Upload
mirceaa21
View
224
Download
0
Embed Size (px)
Citation preview
7/30/2019 Soils Strength
1/41
CE 240
Soil Mechanics & FoundationsLecture 11.2
Shear Strength of Soil II
(Das, Ch. 11)
7/30/2019 Soils Strength
2/41
Class Outlines
Direct shear test
Introduction
Critical void ratio
Triaxial shear test Introduction
Procedure & calculation Critical void ratio
7/30/2019 Soils Strength
3/41
Mohr-Coulomb shear failure criterion
1
1
33n
f
1
3
2
f
= c + n
c
Failed Zone
(, )
ff
7/30/2019 Soils Strength
4/41
Direct Shear Test Direct shear test is Quick and Inexpensive
Shortcoming is that it fails the soil on adesignated 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
7/30/2019 Soils Strength
5/41
Direct Shear Test (cont.)
The test equipment consistsof a metal box in which thesoil specimen is placed
The box is split horizontallyinto two halves
Vertical force (normalstress) is applied through ametal platen
Shear force is applied bymoving one half of the boxrelative to the other to
cause failure in the soilspecimen
Soil
Normal stress n
Shear stress 3
7/30/2019 Soils Strength
6/41
Direct Shear Test
7/30/2019 Soils Strength
7/41
7/30/2019 Soils Strength
8/41
Calculations1. Determine the dry
unit weight, d
2. Calculate the void
ratio, e
3. Calculate the normal
stress & shearstress
1=
d
wGse
A
V
A
N== ;
7/30/2019 Soils Strength
9/41
FiguresPeak Stress
s3
s2
s1
Shear
stress,s
N3 = 30 kg
N2 = 20 kg
N1 = 10 kg
Horizontal displacement, H
7/30/2019 Soils Strength
10/41
Figures (cont)
Shear
Stress,s
(psf)
C
(1,s1)
(3,s3)(2,s2)
Normal Stress , psf
7/30/2019 Soils Strength
11/41
Direct Shear Test Data
Shears
tress
Residual Strength
Peak Strength
7/30/2019 Soils Strength
12/41
Direct Shear Test Data:
Volume change
H
7/30/2019 Soils Strength
13/41
7/30/2019 Soils Strength
14/41
7/30/2019 Soils Strength
15/41
Triaxial Shear Test Developed by Casagrande in an attempt
to overcome some of the seriousdisadvantages of the direct shear test.
Advantages over DST More Versatile
Drainage can be well controlled
There is no rotation of the principal stresses
like the direct shear test
Also the failure plane can occur anywhere
7/30/2019 Soils Strength
16/41
Triaxial Test: Increase the normal
stress in one direction
1
1 major principle stress
n
f3 3
Minor principle stress
Confining stress
7/30/2019 Soils Strength
17/41
7/30/2019 Soils Strength
18/41
Principles of the Triaxial
Compression (TC) Test The triaxial compression test is used to
measure the shear strength of a soil undercontrolled drainage conditions
A cylindrical specimen of soil is encasedand subjected to a confining fluid/air
pressure and then loaded axially to failure.
The test is called "triaxial" because the
three principal stresses are assumed to be
known and are controlled.
7/30/2019 Soils Strength
19/41
Principles of the TC Test
During shear, the major principalstress, 1 is equal to the appliedaxial stress (
= P/A) plus thechamber (confining) pressure, 3
The applied axial stress, 1 - 3is termed the "principal stressdifference" or sometimes the"deviatory stress
The intermediate principal stress,2 and the minor principal stress,3 are identical in the test, andare equal to the confining orchamber pressure
1 = + 3
3
1
3
2 = 3
7/30/2019 Soils Strength
20/41
Triaxial Test Equipment
The Cell (Chamber)
Loading FrameControl Panel
7/30/2019 Soils Strength
21/41
7/30/2019 Soils Strength
22/41
Soil Shear Strength under Drained
and Undrained Conditions .
