Guest column: Next potential worry is landslides

Tag : Stable Tile

The starting point for a landslide may be a stream gully that isdeepening by erosion or a laterally shifting river channel that isundercutting a hill. Man also can have a hand in it, by dumpingfill soil at the top edge of a hill, or by taking soil away fromthe bottom.

Sometimes soil from a walk-out basement is simply pushed out ontoan adjacent hillside, where the addtional weight can trigger alandslide.

Excess water is involved in nearly all landslides, so runoff waterfrom rains should be diverted away from an active slide area.
Our intuition is to fix the problem by putting the soil back whereit came from, but that is a serious mistake. That re-creates thecondition that led to sliding, and eventually the soil will slideagain.

Landslides are hungry, and tend to enlarge by creating a successionof new scarps, or steep cliffs, that run parallel with the first.

One way to stop a landslide is to install tile drains, buttrenching can be extremely dangerous or impossible in unstableground. If sliding continues after a drain line is installed, thetile can be pinched or broken so water is carried and concentratedat the worst place, at the slip zone.
Sometimes the scarp will stop at and expose a basement wall.However, even exposing the foundation can allow it to settle so thewalls crack.

Strange creaks or groans are a wake-up call that it is time todepart the premises. Sometimes the entire house will shake and boomlike thunder. A drop of 3 to 8 feet can happen overnight and leavepart of a house cantilevered out over a void. I investigated onecase where only the front wall of a house was left intact, like amovie set.
What should the homeowner do? First, carefully examine soil abovethe landslide and look for ground cracks that run parallel to theslide area. These little cracks are the future scarps. They must befilled with soil to prevent entry of surface runoff water. Theslide area also can be covered with plastic but it tends to blowaround and at best is a temporary expedient. Put extensions on alldownspouts so roof water is carried away from the slide area.
Then call a local geotechnical engineering firm that deals withlandslides. If the house is in imminent danger, you will be askedto call the utility companies to shut off gas and water.

The engineer may recommend test borings to determine the depth pfthe offending soil layer. The engineer then may suggest remedialmeasures such as drainage or changing the slope geometry.

I developed one of the most successful and least expensiveprocedures in the 1960s. Called drilled lime, it's a specialtyoperation that requires use of chemical lime, which reacts withsoil clay and permanently increases its friction.
Retaining walls are more expensive and normally retain only soilthat has not been involved in sliding.

If all else fails, a house can be moved to a safer location, butonly if it still is on stable soil that can be jacked against. Ahouse situated on actively sliding soil generally is considered atotal loss, which is not covered by most home-insurance policies.

Remember that it is unethical and illegal to sell a house that isaffected by a landslide without revealing the problem toprospective buyers.
A landslide does not mean that all is lost if one acts responsiblyand quickly. I lived in a rescued house for 13 years, and 40 yearslater, the site remains stable.