The basics and safety requirements for the design of foundations in general- part 2
In the previous article (The basics and safety requirementsfor the design of foundations in general) we explained three factors should
considered when design of foundation. Here, we will continue explaining the
remaining factors.
- Safety against sliding.
When exposing foundation to any structure to large
horizontal lateral loads and at the same time smaller depth of the buried
foundation under the ground in addition to the value of the shear resistance of
the soil in contact with the foundation as per the below figure.
Where slipping and movement are likely to occur in the
horizontal direction of the foundation and to prevent this kind of sliding and
movement of the foundation horizontally, a coefficient called a safety factor
against sliding must be provided and this coefficient is greater than one which
is equal Fs
Fs = Hs + Ep / Ea +
H > 1.0 = the sum of the anti-slip forces / the sum
of the forces causing the sliding
Where,
Hs =
shear force at foundation level.
Ep =
passive soil pressure.
Ea = active soil pressure.
H = the horizontal
force transferred from the structure base foundation and is equal to the
horizontal component of the product of the forces acting on the foundation at
the base.
The value of the shear strength of soil (H) can be estimated
from the follow equation according to the type of soil where,
Hs = Q
tan φ + A* Cw
Where,
Q = the sum of the
vertical forces acting at the foundation level with the water pressure at the
base, if any.
Φ = the angle of
intend friction between the foundation and the soil, which is equivalent to
tow-third of the angle of internal friction of the soil.
A = the area of the
foundation subjected to pressure.
Cw = is
the cohesion stress between the soil and the foundation where:
Cw = Cu
in the case of weak and
medium clay. And,
Cw = 0.5 Cu
in the case of cohesive, highly cohesive, and hard clays.
Where, Cu is the cohesive stress of the soil in
the non-drained case.
It should be noted that if the foundation is rested on clay
soil, the part Q tan φ is neglected and equation becomes,
Hs = A * Cw
And if the foundation is rested on sandy clay soil, the part
A* Cw is neglected and equation becomes,
Hs = Q
tan φ
However, in all cases the safety factor against sliding must
not be less than the value mentioned in the below tables:
Load Status |
D.L + L. L |
D.L + L.L + Wind + Pr. |
D.L + L.L +Wind + Earthquake. |
Factor Safety against sliding. |
1.5 |
1.3 |
1.1 |
- Safety against soil failure.
- Define the failure of the soil under the foundation.
The failure of the foundation soil occurs due to the loads
on the foundations are in one of the following images for each of the following
cases:
1- Sliding of soil under the foundation on sliding, curved
surfaces with a specific shape as per figure below, in the event that the value
of shear stresses along curved slip surfaces maximum shear strength of soil
(shear collapse)
2- Soil movement under the foundation and at its sides without forming a specific surface for the slip as per below figure
In both cases 1 and 2 the foundation movement increase
downward at a large rate with increasing the load (vertical settlement) as
shown by the curves of the relationship between the load and the vertical
settlement associated in the below figure
Calculation of the maximum soil bearing capacity for cases 1
and 2 using theories of plasticity and assuming the occurrence of a slip on the
surface shown in the case 1 figure but in the case 2, settlement is often the
ruling factor in design.
It should be noted that the soil may reach its maximum
bearing capacity in the following cases:
. Increasing loads on the foundations (maximum stresses on
the foundation soil)
. Decrease shear resistance for foundation soil.
. Soil viability to settlement.
. Various factors such as the small size of the foundation,
etc.
So that the foundation is intact, the collapse and failure
of the spoil below it must be avoided and failure of the soil below the
foundation occurs when the maximum bearing capacity of the soil at the
foundation level is less than the sum of the forces acting on the foundation at
the foundation level as per the below figure
In order to prevent the collapse and failure of the soil
under foundations, a sufficient safety factor must be achieved against this
failure (Fb) and this factor is equivalent to:
Fb = (Rb) the maximum bearing strength
of the soil at the foundation level/ (R) the sum of the forces at the
foundation level
And the result should be greater than one.
The value of this factor depends on the different loads
states as per the below table.
Load Status |
D.L + L. L |
D.L + L.L +
Wind + Pr. |
D.L + L.L
+Wind + Earthquake. |
Factor Safety
against soil failure. |
2.5 |
2.0 |
1.5 |
- Safety against overall settlement and different settlement.
In general, when the loads move from the foundations to the
soil, it often happens that these foundations settlement. And this settlement
may be equal under all the foundations and may be uneven or relatively under
it.
If the settlement is equal under all the foundation forming
the building, this does not cause damage to the structure elements that make up
the building, as it does regard to the safety of the structure and not result
in generation of there are any cracks or numbness. But if its value is great,
this may affect the safety of the sewage and water connection and the function
of the building.
This is requiring that the value of the maximum total
settlement of shallow foundation does not exceed the value mentioned in below
table which depends on both the type of soil below foundation and the type of
foundations.
Foundation type |
Soil type |
Maximum permissible
total settlement by mm |
Isolated footing |
clay |
70 |
Isolated footing |
sand |
50 |
Raft |
clay |
150 |
Raft |
sand |
100 |