sub grade properties

7/27/2019 sub grade properties

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Subgrade


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Desirable Properties of Sub Grade Soil

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


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Characteristics of Subgrade Soil

Load bearing capacity.

able to support loads transmitted from the pavement structure

without excessive deformation.

Moisture content.

Moisture tends to affect a number of subgrade properties

including load bearing capacity, shrinkage and swelling.

Shrinkage and/or swelling.

soils with excessive fines content may be susceptible to frost

heave in northern climates.


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Subgrade Soil Properties and Functions

one of the principal highway materials

Foundation to the pavement

The main function:

to give adequate support to the pavement and for this the

subgrade should possess sufficient stability under adverse

climate and loading conditions.

To bear the external loads


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Effect of Poor Subgrade

Flexible pavements

The formation of waves

Corrugations

rutting and shoving in black top pavements

Rigid pavements

the phenomena of pumping

blowing and

consequent cracking


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Types of Soil

Gravel: coarse materials with particle size under 2.36 mm with little

or no fines contributing to cohesion of materials.

Moorum: products of decomposition and weathering of the

pavement rock.

Silts: finer than sand, brighter in color as compared to clay, and

exhibit little cohesion.

Clays: finer than silts. Clayey soils exhibit stickiness, high strength

when dry, and show no dilatancy.


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Tests on Soil

Shear tests

These tests are usually carried out on relatively small soil samples in

the laboratory. In order to find out the strength properties of soil, a

number of representative samples from different locations are

tested. Some of the commonly known shear tests are direct shear

test, Triaxial compression test, and unconfined compression test.


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Bearing tests

Bearing tests are loading tests carried out on sub grade soils in-situ

with a load bearing area. The results of the bearing tests are

influenced by variations in the soil properties within the stressed

soil mass underneath and hence the overall stability of the part of

the soil mass stressed could be studied.

Tests on Soil


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Penetration tests

Penetration tests may be considered as small scale bearing tests in

which the size of the loaded area is relatively much smaller and

ratio of the penetration to the size of the loaded area is much greater

than the ratios in bearing tests. The penetration tests are carried out

in the field or in the laboratory.

Tests on Soil


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Stiffness and Strength Tests

Stiffness

the relationship between stress and strain in the elastic range or

how well a material is able to return to its original shape and

size after being stressed

Strength

the stress needed to break or rupture a material


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Stiffness and Strength Tests

California Bearing Ratio (CBR),

Resistance Value (R-value),

Resilient modulus and

Modulus of Subgrade Reaction.


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California Bearing Ratio (CBR) Test

developed by the California Division of Highways around 1930

simple strength test that compares the bearing capacity of a material

with that of a well-graded crushed stone .

To evaluate the strength of cohesive materials having maximum

particle sizes less than 19 mm.

CBR test, an empirical test, has been used to determine the material

properties for pavement design.

Empirical tests measure the strength of the material and are not a

true representation of the resilient modulus.


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It is a penetration test wherein a standard piston, having an area of 3

in (or 50 mm diameter), is used to penetrate the soil at a standard

rate of 1.25 mm/minute.

The pressure up to a penetration of 12.5 mm and it's ratio to the

bearing value of a standard crushed rock is termed as the CBR.

In most cases, CBR decreases as the penetration increases.

The ratio at 2.5 mm penetration is used as the CBR.

In some case, the ratio at 5 mm may be greater than that at 2.5 mm.

If this occurs, the ratio at 5 mm should be used.



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The CBR is a measure of resistance of a material to penetration of

standard plunger under controlled density and moisture conditions.

The test procedure should be strictly adhered if high degree of

reproducibility is desired.

The CBR test may be conducted in re-moulded or undisturbed

specimen in the laboratory.

The test is simple and has been extensively investigated for field

correlations of flexible pavement thickness requirement.



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The laboratory CBR apparatus consists of a mould 150 mm diameter

with a base plate and a collar, a loading frame and dial gauges for

measuring the penetration values and the expansion on soaking.

The specimen in the mould is soaked in water for four days and the

swelling and water absorption values are noted.

The surcharge weight is placed on the top of the specimen in the

mould and the assembly is placed under the plunger of the loadingframe. Load is applied on the sample by a standard plunger with

diameter of 50 mm at the rate of 1.25 mm/min.



