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Engineers and, more specifically, geotechnical engineers, classify soils according to tactile and engineering properties, and how those soil properties relate to the intended use for site development and foundation support. Current engineering classification systems are designed to allow for an easy transition of field observations and predictions of a soil’s engineering properties and behaviours. In the case of soil improvement and foundation construction, each soil type poses its own specific challenges.
There are a lot of “usual suspects” when it comes to building structural foundations, including peat, organics, soft clay and undocumented fill found across Canada. Building on all or a combination of these problematic soils poses serious project risks. It’s one thing to know that each of these soil types come with their own unique challenges. However, it’s even more important to know the “why” behind these challenges, that way they can be mitigated early on in a project. The breakdown in this article aims to clarify the challenges these soils can present on foundation projects.
The 3 major USCS Soil Classification Groups
The most common engineering classification system for natural soils in North America is the Unified Soil Classification System (USCS).
The USCS Soil Classification Groups are:
- Coarse-grained soils (sand and gravel)
- Fine-grained soils (silts and clays)
- Highly organic soils (referred to as ‘peat’)
Therefore, man-made or fill soil, which can contain soil in one or more of the above groups, tends to receive its own classification.
The 3 main types of fill soils
- Engineered fill
- Undocumented fill
- Hydraulic fill
The 6 main types of soil
The main types of soil to consider when selecting a site for building a structural foundation include:
Sand:
- Free-draining soil
- Gritty to the touch
- Dries out rapidly
- Less stable in excavations
Silt:
- Smooth and soapy to the touch
- Can behave like clay or sand
- Retains moisture
- Soil structure is often weak
Clay:
- Feels lumpy and sticky when very wet
- Often rock-hard when dry
- Drains poorly
- Few air spaces
Peat and Organics:
- High organic content
- Usually dark in colour
- Often very high water content
Fill:
- Man-made
- Does not exhibit consistent or reliable engineering properties
- Heterogeneous by nature
- Fill can often have environmental legacy issues (contamination)
Gravel:
- Highly variable in coarseness, thickness and configuration
- Looser state or when mixed with loose sand
- Often found with traces of sand and clay/silt
- Inferior ability to retain moisture
The 6 main soil type foundation challenges
Clay soil
When soft to firm, clay can exhibit large settlements under a load. These settlements are likely to occur slowly due to clay’s poor drainage characteristics. Pore pressures within the clay particle spaces move slowly, leading to long-term consolidation settlement. Regionally, different clays exhibit different and unique properties.
Swelling clay exists in many areas (particularly throughout the Prairie provinces). This clay can shrink or swell upon changes in moisture content. This can lead to subsidence or settlement. Soil heave is also an issue when it comes to clay – swelling clay in particular – having the potential to push the foundation and the structure above upwards.
Another unique clay exists in the Ottawa and St. Lawrence valleys. Leda clay, also known as Champlain Sea Clay or Quick Clay, is highly sensitive clay that exhibits rapid loss of shear strength under excitation, and can also change properties under sufficient stress. Leda clay has a much lower remoulded strength, which is a measure of the residual shear strength measured in a direct vane test following initial shearing. Clay sensitivity is defined as the ratio of the initial shear strength to the remoulded residual shear strength. Typical clays can have a sensitivity in the low range (s = 1 to 3). Leda clays have sensitivities of up to 20.
The main challenge with any soft-to-firm clay is the total settlement it can exhibit under load and time rate of settlement. When loading clays with large area loads, such as embankment fills or area grade raise fill, thicker deposits of soft clay can create settlement and scheduling problems.
Sandy soil
Sandy soil has naturally shifting characteristics, and drainage through sandy soil further increases its shifting properties. This poses a clear challenge for any structures being built on this type of soil.
Loose sand can be subject to large settlements, and these settlements tend to occur quickly due to the free-draining granular nature of sands. This can pose foundation challenges, and loose sand requires densification to provide sufficient settlement control and bearing for new structures.
