Lecture 2: Soil Properties and Groundwater Flow

Transcription

1 9621 Soil Remediation Engineering Lecture 2: Soil Properties and Groundwater Flow Spring 2012 Faculty of Engineering & Applied Science 1

2 2.1 Soil properties What is soil? Each discipline defines soil in a different way, depending on how soil affects it In an engineering sense, soil is the relatively loose agglomerate of mineral and organic materials and sediments found above the bedrock. --- R.D. Holtz and W.D. Kovacs (1981) Soils equation Soil = f (parent material, climate, biota, topography, time) 2

3 Soil composition About 50% of the soil solid particles 45% - Minerals 5% - Organic matter About 50% of soil should be pore space 25% - Air/Pore space 25% - Water Source: Codutoet al., Geotechnical Engineering,

4 Air Water Soil Soil composition by phase: s-soil (dry) w-water a-air v-void (pores filled with water or air) V is volume M is mass 4

5 2.1.2 Physical characteristics of soil (1) Soil profile Soil profile layers of soil are called horizons Typical profile A Horizon topsoil B Horizon subsoil C Horizon bedrock A Horizon B Horizon C Horizon 5

6 (2) Soil texture Soil texture the way the soil feels depends on the amount of each size of mineral particles in the soil Sand, silt, and clay are names that describe the size of individual mineral particles in the soil Sand the largest particles and they fell gritty Silt medium sized, and they feel soft, silky or floury Clay the smallest sized particles, and they feel sticky 6

7 Soil texture: Relative size comparison of soil particles Source: Codutoet al., Geotechnical Engineering,

8 Soils are more cohesive when they have more fine particles Clay Soils are more loose when the have more coarse particles Sand Different combinations of coarse and fine contents produce different soil textures A loam a mixture of sand, silt and clay Source: Codutoet al., Geotechnical Engineering, 2011 Soil texture triangle represents 12 textural classes 8

9 (3) Soil bulk density (ρ d ) Bulk density a measure of soil compaction soil weight per unit soil volume Sample is made of solids and pore spaces 1.33 gms 1 cm (so, there is 1 cubic centimeter of soil) To calculate bulk density: Volume = 1 cm 3 Weight = 1.33 g Bulk density = W T V T Bulk density = Bulk density = 1.33 g/cm 3 9

10 Low BD = high porosity High BD = low porosity Depth Bulk Density (grams/cm 3 ) 0 inches 7 inches 8 inches 9 inches 10 inches Plow layer Compacted zone Uncompacted subsoil Bulk density and compaction zones 10

11 (4) Soil moisture content Soil moisture content the quantity of water contained in a soil Volumetric moisture content, θ defined mathematically as V V w T where V W = the volume of water V T = V S + V V = V S + V W + V a = the total volume (that is soil volume + water volume + air space) 11

12 Gravimetric moisture content, u expressed by mass (weight) as follows: u M M w s where M W = the mass of water M S = the mass of soil 12

13 Soil at different moisture levels Groundwater Pore Spaces are filled with water Soil Pore Space Water on soil particle surface Source: Codutoet al., Geotechnical Engineering,

14 2.1.3 Chemical characteristics of soil (1) Soil ph or Hydrogen-ion activity Soil ph a measure of the relative amount of H + ions indicates the acidity or the alkalinity of a solution (a soil solution) ph meter ph = -log [H + ] In a soil it is driven by the ionization of soil water ph scale ranges from 0 to 14 7 is considered neutral Everything greater than 7 is considered alkaline (basic) Everything less than 7 is considered acidic 14

15 (2) Soil surface charge Coarse-grained soil such as gravel, sand and silt are chemically inert The surface of clay minerals and organic matters (OM) in soil generally carry electro-negative charges Clay Particle Source: Codutoet al., Geotechnical Engineering,

16 Sources of charge on clays Ionization on edges it is ph-dependent, similar to charge on OM just as in the case of a weak acid Isomorphous substitution in clays it is not affected by ph often referred to as permanent charges - Substitution of Al 3+ for Si 4+ in the tetrahedral layer of clays - Substitution of Mg 2+ for Al 3+ in the octahedral layer of clay 16

