Engineered fill creates a firm base to build the house on

As I mentioned in my previous post, the soil under the old house was quite loose and full of organic material. From what I have been told by neighbours, this area used to have a stream or an underground stream running through the backyard and may also have been a floodplain for the Ottawa river long ago before the entire area was developed. That would certainly explain the large amount of organic material in the soil - it was almost like peat.

So, to ensure that the new house would not settle, the builder had to remove all of the organic soil until they reached undisturbed mineral soil - about 60 cm below the level of the bottom of the footings on the new house. Then, about 160 metric tonnes of engineered fill were dumped in the hole, spread and compacted to create a firm flat surface to build on.

Now the surface is ready to build on and the next step is to pour the concrete footings.

Soil troubles

With the old house removed, the site engineer had a chance to inspect the soil on which the new house will be built.

According to Mike Holmes, builders like to take advantage of millions of years of natural compaction by digging down to earth that has not been previously dug up and tamped down. If they don't do this there may be problems with shifting. The foundation has to take the weight of the house, but it also has to stand up against the pressure of the earth around it; if a foundation is going to shift, that pressure is what will cause it.

However, once our old house was removed it became apparent that it was in fact built on loose soil with lots of organic matter in it. No wonder the old house sank in one corner. In fact, I watched them dig up the soil in the corner where the old house had sank, and it was black, stank like sewage and contained logs.

In other words, when they built the original house in the 50s, they did not bother dig down to undisturbed soil and furthermore, it looks like they just piled in old trees and other junk to fill up the space. As this organic matter decomposed, it created a void and the house sank. While further sinking is not usually likely in an older home, given the amount of organic material I saw in the soil, it would not have surprised me, especially after we added extra weight to the house with an addition. All in all, I feel a lot better digging the whole place up.