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Basic Soil Mechanics

ATGB2513

Learning Outcome

Upon completion of this course, students should be able:-

To differentiate the properties and behaviour of various types of soil

To interpret a soil report

To explain various methods of dewatering

To discuss the various methods of soil testing

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Basic Soil Mechanics

Core reading list

Craig, R.F., Craigs Soil Mechanics, 7th Edition, Spon Press (2004)

Whitlow, R., Basic Soil Mechanics, 4th Edition, Prentice Hall (2001)

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Module 6

Testing, Measurement and Evaluation

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Reference Text

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Library ref: 624.151 36 LIU

Chapters 4, 6, 7, 8, 10 12, 14, 18 & 23

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Module 1

Soil Materials

Synonym

AASHTO American Association of State Highway and Transportation Officials

USCS Unified Soil Classification System

ASTM American Society for Testing and Materials

BS British Standards

USDA United States Department of Agriculture

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Soil Materials

Soil and Soil Engineering

To a Pedologist Soil is the substance existing on earths surface, which grows and develop plant life

To a Geologist Soil is the material in the relative thin surface zone within which roots occur and all the rest of the crust is grouped under the term ROCK irrespective of hardness

To an Engineer Soil is the un-aggregated deposits of mineral and/or organic particles or fragment covering large portion of the earths crust

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Soil Materials

Soil and Soil Engineering

Soil Mechanics is one of the youngest disciplines of Civil Engineering involving study of soil, its behavior and application as engineering material

Geotechnical Engineering Is a broader term for Soil Mechanics

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SOIL

Geologic definition: Loose surface of the earth as distinguished from solid bedrock; support of plant life not required.

Traditional definition: Material which nourishes and supports growing plants; includes rocks, water, snow, air.

Component definition: Mixture of mineral matter, organic matter, water, and air.

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Soil Formation/Nature of Soil Deposits

Soil material is a product of rock

May be defined as an accumulation of solid particles produced by mechanical and chemical disintegration of rock

May contain organic material

Soils are derived from the weathering of rocks and are commonly described by external textural terms such as gravels, sands, silts and clays.

What is rock?

Naturally occurring material

Composed of mineral particles firmly bonded

Difficult to separate; blasting, crushing, chemical 12

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Engineering Soils Soil Types Coarse-grained soils Fine-grained soils

Description Boulders Gravel Sand Silt Clay

Feel Hard, Gritty and Bulky Slight Grittiness Smooth

Size (mm) > 75 75 to 4.75 4.75 to 0.075 0.075 to 0.002 < 0.002

Characterization Particle size Particle size and

mineralogy

Basic Geology Knowledge of geology is important for practice of geotechnical

engineering;

The earths surface (lithosphere) is fractured into about 20 mobile plates. Interaction of these plates causes volcanic activities and earthquakes;

The three groups of rocks are igneous, sedimentary and metamorphic. Igneous rocks are formed from magma (molten rock materials) emitted from volcanoes that have cooled and solidified. Sedimentary rocks are formed from sediments, animals and plant materials that are deposited in water or on land on the earths surface and then subjected to pressures and heat. Metamorphic rocks are formed deep within the earths crust from the transformation of igneous and sedimentary rocks into denser rocks;

Sedimentary rocks are of particular importance to geotechnical engineers because they cover about 75% of the earths crust surface area; and

Rock masses are inhomogeneous and discontinuous.

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Categories of rock

Igneous rock solidification of molten material; by intrusion or extrusion to earth surface

Sedimentary rock deposition, under water, disaggregation, preexisting

Metamorphic rock igneous or sedimentary rock, change or metamorphose under heat and pressure

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Geological process that produces soil

General controlling factors

Nature and composition of parent rock

Climatic conditions; temperature, humidity

Topographic and general terrain; degree of shelter or exposure, density, type of vegetation

Length of time under particular prevailing conditions

Interferences by other agencies; storms, earthquakes, action of human

Mode and conditions of transport

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Geological process that produces soil Weathering physical and chemical

Physicalnatural mechanical and abrasion

Coarse soil

Aggregate/gravel

Sand

Retained the same composition of parent rock

Physical weathering causes reduction in the size of the parent rock without change in its composition.

Chemicalwater- acidic, alkaline, oxygen, carbon dioxide

Small particles, crystalline form, two dimensional, flaky

Clayey soil characteristic depends on parent rock, environment, duration of alteration

Chemical weathering causes reduction in size and chemical

composition different from the parent rock.

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Geological process that produces soil

Weathering effects;

Frosts within pore space; splitting, sharp and angular

Wind and water; attrition- rounded

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Type of soil produced by different weathering

Boulders; size

Sand (physical)

Silt (cohesive)

Clay (chemical)

Moisture

Dry, saturated (fully and partially)

Shapes and textures different

Particle size is used to distinguish various soil textures.

