Chapter 2: Water in soil

(a) Pressure head and flow in one dimension

(b) Darcys law, flow of water through a porous medium

porosity

Example:

Exam

ple

:

k = 2.3 x 10-7 cm/sec

(c) Permeability measurement: laboratory and site

(Given: k =

1

0

12

log)(

3.2h

h

ttA

alk

)

Field Permeability Test

Offer the advantage of testing undisturbed soil in the natural location with respect to the ground surface, water table, and other factors that could influence the rate of flow

Field permeability test involves obtaining a record of time that it takes for a volume of water to flow out of, or into the boring casing

Simple field permeability tests are considered appropriate for evaluating granular and silt soils

Low coefficient of permeability associated with clays and silt-clay mixtures would involve a long time period for the field test

Field permeability test (continued.)

Field Permeability test (Continued)

In short

(d) Capillarity

Capillarity

Groundwater table (or phreatic surface) the level which underground water will rise in an observation well, pit or other open excavation in the earth

Soil beneath groundwater table filled with water

Soil moisture any water in soil located above the water table

Capillary rise phenomenon which water rises above the groundwater table against the pull of gravity but is in contact with the water table as its source

Capillary moisture the water associated with capillary rise

Vadose zone the soil region directly above the water table and wetted by capillary moisture

Water in Capillary Tubes

Basic principles of capillary rise in soils related to the rise of water in glass capillary tubes

The rise attraction

between the water and

the glass and to a

surface tension, which

develops at the air-water

interface at the top of

the water column in the

capillary tube

Water in Capillary Tubes (continued)

Value of Ts for water varies according to temperature

As temperature increases, the value of Ts decreases, indicating a lessening height of capillary rise under warm conditions

At room temperature, Ts for water = 0.064 N/m

At freezing, Ts for water = 0.067 N/m

In applying the development of capillary rise in tubes to capillary rise in soils:

hc 31/d mm (McCarthy, D. F., 2002)

(*provided that d is in millimetres)

Water in Capillary Tubes (continued)

Question:

Compute the height of capillary rise for water in a tube having a diameter of 0.05 mm (in SI units)

Solution:

m

mkNmx

mN

d

Th

w

sc 52.0

)/81.9)(105(

)/064.0)(4(435

Water in Capillary Tubes (continued)

The height of capillary rise is not affected by a slope or

inclination in the direction of the capillary tube, or by

variations in the shape and size of the tube at level below

the meniscus

Water in Capillary Tubes (continued)

Capillary rise is not limited to tube, or enclosed, shapes. If

two vertical glass plates are placed so that they touch along

one end and, form a V, a wedge of water will rise up in the

V because of the capillary phenomenon

Capillary rise in soil

Shapes of void spaces between solid particles are

unlike those in capillary tubes

Voids are of irregular and varying shape and size,

and interconnect in all directions, not only the

vertical

The features of capillary rise in tubes are

applicable to soils insofar as they facilitate an

understanding of factors affecting capillarity, and

help to establish an order of magnitude for

capillary rise in the different types of soils

Capillary rise in soil (continued.)

Question:

Limited laboratory studies indicate that for a certain silt soil, the

effective pore size for height of capillary rise is 1/5 of D10,

where D10 is the 10 percent particle size from the grain-size

distribution curve. If the D10 size for such a soil is 0.02mm,

estimate the height of capillary rise.

Solution:

d = effective capillary diameter = 1/5D10 = 1/5 (0.02 mm)

= 0.004 mm

hc 31/d 31/0.004 mm 7750 mm 7.75m

Capillary rise in soil (continued.)

Capillary fringe

Capillary rise in soil (continued.)

Soil Type Meter (m)

Small gravel 0.02-0.1

Coarse sand 0.15

Fine sand 0.3 to 1

Silt 1 to 10

Clay 10 to 30

Table : Representative heights of capillary rise: water in soil

Glossary

One- dimensional flow the velocity at all points has the same direction and (for an incompressible fluid) the same magnitude

Two-dimensional flow all streamlines in the flow are plane curves and are identical in a series of parallel planes

(e) Flow nets and seepage

3.56 x 10-4

No of equipotential drops at point a

Elevation loss

hpw

hpw

hpw

Uplift forces

Static Liquefaction, Heaving, Boiling, & Piping

Static liquefaction the state which the effective stress becomes zero, the soil loses its strength and behaves like a viscous fluid

Boiling, quicksand, piping and heaving are used to describe specific events connected to the static liquefaction state

Boiling the upward seepage force exceeds the download force of the soil

Piping the subsurface pipe-shaped erosion that initiates near the toe of dams and similar structures. High localized hydraulic gradient statically liquefies the soil, which progresses to the water surface in the form of a pipe, and water then rushes beneath the structure through the pipe, leading to instability and failure

Quicksand existence of a mass of sand in a state of static liquefaction

Liquefaction can be produced by dynamic events such as earthquakes

Example:-

(f) In Situ stresses

Stresses in Saturated Soil Without Seepage

The total stress at the elevation of point A, , can be obtained from the saturated unit weight of the soil and the unit weight of water above it

= Hw + (HA H) sat

Stresses in Saturated Soil With Seepage

If water is seeping, the effective stress at any point in a soil mass will be different from the static case

It will increase or decrease, depending on the direction of seepage

Upward seepage

Downward seepage

Effective stress in partially saturated soil

Water in the void spaces is not continuous, and it is a three-phase system

Thus, effective stress, = - ua + (ua uw) * represents the fraction of a unit cross-sectional area of the soil

occupied by water. For dry soil = 0, and for saturated soil = 1

Effective stress in partially saturated soil (Continued)

Bishop intermediate values of depend primarily on the degree of saturation, S

These values are also influenced by factors such as soil structure

(g) Others phenomenon: Quick sand, Frost heave in soils

Quicksand

Dreaded quicksand condition occurs where a sand or cohesionless silt deposit is subjected to the seepage force caused by upward flow of groundwater

The upward gradient of the water is sufficient to hold the soil particles in suspension, in effect creating a material with the properties of a heavy liquid

Elimination of seepage pressure will return the soil to a normal condition capable of providing support

Frost heave in soils

When freezing temperatures develop in a soil mass, most of the pore water in the soil is also subject to freezing. As water cystallizes, its volume expands approximately 9 percent

In considering void ratios and the degree of saturation for soils, expansion of a soil material as a result of freezing might be expected to be on the order of 3 or 4 percent of the original volume

Frost heave in soils (continued)

In the normal frost heave occurrence, the source of water is the groundwater table

Upward movement from a water table to the freezing zone relates to a potential for migration (capillary rise)

Height of capillary rise is quite limited in clean, coarse-grained soil

End of Chapter 2