Why This Foundation?

So when we first started looking at house designs we had planned to do a ‘slab on grade’ foundation. Meaning a simple slab of concrete that we set our house on top of. The frost depth isn’t super deep here so these can often work great. Plus a slab foundation means we could ‘easily’ add radiant floor heating. Alas it was not to be.

After waiting months to hear back from the building department we learned that we were in an Alluvial Fan Hazard Zone and that we would need to get a geotechnical assessment done. Because of going to school to be a dirt nerd I at least knew what all those words meant on their own but was a bit baffled about the whole picture.

Turns out what it means is that sometimes the creek across the street jumps out of its ravine that it carved for itself in the neighbor’s backyard and makes a break for the Nooksack through our property, bringing lots of rocks and debris with it. Not a super comforting thought since fast moving water, mud and rocks are known to move houses right off their slab foundations.

So, we did as we were asked and tracked down a geotech to come check out our property. They were busy but said they could get to our project soon. Then after some months of radio silence, we heard back that they had a bit of a motorcycle accident in another country but would be available soon…but nope. Never heard back. Six+ months down the drain.

So, we found ANOTHER geotech, these guys were great. Our awesome neighbor Chris brought his excavator over and dug some pits. The geotech looked at the various layers of soil that were evidence of water and rocks rocketing down from the hillside repeatedly. This jived with the records research that they did. Not the answer we wanted, but better to know now than to be surprised when I wake up a quarter mile east from where I went to sleep.

Long story short instead of a simple slab we are instead building our house on top of a 6 foot wall of concrete blocks, rebar and cement. It has to go three feet below grade and extend three feet above grade. It also has flow through vents in the western and eastern ends to let flood water pass right through.

Mock up of foundation

A lot more work, a lot more money and a lot more time that we were hoping for I’ll admit. But I can’t say that I’m sad to have a house designed to withstand some of the crazy intense weather events we’ve been having the last couple years. I know I will sleep better knowing we’ll be safe if the creek decides to get unruly. And hey, it has been pretty fun getting to see several feet deep into our soil and learn all sorts of stuff about the history of our property.

This picture shows a bit of the soil history we’ve learned. You can see there are grey sandy layers above and below a dark almost black layer. The lower grey layers are a somewhat coarse river sand, the top 8 inches or so if a very fine silty feeling sand. Below the dark layer is pretty straightforward. River sand as far down as were willing to dig with occasional well sorted layers of gravel or rocks. This is from way back in the day when the Nooksack river was a lot larger and claimed a lot more of this valley as its bed.

Sources/Usage: Image from USGS. View Media Details

The upper finer sand is a different story, that is actually soil almost entirely made of of volcanic ash from Mt Baker eruptions, I believe from 11,000 to 13,000 years ago. Which was my best clue to the dark layer. I believe that the dark layer is evidence of a widespread fire cause by Mt Baker starting to get active and burning down large swaths of forest. All very exciting stuff for this dirt & rock nerd family!

So, that’s how we go here. A couple years later and a more complicated but safer plan. My gratitude to all our friends and family that had to hear us talk about this non stop for about three years. I love you all!

Oh, and on an exciting note foundation building starts this week! Brooks from JB Custom Concepts has already started dropping off materials and we should be pouring footings this week! I’ll post some pics as soon as I have them.