SUB GRADE SOIL INVESTIGATION AND PROPERTIES

Name :- SHAH NIKET RAMESHClass:- B.Tech IVRoll no.:- U07CE044 Faculty:- Dr. Rakesh Kumar

Transportion Engineering DivisionCivil Engineering Department

Sardar Vallabhbhai National Institute of Technology, Surat.

Significance of Subgrade Soil Subgrade soil is an integral part of the road

pavement structure as it provides the support to the pavement from beneath.

The subgrade soil and its properties are important in the design of pavement structure.

The main function of the subgrade is to give adequate support to the pavement and for this the subgrade should possess sufficient stability under adverse climate and loading conditions.

The formation of waves, corrugation, rutting and shoving in black top pavements and the phenomena of pumping, blowing and consequent cracking of cement concrete pavements are generally attributed due to the poor subgrade conditions.

DESIRABLE PROPERTIES

Stability Incompressibility Permanency of strength Minimum changes in volume and stability

under adverse conditions of weather and ground water

Good drainage, and Ease of compaction.

Index Properties of Soil

The soil properties on which their identification and classification are based are known as index properties.

The index properties which are usually used are grain size distribution, liquid limit and plasticity index.

Further the properties which are sometimes used are shrinkage limit, field moisture equivalent centrifuge moisture equivalent and compacted dry density.

GRAIN SIZE ANALYSIS

The grain size distribution is found by mechanical analysis.

The components of soils which are coarse grained may be analyzed by sieve analysis and the soil fines by sedimentation analysis.

It is used to determine the percentage of individual grain size present in a soil sample.

CONSISTENCY LIMITS AND INDICES

Liquid limit may be defined as the minimum water content at which the soil will flow under the application of very small shearing force.

Plastic limit may be defined in general terms, as the minimum moisture content at which the soil remains in a plastic stage.

Plasticity index (P.I.) is defined as the numerical difference between the liquid and plastic limits.

Plasticity index thus indicates the range of moisture content over which the soil is in plastic condition.

Shrinkage limit is the maximum moisture content at which further reduction in water content does not cause reduction in volume.

It is the lowest water content that can occur in clayey soil sample which is completely saturated.

Field moisture equivalent of a soil is the moisture content at which the demands for absorbed water are fully satisfies.

The Centrifuge moisture equivalent of a soil is the moisture content retained against a force of 1000 times gravity for one hour.

Soil Classification Based on Grain Size

There are several classification systems for soil grains based on grain size of soil, according to which soils have been classified as gravel, sand, silt and clay.

The various soil classification systems in use in the field of highway engineering are :

M.I.T. classification system or Indian Standard Classification System

Burmister descriptive classification Casagrande soil classification Unified soil classification of Revised Casagrande soil

classification and I.S. soil classification systems. U.S. Public Roads Administration (PRA) classification Highway Research Board (HRB) or American Association of

Stage Highway Officials (AASHO) classification or Revised PRA classification

Civil Aeronautics Administration (CAA) classification. Compaction classification.

M.I.T. classification system or Indian Standard Classification System

The limits of the grain size for each component as per this system are shown below:

TEXTURAL CLASSIFICATION The textural classification system is based on

grain size distribution of the soil and is helpful in classifying a soil which contains different soil components such as sand silt and clay.

UNIFIED SOIL CLASSIFICATION SYSTEMUnified soil classification system

Coarse Grained

Gravels(G) Sand(S)

UNIFIED SOIL CLASSIFICATION SYSTEM

MAJOR DIVISION GROUP SYMBOLS

DESCRIPTIONS

COARSE GRAINED SOILS More Than Half

Retained on 200

Sieve

GRAVELSMore

Than Half Coarse Fraction Retained on No. 4

Sieve

Clean Gravels(little or no

Fines)

GWWell Graded Gravels,

Gravel – Sand Mixtures,Little or no Fines

GPPoorly Graded Gravels,Gravel - Sand Mixtures,

Little or no FinesGravels with

Fines(Appreciable

Fines)

GM Silty Gravels,Gravel – Sand - Silt Mixtures

GC Clayey Gravels,Gravel – Sand - Clay Mixtures

SANDSMore

Than Half Coarse Fraction Passes a

No. 4 Sieve

Clean Sands(Little or no

Fines)

