Syllabus c d (Geotech Engg)

8/6/2019 Syllabus c d (Geotech Engg)

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UNIT-III PILE FOUNDATION

Piles Foundation

Pile foundations consist of piles that are dug into the soiltill a layer of stable soil is reached. Pile foundations transfer building load to

the bearing ground with greater bearing capacity. Pile foundations are useful

in regions with unstable upper soil that may erode, or for large buildings

History Of Pile Foundations

Pile foundations have been used for many years, for

carrying and transferring of loads to soil considered to be weak in structure

due to the soil conditions. In the early stages of development, villages andtowns were located in the close vicinity of lakes and rivers due to the

availability of water, and, also, to ensure proper protection of the area.

Therefore, the weak bearing ground was reinforced by the use of timber

piles that were either manually forced into the ground, or fixed in holes that

were filled with stones and stand. The primitive methods of pile installation

were modified after the industrial revolution, and techniques of installation

by steam or diesel driven machines were introduced. With the advancement

in the technologies of soil mechanics and other related disciplines, superior

piles and pile installation systems have been developed.

Pile Foundations Design

The pile foundations should be carefully designed

in accordance with the soil and load conditions, and the cost. To ensure the

reliability of the piles foundation that should perform as a unit, the pile caps

should be joined with beams or a reinforced concrete slab that could perform

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in tension and compression. The piles should be designed to carry axial,

shear, and bending stresses that may develop by the relative horizontal

movement of piles between the layers in the soil. Piles can be made from

various materials, like steel, timber, and concrete, each possessing different

characteristics that should be considered.

Pile Foundation:

Piles can be made up of

a. Timber

b. Steel

c. Reinforced cement concrete and

d. Pre-stressed concrete

Types of piles

Timber piles are used only for the temporary structures and not for bridge

foundation. Nowadays R.C.C piles are the most common types used in

major bridge foundations.

Concrete piles are furthur classified asa. Cast in situ

b. Precast piles.

In the case of cast in situ piles, a steel is driven first to the

required depth and concreting is done after placing the reinforcement cage in

the hole. If the shell is left place, then it is called a shell pile. If the shell is

removed, it is referred to as shell less pile. During 1950 to 1960, cast in situ

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piles were commonly adopted since the technique of precasting was not well

developed. With the introduction of better quality cement and precasting

techniques, nowadays precast piles are invariably preferred in place of cast

in situ piles since bridge structures generally involve foundations under

water or in soils with a high water table.

Fig: Precast piles

Precast piles can be made with a high precision regarding dimensions and

strength and thus have superior properties than those cast in situ piles.

Precast piles can be cast to various shapes such as

1. circular

2. square

3. rectangular

4. octagonal

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Generally square and circular section piles are preferred to other shapes.

Load Capacity Of Piles

Following are general methods available to establish load capacity:

1. Static Analysis

2. Dynamic Analysis

3. Load Testing

4. Correlation with field tests (SPT, CPT etc)

Dynamic formulae are used for driven piles. Static formulae are used both

for bored and driven piles. Load testing is the most reliable method to

determine the load capacity of the pile in the field.

They should be performed on all piling projects. However, they are

considerably more expensive than the other methods used to determine pile

capacity, and economic considerations sometimes preclude their use on

projects. Field tests like SPT, CPT are also used to correlate to load carrying

capacity particularly for cohesion less soils.

Dynamic Pile Formulas

Piles are usually forced into the ground by a pile driver or pile hammer. Inmedieval times piles were driven by men manually swinging hammer, which

consists of a weight raised by ropes or cables and allowed to drop freely

striking the top of the pile. After the drop hammer came the single acting

hammer, double acting hammer, differential acting hammer, diesel pile

hammer, and vibratory driver. Dynamic pile formulas are widely used to

determine the static capacity of the driven pile. These formulas are derived

starting with the relation.

Energy Input = Energy Used + Energy Lost

The Energy used equals the driving resistance (Pu) x the pile movement (s).

Energy lost is due to friction, heat, hammer rebound, vibration and elastic

compression of the pile, the pacing assembly, and the soil.

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Energy News Record (ENR) Formula:

This formula takes into account the energy lost due to temporary

compression (C) resulting from elastic compression of the piles. Thus,

Energy Input = Energy Used + Energy Lost

Wr x h = Pu x s + Pu x C

Pu = Wr h / (s + C)

Where Wr = weight of the ram, h = height of fall of the ram, s = penetration

of pile per hammer blow, Pu = average resistance of soil to penetration.

C = 25mm (1 inch) for drop hammer, and

C = 2.5 mm (0.1 inch) for steam hammer (single acting/double acting)

Pa = Pu/SF Where Pa = allowable load on pile and

SF = Factor of safety = 6

For single/double acting hammer, the term (Wr x h) can be replaced by

ɳ hE where

ɳ h = hammer efficiency (see accompanying table) and E = rated energy of

hammer.

