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.

1.2 Bearing capacity and settlements

1.3 Compensated foundation (Floating foundation)

• When structure in soil is nearly equal to the total excavated soil from ground including weight of water in soil.

i.e Excavated weight = Structural weight

• Settlement of sub soil is prevented.
• Rigid raft foundation is provided when floating foundation is needed.

Diffulties during construction of floating foundation:

• Excavation: Should be done carefully.
• Dewatering: If depth of excavation is below water table dewatering is necessary.
• Critical depth: If soil has low shear strength the limited depth is excavated i.e called as critical depth.
• Bottom heave: Excavation of foundation reduce pressure in below of foundation results in heaving of bottom of excavation.

1.4 Conventional method of analysis

• Determine the line of action of all loads acting on the raft.
• Determine the contact pressure.

a. If e = 0 , q = Q/A

b. If resultant has an ecentricity e_x & e_y in x & y direction.

     Then,

      q = (Q/A) ± Q(e_x/I_yy)x ± Q(e_y/I_xx)y

• Divide the mat into number of beam or strip to analyze each beam separately.
• Draw shear force and bending moment diagram of each strip and calculate q_av below the beam.
• Determine modified column load.

Q_av = ½ (downward load + upward load)

or, Q_av = ½ (Q₁ + Q₂ + Q₃ + q_av *B₁*x

Modified average soil pressure

q_av = Q_av/x₁* B

Column load modified factor

F = Q_av / (Q₁+Q₂+Q₃)

All the column load are multiplied by ‘F’ for the strip.

For strip

FQ₁, FQ₂ & FQ₃

• Draw BMD and SFD for modified column load.
• Find maximum bending moment and shear force.
• Find thickness of slab and reinforcement using BM_max and SF_max.

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 Mat foundation 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.

Types of foundation:

A. Shallow foundation

• Width greater than its depth.
• Located just below the lower part of wall or column which it support.

Types of shallow foundation:

1. Strip / Continuous footing

• Provided for load bearing wall.
• L>B.

2. Spread / Isolated footing

• Circular, square or rectangular slab of uniform thickness.

3. Combined footing

• Supports two or more columns in row.
• Preferred in limited space.

4. Strap / Cantilever footing

• Two or more footing connected by beam.

5. Mat / Raft foundation

• Consist of large slab supporting number of column and walls.

B. Deep foundation

• Depth is greater than width.
• Distributes load from super-structure vertically rather than laterally.

Types of deep foundation:

1. Pile foundation

• Column made of wood, steel, concrete of RCC.
• Embedded into the ground to transmit the load of the structure to a hard stratum or compressed soil.

2. Pier foundation

• A cast insitu pile greater than 0.6 m diameter is termed as pier.
• Consists of cylindrical column of large diameter to support and transfer load.

3. Well foundation

• Well is a type of cassion.
• Suitable for soil containing large boulders.

1.2 Factors affecting choice of foundation

• Function of structure.
• Cost of foundation.
• Sub-surface condition of soil.
• Boundry criteria.
• Bearing capacity of soil.
• Types of load of super structure and other load acting on foundation.

1.3 Introduction to machine foundation

• Foundation provided below the super strucutre of a vibrating and rotating machine for installation is called machine foundation.
• Includes the studies of vibration of foundation soil system transmitted by wave energy.
• Used for supporting turbines, large electric motor and generator.
• Wave energy transmitted through the underlined soil from the foundation should not cause harmful effects to machine, structure of people.

1.4 Types of machine foundation

1. Block foundation

• Consists of a pedestal resting on a footing.
• It has large mass and smaller natural frequency.
• Provided for compressor and reciprocating engine.

2. Box or caisson’s foundation

• Use for lighter foundation.
• It has smaller mass and higher natural frequency.

3. Wall type foundation

• Consist of well-column and beam slab.
• Steam turbines are provided with wall type foundation.

1.5 Types of machine

1. High speed machine

• Turbo generator and rotary compressor fall in this category.
• Speed ranging from 3000 to 10000 rpm.

2. Low speed machine

• Compressor and reciprocating engine comes in this category.
• Speed is smaller than 600 rpm.

