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CIE_3008 토질역학 및 실험 Lec.1 Soil Composition 1

Lecture 01

Soil Composition


Outline

• Origin of Soil and Grain (Chapter 2)

• Weight-Volume Relationships (Chapter 3)

• Plasticity and Structure of Soil (Chapter 4)

• Classification of Soil (Chapter 5)


Origin of Soil and Grain Size

Aggregation of weakly cemented mineral grains

Multi-Phase Material: Mineral grain, air, water

Sites With Photos of Minerals:

http://www.theimage.com/mineral/minerals1

.html

http://minerals.usgs.gov/

http://www.theimage.com/mineral/minerals1.html




http://minerals.usgs.gov/



Mineral grains of soil aggregate are the product of rock weathering

Mineral: A mineral is a naturally occurring substance that is solid and

stable at room temperature, representable by a chemical formula,,

and has an ordered atomic structure



Soil-Particle size: easiest method to classify soil

Four types of soils Size cannot alone

describe soil characteristics

Clay is defined as particles

“which develop plasticity when

mixed with a limited amount of

water

Clay particles are mostly

< 1 μm - 2 μm is upper limit



Clay Minerals

Tiny platy crystals

Result in plate-like layers

Negatively charged

Attract positively charged ions (cation)

Absorb or lose water at the surface or interlayer space (swell or shrink)



Clay Structure

Silica tetrahedron (SiO4)

Forms silica sheet (Fig. 2.10b)

Bound by shared oxygen

Alumina octahedron (AlO6)

Forms octahedral (gibbsite) sheet (Fig. 2.10.d)



Clay Minerals



Clay Minerals

Kaolinite 1:1 lattice / length 1,000 – 20,000 A (1A = 0.1 nm) thick =

100 – 1000 A

Illite 1:2 lattice / length 1,000 – 5,000 A thick = 50 – 500 A

Montmorillonite 1:2 lattice / length 1,000 – 5,000 A thick = 10 – 50 A



Clay Minerals

Kaolinite:

One of most common clay minerals in sedimentary soils

Illite

Most common clay mineral in stiff clays and shales, marine and

lacustrine soft clay

Fine grained mica

Montmorillonite

Most common member of a group of clay minerals known as

smectites

Dominant in soils derived from volcanic ash

Lowest permeability

Highest capacity to absorb water

Bentonite (used to limit flow of water)



Clay Minerals

Carry a net negative charge on the surface

Dry clay, negative charge is balanced by exchangeable cations like Ca2+,

Mg2+, Na+, K+

When water is added to clay, these cations and few anions float around

the clay particles this is known as diffuse double layer



Clay Minerals: Mechanism of water attracted to clay

dipolar water attracted to both negatively charged surface of the clay

particles and cations in the double layer

Cations are attracted to clay particles

Hydrogen bonding: hydrogen atom in the water is shared with oxygen

atoms in clay

Clay dictate engineering

behavior if > 50 %

Check Fig. 2.20



Specific Gravity


Mechanical analysis of soil: Sieve analysis



Sieve analysis



Procedure 1.) Pulverize soil with mortar and pedestal to break up clods of silt and clay, then wash the material on a No. 200 sieve. 2.) Using the soil retained on a No. 200 sieve, allow the material to air dry and then weigh the material. 3.) Place a stack of sieves on shake table with the coarsest on top. Place the material in the coarsest sieve and shake. 4.) Make the following computations for a given particle size, % finer = 100 - % retained % retained = (sum of weight retained/total weight of sample) 5.) Plot on grain size distribution curve, % finer by weight Uniform soils are represented by nearly vertical curves. "S" shaped curves which extend over several log cycles are well-graded (contain good representation of sands and gravels)

Sieve analysis



Sample sieve analysis



Procedure is performed as follows:

1.) Soil is air-dried and then pulverized.

2.) The dry soil is then weighed and mixed in a beaker of water

containing a dispersion fluid. In suspension, the electrostatic forces of

attraction between adjacent particles are often greater than the

gravitational forces. The dispersion fluid helps to prevent

development of clay flocs.

3.) After tempering, the fluid is poured into a glass cylinder and mixed

4.) The change in density of the fluid is measured with time.

5.) Based on change in density of fluid with time, an estimate of the

particle size distribution can be obtained.

Hydrometer test



Hydrometer test



Result from Sieve analysis and Hydrometer test



Comments on semi-logarithmic scales




60

10

u

DC

D

2

30

60 10

c

DC

D D

Particle-Size distribution curve

Effective grain size: D10

Uniformity Coefficient :

Coefficient of gradation:



Particle-Size distribution curve

Effective grain size: D10

Uniformity Coefficient :

Coefficient of gradation:


Shape


Weight-Volume Relationships

V= volume S=solid (mineral) w=water g= gas (air), v=voids (water +gas)

Weight (W)= Mass (M) x Acceleration due to gravity



Porosity: n= , Void Ratio: e= , Degree of Saturation: S= vV

Vv

s

V

Vw

v

V

V

Moisture content: w= , Unit weight: , Dry unit weight: w

s

W

W

W

V

sd

W

V



Unit weight : , Dry unit weight: , Void ratio: (1 )

1

s ww G

e

Unit Weight, Void ratio, Moisture content, specific gravity

1

s wd

G

e

1s w

d

Ge

sS e w G

Saturated unit weight : ( )

1

s wsat

G e

e



Unit Weight, Porosity, Moisture content



Various unit-weight relationships


Example (1)

Given: 50 cc Clay, Mt=85 g, Ms=60g, G=2.7 Compute: w, e, S, ρd


Example (2)

Given: w=41.7%, G=2.7, Saturated sample Compute: S, e, γd

Let Vs = 1 Vv=Vw=Mw/ρw=wGs=0.417*2.7=1.13 Vt=Vv+Vs=2.13 e=Vv/Vs=1.13 ρd=Ms/Vt=ρsVs/Vt=GsρwVs/Vt=2.7/2.13=1.27



Relative Density

Minimum Void Ratio (emin)

- Obtained by shaking a sand sample

Maximum Void Ratio (emax)

- Obtained by pouring (raining) oven-dried soil through a sieve

Relative Density (or Density Index)

,max ,minmax

max min ,max ,min

100% 100%d d d

r d

d d d

e eD I

e e


Example (3)

Given: w=41.7%, G=2.7, Saturated sample

Given Lab Measurement: emax=1.5, emin=0.5 Compute: Dr

max

max min

1.5 1.13100% 100% 37%

1.5 0.5r d

e eD I

e e

Given: Ground freezing Compute: frozen sample void ratio

Let Vs = 1 Vv=Vw=Mw/ρw=wGs=0.417*2.7=1.13 Vw_frozen=1.09*1.13=1.23 e=Vv/Vs=1.23

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