ECG426 – SOIL MECHANICS
WEEK 6 - i ~ CHAP. 4
SHEAR STRENGTH OF SOILS
LEARNING OUTCOMES
Week 6 : Coverage ~ Introduction to shear strength of soils.
The friction model. Laboratory determination of shear strength of
soils
Learning outcomes:
At the end of this lecture/week the students would
be able to:
i. Concept of total and effective stress, pore pressure and
excess pore water pressures.
ii. Concept of friction model.
SHEAR STRENGTH OF SOILS
TOPIC 4 : SHEAR STRENGTH OF SOILS I -
INTRODUCTION & LAB TESTING
SHEAR STRENGTH OF SOILS
OUTLINE of PRESENTATION
4.1 Introduction & Overview
4.2 Concept of Total and Effective Stresses
4.3 Definition and the friction model
4.4 Laboratory determination of shear
strength
4.1 INTRODUCTION AND OVERVIEW
- Jan 2007
SHEAR STRENGTH OF SOILS
Introduction and overview
SHEAR STRENGTH OF SOILS
Introduction and overview
SINKHOLE AT ORAN ARMY CAMP, PERLIS (14/10/2001)
SHEAR STRENGTH OF SOILS
Introduction and overview
Rock fall at Bukit Lanjan
on N-S Expressway
Slope failure at Bukit
Antarabangsa
Condominium (15/5/1999)
SHEAR STRENGTH OF SOILS
Introduction and overview
ROAD EMBANKMENT FAILURE
Berita Harian
(01/01/1994)
SHEAR STRENGTH OF SOILS
Introduction and overview
Why does soil fail to perform
its function ??
Is it because of soil erosion?
Degradation of soil materials ?
Loss in strength ?
Reminder from God al-Mighty ?
SHEAR STRENGTH OF SOILS
Total and Effective Stresses
4.2 CONCEPT OF TOTAL AND EFFECTIVE STRESSES
Consider an
element of a
saturated soil
subjected to a
normal stress,
as in the figure.
Stress is called total stress and for equilibrium
(Newton’s third law) the stresses in the soil must be
equal and opposite to .
SHEAR STRENGTH OF SOILS
Total and Effective Stresses
Resistance to , is provided by a combination
of the stresses from the solid called
effective stress (’) and from pore water in
the pores, called pore pressure u.
Thus = ’ + u
’ = - u
This equation is called the principle of effective stress
– first recognized by Terzaghi (1883-1963) – research in
soil consolidation.
Principle of effective stress
SHEAR STRENGTH OF SOILS
Concept of effective stresses
SHEAR STRENGTH OF SOILS
Total and Effective Stresses
`The principle of effective stress is the
most important principle in soil mechanics.
Definition of soils are a function of
effective stresses not total stresses. The
principles of effective stress applies only
to normal stress and not to shear stress‘
`Soil cannot sustain tension. Consequently
the effective stress cannot be less than
zero. The pore pressure can be positive or
negative. The latter is sometimes called
suction or suction pressure.‘
SHEAR STRENGTH OF SOILS
Total and Effective Stresses
For unsaturated soil:
’ = – ua + (ua – uw)
where ua = pore air pressure
uw = pore water pressure
= factor depending on the degree of
saturation: For dry soil = 0, and
for saturated soil = 1
SHEAR STRENGTH OF SOILS
Total and Effective Stresses
Pore water pressure in soil mass can be determined
using either (i) Porewater pressure transducer
or
(ii) Piezometers
SHEAR STRENGTH OF SOILS
Total and Effective Stresses
Stress in the ground; For a case of total stress,
1) no surcharge 2) Ground below water 3) With surcharge load, q
level (lake/river/seabed)
gwt
q
ground surface ground surface
hw
γ h γsat h γ h
o = γh o = γsath + γwhw o = γh + q
The total stress in the The total stress in the The total stress in the
ground in only due to ground in only due to ground in only due to
overburden pressure only overburden pressure + overburden pressure +
hydrostatic pressure surcharge load
SHEAR STRENGTH OF SOILS
Total and Effective Stresses
Stress in the ground; For a case with gwt
occurance, ‘
1) no gwt 2) gwt at surface 3) gwt beneath surface
ground surface ground surface gwt ground surface
γ γsat γ h
h hw gwt
γsat hw
o’ = o – u o’ = o – u o’ = o – u
o’ = γh – 0 = γsathw – γwhw = γh + γ’hw
= γh = (γsat– γw)hw
= γ’hw
SHEAR STRENGTH OF SOILS
Groundwater and pore pressure
suction
23
SHEAR STRENGTH OF SOILS
Definition & the friction model
SHEAR STRENGTH OF SOILS
Definition & the friction model
Shear Strength of Soil
Shear strength between a soil mass is due to the
development of frictional resistance between adjacent
particles, and analyses are based primarily on the
friction model.
The force transmitted between
2 bodies in static contact can be
resolved into 2 components:-
Normal component (N )
Shear component (T )
* both are perpendicular to
each other.
SHEAR STRENGTH OF SOILS
Definition & the friction model
SHEAR STRENGTH OF SOILS
Definition & the friction model
SHEAR STRENGTH OF SOILS
Cohesion
SHEAR STRENGTH OF SOILS
Figure below shows a typical shear stress/displacement
(strain) curve for the shearing of a prismatic element of soil
under constant normal stress ( n)
The maximum value of the shear stress at the yield point is
termed the peak stress and represents the limiting value
corresponding to that value of normal stress.
Peak, ultimate
and residual
limiting stress
SHEAR STRENGTH OF SOILS
The shear stress continue to fall until it levels off again at a
lower value, known as the ultimate stress. The ultimate
stress represents the shear strength of the material at its
critical state and the volume is at critical volume. The
ultimate stress value is usually be reached at strains of
between 10% to 20%.
Peak, ultimate
and residual
limiting stress
SHEAR STRENGTH OF SOILS
At very much larger strains, e.g. on active landslip
surfaces in clay soils, the limiting shear stress falls
further, and at displacements of over a meter it may be
reduced to values as low as 10 % of the peak stress. This
very low large-strain value is referred as the residual
stress.
Peak, ultimate
and residual
limiting stress
