French drains make stable geology under hillside homes
Geology and groundwater saturation around and under homes on sloped building sites affects stability.
The most amazing thing with respect to hand excavated french drains is that cities and counties will issue a building permit for construction of a home on a building site that even goats should not occupy, from a safety stand point, and not mandate the installation of hand excavated french drains to protect the hillside building pad from saturation of groundwater due to hard periodic rains, which cause slumping.
The Portland, Oregon west hills has many homes in neighborhoods perched on stilts or spot footings built over the edge of hillsides literally sloping straight down hundreds of feet, yet in most of these neighborhoods homeowners believe that installing steel posts in the ground will keep their home safe from hillside slumping due to heavy groundwater saturation without hand excavated french drains.
Recent 08 winter rains in the columbia county hwy 30 area of the Oregon coast has shown just how large of an undermined saturated area can be affected. Stilts are not going to protect a home on a heavily damaged hillside location. The ground may be undermined, saturated and cut for a hundred feet or more under the hillside and all move in one motion down the hill.
Talk about blind faith. Wow. I recently looked at a home that is located in an area that meets this description. Under the home the groundwater has cut 3 foot deep slots undermining the spot footings and severely compromising the stability of the hillside.
This home probably should have been built on stilts or not built there at all. Even if the home was built on stilts, the home would still have the same erosion under it however because no hand excavated french drains were installed to protect the hillside from erosion under the home.
It is one thing to have groundwater running around the home, and quite another to have it running under the home.
In the case of the example I used on the hwy. 30 slide this year, groundwater had formed a lake above a group of homes. If the groundwater had been run around the homes in a couple of creeks, tapping the lake and reducing the pressure on the hillside, the hydrostatic pressure would most likely not have caused as much damage, and may have prevented the hillside from slumping as it did.
In a bad rain year when very heavy rains dominate the winter scene in Portland, some of these hillside homeowners on unstable building sites are likely to lose big as their homes slide right over the edge. Many of these homes should not even be built there.
Many of these areas should use hand excavated french drains to remove large amounts of groundwater and flow it around the building site.
If most of the groundwater is running on the bottom of the hand excavated french drain and within the perforated pipe, the groundwater won’t cut deep grooves undermining the homes foundation.
The geology of some of these hillsides makes them even more prone to slumping because of the lack of bedrock and the dominance of clay soils. Some years ago I installed hand excavated french drains around and under a home on Lake Oswego Country Club where the creek that was created from years of no groundwater removal systems installed, and groundwater running under this home, had undermined the spot footings.
The home required new spot footings in various places and had to be jacked and supported during the installation of these new support beams, posts and footings.
If I could show these Portland west hill homeowners with sloped sites some of the properties already severely damaged, they would get it.