SOIL MECHANICS I

What is Soil ? In general sense of engineering , soil is

defined as the Un –Cemented or weakly cemented accumulation of mineral particles ,which are formed by weathering (disintegration) of rocks. The mineral particles can be inorganic (such as sand, silt ,clay)or organic such as peat),in nature.

The weathering action on rocks can be either physical (mechanical) disintegration or chemical decomposition.

Cont.. Physical disintegration of rocks occurs due to actions such

as temperatures changes, spreading of plant roots, abrasion.

The soil formed has the property of the parent rock. Chemical decomposition when chemical decomposition

(chemical weathering ) takes place ,original rock minerals are transformed in to new minerals by chemical reactions.

The soil formed do not have the properties of the parent rock.

All man made structures ,except those which float or fly, are supported by natural soil or rock deposits.

What is Soil Mechanics Soil Mechanics is The branch of Science

that deals with the study of the physical properties of soil and the behavior of soil mass subjected to various types of forces.

In other words ,Soil Mechanics is the Study of both solid and fluid mechanical characteristics of Soils.

Why do ,as civil engineers ,study “soil Mechancis”?

In brief ,all branches of Civil engineering require an Understanding of soil and how it behaves

NamelyStructural Engineering Transportation Engineering Environmental Engineering Hydraulic Engineering

Structural Engineering -virtually all civil Engineering type structures

eventually come into contact with soil via their foundation (i.e. bridges, offices and residence buildings ,chimney towers, transmission line towers etc

Knowledge of soil mechanics is essential to assure that the structures are properly supported . This can avert i)structural damage and failure , loss of life, financial loss

.           

Transportation engineering

-Road beds are often built of soil , and the roadways themselves can often pass through mountains , cuts, fills, etc

Hydraulic Engineering

The design of earthen flow retention structures such as dams ,requires a knowledge of how water is transported through soil.

FORMATION OF SOIL  As defined earlier soil is formed by physical

disintegration or chemical decomposition of rocks.

o Soil can be grouped in to two broad categories (depending on the method of deposition )

Residual – Formed from weathering of rock and remain at the location of their origin

Transported – those materials that have been moved from their place of origin

-by agencies like ,gravity ,water glaciers, or man –either singularity or in combination

Depending on mechanism of transport , transported soil can be divided as

1. Alluvial soils soils that are formed from suspension in running water (river deposits soil ) : sedimentary clays .

2. Lacustrine Soils – soils that has been deposited from suspension in lakes: lacustrine silts and lacustrine clays.

3. Aeolin soils – those transported by wind : loess

4. Glacial soils – those transported by ice : glacial till

Soil Texture and Structure Introduced

1.1  SOIL CONSTITUENTS

SOIL CONSTITUENTS

Soil Constituents

PROPORTIONS OF SOIL CONSTITUENTS

Mineral (Inorganic) Component of the Soil

.

Mineral Component Contd.

.

Sand Particles

(a) Sand particles are visible to the naked eye and vary in size from (2.0 to 0.05 mm).

They are gritty when rubbed between fingers. Sand particles do not adhere to one another and are therefore not sticky.

Silt and Clay Particles

(b) Silt particles are smaller than sand and range from (0.05 to 0.002 mm).

The silt particles are too small to be seen without a microscope. It feels smooth but not sticky, even when wet.

(c) Clays are the smallest class of mineral particles (< 0.002 mm).

They adhere together to form a sticky mass when wet and form hard clods when dry.

Classification of Soil Constituents

USDA ISSS

ROCKS > 2 mm > 2 mm

SAND 0.05 to 2 mm 0.02 to 2 mm

SILT 0.002 to 0.05 mm

0.002 to 0.02 mm

CLAY < 0.002 mm < 0.002 mm

Soil Texture

  This refers to the relative proportions of the various soil separates (sand, silt and clay) in a soil.

Terms such as sandy loam, silty clay, and clay loam are used to identify soil texture.

The various soil components can be separated using Mechanical Analysis.

Sand can be separated by sieving. For silt and clay, separation is by the rate of

settling which can be obtained in a pipette or hydrometer analysis.

Soil Texture Contd.

They are put in a solution and the time of settling is noted.

From the Mechanical analysis, the proportions of sand, silt and clay are obtained.

The actual soil texture is determined using the Soil Textural Triangle ( Fig. 1.1.)

e.g. for a soil with 50% sand, 20% silt and 30% clay, the texture is Sandy Clay Loam.

1.1.1 COLLOIDAL MATERIAL

 The smaller particles (< 0.001 mm) of clay ( and similar sized organic particles) have colloidal properties and can be seen with an electronic microscope.

