September 08

Institut Gramme - LIEGE Dr. Ir. P. BOERAEVE Chargé de cours

Structural Stability

SOIL MECHANICS

Master 2

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 soil and rock. Soil Mechanics is the part concerned solely with soils. From an engineering perspective soils generally refer to sedimentary materials that have not been cemented and have not been subjected to high compressive stresses. As the name Soil Mechanics implies, the subject is concerned with the deformation and strength of bodies of soil. It deals with the mechanical properties of the soil materials and with the application of the knowledge of these properties to engineering problems. In particular it is concerned with the interaction of structures with their foundation material. This includes both conventional structures and also structures such as earth dams1, 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 stability and serviceability. When a structure is built it will apply a load to the underlying soil; if the load is too great the strength of the soil will be exceeded and failure may ensue. It is important to realise that not only buildings are of concern, the failure of an earth dam can have catastrophic consequences, as can failures of natural and man made slopes and excavations. Buildings or earth structures may be rendered unserviceable by excessive deformation of the ground, although it is usually differential settlement3, 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 a factory that contains sensitive equipment is likely to be far more stringent than that for a warehouse.

1 barrage 2 talus 3 tassement

Fig. 1.1 Differential settlement

Soil Mechanics Soils And Their Classification page 3

1.2.2 Effects on adjacent structures Another important aspect to be considered during design is the effect of any construction on adjacent structures, for example the excavation of a basement and then the construction of a large building will cause deformations in the surrounding ground and may have a detrimental effect on adjacent buildings 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 and water. For example, when a basement is excavated water will tend to flow into the excavation. The question of how much water flows in needs to be answered so that suitable pumps can be obtained to keep the excavation dry. The flow of water can have detrimental effects on the stability of the excavation, and is often the initiator of landslides in natural and man made slopes. Some of the effects associated with the interaction of soil and water are quite subtle, for example if an earthquake occurs, then a loose soil deposit will tend to compress causing the water pressures to rise. If the water pressures should increase so that they become greater than the stress due to the weight of the overlying 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 generally there is little control over the material properties of the soil at the site and this is often highly variable. By taking samples at a few scattered locations we have to determine the soil properties and their variability. At this stage in a project knowledge of the site geology and geological processes is essential 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 occurred in 1936 and the mechanical properties of soils are still incompletely understood. The first complete mechanical model for soil was published as recently as 1968. Over the last 40 years there has been rapid development in our understanding of soil behaviour and the application of this knowledge in engineering practice. The subject has now reached a phase of development similar to that of structural 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 disturbed sample. Since these characteristics are independent of the particular conditions in which a soil occurs, 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 and recognize three main types of soil:

(1) coarse soil 6 (2) fine soil7 (3) organic soil.

4 Sables mouvants 5 Granulométrie 6 Sol à grains grossiers 7 Sol à grains fins

Fig.1.2 : soil sample

Soil Mechanics Soils And Their Classification page 4

Coarse soils are classified on the basis of the size and distribution of the particles and fine soils on 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 Clay9 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 in millimetres.

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. The

soil 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. The results 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. The world’s most used set of standard sieves is the ASTM12 set.

8 limon 9 argile 10 Tamis 11 Sieve analysis = analyse granulométrique 12 American Society for Testing and Materials

Soil Mechanics Soils And Their Classification page 5

Fig. 1.3 : Sieves

• Fine To determine the grain size distribution of material passing the 75µm sieve the

hydrometer method is commonly used. The soil is mixed with water and a dispersing agent, stirred vigorously, and allowed to settle to the bottom of a measuring cylinder. As the soil particles settle out of suspension the specific gravity of the mixture reduces. An hydrometer is used to record the variation of specific gravity with time. By making use of Stoke’s Law, which relates the velocity of a free falling sphere to its diameter, the test data is reduced to provide particle diameters and the % by weight of the sample finer than a particular particle size.