17 (3) Cation exchange capacity (CEC) CEC = quantity of exchangeable cations per unit weight of soil The capacity of a soil to adsorb and exchange cations (positively charge ions, Ca 2+, Mg 2+, K +, Na +, NH 4 +, Al[OH] 2 +, Al 3+, and H + ) CEC due to the net negative charge of soil colloids (clays and organic matter) Both ionization and isomorphous substitution impart CEC to clays Total CEC of the soil is dependent upon the amount of these sources and also upon the surface area of clays exposed 17

18 With Magnets In soil Unlikes Attract CLAY CLAY NH 4 + Ammonium K + Potassium Likes Repel CLAY NO 3 - Nitrate Source: Codutoet al., Geotechnical Engineering,

19 Cation exchange the replacement of one adsorbed cation for another from solution Na + + [Ca 2+ ]..Na + Dissolved in soil solution Ca 2+ + [Na + ] [Na + ] Negatively-charged clay 2XNa + + Ca 2+ XCa Na + 19

20 2.2 Groundwater flow Introduction (1) Groundwater It is water that exists beneath the earth's surface in underground streams and aquifers It is found that underground where part/entire void spaces between particles of rock and soil, or in crevices and cracks in rock are filled with water Sand and gravel Igneous rocks limestone 2.2 Groundwater flow Introduction Intergranular Crevice Solution Source: Codutoet al., Geotechnical Engineering,

21 Groundwater an important part of the hydrologic cycle Some of the water from melting snow/rainfall seeps into the soil and percolates into the saturated zone to become groundwater recharge Eventually, groundwater reappears above the ground into streams, rivers, marshes, lakes and oceans or as springs and flowing wells discharge Source: Environment Canada,

22 Groundwater contains 98.7% of the fresh water resources and is a reserve of good quality water Groundwater and the world s freshwater supply Source: Statistics Canada, 1996 Percentage of population reliant on Groundwater in Canada Groundwater faces the threat of contamination from waste sites Properties of subsurface govern both the rate and direction of groundwater flow 22

23 (2) Vertical distribution of groundwater Groundwater can be characterized according to its vertical distribution Zone of aeration consists of interstices occupied partially by water and partially by air Soil water zone extending from ground surface down through the major root zone Vadose zone extending from lower edge of soil water zone to the upper limit of capillary zone Capillary zone extending from the water table up to the limit of capillary rise Zone of saturation all interstices are filled with water under hydrostatic pressure 23

24 Source: Bedient et al., Hydrology and Floodplain Analysis,

25 (3) Aquifer A formation that contains sufficient saturated permeable material to yield significant quantities of water to wells and springs Type of aquifers Confined aquifer (artesian aquifer) groundwater is confined by a relatively impermeable stratum, or confined unit, and water is under pressure greater than atmosphere artesian wells or flowing wells Unconfined aquifer (water table aquifer) an aquifer in which the water table forms the upper boundary the water level in a well tapping an unconfined aquifer will rise only to the level of the water table within the aquifer 25

26 Perched aquifer a perched water table, an example where an unconfined water body sits on top of a clay lens, separated from the main aquifer formed perched aquifer Leaky aquifer upper or lower boundary is semipervious stratum could be confined or unconfined leaky aquifer Piezometric surface an imaginary surface coinciding with the hydrostatic pressure level of the water in the certified aquifer elevation of the surface at a given point can be determined by finding water level in a penetrating well Water table the upper surface of the saturation zone under atmospheric pressure 26

27 Source: Bedient et al., Hydrology and Floodplain Analysis,

28 2.2.2 Subsurface hydraulic properties and groundwater flow (1) Porosity (n) It is the ratio of voids volume to the total volume of medium In the zone of areation Water filled porosity θ w (or n W ) volumetric soil moisture content Air filled porosity θg (or θa, n a ) Total porosity n W a In the zone of saturation porosity is an index of how much total groundwater can be stored in the void space of the saturated medium not indicate how much water the porous medium will yield 28

29 In the zone of saturation Effective porosity (n e ) the ratio of the volume of the void space through which flow can occur to the total volume less than total porosity n Specific yield of an aquifer (S y ) the ratio of the volume of water that drains from saturated material due to the attraction of gravity to the total volume in most cases, n e = S y Specific retention of an aquifer (S r ) the ratio of volume of water that is retained against the force of gravity to the total volume Total porosity n S y S r 29

