Shear Strength of Soil Flashcards
a ________ is one which acts parallel to the normal (i.e. at right angles) to the surface of an object
normal load or force
a _________ is one which acts along the plane of the surface of an object
shear load or force
the ________ acting on a body (either normal or shear)is the appropriate load or force divided by the area over which it acts.
stress
– the net effect of all forces parallel to one direction must be zero– the net effect of all forces orthogonal (at right angles)to the above direction must be zero – the sum of the moments of the forces must be zero
EQUILIBRIUM
When a body is acted upon by external forces then any plane within the body will be subjected to a stress that is generally inclined to
COMPLEX STRESS
The locus of stress condition for all planes through a point
MOHR’S CIRCLE
Soils generally fail in ___
shear
At ______, shear stress along the failure surface(mobilized shear resistance) reaches the shear strength.
failure
The soil grains slide over each other along the failure surface. No crushing of individual grains
Shear Failure
is the maximum shear stress the soil can take without failure, under normal stress
TOTAL STRESSES
Shear strength consists of two components:
______ and _______
cohesive, frictional
c and (reduction nga symbol) are measures of _______.
shear strength
The soil element does not fail if
the ________ is contained
within the envelope
Mohr circle
Initially, Mohr circle is a ____
point
As loading progresses, Mohr
circle becomes ______
larger
failure occurs when Mohr circle touches the _______
envelope
__________
initially subjected to
different isotropic stresses and then loaded
axially to failure
Identical specimens
Most common laboratory tests
to determine the shear strength
parameters are
1.Direct shear test
2.Triaxial shear test
Fieldt ests
- Vane shear test
- Torvane
- Pocket penetrometer
- Fall cone
- Pressure meter
- Static cone penetrometer
- Standard penetration test
___________ is most suitable for consolidated drained tests specially on granular soils (e.g.: sand) or stiff clays
Direct shear test
to
apply vertical load
load framing
to measure vertical and horizontal displacement
dial gauge
to measure shear force
proving ring
Sand is ______
hence c = 0
cohesionless
__________ are drained and pore water pressure are dissipated hence u=0
Direct shear tests
In case of clay, ___________ should be applied at a very slow rate to allow dissipation of pore water pressure(therefore, one test would take several days to finish)
horizontal displacement
In many foundation design problems and retaining wall problems, it is required to determine the _________ between soil and the structural material (concrete, steel or wood)
angle of internal friction
Advantages of direct shear apparatus
-Due to the smaller thickness of the sample, rapid drainage can be achieved
-Can be used to determine interface strength parameters
-Clay samples can be oriented along the plane of weakness or an identified failure plane
Disadvantages of direct shear apparatus
-Failure occurs along a predetermined failure plane
-Area of the sliding surface changes as the test progresses.
-Non-uniform distribution of shear stress along the failure surface
three special types of triaxial tests
Consolidated Drained (CD) test
Consolidated Undrained (CU) test
Unconsolidated Undrained(UU) test
_______ have
no cohesion.
Granular soils
-no excess pore pressure throughout the test
-very slow shearing to avoid build-up of pore pressure
Consolidated Drained (CD) Test
-pore pressure develops during shear
-faster than CD
Consolidated Undrained (CU) Test
-pore pressure develops during shear
-very quick test
Unconsolidated Undrained (UU) Test
Unlike in the CD test, the total and the effective principal test stresses are not the same in the _____. Because the _________ at failure is measured in this test.
CU test, pore water pressure
A simple way to estimate the pore pressure change in undrained loading, in terms of total stress changes ~ after Skempton(1954)
Skempton’s pore pressure parameters
for saturated soils, B =1
B-parameter
For normally consolidated clays Af=1
for heavily overconsolidated clays Af is negative
A-parameter at failure
