Download
1 / 40
500 likes | 734 Views
IDENTIFICATION AND CLASSIFICATION OF PROBLEMATIC SOILS . Pres ented by Kulbir Singh Gill Associate Professor, Deptt. of Civil Engineering,
E N D
IDENTIFICATION AND CLASSIFICATION OF PROBLEMATIC SOILS Presented by Kulbir Singh Gill Associate Professor, Deptt. of Civil Engineering, GNDEC,Ludhiana (kulbirgillkulbir@yahoo.co.in)
Formation of Soil • 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.
Contd…. • 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.
Properties 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
Identification and Classification 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.
Liquid Limit Liquid limit values of soils may be described as low, intermediate, high very high or extra high plasticity as given below in Table .
Plasticity Index • 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
Shrinkage 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
Indian Standard Classification System • The fine-grained soils in ISC system are subdivided into three categories of low, medium and high compressibility . • Coarse-grained soils, when 50% or more of the total material by weight is retained on 75 micron IS sieve. • Fine-grained soils, when more than 50% of the total material passes 75 micron IS sieve. • If the soil is highly organic and contains a large percentage of organic matter and particles of decomposed vegetation, it is kept in a separate category marked as peat (Pt).
0.002 0.075 2.36 63 200 Major Soil Groups Cohesive soils Granular soils or Cohesionless soils Clay Silt Sand Gravel Cobble Boulder Grain size (mm) Fine grain soils Coarse grain soils
Grain Size Distribution Curve • Can find % of gravels, sands, fines • DefineD10, D30, D60.. as above.
Intermediate plasticity Low plasticity High plasticity 60 A-LinePI=0.73(LL-20) 40 Liquid Limit 20 0 100 0 20 35 50 Liquid Limit Classifying Fines • Purely based on LL and PI Clays Silts
water content 0 Shrinkage limit Plastic limit Liquid limit brittle-solid semi-solid plastic liquid Atterberg Limits • Border line water contents, separating the different states of a fine grained soil
FIELD IDENTIFICATION OF SOILS • The soils can be identified in the field by conducting the following simple tests. • The sample is first spread on a flat surface. • If more than 50% of the particles are visible to the naked eye (unaided eye), the soil is coarse-grained; otherwise, it is fine grained
Contd. • To differentiate fine sand from silt, dispersion test is adopted. When a spoonful of soil is poured in a jar full of water, fine sand settles in a minute or so, whereas silt takes 15 minutes or more. • Dilatancy (reaction to shaking) test. • Toughness test. • Dry strength test. • These tests helps in classifying the different types of soil.
CLASSIFICATION OF EXPANSIVE SOILS • Damages to structures, property and life resulting from swell-shrink characteristics of expansive soils have been reported from many parts of the world including India. • India, Africa, Australia, Israel, South America and United States of America possess vast tracts covered with such soils. • Our Black Cotton soil is an expansive soil. • It extends nearly one-fifth of our country, chiefly in the states of Tamil Nadu. Maharashtra, Gujarat, Madhya Pradesh, Southern Uttar Pradesh, Eastern Rajasthan, Karnataka and parts of Andhra Pradesh
Contd. • Most of the expansive soils found in India are black in color and are good for growing cotton. • Some of these soils are reddish brown and yellowish grey in color. • These soils are generally found near the surface, with layer thickness varying from 0.5 m to 10.m, and sometime more than 10 meters.
Contd. • Once an expansive soil is encountered at the project site, the following items need to be given specific attention. • Swell – shrink characteristics of soil encountered. • Thickness and depth of various underlying soil layers • Depths of significant moisture variation. • Local climate and hydrology. • Floor-foundation system; ability to accommodate and tolerate the soil behavior. • Methods of improving the soil behavior
Sensitivity of clays • Some clays have a curious property called sensitivity, which means their strength in a remolded or highly disturbed condition is less than that in an undisturbed condition at the same moisture content. • These highly sensitive clays are called quick clays, are found in certain areas of Eastern Canada, parts of Scandinavia, and else where. • This behavior occurs because these clays have a very delicate structure that is disturbed when they are remolded. • The degree of sensitivity is defined by the parameter S1, the ratio of undisturbed shear strength to the remolded shear strength
Classification of soils on the basis of in situ tests Correlation between N and Denseness of Sand
Contd. Correlation between N and qu
Identification 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.
Contd. • 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.
Contd. • 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.
Contd. • 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.
Contd. • 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.
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.
Contd. • The majority of naturally occurring collapsing soils are aeolinthat 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
Relation of Collapse Potential to the Severity of Foundation Problems
Summary • 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.
