Need advice for house being built on top of a flat cut hill

Hi Guys first time poster here!

My name is Api and I am building a round log post in ground strawbale home in the Bay of Plenty New Zealand. Also looking to have a reciprocal roof with earth on top. We have already excavated a small building pad on top of an established hill. Excavation for the pads were sloped toward the hill for runoff. Excess dirt was pushed to the sides of hill. The common soil type here is loam.

I need advice with building on top of a hill, what I should do for foundations considering location and soil type? Keep in mind New Zealand is in a seismic zone.

We are building with material that is close by, our trees, clay, sand, but straw will be delivered. We have plenty of hay close by but I’m not sure you can use hay? We have endless supply of river rocks and builders mix and natural spring, river and creeks for water.

Pictures will be added so you can see what I’m working with and also an example of the log frame home we hope to build.

Any and all advice would be greatly recommended.

Api

Looks like you burned the bottom of the posts.. good move. By foundation, I’m guessing you mean a contact barrier between the straw bales and ground. I would suggest a nice masonry stem wall of stone. Could be drystacked or mortared, depending on the stone. Congratulations on a good start, and your beautiful site. I spent my early life in Hawaii, but now I live in the desert. All that green gave me a touch of nostalgic feels. Please keep us updated on your progress.

Kia ora Api and haere mai ki Permies! That is a beautiful spot there...I hope it didn't take too much of a hammering the past 24 hours.

In light of the seismic risk, you will probably want a reinforced concrete ring foundation. Consider an inner and an outer wall and then fill the void with stones or rubble, then make a mortared cap. A moisture barrier will go on top of this, and you will need rebar extending from the rings into the bales to tie it all together. This design can be done without too much portland cement, since most of the foundation volume is fill material, but will give good tensile performance when the ground wiggles.

Be caution of doing too much on the fill area because it doesn't look like it was packed and anchored to the hill.  With the right seismic activity and moisture it may become liquefied and "flow" down slope.  

How deep are your posts?  

I second above comment.
I would never build anything on the fill material without proper compaction, especially in seismic or mud flood zones. Even with compaction I would still prefer to build in the cut part of the hill.

Ted Abbey wrote:Looks like you burned the bottom of the posts.. good move. By foundation, I’m guessing you mean a contact barrier between the straw bales and ground. I would suggest a nice masonry stem wall of stone. Could be drystacked or mortared, depending on the stone. Congratulations on a good start, and your beautiful site. I spent my early life in Hawaii, but now I live in the desert. All that green gave me a touch of nostalgic feels. Please keep us updated on your progress.

Thanks Ted,

Appreciate the advice. By foundation I just wanted to see if post inground up on a hill was sufficient. I will be doing a stem wall of mortared river rocks. The depth im thinking of for my log posts is about 1500mm.

Just to clarify, the picture of the of the round post frame is not my build but a reference pic of something similar.

Phil Stevens wrote:Kia ora Api and haere mai ki Permies! That is a beautiful spot there...I hope it didn't take too much of a hammering the past 24 hours.

In light of the seismic risk, you will probably want a reinforced concrete ring foundation. Consider an inner and an outer wall and then fill the void with stones or rubble, then make a mortared cap. A moisture barrier will go on top of this, and you will need rebar extending from the rings into the bales to tie it all together. This design can be done without too much portland cement, since most of the foundation volume is fill material, but will give good tensile performance when the ground wiggles.

Kia ora Phil!

We were inland close to the Uruweras, did not get much of the storm by us, hope you pulled up ok. Keeping that in mind we are quite open on our hill, wind would be something to consider, assuming round homes are pretty good with this thing, not sure reciprocal earth roofs though?

As for concrete ring foundation, I apprefiate thst idea and will probably impleement that. Would the base of the ring be concrete then building walls on top of that? Would you be putting some stone under the concrete ring as well for drainage?

Sourdough Al wrote:Be caution of doing too much on the fill area because it doesn't look like it was packed and anchored to the hill.  With the right seismic activity and moisture it may become liquefied and "flow" down slope.  

How deep are your posts?  

