Of all possible pathologies that can occur in a building, statistics from ASEMAS (architects main insurance company) show that those linked to foundations carry higher overall costs. It is not only a question of money, as any problem associated to foundations also have great social impact because of psychological apprehension that an unsafe foundation may have on the users. Not to mention the inherent complexity of repairing it, altering and even forbidding the use of the property and often it involve the neighbours and of course, the local authorities.
It is urgent to determine the causes.
When there is problem with foundations it is urgent to determine the causes and to establish a solution as soon as possible.
However, the diagnoses of such problems are far from immediate. Symptoms should be analyzed; any cracks and/or movements in the building should be carefully studied so as to figure out what actions, has originated the pathology. Taking into account the existing foundation and any local geotechnical knowledge of the ground supporting the foundations may help in deducting the causes that produced the problem in the first place.
It is very important to follow this methodology, because otherwise the repair will hide from view the problem and any trouble with the foundation will remain unsolved but with the added dilemma that the serious process of deterioration will continue hidden, so that mere cracks in walls misdiagnosed can lead, ultimately, to the collapse of the building.
Once the cause is known it is important to immediately design the solution.
Finally, once the cause is known it is important to immediately design the solution. These can range from acting on the structure itself, or it could be that it is require to changes the geotechnical properties of the soil by treating, improving and reinforcing the soil, or via the common solution of underpinning the existing foundation.
We will try analyze the root causes of foundation failure.
To classify the failures we could analyse the flowing sources:
Variations in the original assumptions of the project.
Variations in environmental conditions of the site.
- Bad design.
There may be errors in the design, or in the foundation calculations.
It must be said that since 2007 when the Spanish New Building Code came into effect, almost all building projects has a geotechnical report carried out by an approved laboratory and fewer mistakes are made into the calculation of foundations as the load bearing capacity of the soil is provided by the lab and no assumption is made by the architect.
However errors are still made according to the official statistics when the architect has assumed a wrong capacity of the soil and therefore an inadequate foundation could has been designed.
If excess loads are not covered by the foundation design because it has been forgotten i.e. some heavy machinery has not been allowed for in the calculations.
The architect has insufficient knowledge of the soil due to lack of awareness of the surrounding soil conditions. As it has been said above, this problem is less common now days. However it may occur because of lack of prior recognition of the land, misinterpretation of an ocular inspection, not considering the phenomena of aggression of chemicals in the soil, and so on.
Special care should be taken when soils of the following characteristics are found:
Rubbish dump sites
Natural soft soils
Underestimating the geotechnical characteristics of the ground could lead us into problem especially when we find that the ground for our foundations is not homogenous.
The appearance of rock layers could erroneously lead us in believing that this is a better foundation ground.
Disregard for the effects of expansive soil, such as clay which could provide the foundation with an uplift or subsidence actions and so on.
Not to take into account ground settlement, when calculating the structure, freeloading efforts on piles foundations, and to forget the effect of loose ground or recent fills of rubble on site, etc.
Not considering the load bulb of the foundation itself, and its extent or its action from adjacent building foundations, which could add a burden onto our foundation. We should considered these factors when calculating our building as we should take into account the effect that our foundations nay cause to any adjacent buildings.
Ignoring the surrounding conditions, could lead us into trouble if we do not consider the environmental conditions of the proposed structure, such as possible level of the water table, and so on.
Not to consider uneven distribution of loads or excessive eccentricity on the same foundation level.
Next week we will continue with the effects that a bad execution of the works may have on our foundations.
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Austin Mickley (Tuesday, 31 January 2017 13:00)
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