ADVANCED FOUNDATION DESIGN(NCE-011)

By- Md Mozaffar MasudAssistant Professor

Civil Engineering DepartmentJIT, Barabanki

UNIT 1

Soils Investigation

Determination of surface and subsurface soil conditions and features in an area of proposed construction that may influence the design and construction and address expected post construction problems.

Soils Investigation contd.

Required to evaluate an area for the construction of a project or evaluate local material as a construction material

Soil Investigation Field Sampling and Testing Laboratory Analysis Report preparation

Planning and evaluation of field work are aided by knowledge of the mechanics of soil deposit’s formation

Soils Investigation contd.

Soil grains are the result of weathering of bedrock physical weathering

granular soil types (gravel, sand, silt) chemical weathering

clays Soil deposits

residual- product of weathering the original bedrock

transported- moved from their place of origin

Soils Investigation contd.

Transportation agents Rivers and streams

gravel sand silt deposited as a fn (water velocity)

Lakesclays and silts settling out

Windsand dunes and loess deposits (silt particles)

Soils Investigation contd.Glacier soil deposits

tills (mixture of gravel sand silt clay)material that has been shoved forward or

picked up from an advancing glacierthis material is deposited when a glacier stops

or retreats as it melts fluvial deposits associated with glaciers

clays from glacier lakesmarine clays deposited from salt watersorted gravel, sand and silt from glacier streams

Requirement of Soils Investigation

Field Investigation Techniques determine bearing capacity for foundations determine water resources find aggregate deposits (road construction) estimate infiltration and seepage rates assess land use capabilities

Information required depth, thickness, properties of each soil layer location of groundwater table depth to bedrock

Soils Investigation contd. Subsurface Investigation

Geophysical methodsseismic or electrical-variations in the speed of sound

waves or electrical resistivity of soil formations Test pits or trenches

shallow depths only Hand Augers

shallow depths only Boring test holes and sampling with drill rigs

principal method for detailed soil investigations

GEOPHYSICAL METHOD Although boring and test pits provide definite results but they are

time consuming and expensive. Subsurface conditions are known only at the bore or test pit

location. The subsurface conditions between the boring need to be

interpolated or estimated. Geophysical methods are more quick and cheaper. They provide thorough coverage of the entire area. results of Geophysical testing however are less definitive and

require subjective interpretation. both methods are important. In case geophysical testing in major

in scope, few borings and sampling will be required for accurate determination of soil properties.

If boring is major in scope then few geophysical lines will be required to know the conditions in-between the borings.

Geophysical Techniques Indirect Methods

Ground Penetrating Radar (GPR) Electromagnetic (EM) Magnetic Utility Locating Seismic Electrical Resistivity Gravity Very Low Frequency (VLF)

Geophysical Techniques Indirect Methods

AdvantagesNon-Destructive Cost Effective Provides Preliminary or Supplemental Information

Geophysical Equipment

Seismograph Spectrum Analyzer

Portable Analyzer Velocity Recorder

Soil Resistivity Method

Resisitivity (ohm-m) is an electrical property. It is the reciprocal of conductivity

Arrays of electrodes used to measure changes in potential.

Evaluate changes in soil types and variations in pore fluids

Used to map faults, karst features (caves, sinkholes), stratigraphy, contaminant plumes.

Soil Resistivity Measurements

Soil Resistivity Measurements

1 10 100 1000 10000

Bulk Resistivity, (ohm- meters)

Clay

Loam

Loose Sands

Sands & Gravels

Glacial Till

Weathered Rocks

Resistivity Values (ConeTec & GeoProbe, 1997)

Seismic Refraction Method

0.000

0.005

0.010

0.015

0.020

Trav

el Tim

e (se

cond

s)

0 10 20 30 40 50 Distance From Source (meters)

Horizontal Soil Layer over Rock

Seismic Refraction Method

Seismic Testing of

Equipments

Westergaard’s Theory of Stress

Westergaard developed a solution to determine distribution of stress due to point load in soils composed of thin layer of granular material that partially prevent lateral deformation of the soil.

