mekanika tanah - atterberg

QUIZ!

Dalam keadaan asli, suatu tanah basah mempunyai volume = 0,41 ft3 Dan Berat 42,02 lb. Setelah dikeringkan dalam oven, berat tanah kering Adalah 37,58 lb. Apabila Gs = 2,69 . Berat Volume air 62,4 lb/ft3 . Hitung Kadar air, Berat Volume, Berat Volume Kering, angka pori, porositas danDerajat Kejenuhan.

2

4. Atterberg Limits and

Consistency Indices

3

Batas Atterberg • Kehadiran air dalam tanah berbutir halus dapat berpengaruh secara

berarti terhadap perilaku keteknikan dari tanah, sehingga kita membutuhkan index acuan untuk menjelaskan pengaruhnya.

(Holtz and Kovacs, 1981)

In percentage

4

Atterberg Limits (Lanjutan)

Batas Cair, LL

Cair

Batas Plastis, PL

Plastis

Batas Susut, SL

Semi Padat

Padat

Tanah Kering

Campuran tanah dan air

Kada

r air

mei

ngka

t

5

4.2 Liquid Limit-LL

Cone Penetrometer Method

(BS 1377: Part 2: 1990:4.3)•This method is developed by the

Transport and Road Research Laboratory, UK.

•Multipoint test

•One-point test

Casagrande Method

(ASTM D4318-95a)•Professor Casagrande standardized

the test and developed the liquid limit device.

•Multipoint test

•One-point test

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4.2 Liquid Limit-LL (Cont.)

Dynamic shear test• Shear strength is about 1.7 ~2.0 kPa.

• Pore water suction is about 6.0 kPa. (review by Head, 1992; Mitchell, 1993).

Particle sizes and water•Passing No.40 Sieve (0.425 mm).

•Using deionized water.

The type and amount of cations can significantly affect the measured results.

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4.2.1 Casagrande Method

N=25 blows

Closing distance = 12.7mm (0.5 in)

(Holtz and Kovacs, 1981)

•Device

The water content, in percentage, required to close a distance of 0.5 in (12.7mm) along the bottom of the groove after 25 blows is defined as the liquid limit

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4.2.1 Casagrande Method (Cont.)

.log

)(/log

,12

21

contNIw

valuepositiveachooseNN

wwIindexFlow

F

F

N

w

•Multipoint Method

Das, 1998

9

4.2.1 Casagrande Method (Cont.)•One-point Method

• Assume a constant slope of the flow curve.

• The slope is a statistical result of 767 liquid limit tests.

Limitations:

• The is an empirical coefficient, so it is not always 0.121.

• Good results can be obtained only for the blow number around 20 to 30.

121.0tan

25

tan

contentmoistureingcorrespondw

blowsofnumberN

NwLL

n

n

10

4.2.2 Cone Penetrometer Method•Device

(Head, 1992)

This method is developed by the Transport and Road Research Laboratory.

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4.2.2 Cone Penetrometer Method (Cont.)•Multipoint Method

Water content w%

Pene

trati

on o

f con

e (m

m)

20 mm

LL

12

4.2.2 Cone Penetrometer Method (Cont.)

44094.140LL,094.1Factor

%,40w,mm15depthnPenetratio

(Review by Head, 1992)

•One-point Method (an empirical relation)

Example:

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4.2.3 Comparison

Littleton and Farmilo, 1977 (from Head, 1992)

A good correlation between the two methods can be observed as the LL is less than 100.

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4.3 Plastic Limit-PL

The plastic limit PL is defined as the water content at which a soil thread with 3.2 mm diameter just crumbles.

ASTM D4318-95a, BS1377: Part 2:1990:5.3

(Holtz and Kovacs, 1981)

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4.4 Shrinkage Limit-SL

Definition of shrinkage limit:

The water content at which the soil volume ceases to change is defined as the shrinkage limit.

(Das, 1998)

SL

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4.4 Shrinkage Limit-SL (Cont.)

