lecture 2: soil properties and groundwater flobaiyu/engi 9621_files/spring 2012/lecture 2.pdf ·...

9621 – Soil Remediation Engineering

Spring 2012

Faculty of Engineering & Applied Science

Lecture 2: Soil Properties and

Groundwater Flow

1

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)

2.1 Soil properties

2.1.1 What is soil?

Soils equation

Soil = f (parent material, climate, biota, topography, time)

2

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

3 Source: Codutoet al., Geotechnical Engineering, 2011

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

(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

2.1.2 Physical characteristics of soil

5

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

(2) Soil texture

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

Soil texture: Relative size comparison of soil particles

7

Source: Codutoet al., Geotechnical Engineering, 2011

Soil texture triangle represents 12 textural classes

Different

combinations of

coarse and fine

contents produce

different soil

textures

A loam a

mixture of sand,

silt and clay

Soils are more cohesive when they have more fine particles

Clay

Soils are more loose when the have more coarse particles

Sand

8


Bulk density a measure of soil compaction soil weight

per unit soil volume

To calculate bulk density:

1 cm (so, there is 1 cubic

centimeter of soil)

Sample is made

of solids and

pore spaces 1.33 gms

Volume = 1 cm3

Weight = 1.33 g

Bulk density = WT VT

Bulk density = 1.33

1

Bulk density = 1.33 g/cm3

(3) Soil bulk density (ρd)

9

Bulk density and compaction zones

8 inches

1.43

0 inches

7 inches

9 inches 10 inches

Depth Bulk Density (grams/cm3)

1.90

1.87

1.84

1.80

1.60

Plow layer

Compacted zone

Uncompacted

subsoil

Low BD = high porosity High BD = low porosity

10

(4) Soil moisture content

Soil moisture content the quantity of water

contained in a soil

Volumetric moisture content, θ defined mathematically

as

where

VW = the volume of water

VT = VS + VV = VS + VW + Va = the total volume (that is soil

volume + water volume + air space)

T

w

V

V

11

Gravimetric moisture content, u expressed by mass

(weight) as follows:

where

MW = the mass of water

MS = the mass of soil

s

w

M

Mu

12

Groundwater Soil

Soil at different moisture levels

Pore Space

Water on soil particle surface Pore Spaces are filled with water


(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

2.1.3 Chemical characteristics of soil

14

(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

- -

- -

- - - - - - - - -


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 Al3+ for Si4+ in the tetrahedral

layer of clays

- Substitution of Mg2+ for Al3+ in the octahedral

layer of clay

Sources of charge on clays

16

(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, Ca2+, Mg2+, K+, Na+, NH4+ ,

Al[OH]2 +, Al3+, 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

With Magnets

-

-

+

-

Unlikes Attract

Likes Repel

In soil

CLAY

NH4+

Ammonium

CLAY

K+

Potassium

CLAY

NO3-

Nitrate

+

-

+ +


Cation exchange the replacement of one

adsorbed cation for another from solution

-

-

-

-

-

-

..Na+

..Na+ [Ca2+]

-

-

-

-

-

-

..Ca2+ [Na+]

[Na+]

Negatively-charged clay

Dissolved in soil solution

+ +

2XNa+ + Ca2+ XCa2+ + 2Na+

19

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

2.2 Groundwater flow

2.2.1 Introduction

20

(1) Groundwater

Sand and gravel Igneous rocks limestone

Intergranular Crevice Solution

2.2 Groundwater flow

2.2.1 Introduction


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

21

Source: Environment Canada, 1990

22

Groundwater contains 98.7% of the fresh water

resources and is a reserve of good quality water

Percentage of population reliant on

Groundwater in Canada

Groundwater and the

world‘s freshwater supply

Source: Statistics Canada, 1996

Groundwater faces the threat of contamination from waste sites

Properties of subsurface govern both the rate and direction of

groundwater flow

(2) Vertical distribution of groundwater

Groundwater can be characterized according to its vertical

distribution

23

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 aeration consists of interstices occupied partially

by water and partially by air

Zone of saturation all interstices are filled with water

under hydrostatic pressure

24

Source: Bedient et al., Hydrology and Floodplain Analysis, 2007

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

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 semi-

pervious 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

27 Source: Bedient et al., Hydrology and Floodplain Analysis, 2007

Water filled porosity θw (or nW) volumetric soil

moisture content

Air filled porosity θg (or θa, na)

Total porosity

28

It is the ratio of voids volume to

the total volume of medium

(1) Porosity (n)

2.2.2 Subsurface hydraulic properties and groundwater

flow

In the zone of areation

aWn

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

Effective porosity (ne) 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 (Sy) 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, ne = Sy

Specific retention of an aquifer (Sr) the ratio of

volume of water that is retained against the force of

gravity to the total volume

Total porosity

29

In the zone of saturation

ry SSn

Air

Water

Soil

30

taa VV

VtWW sVVV

tV nVV

sw uMM

tds VM

Soil composition by phase

(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, h1, and h2. The results showed

31

L

hhKA

dL

dhKAQ

21

32 Head loss through a sand column (z = elevation)

Source: Zhang, Engineering Hydrology, 2003

Where, Q = volumetric flow rate or total discharge K = coefficient of permeability or hydraulic conductivity A = cross-sectional area of flow h = hydraulic head; h1 h2 = head loss L = length of flow path; dh/dL = i = hydraulic gradient

KiAL

hhKA

dL

dhKAQ

21

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


34

An expression for hydraulic conductivity in terms of

fluid and porous media properties

K = cd2g/

Where

c = a dimensionless constant

d = mean grain diameter

= fluid density

= fluid absolute viscosity

g = gravitational acceleration

The product cd2 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 = cd2 and K= kg/

(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 (vS) equals to the Darcy velocity divided by effective porosity since the actual flow is limited to the pore space only vS = v /ne = -Ki/ne

Seepage velocity (vS) usually higher than the Darcy’s velocity

35

(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

T = Kb

Where

T = hydraulic conductivity

b = the saturated thickness of an aquifer

36

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 150.0 cm2.

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

38

Radial flow to a well penetrating a confined aquifer

)/ln(2

w

w

rr

hhTQ

)/ln(2

12

12

rr

hhTQ

or


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

)/ln( 12

2

1

2

2

rr

hhKQ


40

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.

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 m3/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.

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