The document discusses effective stress in soils. It defines total stress, pore water pressure, and effective stress. Total stress is the load carried by the soil grains and water. Pore water pressure depends on depth and water flow conditions. Effective stress is the difference between total stress and pore water pressure, and represents the stress carried by the soil skeleton. Effective stress applies to saturated soils and influences properties like compressibility and consolidation. It is an imaginary parameter that cannot be directly measured but is important in soil mechanics analyses.

1. Effective Stress

2. Geostatic Stresses
Total Stress
Effective Stress
Pore Water Pressure
Total Stress= Effective Stress+ Pore Water Pressure
stotal = seff + u
Bossinisque Equations
1.Point Load
2.Line Load
3.Strip Load
4.Triangular Load
5.Circular Load
6.Rectangular Load
Added Stresses (Point, line, strip, triangular, circular, rectangular)
Stress Distribution in Soils
Influence Charts Newmark Charts
Stress Bulbs
Westergaard’s Method
(For Pavement)
Approximate Method
1:2 Method
A
By: Kamal Tawfiq, Ph.D., P.E
Added
Stress
Geostatic
Stress
sy
sx
txy

3. Introduction
• Water may influence the nature of the mineral surface
chemically and consequently affect the bonding forces
between adjacent soil grain. This kind of interaction between
the soil and water is called chemical interaction.
• The other kind of interaction is physical interaction between
solid and water.
• Volume of the soil skeleton as a whole can change due to the
rearrangement of the soil particles into new positions mainly
by rolling and sliding due to force acting between particle.
This physical interaction is studied when we study the
effective stress concept.

4. • Effective stress concept was developed by Terzaghi.
• Effective stress concept applies to a fully saturated
soil and relates three types of stress
– Total Stress
– Neutral Stress
– Effective Stress

5. Total Stress
• When a load is applied to soil, it is carried by the solid grains
and the water in the pores.
• The total vertical stress acting at a point below the ground
surface is due to the weight of everything that lies above,
including soil, water, and surface loading.
• Total stress thus increases with depth and with unit weight.
Total Stress = unit weight of the soil * depth (Z or H)

6. TOTAL VERTICAL STRESS
in homogeneous soil
z
v 
s 
Ground Level
Depth, z
SOIL
ELEMENT
σv
σv

7. TOTAL VERTICAL STRESS
below a river or lake
w
w
v z
z 

s 

Ground Level
z
Water Level
zw

8. 3
3
2
2
1
1 z
z
z
v 


s 


Ground Level
z1 Soil1
z2 Soil2
Soil3
z3
TOTAL VERTICAL STRESS
in multi-layered soil

9. TOTAL VERTICAL STRESS
with a surface surcharge load
q
z
v 
 
s
Ground Level
z
Very ‘wide’ surcharge, q (kN/m2)

10. Pore Water Pressure
• The pressure of water in the pores of the soil is called pore
water pressure (u). The magnitude of pore water pressure
depends on:
• the depth below the water table.
• the conditions of seepage flow.
Under hydrostatic conditions, no water flow takes place, and the pore pressure at
a given point is given by
u = ɣw.h

11. Ground Level
Water Table
z
Z
u w


PORE WATER PRESSURE
under hydrostatic conditions (no water flow)

12. Effective Stress
• The principle of effective stress was enunciated by Karl Terzaghi in the
year 1936.
• This principle is valid only for saturated soils,
– At any point in a soil mass, the effective stress (represented by σ' or s' )
is related to total stress (s) and pore water pressure (u) as. σ' = σ - u

13. Effective Stress in Unsaturated Zone
• Above the water table, when the soil is saturated, pore pressure will be
negative (less than atmospheric).
• The height above the water table to which the soil is saturated is called the
capillary rise, and this depends on the grain size and the size of pores. In
coarse soils, the capillary rise is very small.

14. EFFECTIVE STRESS CONCEPT
(Terzaghi, 1923)
u

 '
s
s
s
where
= Total Vertical Stress
'
s = Effective Stress
u = Pore Water Pressure

15. VERTICAL EFFECTIVE STRESSES
z
z w


s 

'
Ground Level
z
Water Table
u

 s
s '

16. Salient points about Effective Stress
• It should be noted that some times effective stress is also
called as intergannular stress (
𝐼𝑛𝑡𝑒𝑟𝑔𝑎𝑛𝑛𝑢𝑙𝑎𝑟 𝑓𝑜𝑟𝑐𝑒
𝐶𝑜𝑛𝑡𝑎𝑐𝑡 𝑎𝑟𝑒𝑎
) which is not
actually true.
• The actual contact stress or intergannular stress is very large,
as the contact area between the particles is very small.
Effective stress is an imaginary parameter which is sum of the
contact force divided by the gross area. This is the reason why
effective stress is not measured, hence it is not a physical
parameter.
• Effective stress is a function of normal force acting at the
contact points of grains and pore pressure.

17. • Effective Stress can only be computed by subtracting pore
water pressure form the total stress , both of which are
physical parameters.
• Even though Effective stress is not a measurable quantity, it is
very important parameter in soil mechanics, because Effective
stress is a parameter on which compressibility, consolidation,
settlements, shear stress and bearing capacity depends. These
parameters do not depend on total stress directly.
• Incase of clay mineral particles in soil mass, mineral crystals
are not in direct contact, since they are surrounded by absorbed
layer of water. But it is observed that inter – granular force can
be transformed through the absorbed layer.
• The principal of effective stress is valid for coarse grained and
clayey soils.

18. Effective Stress In soil when No flow takes place

19. STRESSES WHEN FLOW TAKES PLACE THROUGH THE SOIL
FROM TOP TO BOTTOM

20. STRESSES WHEN FLOW TAKES PLACE THROUGH THE SOIL
FROM BOTTOM TO TOP