Presentation on theme: "IDENTIFICATION AND CLASSIFICATION OF PROBLEMATIC SOILS Pres ented by Kulbir Singh Gill Associate Professor, Deptt. of Civil Engineering, GNDEC,Ludhiana."— Presentation transcript:
IDENTIFICATION AND CLASSIFICATION OF PROBLEMATIC SOILS Pres ented by Kulbir Singh Gill Associate Professor, Deptt. of Civil Engineering, GNDEC,Ludhiana (firstname.lastname@example.org)
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.
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.
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
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.
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
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
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<1 Low8 to 10 Intermediate11 to 15 High16 to 20 Very highOver 20
I NDIAN S TANDARD C LASSIFICATION S YSTEM 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).
10 M AJOR S OIL G ROUPS 0.002200632.360.075 Grain size (mm) BoulderClaySiltSandGravelCobble Fine grain soils Coarse grain soils Granular soils or Cohesionless soils Cohesive soils
G RAIN S IZE D ISTRIBUTION C URVE Can find % of gravels, sands, fines Define D 10, D 30, D 60.. as above.
12 C LASSIFYING F INES Purely based on LL and PI 20 100 500 20 0 40 60 Liquid Limit A-Line PI=0.73(LL- 20) Silts Clays High plasticity Low plasticity 35 Intermediate plasticity
13 A TTERBERG L IMITS Border line water contents, separating the different states of a fine grained soil Liquid limit Shrinkage limit Plastic limit 0 water content liquidsemi-solidbrittle-solidplastic
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
C ONTD. 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.
C LASSIFICATION BASED ON FIELD TESTS TestMLCLOLMICIOIMHCHOH Dilatan cy Quick None to very slow Slow Quick to slow NoneSlow Slow to none None None to very slow Toughne ss NoneMediumLowNoneMediumLow Low to medium High Low to mediu m Dry Streng t h None of low MediumLow Medium to high Low to medium High to very high Medium to high
G ENERAL BEHAVIOR OF GRAVELS Soil GroupPermeabilityCompressibilityShear StrengthWorkability GW GP GM GC \ Pervious Very pervious Semi-pervious to impervious Impervious Negligible Very low Excellent Good Good to fair Excellent Good
G ENERAL BEHAVIOR OF SAND Soil GroupPermeabilityCompressibilityShear StrengthWorkability SW Pervious Negligible Excellent SP Pervious Very low GoodFair SM Semi-pervious to impervious Low GoodFair SC Impervious Low Good to fairGood
G ENERAL BEHAVIOR OF SILT AND CLAY OF MEDIUM PLASTICITY Soil GroupPermeabilityCompressibilityShear StrengthWorkability ML, MI CL, CI OL, OI Semi-pervious to impervious Impervious Semi-pervious to impervious Medium Medium Fair Good to fair Fair
G ENERAL BEHAVIOR OF SILTS AND CLAYS OF HIGH PLASTICITY Soil GroupPermeabilityCompressibilityShear StrengthWorkability MH CH OH Semi-pervious to impervious Impervious High Fair to poor Poor
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
C ONTD. 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.
C ONTD. 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
C HECKLIST FOR RECOGNITION OF EXPANSIVE SOIL S.NoItem to be checked Answers Yes No 1 Are the soils near by the project area known to be expansive? 2 Are the evidences of cracks in walls, curb, sidewalks and pavements etc. in nearby construction? 3 Are there shrinkage cracks in the soils in dry season? 4 Does the soil behaves very sticky and sticking to the shoes? 5 If you take a lump of dry soil and try to break it between the fingers, do you find the soil hard and difficult to break.
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 <10 >35 20-35 10-20 <10
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<15
S ENSITIVITY 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 S 1, the ratio of undisturbed shear strength to the remolded shear strength
C LASSIFICATION OF SENSITIVE SOILS Classification Sensitivity, S 1 Low sensitivity 2-4 Medium sensitivity 4-8 High sensitivity 8-16 Quick >16
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°
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
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.
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.
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.
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.
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.
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.
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
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
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.