soilmech ch1 classification

8/20/2019 SoilMech Ch1 Classification

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September 08

Institut Gramme - LIEGE

Dr. Ir. P. BOERAEVEChargé de cours

Structural Stability

SOIL MECHANICSMaster 2


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Soil Mechanics Soils And Their Classification page 2

Contents of this chapter :

CHAPITRE 1. SOILS AND THEIR CLASSIFICATION 2 1.1 REFERENCES : 2 1.2 INTRODUCTION 2

1.3 CLASSIFICATION OF SOILS 3 1.3.1 PROCEDURE FOR GRAIN SIZE DETERMINATION 4 1.3.2 LIQUID LIMIT, PLASTIC LIMIT AND SHRINKAGE LIMIT OF A SOIL SAMPLE 7 1.3.3 UNIFIED SOIL CLASSIFICATION SYSTEM (USCS) 8 1.4 EXAMPLE - CLASSIFICATION USING USCS 10 1.5 EXERCISES 11

Chapitre 1. Soils And Their Classification

1.1 References :

1. Soil Mechanics, University of Sydney, David Airey(http://www.civil.usyd.edu.au/courses/civl2410/ )

2. Solving Problems in Soil Mechanics, B.H.C. Sutton, ISBN 0-582-08971-9

1.2 Introduction

Geotechnical Engineering is that part of engineering which is concerned with the behaviour of soiland rock. Soil Mechanics is the part concerned solely with soils.

From an engineering perspective soils generally refer to sedimentary materials that have not beencemented and have not been subjected to high compressive stresses.

As the name Soil Mechanics implies, the subject is concerned with the deformation and strength ofbodies of soil. It deals with the mechanical properties of the soil materials and with the application ofthe knowledge of these properties to engineering problems. In particular it is concerned with theinteraction of structures with their foundation material. This includes both conventional structuresand also structures such as earth dams

1, embankments

2and roads which are themselves made of

soil.

1.2.1 Effects on stability and serviceability

As for other branches of engineering the major issues are stabilityand serviceability. When a structure is built it will apply a load tothe underlying soil; if the load is too great the strength of the soilwill be exceeded and failure may ensue. It is important to realise

that not only buildings are of concern, the failure of an earth damcan have catastrophic consequences, as can failures of naturaland man made slopes and excavations. Buildings or earthstructures may be rendered unserviceable by excessivedeformation of the ground, although it is usually differentialsettlement

3, where one side of a building settles more than the other, that is most damaging

(Fig.1.1).Criteria for allowable settlement vary from case to case; for example the settlement allowed in afactory that contains sensitive equipment is likely to be far more stringent than that for a warehouse.

1

barrage2 talus

3 tassement

Fig. 1.1 Differential settlement


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1.2.2 Effects on adjacent structures

Another important aspect to be considered during design is the effect of any construction on adjacentstructures, for example the excavation of a basement and then the construction of a large buildingwill cause deformations in the surrounding ground and may have a detrimental effect on adjacentbuildings or other structures such as railway tunnels.

1.2.3 Interaction of soil and water

Many of the problems arising in Geotechnical Engineering stem from the interaction of soil andwater. For example, when a basement is excavated water will tend to flow into the excavation. Thequestion of how much water flows in needs to be answered so that suitable pumps can be obtainedto keep the excavation dry. The flow of water can have detrimental effects on the stability of theexcavation, and is often the initiator of landslides in natural and man made slopes. Some of theeffects associated with the interaction of soil and water are quite subtle, for example if an earthquakeoccurs, then a loose soil deposit will tend to compress causing the water pressures to rise. If thewater pressures should increase so that they become greater than the stress due to the weight of theoverlying soil then a quicksand

4 condition will develop and buildings founded on this soil may fail.

1.2.4 Field investigation

Soil mechanics differs from other branches of engineering in that generallythere is little control over the material properties of the soil at the site andthis is often highly variable. By taking samples at a few scattered locationswe have to determine the soil properties and their variability. At this stagein a project knowledge of the site geology and geological processes isessential to successful geotechnical engineering.

1.2.5 Soil mechanics is young!

Soil mechanics is a relatively new branch of engineering science, the first major conference occurredin 1936 and the mechanical properties of soils are still incompletely understood. The first completemechanical model for soil was published as recently as 1968. Over the last 40 years there has beenrapid development in our understanding of soil behaviour and the application of this knowledge inengineering practice. The subject has now reached a phase of development similar to that ofstructural mechanics a century ago.

1.3 Classification of soils

A description of a soil should give detailed information about its grading5, plasticity, colour, particle

characteristics as well as its homogeneity.Few soils will have identical descriptions. The purpose of classification therefore is to place a soil

in one of a limited number of groups on the basis of the grading and plasticity of a disturbedsample. Since these characteristics are independent of the particular conditions in which a soiloccurs, it gives a good guide to how the disturbed soil will behave as a construction material.

Most systems of soil classification are based on the particle sizes found within the soil mass andrecognize three main types of soil:

(1) coarse soil6

(2) fine soil7

(3) organic soil.

