IDENTIFICATION AND CLASSIFICATION OF PROBLEMATIC SOILS Pres
ented by Kulbir Singh Gill Associate Professor, Deptt. of Civil
Engineering, GNDEC,Ludhiana ([email protected])
Slide 2
F ORMATION OF S OIL Soil is formed either by physical
weathering or by chemical weathering. Because of different
processes of weathering,soils exhibit different characteristics.
Physical weathered rock to some extent represents the parent rock
mass,for instance sand and gravel. Where as chemically weathered
rocks results in the formation totally different material such as
clay. Needless to say that sands and gravels are considered to be
the best material from civil engineering point of view. Except the
situation where permeability is to be controlled. Clays shows huge
volume change when exposed to moisture.
Slide 3
C ONTD . Soils are heterogenious in nature. Soils are also
anisotropic. If the wind is the weathering agent,it results in
aeoline deposits which are cohesion less in nature such as sands.
If water is the agent for movement of weathered rock products, it
results in the formation of alluvial deposit and their suitability
as construction material is varying from poor to fair. Other
deposit are glacial, marine, beach, etc. Residual deposits are the
one which is not transported to farther distances. Suitability of
any soil can be assessed based on its properties.
Slide 4
P ROPERTIES OF SOILS The soil properties include index and
engineering properties. The index properties are specific gravity,
void ratio, liquid limit, plastic limit, shrinkage limit, relative
density, dry density, porosity, initial water content, grains size
distribution etc. The engineering properties are shear strength,
compressibility and permeability. Unlike other material, soil
behavior is influenced by many factors such as mineralogy, water
content, void ratio, soil structure, pore fluid characteristics
(Ion concentration, valancy of ion, dielectric constant),
temperature, drainage, condition, strain rate, aging etc
Slide 5
I DENTIFICATION AND C LASSIFICATION OF SOILS Beside the
complexity of understanding soil, geotechnical engineers made their
best efforts to group the soil based on its specific response to
different environmental conditions Soil can be classified as highly
compressible and soil of low compressibility, expansive & non
expansive, sensitive & insensitive, high plastic & low
plastic, very soft to stiff clay, loose and dense sand etc. In this
note, the identification and classification of different soils are
presented in order to classify the good and poor soil, otherwise
called as Problematic soil.
Slide 6
L IQUID L IMIT Liquid limit values of soils may be described as
low, intermediate, high very high or extra high plasticity as given
below in Table. Plasticity Classified asLiquid limit (%) Low20 to
35 Intermediate35 to 50 High50 to 70 Very high70 to 90 Extra
highOver 90
Slide 7
P LASTICITY I NDEX No uniform standard is adopted in
classifying degree of plasticity of soils. However, the
classification given below is approximately the one which is often
used and hence is recommended Soil classified asPlasticity Index
(%) Non Plastic0-5 Moderately Plastic5-16 Plastic16-35 Highly
plasticOver 35
Slide 8
S HRINKAGE LIMIT Shrinkage limit of soil is an indication of
not only the shrinkage potential of clays but also an indicative of
swelling nature. Swelling and Shrinkage Classification based on
Shrinkage Limit Classified as Shrinkage table / Swelling detail
Shrinkage limit (%) Very Low
I DENTIFICATION OF EXPANSIVE SOILS S.NoDegree of expansion
Liquid limit (%) Shrinkage limit (%) Plasticity index (%) 12341234
Very high High Medium Low 60-70 40-60 30-40 20-30 >30 20-30
10-20 35 20-35 10-20
I DENTIFICATION OF EXPANSIVE SOIL ON THE BASIS OF GSD
S.NoDegree of expansion Clay fraction (%) Colloidal content (%) 1
Very high >28 2 High 20-28 3 Medium 12-2015-20 4 Low 0-12
C LASSIFICATION OF SENSITIVE SOILS Classification Sensitivity,
S 1 Low sensitivity 2-4 Medium sensitivity 4-8 High sensitivity
8-16 Quick >16
Slide 29
C LASSIFICATION OF SOILS ON THE BASIS OF IN SITU TESTS
Correlation between N and Denseness of Sand NDenseness 0-4Very
Loose25 - 32 4-10Loose27 - 35 10-30Medium30 - 40 30-50Dense35 = 45
> 50Very Dense> 45
Slide 30
C ONTD. Correlation between N and q u NConsistencyq u (kN / m 2
) 0 - 2Very Soft< 25 2 - 4Soft25 50 4 - 8Medium50 100 8 -
15Stiff100 200 15 30Very Stiff200 400 > 30Hard> 400
Slide 31
I DENTIFICATION OF DISPERSIVE SOILS Dispersion occurs in soils
when the repulsive forces between clay particles exceed the
attractive forces thus bringing about deflocculating so that in the
presence of relatively pure water the particles repel each other to
form colloidal suspensions. Dispersive soils have a moderate to
high clay material content but there are no significant differences
in the clay fractions of dispersive and non-dispersive soils,
except that soils with less than 10% clay particles may not have
enough colloids to support dispersive piping. Dispersive soils
contain a higher content of dissolved sodium (up to 12%) in their
pore water than ordinary soils.