(Das, 1998)

Soil volume: Vi

Soil mass: M1

Soil volume: Vf

Soil mass: M2

)100)((M

VV)100(

M

MM

(%)w(%)wSL

w2

fi

2

21

i

17

4.5 Typical Values of Atterberg Limits

(Mitchell, 1993)

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4.6 Indices•Plasticity index PI For describing the range of water content over which a soil was plasticPI = LL – PL

•Liquidity index LI For scaling the natural water content of a soil sample to the Limits.

contentwatertheiswPLLL

PLw

PI

PLwLI

LI <0 (A), brittle fracture if sheared0<LI<1 (B), plastic solid if sheared LI >1 (C), viscous liquid if sheared

Liquid Limit, LL

Liquid State

Plastic Limit, PL

Plastic State

Shrinkage Limit, SL

Semisolid State

Solid State

PI

A

B

C

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4.6 Indices (Cont.)

•Sensitivity St (for clays)

strengthshearUnconfined

)disturbed(Strength

)dundisturbe(StrengthSt

(Holtz and Kavocs, 1981)

Clay particl

e

Water

w > LL

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4.6 Indices (Cont.)•Activity A (Skempton, 1953)

mm002.0:fractionclay

)weight(fractionclay%

PIA

Normal clays: 0.75<A<1.25Inactive clays: A<0.75Active clays: A> 1.25High activity:•large volume change when wetted•Large shrinkage when dried•Very reactive (chemically)

•Purpose

Both the type and amount of clay in soils will affect the Atterberg limits. This index is aimed to separate them.

Mitchell, 1993

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• Soil classification (the next topic)

• The Atterberg limits are usually correlated with some engineering properties such as the permeability, compressibility, shear strength, and others.- In general, clays with high plasticity have lower permeability, and they are

difficult to be compacted.- The values of SL can be used as a criterion to assess and prevent the

excessive cracking of clay liners in the reservoir embankment or canal.

4.7 Engineering Applications

- The Atterberg limit enable clay soils to be classified.

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8. ReferencesMain References:Das, B.M. (1998). Principles of Geotechnical Engineering, 4th edition, PWS Publishing

Company. (Chapter 2)Holtz, R.D. and Kovacs, W.D. (1981). An Introduction to Geotechnical Engineering, Prentice

Hall. (Chapter 1 and 2)Others:Head, K. H. (1992). Manual of Soil Laboratory Testing, Volume 1: Soil Classification and

Compaction Test, 2nd edition, John Wiley and Sons.Ifran, T. Y. (1996). Mineralogy, Fabric Properties and Classification of Weathered Granites in

Hong Kong, Quarterly Journal of Engineering Geology, vol. 29, pp. 5-35. Lambe, T.W. (1991). Soil Testing for Engineers, BiTech Publishers Ltd.Mitchell, J.K. (1993). Fundamentals of Soil Behavior, 2nd edition, John Wiley & Sons.

Latihan 1Banyak pukulan Kadar air (%)

15 42,0

20 40,8

28 39,1

Uji batas plastis Kadar air = 18,7

a. Gambar Kurva aliran hasil uji batas cairb. Berapa index Plastisitas tanah?

Latihan 2Banyak pukulan Kadar air (%)

17 42,1

22 38,2

27 36,2

32 34,1

Uji batas plastis Kadar air = 18,7

a. Gambar Kurva aliran hasil uji batas cairb. Berapa index Plastisitas tanah?

Latihan 3Percobaan Batas Cair tanah berbutir halus menggunakan cone penetrometer dengan hasil sebagai berikutCone Penetration (mm) 15.9 17.7 19.1 20.3 21.5

Moisture Content (%) 32.6 42.9 51.6 59.8 66.2

Dalam Uji Batas plastis tanah batas plastis 25%Tentukan Index Plastisitasnya!

Latihan 4Percobaan Batas Cair tanah berbutir halus menggunakan cone penetrometer dengan hasil sebagai berikutCone Penetration (mm) 15.2 17.3 18.9 21.1 22.8

Moisture Content (%) 33.4 42.6 49.2 59.4 66.8

Dalam Uji Batas plastis tanah batas plastis 33%Tentukan Index Plastisitasnya!