Das, B.M. (2004). Principles of Foundation
Engineering, 5th edition, Brooks/Cole
Thomson Learning
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Part One
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Typical Geotechnical Project
Geo-Laboratory
~ for testing
soil properties
Design Office
~ for design & analysis
Soil
mechanics
construction site
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Shallow & Deep
Foundations
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FOUNDATION
load
Foundation
Soil
Condition
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Shallow Foundations
~ for transferring building loads to underlying ground
~ mostly for firm soils or light loads
firm
ground
bed rock
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Deep Foundations
~ for transferring building loads to underlying ground
~ mostly for weak soils or heavy loads
P
I
L
E
weak soil
bed rock
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Perbedaan F. Dangkal & F. Dalam
F. Dangkal
F. Dalam
D/B
Kecil
Besar
Keruntuhan
Sampai
permukaan
tanah
Di dalam
tanah
Digali
Dipancang/
dibor
Instalasi
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Analisis jenis fondasi
Besar
Kecil
Dalam
Fondasi
Dalam
F. Dalam
F. Dangkal
Dangkal
Lapis tanah stabil
Beban
F. Dalam
F. Dangkal
Fondasi
Dangkal
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Pile Foundations
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Pile Foundations
p
Piles are relatively long and slender members used to
transmit foundation loads through soil strata of low
bearing capacity to deeper soil or rock having a higher
bearing capacity.
p
Pile resistance is comprised of
n end bearing
n shaft friction
p
For many piles only one of these components is
important. This is the basis of a simple classification
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Use of pile foundations
When one or more upper soil layers are highly
compressible and too weak to support the load
transmitted by the superstructure. Piles are used to
transmit the load to underlying bedrock or a
stronger soil layer
When bedrock is not encountered at a reasonable depth
below the ground surface, piles are used to transmit the
structural load to the soil gradually. The resistance to the
applied structural load is derived mainly from the
frictional resistance developed at the soil-pile interface
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Use of pile foundations
When subjected to horizontal forces, pile
foundation resist by bending , while still
supporting the vertical load transmitted by the
superstructure
The foundations of some structures, such as
transmission towers, offshore platforms and basement
mats below the water table, are subjected to uplifting
forces. Piles are sometimes used for these foundations
to resist the uplifting force
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Use of pile foundations
Bridge abutments and piers are usually are
usually constructed over pile foundations to
avoid the loss of bearing capacity that a
shallow foundation might suffer because of
soil erosion at the ground surface
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Deep Foundations
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Pile foundation
Tall buildings need
piles down to the
rock bed to transfer
the loads directly to
the solid part in the
earth to avoid
uneven settlement
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Jembatan Suramadu
Sisi Surabaya
Sisi Madura
Total panjang jembatan 5438m
Causeway
Cable Stayed 818m
Approach
Approach
Causeway
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PONDASI CABLE STAYED BRIDGE
20 m
15 m
100 m
100 m
56 Tiang
Diameter 2.4 m
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Sutong Bridge - China
1088m
60m
Pondasi:
Panjang = 130m
Diameter = 3.2m - 60m pertama
2.8m - sisanya
Jumlah = 131 tiang
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Piled Foundations
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Pile
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Jembatan Cikubang
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Jembatan Suramadu
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Ciujung
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Type of Pile Foundations
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Types of Piles
Concrete
Steel
Pipe
Timber
Steel H
Pre-cast
Concrete
Composite
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Steel piles
p
Discription
n
n
p
Advantages
n
n
n
n
p
Usual length 15-60 m
Usual load 300-1200 kN
Easy to handle with respect to cut off and extension to the
desired length
Can stand high driving stress
Can penetrate hard layers
High load-carrying capacity
Disadvantages
n
n
n
n
Relatively costly
High level of noise during driving
Subject to corrosion
H-piles may be damaged or deflected during driving through
hard layers
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Concrete piles
p Precast
piles
n
Using ordinary reinforcement
n
Prestressed : using high-strength steel
prestressing cable
p Cast-in-situ
piles
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Concrete piles
p
Discription
n
n
p
Advantages
n
n
n
n
p
Usual length 10-15m (press : 10-45m)
Usual load 300-3000 kN (press : 7500-8500 kN)
Can be subjected to hard driving
Corrosion resistant
Can be easily combined with a concrete superstructure
High load-carrying capacity
Disadvantages
n
n
Difficult to achieve proper cutoff
Difficult to transport
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Steps in Rational Pile Selection
p
Adequate Subsurface Investigation
p
Soil Profile Development
p
Appropriate Lab/Field Testing
p
Selection of Soil Design Parameters
p
Static Analysis
p
Applied Experience
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Load Magnitude
Deep foundation
type
Typical range of
nominal (ultimate)
resistance (kips)
Typical length
(feet)
Timber pile
75 – 200
20 – 40
Concrete pile
200 – 2,000
20 – 150
Steel H-pile
200 – 1,000
20 – 160
Pipe pile
175 – 2,500
20 – 100
Drilled shaft
750 – 10,000
20 – 160
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What is a Driven Pile?
