABILITY Department of Civil Engineering 34

SOIL LIQUEFACTION QUICK SAND CONDITION Ø Quicksand forms in saturated loose sand when suddenly agitated. Ø When water in the sand cannot escape, it creates a liquefied soil that loses strength and cannot support weight. Ø In the case of upwards flowing water, seepage forces oppose the force of gravity and suspend the soil particles causing lose of strength. Ø The cushioning of water gives quicksand, and other liquefied sediments, a spongy, fluid-like texture. Ø Objects in liquefied sand sink to the level at which the weight of the object is equal to the weight of the displaced soil/water mix and the submerged object floats due to its buoyancy. Department of Civil Engineering 35

SOIL LIQUEFACTION MECHANISM Ø Ø Ø An upward flow opposes the force of gravity and cause to counteract completely the contact forces. Effective stress is reduced to zero and the soil behaves like a very viscous liquid - Quick sand condition. This condition occurs in coarse silt or fine sand subject to artesian conditions. Fig. 12 Quick sand condition - Mechanism Video link : https: //www. youtube. com/watch? v=e. Imt. Yyu. QCZ 8 Department of Civil Engineering Source: Fig. 12 - NPTEL 36

SOIL LIQUEFACTION Contd…. At the bottom of the soil column, During quick sand condition, the effective stress is reduced to zero. where icr = critical hydraulic gradient This shows that when water flows upward under a hydraulic gradient of about 1, it completely neutralizes the force on account of the weight of particles, and thus leaves the particles suspended in water. Department of Civil Engineering 37

SOIL LIQUEFACTION Ø Ø Ø Liquefaction is a special case of quicksand. In this case, sudden earthquake forces immediately increase the pore pressure of shallow groundwater. The saturated liquefied soil loses strength, causing buildings or other objects on that surface to sink. Video link : https: //www. youtube. com/watch? v=ZMWKTu. Rg. Jj. Y Department of Civil Engineering 38

REFERENCES v Arora K R. , “Soil Mechanics and Foundation Engineering”, Standard Publishers, 2011. v Venkatramaiah, C. , “Geotechnical Engineering”, New Age International Publishers, New Delhi, 6 th edition, 2018. v https: //nptel. ac. in/courses. php v https: //en. wikipedia. org/ Department of Civil Engineering 39