Westergaard’s Theory of Stress

2/322z

zr21

1zP

2/322

2z

zra

1z2a.P

• Point Load

2221a

UNIT 2

Shallow foundations: Where the ratio of embedment depth to min plan

dimension is less or equal to 2.5 Embedment depth is the depth below the ground surface

where the base of foundation rests.plain concrete foundation,stepped reinforced concrete foundation, reinforced concrete rectangular foundation reinforced concrete wall foundation.

Shallow Foundation

Steps in Selection of Foundation Types 1 Obtain the required information concerning the nature of the

superstructure and the loads to be transmitted to the foundation. 2. Obtain the subsurface soil conditions. 3. Explore the possibility of constructing any one of the types of

foundation under the existing conditions by taking into account (i) the bearing capacity of the soil to carry the required load, and (ii) the adverse effects on the structure due to differential settlements. Eliminate in this way, the unsuitable types.

4. Once one or two types of foundation are selected on the basis of preliminary studies, make more detailed studies. These studies may require more accurate determination of loads, subsurface conditions and footing sizes. It may also be necessary to make more refined estimates of settlement in order to predict the behavior of the structure.

5. Estimate the cost of each of the promising types of foundation, and choose the type that represents the most acceptable compromise between performance and cost.

Shallow Foundation

The most common (and cheapest) type of shallow foundations are SPREAD FOOTINGS

Shallow Foundation

Strip Footings to support wall loads

Rectangular and Trapezoidal Footings for two columns (combined footing) or machine base

Raft or Mat Foundation

To lower the bearing pressure and reduce differential settlement on soils with low bearing capacity or erratic or variable conditions

Ultimate Bearing Capacity, qf

The least pressure that would cause shear failure of supporting soil immediately below and adjacent to a foundation

Modes of Failure(General Shear Failure)on low compressibility (dense or stiff) soilsplastic equilibrium throughout support and adjacent soil massesheaving on both sides of foundationfinal slip (movement of soil) on one side only causing structure to tilt

Modes of Failure (Local Shear Failure)on highly compressible soilsonly partial development of plastic equilibriumonly slight heaving on sidessignificant compression of soil under footing but no tilting

Modes of Failure (Punching Shear Failure)on loose, uncompacted soilsvertical shearing around edges of footinghigh compression of soil under footing, hence large settlementsno heaving, no tilting

Assumptions for Terzaghi's Method

Depth of foundation is less than or equal to its widthNo sliding occurs between foundation and soil (rough

foundation) Soil beneath foundation is homogeneous semi

infinite massMohr-Coulomb model for soilGeneral shear failure mode is the governing mode (but not the only mode)

Assumptions for Terzaghi's Method

No soil consolidation occurs Foundation is very rigid relative to the soil Soil above bottom of foundation has no shear

strength; is only a surcharge load against the overturning load

Applied load is compressive and applied vertically to the centroid of the foundation

No applied moments present

Terzaghi’s Bearing Capacity EquationNeglecting the shear strength of the soil above depth D implies that this soil is a surcharge: qo= γDTerzaghi’s general equation: qf = 0.5γBNγ + cNc + γDNq

Terzaghi's bearing capacity factors for general shear failure

The General Bearing Capacity Equation.

The General Bearing Capacity Equation.

Other Factors

Other Factors For continuous footing, s = 1 For perpendicular load, i = 1 For level foundation, b =1 For level ground, g =1 Need to compute factors

Bearing Capacity Factor N, Depth Factor d

General Shear Failure of Footings (Ultimate Bearing Capacity)

qccf DNSNcSNγBq )()(5.0

)45(tan 22)tan( eN q

)cot()1( qc NN

)4.1tan()1( qNNFOOTING

TYPE Sγ Sc

Strip 1.0 1.0

Square 0.8 1.2

Circular 1.6 1.2

Rectangular 1-0.2(B/L) 1+0.2(B/L)

theory was developed for strip footings

To adapt square, circular and rectangular shapes, Terzaghi & Peck developed shape factors here which are still widely used today:

Settlement Immediate Settlement: Occurs

immediately after the construction. This is computed using elasticity theory (Important for Granular soil)

Primary Consolidation: Due to gradual dissipation of pore pressure induced by external loading and consequently expulsion of water from the soil mass, hence volume change. (Important for Inorganic clays)

