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PONDASI DANGKAL

MEKANIKA TANAH IIYULVI ZAIKA

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SYARAT- SYARAT PEMILIHAN PONDASI1. KEDALAMAN TANAH KERAS

2. BEBAN YANG DITAHAN

3. BIAYA YANG TERSEDIA

4. PENGARUH-PENGARUH LAIN

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PONDASI TELAPAK

Lapisan Tanah Keras

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Combined footing

2 footings close

to each other

P1 close to property

line and P2 > P1

property line

P1 P2

If P2/P1 < 1/2, use strap combined footing

property line

If 1/2 < P2/P1 < 1

use trapezoidal footing

property line

P1 P2

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BENTUK PONDASI TELAPAK

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PONDASI TELAPAK MENERUS/ LAJUR

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PONDASI RAKIT

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PONDASI RAKIT

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PONDASI RAKIT

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SYARAT PONDASI DANGKAL

Kedalaman tanah pondasi kurang atau sama dengan lebarnya atau kedalaman (Terzaghi, 1943).

Teori lain, kedalaman pondasi dangkal 3-4 kali lebar pondasi.

Pondasi setempat harus memenuhi syarat-syarat:

1. Stabilitas, aman terhadap keruntuhan geser

2. deformasi harus lebih kecil dari yang diizinkan.

Lapisan Tanah Keras

Df

B

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METODA PELAKSANAAN

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KERUNTUHAN PADA PONDASI DANGKALPADA TANAH PASIR PADAT DAN LEMPUNG KAKU

B

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MODEL KERUNTUHAN GESER

» keruntuhan umum geser» pasir padat Dr>67%

» lempung kaku NSPT >12

» Koruntuhan lokal» pasir atau

» pasir kelempungan» kurang padat (medium)

» 30%<Dr<67%

» Keruntuhan penetrasi » Pasir lepas

» Dr< 30%

Beban/luas

Penurunan

qu

Beban/luas

Penurunan

qu1

qu

Beban/luas

Penurunan

qu1

qu qu

Permukaan runtuh

Permukaan runtuh

Permukaan runtuh

(a)

(b)

(c)

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MODEL KERUNTUHAN BERDASARKAN RELATIF DENSITY

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Terzaghi Assumptions

• 1.    Soil under footing is homogeneous and isotropic  • 2. Soil surface is horizontally• 3. The base of footing is rough, to prevent the shear displacement.• 4      The foot is shallow foundation, i.e. the depth of foundation is less than

the width of foot…

• Df      ≤        B• 3.     Shear strength above the level of the base of footing is negligible.

c = 0 above ( F.L ).• 4.     Consider only the surcharge which produced as uniform pressure     •    q = »DF at foundation level.

• 5.      The load on foundation is vertical and uniform.• 6.     The foot is long strip footing (pondasi lajur, B/L » 0). • 7. » = »

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Shear failure happened on many stages

• I) Stage I: The soil in the elastic case and behave as the part of foundation it still that, and by increasing the load performed the region I which called active zone.

• II)     Stage II: At this stage the foundation load effect on the active zone and neighboring soil so perform the region which called arc of logarithmic spiral zone.

• III)    Stage III: By increasing the load performed the third part curve in which part the soil became in the passive case it make to resist the failure.

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PROSES TERJADINYA KERUNTUHAN

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Three components produced to resist the failure of soil.

I)  (Pp)γ   = Component produced by the weight of shear zone II,

III.

II)(Pp)c = Component produced by the cohesive stress.

III) (Pp)q = Component produced by the surcharge q.

Pp     =       (Pp)γ +   (Pp)c  +   (Pp)q

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DASAR PENURUNAN RUMUS TERZAGHI

• Kesetimbangan Gaya qu(2b)= -W+ 2Pp+f sin f• W=(1/2)2bg btan =a b2gtanf• f=c DC= c. b/cosf• Pp=(1/2)g (b tan f)2 Kg + c (b tan f)Kc +q(b tan f) Kq• qu =c{tan (f Kc +1)} +q(tan f Kq) + g B/2{(1/2) tan (f Kg. tan -1)}f

Zona aktif

Zona pasif

Zona geser radial

quA B

J

PpPp

W

» »ff

B=2b

»»

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PERSAMAAN DAYA DUKUNG TERZAGHIUNTUK PONDASI LAJUR/ MENERUS

1

2u c qq cN qN BN

245cos2 2

2

a

Nq =

»= e

cot1qNNc =

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TEORI DAYA DUKUNG TERZAGHI (1943)

Daya dukung:

Pondasi lajur

Pondasi bujursangkar

Pondasi lingkaran

Untuk keruntuhan geser lokal:

c’ = 2/3 c dan f’ = 2/3 f

qu

q=gDf

B

Df

45-f/2 45-f/2 45-f/2 45-f/2a aA C

D

EF

H G

J I

1

2u c qq cN qN BN

1.3 0.4u c qq cN qN BN

1.3 0.3u c qq cN qN BN

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GRAFIK FAKTOR DAYA DUKUNG

General shear failureLocal shear failure

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PERBANDINGAN BIDANG RUNTUH

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RUMUS UMUM DAYA DUKUNG PONDASIMEYERHOF (1963)

• Fcs , Fqs , F»s = faktor bentuk

• Fcd , Fqd , F»d = faktor kedalaman

• Fci , Fqi , F»i = faktor inklinasi

1

2u c cs cd ci q qs qd qi s d iq cN F F F qN F F F BN F F F

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FAKTOR PENGARUH MEYERHOF

• F. BENTUK F. KEDALAMAN F. INKLINASI

245tan

1.01

1.01

2.01

2

p

ps

pqs

pcs

K

L

BKF

L

BKF

L

BKF

B

DKF

B

DKF

B

DKF

pd

pqd

pcd

1.01

1.01

2.01

2

2

190

1

o

ci qi

i

F F

F

b

Q

T

R

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DISTRIBUSI TEGANGAN TANAH AKIBAT TEG. VERTIKAL DAN MOMEN

P

M

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Tegangan kontak akibat beban vertikal dan momen

P

M

P

Me

e = B/6qmaxqmin

e < B/6qmax

qmin

e > B/6qmax

qmin

B

eR

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P

e = B/6qmaxqmin

e < B/6qmax

qmin

e > B/6qmax

qmin

e

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Pondasi Lajur

e

B

B-2e

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Stabilitas Daya Dukung

C

Tegangan kontak : tegangan akibat beban

Menentukan eksentrisitas DPT

XB

eV

MX net

2;