SWWell Graded Sands,

Gravelly Sand,Little or no Fines

SPPoorly Graded Sands,

Gravelly Sand,Little or no Fines

Sands with Fines

(Appreciable Fines)

SM Silty Sands,Sand – Silt Mixtures

SC Clayey Sands,Sand - Clay Mixtures

UNIFIED SOIL CLASSIFICATION SYSTEM

MAJOR DIVISION GROUP SYMBOLS

DESCRIPTIONS

FINE GRAIN

ED SOILS More Than Half

Retained on 200

Sieve

SILTS and CLAYS

Liquid Limit Less Than 50

MLInorganic Silts & very Fine Sands,

Silty or Clayey Fine Sands,Clayey Silts

CLInorganic Clays of Low to Medium

Plasticity,Lean Clays

OL Organic Silts & Organic Silty Clays Of Low Plasticity

SILTS and CLAYS

Liquid Limit Greater Than 50

MHInorganic Silts,

Fine Sand or Silty soils,Elastic silts

CH Inorganic Clays Of High Plasticity ,Fat Clays

OHOrganic Clays Of Medium To High

Plasticity,Organic Silts

Highly Organic Soils PT Peat And Other Highly Organic Soils

HIGHWAY RESEARCH BOARD (HRB) CLASSIFICATION OF SOILS This is also called American Association of Stage Highway

Officials (AASHO) classification of Revised PRA classification (PRA) soil classification system.

A-1 soils are well graded mixture of stone fragments, gravel coarse sand, fine sand and non-plastic or slightly plastic soil binder. The soils of this group are subdivided into two subgroups, A-1-a, consisting predominantly of stone fragments or gravel and A-1-b consisting predominantly of coarse sand. 

A-2 group of soils include a wide range of granular soils ranging from A-1 to A-3 groups, consisting of granular soils and 35% fines of A-4, A-5, A-6 or A-7 groups. Based on the fines content, the soils of A-2 groups are subdivided into subgroups A-2-4, A-2-5, A-2-6 and A-2-7. 

A-3 soils consist mainly, uniformly graded medium or fine sand similar to beach sand or desert blown sand. Stream-deposited mixtures of poorly graded fine sand with some coarse sand and gravel are also included in this group.

A-4 soils are generally silty soils, non-plastic or moderately plastic in nature with liquid limit and plasticity index values less than 40 and 10 respectively.

A-5 soils are also silty soils with plasticity index less than 10% , but with liquid limit values exceeding 40%. These include highly elastic or compressible, soils, usually of diatomaceous of micaceous character.

  A-6 group of soils are plastic clays, having high values of

plasticity index exceeding 10% and low values of liquid limit below 40%; they have high volume charge properties with variation in moisture content.

  A-7 soils are also clayey soils as A-6 soils, but with high

values of both liquid limit and plasticity index, (LL greater than 40% and PI greater than 10%). These soils have low permeability and high volume change properties with changes in moisture content.

FACTORS AFFECTING SUB-GRADE SOIL STRENGTH CHARACTERISTICS Soil type

Moisture content

Dry density

Internal structural of the soil, and

The type and mode of stress application

EVALUATION OF SOIL STRENGTH The tests used to evaluate the strength properties of

soils may be broadly divided into three groups :-

Tests

Shear tests,

Direct Shear Test

Triaxial

compression

test

Bearing tests, and

Plate bearing

test

Penetration tests

California Bearing

Ratio (CBR) test

CALIFORNIA BEARING RATIO (CBR) This is a penetration test developed by the California

Division of Highways, as a method for evaluating the stability of soil, subgrade and other flexible pavement materials. The test results have been correlated with flexible pavement thickness requirements for highways and air fields.