The colloidal particles have a very large area per unit weight so there are enough surface charges to which water and ions can be attracted.

These charges make them adhere together. Humus improves the water holding capacity of the soil.

       WATER

Water is held in the soil at various degrees of tenacity which varies with the amount of water present.

The tension force of water in unsaturated soils has been described by several expressions such as soil-pull, the force of suction and capillary tension. Suction can be defined as the force per unit area that must be exerted to remove water from the soil. Suction or tension is measured in bars.

When the soil is saturated or nearly so, the amount of suction is almost zero, but as the soil water depletes, greater amount of energy must be applied to extract water.

The soil can be initially saturated and if it is drained, field capacity can be reached. Some water is still held by surface tension.

Field Capacity

Field Capacity is greatest amount of water the soil can hold under drainage.

For most soils, it is obtained after two days of drainage after the soil was saturated by heavy rain or irrigation.

It is the optimum amount of water needed for agriculture.

Permanent Wilting Point and Available Water

Below Field capacity, the plant finds it more and more difficult to extract water until the suction or tension reaches 15 atmospheres permanent wilting point is obtained, which is the maximum tension the plant can exert on the soil to extract water.

Available water is the difference between the moisture contents at field capacity and permanent wilting point.

Clay holds more water but the plants exert more tension to extract water more than sand.

SOIL AIR (a) From a mechanical point of view, air is

irrelevant as it cannot support load. Soil air is different from atmospheric air. It is found in voids around the soil solids. 

(b) Soil air has a very high relative humidity when compared with atmospheric air

c) It has high carbon dioxide levels and low oxygen levels in comparison with atmospheric air

Aeration Effects (a)  Aeration affects root development by three main

factors (i)  By oxygen content: There is restricted root

development if oxygen falls below 9%. If it drops to less than 5%, root development ceases.

  (ii) Carbon dioxide content: the roots will survive up

to 9 to 10%. The optimum is < 1%.   By-Products of anaerobic decomposition: Hydrogen

sulphide is very toxic at low concentrations and kills roots. Methane and hydrogen are all right at reasonable concentration

SOIL STRUCTURE This is a field term that describes the

aggregation of the soil primary particles. Managing soil is managing soil structure. The soil structural types include:

  (a) Massive Structure: No aggregation

e.g. big block of clay or mass of sand grains. In clay, the more the dryness, the deeper the cracks from the surface and the following structure is obtained:

Soil Structural Types Contd.

(b)   Prismatic or Columnar Structure : Caused by vertical cracking

If there is secondary horizontal cracks, the following structure is obtained:

  (C) Blocky Structure: (b)   If the blocky structure is worked upon by man,

the spheriodal or granular or crumb structure is obtained.

  (c)   Platelike Structure

Methods of Describing Soil Structure:

1.3.2  Describing structure is difficult but the following methods can be used:

(i) Virtual Account using the above terminologies and stating size ranges can be adopted. Absolute quantification is not possible.

(ii) Clod Size Distribution: Especially using photographs. The sizes can be read using a scale. Aerial, digital or stereo photographs can also be used.

(iii) Dry Sieving: This is aggregate grading and can only be adopted for dry soils as wet soils cannot be dry-sieved. Obtain a grading curve.

Stack of Sieves For Dry Sieving

    Aggregate Stability:  This is practically very important due

to the following effects:  (i)  Rainfall (iii) Tillage tools and wheels (iv) Water tables  There is a threshold value above which

it is impossible to have aggregate stability. Crops need small aggregates so that they can be stable.

  Tests For Aggregate Stability

(i) Wet Sieving: Preparing soil aggregates of a given size e.g. 4 – 5 mm, putting them on a set of sieves and agitating in water. The percentage of soil aggregates that are retained on each sieve are obtained so that water stable aggregates > a particular sieve size e.g. 5 mm can be computed.

Tests For Aggregate Stability Contd. (i) Drop Tests: Water drops, say 3 mm

diameter are dropped on a soil aggregate of a given size say 4 to 5 mm placed on a sieve of about 3 mm size.

The number of water drops needed to break down each aggregate so that it can be washed through the sieve size is used as an index of water aggregate stability. Many replications are needed for each soil.

(ii) Childes/Haines Method: Flood with water from below and compare for various soils. This is not a popular method.

