Understanding the different foundation repair methods
There are four basic foundation repair methods or systems are used for sinking and heaving or where the walls are pushing inward. Regardless which system you will chose for the repair of your home, there are several installation conditions in common. In order to repair you foundation the installer must get underneath your existing foundation wall. Some type of bracketing must be placed under your existing foundation in order to lift or stabilize the walls. Excavation from either the outside or inside must be completed in order to install the brackets. All piering systems must be able to support the load or capacity of the structure. Every home always has two types of load or force pushing downward on the soil. These loads are called “Dead Load” and “Live Load”. Dead load is the amount of weight that is always there – (the walls, roof, floor, furniture). Live load is an ever changing condition. Examples of live load are – (3 feet of snow on the roof, ice, wind, 30 people over for a party).
CONCRETE PUSH PIERS
The concrete piers are installed in one feet section and are installed directly under the existing foundation. The first section sometimes has a cable embedded in it. A hydraulic cylinder is placed on top of the starter section and pushes against the bottom of your existing foundation in order to push it into the dirt. The starter section is open and allows the dirt to go around and in the middle of the pier. Envision the action of a cookie cutter going through dough. Because the pier edges are hard it cuts through the dirt and is pushed downward. As each section is pushed into the ground the next section is placed on top and pushed down. Sections are added until your home lifts (called refusal) or 2000 psi of hydraulic pressure is applied. The capacity of the pier is determined by the friction along the sides of the sections, as they are installed, and either 2” or 3” of end bearing. In most cases the thickness of the steel push piers are ¼” and for concrete piers 1/2” to 3/4” thick. The concrete piers are then filled with grout and used to support your home. Problems with Concrete Push Piers Unfortunately since dirt is pushed up in the middle of the pier sections during installation the mortar does not get into the starter section and many times does not get below 2’ or 3’ from the surface of the dirt. Each section is not mechanically connected and has the potential to wonder and not be in a direct line under your foundation. Failure can occur if the sections are not stacked straight. Even systems that have cable or expanding bell bottoms can not insure the pier is straight. The average depth these concrete piers are installed is 6 to 10 feet. The top 10 feet of soil is considered to be the most expansive by soil engineers. The installation of these piers do not penetrate into the bedrock. The capacity of the piers are determined by the friction of the side walls of each section and the end bearing of the pier ( 5 or 6 inches). The average loading capacity of the piers are 20,000 pounds. In expansive soils, the entire pier will lift with the soil and not protect your home against heaving. The depth of bedrock varies from area to area. The average depth for bedrock is 12 to 18 feet.
STEEL PUSH PIERS
Another foundation repair method is installing steel push piers. Steel push piers are installed along side of the foundation much like helical piers. Each section is normally 2’ long. A steel bracket is bolted to your existing foundation and a hydraulic cylinder pushes the pier section into the dirt. As with the concrete piers the starter section is open and cuts through the dirt. The steel piers are pushed into the ground until your house lifts (refusal) or 2000 psi is reached. To hold the sections together during installation a smaller inner pipe, about 6” long, is tack welded to the extension section and stack on top of each other. Envision Lego’s that stack on top of each other. These sections are not bolted or mechanically connected. The capacity of the pier is determined by the end bearing of the starter section and the friction created along the shaft during installation. Most push piers have a maximum capacity or load of 20,000 pounds.
PROBLEMS WITH STEEL PIERS
The depth of the push piers is totally dependant upon how much your house weights and the condition of your foundation. A broken or weak foundation wall will not have the integrity required when hydraulic forces are applied. Older homes with foundation made of brick or rock do not have the strength to hold together during the installation of the piers. Most houses are not heavy enough and will lift (refusal) before the push pier reaches bedrock. During most installation, no live load is present at your home. Because push piers depend upon the weight of your house for installation there is no safety factor for increased loads. When 2’ of snow or 20 people are present this live load causes the pier to “creep” or pushes the pier farther into to dirt. This is one of the main reasons push piers are failing. Since the capacity of the pier is develop along the shaft friction, expansive soils all ready stuck to the pier, allow the pier to heave upward as the soil does causing your foundation to heave upward.
CONCRETE CAISSONS
Concrete caisson are normally not a foundation repair method. Concrete caissons are normally used in new construction only because of the ability of the drilling rig to access construction sites. Original piles were first constructed of wood trees dig and placed into the ground and were used over 5000 years ago to support structures and to keep them above water lines. Modern building materials (concrete) now replace wood. They are primarily used for new construction. A drilling rig drills an 8” to 20” hole into the dirt. Concrete and steel rebar is then poured into hole and the foundation is put on top. It is important that water not be allow into the hole during the pouring of the concrete. The caisson must also be embedded into bedrock at least 5’. The capacity of the caisson is developed by the end bearing and side friction generated during installation.
WHAT IS COMPACTION GROUTING?
Compaction Grouting is a form of mud-jacking that was first developed by J. W. Poulter in 1933. A slurry solution of filling materials is forced underneath the existing foundation wall, footing or slab. Depending upon the company, the slurry solution can be composed of dirt, gypsum, or a concrete mix. Concrete slurry mix being the best option. A minimum 1000 psi mix is recommended.
