Have you followed the structural problems occurring in the foothills along the Front Range and along C-470? If so, you know that differential ground movement from expansive, steeply dipping bedrock (dipping bedrock) has cost building owners, builders and insurance companies millions of dollars.

Expansive soils are common in our region and are often referred to as bentonite or clay soils. When soil containing high amounts of clay becomes moist, the clay molecules expand and exert tremendous upward force. Newer construction methods leave a void space beneath a structural floor and grade beam. This void allows the soil to expand and contract freely without affecting the structure.


Another common construction method uses a "floating" or isolated slab. In this design, the slab is isolated from all building elements. If these items are not isolated from the slab, any force exerted by the expanding soil will be transmitted to the building's structure, resulting in cracks, unlevel floors, and doors and windows that stick.

A few, but by no means all, of the design elements of an isolated slab include the following:

The floor slab should not attach directly to the foundation wall. Instead, a separation is filled with an expansion joint.

Any basement walls must be "floating" walls. The major structural attachment is at the ceiling, and the walls hang from the ceiling. At the floor, an opening is left in the wall to isolate the wall from the slab.

Door trim and jambs need to be trimmed up higher than normal so they do not rest directly on the floor slab.

Any plumbing fixtures and pipes should be installed using details that will allow the fixtures to remain isolated. Pipes which penetrate the slab are wrapped with isolating material before the slab is placed.

Drainage around a structure is another consideration to help ensure soil stability. Remember: Soils containing clay expand as their moisture content increases. Keeping the soil as dry as possible makes it more stable. Grading around a structure should direct water away quickly. Plants which require frequent watering should not be planted near the structure, and downspouts should have extensions that discharge water away from the backfill zone around the foundation walls.

Dipping bedrock may also cause structural problems and is common along the Front Range of Colorado. Layers of soil and bedrock, tilted when the mountains were formed, may subject uneven forces on a structure and cause greater damage than expansive soil alone. Building a structure on dipping bedrock requires a complete soils report and foundation design.

In steeply dipping bedrock, water can penetrate 30 or more feet below grade as it travels along the fractures and seams between different layers. Additional moisture in the clay layers can result in deep heaving of the foundation caissons, resulting in costly damage.

Copyright 1999 Professional Investigative Engineers. All rights reserved.