Fig. 1.4 A schematic view of the hydrometer test

Soil Mechanics Soils And Their Classification page 6

0.001 10Equivalent Particle Size (mm)

0

10

20

30

40

50

60

70

80

90

100

Percent Finer

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 CoarseMediumSand

Fine Coarse FineMediumGravel

CoarseStone orBoulderClay

" " “ " 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

Plasticityindex

CH

OH

or

MH

CLOL

MLor

CL

ML

"A" line

Comparing soils at equal liquid limit

Toughness and dry strength increase

with increasing plasticity index

Plasticity chart

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

Soil Mechanics Soils And Their Classification page 7

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 of consistency 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 consistency limits (also called Atterberg limits, after the Swedish scientist who devised them) and are expressed as w%. Starting from the liquid state, three consistency limits are met when decreasing the 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 liquid state, 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 consists essentially of a metal cup which can be raised and dropped 10 mm by means of a cam mechanism.

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. The number of such taps required to bring the two halves together is recorded together with the water content. The procedure is repeated on other soil samples with different water contents. From the readings obtained, a graph of water content against the log of the number of taps is plotted. The liquid 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 a glass plate to produce a thread 3 mm thick before the soil just begins to crumble. The water content of the soil in this state is taken as the plastic limit.

Soil Mechanics Soils And Their Classification page 8

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 remains in a plastic state.

PI = LL-PL The shrinkage13 limit (SL) is the water content at which further loss of water in the soil will not cause further reduction in the volume of the soil, i.e. the water content required just to fill the voids of a sample which has been dried. The shrinkage limit is found by measuring the weight and volume of the soil at intervals as it is allowed to air-dry until no further volume change takes place. The volume is found by using a mercury displacement vessel.

1.3.3 Unified Soil Classification System (USCS) The standard system used worldwide for most major construction projects is known as the Unified Soil Classification System. This is based on an original system devised by Cassagrande. Soils are identified 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 the blank 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

Soil Mechanics Soils And Their Classification page 9

Figure 1.9 : USCS flowchart for classifying soils.

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

the uniformity coefficient CD

Du

= 60

10

and the coefficient of curvature CD

D Dc

=×30

2

60 10( )

where Dxx is the maximum size of particle in smallest xx% of sample. The symbols have the following meaning : S Sand G Gravel M Silt C Clay O Organic Pt Peat14 W Well graded (not uniform) P Poorly graded (uniform) H High plasticity L Low plasticity A PDF document describing the different soil groups, and giving some characteristics pertaining to embankments or foundations can be downloaded from the Moodle website.

14 Tourbe

Soil Mechanics Soils And Their Classification page 10

1.4 Example - Classification using USCS Classification tests have been performed on a soil sample and the following grading curve and Atterberg 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, Dual symbols 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 ze (m m )

% Finer

Figure 1.10 : Grading curve.

Soil Mechanics Soils And Their Classification page 11

1.5 Exercises 1. The results of a sieve analysis of two soils are the following :

Sieve size Soil A Soil B (mm) Mass

retained (g)

% retained % finer Mass retained

(g)

% retained % finer

37.50 0.0 20.00 26.0 10.00 31.0 5.00 11.0 0.0 2.00 18.0 8.0 1.18 24.0 7.0

0.600 21.0 11.0 0.300 41.0 21.0 0.212 32.0 63.0 0.150 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 g 0.60 mm 42.1 g 0.21 mm 24.2 g 0.075 mm 16.6 g

Hydrometer, sedimentation analysis:

Amounts finer than 0.03 mm 28.3 g 0.003 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.

Soil Mechanics Soils And Their Classification page 12

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

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

Particle size (mm)

A B C D E F

6.00 2.00 0.60 0.425 0.212 0.150 0.075 0.05 0.01 0.002

100 98 95 92 86 83 82 57 36

100 99 94 89 82 76 74 38 23

100 95 86 77 50 12 0

100 75 55 46 30 19 4 0

100 85 75 69 60 48 35 32 25 10

100 94 89 63 37 10 9 8 8

Liquid limit

67 40 Non-plastic

- 55 40

Plastic limit

27 12 Non-plastic

- 35 15