Civils:- Soil Mechanics Basic characteristics of soils Soils consist of grains (mineral grains, rock fragments, etc.) with water and air in the voids between grains. The water and air contents are readily changed by changes in conditions and location: soils can be perfectly dry (have no water content) or be fully saturated (have no air content) or be partly saturated (with both air and water present). Although the size and shape of the solid (granular) content rarely changes at a given point, they can vary considerably from point to point.
First of all, consider soil as a engineering material it is not a coherent solid material like steel and concrete, but is a particulate material. It is important to understand the significance of particle size, shape and composition, and of a soil's internal structure or fabric.
Soil as an engineering material The term "soil" means different things to different people: To a geologist it represents the products of past surface processes. To a pedologist it represents currently occurring physical and chemical processes. To an engineer it is a material that can be: built on: foundations to buildings, bridges. built in: tunnels, culverts, basements. built with: roads, runways, embankments, dams. supported: retaining walls, quays. Soils may be described in different ways by different people for their different purposes. Engineers' descriptions give engineering terms that will convey some sense of a soil's current state and probable susceptibility to future changes (e.g. in loading, drainage, structure, surface level). Engineers are primarily interested in a soil's mechanical properties: strength, stiffness, permeability. These depend primarily on the nature of the soil grains, the current stress, the water content and unit weight.
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Civils:- Soil Mechanics Size range of grains
The range of particle sizes encountered in soil is very large: from boulders with a controlling dimension of over 200mm down to clay particles less than 0.002mm (2mm). Some clays contain particles less than 1 mm in size which behave as colloids, i.e. do not settle in water due solely to gravity. In the British Soil Classification System, soils are classified into named Basic Soil Type groups according to size, and the groups further divided into coarse, medium and fine sub-groups:
Very coarse BOULDERS soils COBBLES
Coarse soils
60 - 200 mm
coarse 20 - 60 mm G GRAVEL medium 6 - 20 mm fine 2 - 6 mm S SAND
coarse 0.6 - 2.0 mm medium 0.2 - 0.6 mm fine
Fine soils
> 200 mm
M SILT
0.06 - 0.2 mm
coarse 0.02 - 0.06 mm medium 0.006 - 0.02 mm fine
C CLAY
0.002 - 0.006 mm < 0.002 mm
Aids to size identification Soils possess a number of physical characteristics which can be used as aids to size identification in the field. A handful of soil rubbed through the fingers can yield the following: SAND (and coarser) particles are visible to the naked eye. SILT particles become dusty when dry and are easily brushed off hands and boots. CLAY particles are greasy and sticky when wet and hard when dry, and have to be scraped or washed off hands and boots.
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Civils:- Soil Mechanics
Shape of grains The majority of soils may be regarded as either SANDS or CLAYS: SANDS include gravelly sands and gravel-sands. Sand grains are generally broken rock particles that have been formed by physical weathering, or they are the resistant components of rocks broken down by chemical weathering. Sand grains generally have a rotund shape. CLAYS include silty clays and clay-silts; there are few pure silts (e.g. areas formed by windblown Lรถess). Clay grains are usually the product of chemical weathering or rocks and soils. Clay particles have a flaky shape.
There are major differences in engineering behaviour between SANDS and CLAYS (e.g. in permeability, compressibility, shrinking/swelling potential). The shape and size of the soil grains has an important bearing on these differences.
Shape characteristics of SAND grains SAND and larger-sized grains are rotund. Coarse soil grains (silt-sized, sandsized and larger) have different shape characteristics and surface roughness depending on the amount of wear during transportation (by water, wind or ice), or after crushing in manufactured aggregates. They have a relatively low specific surface (surface area).
Rounded: Water- or air-worn; transported sediments Irregular: Irregular shape with round edges; glacial sediments (sometimes subdivided into 'sub-rounded' and 'sub-angular') Angular: Flat faces and sharp edges; residual soils, grits Flaky: Thickness small compared to length/breadth; clays Elongated: Length larger than breadth/thickness; scree, broken flagstone Flaky & Elongated: Length>Breadth>Thickness; broken schists and slates
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Civils:- Soil Mechanics Shape characteristics of CLAY grains CLAY particles are flaky. Their thickness is very small relative to their length & breadth, in some cases as thin as 1/100th of the length. They therefore have high to very high specific surface values. These surfaces carry a small negative electrical charge, that will attract the positive end of water molecules. This charge depends on the soil mineral and may be affected by an electrolite in the pore water. This causes some additional forces between the soil grains which are proportional to the specific surface. Thus a lot of water may be held asadsorbed water within a clay mass.