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Transportation process sorting out process

Not been transported residual soil, usually by chemical means,

flat terrain

Water alluvial soil (river deposit)

Estuarine mixture of marine and alluvial soil; favourable foundation

Lacustrine fresh water; good foundation

Coastal, marine (blackish water); velocity, suspension, deposition

Ice glacial soil (large to smaller)

Wind Aeolin soil

Sand dunes, loess; long distance, arid or coastal areas

Gravity... Colluvial soil (unsorted)

below slide areas, cliff base; future difficulties for foundation 20

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Soil Materials

Types of soils

a) Organic b) Residual c) Alluvial d) Colluvial

a) Organic soil is a mixture of mineral and organic material. Usually dark in colour and with an odour.

b) Residual soil is weathered remains of rock after going through the transportation process.

c) Alluvial soil is material (sand and gravel) deposited by streams and rivers.

d) Colluvial soil is material transported and deposited by gravity like landslides.

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Loading and Drainage History

The current state (i.e. density and consistency) of a soil; influenced by the history of loading and unloading since it was deposited.

Changes in drainage conditions; may have brought about changes in water content.

Initial loading

During deposition the load applied to a layer of soil increases as more layers are deposited over it; thus, it is compressed and water is squeezed out; as deposition

continues, the soil becomes stiffer and stronger.

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Loading and Drainage history

Unloading The principal natural mechanism of unloading is erosion of

overlying layers. Unloading can also occur as overlying ice-sheets and glaciers retreat, or due to large excavations made by man.

Soil expands when it is unloaded, but not as much as it was initially compressed; thus it stays compressed - and is said to be overconsolidated. The degree of overconsolidation depends on the history of loading and unloading.

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Loading and drainage history

Drainage history

Chemical changes

Some soils initially deposited loosely in saline water and then inundated with fresh water develop weak collapsing structure.

In arid climates with intermittent rainy periods, cycles of wetting and drying can bring minerals to the surface to form a cemented soil.

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Loading and drainage history

Drainage history

Climate changes

Some clays (e.g. montmorillonite clays) are prone to large volume changes due to wetting and drying; seasonal changes in surface level occur, often causing foundation damage, especially after exceptionally dry summers.

Trees extract water from soil in the process of evapotranspiration; The soil near to trees can therefore either shrink as trees grow larger, or expand following the removal of large trees.

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Overburden pressure

Pressure/stress caused by weight of all material; more dense for soil located deeper

Normal consolidation

Usually clay, subjected to pressure imposed by the overburden since its formation; a soil which current state corresponds to the maximum consolidation pressure

Over consolidation

Usually clay, subjected to access and extreme pressure besides from the overburden since formation; a soil which have present day overburden pressure less than the highest historic consolidation pressure

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Definition of clay based on stress history

Normally consolidated, whose present effective overburden pressure is the maximum pressure that the soil was subjected to in the past.

Overconsolidated, whose present effective pressure is less that that which the soil experienced in the past. The maximum effective past pressure is called the preconsolidated pressure.

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Clays are composed of three main types of mineral kaolinite, illite and montmorillonite.

The clay minerals consist of silica and alumina sheets that are combined to form layers. The bonds between layers play a very important role in the mechanical behavior of clays. The bond between the layers of montmorillonite is very weak compared with kaolinite and illite. Water can easily enter between the layers in the montmorillonite, causing swelling.

A thin layer of water, called absorbed water, is bonded to the mineral surfaces of soils. This layer significantly influences the physical and mechanical characteristics of fine-grained soils.

Fine-grained soils have much larger surface areas than course-grained soils and are responsible for the major physical and mechanical differences between course-grained and fine-grained soils.

The engineering properties of fine-grained soils depend mainly on mineralogical factors.

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Main components of soil description

Nature of soil

Shape, size and particles distribution

State of soil

Density, relative density, water content

Fabric of soil

Homogeneity or layer sequences

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Soil Color

Indicator of different soil types

Indicator of certain physical and chemical characteristics

Due to humus content and chemical nature of the iron compounds present in the soil

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Major Forms of Iron and Effect on Soil Color

Form Chemical Formula Color

Ferrous oxide FeO Gray

Ferric oxide

(Hematite) Fe2O3 Red

Hydrated ferric oxide

(Limonite) 2Fe2O3 3H2O Yellow

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Field Classification; site exploration; preliminary; with some in-situ testing procedures

Identify particle size; visual and feel; proportion of size range (shaking with water)

Course soils (British Standards)

Seen with unaided eye; Gravel > 2mm, Sand (0.06 mm < 2.0mm); gritty feeling; over 65% (coarse)

Clean sand or gravel; W or P gradation

Dirty sand and gravel; M or C gradation (fines are silty or clayey

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Field Classification

Fine soil (British Classification)

More than 35% < 0.06mm; visually

Need magnification to see grain

Silt (0.002mm < 0.06mm); slightly abrasive

Clay (< 0.002mm); greasy feel

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Field Classification

Compactness (field strength)

Hand spade, wooden peg Loose, dense and slightly cemented

Structure; trial pit, cutting

Homogeneous; one type of soil

Inter-stratified; alternating or bands of different layers

Intact; non fissured fine soil

Fissured; direction, size and spacing

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Field Classification

Cohesion, plasticity and consistency

Remove particles > 2.0mm; squeeze handful; descript feeling; soft, firm, hard and crumbly

