load bearing mechanism of pile - engineering short courses … · 2020-03-24 · – bored pile or...

Load Bearing Mechanism of Pile

Professor Dr. Md. Jahangir Alam

Pile Types Based on Construction Method

• Based on Construction Method– Driven pile (steel, timber or precast RCC pile driven by

hammering)– Pushed or jacked pile– Driven and cast-in-place (closed end steel tube driven

and then filled by concrete in place)– Bored pile or cast-in-situ pile

Pile types based on materials

– RCC– Timber– Steel– Soilcrete (Soil-Cement)

Pile types based on load distribution

4

Pile

FrictionMore than 80% ultimate load taken by skin friction

End bearing

More than 80% ultimate load taken by end bearing

Combination of Friction & End bearing

Ultimate load taken by both skin friction and end

bearing

Floating pile End bearing is neglected

Pile driving methods

5

Pile Driving

Equipment

Jacking

Vibratory

DrivingHammerin

g

Bored Pile vs. Drilled Shaft?

• Same thing• Sometimes large diameter piles called shaft or

pier

6

Load Bearing Mechanism of Pile

Bearing capacity like a footing

Friction and/or adhesion

Service load

h

σ'v

σ‘h = Kσ'v

σ'v

Kσ'v

τ = c + Kσ’v tan φ

Coefficient of lateral earth pressure (Ks)

3 conditions:- At rest, Ko- Active pressure, Ka- Passive pressure, Kp

Typical Distribution of Skin Friction

Typical Load-Settlement Curve of Pile in Different Soils

Dense sand

Loose to medium dense sand

Soft Clay

LoadSe

ttle

men

t

stiff Clay

Loading-unloading effect on pile

Load

Sett

lem

ent

Ultimate Load Vs. Service Load

Mobilized Friction and End Bearing

Axial Force Distribution along the Pile

100 Ton

40 Ton

2 Ton/m30 m

100 Ton

40 Ton

Compression+ve

Axial Force Diagram

Load-Settlement Curve of a Pile

At Service Load

Compression Test of Pile

Main beam

Test

pile

Anch

or P

ileSupporting block

Anch

or P

ile

Dial gauges

Referencebeam

Extended pile main bar

Reaction beam

Bearing plate Jack

Compression Test of Pile

Tension Test of Pile

Limitations of Compressive Load Test Result

• Don’t account negative skin friction• Don’t include consolidation settlement• Don’t consider group action of piles• Don’t consider lateral load capacity of pile

Pile capacity from single pile load testTest pile is loaded until failure (2 to 4 times the design load)Service pile is loaded upto 1.5 times design load

i. Safe Load for Single Pile :i. Two thirds of the final load at which the load

displacement attains a value of 12 mm unlessotherwise required in a given case on the basisof nature and type of structure in which case,the safe load should be corresponding to thestated total displacement permissible

ii. Fifty (50) percent of the final load at which thetotal displacement equals to 10 percent of pilediameter case of uniform diameter piles and 7.5percent of bulb diameter in case of under-reamed piles. 21

BNBC-2017

Load (kN)

sett

lem

ent

12 mm

1200 kN

Allowable Pile Load Capacity = (2/3)*1200 = 800 kN

Single pile

BNBC-2017

Load (kN)

sett

lem

ent

60 mm

1800 kN

Allowable Pile Load Capacity = (1/2)*1800 = 900 kN

Say, pile dia = 600 mm10% = 60 mm

Single pile

BNBC-2017

Pile capacity from group pile load test

i. Safe Load for Pile Group :i. Final load at which the load

displacement attains a value of 25 mm unless otherwise required in a given case on the basis of nature and type of structure.

ii. Two thirds of the final load at which the total displacement attains a value of 40 mm.

24BNBC-2017

Load (kN)

sett

lem

ent

25 mm

5000 kN

Allowable Group Pile Load Capacity = 5000 kN

Group pile

BNBC-2017

Load (kN)

sett

lem

ent

40 mm

9000 kN

Allowable Group Pile Load Capacity = (2/3)*9000 = 6000 kN

Group pile

BNBC-2017

Lateral Load Test of Pile

Lateral Load Test of Pile

Test

Pile

Deadman Dial Gauges

Reference beamJack

Single pile vs. group pile• Failure mechanism is different for single and

group pile• Failure may be initiated by single piles or block

failure as group; factors are– Spacing of pile *** (more spacing >>>> single pile)– Soil type– Pile length

• Settlement of group is more than single pile

Single pile vs. group pile

Single pile vs. group pileQ

Isobar of a single pile

Highly stressedzoneIsobar of

a group

Q

(a) Single pile (b) Group of piles closely spaced

(c) Group of piles with piles far apart

QPile cap

Settlement of group pileQg Qg

Fictitious footing

(a) (b) (c)

∆p ∆p

L𝟐𝟐𝟑𝟑L

L

Qg

Firm stratum

Weak LayerFictitious footing

L1

Weaker layer

L2z2

1

ARRANGEMENT OF PILE

h

h/3

2h/3

ss

s

3 Pile 4 Pile

5 Pile

s

s

ss

s

6 Pile

s

s

s

7 pile

s

s

s

s s s

sss

8 Pile

s

s

s

s

8 pile

s

s

s

s s

sss

s

s

9 Pile

s

s s

10 pile

s

s

s

s s s

sss

s

s

11 pile

s

s

s

ss

s

s s

s s

s s

s

12 Pile

s

s s

s

s

True Distribution of Load in Pile

Frontiers of knowledge

True Distributions of Load in Instrumented Piles

The measurements are analyzed from the assumption that the “zeroreadings”, which are the readings taken at “zero” time.

This assumption is more than a little off. It neglects the existence of locked-in loads—residual load—in the pile and is one of the sources of the myth of the so-called “critical depth”.

Neglect of the residual load distribution is also the main reason for conclusions of instrumented tests that suggest shaft resistance to be smaller when the pile is loaded in tension as opposed to when it is in loaded in compression.

locked-in stress and strain, also called “residual loads”

Residual load will always develop in a pile, be it a driven or a bored pile

Direct Measurements of Residual Load for Piles Driven in Sand

Four instrumented, 8 m and 16 m long, 280 mm diameter precast concrete piles driven into a very loose sand

it both includes measurements of residual load before the start of the static loading test and of the true load distribution in the piles at the ultimate load

CPT and SPT profiles

Residual distributions are measured before static load test in a driven pile

Load and resistance Distributions in pile

The ultimate resistance of Pile are presented figure, showing the measured resistance distribution labeled “True” and a curve fitted to the data by an effective stress calculation, as well as the measured distribution of residual load.

distributions are characterized by that the load in the pile increases below the pile head due to progressively increasing negative skin friction

At a depth of about 6 m or slightly below, a gradual reduction of negative skin friction and transition to positive shaft resistance begins.

An equilibrium (neutral plane) between downward and upward acting forces exists at a depth of about 10 m below which the transition continues with increasing positive shaft resistance.

The values of locked-in toe resistance residual toe load is 50 KN.

The unit negative skin friction along the upper about 6 m length of the piles corresponds to a beta-coefficient of 0.35 in an effective stress analysis.

• The static loading tests on Pile reached ultimate resistances for loads appliedto the pile head of 510 KN.

• The toe resistance of Pile, starting from the locked-in value of about 50 KN,increased linearly to 110 KN.

• The values of total shaft resistance for Pile 400 KN.

A 400-kN ultimate shaft resistance value corresponds to a beta-coefficient of 0.20 determined in an effective stress analysis.

The reduction of the shear stress was about 40% due to several loading and unloading during the test