Drained conditions occur when rate at
which loads are applied are slowcompared to rates at which soil material
can drain (k - dependent) Sands drain fast; therefore under most
loading conditions drained conditions
exist in sands
Exceptions: pile driving, earthquake
loading in fine sands
7/30/2019 Soils Strength
23/41
Soil Shear Strength under Drained
and Undrained Conditions .
In clays, drainage does not occur quickly;
therefore excess pore water pressure
does not dissipate quickly
Therefore, in clays the short-term shearstrength may correspond to undrained
conditions Even in clays, long-term shear strength is
estimated assuming drained conditions
7/30/2019 Soils Strength
24/41
Loose sand and norm.
consolidated clay
dense sand and over-
consolidated clay
7/30/2019 Soils Strength
25/41
Types of Tests
There are three types of tests:1. Unconsolidated-undrained (UU or Q) Test
2. Consolidated-undrained (CU or R) Test3. Consolidated-drained (CD or S) Test
7/30/2019 Soils Strength
26/41
Unconsolidated-undrained Test
This test is also called the quick test.
3 and are applied fast so the soil does
not have time to settle or consolidate. The test is performed with the drain valve
closed for all phases of the test.(Water is not allowed to drain)
UU test simulates short term shear
strength for cohesive soils. For this test, = = 0
s = cu
= Su = (1
-3
)/2 = (1
-3
)/2
7/30/2019 Soils Strength
27/41
UU Test Results
1 3( )2
C lid d d i d T
7/30/2019 Soils Strength
28/41
Consolidated-undrained Test
Apply 3 and wait until the soil consolidates Drainage valves open during consolidation phase but
closed during the shearing phase(Drainage and consolidation is allowed to take place during the application of
the confining pressure 3)
Loading does not commence until the sample ceases todrain (or consolidate).
This test can simulates long term as well as short termshear strength for cohesive soils if pore water pressureis measured during the shearing phase
For this Test, cT c and
T=
From this test we obtain;
c, and u (Effective stress)
cT, T (Total stress)
Skempton pore pressure parameter
7/30/2019 Soils Strength
29/41
Skempton pore pressure parameter
7/30/2019 Soils Strength
30/41
7/30/2019 Soils Strength
31/41
c.
7/30/2019 Soils Strength
32/41
7/30/2019 Soils Strength
33/41
Part c
( ) 41.35 0.257( ) 160.85
d f
f
d f
uA
= = =
C lid t d d i d T t
7/30/2019 Soils Strength
34/41
Consolidated-drained Test
Also called slow test. Drainage valves OPEN during consolidation as
well as shearing phases.
Complete sample drainage is achieved prior toapplication of the vertical load.
The load is applied at such a slow strain rate thatparticle readjustments in the specimen do notinduce any excess pore pressure. (can take up to2 weeks)
Since there is no excess pore pressure totalstresses will equal effective stresses.
This test simulates long term shear strength forcohesive soils.
7/30/2019 Soils Strength
35/41
CD Test Results
7/30/2019 Soils Strength
36/41
Triaxial Test on Sand
Conduct a CD test on sand.
Soil specimens will be loaded to failureunder 3 different confining pressures; 15,
30 and 45 psi
Failure will be defined as the peak or 3
maximum value of principal stress
difference reached.
ASTM D 2850
Test Results and
7/30/2019 Soils Strength
37/41
Test Results and
Calculations
( )
c
c
A
P
A
A
ll
=
=
=
1
0
0
7/30/2019 Soils Strength
38/41
Determining 1
= 2700 psf
32= 1000 psf
psf3700100027001
31
=+=
+=
7/30/2019 Soils Strength
39/41
Mohrs Circles
1 = 3700 psf
3 = 1000 psf 3 = 1000 psf 1= 3700 psf
Triaxial Test on Sand -
7/30/2019 Soils Strength
40/41
Triaxial Test on Sand -
Figures
45 psi
30 psi
15 psi
c
b
a
7/30/2019 Soils Strength
41/41
Reading Assignment:
Das, Ch. 11
HW: Problem 11.11, 11.15