Dilatancy; fine sand and inorganic silt

Dry strength; high (clay), low (inorganic silt, powdery)

Weathering; unweathered, slightly, moderately, highly and fully

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Soil Description and Classification

System to group soil;

Physical characteristic of soil particles

Performance of soil particles when subjected to certain tests or condition of service

Textural classification

Assign descriptive name; e.g. clayey sand

Assign particle size limits to soil fraction and % compositions corresponding to the descriptive names

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Soil Description and Classification

Textural classification

Use more for construction; coarse grain soil (sand and gravel); performance base on relative amount of sizes of particles

Fine soil (silt and clay); produce little information for engineering use; behavior depend more than size eg plasticity characteristic

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Soil Index Properties

As a guide; descriptive nature of soil constituents

Relates to engineering behavior of soil

Behavior of sands and gravels may inferred shape, size and density

Behavior of silts and clays; interaction of particles with water

Properties of soil that indicate the type and conditions of soil

Provide structural properties; strength, compressibility, permeability

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Soil Index Properties

Particle size distribution curve for coarse grain soil

Mechanical; sieve analysis

Plasticity characteristics for fine grain soil and relationship to natural water content

Hydrometer

Phase relationships (air, water, sand and solid) for soil mass

Consistency limits; L.L, P.L and P.I

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Sieve Analysis

A sieve analysis is used to determine the grain size distribution of coarse-grained soils

The particle size distribution plot is used to delineate the different soil textures (percentage of gravel, sand, silt, and clay) in a soil.

For fine-grained soils, a hydrometer analysis is used to find the particle size distribution

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Soil Description

The identification and defining basic soil types, British Classification System;

e.g. boulders, cobbles, gravel, sand, silt and clay

Including organic clay and silt or sand and peat

In terms of particle size range; Figure 1.6

Different percentage of particle sizes defines grouping

Dependent on sizes; two major groups

Coarse grain soil; > 65% sand and gravel sizes

Fine grain soil; > 35% silt and clay sizes

Sand & gravel further subdivided into coarse, medium and fine fraction; Figure 1.6

Names in capital letter in a soil description; e.g. S for Sand

Mixture of basic soil types are referred to as composite type

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British Standard Range of Particle Sizes

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Soil Description Sand & gravel can further described as well graded, poorly graded, uniform or gap-

graded

Well graded; if there is no excess of particles in any size range and if no intermediate sizes are lacking; smooth concave distribution curve

Poorly graded; if high proportion of particles have sizes within narrow limits; uniform graded

Poorly graded; if particles of both large and small sizes are present but relatively low proportion of particles of intermediate particle; gap graded

In the case of gravels, particle shape (angular, sub angular, sub rounded, rounded, flat, elongated) & texture (rough, smooth, polished) can be described

Particle composition can also be described; sandstone in gravel and quartz in sand; Table 1.1

Firmless or strength of in-situ soil description and can also be assessed by means of tests ; Table 1.2

Description of soil structure can also be established; Table 1.3

Example of soil description;

Dense, reddish-brown, sub angular, well graded, gravelly SAND 43

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Table 1.1 Description of Composite Soil Types

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Table 1.2 Firmless or Strength of In-situ Soil Description

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Table 1.3 Descriptions for Structure of Soil Deposit

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Soil Classification System BSCS

British Soil Classification System is shown in Table 1.4.

Soil group in the classification denoted by group symbols composed of main and qualifying descriptive letters having meanings given in Table 1.5

Reference should also to be made to the Plasticity Chart for fine material; Figure 1.7;

Plasticity Index and Liquid Limit parameters; determined from laboratory

Determine the plasticity characteristic of fine soil represented by a point on chart

Classification according to zone in the chart within which the point lies.

If point is above A-line, soil is Clay; and

If point is below A-line, soil is Silt

Any boulders or cobles (particles retained on a 63 mm BS sieve) are to be removed before classification tests are carried out but their percentages in the total sample should be determined or estimated

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48 Table 1.4 British Soil Classification Systems for Engineering Purposes

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Table 1.5 Classifications Qualifying Descriptive Letters (BSCS)

Primary Letters Secondary Letters

Coarse-grained Soil

G = GRAVEL

S = SAND

W = well graded

P = poorly graded

Pu = uniform

Pg = gap graded

Fined grained Soil F = FINES

M = SILT

C = CLAY

L = low plasticity (wL < 35)

I = intermediate plasticity(wL : 35-50)

H = high plasticity(wL :50-70)

V = very high plasticity(wL : 70-90)

E = extremely high plasticity(wL >90)

Organic soils Pt = PEAT O = organic

Sub-group symbols in the British Soil Classification system

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Figure 1.7 Plasticity Chart: British System (BS 5930: 1981)

Soil Classification System USCS

Classification Systems vary from country to country, but most are based on the US system (The Unified Soil Classification System, USCS), or the British Standard Soil Classification System. The Australian Standard Soil Classification System is similar to the British Standard, but the USCS is widely used in Australia and the SE Asia region.

