Soil Mechanics - Civil Engineering

Soil Mechanics 

Civil Engineering

Mostly four types of rollers used are:

• Pneumatic tired roller
• Tamping roller/ sheep foot roller
• Smooth wheel rollers
• Vibratory Roller

Pneumatic tired roller: 

• Pneumatic tired roller has a number of rubber tires at the front and at the rear end. A pneumatic tired roller can be used for highways, construction of dams, and for both fine-grained and non-cohesive soils.

Tamping roller/sheep foot roller:

•  Sheep foot roller also named tamping roller. The front steel drum of the sheep foot roller consists of many rectangular-shaped boots of equal sizes fixed in a hexagonal pattern. In sheep foot, roller compaction is by static weight and kneading of the respective layer. This makes tamping roller better suited for clay soils.

Smooth wheel rollers:

•  Smooth wheel roller and vibratory rollers are the same. Both have the same characteristics. Only the difference in both is vibratory equipment. Smooth wheel roller has no vibrator attached to the drum. This makes smooth wheel roller best suited for rolling of weaker aggregates, proof rolling of subgrades, and compacting asphalt pavements.

Vibratory Roller:

•  Vibratory type rollers have two smooth wheels/drums plus the vibrators. One is fixed at the front and the other one is on the rear side of the vibratory roller. Vibration is to reduce the air voids and to cause densification of granular soils. During the vibration of the soil layer, rearrangement of particles occurs due to the deformation of the granular soil because of the oscillation of the roller in a cycle.

• ∴ Smooth wheel roller is most suitable for proof rolling subgrades and for finishing operation of fills with clayey or sandy soils.

Kaolinite:

• The basic Kaolinite mineral is a two-layer unit that is formed by stacking a gibbsite sheet on a silica sheet
• These basic units are then stacked one on top of the other to form a lattice of the mineral
• These units are held together by hydrogen bonds
• The strong bonding does not permit water to enter the lattice
• Thus, kaolinite minerals are stable and do not expand under saturation

Liquid LImit of Soil

• Liquid Limit of the soil is the moisture content at which water will cause plastic soil to behave as a liquid.

• Liquid limit is measured by Casagrande’s Apparatus and the water content corresponding to the 25 number of blows is its liquid limit.

At the liquid limit, the shear strength of the soil is very low.

• ∴ The liquid limit cannot indicate shear strength.

• Liquid limit is the indication of the compressibility as at water content above the liquid limit, soil structure would flow like a liquid and no compression would be possible.

• Compression of soil structure is possible only for moisture content below the liquid limit.

Retaining walls

• Retaining walls are provided to retain the earth laterally where either there is a difference in earth level on two sides or the earth is situated in slope.

• The retained earth is sometimes called Backfill.

Note:

• For designing a retaining wall, the retained earth or the backfill is assumed as

• Dry, free from moisture, cohesionless, and consists of granular particles.

• These are mainly assumed to simplify the calculation of lateral earth pressure and to provide drainage through the retaining wall.

Soil Types

• The soils formed at a place may be transported to other places by agents of transportation, such as water, wind, ice and gravity.

1) Water transported Soils:

•  Flowing water is one of the most important agents of transportation of soils. Swift running water carries a large quantity of soil either in suspension or by rolling along the bed.

• The size of the soil particles carried by water depends upon the velocity.

• All type of soils carried and deposited by water are known as alluvial deposits. Deposits made in lakes are called lacustrine deposits. Such deposits are laminated or varved in layers. Marine deposits are formed when the flowing water carries soils to ocean or sea.

2) Wind transported Soils:

•  Soil particles are transported by winds. The particle size of the soil depends upon the velocity of wind. Soils deposited by wind are known as aeolian deposits.

• Loess is a silt deposit made by wind. These deposits have low density and high compressibility. The bearing capacity of such soils is very low.

3) Glacier-Deposited Soils:

•  Glaciers are large masses of ice formed by the compaction of snow. As the glaciers grow and move, they carry with them soils varying in size from fine grained to huge boulders.

• Drift is a general term used for the deposits made by glaciers directly or indirectly. Deposits directly made by melting of glaciers are called till. The soil carried by the melting water from the frint of a glacier is termed out-wash.

4) Gravity deposited soil:

•  These are soils transported through short distances under the action of gravity. Colluvial soils such as talus have been deposited by the gravity. Talus consists of irregular, corase particles. It is a good source of broken rock pieces and coarse grained soils for many engineering works.

• D10 = Dia corresponding to which 10% particles are finner

• D30 = Dia corresponding to which 30% particles are finner

• D60 = Dia corresponding to which 60% particles are finner

• The simplified Bishop method uses the method of slices to discretize the soil mass and determine the factor of Safety.

