Chapter 5. Classification of Soil

입도분석(Particle – size distribution Analysis)

- 체 분석 (sieve analysis) : 입경이 0.074mm 이상 snad, silt, gravel

- 침강분석 (hydrometer analysis) : 입경이 0.074 이하 silt, clay

2.5 Mechanical Analysis of Soil

Unified Soil Classification (통일분류법)

- Coarse grained soil (조립토)

Cobbles (조약돌, 호박돌, 사력) > 3 in (76mm)

Gravel (자갈) : 3 in (76mm) ~ #4 sieve(4.76mm)

Sand (모래) : #4 sieve ~ #200 (0.074mm)

- Fine grained soil (세립토)

silt (실트) : #200 ~ 0.002mm (2m)

clay (점토) : < 0.002mm

organics

soluble salts

체 분석 (sieve analysis)

- For particle sizes larger than 0.075 mm in diameter

1. Determine the mass of soil retained on each sieve (i.e., , , ⋯)

and in the pan (i.e., )

2. Determine the total mass of the soil : + + ⋯ + + ⋯ + +=∑3. Determine the cumulative mass of soil retained above each sieve. For

the ith sieve, it is + + ⋯+4. The mass of soil passing the ith sieve is ∑ − ( + + ⋯+)5. The percent of soil passing the ith sieve (or percent finer) is

= ∑ − ( + + ⋯+) ∑ × 100

log scale

• Hydrometer Analysis

Stokes’ Law

For sphere (구형입자);

= where = velocity = unit weight of soil particles = unit weight of water = viscosity of water = diameter of soil particles

If the units of are (g∙ sec)/ , is in g/ , Lis in cm, t is in min, and D is in mm, then

()10 = 18[( ∙ sec)/ 2]( − 1)(/ 3) ( )(min) ∙ 60 D = 30( − 1)

γ = GγD = 18( − 1)

= K( )(min)

K = 30( − 1) where

Stokes’ Law

• 유동이 없는 물속을 구형 토립자가 침강할 때 토립자의 침강속도에 의한토립자의 지름을 구함

0.0002 ~ 0.2 (mm)

0.2 mm > 침강시 교란

0.0002 mm < brown 운동 = • Limitation of stoke’s Law

• - Not perfectly sphere

• - vortex (large particles)

• - Brownian Motion (small particles 0.2)

• - specific gravity was be different

Hydrometer test : conducted in a sedimentation cylinder with 50 g of

oven-dried sample

• Sedimentation cylinder : 18 in. (457.2 mm) high

and 2.5 in. (63.5 mm) in diameter

• Dispersing agent : Sodium hexametaphosphate(규산소다)

• Hydrometers are calibrated for soils that have a specific gravity, , of

2.65; for soils of other specific gravity, it is necessary to make a

correction

• Hydrometer analysis is effective for separating soil fractions down to a

size of about 0.5

= + 12 − where = distance along the stem of the hydrometer from the top of the bulb to

the mark for a hydrometer reading (cm) = length of the hydrometer bulb = 14 cm = volume of the hydrometer bulb = 67 A = cross-sectional area of the sedimentation cylinder = 27.8

log scale

2.6 Particle-Size Distribution Curve

• Parameters are

1. Effective size ; (유효입경)

2. Uniformity coefficient ; (균등계수)

= 3. Coefficient of gradation, Coeff. of curvature (곡률계수)

= − ×

log scale

log scale

5.1 Textural Classification

• texture of soil : surface appearance

※ In most cases, natural soils are mixtures of particles from several size

groups.

• USDA system in Table 2.3 ;

‣ Sand-size : 2.0 to 0.05 mm in diameter

‣ Silt-size : 0.05 to 0.002 mm in diameter

‣ Clay-size : smaller than 0.002 mm in diameter

5.2 Classification by Engineering Behavior

• textural classification : based entirely on the particle-size distribution.

※ It is necessary to consider plasticity, which results from the presence of

clay minerals, in order to interpret soil characteristics.