USCS with primary and secondary descriptive letter and meaning is shown in Table 1.6.

USCS with primary and secondary descriptive letter and laboratory classification criteria is shown in Table 1.7

The associated Plasticity Chart should be used as shown in Figure 1.8

In the USCS system, the divisions are slightly different, but given the wide range of particle sizes, these differences are not important (for example, in the BS system, fines includes silt and clay, and is defined as being < 60 m, but in the USCS, fines is < 75m). Australia and the SE Asia region.

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Table 1.6 Group Symbols with Primary and Secondary Descriptive Letters

(Unified Soil Classification System)

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Table 1.7 Unified Soil Classification System

Unified Soil Classification System

CU 4 1 CC 3 GW < 15% sand Well-graded gravel

15% sand Well-graded gravel with sandCC < 1 or CC > 3 GP < 15% sand Poorly-graged gravel

15% sand Poorly-graded gravel with sandCU < 4

ML or MH GW-GM < 15% sand Well-graded gavel with siltCU 4 1 CC 3 15% sand Well-graded gravel with silt and sand

CL, CH, or CL-ML GW-GC < 15% sand Well-graded gravel with clay (or silty clay)

15% sand Well-graded gravel with clay & sand (or silty clay & sand)

Gravel CC < 1 or CC > 3 ML or MH GP-GM < 15% sand Poorly-graded gravel with silt

% sand < % gravel 15% sand Poorly-graded gravel with silt and sand

CL, CH, or CL-ML GP-GC < 15% sand Poorly-graded gravel with clay (or silty clay)CU < 4 15% sand Poorly-graded gravel with clay & sand (or silty clay & sand)

ML or MH GM < 15% sand Silty gravel

15% sand Silty gravel with sand

CL or CH GC < 15% sand Clayey gravel

15% sand Clayey grvel with sand

CL-ML GC-GM < 15% sand Silty, clayey gravel

15% sand Silty, clayey gravel with sand

CU 6 1 CC 3 SW < 15% gravel Well-graded sand

15% gravel Well-graded sand with gravelCC < 1 or CC > 3 SP < 15% gravel Poorly-graded sand

15% gravel Poorly- graded sand with gravelCU < 6

ML or MH SW-SM < 15% gravel Well-graded gsand with siltCU 6 1 CC 3 15% gravel Well-graded sand with silt and gravel

CL, CH, or CL-ML SW-SC < 15% gravel Well-graded sand with clay (or silty clay)

15% gravel Well-graded sand with clay & gravel (or silty clay & sand)

Sand CC < 1 or CC > 3 ML or MH SP-SM < 15% gravel Poorly-graded sand with silt

% sand % gravel 15% gravel Poorly-graded sand with silt and gravel

CL, CH, or CL-ML SP-SC < 15% gravel Poorly-graded sand with clay (or silty clay)

CU < 6 15% gravel Poorly-graded sand with clay & gravel (or silty clay & sand)

ML or MH SM < 15% gravel Silty sand

15% gravel Silty sand with gravel

CL or CH SC < 15% gravel Clayey sand

15% gravel Clayey sand with gravel

CL-ML SC-SM < 15% gravel Silty, clayey sand

15% gravel Silty, clayey sand with gravel

Flow chart for classification of coarse-grained soils

Coduto, D.P., Yeung, R.M. and Kitch, W.A. 2011. Geotechnical Engineering: Principles and Practices . 2nd edn. USA: Pearson(Adapted from ASTM D2487).

50% or more of

coarse fraction

passes the 4.75

mm (#4) sieve

50% or more of

coarse fraction

retained on the

4.75 mm (#4)

sieve

> 12% pass #200

> 12% pass #200

More than

50%

retained on

the 0.075

mm (#200)

sieve

5 - 12% pass #200

< 5% pass #200

Course-

Grained

Soils

5 - 12% pass #200

< 5% pass #200

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Table 1.7 Unified Soil Classification System