SOIL PHASE RELATIONS

Air

Water

Solids

Va

Vw

Vs

Vp

V

Va is the Volume of air, Vw is the Volume of waterVs is the Volume of solids, Vp is the Volume of pores (water & air)

V is the total volume (air, water and solids)

Definitions: Density of Solid

Particles, Dp (Particle density):

(a) It is defined as the mass per unit volume of soil solids usually expressed as g/cm3 or Mg/m3 .

i.e. Particle density = Ms/Vs where Ms is the mass

of dry soil Particle density is essentially the same as the

specific gravity of a soil substance and varies within a narrow range of 2.60 to 2.75 g/cm3 with the average being assumed as 2.65 g/cm3 for a typical soil with 1 to 5% organic matter content.

Montmorillonite can have a particle density of 2.74 g/cm3, quartz , 2.65 and kaolinite, 2.61 g/cm3.

(b) Bulk Density, Db:

It is defined as the mass of a unit volume of dry soil. This includes both solids and pores. i.e. bulk density = Ms/V ; Ms is the mass of dry soil and V is the total volume of undisturbed soil.

The major method of measuring bulk density in the field is to collect a known volume of undisturbed soil (V) in a soil core, and drying it in the oven to remove all the water to obtain Ms. The values of bulk density range from 1.0 for loose open soil to 1.7 g/cm3 for compacted soil.

Values of bulk density are mainly affected by soil texture (sandy soils have more density than silty and clay soils), degree of soil aggregation and is reduced by soil organic matter content.

c) Porosity

It is defined as the proportion of the volume of soil pores (air and water) in comparison with the total volume of soil.

i.e. Porosity, P = Volume of Pores, Vp / V It normally ranges from 0.2 (20%) to 0.6

(60%). One of the main reasons for measuring soil

bulk density is that this value can be used to calculate soil porosity.

For the same particle density, the lower the bulk density, the higher the porosity.

Derivation of Formula Used to Calculate Porosity of Soil

Solving for Ms gives: Ms = Dp x Vs and Ms = Db (Vs + Vp)

Therefore: Dp x Vs = Db (Vs + Vp) and

`Since;

Since: Pore space + Solid space = 1 and Pore space (Porosity) = 1 – Solid space , then:

By definition Particle density DM

Vand Bulk density D

M

V Vps

sb

s

s p

: , ,

V

V V

D

Ds

s p

b

p

V

V Vsolid space then solid space

D

Ds

s p

b

p

Porosity PD

Db

p

, 1

More Definitions of Soil Phase Relations

(d) Void Ratio (e): It is defined as the volume of Pores (Vp) divided by the volume of solids, Vs. It

can be shown that:

(e) Degree of Saturation, S: This is the volume of water divided by the volume of voids. A soils is said to be saturated when all pores are filled with water i.e. when: Vw = Vp

eP

Pand P

e

e

1 1

Soil Wetness This can be expressed by mass or by

volume. (i) By Mass (Pm) – gravimetric system: This

is equal to: Weigh soil, put in oven at 105 oC for about

48 hours; weigh again and obtain the weight of water by subtraction.

A good soil should have moisture contents between 5 and 60% and for peat or organic soils, it can be greater than 100%.

Soil Wetness by Volume

(ii) Volumetric water content, Pv. This is equal to:

Recall that volume = mass/density i.e.

Since Dw = 1

i.e. Pv = Pm x Db where Db is the bulk density of the soil.

PV

V V V

V

V

Volume of water

Total volume of undisturbed soil samplevw

s a w

w

PM D

M Dand P

M

Mx Dv

w w

s bv

w

sb

/

/

Example

A moist sand sample has a volume of 450 cm3 and a wet mass of 786 gm. The particle density is 2.65 g/cm3 and the dry mass is 731 gm. Determine the void ratio, porosity, percentage water content and the degree of saturation.

Solution:

Mass of water = 786 – 731 = 55 gm Volume of water = Mw/Dw = 55 cm3,

assuming Dw = 1gm/cm3

Mass of solids = 731 gm and Volume of solids = Ms/Ds = 731/2.65 = 275.85 cm3

Volume of air, Va = V – Vs – Vw = 450 – 275.85 – 55 = 119.15 cm3.

Volume of pores, Vp = Vw + Va = 55 + 119.15 = 174.15 cm3

Solution Concluded

(a) Void ratio, e = Vp/Vs = 174.15/275.85 = 0.63

(b) Porosity, P = Vp/V = 174.15/450 = 0.30

c) Percentage water content = Mw/Ms = (55/731) x 100 = 7.52%

(d) Degree of saturation = Vw/Vp = 55 / 175.15 = 31.4%

SOIL PROPERTIES AND MOISTURE CONTENT

Moisture content is a critical factor for soil behaviour.

