Before undertaking any new build or foundation work, it is important to understand the makeup of the soil at the site. Across the Ottawa region, the soil composition can be quite varied and the only way to determine the actual soil makeup of your site is by drilling boreholes and taking samples of these materials. However, we can also get a good idea of the types of soil by consulting soil maps published by the Geologic Survey of Canada.For example, here are the general soil profiles for a few areas in Ottawa:
- Downtown Ottawa: Much of the downtown area is underlain by bedrock and there is very little natural soil cover.
- Westboro: Underlain by glacial till over bedrock.
- Sandy Hill: Underlain by a thin sand cap, over a thick clay deposit, over glacial till, over bedrock.
Generally, not all of the soil types described in the list below will be present at each site, but considering Ottawa’s geology, the site may consist of the following, in approximate sequence, from youngest to oldest:
Random and Engineered Fill Materials
Fill materials form the youngest part of the soil profile and consist of soils excavated from their natural location and then transported to another location through human activity. There are two broad categories of fill materials:
- Random fills: Not typically controlled, these come in varying compositions (they can contain any portion of clay, silt, sand, gravel, cobbles and boulders). This material often contains organic matter and refuse (garbage) and is placed without compaction.
- Engineered fills: Made of premium materials (such as crushed bedrock, clean sand, or clean sand and gravel). Placed and compacted under controlled conditions. Typically used to support other structures (e.g., a house or a pool).
Organic Soils (topsoil, peat)
Recent decomposition of vegetation will form soils that may consist partly or almost entirely of organic matter (instead of mineral particles only). Common types of organic soils include:
- Topsoil: Generally a mineral soil (like sand or clay) with up to about 10 percent organic matter; it is darker soil found at the upper-most level of the soil profile and is mostly found in undeveloped areas.
- Peat: Generally found in swampy areas, it is formed by the accumulation and decomposition of organic matter; it is almost entirely organic (i.e., no mineral soil).
- Alluvium: Made up of the soils deposited along river and creek banks (generally silt and sand) ; it looks like a dark, silty mud.
- Marl: Although it does not typically have a high percentage of organic composition, it is commonly overlain by peat and is usually biologic in origin (containing numerous shells). Marl is white or grey and feels like a soft, wet clay that behaves like jello when shaken.
Sandy Soils (silty sand, gravel)
Sand deposits in the Ottawa area are found in two main areas. First, in a large triangular patch extending across the Ottawa airport site and the lands to the south, and second, along beaches of old channels of the Ottawa River, such as Mer Bleu, an area southeast of the City. There are also sand deposits in many other areas within the City. The composition of these sand deposits varies greatly (from clean, beach-like sand, to silt, silty sand, sandy silt, as well as sand and gravel), but the way that they behave, from an engineering perspective, is similar.
Sensitive Silty Clay (10,000 years)
About 10,000 years ago, eastern Ontario was flooded by the Atlantic Ocean and the Ottawa area was covered by what has been named the Champlain Sea. When this Sea dried up, it left thick deposits of silty clay which now form the geology of eastern Ontario. This deposit is known as Champlain Sea Clay or Leda Clay. Since the groundwater level is generally within about 2 to 5 metres of ground surface, the silty clay to that depth has been exposed to air, then oxidized, and finally dried to form a relatively dry and stiff brown or grey-brown crust. The silty clay below this has never been exposed to air and so has different properties. It is grey, wet and compressible, and it is relatively weak.
Glacial Till (12,000 years)
About 12,000 years ago, the glaciers melted and formed our current landscape. The soil and rock that the glaciers picked up along their journey were deposited as the glaciers melted as a sheet of glacial till. Because of this, glacial till consists of all particle sizes: boulders, cobbles, gravel, sand, silt, and clay. Glacial till is not necessarily present at every location in the Ottawa area, but when it is, it always overlies the bedrock surface.
There are two kinds of bedrock present in the Ottawa area, the granite bedrock of the Canadian shield and the sedimentary rocks present across the rest of southern Ontario (e.g., limestone, dolomite, sandstone, shale). Granite bedrock forms the Gatineau Hills, but also extends to areas within the City of Ottawa, including parts of the Kanata Lakes community and along the ridge line that extends to the north, past Carp. In addition, granite underlies all of the younger sedimentary bedrock types that exist in the rest of the Ottawa area. It's often up to 200 to 300 metres deep.
The differential movement that causes foundation distress is largely caused by changes in soil moisture. Variations in soil moisture can cause serious shrinkage or swelling of the soil surrounding the foundation — and water is almost always the main culprit. The extent of movement resulting from the shrinking or swelling of the soil will vary from one soil to another; in some areas the movement is insignificant, whereas in others it is quite noticeable. Soils with high clay content are generally more susceptible to this type of shrinking & swelling, while those with lower clay content are the least affected. When unstable soils are used as a base for a foundation, their tendency for movement is transmitted to the foundation. The problem shows up in both slab and pier and beam foundations.
Settlement cracks are nearly always vertical, and they should not be confused with cracks that occur when a wall is subjected to lateral movement from soil pressure. If the frame of a house hasn’t been distorted after three or more years of satisfactory performance, it is doubtful that the distortion is caused by full-depth foundation settlement, which is always evidenced by matching cracks. Matching cracks occur on each side of a portion of a foundation wall that is being moved downward because of soil bearing failure.
Whether your home has sunk just a few inches or nearly a foot, call Blueprint Ltd. for an assessment of your property. We will be able to evaluate the amount of damage and determine the best course of action for your particular situation.