Unified Soil Classification System

85% pass #200 Lean clay

70% pass #200 70 - 84% pass #200 % sand % gravel Lean clay with sand

% sand < % gravel Lean clay with gravel

% sand % gravel < 15% gravel Sandy lean clay

50 - 69% pass #200 15% gravel Sandy lean clay with gravel

% sand < % gravel < 15% sand Gravelly lean clay

15% sand Gravelly lean clay with sand

85% pass #200 Silty clay

70% pass #200 70 -84% pass #200 % sand % gravel Silty clay with sand

Silts and Clays % sand < % gravel Silty clay with gravel

Liquid Limit < 50% % sand % gravel < 15% gravel Sandy silty clay

50 - 69% pass #200 15% gravel Sandy silty clay with gravel

% sand < % gravel < 15% sand Gravell silty clay

15% sand Gravelly silty clay with sand

85% pass #200 Silt

70% pass #200 70 -84% pass #200 % sand % gravel Silt with sand

% sand < % gravel Silt with gravel

% sand % gravel < 15% gravel Sandy silt

50 - 69% pass #200 15% gravel Sandy silt with gravel

% sand < % gravel < 15% sand Gravelly silt

15% sand Gravelly silt with sand

85% pass #200 Fat clay

70% pass #200 70 -84% pass #200 % sand % gravel Fat clay with sand

% sand < % gravel Fat clay with gravel

% sand % gravel < 15% gravel Sandy fat clay

50 - 69% pass #200 15% gravel Sandy fat clay with gravel

Silts and Clays % sand < % gravel < 15% sand Gravelly fat clay

Liquid Limit 50% 15% sand Gravelly fat clay with sand

85% pass #200 Elastic silt

70% pass #200 70 -84% pass #200 % sand % gravel Elastic silt with sand

% sand < % gravel Elastic silt with gravel

% sand % gravel < 15% gravel Sandy elastic silt

50 - 69% pass #200 15% gravel Sandy elastic silt with gravel

% sand < % gravel < 15% sand Gravelly elastic silt

15% sand Gravelly elastic sitl with sand

Flow chart for classification of inorganic fine-grained soils

Coduto, D.P., Yeung, R.M. and Kitch, W.A. 2011. Geotechnical Engineering: Principles and Practices . 2nd edn. USA: Pearson(Adapted from ASTM D2487).

Fine-Grained

Soils

ML

CH

MH

More than

50% passes

the 0.075 mm

(#200) sieve

CL

CL -ML

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Figure 1.8 Plasticity Chart (Unified Soil Classification System)

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Basic characteristic of soil constituents (British Soil Classification System);

Majority consist of mixtures of inorganic mineral particles, with water and air(void); solid, water and gas phases

Rock fragments

Fairly large (> 2mm) eg sand to gravel(stone)

Soundness depends on extent of mineral decomposition

Mineral grain

Separate particles of mineral eg sand (quartz)

Size from 2mm to clay (1m)

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Basic characteristic of soil constituents(British Soil Classification System)

Dependent on sizes; two major groups

Coarse grain soil; > 65% sand and gravel sizes

Fine grain soil; > 35% silt and clay sizes

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Coarse grain

Individual grain; wet or dry condition

Particle size > 0.06mm; sands and gravels

Rounded and angular (depend on degree of wear)

Fragments of rock, quartz or jespar, iron oxide, calcite, mica

Equidimentional shape; crystalline structure of mineral

Cohesionless

Fine grain

Particle size < 0.06mm; silts and clays

Flaky shape; large surface area

Very fine sulphides and oxides

Organic (infrequent)

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British Standard Range of Particle Sizes

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Very coarse

soils

BOULDERS > 200 mm

COBBLES 60 - 200 mm

Coarse

soils

G

GRAVEL

coarse 20 - 60 mm

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

Fine

soils

M

SILT

coarse 0.02 - 0.06 mm

medium 0.006 - 0.02 mm

fine 0.002 - 0.006 mm

C CLAY < 0.002 mm

Soil Classification- Basic Soil Type Group

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Gravel fraction(BSCS)

The fraction of a soil composed of particles between the sizes of 60 mm and 2 mm. The gravel fraction is subdivided as follows:

Coarse gravel 60 mm to 20 mm

Medium gravel 20 mm to 6 mm

Fine gravel 6 mm to 2 mm

Sand fraction(BSCS)

The fraction of a soil composed of particles between the sizes of 2.0 mm and 0.06 mm. The sand fraction is subdivided as follows:

Coarse sand 2.0 mm to 0.6 mm

Medium sand 0.6 mm to 0.2 mm

Fine sand 0.2 mm to 0.06 mm

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Silt fraction(BSCS)

The fraction of a soil composed of particles between the sizes of 0.06 mm (63 m) and 0.002 mm. The silt fraction is subdivided as follows:

Coarse silt 0.06 mm to 0.02 mm

Medium silt 0.02 mm to 0.006 mm

Fine silt 0.006 mm to 0.002 mm

Clay fraction(BSCS)

The fraction of a soil composed of particles smaller

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Very coarse

soils

BOULDERS > 200 mm

COBBLES 60 - 200 mm

Coarse

soils

G

GRAVEL

coarse 20 - 60 mm

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

Fine

soils

M

SILT

coarse 0.02 - 0.06 mm

medium 0.006 - 0.02 mm

fine 0.002 - 0.006 mm

C CLAY < 0.002 mm

British Soil Standard Range of Particle Sizes and Classification

Unified Soil Classification System

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ASTM Particle Size Classification (ASTM D2487)

Sieve Size Particle Size

Passes Retained on (inch) mm

12 in. >12 >300 Boulder

Rock Fragment 12 in (300 mm) 3 in. 3 12 75 300 Cobble

3 in. (75 mm) in 0.75 3 19.0 75 Coarse gravel

Soil

in. (19 mm) #4 0.19 0.75 4.75 19.0 Fine gravel

#4 (4..75 mm) #10 0.079 0.19 2.00 4.75 Coarse sand

#10 (2.00 mm) #40 0.017 0.079 0.425 2.00 Medium sand

#40 (0.425 mm) #200 0.003 0.017 0.075 0.425 Fine sand

#200 (0.075 mm) < 0.003 < 0.075 Fines (silt + clay)

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Plasticity of fine grain soils

Ability to undergo unrecovered deformation without cracking or crumbling

Due to presence of high clay content

Ratio of mass of water to mass of soil

Water reduction; Liquid>plastic>semi-solid

Cohesion; negative pressure>capillary action>suction

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Consistency Relationship of Cohesive Soil

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Liquid limit Two main types of test are specified.

