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# Shear Strength of Soils - geotechnical engineering

Module 7 of Geotechnical Engineering
by

## Aidan Bigham

on 29 October 2015

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#### Transcript of Shear Strength of Soils - geotechnical engineering

Geotechnical Engineering
Shear Strength
Cohesion
Coulomb's Law
Friction
The mutual attraction between fine particles
e.g. clays and silts
This is the combination of the two components to form one equation
Shear Strength of Soils
The shear strength of a soil mass is the
INTERNAL RESISTANCE
that the material can offer against possible sliding along
ANY PLANE
Shear strength (tau) is expressed in kPa.
What do you think are the two most important parameters when discussing shear stress?
Interlocking and friction between particles
Apparent in course grained soils
e.g.
This parameter is measured as an angle
Called either
The angle of internal friction
or
angle of shearing resistance
The
angle of internal friction
will decrease with:
decreasing angularity of particles
decreasing grain size distribution (smaller Cu)
increasing the void ratio
WHY?
Moisture only has a small effect on the frictional strength of the coarse particles.
Cohesion is denoted by "c" and is expressed in stress units kPa
The larger the particle size, the larger the shear strength
As water content increases, the shear strength decreases
So friction and cohesion are the most important factors (for now) that inhibit shear failure.
To test for shear strength
Need to establish conditions in laboratory which represent site conditions
Undisturbed Sample
Undrained shear strength (pore water pressure)
Drained shear strength (no pore water pressure)
Consolidated undrained shear strength
Why do we need to test undrained and drained strength?
Shear strength (kPa) =
cohesion (kPa)
+ stress normal to the plane (kPa) x tan
(angle of internal fiction (degrees)
)
Increasing angularity
Particles less rounded, more ‘sharp’ hence better interlock with each other

Increasing grain size distribution
A bigger Cu (uniformity coefficient = D60/D10 ) this means a bigger spread of granular particles

Decreasing void ratio
Less voids corresponds to a greater Cu, and also is caused by compaction
On this graph:
Label the axes
Include the formula on the chart
Show where the cohesion value is
Show where failure occurs
Show Where failure doesn't occur
Show where failure is about to occur
When designing a geotechnical structure, both undrained and drained conditions must be considered to determine which one is more critical.

For an excavated slope, the long- term or drained condition is more critical because the drained strength of soil is lower than its undrained strength.

On the other hand, for an embankment, the short-term or undrained condition is more critical because the undrained strength of soil is lower than its drained strength.
We determine the values for coloumbs equation
By testing the soil – Lab
Shear box test
Triaxel compression test
Unconfined Compression Strength (UCS) test
Shear Vane (lab)
By testing the soil – Field
Shear Vane (field)
The Shear Box Test
Self Evaluation Exercise 1
Page 9
How does
compaction

effect the
Shear Box test?
Read page 11 and do Self Evaluation Exercise 2 (page 12)
Have a perusal of the Unconfined Compressive Strength test (pg 13)
Shear Vane Test
Primarily used for the field determination of the undrained shear strength of fully saturated clays
Basically the torque needed to fail the clay is corresponded to a shear strength using the formula
Self Evaluation Exercise 3
page 15
What is Mohrs circle?
3 minutes: Find out
Note the mistake in the book
Determine normal stress (x axis)
Determine Shear stress (y axis)
Graph and line of best fit
Detemine y intercept - c
Determine gradient and convert to angle (theta)
Full transcript