We never know what is underneath us, there are always uncertainties exist below ground. These uncertainties may be significant to the design of foundation and underground structure.
In this shallow foundation project, we had gone through an interesting experience about the contrary of expected and actual condition beneath our site. The challenging part about this project is the existing SMART tunnel is right below the proposed building. Desk studies were carried out to understand more about the location and dimensions of the tunnel based on available information and drawings given to us. The original design of strip foundation was proposed to be sitting on top of rock. To confirm the feasibility of our design, soil investigations and rock probing tests were carried out. The results showed it was desirable in a way it is within our design assumptions.
Everything seemed to be going smoothly until the contractor started excavation to begin the construction of strip footing. The actual site condition contradicted to our expectation, there were many abandoned structures from the previous construction of SMART tunnel such as RC wall, gabion wall, pipes and even concrete column. Worse still, the rock edges alignment located away from our design assumptions, in which part of the proposed strip footing is not supported directly on rock, due to the over-excavation during construction of SMART tunnel. To solve this issue, we had to revise our design as well as proposing ground treatment work to suit the actual site condition.
From this experience, we can learn that uncertainties always exist below the ground since we have no clear vision on it, even though quite number of investigation works were carried out. Hence, we are encouraged to have more careful considerations on the uncertainties below ground in foundation and underground structures designs and always be prepared for design changes to suit the actual site condition during construction.
Author: Si Jet