Secondary Consolidation: Occurs at constant effective stress with volume change due to rearrangement of particles. (Important for Organic soils)

Elastic Settlement of Rectangular footings

Flexible vs Rigid

Flexible vs Rigid

Elastic Settlement

Elastic Settlement [Schmertman Method (1978)]

𝜌=∁1 ∁2∆ 𝜎∑𝑖=1

𝑛

( 𝐼𝑧𝐸 )𝑖∆𝑍 𝑖

Primary Consolidation

Expulsion of water from soils accompanied by increase in effective stress and strength

Amount can be reasonably estimated based on lab data, but rate is often poorly estimated

Consolidation Settlement

This method makes use of the results of the conventional oedometer test where the consolidation parameters of the soil are measured.

To compute the stress changes within the soil mass. The stress changes are computed using a Boussinesq type approach assuming elasticity.

The important parameter for consolidation settlement calculation is the net effective stress change in the soil.

Usually the settlements are calculated for the soil divided into a number of sub-layers and the final total settlement is the sum of individual sub-layer settlements

Consolidation Settlement

Secondary Consolidation

At the end of primary settlement, settlement may continue to develop due to the plastic deformation (creep) of the soil.

The stage of consolidation is called secondary consolidation.

UNIT 3

Deep Foundation

Deep Foundations are those - in which the depth of the foundation is very large

in comparison to its width. Which are not constructed by ordinary methods of

open pit excavations.

When Used?

In cases where - The strata of good bearing capacity is not available near

the ground The space is restricted to allow for spread footings

In these cases the foundation of the structure has to be taken deep with the purpose of attaining a bearing stratum which is suitable and which ensures stability and durability of a structure.

The bearing stratum is not the only case. There may be many other cases. For example, the foundation for a bridge pier must be placed below the scour depth, although suitable bearing stratum may exist at a higher level.

Pile Foundations

BS8004 defines deep foundation with D>B or D>3m. Pile foundation always more expensive than shallow

foundation but will overcome problems of soft surface soils by transferring load to stronger, deeper stratum, thereby reducing settlements.

Pile resistance is comprised of end bearing shaft friction

For many piles only one of these components is important. This is the basis of a simple classification

End Bearing Piles

End bearing pile rests on a relative firm soil . The load of the structure is transmitted through the pile into this firm soil or rock because the base of the pile bears the load of the structure, this type of pile is called end bearing pile

Piles

Soft Soil

Rock

Types of Pile The pile installation procedure varies considerably, and has an

important influence on the subsequent response Three categories of piles are classified by method of

installation as below: Large displacement piles

They encompass all solid driven piles including precast concrete piles, steel or concrete tubes closed at the lower end

Small displacement pilesThey include rolled steel sections such as H-pile and

open-end tubular piles Replacement piles

They are formed by machine boring, grabbing or hand-digging.

Ultimate capacity of axially load single pile in soil

Estimated by designer based on soil data and somewhat empirical procedures. It is common practice that the pile capacity be verified by pile load test at an early stage such that design amendment can be made prior to installation of the project piles. The satisfactory performance of a pile is, in most cases, governed by the limiting acceptable deformation under various loading conditions. Therefore the settlement should also be checked.

Basic Concept

Qu

WQs

Qb

The ultimate bearing capacity (Qu )of a pile may be assessed using soil mechanics principles. The capacity is assumed to be the sum of skin friction and end-bearing resistance, i.e

Qu =Qb+Qs-W ……………………….(1)

Where, Qu is total pile resistance, Qb is the end bearing resistance and Qs is side friction resistance

General behaviour Shaft resistance fully mobilized at

small pile movement (<0.01D) Base resistance mobilized at large

movement (0.1D)

End Bearing resistance for Bore pile in granular soils

Due to the natural of granular soil, the c’ can be assumed equation to zero. The ultimate end bearing resistance for bored pile in granular soils may be express in terms of vertical effective stress, ’v and the bearing capacity factors Nq as :

QB=AB Nq ’v

Nq is generally related to the angle of shearing resistance f’. For general design purposed, it is suggested that the Nq value proposed by Berezantze et al (1961) as presented in Figure ?? are used. However, the calculated ultimate base stress should conservatively be limited to 10Mpa, unless higher values have been justified by load tests.

