Soil mechanics - Consolidation of soils - Settlement along time - Degree of consolidation

1. 2017
“ Soil Mechanics “
Consolidation of Soils
Name : Moussa Amr Hussein
Assr Mohamed
Dr Alexander Kislyakov

2. What is Consolidation ?
Consolidation is a process by which soils decrease in
volume. According to Karl Terzaghi consolidation is
any process which involves decrease in water
content of a saturated soil without replacement of
water by air . In general it is the process in which
reduction in volume takes place by expulsion of
water under long term static loads. It occurs when
stress is applied to a soil that causes the soil
particles to pack together more tightly, therefore
reducing its bulk volume. When stress is removed
from a consolidated soil, the soil will rebound,
regaining some of the volume it had lost in
the consolidation process. If the stress is reapplied,
the soil will consolidate again along a recompression
curve, defined by the recompression index. The soil
which had its load removed is considered to be
overconsolidated. The highest stress that it has been
subjected to is termed the preconsolidation stress .
The over consolidation ratio or OCR is defined as
the highest stress experienced divided by the current

3. stress. A soil which is currently experiencing its
highest stress is said to be normally consolidated
and to have an OCR of one. A soil could be considered
underconsolidated immediately after a new load is
applied but before the excess pore water pressure has
had time to dissipate . In case of coarse grained soils
like sands and gravels, the removal of this pore
water is easy since water freely moves from one
region to another within these soil types.
However, in case of fine grained soils like silty or
clayey soils, consolidation is a time consuming process.
In case of fine grained soil on which a structure
is to be built, high water content is not desired as
the weight of the structure may cause sinking
(consolidation settlement) of the structure in due
time. Typically the permeability (ability of water to
move through the soil voids) of fine grained soils is
low, hence it takes a long time for consolidation
process.

4. So two aspects of consolidation settlement are
Important
The rate at which the consolidation is taking place and
The total amount of consolidation. It is very important to
note that unlike settlement in sands and other coarse
grained soil, consolidation settlement of fine grained soil
does not occur immediately . Hence, it is common
practice to ensure that the consolidation process is
expedited and that most of the consolidation takes place
during the various phases of construction. If the soil is
such that it has never experienced pressure of the
current magnitude in its entire history, it is called a
normally loaded soil. The soil is called pre-consolidated
(or overconsolidated) if at any time in history, it has
been subjected to a pressure equal to or greater than
the current pressure applied to it. In case of normally
consolidated soils, the consolidation will be greater than
that for a pre-consolidated soil. That is because the pre-
consolidated soil has previously experienced greater or
equal pressure and has undergone at least some
consolidation under that pressure. So a pre-consolidated
soil is preferred over a normally consolidated soil.

6. Types of Consolidation
Primary Consolidation:
It is the reduction in volume due to expulsion of
water from the voids. Expulsion of water from the
voids depends on permeability of soil and it is
therefore time dependent.
Secondary Consolidation:
When all the water is squeezed out of the voids
and primary consolidation is completed, further
reduction in volume of soil is called secondary
consolidation. It may be due to plastic deformation
of the soil particles or some other reasons. The value
is however very small and commonly neglected.

7. ► The process of consolidation is often confused
with the process of compaction.
► The difference between consolidation and
compaction can be appreciated using three-phase
diagrams as shown below:

10. Elastic Settlement or Immediate Settlement
This settlement occurs immediately after the load is
applied. This is due to distortion (change in shape)
at constant volume. There is negligible flow of
water in less pervious soils. In case of pervious
soils the flow of water is quick at constant
volume. This is determined by elastic theory (E &
μ are used). It occurs due to expulsion of pore water
from the voids of a saturated soil.

11. The movement of pore water depends on the
permeability and dissipation of pore water pressure.
With the passage of time the pore water pressure
dissipates, the rate of flow decreases and finally
the flow of water ceases. During this process there
is gradual dissipation of pore water pressure and a
simultaneous increase of effective stress as shown
in Fig above. The consolidation settlement occurs
from the time water begins move out from the
pores to the time at which flow ceases from the
voids. This is also the time from which the excess
pore water pressure starts reducing (effective stress
increase) to the time at which complete dissipation
of excess pore water pressure (total stress equal to
effective stress). This time dependent compression is
called “Consolidation settlement”.