3. Impact type machine

• Produce impact loading.
• Speed is usually 60 to 150 blows per minute.

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 Introduction to Foundation and Machine Foundation appeared first on OnlineEngineeringNotes.

Soil investigation:

The process of collecting information about subsurface and taking sample to determine profile of natural soil deposits and engineering properties of soil.

Soil exploration:

The field and laboratory studies carried out for obtaining necessary information about sub soil characteristics including the position of ground water table.

Objective of soil exploration:

• To determine ground water condition in the site.
• To select suitable construction technique.
• To predict and solve potential foundation problem.
• To investigate safety of existing structure.
• To conduct in-situ test to asses appropriate soil characteristics.

Method of soil exploration:

1. Direct method

• It helps to conduct visual inspection of the site locating different strata of the soil and its boundaries.
• It helps to obtain undisturbed sample.

Types of direct method:

a. Test pit

• Depth up to 3 m.
• Uneconomical at greater depth.
• Open type exploration.

b. Trial pits

• Used to recover large bulk sample of soil.

c. Trench

• Used for searching and excavating ancient ruins.

2. Semi-direct method

• Drilling a hole into the soil strata to specified depth is known as boring.

Types of boring:

a. Auger boring

• Trenchless technology used to install steel casing pipe in a variety of soil condition.

b. Auger and shell  boring

• Useful in obtaining sample of sand and gravel from below the water table.

c. Wash boring

• Simple and fast method of soil exploration for cohesive soil and sand or gravel without boulders.

d. Rotary drilling

• Used to form a deep observation.

e. Percussion drilling

• Used for making holes in rocks, boulder and hard strata.

3. Indirect method

• It is also known as geophysical method.
• It helps to measure physical properties of soil such as electrical resistive, vibration and gravitational field.

1.2 Soil sampling, types of sample, soil samplers and its basic requirement for cohesive soil

Soil sampling

• Process of extracting soil sample from drilling tools and sampling equipment.

Types of sample:

1. Disturbed sample

• Natural structure of soil gets modified partly or fully during sampling.

a. Non-representative sample

• Sample obtained from auger boring and wash boring.
• Provides information of major change in sub-surface strata.
• Gives rough idea about soil and its stratification.

b. Representative sample

• Suitable for identification and determination of certain physical properties such as Atterberg limit, specific gravity of soil.

2. Undisturbed sample

• Original soil structure is preserved without modification.
• Practically impossible to get undisturbed sample.
• Tube and chunk sample are considered as undisturbed sample.

Types of samplers:

1. Open driven sampler

• Thin walled tube which can be pushed or driven into soil at the bottom of the hole and then rotated to detached the lower end of the sample from the soil.

2. Split spoon sampler

• Used in standard penetration test (SPT).
• It is tube split into two equal halves length wise.

3. Thin wall sampler

• A sampler introduce very less disturbance to the soil sample.
• Used for collecting undisturbed soil sample.

4. Piston sampler

• Special type of thin walled sampler with piston inside.
• Sampling is pushed into the soil hydraulically by keeping the piston stationary.
• Excellent tool for obtaining very fine undisturbed sample.

5. Rotary sampler

• It is a double walled tube sampler with an inner linear removable.
• Sample is collected in inner liner.

Basic requirement of sampler for cohesive soil:

Let,

D₁ = Inner diameter of cutting edge

D₂ = Outer diameter of cutting edge

D₃ = Inner diameter of sampler tube

D₄ = Outer diameter of sampler tube

Basic requirement of sampler for cohesive soil are as follows

1. Area ratio (A_r)

• A_r = (D₂² – D₁²) * 100% / D₁²
• For obtaining good quality undisturbed sample, the area ratio should be less than 10%.

2. Inside clearance ratio (C_i)

• C_i = (D₃ – D₁)* 100% / D₁
• Reduce the friction between the soil sample and the sampler when soil enters the tube allowing elastic expansion of soil.
• For undisturbed sample: C_i = (0.5 to 3)%

3. Outside clearance ratio (C_o)

• C_o = (D₂ – D₄) * 100% / D₄
• Lies between (0 to 2)%
• For reducing driving force it must be minimum.

