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