Hillside homes are subject to slumping in heavy rains. The term slumping is a term of geology that means when a large portion of a groundwater saturated hillside or mountain can no longer support the weight of the mud under it, the whole portion, many times acres, just flow is one move down hill like a thick milk shake that was spilled out over the kitchen counter and down to the floor below. Large sections of land can fall away when saturated, and slide your home right down the hill with it. No matter the engineered pilings, oversized footings and earthquake strapping. None of that matter when the whole hillside goes. Groundwater that forms ponds above your home many times cause this to happen. Two winters ago we had an event in a mountainous region where creeks were dammed up by debris creating this very situation. It wiped out homes, boats, and a mile stretch of Oregon coast highway.
The way you can protect your homes foundation footprint from saturation of rain created groundwater is to install hand excavated french drains on the uphill side of your home to flow the groundwater around the homes foundation. Install them many feet away from the foundation, as well as around 2 feet from the foundation.
That fact is that the french drains installed around your home are not going to, by themselves, give you overall ultimate large scale groundwater protection from a large geological event such as slumping of large portions of mountain side if it is above you, large, and super saturated. The upshot of the whole deal at that point is to admit that just because the home plans passed the muster of the city or county planning authorities, it still might not have been such a great choice from a home drainage and safety perspective.
You would not catch me building a home on a hillside without the inclusion of industrial sized groundwater removal hand excavated french drains. Why risk it when history is not on your side.
Installing a hand excavated french drain to remove groundwater caused by rain, while it is running on the ground itself and within the top 18″ of soil depth, can literally save your life and property. Many times it really does not matter in this instance if the home has pilings driven in the ground deeply. The slump line is perhaps 100 feet or more below your footings. Bye Bye.
The entire section of mountain can let loose once saturated. Foundation footings can be undermined causing the home to lose subjacent and lateral support. In plain terms. The support under the home created by the foundation and the support from the side of the home which prevents it from shifting to the side or down.
Hand excavated french drains installed perpendicular to the direction of the groundwater flow, about 10-20 feet apart are your best defense for slumping. Sometimes many of the french drains are needed. Flow the groundwater around the home, and not too close to it faster than it can saturate your foundation and you will achieve success.