SOIL MECHANICS: UNIT 4 SOIL CLASSIFICATION

    

Unit 4 soil classification

Theoretical and scientific purpose: 

➢ For generalization and advancement of soil science by engineering different idea➢ To provide a common language to engineers and scientists for the exchanging of

information and experience regarding soil. 
➢ For the systematic study, study, communication, and discussion about soil. 

Practical purpose 

➢ For gathering data in the different schemes in an easy way. 
➢ To correlate with behavior of soil under a specific use. 

Basic requirement of soil classification 

➢ Should have united number of groups 
➢ Should have based on engineering properties 
➢ Should be simple and uses easily understandable terms. 

Soil classification 

Soil classification is a separation of soil into classes or groups each having similarcharacteristics and potentially similar behavior. A classification for engineering purposes should be based mainly on mechanical properties, e.g. permeability, stiffness, strength. In general, there are two major categories into which the classification systems can be gr-ouped. 

 1. The textural classification is based on the particle-size distribution of the

percent of gravel, sand, silt, and clay size fractions present in a given soil.

Such as Massachusetts Institute of Technology  classification system

(M.I.T classification) and the U.S. Department of Agriculture. 

 

2. The other major category is based on the engineering behavior of soil and takes into

consideration  the particle-size distribution and the plasticity (i.e., liquid limit and

plasticity index). Under this category,  there are two major classification systems in

extensive use now: 

a. The American Association of State Highway and Transportation classification      system  (AASHTO) (not included in our syllabus), and 

b. The Unified classification system (USCS).

Mechanical Analysis of Soil: 

The mechanical analysis is the determination of the size range of particles present in the soil,  expressed as a percentage of the total dry weight. Two methods are used to find the particle-size  distribution of soil:  

(1) Sieve analysis—for particle sizes larger than 0.075 mm in diameter, and  
(2) hydrometer analysis—for particle sizes smaller than 0.075 mm in diameter 

(1) Sieve Analysis :  
                                Sieve analysis consists of shaking the soil sample through a set of sieves that haveprogressively  smaller openings. 

(2) Hydrometer analysis: 

                                is based on the principle of sedimentation of soil grains in water. When a soil specimen is dispersed  in water, the particles settle at different velocities,

depending on their shape, size, weight, and the  viscosity of the water. For simplicity, it is assumed that all the soil particles are spheres and that  Stokes' law can express the

velocity of soil particles. Used for soil particles less than 0.075mm, (U.S.  No. 200 sieve)

Grain size distribution curve:

It is the semi-log graph plotted between % passing of soil  sample and particle size diameter(mm).

Uniformity coefficient (Cu) = D60/D10 

Curvature coefficient (Cc) = (D30)2/(D60D10)

M.I.T CLASSIFICATION SYSTEM OF SOIL: 

This system was developed by prof. G. Gilbay at Massachusets institute of technology in USA. This  soil classification system based on sieve analysis and divided into main four groups” clay, silt, sand  and gravel. 

where, 
            f= fine 
            m= medium 
            c= course

TRIANGULAR TEXTURAL SOIL CLASSIFICATION SYSTEM:

US Bureau has developed a triangular  classification giving name of soil, containing different percentage of sand, silt, and clay size of  particles as shown in figure:

1. Texture indicates the relative content of particles of various sizes, such as sand,

silt and clay  in the soil 

2. textural classification is based on particle size on which percentage of sand, silt

and clay are plotted an equilateral triangle which is already divided into 10 zo

3. In plotting, sand % is parallel to clay, silt % is parallel to sand and clay % is parallel

to sil

4. The meeting point of these three lines in one zone out of 10 zone in which point

    meet, that  is types of that soil.

    UNIFIED SOIL CLASSIFICATION SYSTEM: 

1. It was firstly developed by Cassagrande in 1948 and later in 1952 modified by Bur of  Reclamation And Crops Of Engineer Of USA. 

SOIL TYPE	PREFIX	SUB GROUP	SUFFIX
GRAVEL	G	Well graded	W
SAND	S	Poorly graded	P
SILT	M	silty	M
CLAY	C	clayey	C
ORGANIC	O	Low plasticity	L
PEAT	Pt	High plasticity	H

2. This system based on particle size distribution and plasticity characteristics of soil. 3. In this system soil is classified in 15 groups. All of them main 3 groups coarse grain, fine grain soil  and organic soil. 