12. Primary consolidation is a major component of
settlement of fine grained saturated soils and this
can be estimated from the theory of
consolidation.
In case of saturated soil mass the applied stress is
borne by pore water alone in the initial stages:
At t = 0 ∆σ= ∆u ∆σ´= 0
With passage of time water starts flowing out from
the voids as a result the excess pore water pressure
decreases and simultaneous increase in effective stress
will takes place. The volume change is basically due to
the change in effective stress ∆σ´. After considerable
amount of time (t = ∞) flow from the voids ceases the
effective stress stabilizes and will be is nequal to
external applied total stress (∆σ ) and this stage
signifies the end of primary consolidation.
At t = t1 ∆σ = ∆σ ′ + ∆ u
At t = ∆σ = ∆σ ꞌ
∆u = 0 (End of primary consolidation)

13. Secondary Consolidation Settlement
This is also called Secondary compression (Creep).“It is
the change in volume of a fine grained soil due to
rearrangement of soil particles (fabric) at constant
effective stress”. The rate of secondary consolidation
is very slow when compared with primary
consolidation.

14. Settlement of a Soil Layer
The settlement is defined as the compression of a soil
layer due to the loading applied at or near its top
surface. The total settlement of a soil layer consists of
three parts:
- Immediate or Elastic Compression
- Compression due to Primary Consolidation
- Compression due to Secondary Consolidation
The immediate or elastic compression can be calculated
using the elastic theory if the elastic modulus of the soil
layer is known.
Compaction increases the density of an unsaturated soil
by reducing the volume of air in the voids.
Consolidation is a time-related process of increasing
the density of a saturated soil by draining some of the
water out of the voids. Consolidation is generally
related to finegrained soils such as silts and clays.
Coarse-grained soils (sands and gravels) also undergo
consolidation but at a much faster rate due to their
high permeability.

15. Saturated clays consolidate at a much slower rate due
to their low permeability.
Consolidation is gradual reduction in volume, on load
application, of a fully saturated clay due to drainage of
pore water
Consolidation continues until all the excess pore
pressure generated by the increase in total stress has
completely dissipated
The consolidation test is used to find compressibility
parameters (σꞌvp, Cc, Cs, and Cv)

16. Consolidation Settlements
Preconsolidation Pressure
The preconsolidation pressure for an overconsolidated soil
should not be exceeded in construction, if possible.
Consolidation settlements will small if the effective
vertical stress in the soil layer remains below its
preconsolidation pressure. If effective vertical stress in
the soil layer exceeds its preconsolidation pressure, the
consolidation settlements will be large due to further
yielding of the soil layer .The estimation of preconsolidation
pressure is greatly affected by the amount of disturbance
experienced by the soil sample.
Estimation of Preconsolidation Pressure
Preconsolidation
pressure (σ’c) for an OC
soil can
be estimated from the e-
log( σ’v) curve using
Casagrande’s procedure.

17. Consolidation Settlement – 1-D Method
For the calculation of consolidation settlement
using the 1-D method, the value of either the
coefficient of volume compressibility (mv) or the
Compression Index (Cc) is required. Let’s consider a
layer of saturated soil of thickness H subjected to
an increase in total vertical stress of Δσ as shown in
the figure on the right. At the completion of
consolidation, the soil layer will experience an
increase in effective vertical stress of Δσ’ and as a
result, its void ratio will reduce from e0 to e1.

18. Degree of Consolidation
For an element of soil at a depth z, the progress of
the consolidation process under a particular total
stress increment can be measured in terms of the
void ratio at an instant of time t as :
eo–void ratio before the start of
consolidation
e1–void ratio at the end of consolidation
e –void ratio at time t after the start of
consolidation
• Uz is defined as the degree of consolidation at a
particular instant of time tat depth z.
• Uz takes a value between 0 and 1. At the start of
consolidation Uz= 0 and at the end of
consolidation, Uz= 1.

19. If we assume the e-σ’
curve to be linear over
the range of stresses in
the field (see figure), the
degree of consolidation
can be expressed in
terms
of effective stresses as :
where σ’ is the effective
stress in the soil at an
instant of time t and is
given by :

20. Since the excess pore
pressure is zero at the
end
of consolidation, the
effective stress at the
end of
consolidation can be
expressed as :
Therefore, the degree of consolidation can be expressed
in terms of changes in excess pore pressure as: