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GeoInsighter Fall / Winter 2000 Newsletter
Volume 5 Number 3

That Sinking Feeling

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Some contractors would have you believe that settlement of buildings and slabs and cracks in foundations and slabs are to be expected as a normal occurrence of construction; "the building coming to rest." This notion is not only false, but may represent the automatic reply of a contractor used to "cutting corners" on a project. Building foundations must include proper designs to transfer expected building loads to the underlying soil with shallow or deep foundation systems. If done correctly, the weight of the building will be carried by the underlying soil such that settlement of the building will not likely be noticeable or measurable without special equipment. Proper foundation preparation most commonly includes compaction of soil surfaces to support cast-in-place concrete spread footings and slabs and is easy and inexpensive to do correctly during the sitework phase of new construction. Attempting to address settlement after a building has been constructed is difficult and typically quite expensive. 

Common evidence of possible foundation settlement may include:

  • new or expanding cracks in the foundation walls and slabs;

  • water infiltration into basement areas;

  • walls or sills becoming unplumb or unlevel;

  • separation of structural framing members;

  • sticking doors and windows that are not associated with humidity; and

  • new cracks in finished sheetrock and plaster. 

While it is true that some normal shrinkage of concrete may occur during its placement that could cause hairline cracks in foundation walls and slabs, proper concrete placement techniques minimize or eliminate these cracks altogether. Cracks in walls or slabs that are wider than approximately 1/16 of an inch, or more frequent than one vertical crack per 8 feet of wall, may indicate a problem with either settlement or poor concrete placement, or both. If foundation settlement is suspected, a monitoring program should be designed to evaluate the length and width of cracks (to see if they are growing) and to measure the elevation of benchmarks set on the structure to identify if downward movement is occurring and the rate of such movement.

If settlement is not occurring, repair of foundation and slab crack damage can be as simple as implementing an epoxy injection program. Wall cracks that are wider than approximately 1/4 inch should be assessed carefully to determine if the integrity of the foundation wall has been lost; some large cracks need to be held together with structural steel bolted across the crack to ensure that the wall continues to function properly. If settlement is occurring, bonding a crack with epoxy will not usually stop the crack from re-occurring. 

To address and remedy building settlement, an investigation is required to determine the cause of the settlement and then a new support scheme is designed. Because of the typical disturbance associated with repairing an existing building (i.e., disruption to the use of the structure and isolated destruction and reconstruction of certain building components to effect a repair), significant costs are often incurred. Re-support of existing foundation systems may include underpinning, installation of piles, or soil improvement to stop additional settlement from occurring. Repair of settled slabs may include pressure grouting to lift slabs back into place, repouring slabs to obtain a level surface again, or slab removal and replacement with lightweight fill and a new slab to prevent further settlement. The proper remedy for each building is site specific, dependent upon subsurface conditions, the age and use of the building, and the owner's objectives. 

Because of the potential order-of-magnitude cost increase associated with post-construction repair to address settlement, money spent during construction is a very wise investment. Prevention is not a simple matter for a pre-existing building and, unfortunately, if not identified during a pre-purchase building inspection, recovery of costs associated with repair is very difficult and should, therefore, be an important part of a real estate transaction for certain properties. 

Michael C. Penney, P.E., L.S.P.
mcpenney@geoinc.com


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