Slide 32
C ONTD. The sodium adsorption ratio (SAR) is used to quantify
the role of sodium where free salts are present in the pore water
and is defined as: SAR= Na/ 0.5(Ca+Mg ) with units expressed in
meq/litre of the saturated extract.. Gerber and Harmse (1987)
considered an SAR value greater than 10 indicative of dispersive
soils, between 6 and 10 as intermediate, and less than 6 as
non-dispersive.
Slide 33
C ONTD. The presence of exchangeable sodium is the main
chemical factor contributing towards dispersive behavior in soil.
This is expressed in terms of the exchangeable sodium percentage
(ESP): ESP= Exchangeable sodium x 100/cation exchange capacity.
Where the units are given in meq/100 g of-dry clay.
Slide 34
C ONTD. Soils with ESP values above 15% are highly dispersive
(Bell and Maud, 1994). Those with low cation exchange values (15
meq/100 g of clay) have been found to be completely non- dispersive
at ESP values of 6% or below. Unfortunately, dispersive soils
cannot be differentiated from non- dispersive soils by routine soil
mechanics testing. Although a number of special tests have been
used to recognize dispersive soils, no single test can be relied on
completely to identify them (Bell and Maud, 1994). These can be
divided into physical and chemical tests. The former include the
crumb test, the dispersion or double hydrometer test, the modified
hydrometer or turbidity ratio test and the pinhole test.
Slide 35
C ONTD. Serious piping damage to embankments and failures of
earth dams have occurred when dispersive soils have been used in
their construction (Bell and Maud, 1 994). Severe erosion damage
also can form deep gullies on earth embankments after rainfall. In
many areas where dispersive soils are found there is no economical
alternative other than to use these soils for the construction of
earth dams. However, experience indicates that if an earth dam is
built with careful construction control and incorporates filters,
then it should be safe enough even if it is constructed with
dispersive soils.
Slide 36
COLLAPSIBLE SOIL Collapsible soils, which are sometimes
referred to as metastable soils, are unsaturated soils that undergo
a large volume change upon saturation. This volume change may or
may not be the result of the application of additional load.
Foundations that are constructed on such soils may undergo large
and sudden settlement if and when the soil under them becomes
saturated with an unanticipated supply of moisture. This moisture
may come from several sources, such as (a) broken water pipelines,
(b) leaky sewers, (c) drainage from reservoirs and swimming pools,
(d) slow increase of groundwater, and so on. This type of
settlement generally causes considerable structural damage. Hence
identification of collapsing soils during field exploration is
crucial.
Slide 37
C ONTD. The majority of naturally occurring collapsing soils
are aeolin that is, wind- deposited sand and/or silts, such as
loess, aeolic beaches, and volcanic dust deposits. These deposits
have high void ratios and low unit weights and are cohesionless or
only slightly cohesive. Loess deposits have silt-sized particles.
The cohesion in loess may be the result of the presence of clay
coatings around the silt-size particles, which holds them in a
rather stable condition in an unsaturated state. In the United
States, large parts of the Midwest and arid West have such types of
deposit. Loess deposits are also found over 1 5%-20% of Europe and
over large parts of China
Slide 38
R ELATION OF C OLLAPSE P OTENTIAL TO THE S EVERITY OF F
OUNDATION P ROBLEMS Cp(%)Severity of problem 0-1No Trouble
1-5Moderate Trouble 5-10Trouble 10-20Severe Trouble > 20Very
Severe Trouble
Slide 39
S UMMARY Stability of any civil engineering structures lies
primarily with the response of soil under the influence of external
loading. It is a must for any civil engineer to understand the type
of soil and their engineering characteristics prior to the use of
same for any applications. If there is no proper importance given
to the soil before start of construction activities in the
beginning itself, then the rectification of damage to the
structure, because of soil movement, if any would be much higher
than the cost of the project itself.