Prevailing assumptions about slab design may not hold water.
By Gary K. Munkelt
This is the second of two technical articles discussing the effects of groundwater pressure on bottom slab design for buried precast concrete structures. The first article, published in Precast Solutions Spring 2010 issue, ended with the conclusion that the load on the bottom slab is the same in the dry condition as it is in the saturated condition when water level is at or below grade. This article will discuss the effect of flood conditions on the load applied to the bottom slab when water level is above grade (see Figure 1).
Flood conditions create a situation for buried precast structures that is different than the condition where water level is at or below grade. A submarine presents a good analogy as it dives deeper into the ocean (see Figure 2). Water pressure on the ship’s hull increases as depth from the surface increases. Likewise, during floods, the buried concrete structure (plus the soil above the structure) is experiencing a greater depth of water.
Pressure increases on an underground precast structure as flood levels rise. The load on the bottom slab will be greater than the condition when water level in the ground is at grade. When the water level is at grade (see Figure 3), design includes the effect of a live load (LL) caused by a truck with impact due to a speed of 60 mph (97 km/h). But, realistically, we know traffic comes to a halt during high flood conditions (see Figure 4). It is logical that LL does not have to be considered in the precast design for flood conditions.
The formula for loads on the bottom slab is derived by assuming flooding has risen to h3 (see Figure 1 and Figure 4) above grade and LL from a truck is neglected. The load will be influenced by the weight of the precast concrete vault (Wv), the weight of the soil above the vault (We) and the pressure from water at a depth of (h1 + h2 + h3),
LL = Live Load of truck in pounds
h1 = height of underground precast vault in feet
h2 = height of earth above precast vault in feet
h3 = height of flood waters above grade in feet
Wv = weight of precast concrete vault in pounds
We = weight of earth above precast vault in pounds
62.4 pcf = weight of one cubic foot of water
It is easy to see that water pressure will add 936 psf (62.4 psf × 15 ft) to design load on the bottom slab when flooding is at a level of 15 ft above grade.
Design of structures needs to consider the worstcase loads of anticipated conditions. There are two cases involved when flooding is possible. Case I occurs when there is no flooding and the design truck LL is applied. Case II occurs when there is flooding and LL = 0. The design procedure must include load calculations on the slab (ps) for both Case I and Case II. Final design will be to make the slab capable of withstanding the worst case loading.
In conclusion, flood waters can add a significant load to the bottom slab of an underground precast structure. The additional load is due to pressure from water above grade.
Gary Munkelt is a consulting engineer with Gary K. Munkelt & Associates in North Wales, Pa. Contact him at [email protected].