Restoration Guide: Shoring, Underpinning and Foundation Repair
Editor's Note: This is article 8 of 9 in Chapter 1: Foundations of the Old House Web Restoration Guide. This guide was developed and edited for old homes from original material in the U.S. Housing and Urban Development (HUD) Rehab Guide.
8. SHORING, UNDERPINNING, AND REPAIR
Section 1--The Essentials
When beginning a home restoration project in an old house, one of the greatest fears is finding foundation problems. These problems can range from cracks to water damage to settling and displacement. Foundation damage can result from a variety of causes, including poor soil conditions, moisture, or problematic design and engineering. Whatever the cause, foundation problems threaten the structural integrity of your home and should be taken seriously.
Fortunately, these problems do not happen immediately, and there is usually time to effect repairs before catastrophic failure occurs. Unfortunately, these are not repairs that are within the capabilities of the average homeowner. Structural engineers, soil engineers, and specialty contractors are typically needed to correct these problems.
Section 2--Foundation Work: Stabilizing and Raising Settled Foundations
Stabilizing and re-positioning a cracked or displaced foundation may require one or more of the following techniques, all of which should be completed by professionals.
2.1: Reinforced Concrete Piers
In this process, a reinforced concrete pier with a bracket or cap is poured next to the settled portion of the foundation. This takes 28 days to cure, at which time the settled foundation section is jacked up and the load distributed to the new pier. Depending on the length of the settled foundation section, several new piers may be required.
2.2: Steel Mini Piers
Hydraulic or pneumatic steel mini piers, ranging in size from 2" to 8" diameter, are set close together next to the face of the foundation footing. They must then be jacked down to bedrock, usually to a depth of 15' to 25', but deeper in many instances. A metal bracket on the mini pier engages the foundation and provides stability. A variation on this technique shoots concrete grout down a hollow pipe to provide a bearing surface. Mini piers can also be used to raise settled slabs.
Mini piers are an economical and durable solution provided a bearing layer is no deeper than 60 feet beneath your foundation--any deeper and this technique is less likely to provide the results you want.
2.3 Helical Piers
This patented system uses a steel shaft with a helical plate that is screwed into stable soil. The plate's rotation creates enough resistance to stabilize or raise the foundation. Use of helical piers requires training and certification by the manufacturer. Helical piers can also be used as wall anchors or to raise slabs.
2.4: Pressure Grouting or Mud Grouting
This is a simple process in which a slurry of cement, water, and sand is injected under pressure into the soil to raise a slab or to stabilize a foundation. This process can be unpredictable, and if it doesn't work, it may crack the slab, and the slurry can migrate through the soil to other areas.
2.5: Compaction Grouting
If a house was built on loose or poorly compacted soils, this technique can be used to remedy the situation. Compaction grouting involves pressure injection of very low-slump mortar grout through a buried pipe. It is used to compact soil, fill sink holes, prevent seismic soil liquefaction, re-level settled foundations, and form a structural base for mini-piles or underpinning. Unfortunately, the process is too costly for use on most residential structures.
2.6: Other Specialized Grouting Techniques
Grouting techniques similar to mud grouting and compaction grouting include:
- Jet grouting
- Chemical grouting using sodium silicate
- Urethane grouting for waterproofing
- Vibro compaction
- Injection systems using a mixture of potassium, lime, and water.
Like compaction grouting, the cost of these techniques is beyond the budget of most residential home restoration projects.
2.7: Enlarged Footing to Underpin a Masonry or Concrete Stem Wall
This technique can be used if only a small portion of a foundation has settled. By enlarging the footing, the weight of the house can be more evenly distributed over a larger surface. This repair involves jacking up the house to take the weight off the settled section, excavating a section of the footing, and filling the excavation with reinforced concrete. Use of this technique requires calculations by an architect or engineer to ensure that it will work with the soil conditions present.
2.8: Stabilize Stone Foundation Walls
Stone foundation walls, particularly those built without mortar, are subject to damage from temperature extremes, water, and expansive soil. To stabilize and waterproof a stone foundation wall, excavate either the inside or outside, construct plywood forms, and pour a concrete facing between the form and the foundation wall. When cured, the concrete reinforces the wall and provides a surface for application of waterproofing.
2.9: Foundation Replacement Projects
This is the most drastic and expensive step and is only undertaken when the foundation is severely damaged. Supporting girders or needle beams, depending on the orientation of the framing, are placed to support the weight of the house if one foundation wall is being replaced. To replace the entire foundation, a system of beams and jacks is designed to support the house while the foundation is replaced.
Replacing the foundation requires extensive repairs, including replacing plumbing and electrical wiring. However, the results can be dramatic. This repair should only be performed by a specialty contractor experienced in these techniques.