Foundation soil improvement:

• Process of improving engineering properties of soil to make more stable foundation.
• Reduce permeability and compressibility of soil.
• Increase shear strength of soil.
• Increase bearing capacity of foundation of soil.

Method of soil improvement:

• Mechanical compaction
• Dynamic compaction
• Soil stabilization by use of admixture
• Sand compaction piles
• Soil stabilization by injection of suitable grout

1.2 Mechanical compaction

• It is the process of increasing density of soil by application of mechanical energy.
• For cohesive soil compaction is done till optimum moisture content.
• For cohesion less soil compaction is done by vibrating.

Purpose:

• Increase shear strength.
• Reduce compressibility.
• Reduce permeability.

Equipment:

1. Smooth wheeled roller

2. Vibratory roller

3. Impact rammer

1.3 Preloading

• Soil improvement by applying compressive load to reduce settlement and increase bearing capacity.
• It is also known as tempory loading.

1.4 Sand compaction pile and stone column

Sand compaction pile:

• This method is used for improving ground stability, preventing liquefaction and reducing settlement.
• Process: By installing sand into soft ground by casting pipe and vibrating the sand to produce firmly compacted sand piles in the ground.

Stone column:

• This method is used for installing and compacting pile to reduce liquefaction.
• Size of stone are 6 mm to 40 mm.

1.5 Soil stabilization by the use of admixture

• Physical properties of soil can be improved economically by use of admixture like lime, portland cement and asphalt.
• Applicable only in shallow foundation.

Types of admixture:

1. Soil cement admixture

• Mixture of cement and water with soil and compacted to high density.
• PPC cement is used.
• Used in sub base and base course on road.

2. Soil lime stabilization

• Improves strength, stiffness and density.
• Reduce plasticity index.
• Used in canal lining.

1.6 Soil stabilization by injection of suitable grout

• It is the process by which fluid like material either in suspension or in solution is injected in void space of underground soil or rock.

Effective in following case:

• When foundation has to be constructed below ground water table.
• Difficult for access of foundation level.
• Geometric dimension of foundation is complicated.

Types of soil stabilization by injection of suitable grout:

• Chemical grouting ( silica and resins)
• Cement grouting

References:

• Terzaghi, Karl, Peck, R.B & John, Wiley (1969) Soil mechanics in engineering practice, New York.
• Arora , K.R (2008), Soil mechanics and foundation engineering, Delhi: Standard Publisher Distribution.

The post Foundation Soil Improvement Methods appeared first on OnlineEngineeringNotes.

• Deep foundation provided below water level.
• Also known as caissons foundation.
• Used for bridge construction.

Types of well foundation

1. Open well

• Both top and bottom are open during construction.
• Cost is cheap.
• Rate of progress is slow.

2. Pneumatic caissons

• Open at bottom and closed at top.
• Sunk vertically.
• Construction cost is quite high.

3. Box caissons

• Open at top and closed at bottom.
• Cost of construction is low.

Shape of well foundation

1. Circular well

• Commonly used shape.
• Maximum diameter is 9 m.

2. Doub D-well

• Sunk easily.
• Used for pier.

3. Double octagonal well

• Shape of well is better than double D-well.

4. Twin circular well

• Two indepent wall.
• Small depth of sinking.

5. Rectangular wall

• Used for bridge foundation.
• Large foundation, double rectangular well.

1.2 Component of well foundation

• Well cap: Transmit load of super structure to steining.
• Steining: Transmit load to subsoil.
• Well curb: Facilitates process of sinking.
• Cutting edge: Cuts soil during sinking.
• Bottom plug: Transmit load to sub soil.
• Dredge hole: Hole formed during excavation.
• Top plug: Concrete plug constructed at top.