In a soil tillage context, it is not a good measure e.g. 40% moisture content does not say anything about the degree of wetness.

For clay, the soil may be moist at 40% moisture content, but for sandy loam soil, it can be flowing as a liquid.

Soil tension is also not useful.

Soil Consistency

Soil consistency is the best parameter for predicting soil behaviour during tillage operations.

For instance, all soils behave in the same way at the same consistency limit.

If the plastic limit of the soil is 35% and it is cultivated at 30%, then the condition is friable.

Soil Consistency Contd.

Soil Consistency describes how a soil behaves in contact with water.

It can be referred to as rheology. Consistency is the nature and behaviour of a soil.

It states whether the soil is hard, friable, firm, plastic or liquid when it comes in contact with water.

Diagram Depicting Soil Consistency

CementedHarshHard

FriablePlastic

StickyLiquidMud

VeryHard

Very WetIncreasing Moisture Content

Shrinkage limit

Lower PlasticLimit (Plastic Limit)

Upper PlasticLimit (Liquid Limit)

Soil Consistency

Soil Consistency Contd.

The soil can behave as a solid or fluid depending on moisture content.

When dry, the soil has a cemented, harsh or hard consistency.

When very wet, it is sticky, liquid and muddy. The two extremes are not good for soil

cultivation, one too wet and the other too dry. In-between is better.

First, there is the friable zone in which the soil can be manipulated easily. It is the ideal soil working zone.

Soil Consistency Concluded

Most soils with clay have plastic limits between the friable and sticky or liquid zones.

Sand has not got any plastic limit because sand particles are inert and have no links with water.

Same goes for silt. For a soil with up to 10% clay content, clay

begins to dominate soil behaviour and plasticity sets in.

Atterberg Limits

Atterberg defined limits of the plastic limits as follows:

(a) Lower Plastic Limit: (LPL): Also called Plastic Limit. Significant water has been added to provide a film around each particle. Film cohesion is at a maximum just above the lower plastic limit. It is the minimum moisture content at which the soil may be puddled.

In the laboratory, the LPL or plastic limit is determined by adding soil moisture and at each point, rolling the soil out and remoulding up until it breaks out at 10 mm length and 3 mm diameter. For higher moisture content, it can be longer.

Diagrams For Lower Plastic Limit

Moisture Content

Cohesion

LPL UPL

3 mm diameter10 mm

Upper Plastic Limit (UPL)

This is also called liquid limit. This signifies the moisture content at which the water films are so thick that the film cohesion is decreased and the soil mass can flow under an applied load.

The point of maximum adhesion called the sticky point is almost the same as the UPL.

Diagram of Upper Plastic Limit

Sticky Point

Adhesion Lubrication

FrictionPhase

Force

Moisture Content

Determination of Upper Plastic Limit

(i) Drop Cone Penetrometer: The soil is prepared in a core. A cone penetrometer is dropped and allowed to penetrate the soil. The UPL is the soil state or condition where the depth of penetration is 20 mm measured with standard methods.

(ii) Cassangrande Apparatus: The soil is put into a cup. A V-shaped groove of given dimensions is made on the soil and the cup is cranked so that it falls through 1 cm onto a rubber base, twice per second. The impact blows causes the soil to flow and close the groove. The UPL is reached when the soil is closed a distance of 12.5 mm by 25 blows.

Liquid Limit Apparatus

Plasticity Number or Index

It is the difference between the upper and lower plastic limits.

It gives the range of soil moisture where the soil will behave plastically.

The larger the plasticity index, the smaller the apparent friable zone.

If it is ten percent for instance, the soil is good for cultivation.

If it is 100 to 150, then the soil is bad for working.

Factors that Affect the Atterberg Limits

(a) Clay Content: The higher the clay content, the higher the LPL, UPL and the plasticity number. Increase in plasticity number is due to the greater rate of increase of UPL more than LPL.

(b) Nature of Exchangeable Cations: The ions linked to clay affect the orientation of water molecules.

Some like phosphate lowers LPL as it discourages thick oriented water layers while others like sodium encourages thick water layers and therefore increase the three Atterberg limits.

Factors that Affect the Atterberg Limits Concluded (c) Nature of the Clay Mineral: The nature

of the clay mineral affects the swelling and shrinking nature of the clay and the Atterberg limits. For montmorillonite, there is a lot of swelling and shrinkling and the plastic limits are very high in comparison with the non-swelling illite or kaoline clays.

(d) Organic Matter: For higher Organic matter content of the soil, higher lower plastic limits are obtained. The soil becomes difficult to work as organic matter level falls.