I've got an underground tank (reinf. concrete) that we've initially designed. The design water table is assumed at grade level. There is about 3 feet of earth over the tank lid.
The base slab thickness was calculated to provide enough weight to counter buoyancy uplift of the overall tank as there will be times when the tank is empty.
We included the weight of the tank walls and lid as well. We wanted to use a 1.5 safety factor against uplift. We also included the weight of the earth on top of the lid.
The base slab extends out from the perimeter walls a small amount but we did not include any weight of soil directly above these lips.
The question is, if we extend the base slab further out to create larger lips, can we include any weight of soil above these lips to help resist uplift forces from buoyancy?
My initial thought is no - since the soil would be fully saturated with the water table at ground level and the soil would have a highly unknown shear capacity - much like a tank submerged in quicksand. However, I was curious what you all thought about counting on some amount of "help". This would allow a bit smaller, more economical base slab thickness.
The soil is comprised of various strata of lean clays, sandy silts, some trace gravel, N counts about 10 blows/ft.
The base slab thickness was calculated to provide enough weight to counter buoyancy uplift of the overall tank as there will be times when the tank is empty.
We included the weight of the tank walls and lid as well. We wanted to use a 1.5 safety factor against uplift. We also included the weight of the earth on top of the lid.
The base slab extends out from the perimeter walls a small amount but we did not include any weight of soil directly above these lips.
The question is, if we extend the base slab further out to create larger lips, can we include any weight of soil above these lips to help resist uplift forces from buoyancy?
My initial thought is no - since the soil would be fully saturated with the water table at ground level and the soil would have a highly unknown shear capacity - much like a tank submerged in quicksand. However, I was curious what you all thought about counting on some amount of "help". This would allow a bit smaller, more economical base slab thickness.
The soil is comprised of various strata of lean clays, sandy silts, some trace gravel, N counts about 10 blows/ft.