• Two Classification System :

1) AASHTO Classification System - Highways

2) Unified Classification System - Geotechnical Engineers

5.3 AASHTO Classification System

- 미국 공로국 (U.S. Public Road Administration),1929 발표 후 수 차례 개정

- 입도분석, Atterberg Limit을 사용하는 점에서는 USCS 법과 동일하지만군지수(Group Index)를 사용하는 점이 특징이다.

※ Soil is classified into seven major groups : A-1~A-7

• A-1, A-2, and A-3 :

Granular materials where 35% or less of the particles pass through the

No. 200 sieve.

• A-4, A-5, A-6 and A-7 :

More than 35% pass through the No. 200 sieve.

5.4 Unified Soil Classification System

• 미국에서 개발 by Casagrande for use in wwǁ for the U.S. army

• ASTM designation D-2487

- 군 기호를 1차와 2차 특성문자로 표시

v 흙의 분류 · 토류구조물을 통한 침투

· 전단강도에 대한 연구

v 수상경력 및 주요 활동

§ Karl Terzaghi Award (최초 수상자 )

§ 국제토질역학 및 기초공학회 회장

(1961~1965)

§ Institute of Civil Engineers

– Rankine Lecturer

§ ASCE – Terzaghi Lecturer

Arthur Casagrande(1902-1981)

지반공학 인물 평전

Primary letter

G

S

M

C

O (organic silts and clays) (peat, muck and highlyorganic soils)

WP #200 passing < 5%

GP-GMSW-SM

dualsymbol

5 ~ 12 %

M : 비소성 세립질C : 소성 세립질

#200 passing > 12%

Secondary letter

L : 저소성 (LL < 50%)

H : 고소성 (LL > 50%)

50% 이하이상

50% 이하이상

#4

#200

① Sieve analysis, , , 2중 1

① #4(4.76mm) 통과 or 걸린 량

② GW, > 41 < < 3SW, > 41 < < 3 나머지는

L H

CHCL

MLOL

MHOH

CL

CL-ML

ML

7

4

A Line = 0.73(-20)

# 200 passing > 12%

5020

SM, A선 아래 , < 4

GM

GC, A선 위

, > 7

SC

④

• For proper classification according to this system, some or all of the following need to be known :

1. Percent of gravel – that is, the fraction passing the 3 in. sieve (76.2mm opening) and retained on the No.4 sieve(4.75 mm opening)

2. Percent of sand – that is, the fraction passing the No.4 sieve (4.75 mm opening) and retained on the No.200 sieve (0.075 mm opening)

3. Percent of silt and clay – that is, the fraction finer than the No.200 sieve (0.075 mm opening)

4. Uniformity coefficient(Cu) and the coefficient of Gradation(Cc)

5. Liquid limit and plasticity index of the portion of soil passing the No.40 sieve

5.4 Unified Soil Classification System

• Proposed by Casagrande in 1942

• ASTM designation D-2487

5.5 Comparison Between the AASHTO and Unified Systems

• AASHTO system :

• 35% passes through the No. 200 sieve

• No. 10 sieve is used to separate gravel from sand.

• Gravelly and sand soils are not clearly separated.

(The A-2 group, in particular, contains a large variety of soils.

• Unified system :

• 50% passes through the No. 200 sieve

• No. 4 sieve is used to separate gravel from sand.

• Gravelly and sandy soils are clearly separated.

※ Symbols like GW, SM, CH, and others that are used in the Unified

system are more descriptive of the soil properties than the A symbols

used in the AASHTO system.

※ The classification of organic soils as OL, OH, and Pt is provided in the

Unified System. Under the AASHTO system, there is no place for

organic soils.

• USCS vs AASHTO의 비교

① 흙의 입도분포와 #200 sieve 통과량을 기준으로 조립토와 세립토를 구

분하는 것은 동일하나 USCS는 50%, AASHTO는 35%를 기준

② AASHTO는 모래 자갈을 2.0mm를 기준으로 하나 USCS는 4.76mm를 한

계로 함

③ USCS는 모래, 자갈을 세밀히 구분하고 있으나 AASHTO는 개략적으로

구분

④ 유기질토는 USCS로서 가능하나 AASHTO는 제외