Know Your Sub Soils Before You Begin Your Straw Bale Build

Written by Andrew Morrison

subsoilI just went through a humbling process on a new home construction project. I used a sub soils report prepared by a geotechnical engineer and provided by the land owner for a home site I was to build a custom home on. The sub soils report showed that there was fill from ground level to 3 feet down and that below that (from 3′ to 7′, the depth of the test pit) was “medium stiff, red-brown clayey SILT; moist.” That spelled out native soil to me and the report further stated that the foundation requirements for such soils were standard spread footers over a compacted rock base of roughly 4″ thickness.

Here’s where the humility comes in: I actually proceeded as if this engineered report was accurate. That seems like a fair assumption, but proved very expensive. The report was a couple years old, and when I compacted my gravel base on the site, the City wanted a new geotechnical report report they would approve the compacted fill and allow me to form my footers. I called my own geotech and asked him to come out for a site visit. he had us dig a test pit and when we did so, he was not convinced the soils that were quoted at 3′-7′ were in fact native. He asked my backhoe driver to keep digging. It was not until 13′ down that we knew he was right. We starting bringing up 40 year old garbage! The “soil” the previous geotech had identified was in fact 40 year old fill over an old personal garbage pit in the old property owner’s back yard.

A simple foundation changed to something very elaborate. We had to follow the native soils below the fill line all the way around the house and pour a sand/cement slurry to fill the now very deep, three feet wide holes. All of the rock had to be removed from the site so it would not end up contaminated and then replaced on the finished grade after the slurry was poured. The ultimate cost of this work was over $7000 and a time delay of one month ensued from the date of the geotech report request to that of completion of the new requirements. Yikes!

So, know your sub soils. Know what is under your ground. Be certain that what you are building on is solid and can hold your structure. Although this was a slow pain in the rear end to deal with, I always prefer to deal with potential problems up front rather than having to fix them after the fact. The time and money spent on this project far outweighs the potential disaster that could have occurred had we not been required to get a new report. Of course, the fact that I relied on a previous geotech report is something of a frustration. The report, although 2 years old, should have been accurate and it was not. You can be sure I will write a letter to the company owner of the old geotechnical engineers on that one!

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2 Responses

  1. Hi Andrew
    We have a dream to build an eco retreat and healing space in our small town of Geraldton Western Australia. The Block is situated right on the beach and is 20acres The dream is to build two community structures at top of block a venue structure mid way with 4 acres of permaculture garden and 5 small homes at the beach side which is in dunes and I do not want to destroy the dunes but build floating over top somehow. We have chosen strawbale and cob for all construction. We will be using steel frame due to the horrific termite population where we live and box the frame. I’m hoping to raise the floor and to have recycled jarrah flooring. The property has a serious slope mid way and has alot of limestone and the steepest part of the slope. This is where we are looking to build the venue which is a large widespan dedign. 15mtrs by 30.
    Can you please give is some tips for building on stilts in heavy limestone. And is there any chance you may be interested in mentoring us through this amazingly beautiful project named Sacred sands in approximately 12months time

  2. Hi Brooke. What a cool sounding project! The good thing about straw bale construction is that it doesn’t really care what foundation it sits on as long as the engineering has been done. I’ve built houses on stilts and steep hillsides in the past with great success. The key is making sure that the engineering manages the vertical loads as well as the potential for lateral movement. Anytime you get heavy materials like plastered bale walls up in the air, there is a risk of them building momentum under a load which can be hard to stop once initiated. A good engineer will be able to manage all of that.

    Please let me know when you are ready to get started. If I have space in my consulting schedule, I’d love to help with the project.


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