The first is the cone penetrometer method, which is fundamentally more satisfactory than the alternative because it is essentially a static test depending on soil shear strength. It is also easier to perform and gives more reproducible results.

The second is the much earlier Casagrande type of test which has been used for many years as a basis for soil classification and correlation of engineering properties. This test introduces dynamic effects and is more susceptible to discrepancies between operators.

For both types of test an alternative rapid one-point procedure is given, which may give less accurate results.

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Liquid limit Cone penetration (Penetrometer) Method;

This method covers the determination of the liquid limit of a sample of soil in its natural state, or of a sample of soil from which material retained on a 425 m test sieve has been removed.

Proceed from drier to wetter state; range approx. from 15mm to 25mm; water content against penetration graph (20mm defines the liquid limit)

The amount of water added shall be such that a range of penetration values of approximately 15 mm to 25 mm is covered by the four or more test runs and is evenly distributed.

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Penetration Water Content Graph

Liquid limit Casagrande Method

This is an alternative method for the determination of the liquid limit of a sample of natural soil, or of a sample of soil from which material retained on a 425 m test sieve has been removed.

Flat metal cup; grove; dropping of cup

Turn the crank handle at the rate of 2 revolution/sec so that the cup is lifted and dropped, counting the number of bumps. Continue until the two parts of the soil come into contact at the bottom of the groove along a distance of 13 mm, measured with the end of the grooving tool or with a ruler. Record the number of bumps at which this occurs.

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Flow Curve - Interpolate Liquid Limit water content at 25 blows

Log N

Plastic limit This method covers the determination of the plastic limit of a soil

sample, i.e. the lowest moisture content at which the soil is plastic. The sample shall be of soil in its natural state, or of soil from which material retained on a 425 m test sieve has been removed.

flat, glass plate, smooth and free from scratches, on which threads are rolled. A convenient size of plate is about 10 mm thick and 300 mm square.

A length of rod, 3 mm in diameter and about 100 mm long.

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Plastic limit Roll the thread between the fingers, from finger-tip to the second

joint, of one hand and the surface of the glass rolling plate. Use enough pressure to reduce the diameter of the thread to about 3 mm in five to 10 complete, forward and back, movements of the hand. Some heavy clays will require 10 to 15 movements when the soil is near the plastic limit because the soil hardens at this stage. It is important to maintain a uniform rolling pressure; do not reduce the pressure as the thread diameter approaches 3 mm.

Pick up the soil, mould it between the fingers to dry it further, form it into a thread and roll it out again

Repeat until the thread shears both longitudinally and transversely when it has been rolled to about 3 mm diameter, as gauged by the rod. Do not gather the pieces of soil together after they have crumbled, in order to reform a thread and to continue rolling; the first crumbling point is the plastic limit.

Determine water content

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Organic matter

Originates from plant and animal; decomposed

Topsoil < 0.5m from surface

Peat fibrous organic material

Undesirable properties (engineering)

High absorbance and compressibility (why), low bearing capacity, settlement, shear failure

High cost for stabilization

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41

Water

Ever present, fundamental,

Substantial influence on soil properties

Seepage and permeability (pros and cons)

Compressibility, containment, drainage

Pressure on retaining system

Shear strength

Chemical reaction (sulphate ions; Portland cement concrete), harmful to structures

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Nature and structure of clay minerals

Weathering of felspars and micas

Layer-lattice minerals; small, flaky; ion of silicon, magnesium, aluminium

Four main groups; kaolinite, illite, montmorillonite, vermiculite

Kaolinite weathering of felspar; kaolin and china clay

Illite degradation of micas under marine conditions; shale and marine clay

Montmorillonite further degardation of illite; high swelling and shrinkage

Vermiculite weathering from biotite and chloride; swelling and shrinkage

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Some important properties of clay minerals

Surface area; large with lesser weight

Surface charge and absorption; ability to absorb water

Base exchange capacity; ability to absorb water

Flocculation and dispersion; thin layer structure and high liquid limit

Swelling and shrinkage; absorption and dispersion of water

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Soil quality

Detrimental effects on embedded structures; testing samples of ground water

Soluble sulphates

Reacts with certain constituents of Portland cement; inhibits hardening and disruption to aggregate binding process

Organic acids; in peat soil

Reacts with lime in cement to form calcium salts; deteriorate concrete with high water/cement content

pH value; presence of industrial waste or other pollutants

Corrosion of buried iron, steel and some concrete 84

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Physical Properties of Soil

Soil texture

Soil structure

Soil color

Bulk density

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Particle size distribution; sieve analysis; description of type of soil eg poorly or well graded, sandy, silty, gravel