Shaft Friction Resistance

The ultimate shaft friction stress qs for piles may be expressed in terms of mean vertical effective stress as :

qs =c’+Ksv’tands

qs =b v’ (when c’=0) Where, Ks= coefficient of horizontal pressure which depends on the relative density and state of soil, method of pile installation, and material length and shape of pile. Ks may be related to the coefficient of earth pressure at rest, K0=1-sin Qv’ = mean vertical effective stress s’ = angle of friction along pile/soil interface b = shaft friction coefficient Qs = pLqsWhere p is the perimeter of the pile and L is the total length of the pile

Bored pile in Clays

The ultimate end bearing resistance for piles in clays is often related to the undrained shear strength, Cu, as

qB=NcCu

QB=ABNcCu

where, Nc= 9 when the location of the pile base below ground surface exceeds fours times the pile diameter

Bored pile in Clays

The ultimate shaft friction (qs) for soils in stiff over-consolidated clays may be estimated on the semi-empirical method as:

qs=aCu

a is the adhesion factor (range from 0.4 to 0.9)

Design of Pile Groups

The efficiency of a pile group is the ratio of the ultimate capacity of the group to the sum of the candidates of the individual piles.

ɳg = Qug

nQu

Where: g = Pile group efficiency.

Qug = Ultimate capacity of the pile group.

n = Number of piles in the pile group. Qu = Ultimate capacity of each pile in the pile group

Design of Pile Groups

The group efficiency may be less than 1 for a pile group driven into a compressible cohesive soil, or into a dense cohesionless soil underlain by a weak cohesive deposit.

The group efficiency in cohesionless soils is generally greater than 1.

The settlement of a pile group is likely to be many times greater than that of a single pile carrying the same load as each pile in the pile group.

Stress Zones from a Single Pile and Pile Group

Overlap of Stress Zones

Group Capacity in Cohesionless Soils

The ultimate axial compression capacity of a pile group driven in a cohesionless soil may be taken as the sum of the individual capacities, unless underlain by a weak deposit, jetted, or predrilled.

If underlain by a weak deposit, the ultimate group capacity is the lesser of the 1) sum of the individual pile capacities, or 2) the group capacity against block failure.

A minimum center-to-center pile spacing of 3 diameters is recommended.

Group Capacity in Cohesive Soils

For pile groups in clays with undrained shear strengths less than 95 kPa (2 ksf), and the cap not in firm contact with the ground, use a group efficiency ranging from 0.7 for c-t-c spacings of 3 diameters, to 1.0 for c-t-c spacings of 6 diameters (interpolate in between).

For pile groups in clays with undrained shear strengths less than 95 kPa (2 ksf), and the cap in firm contact with the ground, a group efficiency of 1.0 may be used.

For pile groups in clays with undrained shear strengths greater than 95 kPa (2 ksf), regardless of pile cap/ground contact, use a group efficiency of 1.0.

Group Capacity in Cohesive Soils

Calculate the ultimate pile group capacity against block failure, and use the lesser capacity.

A center-to-center spacing less than 3 diameters should not be used

Short-term group efficiencies in cohesive soils 1 to 2 months after installation may be as low as 0.4 - 0.8 due to high driving-induced excess porewater pressures (results in decreased effective stress).

Pile groups in clays which are loaded shortly after pile installation should consider the reduced short-term group capacity.

In critical cases, piezometers should be installed to monitor porewater pressure dissipation with time

Settlement of pile group

Block failure of pile groups is generally only a design consideration for pile groups in soft cohesive soils or in cohesionless soils underlain by a weak cohesive layer.

The bearing capacity factor, Nc, for a rectangular pile group is generally 9.

However, Nc should be calculated for pile groups with small pile embedment depths and/or large widths

Nc = 5 [ 1+D/5B ] [ 1+B/5Z ] ≤ 9

UNIT 4

Expansive Soils

Vertisol Soils, or known as Shrink Swell Soils The Soil contracts due to its clay minerals and the

structure of the clay allowing water to be imbedded in-between the clay layers

Process is reversible, and causes contraction of the soil

Characteristics of expansive soils

The expansive properties of soils depend on the grain size, mineralogy and water content.