1.3 Depth of well foundation

• Depth is dependent on:

1. Minimum grip length below scour depth.

2. Base pressure to be within permissible load.

• Normal depth of scour is calculated by lacy’s formula.

d = 0.473 (Q/f)^1/2

Where,

Q = Design discharge in cumecs

f = Lacy’s factor = 1.76m^1/2

d = Scour depth

m = mean particle size

• Regime width of water way (W)

W = CQ^1/2

Where,

W = Regime width

C = Constant

• Actual water way length (L) is less than regime width.
• The actual depth(d^|) = d(W/L)0.67
• Grip length: Depth of bottom of well below maximum scour level.

1.4 Force acting on well foundation

• Live load: Load is not constant and change with time.
• Impact load: Sudden load.
• Wind load: Horizontal load.
• Force due to water.
• Seismic force.
• Earth pressure.

1.5 Construction and sinking of well

1. Sinking of a well

Steps:

a. Laying of curb

• If river bed is dry the cutting edge is placed.
• If water table is upt 5 m sand is land is created.
• If water table is more than 5 m more economical curb is built.

b. Construction of well steining

• Steining is constructed with a height of 1.5 m at time of sinking.

c. Sinking operation

• Material is excavated mechanically or manually. Manual work can be done upto 1 m height.
• Well os allowed to remain vertical.
• Sinking in well increase skin friction.

2. Tilt and shift of well

• Objective is to well sunk should sunk straight and vertical.

Cause of tilt and shift:

• No – uniform bearing capacity.
• Obstraction on one side.
• Unequal removal of soil.

Precausion:

• Uniform thickness cutting edge should be provided.
• Tilt and shift should be carefully noted.

Remedial measure of tilt and shift:

1. Control dredging

• Done more on higher side.

2. Eccentric loading

• Provide greater sinking effort on higher side of wall.

3. Pushing the wall

• Applied on lower side of wall.

4. Pulling the wall

• Applied on higher side of wall.

5. Water jetting

• Used for outer face of higher side.

References:

• Terzaghi, Karl, Peck, R.B & John, Wiley (1969) Soil mechanics in engineering practice, New York.
• Arora , K.R (2008), Soil mechanics and foundation engineering, Delhi: Standard Publisher Distribution.

The post Well Foundation Types and Construction appeared first on OnlineEngineeringNotes.

Lateral earth pressure:

• The pressure exerted by soil in horizontal direction.

Lateral earth pressure = K * over burden stress

or, σ_h = K* σ_v

or, σ_h = K* γZ

where,

K = Coefficient of lateral pressure

Types of lateral earth pressure:

1. Earth pressure at rest

• When soil mass is not subjected any lateral yielding or movement the pressure in this condition is known as earth pressure at rest.

2. Active earth pressure

• Occurs when soil mass yields in such a way that it tends to stretch horizontally.
• It is the state of plastic equilibrium as the entire soil mass is on verge of failure.

3. Passive earth pressure

• When retaining wall moves inward to the backfill the soil gets compressed and failure due to upward movement of wedge occurs.

1.2 Rankine’s earth pressure theory for active and passive state

Rankine’s earth pressure theory:

Assumption

• Soil is homogenous and semi-infinite.
• Back of retaining wall is vertical and smooth.
• Ground surface is plane which may be horizontal or inclined.
• Soil is dry and cohesion less.
• Wall movement is sufficient so that plastic equilibrium is fulfilled.

Rakine’s various backfill condition are:

A. Rankine theory for cohesionless soil (C=0)

1. Rankine theory for horizontal backfill

a. Active earth pressure

Consider,

• σ_v = γZ
• Intially there is no lateral movement.

i.e σ_h = K_o* σ_v

• As the wall moves away from the soil σ_v remains same but σ_h decreases till failure occurs i.e σ_h  → σ_a.

As wall moves away

σ_a = K_a σ_v

Now, for expression of K_a

References:

• Terzaghi, Karl, Peck, R.B & John, Wiley (1969) Soil mechanics in engineering practice, New York.
• Arora , K.R (2008), Soil mechanics and foundation engineering, Delhi: Standard Publisher Distribution.

The post Lateral Earth Pressure Theories appeared first on OnlineEngineeringNotes.