4 Sables mouvants

5

Granulométrie6 Sol à grains grossiers

7 Sol à grains fins

Fig.1.2 : soil sample


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Coarse soils are classified on the basis of the size and distribution of the particles and fine soilson the basis of their plasticity, using a chart.

A coarse soil is one in which less than 50% of the material is finer than 0.075 mm.

A fine soil contains more than 50% of material finer than 0.075 mm.Both types are further sub-divided on the basis of grain size as shown on the following table.

Gravel Sand Silt8 Clay

9

C M F C M F C M F C M F

60 20 6 2 0.6 0.2 0.06 0.02 .006 .002 .0006 .0002

where C, M, F stand for coarse, medium and fine respectively, and the particle sizes are inmillimetres.

1.3.1 Procedure for grain size determination

Different procedures are required for fine and coarse-grained material.

• Coarse Sieve10

analysis11

is used to determine the distribution of the larger grain sizes. Thesoil is passed through a series of sieves with the mesh size reducing progressively,and the proportions by weight of the soil retained on each sieve are measured. Theresults are then plotted on a graph as shown on Fig. 1.5. There are a range of

standard sieve sizes that can be used, and the finest is usually a 75 µm sieve. Theworld’s most used set of standard sieves is the ASTM

12 set.

8 limon

9 argile

10

Tamis11 Sieve analysis = analyse granulométrique

12 American Society for Testing and Materials


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Fig. 1.3 : Sieves

• Fine To determine the grain size distribution of material passing the 75µm sieve thehydrometer method is commonly used. The soil is mixed with water and adispersing agent, stirred vigorously, and allowed to settle to the bottom of ameasuring cylinder. As the soil particles settle out of suspension the specific gravityof the mixture reduces. An hydrometer is used to record the variation of specificgravity with time. By making use of Stoke’s Law, which relates the velocity of a freefalling sphere to its diameter, the test data is reduced to provide particle diametersand the % by weight of the sample finer than a particular particle size.

Fig. 1.4 A schematic view of the hydrometer test


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0.001 10Equivalent Particle Size (mm)

0

1020

30

40

50

60

70

80

90

100

P e r c e n t F i n e r

200 140 100 70 50 40 30 20 16 12 8 6 4

ASTM SIEVE SIZES

B.S. SIEVE SIZES

300 200 150 100 72 52 36 25 18 14 10 7

0.002 0.006 0.01 0.1 10.02 0.06 0.2 0.6 2 6 20 60

MediumSilt

Fine CoarseMedium

SandFine Coarse Fine

MediumGravel

CoarseStone or Boulder Clay

" " “ " 1" 1 " 2"3 8/"

" " “ " 1" 1 " 2"3 8/"1 8/" 3 16/ "

Fine soils are described by reference to their position on the plasticity chart shown on Fig. 1.5,

0 10 20 30 40 50 60 70 80 90 100Liquid limit

0

10

20

30

40

50

60

P l a s t i c i t y i n d e x

CH

OH

or

MH

CLOL

MLor

CL

ML

" A "

l i n e

Comparing soils at equal liquid limit

Toughness and dry strength increase

with increasing plasticity index

Figure 1.6 : Plasticity chart for laboratory classification of fine grained soils

To use that chart, we need to know the liquid limit and the plasticity index.

Fig. 1.5 : Grading curve sheet


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1.3.2 Liquid limit, plastic limit and shrinkage limit of a soil sample

When a fine soil is deposited from suspension in a liquid it passes through four states ofconsistency depending on the water content :

(1) liquid state;

(2) plastic state;(3) semi-solid state;(4) solid state.

The water content at which the soil passes from one state to the next state called consistencylimits (also called Atterberg limits, after the Swedish scientist who devised them) and areexpressed as w%. Starting from the liquid state, three consistency limits are met when decreasingthe water content :

the liquid limit, the plastic limit and the shrinkage limit.

The liquid limit (LL) is the water content at which the soil passes from the plastic to the liquidstate, i.e.. begins to behave like a viscous mud and flow under its own weight.

A method of measuring the liquid limit is by means of the Casagrande apparatus. This consistsessentially of a metal cup which can be raised and dropped 10 mm by means of a cammechanism.

Figure 1.7 : Cassagrande apparatus for Liquid Limit measure of a fine soil.

Wet soil is placed in the cup and divided into two halves by means of a Standard grooving tool.The cup is then raised and tapped by being dropped twice a second onto the rubber base. Thenumber of such taps required to bring the two halves together is recorded together with the watercontent. The procedure is repeated on other soil samples with different water contents. From thereadings obtained, a graph of water content against the log of the number of taps is plotted. Theliquid limit is then taken as the water content corresponding with 25 taps.

The plastic limit (PL) is the lowest water content at which the soil remains in a plastic state, i.e.when it is about to change from a plastic state to a crumbly semi-solid.

