Foundation options for steep slopes 1

[rdemyan]

I am considering buying about 0.3 acres of which, according to a previous survey, over 93% is > 40% slope. The property has a "killer" water view and is close to an 80 year old house I already own (also in a > 40% slope area). A 40 foot (North to South) by 60 foot (East to West) rectangular building plot would have the following slopes:

East to West on the North side: 42%
East to West on the South side: 53%
North to South on the East side: 17.5%
North to South on the West side: 35%

The property has been on the market for over two years and the price is starting to drop. The real estate agent says that the problem is that homeowners don't want to buy land where the house foundation has to be a "pole" type of foundation. Note that my current house in this area does not have a "pole" foundation; but of course it is about 80 years old and, according to engineers, has undergone differential settlement (presumably because the soils were not compacted back then).

I have been trying to find out what the foundation options are for a steep slope. Can soil reinforcement with geotextiles be used? What about soil nailing?

Unfortunately, I don't have a soils report yet. Also, I checked with the building department and they pretty much take the recommendation of the geotechnical engineer for the construction project with regards to the foundation: hence, there is no particular requirement that a foundation type be chosen to result in the least excavation in this environmentally sensitive area.

Can someone provide me with the potential foundation types for house construction on a > 40% slope along with relative cost. Also, how important is the quality of the soil (i.e. sand, clay, etc.) on determining the choice from these options.

I don't trust real estate agents with providing me with accurate information, even if they have reports available. I fully understand that I may want to hire a geotech engineer before I purchase the property; but, this may be a project that I want to walk away from right now based even on the limited information presented above. After all, apparantly a number of other potential buyers have walked away.

Thanks for providing me with advice. I realize that this is a technical forum and this is not a technical thread. But it's not easy to find basic information on such questions.

 

[BigH]

Can you do a quick survey of other homes in similar conditions and see what they used? Would the building dept let you see the soils reports for some of the homes built in similar conditions, etc? This might get you started while you imperatively seek out the advice of a qualified geotechnical engineer. You can get the names of the ones used before from the bldg dept soils reports. I worked on homes against slopes in Ontario Scarborough Bluffs - it is tricky and, sometimes, too, the city conservation authority might wish to ensure that no more building be done - political rather than technical.

 

[Focht3]

You didn't tell us where the property is located; geology will play a large factor in evaluating your options. You should research this as a part of your decision-making process.

Those are pretty steep slopes. And a relatively small site (given those slopes.) My gut says that your best bet will be a "pole" type support system, since any fill will shrink somewhat.

If your dream house is a 1[½] story home 35 feet by 50 feet in plan with the ground floor at a single elevation, you would be looking at 35*.53=12.25 feet to 50*.35=17.5 feet of fill at the southwest corner of the home. If you use a high quality rock fill and compact it heavily (i.e. to highway standards), you could still see at least an inch of fill shrinkage at the "tall" corner. Of course, that assumes the contractor can achieve heavy compaction on such a steep slope, which I seriously doubt. As a result, I'd expect to see more like a minimum of 2 to 5 inches of settlement at the southwest corner for high quality fill. As the fill quality decreases, the expected settlement increases significantly.

I wouldn't gamble with fill - unless you could tolerate a multi-level home with small stairs between levels. That could drastically cut the fill requirements.

Are you in earthquake country?



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[rdemyan]

Actually, I had the potential plot plan as 60 feet east to west and 40 feet north to south.

Changing the rectangular plot plan to 60 feet from north to south and 40 feet from east to west results in slopes as follows:

East to West on the North side: 45%
East to West on the South side: 67.5%
North to South on the East side: 18.3%
North to South on the West side: 33%

Unfortunately, I don't know very much about foundations. Am I correct in assuming that your discussion on fill is in connection with putting in a different type of foundation than a "pole" foundation.

The property is located in Seattle.

 

[rdemyan]

In rereading your reply, I'm not sure if there may be a misunderstanding. The actual property site is about 0.3 acres measuring roughly 130 ft by 115 ft. The building plot area of 40 ft by 60 ft is what I selected as a good area for excellent views and also where the topography map shows that the contour lines are not as close together (this is also the same area that an architect created a few preliminary, conceptual house drawings ).

My current investment property is in a similar area (not quite as steep). Back in the 1920's they cut into the hillside to build two stories below the street grade. For the potential property that I'm interested in, I was thinking that maybe one story could be cut into the hillside with another story above.

 

[cvg]

It sounds like this property could be quite valuable once you have built the house. It would seem worthwhile to just hire a geotech to perform a quick assessment. For a few bucks you could get a good professional opinion on the recommended construction methods.