Backfilling against the foundation

Backfilling of the house occurred last week. It took about two days for them to complete it, with several large trucks of fill delivered.

Waterproof membrane applied to foundation

In the last week an a half, a lot of progress has been made, starting with waterproofing the foundation.

Once the foundation was poured, a Platon membrane was applied to the outside of the foundation against living areas. Platon is a tough, waterproof, double dimpled, high-density polyethylene plastic membrane that keeps wet soil away from the foundation wall. The dimples create an air gap between the wall and the waterproof membrane. Any water that does find its way past the membrane flows to the footing drain. As an added benefit, wall moisture condenses on the back of the membrane and flows to the footing drain. This dries out the wall. Because Platon is not adhered to the wall, the wall can crack, shift and settle over the years without affecting Platon’s performance (although hopefully that wont be an issue).

Next step, back filling.

Foundation poured

On Thursday or Friday, they poured the concrete for the foundation walls. They removed the forms on Friday.

Forming the basement

Last Thursday, they formed the basement.

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weeping tile and stone fill

Once the footings were poured, weeping tile was placed around the footings and a stone slinger deposited stone fill all around the footings in preparation for the concrete.

concrete footings poured

The footings for the house were poured yesterday. Today, they had hoped to install the weeping tile around the outside of the footings, spread stone around the footings and erect the forms for the basement walls. However, with all the rain today, things were delayed. As you can see from the photo, some of the plumbing has also been installed.

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.

Foundation repairs

Here are a few pictures of the foundation problems we discovered in the house after we were able to rip down some of the walls in the basement. These and some other problems are what conviced us to build new rather than renovate. While anything can be fixed for a price, it was hard to justify the cost of these repairs while still ending up with a seven foot basement and a sloping floor.



In the image below, this crack was directly beneath a window. The wall did not have enough strength to hold back the force of the ground on the other side. To fix it would have required pulling out the window and pouring a new concrete sill heavily reinforced with steel, which would prevent the wall from buckling in. The cost was estimated to be as high as $5000.









In the two images below, a large crack (about 1" wide) can be seen running down the wall from the ceiling to the floor. Amazingly, the basement window is also bent, but the glass is not broken.

The image below is from the garage, where frost has made its way under the insufficiently insulated floor. It's hard to tell from the photo, but this whole section of wall is buckling inwards due to the uplift of the floor and the force of the ground on the other side of the wall pushing in. To fix it would require supporting the house above while the entire wall is ripped out and rebuilt. The cost for this repair alone was estimated at $15,000 - $20,000

It was interesting to hear from our builder some of the history of these types of cinder-block foundations. Apparently, when these houses were built in the '50s, poured concrete foundations were available, but were more expensive and required a trained crew. A builder could save some money by using cinder blocks because the blocks were cheaper than the equivalent volume of poured concrete and the builder's own workment could usually do the job.

Interestingly, today the situation is reversed and it would now cost more to build a cinder-block foundation (not that I think you ever would want to).