• In this method, it is assumed that the resultant forces on the sides of the slices are horizontal. Bishop’s simplified method considers the interslice normal forces but neglects the interslice shear forces.

It further satisfies vertical force equilibrium to determine the effective base normal force and it is assumed that for each slice the resultant of the interslice forces is zero.

Piezometric head (H) = Pressure head (h’) + datum head (z)

At critical condition,

• Uplift pressure = downward pressure

The Indian Standard Equivalent of the Standard Proctor Test is called the light compaction test (IS: 2720 Part VII - 1974).

The Indian Standard Equivalent of the Modified Proctor Test is called the heavy compaction test (IS: 2720 Part VII - 1983).

In compaction test:

Property IS light compaction test IS Heavy Modified compaction test

 Weight of hammer         2.6 kg 4.9 kg

Number of Layers 3          5

Number of blows 25         25

Hight of fall         310 mm 450 mm

Volume of mould 1000 cc 1000 cc

Penetration Test can be conducted to determine:

• Relative Density of sands
• Ultimate bearing capacity on the basis of shear criteria
• Allowable bearing pressure on the basis of settlement criteria
• Hydrometer Test can be used to determine:
• Distribution of grain size

Proctor Test can be used to determine:

• The compaction of the soil

Vane Test can be used to determine:

• In-situ shear strength of clay.

:

Plasticity index	Soil type	Degree of plasticity	Degree of cohesiveness
0	Gravel and sand	Non - plastic	Non-cohesive
< 7	silty	Low plastic	Partly cohesive
7 - 17	silt clay	medium plastic	Cohesive
> 17	clay	High plastic	Cohesive

Coefficient of Active Earth Pressure Coefficient:

Ka=1−sin(ϕ)1+sin(ϕ)=tan2(45−ϕ2)

Coefficient of Active Earth Pressure Coefficient:

Kp=1+sin(ϕ)1−sin(ϕ)=tan2(45+ϕ2)

Using Allen Hazen’s equation:

k=CD210

• Where is C is constant and 100 ≤ C ≤ 150

• Generally, we take C = 100, D10 is effective grain size in cm and k = Permeability in cm/sec

• Weathering is the process of breaking down of rocks by mechanical or chemical process into smaller pieces.

• Soil can be transported from its place of origin by wind, water, ice or any other agency and has been redeposited at another place.

• Soils that had been transported by running water and deposited in the river bed and hence that will be known as alluvial soil.

• Soils that have been deposited from suspension in sea water is known as Marine deposit.

Porosity (η) is defined as the ratio of volume of voids to the total volume of the soil sample in a given soil mass.

The porosity of soils can vary widely.

• The porosity of loose soils can be about η = 50 to 60%.
• The porosity of compact soils is about η = 30 to 40%.

Coefficient of Uniformity / Uniformity Coefficient:

• It is defined as the ratio of D60 and D10 sieve sizes in sieve analysis of granular material.

• Higher is the value of Cu larger is the range of the particle size.

• For uniformly graded soil, Cu = 1

• For well graded sand, Cu > 6

• For well graded gravel Cu > 4

Coefficient of Curvature / Curvature Coefficient:

• For well graded soil, 1 < Cc < 3

• For gap graded soil, 1 < Cc or Cc > 3

Oven dry method is the most accurate and simplest method for water content determination.

In this method complete drying of soil sample occur and water content in sample is calculated accurately by a maintained temperature in the oven ( 105° C to 110° C) for 24 hours.

Note:

For highly organic soils a low temperature of about 60° C is preferable.

If Gypsum is present, the temperature should not be more than 80° C but for a long time.

Radiation method is based on energy loss during radio active isotropes material (E.g. cobalt-60) emmites on one end and received by detector on other end. This is an approximate method.

In calcium carbide method, Acetylene gas exerts pressure on the pressure gauge which indicate the water content present in soil sample. This is not an accurate method since no control over chemical reactions.

Sand bath method is field method and there is no control over heat given to soil sample. Therefore, it is not suitable for organic soil and soil having higher gypsum content. 

Hydraulic gradient 

Hydraulic gradient (i) is defined as the head loss per unit length of the specimen.

It is given by the slope of the hydraulic head between two points.

Assumptions given by Rankine are:

1. The soil is homogeneous and isotropic, which means c, φ and γ have the same values everywhere, and they have the same values in all directions at every point.

2. The most critical shear surface is a plane.

3. The ground surface is a plane.

4. The wall is infinitely long so that the problem may be analysed in only two dimensions.

5. The wall moves sufficiently to develop the active or passive condition.

6. The wall surface at the back is vertical and smooth.

• This method is based on the fact that seismic waves have different velocities in different types of soils (or rock) and besides the wave refract when they cross boundaries between different types of soils.