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A load penetration curve is drawn. The load values on standard

crushed stones are 1370 kg and 2055 kg at 2.5 mm and 5.0 mm

penetrations respectively.

CBR value is expressed as a percentage of the actual load causing

the penetrations of 2.5 mm or 5.0 mm to the standard loads

mentioned above. Therefore,



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Two values of CBR will be obtained. If the value of 2.5 mm is

greater than that of 5.0 mm penetration, the former is adopted.

If the CBR value obtained from test at 5.0 mm penetration is higher

than that at 2.5 mm, then the test is to be repeated for checking.

If the check test again gives similar results, then higher value

obtained at 5.0 mm penetration is reported as the CBR value.

The average CBR value of three test specimens is reported as the

CBR value of the sample.



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Modulus of Subgrade Reaction

The modulus of subgrade reaction (k) is used as a primary input for

rigid pavement design.

It estimates the support of the layers below a rigid pavement surface

course (the PCC slab). The k-value can be determined by field tests or by correlation with

other tests. There is no direct laboratory procedure for determining k-

value.

The modulus of subgrade reaction came about because work done by

Westergaard during the 1920s developed the k-value as a spring

constant to model the support beneath the slab.


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Plate bearing test

Plate bearing test is used to evaluate the support capability of sub-

grades, bases and in some cases, complete pavement.

Data from the tests are applicable for the design of both flexible and

rigid pavements.

In plate bearing test, a compressive stress is applied to the soil or

pavement layer through rigid plates relatively large size and the

deflections are measured for various stress values.


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Plate bearing test

The deflection level is generally limited to a low value, in the order

of 1.25 to 5 mm and so the deformation caused may be partly elastic

and partly plastic due to compaction of the stressed mass with

negligible plastic deformation. The plate-bearing test has been devised to evaluate the supporting

power of sub grades or any other pavement layer by using plates of

larger diameter.

The plate-bearing test was originally meant to find the modulus of

sub grade reaction in the Westergaard analysis for wheel load

stresses in cement concrete pavements.


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Plate bearing test

The test site is prepared and loose material is removed so that the 75

cm diameter plate rests horizontally in full contact with the soil sub-

grade.

The plate is seated accurately and then a seating load equivalent to a

pressure of 0.07 kg/cm2 (320 kg for 75 cm diameter plate) is applied

and released after a few seconds.

The settlement dial gauge is now set corresponding to zero load.

A load is applied by means of jack, sufficient to cause an average

settlement of about 0.25 cm.


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Plate bearing test

When there is no perceptible increase in settlement or when the rate

of settlement is less than 0.025 mm per minute (in the case of soils

with high moisture content or in clayey soils) the load dial reading

and the settlement dial readings are noted.

Deflection of the plate is measured by means of deflection dials;

placed usually at one-third points of the plate near it's outer edge.

To minimize bending, a series of stacked plates should be used


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Plate bearing test

Average of three or four settlement dial readings is taken as the

settlement of the plate corresponding to the applied load.

Load is then increased till the average settlement increase to a

further amount of about 0.25 mm, and the load and average

settlement readings are noted as before.

The procedure is repeated till the settlement is about 1.75 mm or

more.


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Plate bearing test


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Plate bearing test

Allowance for worst subgrade moisture and correction for small

plate size should be dealt properly.

Calculation

A graph is plotted with the mean settlement versus bearing pressure

(load per unit area) as shown in Figure .

The pressure corresponding to a settlement is obtained from this

graph. The modulus of subgrade reaction is calculated from the

relation.


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Modulus of subgrade reaction (k)


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The reactive pressure to resist a load is thus proportional to the

spring deflection (which is a representation of slab deflection) and k



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Relation of Load, Deflection and Modulus ofSubgrade Reaction (K)


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The value of k is in terms of MPa/m (pounds per square inch per

inch of deflection, or pounds per cubic inch - pci) and ranges from

about 13.5 MPa/m (50 pci) for weak support, to over 270 MPa/m

(1000 pci) for strong support.

Typically, the modulus of subgrade reaction is estimated from other

strength/stiffness tests, however, in situ values can be measured

using the plate bearing test.



