Foundations
Foundation Design Specifications
The International Building Code (IRC - Chapter 18) and the International Residential Code (IBC-Chapter 4) specify that a Building Official can dictate the requirements regarding whether or not a soils (geotechnical) report is required for a structure. Most jurisdictions require a soils report to pull a permit.
Soil (geotechnical) reports are produced by engineers who have a geotechnical specialty. This means that these professionals have additional education and experience in soil mechanics and structural design.
Common Foundation and Basement Conditions
FOUNDATION WALL CRACKS
Cracks in the wall of a foundation does not necessarily mean structural problems or failures. It depends on the location, width, length, severity, orientation, displacement, and other factors.
BASEMENT SLAB-ON-GRADE CRACKS AND MOVEMENT
Cracks in the basement slab-on-grade floor do not necessarily mean structural failure. It depends on the location, width, length, severity, orientation, vertical movement, and other factors.
Click here for more information on basements.
DRYWALL CRACKS AND OUT-OF-SQUARE DOORFRAMES
Drywall cracks and doorframe problems can be caused by foundation movement, basement slab on the grade floor movement, or a combination of the two.
SEPARATIONS IN TRIM
Baseboard and trim damage can be caused either by foundation movement or basement slab-on-grade movement.
BUILDING ENVELOPE DISTRESS
Visible separations or cracks in building envelope elements such as siding, masonry, stucco, or stone facades are indicative of foundation movement.
MOISTURE
Water (moisture) on a foundation wall or basement slab-on-grade surface can indicate problematic drainage conditions in the "foundation protective zone" of the foundation.
Water can also migrate into the foundation through pipe trenches, such as along water and sanitary service lines, and enter the basement or crawlspace through the foundation wall penetration.
EXCESSIVE SUMP PUMP DISCHARGE
Excessive or constant cycling of a sump pump and/or large amounts of water discharge from the pipe daylight outside the home indicate water problems and should be investigated by an engineer.
FLOORING
Heave and cracking of the basement slab-on-grade can cause severe damage to flooring.
SULFATE DAMAGE
Sulfate is a substance in soil that can deteriorate concrete. During a site soil investigation performed by a geotechnical engineer, the soil is tested in the laboratory for sulfate content, and recommendations to mitigate this problem are specified in the soil report. If sulfate is present in the soil, special chemicals are added to the cement mix at the concrete plant. Chemical additives to cement mix add cost to the concrete mix but help prevent sulfate damage.
FOUNDATIONS STRUCTURAL DISTRESS
WHY IT COULD BE HAPPENING
1. SWELLING OR COMPRESSIVE SOIL
It is essential that swelling or compressive soil conditions be adequately identified in the soil laboratory. Valid soil test results provide the geotechnical engineer with data so they may provide a design that protects the stability of the structure against forces in these types of soils. If an engineer is unaware of the magnitude of expansive or compressive characteristics of the soils, this may lead to a problematic foundation design.
2. INVALID SOIL DATA AND TEST RESULTS
In some cases, there may have been a faulty collection of samples or mistakes in calculating test data. It is possible that not enough samples were tested or there were errors in the soil laboratory testing results.
3. DRAINAGE PROBLEMS
Water intrusion due to drainage problems or a waterline break can affect soils that are prone to swelling or consolidation when wetted. Water enters the soil in the foundation backfill or below the structure, and the soil reacts by pushing on or receding from the foundation components, causing the structure to become unstable.
4. ENGINEER'S ERRORS OR OMISSIONS
The design of a foundation requires the application of knowledge related to geotechnical and structural analysis. Geotechnical engineers specialize in soils and the design of foundations and specify the type (either footing or pier) and provide structural engineers with load parameters and other guidelines for building on a particular site. Structural engineers use the guidelines from a geotechnical engineer to produce foundation drawings. Someone who is not knowledgeable in geotechnical and structural disciplines would not have the ability to perform this type of work adequately, likely resulting in unintended negative results.
Engineers are human and may make mistakes in collecting facts and data or performing calculations. It is possible for engineers to make inadvertent errors or omissions in designing a foundation.
5. OVER-EXCAVATED SOIL
The over-excavation of soil is often used by builders to avoid costs associated with having to construct piers on unstable soil. The process involves replacing a soil mass below a foundation with soil that has better qualities. It allows the foundation to be constructed on less costly footings instead of piers anchored in bedrock.
Structures on footings constructed upon a depth of over-excavation instead of piers can be especially prone to problems. Although over-excavated soil is replaced under supervision to ensure it is of the right consistency, moisture content, and density, substandard construction means and methods often occur.
Construction defect claims associated with over-excavated soil can be common. Problems with over-excavated soil often include:
Reduced strength due to a bad soil type used,
Soil layers not compacted enough,
Intrusion of water into the soil,
Excavations that are not deep enough.
6. COSTS
Builders and developers are known for wanting to save money and may pressure engineers to design footings instead of piers because they are cheaper to construct. However, not all soils can adequately support footings, and piers are specified instead.
7. CONTRACTOR'S MEANS AND METHODS
To save money on concrete, contractors may reduce the number of piers or construct piers that are shorter than the depth recommended by the engineer. Money is saved on concrete because short piers require less concrete than longer piers. The contractor's means and methods may also be faulty. For example, the contractor may use concrete that does not have the proper structural strength. Or perhaps the piers do not properly align with the foundation walls. There are many possible causes.
P.T. SLABS
PT slabs are regularly implemented in commercial construction due to the reduced cost, flexibility in design, lesser usage of materials, and durability. Unfortunately, when it comes to residential construction, there are too many serious potential problems with PT slabs, and they are not worth the risk for the average homeowner. The PT slab is used in the modern architecture of skyscrapers, bridges, and marvelous structures. It requires an experienced contractor that has skilled workers to construct PT slabs, and the workers need close supervision. PT slabs should not be constructed by a typical foundation crew.
IMPORTANT NOTE FROM ENGINEER MIKE: I do not recommend, ever, purchasing a single-family, townhome, or condominium constructed on a post-tensioned concrete slab-on-grade foundation (a PT slab). I would never buy one for myself or recommend it to anyone. Builders try to sell this as a superior foundation for residential construction, which it is not for many reasons.