There are four major methods of foundation repair in the southern part of the United States. They are known as concrete pressed pilings, concrete pressed pilings with insert, steel pressed pilings, and poured concrete piers (Bell Bottom Piers). It should be noted that the Federal Housing Authority has not certified or expressly established “approved foundation repair methods.”
A comparison of foundation repair methods must include the poured concrete piers or Bell Bottom Pier method. It was the first foundation repair method developed to stabilize concrete slab foundations. It was developed in the 1960s and its primary disadvantages are higher cost and longer construction time. However, this method has impressive advantages. It is based on the same construction concept that is used to build support columns for highway overpasses. It is a thoroughly researched concrete slab repair method that is recommended by the vast majority of structural engineers. Large holes are dug under the areas that need foundation support. Concrete is poured into the holes and steel rebar is placed in the wet concrete. When the concrete is dry the building structure can be leveled. The Bell Bottom Piers are extremely strong and will resist all horizontal and vertical soil movement. It is the most permanent foundation repair method available.
In the 1980s a lower cost but remarkably inferior method of foundation repair was developed called concrete pressed pilings. It is probably the simplest and lowest cost of all the methods of foundation repair. It is still used today because in can be completed in a short amount of time and there is no waiting time for the concrete to cure. However, it has major disadvantages. This process uses the weight of the building structure, or a portion of it, to leverage or vertically drive precast concrete cylinders into the ground. The depth of the driving process is limited by both soil conditions and the weight of the building structure. There is a point called the refusal point, which is the point where it takes more force to drive additional piles than it does to lift the building structure. The refusal point is the limit that piles can be driven into the soil because any additional force would raise the foundation of the building structure rather than driving the piles further into the soil. Therefore, the precast pressed pilings, which are driven-in vertically, many not reach bedrock or stable soil. Perhaps the greatest disadvantage of this method is the fact that there is nothing preventing the misalignment of the precast concrete cylinders. There is no way to determine if piles have been driven in the ground in a straight vertical column. If the first pile hits a big rock it could break or become misdirected and continue at an angle. Any of the concrete piles could fracture and break and misalign the entire column. Even if the concrete piles are aligned in a vertical column they have no way to resist future horizontal soil movement, which is very common with any building structure that is experiencing foundation problems. And another potential drawback is that the repair crew may improperly exceed the driving limitation while forcing these concrete piles into the ground and thus severely crack the foundation they are supposed to be stabilizing. This method is an unproven foundation repair method and typically no soil test is performed prior to installation. For these reasons, the concrete pressed pilings method is recommended far less frequently than in the past.
Because of the severe deficiencies of the concrete pressed pilings method, a variation was developed called the concrete pressed pilings with insert method. A comparison of these methods of foundation repair will reveal that they are very similar. The minor difference is that the precast concrete piles have a hole in the center for placement of a steel rod. The steel rod, if inserted during the driving process, will help keep the concrete piles in a vertically aligned column. However, the steel rod will not prevent all potential misalignment problems. The steel rod will help the entire column resist horizontal soil movement. Unfortunately, this new method does not address the other disadvantages mentioned earlier. The depth of the driven vertical concrete piles is still limited by the weight of the building structure. And the foundation of the building structure is still as risk of damage if the repair crew exceeds the driving limitations. In addition, this method introduced a center hole in the vertical column of the concrete piles that will accumulate water. An accumulation of water in expansive soils will put additional forces on the vertical column as well as the foundation.
The steel piling method is the latest version of the pressed piling method. A comparison of these three similar methods of foundation repair will reveal some improvement with the latest version. Steel pilings are stronger than concrete piles and can be driven deeper into the soil. However, the driving depth is still limited by the weight of the building structure and the refusal point. And the steel pilings are thinner than the precast concrete pilings and less able to resist the forces of vertical and horizontal soil movement. And steel piling also share the other disadvantages of concrete piling such as misalignment that cannot be detected and unconnected piling segments that can not resist uplift forces of soil movement.