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Plate Load Test

The plate load test presses a steel bearing plate into the surface to be

measured with a hydraulic jack.

The resulting surface deflection is read from dial micrometers near

the plate edge and the modulus of subgrade reaction is determined

by the following equation:


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Schematic Diagram of Plate Load Test


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Relation between k and MR

The 1993 AASHTO Guide offers the following relationship between

k-values from a plate bearing test and resilient modulus (MR)

k = MR/19.4


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Stabilisation of Subgrade Soil

Stabilization with a cementitious or asphaltic binder

Lime (Lack of Stability)

Portland Cement (Plasticity Index 10)

Bitumen Emulsion (Sandy Soil & fines 0.075 mm)


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Soil stabilisation with Lime


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Soil stabilisation with Cement


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Soil stabilisation with Bitumen Emulsion


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Frost action


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Freezing and Thawing


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Climate Change


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Climate Change


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Effects of Climate Change

Both pavement and supporting layers are exposed to strong climatic

influences.

The factors are

Air temperature Soil Moisture

The effects are

Cracking

Spalling or even blow out of some slabs.


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Effect of Low Temperature

Soil Shrinkage in cohesive soil, then cracks will be filled with waterduring next period of rain.

Water in cracks is frozen due to depressed air temperatures, then a

break-up in the soil mass may result.

A temperature potential applied to soil will cause the movement of

soil moisture from warmer regions to colder ones, thus changing the

moisture distribution in the soil.

Colder region is then exposed to freezing temperatures, themigrating moisture acts as a supply, which causes the growth of ice

lenses under the pavement and may contribute to frost damage.


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Effect of Rainfall

Rain has influence on the stability and strength of the supporting

medium because it affects the moisture content of the subgrade and

sub-base.

Also affects

The elevation of the water table,

The intensity of frost action, erosion, pumping and infiltration,

Loss of strength during thaw period.


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Moisture content will also vary with rainfall, this will in turn affect

the expansion and contraction of the pavement.

Long periods of rainfall of low intensity can be more adverse than

short periods of high intensity.

Because the amount of moisture absorbed by the soil is greatest

under the former conditions.

Effect of Rainfall


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Moisture Condition


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It includes both heave and loss of sub grade support during the

frost-melt period.

It may cause a portion of pavement to rise, due to ice crystal

formation in a frost susceptible subgrade or base course.

Thawing of the frozen soil and ice crystals during the spring period

may cause pavement damage under loads.

This damage usually results in high maintenance costs, requires

heavy loads may prohibited during the critical period.

It will result in economic loss for trucks and aircraft operation

during this period.

Effect of Frost action


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Cracks From Excessive Pavement Contraction


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Frost action can be quite detrimental to pavements and refers to two

separate but related processes:

Frost heave

An upward movement of the subgrade resulting from the expansion

of accumulated soil moisture as it freezes.

Thaw weakening

A weakened subgrade condition resulting from soil saturation as ice

within the soil melts.

Effect of Frost action


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Formation of

Ice Lenses in a

Pavement

Structure


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Frost Heave


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Where subgrade change from clean not frost susceptible (NFS)

sands to silty frost susceptible materials.

Abrupt transitions from cut to fill with groundwater close to the

surface.

Where excavation exposes water-bearing strata.

Drains, culverts, etc., frequently result in abrupt differential heaving

due to different backfill material or compaction and the fact thatopen buried pipes change the thermal conditions (i.e., remove heat

resulting in more frozen soil).

Frost Heave - Locations


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Additional factors which will affect the degree of frost susceptibility

(or ability of a soil to heave):

Rate of heat removal

Temperature gradient

Mobility of water (e.g., permeability of soil)

Depth of water table

Soil type and condition (e.g., density, texture, structure, etc.)

Frost Heave - Factors


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Thaw Weakening

Thawing is essentially the melting of ice contained within the

subgrade.

As the ice melts and turns to liquid it cannot drain out of the soil

fast enough and thus the subgrade becomes substantially weaker

(less stiff) and tends to lose bearing capacity.

