The basics and safety requirements for the design of foundations in general- part 2
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The basics and safety requirements for the design of foundations in general- part 2

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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.

E = 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







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