30 Air Water Soil V V a W V a W t V VV nv t Soil composition by phase t sv V M w um s M s V d t 30

31 (2) Hydraulic conductivity (K) Hydraulic conductivity (or permeability) is defined as the property of a porous media that permits the transmission of water through it K can be obtained through using Darcy s Law In 1856, Henri Darcy investigated the flow of water through beds of permeable sand. The followed figure shows the experimental set-up for determining head loss through the sand column Darcy experimented with different soils and with different values of L, h 1, and h 2. The results showed Q dh h1 h2 KA KA dl L 31

32 Where, Q = volumetric flow rate or total discharge K = coefficient of permeability or hydraulic conductivity A = cross-sectional area of flow h = hydraulic head; h 1 h 2 = head loss L = length of flow path; dh/dl = i = hydraulic gradient Q KA dh dl h1 h KA L 2 KiA Source: Zhang, Engineering Hydrology, 2003 Head loss through a sand column (z = elevation) 32

33 Hydraulic conductivity a measure of the permeability of the porous media or, say, an indication of an aquifer s ability to transmit water Its value usually depends on the size and number of pores in the soil or aquifer material It has the dimensions of length/time (L/T) or velocity, such as cm/sec, ft/day Source: Bedient et al., Hydrology and Floodplain Analysis,

34 An expression for hydraulic conductivity in terms of fluid and porous media properties K = cd 2 g/ Where c = a dimensionless constant d = mean grain diameter = fluid density = fluid absolute viscosity g = gravitational acceleration The product cd 2 is a function only of the porous media and are functions of the fluid The intrinsic permeability k is a property of the medium (soil or rock) only, independent of fluid properties k = cd 2 and K= k g/ 34

35 (3) Groundwater movement velocity Darcy s velocity (v), or discharge velocity an average discharge velocity through the entire cross section of the column v = Q/A = -Kdh/dL = -Ki Seepage velocity (v S ) equals to the Darcy velocity divided by effective porosity since the actual flow is limited to the pore space only v S = v /n e = -Ki/n e Seepage velocity (v S ) usually higher than the Darcy s velocity (4) Transmissivity (T) Transmissivity a measure of the water amount that can be transmitted horizontally through a unit width by the fully saturated thickness of an aquifer under a hydraulic gradient equal to 1 35

36 T = Kb Where T = hydraulic conductivity b = the saturated thickness of an aquifer Example 2-1: Calculate the discharge and seepage velocities for water flowing through a pipe filled with sand with a hydraulic conductivity of 1.5 x 10 6 cm/s and a porosity of 0.2. the hydraulic gradient is 0.01 and the cross-sectional area of the pipe is cm 2. 36

37 2.2.3 Groundwater flow toward a pumping well (1) Steady flow to a well in a confined aquifer When a well is pumped, water levels in its neighborhood are lowered this lowering amount at a given point defines the drawdown at that point At the given point in time, the variation of drawdown with distance from the well describes the drawdown curve (or cone of depression). The steady-state flow to a well means the variation of head occurs only in space and not in time The steady radial flow to a well fully penetrating a homogeneous confined aquifer can be expressed as 37

38 Q 2 T h h ln( r / r w w ) or Q 2 T h2 h1 ln( r / r ) 2 1 Radial flow to a well penetrating a confined aquifer Source: Bedient et al., Hydrology and Floodplain Analysis,

39 (2) Steady flow to a well in a unconfined aquifer The steady radial flow to a well fully penetrating a homogeneous unconfined aquifer can be expressed as h2 h1 Q K ln( r2 / r1 ) 2 2 Source: Bedient et al., Hydrology and Floodplain Analysis,

40 Example 2-2: a well is constructed to pump water from a confined aquifer. Two observation wells, OW1 and OW2, are constructed at distances of 100m and 1000m, respectively. Water is pumped from the pumping well at a rate of 0.2 m 3 /min. at steady state, drawdown s is observed as 2m in OW2 and 8m in OW1. Determine the hydraulic conductivity K and transmissivity T if the aquifer is 20 m thickness. Example 2-3: A well is drilled through an unconfined aquifer. Original water table was 50 ft below ground surface and bedrock was reached at 150 ft below ground surface. After pumping until equilibrium conditions at 1700 gpm, the water table was lowered by 10 and 20 ft at observation wells located at 1000 and 100 ft respectively. Determine hydraulic conductivity. 40