The plastic limit of the soil is found by rolling a ball of wet soil between the palm of the hand and aglass plate to produce a thread 3 mm thick before the soil just begins to crumble. The watercontent of the soil in this state is taken as the plastic limit.


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Figure 1.8 : Plastic Limit measure of a fine soil.

The plasticity index (PI) is a measure of the range of water contents over which the soil remainsin a plastic state.

PI = LL-PL

The shrinkage13

limit (SL) is the water content at which further loss of water in the soil will notcause further reduction in the volume of the soil, i.e. the water content required just to fill the voidsof a sample which has been dried.The shrinkage limit is found by measuring the weight and volume of the soil at intervals as it isallowed to air-dry until no further volume change takes place. The volume is found by using amercury displacement vessel.

1.3.3 Unified Soil Classification System (USCS)

The standard system used worldwide for most major construction projects is known as the UnifiedSoil Classification System. This is based on an original system devised by Cassagrande. Soils areidentified by symbols determined from sieve analysis and Atterberg Limit tests.

The USCS flowchart herebelow shows how to classify the soil.

Note : The chart mentions two ASTM sieves by their N°. These numbers are on the top of theblank grading curve sheet. (Sieve No. 200 corresponds to a 0.075mm sieve opening and sieve No.40 corresponds to a 0.425mm sieve opening.)

13 retrait


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Figure 1.9 : USCS flowchart for classifying soils.

In that flowchart, two coefficients are deduced from the grading curve

the uniformity coefficient C D

Du =

60

10

and the coefficient of curvature C D

D Dc =

×

30

2

60 10( )

where D xx is the maximum size of particle in smallest xx% of sample.

The symbols have the following meaning :S SandG Gravel

M SiltC ClayO OrganicPt Peat

14

W Well graded (not uniform)P Poorly graded (uniform)H High plasticityL Low plasticity

A PDF document describing the different soil groups, and giving some characteristics pertaining toembankments or foundations can be downloaded from the Moodle website.

14 Tourbe


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1.4 Example - Classification using USCS

Classification tests have been performed on a soil sample and the following grading curve andAtterberg limits obtained. Determine the USCS classification.

Atterberg limits: Liquid limit LL = 32, Plastic Limit, PL =26

Step 1: Determine the % fines from the grading curve

%fines (% finer than 75 µm) = 11% (between 5% and 12%)-> Coarse grained, Dualsymbols required

Step 2: Determine % of different particle size fractions (to determine G or S), and D10, D30, D60 from grading curve (to determine W or P)

D10 = 0.06 mm, D30 = 0.25 mm, D60 = 0.75 mm

Cu = 12.5, Cc = 1.38, and hence Suffix1 = W

Particle size fractions: Gravel 17%Sand 73%

Silt and Clay 10% In the coarse fraction (~Sand+Gravel) Sand is dominant, hence Prefix is S

Step 3: From the Atterberg Test results determine its Plasticity chart location

LL = 32, PL = 26. Hence Plasticity Index Ip = 32 - 26 = 6

From Plasticity Chart point lies below A-line, and hence Suffix2 = M Step 4: Dual Symbols are SW-SM

Step 5: Complete classification by including a description of the soil : Well graded Silty Sand

0 .0 0 0 1 0 .0 0 1 0 .0 1 0 .1 1 1 0 1 0 0

0

2 0

4 0

6 0

8 0

1 0 0

P a r ti c l e s i z e ( m m )

% F

i n e r

Figure 1.10 : Grading curve.


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1.5 Exercises

1. The results of a sieve analysis of two soils are the following : Sieve size Soil A Soil B

(mm) Massretained

(g)

% retained % finer Massretained

(g)

% retained % finer

37.50 0.020.00 26.010.00 31.05.00 11.0 0.02.00 18.0 8.01.18 24.0 7.00.600 21.0 11.00.300 41.0 21.00.212 32.0 63.00.150 16.0 48.0

0.063 15.0 14.0

Rest (< 0.063mm) 15.0 3.0

Draw the grading curve and calculate D10, Cu, Cc for both soils.

2. A mass of 127.62 g of a dried soil was subjected to a grading analysis:

Sieve analysis:

Retained on sieve 2.36 mm 0 g0.60 mm 42.1 g0.21 mm 24.2 g0.075 mm 16.6 g

Hydrometer, sedimentation analysis:

Amounts finer than 0.03 mm 28.3 g0.003 mm 17.2 g

Atterberg limits: Liquid limit LL = 42, Plastic Limit, PL =33

Draw the grading curve and classify the material according to the USCS.


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3. Draw a grading curve for each of the soils A to F and classify each one according to the UnifiedClassification System.

The values given in the table are the percentages finer than the given particle size.

Particlesize (mm)

A B C D E F

6.002.000.600.4250.2120.1500.075

0.050.010.002

1009895928683

825736

1009994898276

743823

1009586775012

0

10075554630194

0

100857569604835

322510

1009489633710

988

Liquidlimit

67 40 Non-plastic

- 55 40

Plasticlimit

27 12 Non-plastic

- 35 15

soilmech ch1 classification

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