Lecture 1 Soil Mechanics

8/6/2019 Lecture 1 Soil Mechanics

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Civil EngineeringCivil Engineering

Soil MechanicsSoil Mechanics

Flow MeasurementsFlow Measurements

SurveyingSurveying



SOIL MECHANICSSOIL MECHANICS

y Soil Classificati

on

y S

ieve Analysis



INTRODUCTIONINTRODUCTION

Soils:

All naturally occurring relatively unconsolidated earth

material (organic or inorganic) that lies above bedrock.

They can be broken down into constituent particles

relatively easily (distinct from rocks, hard soil at the

boundary).

Soil mechanics:

Concerns the application of principles of mechanics,

hydraulics & chemistry to engineering problems related

to soils & to under stand soil behavior.



INTRODUCTIONINTRODUCTION

In most of the civil engineering applications, soil is

subjected to stress due to external loading.The external

loading is caused by buildings (residential, commercial,

industrial), highways, dams, embankments etc.

In the present course, soil is studied as engineering

material and it is classified (gravel, sand, silt, clay) based

on certain experimental parameters. Based on this

classification proper use of soil as civil engineering

material is determined (in foundation, dam, highway,

embankment construction etc.)



CLASSIFICATION OF SOILSCLASSIFICATION OF SOILS

y article Size Classification

y Textural Classification

y Public Roads Administration Classification

System

y Unified Soil Classification System



Different types of soil classification:

(i) Particle size classification: Soil is classified as gravel, sand, silt &

clay based on particle size.

Grain size distribution of soil is determined.

(ii) Textural classification:

(a) IS:1498-1970 system: Grain size distribution of soil is obtained.

Weight of soil grain within each size group is computed.

% of soil in each size group is obtained (as per this system, mixed soilmight contain 10% gravel, 52% sand, 38% silt & clay).

(b) US bureau of soils & chemistry system: First % of sand, silt and

clay is found.




y Based on this a point is located on the triangular chart which indicates soil

type (Fig. 1 on page 14).




(iii) Public roads administration classification system [AASHTO

(1978) system]:Classification is based on particle size composition

as well as on plasticity characteristics.

Group index of soil is determined and soil is rated as subgrade material.

Higher value of group index indicates poor quality of material.

(iv) Unified soil classification system (USC): This method is based

on grain size distribution and plastic properties. It is applicable for

any use.

Modified USC has been adopted in IS:1498-1970.

Plasticity chart for classifying fine grained soil is given in Fig. 2 (Page 14)

Soils belowA line are inorganic silt, organic silt or organic clay.




y Coarse grained: gravel has 80mm to 4.75mm particle size.

sand has 4.75mm to 0.075mm size.

y Fine grained: silt has 0.075mm to 0.002mm particle size.

clay has less than 0.002mm particle size.

y Organic fine grained soil has appreciable amount of

organic matter in soil. It has dark brown colour and

odour of decaying vegetation.

y Inorganic fine grained soil is classified based on dry

strength, shaking, plasticity and dispersion test.

FIELD IDENTIFICATION OF SOILSFIELD IDENTIFICATION OF SOILS



In dry strength test, 3mm size dry lump of fine grained

soil is broken between thumb and forefinger. Silt breaks

with little effort and clay requires lot of effort.

In shaking test, fine grained soil of very soft consistency

with smoothened top surface is kept in hand. It is shaked

in hand by tapping. If shiny appearance comes at top, then

it is silt. If dull appearance comes at top, then it is clay.




y In plasticity test, small fine grained soil paste is rolled

into 3mm thread. Clay can easily be rolled into thread,

whereas silt can·t be.y In dispersion test, small soil sample is dispersed in 10cm

water depth. Sand settles in 30sec, silt in 0.5 to 240

minutes and clay takes several hours to several days.




SieveAnalysis

WetAnalysis (HydrometerAnalysis)

Pipette Method

GRAIN SIZE ANALYSISGRAIN SIZE ANALYSIS



SIEVE ANALYSIS



SIEVE DESIGNATIONSIEVE DESIGNATION -- LARGELARGE

Sieves larger

than the #4

(4.75mm) sieve

are designated

by the size of the

openings in the

sieve

(Example:

31.5mm Size)



SIEVE DESIGNATIONSIEVE DESIGNATION -- SMALLERSMALLER

10

openings

per inch

# 10 sieve

1-

inch

Smaller sieves are numbered according to the number of openings per inch



SIEVE ANALYSIS

y Write down the weight of each sieve as well as the

pan to be used in the analysis.

y Record the weight of the given dry soil sample.

y Make sure that all the sieves are clean, and assemble

them in the ascending order of sieve numbers. Placethe pan below the smallest sieve. Carefully pour thesoil sample into the top sieve and place the cap over it.

yPlace the sieve stack in the mechanical shaker andshake f or 10 minutes

y Remove the stack fr om the shaker and carefully weigh and record the weight of each sieve including

the pan with its retained soil.



SIEVES AS PER IS CODE OF PRACTICE

4.75mm

2.36mm

1.18mm

0.6mm

0.425mm

0.3mm0.15mm

0.075mm

PAN

(Weight of the Sample ± Depends on the Nominalsize of the Mix and the Sieve Aperture (For thecurrent Experiment ± 1200Gm)

Lecture 1 Soil Mechanics

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