Soil Settlement.
In the previous article (Mechanical properties of soil- Distribution of vertical and horizontal stresses in the soil.) we took the mechanical properties of soil and explain distribution of vertical and horizontal stresses in soil, now we will explain another mechanical property of soil which is soil settlement.
- What is Soil settlement
When the loads affect the soil, the stresses arising from it
cause the compression of the soil particles directly by simply affecting the
load, which is called the immediate settlement.
This is followed by a settlement as a result of the water
dropping out from between the soil particles if the soil is soft and saturated
with water, which is known as Consolidation settlement, and this type of
subsidence takes a long time due to the lack of permeability of soft soil to
water.
- Reasons for the occurrence of soil settlement
- Soil is subjected
to static loads such as the weight of the structure itself, or as a result of
large storage, or as a result of moving loads or vibrations during piling, or
as a result of vibrations of the machines.
- Reducing the groundwater level below the foundations, which leads to a decrease in the volume of the soil associated with the exit and withdrawal of water from the soil.
- Swelling and shrinkage of the clay soil.
- Excavation adjacent to foundations or because of work and construction of tunnels.
- Types of soil settlement under foundations.
The total subsidence that occurs to foundations (especially
the shallow foundations) can be divided into three components as follows:
- Immediate settlement
- Consolidation settlement (primary settlement)
- Creep settlement (secondary settlement)
The total soil settlement under foundations is the sum of
both immediate and consolidation settlement and that with a merging of the
consolidation and creep settlement with each other.
- Immediate settlement
This type of soil settlement occurs immediately once the
effect by load. In the case of inconsistent soils (sandy or slippery) the
immediate settlement is almost equal to the expected total settlement.
As the high permeability that characterizes this type of
soil is sufficient to dissipate the interstitial pressure upon application and
effect of the load, but in the case of cohesive soils (clay) the immediate
settlement it is the elastic compression of the clay layers and it is usually
represents a small percentage of the total settlement in the event that the
soil is saturated and this occurs with a constant volume.
Whereas, if the soil is not saturated with water or is
highly fortified then the immediate settlement represents a large part of the
expected total settlement.
- Immediate soil settlement under foundations calculation.
Immediate soil settlement under foundation on infinitely
thick soils as follows:
Si = PB * 1- µ2 / Es *
I
Where,
Si = the amount of immediate settlement in cm.
P = the amount of uniform pressure affecting the soil at the
foundation level (kg/cm).
B = the width of the foundation (cm).
μ = the Poisson’s ratio of soil (0.5 for saturated clay, 0.3
for silt and sand).
Es = soil compressibility coefficient (kg/cm 2).
I = coefficient that take into account the shape and
stiffness of the foundation.
For immediate settlement under foundation on soil of depth
(H) as follows:
Si = P/Es * H
Where,
Si = the amount of immediate settlement in cm.
P = the stress value of the foundation acting at the middle
of the thickness of the soil layer (kg/cm 2).
Es = soil compressibility coefficient (kg/cm 2).
H = it is the thickness of the soil layer (cm).
- Consolidation settlement.
This settlement occurs due to the decrease in the size of
the soil voids as a result of the dissipation of the mini pressure generated by
the effect of the added loads to it, and in this case the relationship between
stress and strain the soil varies a lot according to the previous degree of consolidation. So, it is necessary to study the geology of
the site and the loads to which it was exposed in the past.
- Calculation of settlement
resulting from soil consolidation for normal consolidation soils.
Normal consolidation soils are called soils that are not
affected by pressure greater than that of the effective soil burden affecting
them at the current time when calculating the settlement.
In this case, the settlement resulting (Sc) from the
effective load is calculated as follows:
Sc =
Cc / 1 + eo * H * log (Po
+ ΔP/ Po)
Where,
Cc = the value of the index of compression and is shown from
results of the initial consolidation experiment and is determined from drawings
the relationship (e- log P) between the ratio of voids (e) and the vertical
effect stress (P) as per the below figure:
eo = the
ratio of the primary voids.
Po = the value of the effective
and effective overhead load at the middle of the thickness of the layer
subjected to compression.
ΔP = the value of the vertical additional stress
at the middle of the layer thickness.
H = the thickness of the layer.
It should be
noted that the previous equation should be used only in the event that the
thickness of the compressible layer (H) is less than half the width of the
foundation.
If the thickness of the layer exceed half the width of the
foundation or a change in the compressive properties occurs, then the compressed
layer must be divided into a group of successive layers and the settlement is
calculated for each layer separately using the previous equation, and the total
settlement is the algebraic sum of the settlement value of the multiple layers.
- Calculation
of settlement resulting from soil consolidation for pre-consolidated soil.
Soil is
called pre-consolidated if it has previously been subjected to pressure greater
than the effective pressure of the current soil burden.
pre-consolidated
soil occurs because of several reasons. For example:
- The water table has fallen at some point in the past.
- Removing weights from the soil by erosion or excavation, etc.
The calculation
of settlement in this case depends on the amount od additional stress that
affects the soil in the manner as follows:
- If the sum
of the additional stress and the stress of the current effective soil burden is
less than the pressure value which causes of the pre-consolidated (Pc).
As per the
figure below, calculation the settlement amount (Sc) in this case as follows:
Sc = = mv * ΔP * H
Where,
mv = is the value of the coefficient of
volumetric compressibility of the soil and it is determined from the
entertaining consolidation experiment.
ΔP = is the value of the vertical additional stress at the middle
of the layer thickness.
H = the
thickness of the layer.
- If the sum
of the additional stress and the stress of the current effective soil burden is
greater than the pressure value which causes of the pre-consolidated (Pc).
As per the
figure below, calculation the settlement amount (Sc) in this case as follows:
Sc = Sc 1 + Sc 2
Where,
Sc1 = mv * ΔP1 * H
And,
Sc2 =
Cc / 1 + eo * H * log Pc + ΔP2 / Pc
- Creep settlement
As it known
that according to the consolidation theory, the settlement usually stops when
the intra-pressure generated
by the effect of the added loads is dissipated. However, the decline usually
continues with time, because of creep.
Secondary
depression usually represents a significant value of total settlement, especially in organic soils and
highly compressed clay soils.