Grading and composition The recommended standard for soil classification is the British Soil Classification System, and this is detailed in BS 5930 Site Investigation. Coarse soils Coarse soils are classified principally on the basis of particle size and grading. > 200 mm Very coarse BOULDERS soils COBBLES 60 - 200 mm
Coarse soils
coarse 20 - 60 mm G GRAVEL medium 6 - 20 mm fine 2 - 6 mm S SAND
coarse 0.6 - 2.0 mm medium 0.2 - 0.6 mm fine
0.06 - 0.2 mm
Particle size tests The aim is to measure the distribution of particle sizes in the sample. When a wide range of sizes is present, the sample will be sub-divided, and separate tests carried out on each sub-sample. Full details of tests are given in BS 1377: "Methods of test for soil for civil engineering purposes". Particle-size tests Wet sieving to separate fine grains from coarse grains is carried out by washing the soil specimen on a 60mm sieve mesh. Dry sieving analyses can only be carried out on particles > 60 mm. Samples (with fines removed) are dried and shaken through a nest of sieves of descending size. Sedimentation is used only for fine soils. Soil particles are allowed to settle from a suspension. The decreasing density of the suspension is measured at time intervals. Sizes are determined from the settling velocity and times recorded. Percentages between sizes are determined from density differences. Particle-size analysis The cumulative percentage quantities finer than certain sizes (e.g. passing a given size sieve mesh) are determined by weighing. Points are then plotted of % finer (passing) against log size. A smooth S-shaped curve drawn through these points is
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Civils:- Soil Mechanics called a grading curve. The position and shape of the grading curve determines the soil class. Geometrical grading characteristics can be determined also from the grading curve.
Typical grading curves Both the position and the shape of the grading curve for a soil can aid its identity and description. Some typical grading curves are shown in the figure: A - a poorly-graded medium SAND (probably estuarine or flood-plain alluvium) B - a well-graded GRAVEL-SAND (i.e. equal amounts of gravel and sand) C - a gap-graded COBBLES-SAND D - a sandy SILT (perhaps a deltaic or estuarine silt) E - a typical silty CLAY (e.g. London clay, Oxford clay)
Grading characteristics
A grading curve is a useful aid to soil description. Grading curves are often included in ground investigation reports. Results of grading tests can be tabulated using geometric properties of the grading curve. These properties are called grading characteristics First of all, three points are located on the grading curve: d10 = the maximum size of the smallest 10% of the sample d30 = the maximum size of the smallest 30% of the sample d60 = the maximum size of the smallest 60% of the sample From these the grading characteristics are calculated: Effective size d10
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Civils:- Soil Mechanics Uniformity coefficient Cu = d60 / d10 Coefficient of gradation Ck = d30² / d60 d10 Both Cu and Ck will be 1 for a single-sized soil Cu > 5 indicates a well-graded soil Cu < 3 indicates a uniform soil Ck between 0.5 and 2.0 indicates a well-graded soil Ck < 0.1 indicates a possible gap-graded soil
Sieve analysis example The results of a dry-sieving test are given below, together with the grading analysis and grading curve. Note carefully how the tabulated results are set out and calculated. The grading curve has been plotted on special semi-logarithmic paper; you can also do this analysis using a spreadsheet. Percentage Sieve mesh Mass Percentage finer size (mm) retained (g) retained (passing) 14.0
0
0
10.0
3.5
1.2
6.3
7.6
2.6
5.0
7.0
2.4
3.35
14.3
4.9
2.0
21.1
7.2
1.18
56.7
19.4
0.600
73.4
25.1
0.425
22.2
7.6
0.300
26.9
9.2
0.212
18.4
6.3
0.150
15.2
5.2
0.063
17.5
6.0
Pan
8.5
2.9
TOTAL
292.3
100.0
What is the classification?
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Civils:- Soil Mechanics British Soil Classification System The recommended standard for soil classification is the British Soil Classification System, and this is detailed in BS 5930 Site Investigation. Its essential structure is as follows: Soil group Symbol Recommended name Coarse soils
Fines % G GF
GRAVEL GF
S
GW
0-5
Well-graded GRAVEL
GPu/GPg
0-5
Uniform/poorly-graded GRAVEL
GWM/GWC 5 - 15
Well-graded silty/clayey GRAVEL
GPM/GPC
5 - 15
Poorly graded silty/clayey GRAVEL
GML, GMI...
15 - 35
Very silty GRAVEL [plasticity subgroup...]
GCL, GCI... 15 - 35
Very clayey GRAVEL [..symbols as below]
SW
0-5
Well-graded SAND
SPu/SPg
0-5
Uniform/poorly-graded SAND
S-F SWM/SWC 5 - 15 GPM/GPC
SAND
5 - 15
SF SML, SMI... 15 - 35 SCL, SCI... 15 - 35
Fine soils
SILT
CLAY
>35% fines
M
C
Well-graded silty/clayey SAND Poorly graded silty/clayey SAND Very silty SAND [plasticity subgroup...] Very clayey SAND [..symbols as below]
Liquid limit%
MG
Gravelly SILT
MS
Sandy SILT
ML, MI...
[Plasticity subdivisions as for CLAY]
CG
Gravelly CLAY
CS
Sandy CLAY
CL
<35
CLAY of low plasticity
CI
35 - 50
CLAY of intermediate plasticity
CH
50 - 70
CLAY of high plasticity
CV
70 - 90
CLAY of very high plasticity
CE
>90
CLAY of extremely high plasticity
Organic soils
O
[Add letter 'O' to group symbol]
Peat
Pt
[Soil predominantly fibrous and organic]
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Civils:- Soil Mechanics
Fine soils In the case of fine soils (e.g. CLAYS and SILTS), it is the shape of the particles rather than their size that has the greater influence on engineering properties. Clay soils have flaky particles to which water adheres, thus imparting the property of plasticity.
Consistency limits and plasticity
Consistency varies with the water content of the soil. The consistency of a soil can range from (dry) solid to semi-solid to plastic to liquid (wet). The water contents at which the consistency changes from one state to the next are called consistency limits (or Atterberg limits). Two of these are utilised in the classification of fine soils: Liquid limit (wL) - change of consistency from plastic to liquid Plastic limit (wP) - change of consistency from brittle/crumbly to plastic Measures of liquid and plastic limit values can be obtained from laboratory tests.
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Civils:- Soil Mechanics
Plasticity index
The consistency of most soils in the ground will be plastic or semi-solid. Soil strength and stiffness behaviour are related to the range of plastic consistency. The range of water content over which a soil has a plastic consistency is termed the Plasticity Index (IP or PI). IP = liquid limit - plastic limit = wL - wP The plasticity chart and classification In the BSCS fine soils are divided into ten classes based on their measured plasticity index and liquid limit values: CLAYS are distinguished from SILTS, and five divisions of plasticity are defined: Low plasticity wL = < 35% Intermediate plasticity
wL = 35 - 50%
High plasticity
wL = 50 - 70%
Very high plasticity
wL = 70 - 90%
Extremely high plasticity wL = > 90% A plasticity chart is provided to aid classification.
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Civils:- Soil Mechanics
Volumes of solid, water and air: the soil model
The soil model is given dimensional values for the solid, water and air components: Total volume, V = Vs + Vw + Va Since the amounts of both water and air are variable, the volume of solids present is taken as the reference quantity. Thus, the following relational volumetric quantities may be defined:
Note also that: n = e / (1 + e) e = n / (1 - n) v = 1 / (1 - n) Typical void ratios might be 0.3 (e.g. for a dense, well graded granular soil) or 1.5 (e.g. for a soft clay).
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Civils:- Soil Mechanics
Masses of solid and water: water content
The mass of air may be ignored. The mass of solid particles is usually expressed in terms of their particle density or grain specific gravity. Grain specific gravity
Hence the mass of solid particles in a soil Ms = Vs .Gs .rw (rw = density of water = 1.00Mg/m続) [Range of Gs for common soils: 2.64-2.72] Particle density rs = mass per unit volume of particles = Gs .rw The ratio of the mass of water present to the mass of solid particles is called the water content, or sometimes the moisture content.
From the soil model it can be seen that w = (Sr .e .rw) / (Gs .rw) Giving the useful relationship: w .Gs = Sr .e
Laboratory measurements It is important to quantify the state of a soil immediately it is received in the testing laboratory and just prior to commencing other tests (e.g. shear tests, compression tests, etc.). The water content and unit weight are particularly important, since these could change during transportation and storage.
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Civils:- Soil Mechanics Some physical state properties are calculated following the practical measurement of others; e.g. void ratio from porosity, dry unit weight from unit weight & water content. Water content The most usual method of determining the water content of soil is to weigh a small representative specimen, drying it to constant weight and then weighing it again. Drying can be carried out using an electric oven set at 104-105° Celsius or using a microwave oven. Example: A sample of soil was placed in a tin container and weighed, after which it was dried in an oven and then weighed again. Calculate the water content of the soil. Weight of tin empty
= 16.16 g
Weight of tin + moist soil
= 37.82 g
Weight of tin + dry soil
= 34.68 g
Water content, w
= (mass of water) / (mass of dry soil) = (37.82 - 34.68) / (34.68 - 16.16) = 0.169
Percentage water content = 16.9 % Liquidity index
In fine soils, especially clays, the current state is dependent on the water content with respect to the consistency limits (or Atterberg limits). The liquidity index (IL or LI) provides a quantitative measure of the current state:
where wP = plastic limit and wL = liquid limit
Significant values of IL indicating the consistency of the soil are: IL < 0 Ă&#x17E; semi-plastic solid or solid 0 < IL < 1 Ă&#x17E; plastic
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Civils:- Soil Mechanics 1 < IL Ă&#x17E; liquid BS system for description and classification BS 5930 Site Investigation recommends the terminology and a system for describing and classifying soils for engineering purposes. Without the use of a satisfactory system of description and classification, the description of materials found on a site would be meaningless or even misleading, and it would be difficult to apply experience to future projects.