The C.B.R. value is calculated using the relation:

[ Load (or pressure) sustained by the specimen at 2.5 or 5.0 mm penetration ]

CBR, % = ------------------------------------------------------------------------ x 100 [ Load (or pressure sustained by standard

aggregates at the corresponding penetration level ]

CBR VALUE 3% and less 3% - 5% 5% - 15%SUBGRADE STRENGTH

Poor Normal Good

INVESTIGATING PROCEDURE FOR CLASSIFICATION OF SUB GRADE

SOIL In particular , soil and material surveys are required:

(i) to determine the nature and physical characteristics of soil and soil profile for design of embankment and pavement.(ii) to determine the proper method(s) of handling soils;(iii) to classify the earth work involved into various categories such as rock excavation, earthwork in hard soil etc.(iv) to gather general information regarding subsoil water level and flooding; and(v) to locate sources for pavement construction materials and to ascertain their availability and suitability for use in the different pavement courses.

SOIL INVESTIGATION FOR LOW EMBANKMENT

The first operation in the detailed soil survey is to demarcate the possible borrow areas along or off the road.

While demarcating borrow areas within the road land, the directions contained in IRC : 10-1961 “Recommended Practice for Borrowpits for Road Embankments Constructed by Manual Operation“ should be kept in view.

The extent of borrow areas should be commensurate with the volume of work involved in the embankment.

Test pits 0.5 to 1.0 m.sq. should be dug in the borrow areas from where the embankment material is to be obtained.

For borrow pits along the road land, the test pits should be at intervals of 200 metres but the intervals may be varied depending on the uniformity or variability of the soil.

When embankment material is to be obtained from off-road locations, adequate number of samples should be taken from each such area.

The depth of test pits should not exceed the likely depth of the borrow pits by more than 15 cm.

The general character of material excavated from the test pits should be recorded and tests conducted on it in the laboratory for properties mentioned further on.

Where the type of material varies in a single pit, the tests should be conducted on each type of soil separately and the horizon of occurrence noted.

SPECIAL INVESTIGATIONS FOR HIGH EMBANKMENTS

The basic objective of investigations in such cases is to obtain engineering data for soil and rock that are necessary for a quantitative design of embankment at the chosen sites.

The details of investigations depend upon the requirements of design.

Generally for checking stability against slip failures, the basic properties to be investigated are shear parameters, unit weight and moisture conditions.

For safety against excessive settlement, consolidation properties are important.

For investigation of such cases, services of specialists will often be needed.

Reference may also be made to IRC: 75-1979 ‘Guidelines for the Design of High Embankments.

SOIL INVESTIGATIONS FOR CUT SECTIONS

In the same manner as described in paragraph for embankment materials, soil in cut sections along the centre line of the road at an elevation corresponding to the design subgrade level should be tested for the following general properties :

(i) Gradation(ii) Atterberg limits(iii) Field dendity and moisture content(iv) Proctor denstity.

In the case of rock cuts where necessary, trial pits or boreholes should be carried out at the foundation level to make realistic estimates of the type of cutting involved.

The interval of trial bores may be 30-50 metres or as otherwise decided by the Engineer-in-Charge depending on specific requirements.

GENERAL GUIDE TO THE SELECTION OF SOILS ON BASIS OF ANTICIPATED EMBANKMENT

PERFORMANCEP.R.A

.Classification

Comparable soil groups in Indian Standard Soil Classification System

Visual description

Max. Dry density

range gm per c.c.

Optimum moisture content range

percent

Anticipated embankme

nt performanc

e.Most probable Possible

1 2 3 4 5 6 7

A-1GW , GP, GB ,

GM , SW , SP, SB , SM

--- Granular materials

1.84-2.28 (115-142) 7-15 Good to

excellent

A-2 GM , GC , SM , SC ---

Granular materials with soil

1.76-2.16 (110-135) 9-18 Fair to

excellent

A-3 SP --- Sand 1.76-1.84 (110-115) 9-15 Fair to good

A-4 ML , MH , OL , OH

CL , SM , SB , SC

Sandy silts and silts

1.52-2.08(95-130) 10-20 Poor to good

A-5 MH , OH --- Elastic silts and silts

1.36-1.60(85-100) 20-35 Unsatisfacto

ry

A-6 CL , CI MH , OH , SC Silt clay 1.52-1.92 (95-120) 10-30 Poor to good

A-7 MH , CI , CH , OH SC Clay 1.36-1.84

(85-115) 15-35 Poor to fair

* For guidance in this respect see Standard No. IRC : 37 – 1970 “Guidelines for the Design of Flexible Pavements”.

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