The 2:1 sheet smectite group include expansive monmorillonite clay.

Montmorillonite swell and shrink at different moisture content

Foundation on expansive soil

Foundation is the lowest load-bearing part of engineering infrastructures, typically below ground level.

Foundations are affected by engineering properties and characteristic of the soil.

Engineering problems and type of foundation support are vital in construction of foundation.

Foundation on expansive soil contd.

Foundation on expansive soils is affected by the behaviour of soil under different moisture content.

The swelling tendency of expansive soils on foundation can be quantified by the swell potential and swelling pressure parameters.

The major engineering problem of expansive soils on foundation is shrink-swelling characteristics of the soil.

Foundation types that can be utilised on expansive soils are pile, raft, shallow and caissons foundation.

Engineering problems

Swelling and shrinking of soil

Damage Done

Damage Done

What can be done?

Test soil before building If expansion is greater then 10 %, it is critical Remove soilMix soil with material that does not expand Keep consistent soil moistureHave strong foundations in buildings that can handle

the changes in volume.

Engineering solutions

Post-wet and pave the area with bricks or blocks laid on the plastic membrane.

Total removal of expansive soils.Under pinning with piles. Reinforcement of with tie-bars. Caissons foundation.

Well Foundation

Well foundation is the most commonly adopted foundation for major bridges in India. Since then many major bridges across wide rivers have been founded on wells.

Well foundation is preferable to pile foundation when foundation has to resist large lateral forces.The construction principles of well foundation are similar to the conventional wells sunk for underground water.But relatively rigid and engineering behaviour.Well foundations have been used in India for centuries. The famous Taj Mahal at Agra stands on well foundation.

Well Foundation

Benefits of Well Foundation

Provides massive and solid foundation. Possible to sink well through boulders,logs of wood

found at depth. Large section modulus with minimum cross sectional

area is advantageous. The strata through which well passes is known

exactly.Well raising and stiening is done in steps so

foundation level can be varied. Economical to provide it for unstable soil mass

Shapes of well foundations

Wells have different shapes and accordingly they are named as:-

Circular well, Double D well, Twin circular well, Double octagonal well, Rectangular well.

Shapes of well foundations

Types of Well Foundation

Open caisson or wellBox CaissonPneumatic Caisson

Types of Well Foundation

Open caisson or well: The top and bottom of the caisson is open during construction. It may have any shape in plan.

Box caisson: It is open at the top but closed at the bottom.

Pneumatic caisson: It has a working chamber at the bottom of the caisson which is kept dry by forcing out water under pressure, thus permitting excavation under dry conditions.

Open Caisson

Open Caisson

Box Caisson

Box Caisson

Pneumatic Caissons

Construction Procedure

Layout Fabrication of cutting edge.Well curb. Construction of stieining. Island constructionWell Sinking. Plugging. Sand filling. Casting of well cap.

Sinking Operations

Erect Cutting Edge. Erect inside shuttering of curb. Fix reinforcement for the curb. Erect outside shuttering of curb. Concrete the curb and ground it. Remove the shuttering. Fix reinforcement in steining Erect reinforcement for one lift.

Sinking Operations Contd.

Concrete the steining.Dredge inside the well. Sink the well in stages. Sinking is done by uniform excavation of material.Use of water jetting and explosives may be done.Normally dewatering should not be done. Tilts must be rectified wherever necessary

Precautions

When two wells sunk near each other, they should be sunk alternately.

Least possible area must be disturbed in vicinity. In sinking of dumb bell shaped well, excavation must

be done simultaneously.Dredged material must not be accumulated near

well. In sinking of two wells through sand, timber logs are

provided between steining. Care must be taken when cutting edge approaches

junction of strata.

Sinking Well Through Clay Strata

It is one of the tough situations to face as well becomes stationary.

Tilting occurs due to horizontal force by water. The well becomes vulnerable to tilt if a step is

provided on outside face of the well steining to reduce

It may lead to a very expensive and time-consuming affair for attempting to make well straight and vertical.

Measures Adopted

Remove soil in contact with the outside surface of the well by grabbing to a certain depth.

Continue grabbing much below the cutting edge level of the well.