BS description system A recommended protocol for describing a soil deposit uses nine characteristics; these should be written in the following order: compactness e.g. loose, dense, slightly cemented bedding structure e.g. homogeneous or stratified; dip, orientation discontinuities spacing of beds, joints, fissures weathered state degree of weathering colour main body colour, mottling grading or consistency e.g. well-graded, poorly-graded; soft, firm, hard SOIL NAME e.g. GRAVEL, SAND, SILT, CLAY; (upper case letters) plus silty-, gravelly-, withfines, etc. as appropriate soil class (BSCS) designation (for roads & airfields) e.g. SW = well-graded sand geological stratigraphic name (when known) e.g. London clay Not all characteristics are necessarily applicable in every case.
Example: (i) Loose homogeneous reddish-yellow poorly-graded medium SAND (SP), Flood plain alluvium (ii) Dense fissured unweathered greyish-blue firm CLAY. Oxford clay.
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Civils:- Soil Mechanics
Definitions of terms used in description A table is given in BS 5930 Site Investigation setting out a recommended field indentification and description system. The following are some of the terms listed for use in soil descriptions: Particle shape angular, sub-angular, sub-rounded, rounded, flat, elongate Compactness loose, medium dense, dense (use a pick or driven peg, or density index ) Bedding structure homogeneous, stratified, inter-stratified Bedding spacing massive(>2m), thickly bedded (2000-600 mm), medium bedded (600-200 mm), thinly bedded (200-60 mm), very thinly bedded (60-20 mm), laminated (20-6 mm), thinly laminated (<6 mm). Discontinuities i.e. spacing of joints and fissure: very widely spaced(>2m), widely spaced (2000-600 mm), medium spaced (600-200 mm), closely spaced (200-60 mm), very closely spaced (60-20 mm), extremely closely spaced (<20 mm). Colours red, pink, yellow, brown, olive, green, blue, white, grey, black Consistency very soft (exudes between fingers), soft (easily mouldable), firm (strong finger pressure required), stiff (can be indented with fingers, but not moulded) very stiff (indented by sharp object), hard (difficult to indent). Grading well graded (wide size range), uniform (very narrow size range), poorly graded (narrow or uneven size range). Composite soils In SANDS and GRAVELS: slightly clayey or silty (<5%), clayey or silty (5-15%), very clayey or silty(>15%) In CLAYS and SILTS: sandy or gravelly (35-65%)
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Civils:- Soil Mechanics British Soil Classification System The recommended standard for soil classification is the British Soil Classification System, and this is detailed in BS 5930 Site Investigation. Its essential structure is as follows: Soil group Symbol Recommended name Coarse soils
Fines % G GF
GRAVEL GF
S
GW
0-5
Well-graded GRAVEL
GPu/GPg
0-5
Uniform/poorly-graded GRAVEL
GWM/GWC 5 - 15
Well-graded silty/clayey GRAVEL
GPM/GPC
5 - 15
Poorly graded silty/clayey GRAVEL
GML, GMI...
15 - 35
Very silty GRAVEL [plasticity subgroup...]
GCL, GCI... 15 - 35
Very clayey GRAVEL [..symbols as below]
SW
0-5
Well-graded SAND
SPu/SPg
0-5
Uniform/poorly-graded SAND
S-F SWM/SWC 5 - 15 GPM/GPC
SAND
5 - 15
SF SML, SMI... 15 - 35 SCL, SCI... 15 - 35 Fine soils
SILT
CLAY
>35% fines
M
C
Well-graded silty/clayey SAND Poorly graded silty/clayey SAND Very silty SAND [plasticity subgroup...] Very clayey SAND [..symbols as below]
Liquid limit%
MG
Gravelly SILT
MS
Sandy SILT
ML, MI...
[Plasticity subdivisions as for CLAY]
CG
Gravelly CLAY
CS
Sandy CLAY
CL
<35
CLAY of low plasticity
CI
35 - 50
CLAY of intermediate plasticity
CH
50 - 70
CLAY of high plasticity
CV
70 - 90
CLAY of very high plasticity
CE
>90
CLAY of extremely high plasticity
Organic soils
O
[Add letter 'O' to group symbol]
Peat
Pt
[Soil predominantly fibrous and organic]
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Civils:- Soil Mechanics
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