A. Coarse grain soil: more than 50% retain on 75 micron sieve. 

a) Gravel: more than 50% of retain on 4.75mm sieve. 

1) Clean gravel: less than 5% pass on 75 micron sieve. 

i. Well graded gravel (GW):Cu =4 and Cc = 1-3. 

ii. Poorly graded gravel(GP): not meet above condition. 

2) Gravel with fine - more than 12% passing on 75 micron sieve.  

i. Silty gravel (GM): Atterberg limit below A line and Ip < 4 

 A LINE equation: 0.73(Wl-20) 

ii. Clayey gravel (GC): Atterberg limit above A line and Ip > 7 

b) Sand: more than 50% pass through 4.75 mm sieve 

    1) Clean sand: less than 5% fine particles. 

     i. Well graded sand (SW): Cu =6 and Cc= 1-3                       
ii. Poorly graded sand (SP) :not meet above cond. 
2) Sand with fine: -more than 12% fine. 
i. Silty sand (SM): atterberg limit below A-line and Ip < 4 
 A LINE equation: 0.73(Wl-20) 
ii. Clayey sand (SC) : Atterberg limit above A line and Ip > 7 

❖ Note:                 Gravels with 5-12% fine particles requires dual symbols: GW-GM, GW-GC, GP-GM,GP-GC.Sand with 5-12% fine particles requires dual symbols: SW-SM, SW-SC, SP-SM, SP-SC. 

  

 B. Fine grain soil: more than 50% passing on 75 micron sieve. 

                    Fine grain soil can be divided in a 2 main group i.e. 

            a. Low plasticity 

b. High plasticity 

            a) Low plasticity fine soil- Wl < 50% 

            i. Inorganic silt of low plasticity- ML 

           ii. Inorganic clay of low plasticity- CL 

          iii. Organic silt of low plasticity- OL 

           b) High plasticity fine soil - Wl > 50% 

           i. Inorganic silt of high plasticity- MH 

           ii. Inorganic clay of high plasticity- CH

                     iii. Organic silt of high plasticity- OH

               c) Highly organic soil: they are identified by visual inspection . termed as peat                          soil(पात पतीङगर कूहिएर बनेको), muck soil( body decay भएर).

Field identification of soil: 

Generally, most natural soil are mixture of two or more varieties soil. They may also contain some  organic materials in a partially or fully decomposed state. The mixture is then named major portion  as sand and minor portion called silty sand. 

Methods of field identification of soil convinently be discussed under the heading of  coarse grain and fine grain soil, gravel and sand ,silt and clay. 

Thumb rule to describe soil according to consistency:

CONSISTENCY(WATER CONTENT)	FIELD IDENTIFICATION
Very stiff	Brittle or very hard
Stiff	Cannot be moulded in finger
firm	Can be mould in finger by strong pressure
soft	Easily moulded in finger
Very soft	Exude between finger when squeezed.

a. Gravel versus sand: 

Technique: visual inspection  

Criteria: If presence % of gravel is more than sand, soil is gravel, else sand. 

b. Coarse versus fine soil: 

Technique: visual inspection  

Criteria: the particle smaller than 0.075mm are not visible individually. 

Decision: First, soil is spread on flat surface and shown, if more than 50% particle are visible to  the naked eye, this soil is coarse grain otherwise fine grain soil.

c. Sand vs. silt: 

Technique: sedimentation 

Criteria: the fine sand settles very fast in water while silt takes some time 

Decision: if particle settle within 30 sec. is about 10 cm that is sand .silt has a longer time to  settle. 

d. Silt vs. clay: 

➢ By taking moist soil on finger, if we touch finger each other, feel rough if it is sandy or  silty but smooth if it is clayey. 

➢ Also if we squeezed soil between finger moisture come out and dries fairly quickly that  is silt but clay attaches to the finger and dries slowly. 

e. Plastic vs. non plastic clay: 

Technique: rolling soil in between palm 

Criteria: for plastic clay soil can be rolled up to very fine tread about 3 mm. 

 For non plastic clay it crumbles while rolling.