2. STRESS
When some external system of forces
or loads act on a body, the internal
forces equal and opposite are set up at
various sections of the body, which
resist the external forces. This internal
force per unit area at any section of
the body is known as Unit Stress or
Simple Stress.
3. ð = P/A
P= Force or load acting on the body.
A= Cross section area of the body.
SI units is Pascal. (Pa).
1Pa = 1N/ m2.
4. SOURCES OF STRESS
To evaluate a stress in the ground, we
need to know the location, magnitude
and directions of forces that causes
them.
So we divide these sources in to two
broad categories;
1.
Geo-Static Stress
2.
Induced Stress
5. GEOSTATIC STRESS
Geostatic stress are some time called Body
stress, are those occur due to the weight of
soil above the point being evaluated.
These stresses are caused by gravity acting
on the soil or rock, so the direction result in
vertical normal stress, σZ.
Such stresses are naturally present in the
ground.
This stress has a significant impact on the
engineering behavior of soil.
6. To compute the
geostatic stresses
at a point below the
ground surface,
consider a column
as shown in the
figure.
This column
intercepts the soil
strata with unit
weight γ1, γ2 & γ3.
7. So its weight is;
W = dxdy Σ γH
So the total geostatic stress at the bottom of
the column is then;
σZ = w/A
σZ = dxdy Σ γH /A
σZ = Σ γH
8. INDUCED STRESSES
Induced stresses are those which are
caused by external loads, such as
structural foundations, vehicles or fluid
in a storage tank.
These are usually caused by human
activities.
9. PORE PRESURE
This is the pressure induced in the fluid (either
water, or vapor and water) filling the pores.
Pore fluid is able to transmit normal stress, but
not shear, and is therefore ineffective in
providing the shear resistance.
For this reason it is sometime referred as
Normal Pressure.
U = γw × h
U = Pore water pressure (KN/m³)
γw = unit weight of water (9.81 KN/m³)
h = height of water.
10. EFFECTIVE STRESS (σ’)
This is the stress transmitted through the
soil fabric via inter-granular contacts.
It is this stress component that is effective in
controlling both volume change deformation
and the shear strength of soil since normal
and shear stress is transmitted across grainto-grain contact.
According to Terzaghi showed that for a
saturated soil, effective stress may be
defined as difference b/w Total stress and
Pore pressure.
σ’ = σ - U
11. TOTAL STRESS
The sum of effective stress and pore
water pressure is known total stress.
σ = σ’ + U
m.irfan B-15952