Engineering properties

Uniformity Coefficient CU ; measure particle size range CU = d60 / d10 ; Hazen Coefficient; permeability; d60 maximum

size of the smallest 60 % of the sample

Coefficient of Curvature or Gradation CC or CZ or CG = ( d30 )

2 / ( d60 x d10 ) ; measure the shape of the particle size curve

Coefficient of Permeability k = CK ( d10 )

2 m/sec ; CK is coefficient of permeability range from 0.01 to 0.015

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87

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 , d30 and d60 Uniformity coefficient CU = d60 / d10 Coefficient of gradation CC or CG or CZ = ( d30 )

2 / ( d60 x d10 )

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Grading characteristics

45

Both CU and CC or CG or CZ will be 1 for a single-sized soil CU > 5 indicates a well-graded soil CU < 3 indicates a uniform (one type) soil

CC between 0.5 and 2.0 indicates a well-graded soil CC < 0.1 indicates a possible gap-graded soil

Two coefficients the uniformity coefficient, CU , and the coefficient of curvature CC or CG or CZ , are used to characterize the particle size distribution.

Poorly graded soils have uniformity coefficient < 4 and steep gradation curves (USCS).

Well-graded soils have uniformity coefficients > 4, coefficients of curvature between 1 and 3, and flat gradation curves(USCS).

Gap-graded soils have coefficients of curvature < 1 or > 3, and one or more humps on the gradation curves(USCS).

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A - a poorly-graded medium SAND B - a well-graded GRAVEL-SAND (i.e. equal amounts of gravel and sand) C - a gap-graded COBBLES-SAND D - a sandy SILT E - a typical silty CLAY

Typical grading curves

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46

Very coarse

soils

BOULDERS > 200 mm

COBBLES 60 - 200 mm

Coarse

soils

G

GRAVEL

coarse 20 - 60 mm

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

Fine

soils

M

SILT

coarse 0.02 - 0.06 mm

medium 0.006 - 0.02 mm

fine 0.002 - 0.006 mm

C CLAY < 0.002 mm

Soil Classification- British Basic Soil Type Group

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Figure 1.7 Plasticity Chart: British System (BS 5930: 1981)

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Soil Classification Example Using British and Unified Soil Classification System

Example Using British and Unified Soil Classification System

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For classification of Fine grained soils and Fine-grained fraction of Coarse-Grained

Soils Equation of A Line: Horizontal at PI = 4 to LL = 25.5, then PI = 0.73 (LL-20) Equation of "U" Line: Vertical at LL = 16 to PI = 7, then PI = 0.9 (LL-8)

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Fine- grained soils can exist one of four states: solid, semisolid, plastic and liquid.

Water is the agent that is responsible for changing the states of soils.

A soil gets weaker if its water content increases.

Three limits are defined based on the water content that causes a change of state.

These are the liquid limit the water content that caused the soil to change from liquid to a plastic state;

The plastic limit the water content that cause the soil to change from plastic to semisolid; and

The shrinkage limit the water content that caused the soil to change from a semisolid to a solid state.

All these limiting water contents are found from laboratory tests

The plasticity index defines the range the water content for which the soil behaves like a plastic material.

The liquidity index gives a measure of strength.

The soil strength is the lowest at the liquid state and the highest at the solid states.

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99

Liquid Limits

Ranges from 35% to 55% moisture content

Normally consolidated soils, (consolidated or densified under their own weight). If deposited through gentle agitation from slow moving rivers, deltaic fans, ice melt or quiet marine shore conditions, the LL value can be much lower. The soil can be classed as sensitive.

Ranges from 60% to 100% moisture content

Over consolidated soils, (subjected to high surcharge loads, such as a thick layer of ice, rock formations that have been denuded or eroded away, or subject to many wetting and drying cycles).

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Plastic Limits

Low values below 10% tell us that the soil is normally consolidated, but sensitive.

Values between 10% and 25% tell us that the soil is normally consolidated and medium to low sensitivity.

Values above 30% moisture content tell us that the soil is over consolidated and insensitive.

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Low Plasticity WL= < 35%

Intermediate High Plasticity WL= < 35% - 50%

High Plasticity WL= < 50% - 70%

Very High Plasticity WL= < 70% - 90%

Extremely High Plasticity WL= > 90%

Plasticity Chart and Classification

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Soil Description and Classification

Textural classification

Textural soil classification chart (USDA)

Index line for clay is horizontal

Index line for silt is down and towards the left

Index line for sand is upwards towards the left

When all index lines meet, the intersection point descript the soil

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Sieve No Opening (mm) Sieve No Opening (mm)

4 4.75 35 0.500

5 4.00 40 0.425

6 3.35 50 0.355

7 2.80 60 0.250

8 2.36 70 0.212

10 2.00 80 0.180

12 1.70 100 0.150

14 1.40 120 0.125

16 1.88 140 0.106

18 1.00 170 0.090

20 0.850 200 0.075

25 0.710 270 0.053

30 0.600 0.6 mm

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U.S. Standard Sieve Sizes

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US Standard Sieve Size British Standard Sieve Size