Therefore, loading that would not normally damage a given

pavement may be quite detrimental during thaw periods (e.g.,spring thaw).

i l d fl i ill i


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Typical pavement deflections illustratingseasonal pavement strength changes


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Freeze-thaw damage


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Thawing process

Thawing can proceed from the top downward, or from the bottom

upward, or both.

It depends mainly on the pavement surface temperature.

During a sudden spring thaw, melting will proceed almost entirely

from the surface downward.

This type of thawing leads to extremely poor drainage conditions.

The frozen soil beneath the thawed layer can trap the water releasedby the melting ice lenses so that lateral and surface drainage are the

only paths the water can take.


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Effect of Thawing

Tabor (1930) also noted an added effect:

"The effects of refreezing after a thaw are also accentuated by the

fact that the first freeze leaves the soil in a more or less loosened or

expanded condition."

This observation shows that

the reduced density of base or subgrade materials helps to

explain the long recovery period for material stiffness or

strength following thawing, and

refreezing following an initial thaw can create the potential for

greater weakening when the "final" thaw does occur.


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Estimation the depth of frost penetration

Modified Berggren Formula

Stefan Formula


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Stefan Formula

the first studies of freeze/thaw depth were made by Josef Stefan in

1889, in connection with ice formation and melting in the Polar

oceans.

It is assumed that the latent heat of soil moisture is the only heat

that must be removed when freezing the soil.

Thus, thermal energy stored as volumetric heat and released as soil-

temperatures drop to and below freezing is not considered. Because

volumetric heat is neglected,


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the Stefan Formula tends to overestimate frost depth in temperate

zones .

The latent heat supplied by the soil moisture as it freezes a depth dx

in time dt = rate at which heat is conducted to the ground surface.

Heat removal process can be represented by (heat released by

freezing a layer of soil dx thick in time dt)

Stefan Formula


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Stefan Formula


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(heat conducted through frozen layer)

and Q1 = Q2 so

by integrating and solving for x,

is in units of F hr and is called surface freezing index. The

freezing index is normally expressed as F days. Thus, rewrite the

equation and add an "n" factor which results in the Stefan formula:

Stefan Formula


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The modified Berggren formula was developed in the early 1950s to

address the shortcomings of the Stefan formula.

The modified Berggren formula assumes that the soil is a semi-

infinite mass with uniform properties and existing initially at a

uniform temperature (Ti).

It is further assumed that surface temperature is suddenly changed

from Tito T

s(below freezing).


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The modified Berggren formula is simply the Stefan formula

corrected for the effects of temperature changes in the soil mass:

where:

X = depth of freeze or thaw, (ft))

= dimensionless coefficient which takes into consideration the

effect of temperature changes in the soil mass (i.e., a fudge factor).Corrects the Stefan formula for the neglected effects of volumetric

heats (accounts for "sensible heat" changes)



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K avg = thermal conductivity of soil, average of frozen and unfrozen

(BTU/hr ft F)

n = conversion factor for air freezing (or thawing) index to

surface freezing (or thawing) index

FI = air freezing index (F days)

TI = air thawing index (F days)

L = latent heat (BTU/ft3)


1 BTU = 1 055.05585 joules


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Determination of

lcan be determined by chart based on inputs of a (thermal ratio) and

m (fusion parameter).

= f (FI (or TI), mean annual air or ground temperature,

thermal properties of soil)

= f(m , a) and can be read from chart

= fusion parameter

C = average volumetric heat capacity of a soil (BTU/ft3 F)

L = latent heat (BTU/ft3)


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|T f - Ts| = surface freezing (or thawing) index, nFI (or nTI)

divided by length of freezing (or thawing) season. Represents

temperature differential between average surface temperature and

32 F taken over the entire freeze (or thaw) season.

=

d = length of freezing or thawing duration.

For example, if the winter freezing season is December throughFebruary, then the duration of freezing (d) equals about 90 days.

Determination of


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T f = 32 F

Ts = average surface temperature for the freezing

(or thawing) period

= thermal ratio

T = average annual air or ground temperature

|T T f| = represents the amount that the mean annual

temperature exceeds (or is less than) the freezing

point of the soil moisture (assumed to be 32 F).

Determination of


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Coefficient in the Modified Berggren Formula.


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Pictorial representation of variables involved with l


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Signal control for vehicle


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Result?...


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sub grade properties

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