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US-10: Under slab perimeter drain failure

Report ID: 907

Published: CROSS-US Newsletter 2 - September 2020

Report Overview

Foundation, wall, and basement slab settlement damage due to high water table and layers of subsurface fine (sugar) sand.

Report Content

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 for 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.


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.

Additional references1&2 are included below.

1. A guide to footing drain pipes: (https://basc.pnnl.gov/resource-guides/footing-drain-pipe).
2. Bulk Water Control Methods for Foundations (https://www.buildingscience.com/sites/default/files/migrate/pdf/BA-1015_Bulk_Water_Control.pdf).

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