A Driven Pile is a deep
foundation that is constructed
by driving a concrete, steel or
timber pile to support the
anticipated loads in competent
subsurface material.
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Driven Low Displacement Piles
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Driven High Displacement Piles
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Drilled Shafts (bored piles)
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Driven & Bored Pile
Jenis
Keunggulan
Kekurangan
Driven pile
(Precast pile)
Kualitas terjamin
Dynamic pile capacity
Pelaksanaan singkat
Displacement pile
Human error kecil
Vibrasi saat driving
Tanpa vibrasi
Non displacement pile
Kualitas perlu ketelitian
Non dynamic pile capacity
Pelaksanaan cukup lama
Human error relatif besar
f = Unit Frictional
Resistance
AS = Shaft Area
qP = Unit Bearing
Capacity
AP = Area of Point
QP = qPAP
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Qu
ΔL1
QS1
ΔL2
QS2
Layer 2
ΔL3
QS3
Layer 3
QS4
Layer 4
ΔL4
Layer 1
Qu = ΣQs+Qp
Qp
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End Bearing or Friction?
END BEARING
FRICTION
LOAD
LOAD
L
O
A
D
SANDS
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SOFT
CLAYS
L
L
O
O
A
A
D
D
SANDS
SANDS
CLAYS
CLAYS
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ROCK
SAND
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Method of Support
End Bearing
Side Friction
Combined
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Mekanisme trasfer beban
p
Tahanan friksi (gesekan permukaan) termobilisasi penuh
jika telah terjadi displacement sebesar :
● 5-10 mm (0,2-0,3 inch)……………..B.M. Das
● 0,30 – 1% lebar/diameter tiang …..Tomlinson
p
Tahanan ujung termobilisasi penuh jika telah terjadi
displacement sebesar
● 10-25% lebar/diameter tiang ……….B.M. Das
● 10-20% lebar/diameter tiang ……….Tomlinson
Qs = fAs
f = Unit Frictional
Resistance
AS = Shaft Area
qP = Unit Bearing
Capacity
AP = Area of Point
QP = qPAP
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End Bearing Piles
PILES
SOFT SOIL
ROCK
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Friction Piles
PILES
SOFT SOIL
Strength
increases
with depth
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Mekanisme keruntuhan
Terzaghi
Meyerhof
Vesic
Skempton
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Luthfi Hasan (1998)
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Arching at Pile Tip
Ground Surface
B
Arching Action D
f
Zone of
Shear &
Volume
Decrease
PO = αγDf
γDf
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Loads applied to Piles
V
p
Combinations of vertical, horizontal and moment
loading may be applied at the soil surface from
the overlying structure
p
For the majority of foundations the loads applied
to the piles are primarily vertical
p
For piles in jetties, foundations for bridge piers,
tall chimneys, and offshore piled foundations the
lateral resistance is an important consideration
p
The analysis of piles subjected to lateral and
moment loading is more complex than simple
vertical loading because of the soil-structure
interaction.
