Foundation Failure may occur due to many reasons, starting from the bad soil condition to Structural faults, but among the many few are much more important than other, and also at the same time they are much more easy to control. Foundation Failure may occur due to many reasons, starting from the bad soil condition to Structural faults, but among the many few are much more important than other, and also at the same time they are much more easy to control.
Failure of Foundation due to exceeding of Bearing Capacity of Soil :-
Bearing Capacity of the soil (rather say Ultimate Bearing Capacity) is Ultimate Load Intensity that a soil can take without having any Shear Failure of the Soil. This Bearing Capacity is the factor upon which the size of the footing in terms of its size as Length and Breadth or Diameter depends.
A Foundation/ Footing of a structure must have sufficient size that the intensity of load coming through that footing from the structure to the soil beneath shall not increase the bearing capacity of that soil. If the Bearing capacity of soil is low or load acting is heavy and we put a footing size comparatively small than what is required then the result will be that the load intensity (i.e. load per unit area say KN/sq.m. etc.) induced on the soil may be higher than the bearing capacity of the soil, hence soil will undergo a Shear Failure.
For being at safe side for determination of footing size it is a practice to use Safe Bearing Capacity instead of Ultimate Bearing Capacity to have Margin of safety against any accidental loading and also unusual behavior of the soil of varying its bearing capacity. Safe Bearing Capacity is nothing but Ultimate Bearing Capacity divided by a suitable factor of safety generally ranging from 2 to 4.
Failure of Foundation due to Sudden Withdrawn of Moisture from Soil :-
This is one of the very very very important reason which most of the Engineer are not concern about and neglects very much. Moisture withdrawn can happen in many ways
- By Pumping out water from the adjoining well
- By De-watering adjoining Pond
- Lowering of Ground Water Table previously which was very near to the ground.
Soil in most of the general cases consist of Three Phase System that is Soil Solid+Water+Air and prior to Full Consolidation of the soil which takes a very long rime the Pore Water within the soil also takes up the super structural loads coming to the soil.
Suppose for an example there is a pond full of water adjoining to a building, and you are dewatering that pond, then the following things will happen leading to the failure of foundation of the adjoining building.
- Water is pumped out from the pond, hence pond which is like a big depression on the ground will be empty or with very less water.
- Due to the pumping, out huge quantity of water, the previous pressure of the pond water towards the soil around the Bank of the pond will drastically reduced.
- Surrounding soil of the pond which in this as it is having adjoining building, the soil around and below the foundation of the building or any other such structure will be in high pressure zone of pore water as compared to the pond having no or very less water.
- Due to this water pressure difference between the pond and its surrounding soil, Pore water from the surrounding soil will start laterally flowing through the soil towards the Pond and try to fill in the pond.
- Therefore, pore water from side and bottom of the foundation soil is being withdrawn and moving towards the pond continuously.
- As the soil generally consist of Soil Solid + Water + Air and super structural load on soil is borne by the both Soil Solid and Pore Water, after withdrawn of pore water it will create voids within the soil system around and at the bottom of the foundation soil, causing subsidence of the soil along withe supporting structure on with and leading to a Foundation Failure.
It is very much dangerous as the withdrawn of moisture in the above phenomenon may not be same at all points around and bottom of the foundation and will lead to Differential Settlements which is very much dangerous. It is many times due to the above phenomenon seen that floor cracks, Roads collapses hen an adjoining water body is being De-watered.
Failure of Foundation due to Differential or Unequal Settlement :-
Differential Settlement or Unequal Settlement is the worst case that can occur and which surely leads to a massive failure of foundation involving costly restoration works.
First of all we must understand what is Differential Settlement? When the portion of the soil underside a structure subsides or settles in different magnitude under different places of the structure causing the supported structure also to subside along with the soil is known as the Differential Settlement or Unequal Settlement.
Now as due to this different magnitude of settlement at different portion of the structure occurs or in other words one portion of the structure settles more or less than the another point it creates a huge amount of diagonal stress to be induced into the structure due to the super structural load as they tend to settle along with the soil on which it rests. These Diagonal Stresses so induced results in cracking.
Crack so develop may be very wide also and may go on increasing as the settlement goes on continuing. This leads to the instability in the structure and ultimately Collapse or Failure of the Structure.
Failure of Foundation due to Uneven Progress and Pattern of Construction:-
Though most of the people neglect this point, but it is very important not to load the soil unevenly by uneven progress and pattern of construction, or in other words the structure should be constructed or raised uniformly that is we must work on the entire area of a particular structure say building upto certain level of height, after when upto certain level throughout the area has been constructed then only we must proceed to the raising more height.
I know you are not understanding what I was trying to say, Okay lets make it simplified with an example.
Suppose a RCC Framed Structure Building is being constructed having having 10 Nos. of Columns along x axis and 4 Nos. of column along y axis, that is total of 40 columns. Now, uniformly progress and pattern of construction means that all columns should be raised simultaneously, and step by step construction should be done simultaneously at all parts. say first all 40 columns should be founded, then all 40 columns should be raised upto plinth beam then all 40 columns upto lintel bottom level and so on.
It should not be non-uniform like say out of 40 columns you are founding 25 columns or so haphazardly at different locations, and then raising only those up plinth beam, then upto lintel bottom and son on, and after those you are starting construction by founding of remaining 15 columns. If non-uniform pattern of construction will be adopted then soil will have to bear different quantum of loads at different pockets packed closer and ultimately these uneven stress distribution in the soil mass may lead to the shear failure of the soil, which also translates as failure of the structure as a whole.
Failure of Foundation due to Movement of Adjoining Soil :-
Foundation failure will also occur if soil surrounding the foundation is moved away. This may happen if excavation has been resorted to very near an existing foundation, like in case of a new building is being constructed just by side of an existing building structure. Due to an open excavation the soil by the sides of the old foundation will get loosened and move away, reducing the lateral support which was being provided by the soil to that existing foundation. as the lateral support reduces, the existing foundation may tend to slide, and also the soil from below the foundation which is actually directly bearing the load may get eroded due to lack of lateral support from the surrounding soil, and this whole sequence of outcomes will definitely cause a massive failure of the foundation and the structure which is being supported.
This situation can be prevented by providing adequate numbers of Sheet Piling along the sides and upto certain depth below the deepest point of the shallow foundation, so that even the soil from the below the foundation cannot escape away, in fact move.