Dewatering well results into increasing effective weight.

Flushing with jet of water on the outside face of well. By Kentledge loading on the well

General Measures for Ease of Sinking.

Appropriate choice of cutting edge and adoption of proper detailing. The "Angle iron" cutting edge works well when the well passes

through alluvial soil strata without any hard obstruction. A "V type" cutting edge is more appropriate in meeting various

obstructive situation provided correct detailing is adopted. The inclined plate should be stopped about 25 mm above the

bottom tip of vertical plate. Adequate no. of Borelogs must be taken in the location of each well. Presence of very large boulder covering a part of the well at some

depth in the bridge over Brahmaputra at Jogighopa. Similar type of problems including sudden change of bed profile are

encountered in various rivers in India.

UNIT 5

Types of slopes

Two Types: Natural slopes: Due too natural causes Man made slopes: Cutting and embankments

The slopes whether natural or artificial may be Infinite slopes Finite slopes

Causes of Failure of Slopes

The important factors that cause instability in a slope and lead to failure are: Gravitational force Force due to seepage water Erosion of the surface of slopes due to flowing

water sudden lowering of water adjacent to a slope Forces due to earthquakes

COMMON FEATURES OF SLOPE STABILITY ANALYSIS METHODS

Safety Factor: F = S/Sm where S = shear strength and Sm = mobilized shear resistance. F = 1: failure, F > 1: safety

Shape and location of failure is not known a priori but assumed (trial and error to find minimum F)

Static equilibrium (equilibrium of forces and moments on a sliding mass)

Two-dimensional analysis

Factors Affecting Slope Failure

Geological discontinuities Effect of Water Geotechnical Properties of Material Mining Methods State of stress Geometry slope: Temperature Erosion Seismic effect Vegetation

Types of Rock Slope Failure

Plane failureWedge Failure Toppling failure Rockfalls Rotational Failure

Rock Slope Stability Analysis: Limit Equilibrium Method

Planar Failure Analysis Sliding analysis of a block Plane failure analysis along a discontinuity Water is filled in discontinuities Tension crack present in the upper slope surface Tension crack present in the slope surface The tension crack is filled with water with upper slope angle Effect of rock bolts Wedge Failure Analysis Analysis of wedge failure considering only frictional resistance Analysis of wedge failure with cohesion and friction angle Toppling Failure Analysis Kinematics of block toppling failure Limit equilibrium analysis for toppling failure Stability analysis of flexural toppling

Infinite Slope Analysis

Translational failures along a single plane failure surface parallel to slope surface

The ratio of depth to failure surface to length of failure zone is relatively small (<10%)

Applies to surface raveling in granular materials or slab slides in cohesive materials

Equilibrium of forces on a slice of the sliding mass along the failure surface is considered

Infinite Slope Analysis contd.

F = f(c’, ’, , b, d, u) F = (c’/ d) secb cosecb + (tan’/tanb)(1-ru sec2b)

where ru = u/d (different ru for seepage parallel to slope face, seepage emerging, seepage downward, etc)

For Granular Soil: F = (tan’/tanb)(1-ru sec2b) Dry Granular Soil (ru = 0): F = (tan’/tanb)

For Cohesive Soil: F decreases with increasing depth to failure plane; if c is sufficiently large, dc for F = 1 may be large and infinite slope failure may not apply.

Finite Slopes: Plane Failure Surface

Translational Block Slides along single plane of weakness or geological interface

F = c’L + (W cos uL) tan’ / W sin + Fw

Block Slides

Method of Slices

Assumes that resultant of side forces on each slice are collinear and act parallel to failure surface and therefore cancel each other

F = [cn ln + (Wn cosan - un ln) tann] / Wn sinan

Undrained analysis: F = [cn ln] / Wn sinan

Side Forces in Method of Slices

Bishop’s Simplified Method

Assumes that resultant of side forces on each slice act in horizontal direction and therefore vertical side force components cancel each other

F = [cn bn + (Wn - un bn) tann](1/ma) / Wn sinan

ma = cosan + (sinan tanan)/FUndrained analysis: F = [cn ln] / Wn sinan

Wedge Method

Failure surface consists of two or more planes and applicable to slope containing several planes of interfaces and weak layers

Force equilibrium is satisfiedAssumes that resultant of side forces on each slice

either acts horizontally or at varying angles from horizontal (typically up to 15o)

Wedge Analysis

Equilibrium of Forces in each slice is considered to adjust the inter-slice forces and balance them resulting in a correct solution.