M
H
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Estimation of Pile Capacity
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Tahapan desain
p
Mengusahakan data tanah melalui soil investigation,
berupa :
- Cone Penetration Test (CPT = Sondir)
- Standard Penetration Test (SPT)
- Boring (pengambilan sampel tanah)
p
Melakukan survei tentang kedalaman fondasi tiang pada
bangunan sekitarnya
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Tahapan desain (lanjutan)
p
Melakukan estimasi kapasitas fondasi tiang tunggal
menggunakan static formula, berdasarkan data:
- Cone Penetration Test (CPT)
- Standard Penetration Test (SPT)
- Hasil uji laboratorium
- Korelasi dari berbagai data diatas
p
Melakukan estimasi kelompok tiang berdasarkan hasil
estimasi tiang tunggal dan beban kolom yang harus
ditahan
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Tahapan desain (lanjutan)
p
Melaksanakan pile driving dengan menggunakan
dynamic formula berdasarkan estimasi nilai static
formula. Menentukan kapasitas tiang yang digunakan
p
Melaksanakan pile load test bagi fondasi tiang yang
meragukan.
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Estimasi kapasitas tiang
Q u = Q p + Qs − ( W )
Q u = A p .q p + A s .q s
Qall =
A p .q p
SF1
As .q s
+
SF2
Qp
Tahanan ujung end bearing)
Qs
Tahanan friksi (friction resistance)
qp
Unit daya dukung
qs
Unit tahanan friksi
SF1 Angka keamanan untuk tahananujung
SF2 Angka keamanan untuk tahanan friksi
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Menghitung tahanan ujung (end bearing)
Q p = A p .q p
Terzaghi
_
q u = 1,3.c.N c + q N q + 0,4.B.γ.N γ
_
q u = 1,3.c.N c + q N q + 0,3.B.γ.N γ
Square footing
Circular footing
Meyerhof
_
q u = c.N c .Fcs .Fcd + q N q .Fqs .Fqd + 0,5.B.γ.N γ .Fγs .Fγd
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Menghitung tahanan ujung (end bearing)
Deep foundation
General equation
q p = c.N*c
_
+ q .N*q + γ.B.N*γ
N*c , N*q , N*γ Bearing capacity factors
Nilai B atau D kecil
γ.B.N*γ ≈ 0
*
q
=
c
.
N
Sehingga : p
c
_
+ q .N*q
*
Q p = A p (c.N c
_
*
+ q .N q )
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DAYA DUKUNG AKSIAL
Qs =Σ2πr Δl (α C)
+ Σ2πr Δl (k σv tanδ)
Δl
κ σv
Qu = Qp + Qs
Qall =
Qu
F.S.
σv
Qp =Ap(c Nc +q Nq)
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Bearing Capacity Factors for Deep Foundations (Meyerhof, 1976)
1000
800
600
400
200
and
100
80
60
40
20
10
8
6
4
2
1
0
10
20
30
40
45
S oil friction a ngle,
Ø
(deg)
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Tahanan ujung tiang pada tanah pasir
Tanah pasir c = 0 , sehingga :
Q p = A p .q p
_
Q p = A p . q .N*q
_
q = ∑ γh
Meyerhof’s
Method :
Loose
L=LB
L
LB
Dense
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Tahanan ujung tiang pada tanah pasir
qp akan naik sejalan dengan naiknya LB dan akan maksimum pada :
L B ⎛ L B ⎞
= ⎜
⎟
D ⎝ D ⎠critic
Dibawah (Lb/D)cr digunakan qp
Diatas (Lb/D)cr
digunakan qp = qL (limit/batas)
_
Sehingga :
Q p = A p . q .N*q ≤ A p .q L
q L = 50.N*q . tan φ
kN/m2
q L = 5.N*q . tan φ
T/m2
q L = 1000.N*q . tan φ lb/ft2
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Cases
Case-1
Kedalaman tiang 305x305 mm adalah 12 m. Tanah pasir homogen dengan
γb=16,8 kN/m3, φ = 35o. Hitung nilai tahanan ujung tiang (Qp) dengan cara
Meyerhof
Case-2
5m
loose
∇
⊆
13 m
4m
γb=15,7 kN/m3
φ = 30o
c=0
kN/m3
loose
γsat=18,1
φ = 30o
c=0
dense
γsat=19,4 kN/m3
φ = 40o
c=0
Dimensi fondasi : 309 X 309 mm2
Hitunglah : Qp
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Menghitung tahanan friksi (friction)
General :
Qs = ∑ p.ΔL.f
p
= perimeter (keliling tiang)
ΔL
= unit panjang tiang
∑p. ΔL = luas selimut tiang
f =qs
= unit tahanan friksi
f = K.σ'v . tan δ
K = Koefisien tekanan tanah
σ’v = Tegangan efektif vertikal pada kedalaman yang
ditinjau, dianggap konstan setelah kedalaman 15D
(Meyerhof) atau 10D (Schmertmann)
δ
= Sudut gesek permukaan (tanδ = µ)
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DAYA DUKUNG AKSIAL
Qs = Σ2πr Δl (k σ tanδ)
v
Δl
κ σv
Qu = Qp + Qs
Qall =
Qu
F.S.
