Underpinning is the process of modifying an existing foundation system by extending into subsurface stratum that is deeper and more stable than the near surface soil that supports the existing foundation system. This is done to provide vertical support that is not present in the existing design. Methods of underpinning include the construction of footings, stem walls and driven pilings for drilled piers.
The underpinning, if properly designed and installed, provides the basis to lift the structure to a more acceptable elevation and provides vertical support to prevent the underpinned area from settling.
There are many systems and many contractors that perform foundation repair work. The buyer must be educated and selective when deciding on the repair method for their home. Improper repair methods could cause tens of thousands of dollars in additional defects.
A Foundation Failure Report is an investigation by a non-engineering firm - usually a foundation repair specialist - of a structure's foundation integrity and repair alternatives. The survey is conducted on site and can take several hours to complete. The report involves an interior and exterior examination of the structure, interior floor elevations, load calculations and a recommendations on the repair methods along with a price. Contractors may charge from $200 to $1,000 for a complete survey and Foundation Failure Report. This fee is typically deducted if you order.
This is a comprehensive report prepared by a licensed engineering firm. It involves on-site soil testing by taking soil samples at various depths around the structure, looking at the geographical features and history of the area and drawing conclusions from that data. These reports are often required by structural engineers when determining how to repair a foundation. The reports can cost in the range of $1,500 to $5,500.
is a process of restoring your home near its original position. Leveling is required when the foundation has shifted. In order to fortify, underpin or lift your foundation, you must go through a series of steps to raise it. First, you must add support through a process called shoring. During this step, beams are inserted under your home with jacks underneath. Slowly, these jacks are raised evenly, which prevents damage to your home. When the jacks are raised to the correct height, new piers or caissons are added; these new installations secure the original foundation to firm soil as perscribed by structural & soils engineering.
Underpinning is a generic term to describe the process of modifying an existing foundation by adding support. This can be done by various means, such as Piering (installing steel piers), concrete caissons or piles. Each have their own advantages and disadvantages.
Piering a foundation is the process of installing steel piers under the footing of a foundation and driving the piers through the soil down to load bearing stratum. The piers can be used to either support the structure or lift the structure.
Typically From 10" or 3' diameter holes are drilled/ excavated into the earth and embedded into bedrock 2 to 5+ feet. Usually used for the structural support for a type of foundation wall, porch, patio, monopost, or other structure. Various "sticks" of reinforcing bars (rebar) are inserted into and run the full length of the hole and then concrete is poured into the caisson hole. A caisson is designed to rest on an underlying stratum of rock or satisfactory soil and is used when unsatisfactory soil exists.
If the mudsill of a house (the pink area in the illustration at right) is not bolted, the lateral loads (back and forth motions) of an earthquake can jerk the house off its foundation and cause it to collapse. At- tachment of the mudsill to the foundation is accomplished with the use of foundation bolts. This illustration shows earthquake forces pushing on the mudsill and the placement of bolts that pre- vent movement. The following topics are discussed in this article. You can click on a topic to go directly to it or scroll down this page to view them all. The next portion of this Retrofit Design series is Shear walls - bracing of the cripple walls. Code requirements for bolting:
The Uniform Building Code (UBC) which is designed for new construction and is not intended for retrofit, specifies that only 5/8 inch bolts with plate washers may be used. They should be 6 ft. o.c.
(on center) on single and two story homes. Three story homes require an engineer. Older building codes, 1994 and earlier, required 1/2 inch bolts spaced 6 ft. apart no matter how many stories.
The Uniform Code of Building Conservation (UCBC), which is a retrofit code applicable to existing homes, requires 1/2 inch a bolts with plate washers be installed 6 ft. o.c. on one story buildings, 1/2 inch bolts with plate washers installed 4 ft. o.c. on two story buildings, and 5/8 inch bolts with plate washers installed 4 ft. o.c. on three story buildings. The importance of plate washers will be discussed in this article. Types of bolts: The most common foundation bolts are either mechanical wedge anchors or epoxy bolts, 1/2 inch or 5/8 inch in diameter. The mechanical wedge anchor is installed by drilling a hole through the mudsill into the concrete, beating the wedge anchor into the hole with a sledge hammer and then tightening the bolt. A "wedge" on the bottom of the bolt expands while it is being tightened which secures the bolt into the concrete. An epoxy bolt is installed in the same manner except that it is glued into the hole with epoxy. All seismic retrofit codes i.e., The International Conference of Building Officials (ICBO), The Uniform Code of Building Conservation (UCBC), and the Los Angeles Retrofit Code recognize no difference.
in earthquake resistance between wedge anchors and epoxy bolts as long as they are installed per manufacturer's installation instructions. Generally speaking, epoxy bolts are installed when the con- crete is in poor condition or the bolts are being installed in brick. They are also the most common type of bolt installed in the South Bay Area, while mechanical wedge anchors are the most com- mon type of bolt installed in Southern California. This difference merely reflects a "common construc- tion practice" for these geographic areas. However epoxy bolts are always used when anticipated forces will be trying to pull the bolt upward out of the concrete, such as in the installation of hold- downs. Rusting of bolts:
A common problem that occurs in areas with high moisture involves rusting of the bolts, regardless of which type is used. If a bolt is rusted to the point that the nut is corroded fast to the bolt and cannot be removed, the bolt should be replaced. When trying to remove the nuts and replace the washers, we have snapped off many rusted bolts. Even though the bolts look sound, they can be rusted away where the bolt and the concrete meet. We have seen bolts where less than 1/8 of an inch of the origi- nal bolt was left even though the nut and bolt looked intact. We recommend bolts showing severe rust be replaced with hot-dipped galvanized wedge anchors or epoxy bolts. Recessed bolts:
Another problem is bolts that are recessed into the mudsill. This occurs because the bolts were not sticking up far enough when the foundation was poured. When the contractor installs his dimensional 2-by mudsill on a foundation that has bolts sticking up only 1-1/2 inches, he has to chisel out the mudsill around the bolts in order to get the nuts on. In this situation it is rarely possible to adequately tighten the nuts and it is impossible to install plate washers and mudsill plates. Recessed bolts should be considered marginally functional and should be replaced.