Machine Foundations

Machine foundations require a special consideration because they transmit dynamic loads to soil in addition to static loads due to weight of foundation, machine and accessories.

The dynamic load due to operation of the machine is generally small compared to the static weight of machine and the supporting foundation.

In a machine foundation the dynamic load is applied repetitively over a very long period of time but its magnitude is small and therefore the soil behaviour is essentially elastic, or else deformation will increase with each cycle of loading and may become unacceptable.

The amplitude of vibration of a machine at its operating frequency is the most important

parameter to be determined in designing a machine foundation, in addition to the natural frequency of a machine foundation soil system.

Machine Foundations: Block Foundation

Block foundation consists of a massive block of concrete resting directly on soil or supported on piles or a pedestal resting on a footing.

If two or more machines of similar type are to be installed in a shop, these can profitably be mounted on one continuous mat.

A block foundation has a large mass and, therefore, a smaller natural frequency.

The block has large bending and torsional stiffness and easy to construct. To modify the block foundation at a later time is extremely difficult.

Machine foundations: Box or Cassion Foundation

However, if a relatively lighter foundation is desired, a box or a caisson type foundation may be provided.

The mass of the foundation is reduced and its natural frequency increases.

Box or Caisson foundation consists of a hollow concrete block (can be used as operational space) that supports the machine on its top. Hammers may also be mounted on block foundations, but their details would be quite different than those for reciprocating machines.

It has high static stiffness just like a plate foundation and is not easily amenable to alterations at a later date.

Machine foundations: Wall type Foundation

Steam turbines have complex foundations that may consist of a system of walls columns, beams and slabs.

This type is usually adopted for very high-speed machines requiring large operational space below for connecting pipes and additional equipment.

It can be made or either RCC or steel frames. Although the frame made of steel is easy to alter at a later date, its behaviour under dynamic loading is not as good as that of an RCC frame.

Each element of such a foundation is relatively flexible as compared to a rigid block and box or a caisson-type foundation.

Design Criteria for Machine Foundations It should be safe from a bearing capacity failure under static and dynamic

loads, The settlement must be less than the prescribed ones, The dynamic amplitudes of the machine-foundation-soil system must be

within the prescribed limits under service conditions. There should be no resonance, i.e. the natural frequency of the machine-

foundation-soil system should not coincide with the operating frequency of the machine,

Preferably, the Centre of gravity of the machine should lie in the same vertical line as the Centre of gravity of the foundation system.

When design criteria (iii) to (v) are satisfied then the machine itself is not damaged by the vibrations generated, the structure in which the machine is housed and adjacent structures do not

suffer any vibration induced damage, performance of machines located in its vicinity is not impaired, and employees working around the machine are not bothered by the vibrations

Vertical Vibrations of a Machine Foundation

Vertical Vibrations of a Machine Foundation (a) Actual Case (b) Equivalent model with damping ( c) Model without damping

Design of Machine Foundation

In order to calculate the natural frequency and amplitude of vibrations for a particular machine-foundation-soil-system, you need to know the local soil profile and soil characteristics as also the dynamic loads generated by the machine that are provided by the manufacturer. Empirical Elastic half space method Linear elastic weightless spring method, and

Empirical Method On such design guideline, rather a rule of thumb was the weight of

the foundation should be at least three to five times the weight of machine being supported.

There are some empirical formulae available in literature for estimating the natural frequency, mostly for the vertical mode of vibration.

In these formulae, it is assumed that a certain part of the soil, immediately below the foundation, moves as a rigid body along with the foundation and is called apparent soil mass or in-phase mass.

For example, D.D.Barken in 1962 suggested that the mass of the vibrating soil should be between 2/3 to 3/2 times the weight of foundation and machine.

These guidelines/formulae do not take into account the nature of subsoil, type of excitation force (harmonic/impact), contact area and mode of vibration.

THANKS