Transcript Slide 1

LINTON UNIVERSITY COLLEGE
SCHOOL OF CIVIL ENGINEERING
GEO-MECHANICS
(CE2204)
Soil Stress and Pore Water Pressure
Lecture Week No 2
Mdm Nur Syazwani Noor Rodi
TOTAL VERTICAL STRESS
• The total vertical stress (σv) acting at a point
below the ground surface is due to the weight of
everything lying above i.e. soil, water, and
surface loading
• Total vertical stresses are calculated from the
unit weight of the soil
• Any change in total vertical stress (σv) may also
result in a change in the horizontal total stress
(σ h) at the same point
• The relationships between vertical and
horizontal stress are complex (Δσv ≠ Δσh)
TOTAL VERTICAL STRESS
in homogeneous soil
Ground Level
σv
Depth, z
SOIL
ELEMENT
 v  z
σv
TOTAL VERTICAL STRESS
below a river or lake
Water Level
zw
Ground Level
z
 v  z   w zw
TOTAL VERTICAL STRESS
in multi-layered soil
Ground Level
z1
Soil1
z2
Soil2
z3
Soil3
 v   1z1   2 z2   3 z3
TOTAL VERTICAL STRESS
with a surface surcharge load
Very ‘wide’ surcharge, q (kN/m2)
Ground Level
z
 v  z  q
PORE WATER PRESSURE
• The water in the pores of a soil is called pore
water.
• The pressure within this porewater is called pore
water pressure (u)
• The magnitude of pore water pressure depends
on:
a) the depth below the water table
b) the conditions of seepage flow
PORE WATER PRESSURE
under hydrostatic conditions (no water flow)
Ground Level
Water Table
z
u   wZ
EFFECTIVE STRESS CONCEPT
(Terzaghi, 1923)
  u
'
where
=
'
 =
u
Total Vertical Stress
Effective Stress
= Pore Water Pressure
VERTICAL EFFECTIVE STRESSES
Water Table
Ground Level
z
  u
'
  z   w z
'
EXAMPLE 1
Plot the variation of total and effective vertical stresses,
and pore water pressure with depth for the soil profile
shown below
Ground Level
Water Table
 sat  18.5 kN/m3
4m
GRAVELY
SAND
4m
SAND
 sat  19.5 kN/m3
5m
SAND GRAVEL
 sat  19.0 kN/m3
2m
 B  17.8 kN/m3
EXAMPLE 2
The soil layers on a site consists of:
0 – 4 m Gravel-sand (ρsat= 2038 kg/m3; ρB 1957 kg/m3)
4 – 9 m Clay (ρsat= 1835 kg/m3)
Draw an effective stress and total stress profile between
0 – 9m, when the water table is 1m above the top of the
clay
EXAMPLE 3
On a certain site a surface layer of silty sand is 4m thick
and overlies a layer of peaty clay 7m thick, which in turn
is underlain by impermeable rock. Draw effective and
total stress profiles for the following condition:
a) Water table at the surface
b) Water table at a depth of 5m, with the silty sand
above the water table saturated with capillary
water
Unit weight:
Silty Sand = 18.5 kN/m3
Clay = 17.7 kN/m3
EXAMPLE 4
A confined aquifer comprises a 5m thick of sand overlain
by a 4m thick layer of clay and underlain by impermeable
rock. The unit weight of the sand and clay respectively
are 19.6 kN/m3 and 18.4 kN/m3. Determine effective
overburden stress at the top and bottom of the sand
layer, when the levels of the water in a standpipe driven
through the clay into the sand layer are:
a) at ground surface
b) 1.5m below the ground surface
c) 3.0m below the ground surface
d) 1.5m above the ground surface
e) 3.0m above the ground surface
and hence comment on the effect of changing
water table
EXAMPLE 5
A sediment settling lagoon has a depth of water of 4m
above the clay base. The clay layer is 3m thick and this
overlies 4m of a medium sand, which in turn overlies
impermeable rock. Calculate the effective stresses at the
top of the clay and at the top and bottom of the second
layer under the following condition:
a) Initially, before any sediment is deposited
b) After a 3m layer of sediment of silty fine sand
has been deposited
c) After draining the lagoon down to base level,
with same thickness (3m) of sediment still in
place
Unit weight:
Sand = 20 kN/m3; Clay = 18 kN/m3; Sediment = 16 kN/m3
EXAMPLE 6
Plot the variation of total and effective vertical stresses,
and pore water pressure with depth for the soil profile
shown below for the following condition:
a)initially before construction
b)immediately after construction
c)few days after construction
d)many years after construction.
Surface surcharge, q (100 kN/m2)
Ground Level
Water Table
4m
SAND
2m
CLAY
 sat  18.5 kN/m3
 B  17.8 kN/m3
 sat  19.5 kN/m3
SHORT TERM & LONG TERM STRESSES
a) Initially before construction
Stress distribution profile at its original stage
b) Immediately after construction
The immediately effect after the construction is an increasing in the
pore water pressure – loading is too rapid and not allow any
significant out flow of pore water and the soils are in an
UNDRAINED stage
c) Few days after construction
Few days after the construction, the out flow of pore water takes place
at the Sand layer due to its high permeability and the sand is in
DRAINED stage. i.e. excess PWP is dissipated at the Sand layer
whereas Clay Layer is in contrast
d) Many years after construction
After many years, excess PWP will dissipated in clay layer despite its