• In this method, an artificial impulse are produced either by detonation of explosive or mechanical blow with a heavy hammer at ground surface or at the shallow depth within a hole.

• For Seismic Method:

• a) If seismic velocity of upper strata is more than lower strata, then wave refracts more towards normal then the thickness of the strata is neglected, called blind zone effect.

• b) It can analyse the ground even if present with irregular water table, but cannot be used if the water table has frozen.

• c) It can also be used for the areas covered by concrete or asphalt pavement having a high seismic velocity.

• There are two basic design criteria’s. One is the bearing capacity criteria or the shear failure criteria another is the settlement criteria. When we design the footing, it should take the load coming from the superstructure without shear cracking.

• At the same time there should not be an excessive amount of settlement in the footing. So the pressure that we can apply on a soil is the minimum pressure on a soil in terms of shear criteria or in terms of the settlement criteria.

• Tension failure, compression failure or bond failure hardly occur in foundation structures and are never a serious concern.

Caissons

• Caissons are recommended if the area of foundation is very less than the depth of the foundation. In other words, caissons are suitable for deeper depth of water of about 12 m to 15 m.

• Caisson are acted upon by vertical loads (loads of bridges, columns, etc.), lateral loads (earth pressure and water pressure) and sinking loads (Friction, Internal air pressure, Water pressure, hogging and sagging stresses).

• Open Caisson are carried to hard stratum such as compact sand, gravel, and rock bed. Piers are drilled to the soil and hence it has high load bearing capacity.

• Floating caisson are large hollow boxes with top open and bottom closed. These are floated to the place where they are needed to be installed.

• Floating caisson have much lower load-carrying capacity than open caisson.
• Floating caisson are less expensive than open caisson.
• Its installation is quick and convenient.

Bearing capacity of soil is the maximum load per unit area. Ultimate bearing capacity of soil shown in table. Dividing the ultimate soil bearing capacity by a safety factor maximum safe bearing capacity of soil for design of foundations can be obtained.

.

Types of Soil	Bearing Capacity (kg/m2)	Bearing Capacity (kN/m2)
Soft, wet clay or muddy clay	5000	50
Soft clay	10000	100
Fine, loose and  dry sand	10000	100
Black cotton soil	15000	150
Moist clay and sand clay Mixture	15000	150
Loose gravel	25000	250
Medium clay	25000	250
Medium, compact  and dry sand	25000	250
Compact clay	45000	450
Compact sand	45000	450
Compact gravel	45000	450
Soft rocks	45000	450
Laminated rock  such as sand stone  & Lime stone	165000	1650
Hard rocks  such as granite,  diorite, trap	330000	3300

Gross pressure(qg):

•  It is the total pressure generated at the base of foundation due to self-weight of footing, self-weight of overlying soil and applied loads

qg=PB2+γDf

Net bearing capacity (qn­):

•  It is the loading intensity at the base of footing in excess of loading intensity that was originally subjected to (γDf).

qn=qg−γDf

Ultimate bearing capacity:

•  It is the maximum gross loading intensity that the soil can support before it fails in shear.

Net safe bearing capacity (qns): 

• is the maximum net pressure intensity to which the soil at the base of the foundation can be subjected without risk of shear failure.

qns=qnFOS

Allowable bearing pressure (qa)

• it  is the maximum gross pressure intensity to which the soil at the base of foundation can be subjected without risk of shear failure and excessive settlement which may be detrimental to the structure.

qa=qna+γDf

Type of Footing and Soil	Settlement and contact pressure
Rigid footing and sand	Settlement : Uniform Contact Pressure:  Zero at edges and maximum at center
Rigid footing and Clay	Settlement : Uniform Contact Pressure:  maximum at edges and minimum at center.
Flexible footing and sand	Settlement:  Maximum at edges and minimum at center. Contact Pressure:   Uniform
Flexible footing and Clay	Settlement :  Minimum at edges and maximum at center Contact Pressure:  Uniform

Squre footing

qu=1.3×cNc+γDfNq+0.4γBNγ

Circular Footing

qu=1.3×cNc+γDfNq+0.3γBNγ

According to IS 1904 - 1986, the permissible values for settlement of isolated foundation is given in following table.

Type of Structures	Maximum Settlement(mm) for Isolated Foundations
	Sand and hard clay	Plastic clay
For steel	50	50
For RCC	50	75

settlement of soil

For Granular soil

SfSp=[Bf(Bp+0.3))Bp(Bf+0.3)]2

For Clay soil

SfSp=BfBp

h