"Hydro static pressure" is a term used for several different situations from water tanks to dams. In this case we are discussing damage to buildings caused by excessive water building up around foundations. When a medium such as soil becomes soaked with water the medium becomes much more liquid and acts more like a liquid on its structural surroundings. This effect has been seen dramatically in farming operations when green silage has been put into a silo too quickly causing the silo to buckle and collapse. Moist silage tends to act more and more like a liquid as its moisture content increases and these structures are usually not designed to withstand these kinds of loads. That is another story but it illustrates how the presence of water can greatly increase the pressure on building walls that are exposed to soil. In the case of a building the soil/fill around the structure is the medium that becomes more like a liquid.
Photo 1: A residential house basement with walls damaged by hydro static pressure.
In my forensic engineering practice I have seen many cases where house basement walls have been damaged by hydro static pressure. This damage is in the form of cracks along the uphill side of the house. These cracks are usually horizontal in the middle of the wall and “stair stepped” or diagonal near the corners. These are cases where the uphill side of the basement is back-filled to match the prevailing grade of the lot. These types of problems can exist even when there is a proper slope away from the house for a few feet near the house (see figure 2).
The following figures help explain the conditions to be aware of when it comes to hydro static pressure damage.
Figure 1 shows some typical conditions that can contribute hydro static damage to walls in a below grade basement.
The foundation has inside corners that tend to trap water near the house. Figure 1.
Large paved areas on the uphill side of the building. Pavement can collect water and direct it toward the building. Also, large paved areas tend to concentrate water in to a few smaller areas.
Walls built without steel reinforcement bars. This reduces the load capacity of the walls.
Walls that are not well supported at the top by building floor trusses. Figure 3.
Vehicles parking near the house especially when they have to brake hard at the bottom of a slope.
Poor management of gutter drainage. This allows roof runoff to accumulate near the building.
Figure 2: When floor joists end on the foundation wall it provides maximum support to support the top of the wall against soil loads however walls can still crack as shown in the photo at the top of this article.
Figure 3: When the floor joists run parallel to the wall, high soil loads can cause the wall to tip inward. Either by a broken connection or by bending or damaged framing.
This article provides a few things that you can look for when evaluating below grade wall damage and it's causes.