3 in. (75mm) 75 mm

2 in. (50.0mm) 63 mm

1.5 in. (38.1mm) 50 mm

3/4 in. (19.0mm) 37.5 mm

in. (12.5mm) 20.0 mm

3/8 in. (9.5mm) 14.0 mm

No. 4 (4.75mm) 10.0 mm

No. 10 (2.0mm) 6.3 mm

No. 20 (0.85mm) 5.0 mm

No. 40 (425mm, ) 2.0 mm

No. 60 (0.25mm) 1.18 mm

No. 100 (0.15mm) 0.6 mm

No. 200 (75m, 0.075mm) 0.425 mm

0.3 mm

0.212 mm

0.15 mm

0.063 mm

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BSS, ASTM and ISS Aperture

APERTURE SIZE

BSS Mesh

No.

(410/1969)

ASTM Mesh

No.

(11-70)

ISS

(469/1972)

Microns

4 5 4.00mm 4000

5 6 3.35mm 3353

6 7 2.80mm 2812

7 8 2.36mm 2411

8 10 2.00mm 2057

10 12 1.70mm 1680

12 14 1.40mm 1405

14 16 1.18mm 1204

16 18 1.00mm 1003

18 20 .850mm 850

22 25 .710mm 710

25 30 .600mm 600

30 35 .500mm 500

36 40 .425mm 420

44 45 .355mm 355

52 50 .300mm 300

60 60 .250mm 250

72 70 .212mm 210

85 80 .180mm 180

100 100 .150mm 150

120 120 .125mm 120

150 140 .106mm 105

170 170 .090mm 90

200 200 .075mm 75

240 230 .063mm 63

300 270 .053mm 53

350 325 .045mm 45

400 400 .037mm 37

500 .025mm 35

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Phase Relationship/Basic Properties

Serve as indices for better description of soils; physical state

Soil can be in 2 or 3 phases composition

Solid, liquid and gas; phase diagram

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108

(a) Soil element in natural state; (b) three phases of soil element

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109

Three separate phases of soil element with volume of solids equal to one

Saturated soil element with volume of soil solids equal to one

S : Solid Soil particle

W: Liquid Water (electrolytes)

A: Air Air

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Phase Diagram

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112

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113

114

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115

Phase Diagram

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59

Compaction

Compaction is the densification of a soil by the expulsion of air and rearrangement of soil particles.

The Proctor test is used to determine the maximum dry unit weight and the optimum water content and serves as the reference for field identifications of compaction.

Higher compaction effort increases the maximum dry unit weight and reduces the optimum water content.

Compaction increases strength, lowers compressibility and reduces the permeability of soils.

A variety of field equipment is used to check the dry unit weights achieved in the field. Popular field equipment includes the sand cone apparatus, the balloon apparatus and the nuclear density meter.

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Compaction Rolling and tampering

Reduction of air-void volume

No change in solid volume and water content; increase in density

Effect permeability

Increase shear strength; bearing capacity

Reduce settlement and damage to structures

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Compaction

Effectiveness

Nature and type of soil

Water content during compaction

Maximum possible state of compaction; attainable and field conditions

Type of construction plant

Sheep foot roller

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Compaction

Degree of compaction

Depend on maximum dry/water content

Increase in water allows soil particles to be pack more closely; increase in density; beyond certain water limit/content density reduces

Maximum dry density; at optimum moisture content

Maximum possible state of compaction; attainable and field conditions

Construction specification; 90% to 95% of the optimum moisture content; locations and usage of fill ground

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Degree of compaction

Proper compaction is very important. The degree of compaction depends on the soil type, compaction method, compactive effort and the as-compacted moisture content.

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Compaction When clay is compacted than optimum moisture content, clay tends to

have a flocculated fabric consisting of platy particles oriented randomly. When is compacted wetter than optimum moisture content, clay tends to have a more oriented or dispersed fabric, in which platy particles are aligned parallel to one another.

Difference in soil fabric leads to differences in various soil properties,

eg. Drier than optimum moisture content will give higher hydraulic conductivity than clay with wetter than optimum moisture content

Eg. Drier than optimum moisture content will have a greater shear strength than wetter than clay with wetter than optimum moisture content

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123

Compaction

Effect of increased compaction effort

Maximum dry density increases

Optimum moisture content decreases

Air-void content remain the same

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Soil type and its effect on the optimum moisture and density

Well graded; high density

Plasticity and % of fine increase, lower density

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Field Density Measurement

Core cutter method

Sand material

Steel rammer and dolly; 100mm dia. and 130mm long

Sand replacement method

Dig a hole ; 150mm

Place in sand to measure volume of hole

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Field Density Measurement

127

Core cutter method Sand material Steel rammer and dolly; 100mm dia. and 130mm long

Sand replacement method Dig a hole ; 150mm Place in sand to measure volume of hole

End of Module 1