4. Recovery length ratio (L_r)

• L_r = L/H [where, L = Length of sampler & H = Depth of penetration of sampling tube]
• Condition:
• L_r = 1; Indicates a good recovery.
• L_r  < 1; Soil in sample is compressed.
• L_r > 1; Soil has swelled.
• It lies between (96 to 98)%

5. Inside wall friction

• Wall of sampler should be smooth and properly oiled.

6. Non – return value

• It should permit easy and quick escape of water and air when sample is driven.

1.3 Planning of exploration, number of bore holes, depth of exploration

Planning of exploration:

• A detailed study of the geographical condition of the area which includes collection of topographical and geographical maps.
• Collection of hydraulic condition such as water table fluctuation, flooding of site.
• Preparation of layout plan of project.
• Preparation of borehole layout plan.
• Marking on the layout plan for addition soil investigation if required.
• Preparation of specification and guideline for field execution.
• Preparation of specification and guideline for testing and collecting sample.

Number of bore holes:

• More number of borehole sunk more we will know about site condition and greater economy is achieved in foundation design.
• For small structure at least 2 or 3 bore holes should be shank.

Depth of exploration:

• Depth of boring depends upon the depth of soil affected by the load transmitted by foundation.
• Guideline regarding depth of boring are :

1.4 Field penetration test and their suitability

1. Standard penetration test

• Most commonly used for cohesion less soil.
• Split spoon sampler is used.
• This test is done to determine relative density.
• The sampler is driven into the soil by dropping hammer into the soil falling vertically through a height of 750 mm at the rate of 30 blows per minute.
• The number of hammer blows required to driven 150 mm of the sampler is counted. Again, the sampler is driven by 150 mm and the number of blows is recorded. Further the sampler is driven by 150 mm and the number blows is recorded.
• The number of blows is recorded for the last two 150 mm interval are added to give the SPT value.

Merit:

• Quick and simple to perform.
• Able to penetrate dense layer, gravel and fill.

Demerit:

• Not continuous.
• Sample obtained is disturbed.

SPT value correction:

a. Dilatancy correction

• Applied when the test is performed in fine or silty saturated sand.
• To reduce over estimate value of SPT.

The correction penetration number;

N^|_c = 15 + (N_R – 15)/2

If N_R ≤ 15, N_c = N_R

Where,

N_R = Recorded N- value

N_c = Corrected N-value

b. Overburden pressure correction

• Penetration resistance  of soil depends on over burden pressure.
• In deeper depth overburden pressure is high and its response to SPT test will better compare to the same soil at shallow depth.

Correction SPT- N value is

N_c = C_N * N^|_c

Where,

N_c = Corrected N value of overburden pressure

N^|_c = Observed N value

C_N = Correction factor = 0.77 log₁₀ (2000/ σ_o)

2. Static cone penetration test (SCPT)

• Widely used in place of SPT test for soft clay and silt.
• Standard cone is pushed into soil by applying force. For obtaining cone resistance cone is pushed downward at a steady rate. Now, both cone and sleeve are pushed into soil and combined resistance is determined.

∴ Resistance of sleeve = Combined resistance – Cone resistance

Merit:

• Simple and quick.
• Economical to perform.
• Help in identifying problem in soil.

Demerit:

• Soil sample is not obtained.
• Requires special equipment and skill manpower.

3. Dynamic cone penetration test (DCPT)

• If hard layer is found DCPT is done.
• The sampler is driven into the soil by dropping hammer into the soil falling vertically through a height of 750 mm at the rate of 30 blows per minute.
• The number of hammer blows required to driven 150 mm of the sampler is counted. Again, the sampler is driven by 150 mm and the number of blows is recorded. Further the sampler is driven by 150 mm and the number blows is recorded.
• The number of blows is recorded for the last two 150 mm interval are added to give the SPT value.

Merit:

• Doesn’t need borehole.
• Economical.

Demerit:

• Cannot be performed in cohesive soil.
• Not possible to determine mechanical properties of soil.

