mekanika tanah 1 - Spada UNS

MEKANIKA TANAH 1

Pertemuan ke 15

Uji Tekan bebas, Uji Gesek langsung

1

Disusun oleh:

Tim KBK Geoteknik Prodi Teknik Sipil FT UNS

Lab. Mekanika Tanah FT UNS, Jl Ir Sutami 36 a Surakarta

Slope Failure in Soils

Failure due to inadequate strength at shear interface

Shear Failure in Soils

Civil Engineering - Texas Tech University

Bearing Capacity Failure

Transcosna Grain Elevator Canada (Oct. 18, 1913)

Civil Engineering - Texas Tech University

West side of foundation sank 24-ft

Significance of Shear Strength

 Engineers must understand the nature of shearing resistance in order to analyze soil stability problems such as;



Bearing capacity



Slope stability



Lateral earth pressure on earth-retaining structures



Pavement

Shear Strength of Soils; Cohesion



Cohesion (C), is a measure of the forces that cement particles of soils

 Dry sand with no cementation

 Dry sand with some cementation

 Soft clay

 Stiff clay

Shear Strength of Soils; Internal Friction



Internal Friction angle ( f ), is the measure of the

shear strength of soils due to friction

Mohr-Coulomb Failure Criteria

• This theory states that a material fails because of a critical

combination of normal stress and shear stress, and not from their either maximum normal or shear stress alone.

• The relationship between normal stress and shear is given as

f

 ^{ }

 

 c tan

s

^{c cohesionangleofinternal friction}

strength shear

s

 

 

 f

Shear Strength,S

Normal Stress,



_n

=  =

g h

C

f = f

Mohr-Coulomb Failure Criterion

General State of Stress

σ

₁

σ

₁ major principle stress

σ

₃

σ

₃

Minor principle stress Confining stress

State of Stresses in Soils

σ

₁

Shear

stress σ

₃

σ

₃

Normal stress σ

_n

Consider the following situation:

- A normal stress is applied vertically and held constant

- A shear stress is then applied until failure

Determination of Shear Strength Parameters

• The shear strength parameters of a soil are determined in the lab primarily with two types of tests;

• Direct Shear Test

• Triaxial Shear Test

Soil

Normal stress σ_n

Shear stress σ₃

₃

₁

Direct Shear Test

• Direct shear test is Quick and Inexpensive

• Shortcoming is that it fails the soil on a designated plane which may not be the weakest one

• Used to determine the shear strength of both cohesive as well as non-cohesive soils

• ASTM D 3080

Direct Shear Test (cont.)

• The test equipment consists of a metal box in which the soil

specimen is placed

• The box is split horizontally into two halves

• Vertical force (normal stress) is applied through a metal platen

• Shear force is applied by moving one half of the box relative to the other to cause failure in the soil specimen

Soil

Normal stress σ_n

Shear stress σ₃

Direct Shear Test

Direct Shear Test

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Direct Shear Test

Direct Shear Test Data

Shear stress

Residual Strength Peak Strength

Direct Shear Test Data Volume change

DH

Direct Shear Test (Procedure)

1.Measure inner side or diameter of shear box and find the area

2.Make sure top and bottom halves of shear box are in contact and fixed together.

3.Weigh out 150 g of sand.

4.Place the soil in three layers in the mold using the funnel. Compact the soil with 20 blows per layer.

5.Place cover on top of sand 6.Place shear box in machine.

7.Apply normal force. The weights to use for the three runs are

2 kg, 4 kg, and 6 kg if the load is applied through a lever arm, or 10 kg, 20 kg, and 30 kg, if the load is applied directly.

Note: Lever arm loading ratio 1:10 (2kg weight = 20 kg)

Direct Shear Test (Procedure)

8. Start the motor with selected speed (0.1 in/min) so that the rate of shearing is at a selected constant rate

9. Take the horizontal displacement gauge, vertical displacement gage and shear load gage readings. Record the readings on the data sheet.

10. Continue taking readings until the horizontal shear load peaks and then falls, or the horizontal displacement reaches 15% of the

diameter.

Civil Engineering - Texas Tech University

Calculations

1. Determine the dry unit weight, g

_d

2. Calculate the void ratio, e

3. Calculate the normal stress & shear stress

 1



d

Gs

w

e g g

A V A

N 

 

 ;

Figures

Shear stress, s

Peak Stress

N₁= 10 kg N₂= 20 kg N₃ = 30 kg

Horizontal displacement, DH s₃

s₂ s₁

Figures (cont)

ShearStress, s (psf)

C

f

(₁,s₁)

(₃,s₃) (₂,s₂)

Normal Stress

,

psf

Figures (cont)

Verticaldisplacement

Horizontal displacement (Sample memampat selama di-uji geser)

Sample mengembang selama di-uji geser

Sample contracts during sheared

Sample dilates during sheared

ROTATION OF PRINCIPAL STRESSES IN

DIRECT SHEAR TEST DURING SHEARING (1)

ROTATION OF PRINCIPAL STRESSES IN

DIRECT SHEAR TEST DURING SHEARING (2)

Uji Tekan Bebas

Unconfined Compression Test

• For clay soils

• Cylindrical Test specimen

• No confining stress (i.e. 

₃

= 0)

• Axial stress, D = 

₁

• ASTM D-2166



₃

= 0



₁

Mohr’s Circles



₃

=0



₁

Uniaxial

Compression

UC Test Data

 

c c

A P A A

l l



 

 D







1

0 0

psf q psi

Strngth Shear

Undrained S

psi Strength

n Compressio Unconfined

q

u u

u

972 75

. 2 6

5 . 13 2

5 . 13















UC Test (cont.)

= 6.75 psi

q_u = 13.5 psi



₃



₁



₁

45 ° + Ø /2

Most possible shear plane

 ^{max =} c ⁺  ^{’ n tan Ø}

DAFTAR PUSTAKA

Das, Braja M. 2002. Soil Mechanics Laboratory Manual. New York:

Oxford University Press.

Shafiq, Hossam. 2015. Slideshare, Class 6 Shear Strength – Direct Shear Test. Texas Tech University.