Thanks Sourdough,

I'm looking to get posts 1500mm deep. Dig holes, compact bottoms, place big rock at the bottom, place log in hole, fill sides with sharp rock to the top. Sound ok? Would there be another way to keep posts out of the ground and build on a hill like that without problems? Be good to avoid logs making contact with soil, but not sure what would be best.

cheers

Cristobal Cristo wrote:I second above comment.
I would never build anything on the fill material without proper compaction, especially in seismic or mud flood zones. Even with compaction I would still prefer to build in the cut part of the hill.

Hi Cristobal,

I will be building on the cut part (most solid part) of the hill.

cheers

That sounds good.  

We are planning to start building an in-ground geo-thermal greenhouse (similar to a Walipini) this year.  The engineered design we got show attaching the greenhouse frame to 4-foot concrete pillar/posts.  The reason being is to secure it from winds and swelling ground due to harsh freezing weather we get here in Montana.  Also, that keeps our wooden beams above the dirt and supported by the concrete posts.  

The greenhouse design calls for not less than 12" (305 mm) diameter "sonotube" which we can get from our local hardware store to make the concrete pillars/posts.  They go up to 30" (762 mm) wide x 4ft (1,220 mm) long at our store, but the design only calls for 12".  If we had different soil, we would have to go with the wider ones.

When I was a kid, my parents moved a big 2-story house to a hillside cut.  Most of the house was on the fresh cut (which was hardpan/compacted clay and gravel), but the west wall was partly on fill.  Every summer dad had to re-level the whole house so mom's cupboard doors would close again.  It took about 10 years to finally stop shifting.  Dad just poured footers and walls to set the house on.  I suspect if he poured some 3 or 4 foot pillars under the footers, the house would have been more stable.

In the 90's, my wife and I bought in a 1950's house in the Helena Valley in Montana - USA.  That valley is filled with 1,000's of feet of loose alluvial gravel.  We had a small 4.2 earthquake one spring and the kitchen area sunk ~ 1".  I had to shim that area up so doors would work again.  What was strange was none of the other rooms had any noticeable effects, just the kitchen.  

Later that year, we visited a guy across the valley from us and his new house (~5 yrs old) had massive cracks and ripped sheetrock throughout the whole house.  It looked to have differentially sunk at least an inch or two (25-50mm).  We felt so sorry for him...    

So there are my fun experiences with "shifting" houses....

In closing.  From our experiences with 3 different building sites in two different states, a lot of the final details will come to you just by taking time to walk around your area multiple times in multiple directions to "listen" to what it tells you.  Sounds weird, but it does happen.   Things will just seem to pop into your mind from out of the blue...

Enjoy and wishing you a lot of fun.  It looks like a beautiful area!

From the bottom: Undisturbed earth -> reinforced concrete footer with stone infill -> mortar cap -> waterproof layer e.g. bitumen -> straw bales.

You could put some gravel in the bottom of the trench before you pour the ring wall, but I don't think you need to do that. Your roofline should extend well outside the foundation perimeter and any water that lands needs to flow away from the structure, so drainage is solved at that level.

PS: It's still raining here but the wind died down. The river below our property burst its banks and went through a few houses yesterday, and took out at least two bridges up the valley. More rain forecast tomorrow...still looks wet. I'm glad you have a spot on high ground...the Ureweras are beautiful.

Hi Guys,

Just an update, posts are in. Packed in with 65mm sharp rock and posts were put on top of a large rock. Posts were treated with a bio oil, hoping they last longer in the ground then my lifetime! The diametere of the large home is 8m. I have sourced strawbales and hope to move onto walls by late Jan 2024.  Its been a long year and we are glad o be heading onto the roof stage. My sarking timber inbetween rafters will be poplar as it was the best i could afford apart from trying to mill my own, which by the way takes a lot of time. If you are married like me, my wife lacks patience and wants in the house yesterday. We will work on the stem wall once the roof is up 200-300mm deep with loose rock and building up from there 250mm with mortared river rocks. Looking to break the river rocks to roughen up the edges so they are not so smooth and round, hoping this works. Its a free source from our river and plenty of it. I will post a picture of the rough design when i get a chance so you can see the windows and layout of the home. Hope you are well, witht he holiday season upon us.