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KARAKTERISTIK KEMAMPATAN (COMPRESSIBILITY
CHARACTERISTICS)CHARACTERISTICS)
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PENURUNAN AKIBAT KONSOLIDASI
Sc
z ∆б
H
LAPISAN TANAH KERAS
dz
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ANGGAPAN YANG DIPAKAI DALAM MENGHITUNG SETTLEMENT
1. LAPISAN TANAH DIANGGAP KENYANG AIR
2. TAMBAHAN TEGANGAN Δб BEKERJA PADA LAPISAN dz DI
KEDALAMAN Z
3. TIDAK ADA PERUBAHAN BENTUK LATERAL3. TIDAK ADA PERUBAHAN BENTUK LATERAL
4. SETELAH KONSOLIDASI BERAKHIR, TAMBAHAN Δб’ SAMA
DENGAN TAMBAHAN BEBAN DARI бo‘ KE б1'
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∆v
Vo
e1eo
dSc
dZVo
1
dZ
PENGURANGAN VOLM PER UNIT VOLM = PENGURANGAN TEBAL / SATUAN TEBAL
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Karakteristik Kemampatan (compressibility
characteristic)
• Koef Perubahan Volume : mv
−−
+=
,0
,1
10
01
1
σσee
emv
• Indek Kompresi (compression index): Cc
� kemiringan VCL
• Swelling Index : Cs
� kemiringan swelling line
,0
,1
10
logσσee
Cc
−=
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Koefisien perubahan volume
~ notasi mv
~ regangan volumetrik elemen lempung per satuan kenaikan tegangan
volumeoriginal
volumein change=
σ∆
∆= V
Vmv
i.e.,
no units
F/L^2L^2/F
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Compression and recompression indices
void ratio
Cc ~ compression index
1
Cr
log σv’
void ratio
1
Cc
Cr ~ recompression index
(or swelling index)
1Cr
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Tekanan Prakonsolidasi
void ratio
Tegangan vertikal
maksimum efektif yg pernah dialami tanah
log σv’
void ratio
σp’Tekanan prakonsolidasi
dialami tanah
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Virgin Consolidation Line
virgin consolidation lineeo
original state
log σv’
void ratio
σp’σvo’
eo, σvo’
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Overconsolidation ratio (OCR)
virgin consolidation lineeo
original state
log σv’
void ratio
σp’σvo’Field
σvo’'
'
vo
pOCRσσ
=
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Overconsolidation ratio (OCR)
OCR=1
OCR=13
~current state
VCL
Normally consolidated
log σv’
void ratio
OCR=2
OCR=2
OCR=13
consolidated clay
Slightly overconsolidated clay
Heavily overconsolidated clay
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• Preconsolidation stress (pressure) σc’Tegangan efektif maksimum yang pernah bekerja padaelemen tanah
• Over - consolidated Tanah disebut over-consolidated (OC) jika:
Terminology
Tanah disebut over-consolidated (OC) jika:
• Normally consolidatedTanah disebut normally consolidated (NC) jika:
Tegangan efektif saat ini σ0’ Preconsolidation Stress σc’<
Tegangan efektif saat ini σ0’ Preconsolidation Stress σc’=
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Terminology
e
Keadaan tidak mungkin untuk
Teg efektif saat ini Preconsolidation Stress>
Normal Consolidation
Impossible states
log10 (σ’)
Consolidation Line
Over-consolidated
states
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e
P
Deposition
Normally consolidated soil
log10 (σ’)
Loading
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Over - consolidated soil
Q
P
e = e0e = ef
O
F
R
′σσσσ0 ′σσσσ f′σσσσpc logarithmic scale
Overconsolidation ratio, (5)
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A C
D
F
e
Menentukan preconsolidation pressure
B
F
′σσσσ pc log (σ’)
Cara Casagrande
E
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Menentukan Tekanan Prakonsolidasi
R min
Cr
σσσσc
Cc
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Idealisasi perilaku tanah 1-D
e
log (σ’)
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I
F
eI
eF
Slope IF
Respons idealisasi normally consolidated
∆e
e eF I
F I
−′ − ′log ( ) log ( )10 10σ σ (6a)
e e CF I cF
I
= −′′
log ( )10σσ
log ( )10 ′σ I log ( )10 ′σF
∆e
F Ilog ( / )10 ′ ′σ σ
- Cc (6b)
(6c)
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I
F
eI
eF
Slope IF
Respons idealisasi over - consolidated
∆e
e eF I
F I
−′ − ′log ( ) log ( )10 10σ σ
(7a)
log ( )10 ′σ I log ( )10 ′σF
∆e
F Ilog ( / )10 ′ ′σ σ
- Cr (7b)
(7c)e e CF I rF
I
= − ′′
log ( )10σσ
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(1)
(2)
(3)
e1
e2
e3
e e Cc3 2 103
2
==== −−−−′′′′′′′′
log ( )σσσσσσσσ
e e Cr2 1 102
1
==== −−−−′′′′′′′′
log ( )σσσσσσσσ
Perubahan angka pori utk tanah dari OC ke NC
(8a)
(8b)
(3)e3
log ( )10 1′σσσσ log ( )10 2′σσσσ log ( )10 3′σσσσ
The initial value of σ´pc = σ´2
The final value of σ´pc = σ´3
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CONTOH SOAL
Suatu bangunan dengan pondasi raft 45x30 m akan
menimbulkan tegangan pada dasar pondasi sebesar 12,5 t/m2
mv = 0,0035 m²/t . Hitunglah settlemen akibat proses
konsolidasi ditengah tengah pondasi.
30 m
12,5 t/m²3,5m
12,5 t/m²
MAT
CLAY 4 m
21,5 mPASIR
3,5m
PASIR
23,5 m
30 m
45 m
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PENYELESAIAN
n
m
67,05,23
15
96,05,235,22
==
==
mm
HmvSc
mt
VadumgrafikdariI
98
098,04*7*0035,0**
/75,12*14,0*4
)(140,0
'
2'
=
==∆=
==∆
→=
σ
σ
σ