σv
Qp =Ap(c Nc +q Nq)
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Nilai K dan δ
Nilai K :
Metoda instalasi
K
Tiang pancang, displacement besar
(1-2)Ko
Tiang pancang, displacement kecil
(0,75-1,75)Ko
Bored pile
(0,75-1)Ko
Ko = 1-sinφ
Nilai δ :
Interface
δ
Baja halus
(0,5-0,7) φ
Baja kasar
(0,7-0,9) φ
Precast concrete
(0,8-1) φ
Cast in place
φ
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Menghitung tegangan effektif (σv’)
σ’v akan naik sejalan dengan kedalaman tiang
hingga mencapai kedalaman L’ = 15D (asumsi,
tergantung dari nilai φ, Cc dan Dr), selanjutnya
konstan.
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Case-3
5m
γb=15,7 kN/m3
φ = 30o
c=0
Dimensi fondasi = 400X400 mm ,
loose
γsat=18,1 kN/m3
φ = 30o
c=0
Hitung tahanan friksi tiang (Qs).
dense
γsat=19,4 kN/m3
φ = 40o
c=0
loose
∇
⊆
13 m
4m
K = 1-sin φ , δ = 0,6 φ
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Tahanan ujung tiang pada clay (lempung)
Q p = A p (c.N*c
_
+ q .N*q )
_
Tanah lempung : φ = 0
;
q N q ≈ kecil
Nc = 9
Q p = A p .9.c u
cu = undrained cohesion
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Menghitung tahanan friksi (friction)
Banyak metoda diperkenalkan untuk mencari tahanan
friksi pada lempung : Metoda
Metoda
f
α
α, metoda λ dan metoda β
f = α.cu = α.Su
= unit friksi ; α = adhesion factor ;
cu = undrained cohesion ; Su= undrained strength
α dicari dengan beberapa cara, yang banyak digunakan
adalah API (American Petroleum Institute, 1981) dan
Randolph & Murphy (1985)
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DAYA DUKUNG AKSIAL
Qs =Σ2πr Δl (α c)
Δl
Qu = Qp + Qs
Qall =
Qu
F.S.
Qp =Ap.c Nc
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Faktor penentu nilai α
p
Konsolidasi tanah selama pelaksanaan
p
Dragdown lapisan diatasnya saat pemancangan
p
Cara mendapatkan Su atau cu
p
Tipe instalasi fondasi tiang
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Menentukan α
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Menentukan α
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Nilai undrained shear strength (Su) :
Clay
Su (kPa)
Su (kg/cm2)
Very soft
0-12
0-0,12
Soft
12-24
0,12-0,24
Medium
24-48
0,24-0,48
Stiff
48-96
0,48-0.96
Very stiff
96-192
0,96-1,92
Hard
> 192
> 1,92
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Case-4
5m
∇
⊆
5m
20m
cu =30 kN/m2
γ = 18kN/m3
cu =30 kN/m2
γsat = 19,2 kN/m3
cu =100 kN/m2
γsat = 19,8 kN/m3
Hitung :
Kapasitas tiang ijin (Qall)
Jika diamater tiang 315 mm
dan FS = 4