- Hydrostatic pressure beneath the building could bend a normal floor slab or even lift the building from the ground.
- Flood waters might rise over the floor level flooding the building – we used 5 feet above grade (the floor) as our flood level (13 feet above mean sea level).
How do we solve these problems? We use simple buoyancy principles, meaning we must keep the building from floating away in a flood condition of water levels of 13 feet. We know that as a concrete mat gets deeper the hydrostatic uplift pressure at the bottom of the mat increases. However, so does the mat weight. So, it becomes a balancing act.
But we can’t forget our second challenge – the 5 feet of flood water above the floor level. We designed the building to be literally flood proof! Special doors, windows and walls will hold out the high water and keep the building interior and its contents dry.
First, we use a hydrostatic uplift equation to determine the elevation of the point where the two forces would balance. In our case it is located 3’-9” below the finished floor. Therefore, the concrete mat thickness is 3’ 9” (remember that the top of the mat is the unfinished floor). Working in our favor on this site was a need to over-excavate the foundation due to poor soils. The soils test report told us that we had to dig 6 feet of the existing soils and replace and compact before our new foundation could be completed. The hole was already there!
- Water is 62 lbs. per cubic foot, and 8.75 comes from 5 feet of water above the floor plus 3.75 feet of mat thickness.
- 62 lbs. per cubic foot x 8.75 feet of water depth = 542 lbs. per square foot.
- The weight of our mat is 544 lbs. per square foot, offsetting the force of hydrostatic pressure
While due diligence is paramount in selecting the right place for your new building, in our case the owner had no choice because the tenant was moving out of an outdated building on the same property and did not want to change locations. The site conditions will always dictate your building design - and the resulting challenges to your architect.