Engineering News Formula

Qc= Wh / Fs (S+ c)

C=1.0 for single acting hammer

• Work poor, but used a lot because of

convenient

•We can run a pile test at the site and relate to

this formula

Pile load test (INTRODUCTION)

The Foundation Testing Branch (FTB) is the premier

conventional Pile Load Testing (PLT) service, possessing the highest static

axial test capacity in the world. FTB provides static pile load testing services

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for deep foundations throughout the State of California transportation

network. PLT accurately provides the actual capacity and load-deflection

values that only could be estimated by other means. These values can assist

pile designers in calibrating design models, eliminating excess conservatism,

shortening pile length, quantifying capacity in difficult soil and rock

conditions, and generating data for future research.

Static Load Tests

• Most reliable method to determine capacity

• Very expensive & takes a lot of time

• Lower FS can be used

• Most effective when

o Large project with many pile (Lower FS)

o Erratic soil conditionso Pile in soft clay

o Structure sensitive to settlement

o Uplift Capacity

• Before construction- test pile program

• Proof testing during construction

Bearing capacity of piles in clay

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Negative skin friction along piles

Lateral Load Capacity of Piles on Cohesionless Soil

The behaviour of pile under lateral load is studied through laboratory

experiments on model mild steel and

aluminum pipe piles driven into dry river sand. The load-displacement

diagrams were drawn to study theeffect of pile diameter, pile length, weight of pile, pile material, density of

sand, and roughness of pile on

the lateral load capacity of pile. It was found that the lateral load capacity of

piles increases with increase in

length, pile diameter, weight of the pile, pile roughness and increase in sand

density. It is also found that

lateral load capacity is more for steel as compared to that of aluminium.

Several methods are available for predicting the lateral load capacity of piles

in cohesionless soil.

Numerous tests have been carried out on model piles and pile groups

to study the efficiency of pile underlateral loading. Gandhi and Selvam

(1997) studied the behaviour of pile groups of different configuration under

fixed head condition subjected to lateral load through laboratory experiments

on aluminum pipe piles in cohesionless soil. They studied the effect of pile

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driving by comparing the behaviour of single driven pile with that of a bored

pile [2]. Ranjan and Jagannath (2001) conducted model tests on pile groups

of different configuration subjected to lateral loads, in dry Ennore sand.

They investigated the load-displacement response, ultimate resistance, and

group efficiency with spacing and number of piles in a group

Negative skin friction in pile group

When a fill or load is placed on compressible soil deposit, consolidation of

the compressible material takes place. When the piles are driven through

compressible soils before consolidation is complete or the site has newly

placed fill or will be placed in future, the compressible soil will downward

relative to the pile. The downward movement of soil develops skin friction

between the pile and the surrounding soil and it is termed as negative skin

friction. Negative skin friction can be developed from lowering on water level in compressible soils such as clay, peat, mud and soft soil and also due

to increase in stress by some means (e.g. filling).

The net ultimate load carrying capacity of the pile is given by the equation

= negative skin friction

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= net ultimate load

Where it is anticipated that negative skin friction would impose undesirable,

large downward drag on a pile, it can be eliminated by providing a

protective sleeve or a coating for the section which is surrounded by thesettling soil.

To compute negative skin friction on group of piles, the minimum value

from the following equations should be used.

(i) The negative skin friction as the sum of individual piles

Where n – number of piles in a group

– negative skin friction on each pile

= S x p x L (for cohesive soils)

(for granular soils)

(where c – cohesion, - reduction factor)

Where p – perimeter of the pile

L – depth of fill

= earth pressure coefficient

= unit weight of fill

f – coeffieicent of friction =

= angle of friction between pile and soil

(ii) The block skin resistance:

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When the piles are placed close to each other, the negative skin friction

resistance may act effectively on the block perimeter of the pile group.

S – shear resistance of soil

L – depth of fill

p – perimeter of pile group

– unit weight of soil

A – area of pile group enclosed in perimeter p.

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5.4.9 BARRETTES

Barrettes, or barrette piles are installed by machine excavation into a bentonite

slurryfilled trench down to the founding level, inserting the reinforcement cage and

concretingthe excavated trench by tremie method. They are usually of rectilinear

sections andfounded by end bearing on rock. Other founding criteria may also be used

provided thattheir suitability is demonstrated. The design of the pile should be in

accordance withthe provision of Clauses 5.1 to 5.3.The bentonite slurry in the

excavated trench should be of sufficient hydraulic head tomaintain the stability of thetrench, including any surcharge from adjacent structures andconstruction loads. Rigid

reinforced concrete guide walls are usually provided tomaintain alignment and

verticality of the excavation.

