shear strength i - ub

SHEAR STRENGTH IYULVI ZAIKA

MATERIKeruntuhan mohr coulomb,

stress paths,

kuat geser tanah non kohesif dan kohesif,

evaluasi kuat geser di lapangan,

tegangan normal dan tegangan geser pada sebuah bidang

Strength of different

materials

Steel

Tensile

strength

Concrete

Compressive

strength

Soil

Shear

strength

Presence of pore waterComplexbehavior

Shear strength of soilSoil cannot resist tension

Soil can resist compression.

For excessive compression failure occurs in the form of shearing along the internal surface within the soil

Structural Strength of soil = f ( soil’s shear strength)

Shear Strength◦ Soil’s ability to resist sliding

◦ Important for

◦ foundation design

◦ Lateral earth pressure calculations

◦ Slope stability analysis

Shear Strength due to sliding friction

SlidingForce

Normal Force

Shear Strength due to sliding friction

Normal Force

SlidingForce

What is Shear Strength?

Shear strength in soils is the resistance to movement between particles due to physical bonds from:

A.Particle interlocking

b.. Atoms sharing electrons at surface contact points

C.. Chemical bonds (cementation) such as crystallized calcium carbonate

Influencing Factors on Shear Strength

The shearing strength, is affected by:

––soil composition,: mineralogy, grain size and grain size distribution, shape of particles, pore fluid type and content, ions on grain and in pore fluid.

–– Initial state: State can be describe by terms such as:

loose, dense, over--consolidated, normally

consolidated, normally

consolidated, stiff, soft, etc.

––Structure : Refers to the arrangement of particles within the soil mass; the manner in which the particles are packed or distributed. Features such as layers, voids, pockets, cementation, etc, are part of the structure.

Embankment

Strip footing

Shear failure of soilsSoils generally fail in shear

At failure, shear stress along the failure surface(mobilized shear resistance) reaches the shear strength.

Failure surface

Mobilized shear resistance

Retaining wall

Shear failure of soilsSoils generally fail in shear

Retaining wall

Shear failure of soils

At failure, shear stress along the failure surface(mobilized shear resistance) reaches the shear strength.

Failure surface

Mobilized shear resistance

Soils generally fail in shear

Shear failure mechanism

The soil grains slideover each other alongthe failure surface.

No crushing ofindividual grains.

failure surface

Shear failure mechanism

At failure, shear stress along the failure surface ()reaches the shear strength (f).

Mohr-Coulomb Failure Criterion(in terms of total stresses)

f is the maximum shear stress the soil can take without failure, under normal stress of .

tan cf

c

CohesionFriction angle

f

Mohr-Coulomb Failure Criterion(in terms of effective stresses)

f is the maximum shear stress the soil can take without failure, under normal effective stress of ’.

’

'tan'' cf

c’

’

Effective cohesion Effective

friction anglef

’

u '

u = pore water

pressure

Mohr-Coulomb Failure Criterion

'tan'' ff c

Shear strength consists of two components: cohesive and frictional.

’f

f

’

'

c’ c’

’f tan ’ frictional component

c and are measures of shear strength.

Higher the values, higher the shear strength.

NOTE

Mohr Circle of stress

Soil element

’1

’1

’3’3

q

’

q

q

2cos22

2sin2

'

3

'

1

'

3

'

1'

'

3

'

1

Resolving forces in and directions,

2'

3

'

1

2'

3

'

1'2

22


2'

3

'

1

2'

3

'

1'2

22

’

2

'

3

'

1

2

'

3

'

1

'

3 '

1


2'

3

'

1

2'

3

'

1'2

22

’

2

'

3

'

1

2

'

3

'

1

'

3 '

1

PD = Pole w.r.t. plane

q

(’, )

Soil elements at different locations

Failure surface

Mohr Circles & Failure Envelope

X X

X ~ failure

YY

Y ~ stable

’

'tan'' cf


Y

c

c

c

Initially, Mohr circle is a point

c+

The soil element does not fail if the Mohr circle is contained within the envelope

GL


Y

c

c

c

GL

As loading progresses, Mohr circle becomes larger…

.. and finally failure occurs when Mohr circle touches the envelope

’

2

'

3

'

1 '

3 '

1

PD = Pole w.r.t. plane

q

(’, f)

Orientation of Failure Plane

’

’1

’1

’3’3

q

’

’1

’1

’3’3

qq

’

Failure envelope

(90 – q)

Therefore,

90 – q ’ = q

q 45 + ’/2

Mohr circles in terms of total & effective stresses

= X

v’

h’X

u

u+

v’h’

effective stresses

uvh

X

v

h

total stresses

or ’

Failure envelopes in terms of total & effective

stresses

= X

v’

h’X

u

u+

v’h’

effective stresses

uvh

X

v

h

total stresses

or ’

If X is on failure

c

Failure envelope in

terms of total stresses

’

c’

Failure envelope in terms

of effective stresses

Mohr Coulomb failure criterion with Mohr circle

of stress

X

’v = ’1

’h = ’3

X is on failure ’1’3

effective stresses

’’ c’



c’ cot’ (’1 ’3)/2

(’1 ’3)/2

2'sin

2'cot'

'

3

'

1

'

3

'

1

c

Therefore,

Mohr Coulomb failure criterion with Mohr circle

of stress

2'sin

2'cot'

'

3

'

1

'

3

'

1

c

( ) ( ) 'cos'2'sin'

3

'

1

'

3

'

1 c

( ) ( ) 'cos'2'sin1'sin1 '

3

'

1 c

( )( ) ( )'sin1

'cos'2

'sin1

'sin1'

3

'

1

c

2

'45tan'2

2

'45tan2'

3

'

1

c

If c=0 then

𝑠𝑖𝑛𝜑 =𝜎1𝑓−𝜎3𝑓

𝜎1𝑓+𝜎3𝑓

Or

𝜎1

𝜎3=

1+𝑠𝑖𝑛𝜑

1−sin 𝜑= 𝑡𝑎𝑛2 45 +

𝜑

2

’1’3

effective stresses

’’



(’1 ’3)/2

exercisesA cylindrical soil (sand) sample was subjected to axial principal stress (1) and radial principal stress (3). The soil can not support additional stress when 1’= 300kPa and 3’=100 kPa. Determine the friction angle and the inclination of the slip plane to the horizontal.

Stress Point

31

t

s

hv

(v-h)/2

(v+h)/2

stress pointstress point

2hvs

2hvt

X

v

h

Stress Path

32

t

s

Stress path adalah tempattitik tegangan

Stress path

Stress path adalah cara yang lebih mudah digunakan untuk melihatProses pembebanan dari awal sampai mencapaiselubung keruntuhan

During loading…

SELUBUNG KERUNTUHAN

33

t

s

c

c cos

tan-1 (sin )

failure

During loading (shearing)….

stress path

FIELD TESTVANE SHEAR

Measure undrained shear strength of soil

Suitable for very soft clay to medium stiff clay

Principle of equipment operation : vane pushed and rotated

The vane shear equation :

𝑠𝑢 =2𝑇

𝜋𝑑3 ℎ 𝑑+1

3

Where:

T : Maximum torque

H : height of vane

d : diameter of vane

Standard Penetration Test (SPT)

shear strength i - ub

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