The Danger of open excavation near your foundation - Engr. Osaz' Enobakhare

There is naturally a danger of having an open excavation anywhere at all; from being a dirt trap to becoming a death trap hence protection of open excavation is a compulsory item in basic construction practice. On typical construction sites, it is often the responsibility of the Safety Engineer to enforce such vital regulation on site. They would go hard on any worker who do not obey this regulation due to the numerous potential dangers; but what happens when the site is now built up, handed over, the ‘dreaded’ safety Engineers are gone, and someone excavates close to the foundation and leaves it open? No doubt, there is equally a latent danger in having an open excavation near a foundation. Here is why –Foundations relate with the subsoil on which they are built and around them by constantly pushing against the active soil pressure in a relatively balanced manner in order to remain stable throughout their life span. If there is a substantial counter balance from either of the foundation itself or the supporting earth at any point, it will impede on the stability of the foundation and might cause it to settle (or sink) rapidly.

The rapid settlement of some defective buildings studied over the years is a result of fully or partially-open excavation at close distance to the foundation. Open excavations up to 4-metres from the foundation line can still affect your foundation depending on the soil strata. When there is an excavation near a foundation, the active soil pressure acting on the foundation at that region reduces, causing the soil to slip. Subsequently when there is natural earth movement, it drops further, making the foundation not to hold firm to the ground. If not checked on time, the vertical load of the building acting on it would cause the building to settle at that region and further apart. Although there is nothing particularly wrong in having an excavation close to a foundation but care must be taken to ensure that the walls of the excavated areas are protected or embarked to stay the adjourning earth and prevent the devastating impact on the neighboring structure especially if such structures are on shallow foundations (i.e. foundations of 5-metres or less in depth from natural ground level mostly raft, strip, pad, etc.). Such impact could also include poor resistance against ground vibrations and burrowing by small animals that may attack foundation walls.  

Reinforced Concrete can be used in protecting the walls of an excavation close to a foundation but it is important to construct them properly else they fail to serve the purpose intended. Where a tree is fell close to a foundation, it is important to refill the excavated portion and stabilize the earth firmly at that point. Leaving an open excavation close to a foundation unattended to also allows it to collect surface water which might constantly soak-away into the foundation and damage its fabric rapidly. This is particularly the case where due to low water table a traditional ‘block’ strip foundation was used. Whichever way it is advisable not to allow especially deep open excavation close to your foundations to prevent foundation cracks and the attendant regrets!