Last week we were looking at the different foundation problems due to bad design and we explained how important it was to determine the causes swiftly and proposing rapidly a solution which could range from acting on the structure itself, making changes on the geotechnical properties of the soil by treating, improving and/or reinforcing the soil, via the common solution of underpinning the existing foundation, etc.
Foundations problems due to bad execution.
This week we will look at the problems on foundations due to bad execution of the works i.e. poor quality control, which in principle is totally avoidable.
Let us take a look at some execution defects.
Making an incorrect assessment of the depth of the resistant strata could lead to future foundation problems. Some times we find on site thin layers of soil and do not investigate further, believing that the depth of a particular variety of soil is deeper than actually it is.
Error in setting out on site the correct width and/or depth of the foundations.
To use on site poor quality materials, especially concrete. Impairment of concrete pads foundations, reinforced concrete slabs and load bearing walls due to the use of low quality concrete: shrinkage, cracks, bad dosage of concrete materials with lower strength than what it is required in the project, and so on.
Material degradation, often associated with a previous poor quality laid concrete which is not resistant to the aggressiveness of the surrounding soil, inadequate reinforcement cover, etc.
Permitting the wash off the cement in the concrete due to shallow water table or in deep foundations, owing to the presence of moving water.
Errors in the placement of reinforcement bars, by confusing the use of different diameters of reinforcement bars, lack of spacers, i.e. Plastic spacers which keep the bars away from the ground permitting the concrete to cover the reinforcement bars and avoiding the bars to be in direct contact with the ground and not letting the these to rust.
Problems due to incorrect cured concrete coupled with errors of wrong dosage of cement, etc.
Deep foundations poorly executed i.e. background not properly cleaned, cut, or washed, or excessive load bearing on the wrong ground.
Broken shafts of deep pile foundations, due to excessive hammering during the driving of the piles into the ground.
Failure of deep pile foundation to anchored in soft clay if they have not been properly driven. Failure of connections between the head of the piles and the steel reinforcements of the columns.
Problems when removing the shattering tubes in the case of in-situ concrete piles, etc.
Structural fills. Defects owing to wrong compaction on structural fills in soil substitution or improvements of foundations ground.
External causes of foundation failures
Are those derived from human activity over time, so by definition are variable and difficult to predict.
They include both changes in the construction and in the environment after the building has been built; when the conditions has been substantially altered for which the foundation has been designed.
These amendments to the design of the building can affect the foundation. And this can happen in several ways, such as:
When the number of floors is increased above foundation level and the capacity of load bearing for the foundation is not taken into account.
To increase live loads in the building by escalating the number of activities within the building that requires the addition of elements not taken into account in the original structural calculations such as large heavy files for instance or the introduction of a swimming pool at roof level, or a change of activity in the edifice and consequently in the loads. Here we may include dynamic stress produced by machinery, not initially foreseen.
Rehabilitation of ground floors converting this part of the building into commercial units and changing the structure and use of this area which usually involves a redistribution of loads, with a new allocation of columns, opening in façades etc.
It is essential to proper diagnosis the causes of any foundation failure.
If the structural damage it is originated in the foundation, we should propose rapidly a solution to repair it properly and definitely as any hitch which is not solved correctly it may turn into further difficulty increasing progressively later on.
The damage to the foundation can have serious consequences for the building, and could even produce a building collapse. Some times a problem in our foundations could also propagate or involve our neighbours’ foundations.
Even when proper care has been taken into the execution control, the numbers of claims incurred in the Spanish construction industry, more than justify the cost of quality control tests. Thanks to these tests any main construction defects is detected and can easily be corrected.