1.5 Ground – water observation

• The variation of ground water affect the soil characteristics such as shear strength.
• Important to find maximum and minimum water level for proper design of structure.
• In highly permeable soil depth of water table is measure by chalk coated tape.
• In low permeable soil depth of water table is measured by Casagrande piezometer.

1.6 Borehole logs

• Detailed record of boring operation and other tests carried out in the field.
• Provided information in bore log is useful to give information of soil type, consistency of soil and water table.

1.7 Site investigation report

• Result/ conclusion of the investigation, exploration and testing program.
• Report should be comprehensive, clear and to the point.
• Typical report includes following point:
• Introduction
• Borehole log
• Field and laboratory test result
• Analysis of data
• Recommendation
• Reference

References:

• Terzaghi, Karl and peck, R.B. John Wiley.(1967). Soil mechanics in engineering practice, New York.
• Arora K.R. (1997). Soil Mechanics and foundation engineering, India: Standard Publisher Distribution.

The post Site investigation and Soil exploration: Field penetration tests and their suitability appeared first on OnlineEngineeringNotes.

Purpose / Importance:

• To determine the type of foundation.
• To determine design parameter for the foundation such bearing capacity and allowable soil bearing pressure.
• To calculate potential settlement of foundation.
• To determine expansion potential at the site.
• To investigate the stability of slope and their effect on adjacent structure.
• To investigate possible way of improving the soil to increase the foundation bearing capacity.

Types of foundation:

A. Shallow foundation

• Width greater than its depth.
• Located just below the lower part of wall or column which it support.

Types of shallow foundation:

1. Strip / Continuous footing

• Provided for load bearing wall.
• L>B.

2. Spread / Isolated footing

• Circular, square or rectangular slab of uniform thickness.

3. Combined footing

• Supports two or more columns in row.
• Preferred in limited space.

4. Strap / Cantilever footing

• Two or more footing connected by beam.

5. Mat / Raft foundation

• Consist of large slab supporting number of column and walls.

B. Deep foundation

• Depth is greater than width.
• Distributes load from super-structure vertically rather than laterally.

Types of deep foundation:

1. Pile foundation

• Column made of wood, steel, concrete of RCC.
• Embedded into the ground to transmit the load of the structure to a hard stratum or compressed soil.

2. Pier foundation

• A cast insitu pile greater than 0.6 m diameter is termed as pier.
• Consists of cylindrical column of large diameter to support and transfer load.

3. Well foundation

• Well is a type of cassion.
• Suitable for soil containing large boulders.

1.2 Factors affecting choice of foundation

• Function of structure.
• Cost of foundation.
• Sub-surface condition of soil.
• Boundry criteria.
• Bearing capacity of soil.
• Types of load of super structure and other load acting on foundation.

1.3 Introduction to machine foundation

• Foundation provided below the super strucutre of a vibrating and rotating machine for installation is called machine foundation.
• Includes the studies of vibration of foundation soil system transmitted by wave energy.
• Used for supporting turbines, large electric motor and generator.
• Wave energy transmitted through the underlined soil from the foundation should not cause harmful effects to machine, structure of people.

1.4 Types of machine foundation

1. Block foundation

• Consists of a pedestal resting on a footing.
• It has large mass and smaller natural frequency.
• Provided for compressor and reciprocating engine.

2. Box or caisson’s foundation

• Use for lighter foundation.
• It has smaller mass and higher natural frequency.

3. Wall type foundation

• Consist of well-column and beam slab.
• Steam turbines are provided with wall type foundation.

1.5 Types of machine

1. High speed machine

• Turbo generator and rotary compressor fall in this category.
• Speed ranging from 3000 to 10000 rpm.

2. Low speed machine

• Compressor and reciprocating engine comes in this category.
• Speed is smaller than 600 rpm.

3. Impact type machine

• Produce impact loading.
• Speed is usually 60 to 150 blows per minute.

References:

• Terzaghi, Karl and peck, R.B. John Wiley.(1967). Soil mechanics in engineering practice, New York.
• Arora K.R. (1997). Soil Mechanics and foundation engineering, India: Standard Publisher Distribution.

The post Introduction to Foundation and Machine Foundation appeared first on OnlineEngineeringNotes.