5.4.10 HAND-DUG CAISSON

Hand-dug caisson works have been banned for general use because of safety and

healthreasons. However foundation plans including hand-dug caisson works may still

beapproved when it can be demonstrated that any of the following circumstances

exists :(a) the depth of the hand-dug caisson does not exceed 3 m and the diameter of

theinscribed circle of the hand-dug caisson is not less than 1.5 m;(b) for the site

concerned :(i) the use of a hand-dug caisson is the only practical constructionmethod;or(ii) there is no other safe engineering alternative.

5.4.11 STEEL H-PILES DRIVEN TO BEDROCK(1) General Considerations

Where the bedrock is relatively shallow and the soil strata do not

haveadequate strength to allow the founding of piles, steel H-piles driven to

refusalwith pile bases terminated on or very close to bedrock are

sometimesproposed. However, this type of piles often poses the following

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problems:(a) the pile bases are susceptible to damage due to hard driving at

or nearthe bedrock;(b) the pile bases could easily be deflected at the rock

surface particularlywhere the bedrock profile is sloping or undulating;(c) the

piles are prone to have buckling or stability problem, as therelatively

shallow and weak soil strata above bedrock may not provideadequate lateral

resistance to the piles.Where the above problems could not be satisfactorily

coped with, otherfoundation options such as rock-socketed piles should be

considered. Wherethe RSE has confidence in the successful installation of

this type of piles, he orshe may either use any recognized engineering

method to cope with the above

38problems, or follow the guidance on design principles and

constructionrequirements given in items (2) and (3) below.

(2) Design Principles

(a) The maximum allowable axial working stress should not exceed 30%of the yield stress of the steel H-pile;(b) The maximum combined stress due to

the axial load and bendingmoment should not exceed 50% of the yield stress

of the steel H-pile;(c) Piles should be founded on or close to rock not

inferior to category 1(d)defined in Table 2.1. Piles may be considered as founded

on rock whendriven to refusal by using sufficient driving energy (see

Clause5.3.2(1)). Driven to refusal means the actual penetration of a pile is

notmore than 10mm per 10 blows. To avoid overdriving of piles,monitoring

of the peak driving stress by using Pile Driving Analyser orother suitable

method should be carried out as appropriate;(d) The pile bases should be

designed for the hard driving on or close torock; where appropriate, the pile

bases should be strengthened bysuitable means such as welded-on shoes.(e)

The pile bases should be designed to avoid deflection of piles when therock

surface is encountered; where penetration or key into rock isrequired, the

pile bases should be provided with a rock point or othersuitable means;(f)

The stability of the pile foundations should be carefully assessed,particularly

where the piles are short and the embedded soils are weak;in this connection, it is

recommended that the piles should be designedand properly detailed as fixed

head;(g) The buckling behaviour of the piles should be checked, taking

intoaccount the lengths, any lateral load, the embedded conditions andendconnections of the piles; and(h) In the special circumstances that this

type of piles are used on site withsloping or undulating rock surface, the

stability of individual pile aswell as the whole pile group, and that of the

rock under the foundationsshould be assessed, taking into account the joint

orientation of the rock.

(3) Construction Requirements

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(a) As an accurate estimation of the anticipated founding levels isimportant

to the successful installation of this type of piles, the groundinvestigation

should provide a reasonably accurate estimation of thebedrock profile;

39(b) Verticality of the steel H-piles should be checked during

installation;and(c) The RSE must provide adequate supervision of the piling

operation sothat where any set reading or other sign has indicated a damage

ordeflection of the pile base, the pile should be abandoned and replaced.(d)

See Clause 5.4.1 for pile splices and pile head design.

(4) Trial Piles

Trial piles may be required to demonstrate that this type of piles is suitable

insituations such as the following:(a) the bedrock profile is sloping or

undulating;(b) the foundations include piles shorter than 10 m (measured

from cut-off level); or(c) the foundations include piles driven through weak

strata. Note: In the absence of better criteria, strata that do not have a 5 m thick soil

layer that has anaverage SPT N-value not less than 10 and no individual SPT

N-value less than 5 maybe considered as weak strata.

The integrity and capacity of the trial piles should be ascertained by means

of:(a) Pile Driving Analysis (PDA) tests together with modelling by

pilewave analysis program; and(b) static load tests.

(5) Additional Testing Requirements

In addition to the normal testing requirements for piling works, the

testingrequirements specified below are required for this type of piles.Prior

to the commencement of the piling operation, the RSE should confirmthe

design assumptions and that this type of piles could be successfullyinstalled

by test driving a sufficient number of piles, which should in no casebe less

than 2.More stringent proof test requirement than normal piling works may

beimposed; PDA tests to verify the integrity and capacity of at least 10% of

theworking piles are usually required.

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Syllabus c d (Geotech Engg)

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