**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)