CROSS Safety Report
Under slab perimeter drain failure
Local homeowners installed perimeter drains and sump pumps after high water tables caused water leakage into their basements.
Unfortunately, the homes were in areas of deposits of fine (sugar) sands, and such sands migrated through the standard filter fabrics of the perimeter drains, resulting in loss of soil beneath the footings and slabs and causing settlements.
Key Learning Outcomes
For building owners, civil engineers, and contractors:
Fine (sugar) sands may migrate through standard filter fabrics in foundation perimeter drain systems, causing loss of soil and settlement
Special filter fabrics or other means of addressing such soils are required
It is highly desirable to consult a qualified geotechnical engineer in such situations
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Local homeowners were experiencing movement and settlement of foundations, walls, and slabs after installing interior perimeter wall drains with sump pumps in their basements in response to water infiltration due to recent high water tables. Under slab perforated pipe drains wrapped in filter fabric are a standard method of groundwater control. The area that is the subject of this report has large deposits of fine (sugar) sands, which migrate through the standard filter fabric and are pumped out to the discharge area resulting in loss of soil beneath the footings and slabs, causing settlement and damage to foundations, basement walls, and slabs.
A solution was provided via a local municipality ordinance consideration to use filter fabrics with Apparent Opening Sizes of a #100 sieve and a surrounding layer of poorly graded course sand (e.g., Soil Classification SP) around the perforated pipe to prevent the fine (sugar) sands from migrating into the pipe and stop the erosion of the soil beneath the foundation and slab.
Expert Panel Comments
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The report describes how a common solution to a rise in water table elevation to prevent water in a basement did not work in an area with fine sand layers.
The solution proposed seems appropriate, at least in concept, but it should be reviewed and approved by a competent geotechnical engineer. There are other possible solutions such as waterproofing by chemical injection just beneath the basement floors and just outside the basement walls. Chemical injection might also be a method to arrest, and perhaps to reverse, the observed settlement.
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