Clean sands (sands with fewer fine particles mixed in) underwater can be prone to liquefaction when subjected to excitation such as vibrations or seismic forces. Seismic forces causing liquefaction increase pore pressure to a point where the sand becomes suspended, and the natural angularity of the sand no longer affords high shear strength. The temporarily suspended sand-in-water mixture behaves as a liquid and any objects sink to a point where the weight of the object is equal to the weight of the sand-water mixture. Once the excitation stops and pore pressure dissipates, the sand again becomes frictional and the settlement stops. By then, the damage has been done.
Silty soil
Silt can be a challenging soil to characterize as it can behave more like sand when most of the silt particles are in the larger range of the silt particle-size band. More often, silt behaves more clay-like and often silts and clays are mixed. A large percentage of clays encountered in Canada are in fact, silty clays – where silt comprises a large fraction of the total weight of the soil, but clay is the predominant particle. Soils with large volume fractions of silt and clay are often the most troublesome to the geotechnical engineer.
Due to similarities in appearance, silts are often mistaken for clay, often with unfortunate results. Certain field tests can help broadly differentiate the clays from silts. However, often a hydrometer test must be conducted to define the particle size fraction of a silt-clay properly. There are several important differences in the behaviour of silts compared to clays.
Pure silt or soils with a very large fraction of silt-sized particles do exist in regions of Canada. These silts tend to exhibit dilantency (change of volume with change of shape) where more clayey materials tend to exhibit plasticity (retention in volume with change of shape). Therefore, silts can create significant challenges in construction and the dilatancy also makes them more difficult to improve.
Silt soils tend to hold moisture and are difficult to drain. It is difficult to dewater in silt soils and high moisture silts tend to flow or run with sometimes catastrophic results.
Peat and organic soil
Peat and organics are typically problematic for construction. These soils easily become waterlogged and are often extremely acidic in nature. Peat/organic soils exhibit extremely high compressibility characteristics. The shear strength of peat can be difficult to predict and is often low, although fibrous peat tends to exhibit higher shear strength due to wood fibres in the peat as compared to amorphous peat. Engineering properties are challenging to determine in peat and due to the possibility of future volume loss due to decomposition, it is difficult to rely on engineering properties in peat soils. Peat also tends to off-gas during decomposition and methane becomes a concern for buildings where peat is left in place. Therefore, peat and highly organic soils are typically removed and replaced with engineered fills.
Fill soil
In urban centres, infilling of built areas is occurring at an increased rate. Often, remaining sites in urban centres are more difficult to develop and have been passed over in favour of sites with lower development costs. As these areas are built out, and as municipalities struggle to find room for burgeoning populations, the leftover sites are targeted for development. Such sites often require extensive remedial effort to develop, including massive excavation and soil replacement or disposal.
There are a few types of fill soils: engineered fill, undocumented fill or hydraulically deposited fill.
Engineered fill is used widely for replacing other non-engineered fills for the purpose of supporting structures. It consists of granular materials or select subgrade soils compacted in thin lifts to a minimum level of compaction. When properly constructed and monitored, engineered fill can be relied on for foundation support with perhaps somewhat limited bearing capacity.
Dumped or undocumented fill is by nature highly variable and the typical reaction is to remove and either re-engineer it back in place, or, if it cannot be re-engineered, replace it with approved engineered fill soils.
Hydraulic fill is typically placed using water in some way, either through stream or river action, or by land reclamation in water bodies, such as what occurred along Toronto’s waterfront over many years. Hydraulic fill is often characterized by either select particle size or by heterogeneous or multilayered soils that could include organics and deleterious materials. Old hydraulic fill deposits are sometimes treated as native soil when reasonable compressibility characteristics are expected.
Silt can be a challenging soil to characterize as it can behave more like sand when most of the silt particles are in the larger range of the silt particle-size band.
Gravel soil
In areas where gravelly soil is the predominant particle, stabilizing plant life often cannot grow. Depending on the type of gravel in which the work is occurring, there can be significant challenges from a structural foundation perspective. Depending on grading and state of natural density, gravel can be unstable, and if evenly graded or rounded, the gravel can be easily rearranged by external forces. This can restrict the ability to build on a truly sound foundation in non-engineered gravel soils.
It should be noted that some of the better-engineered fill materials available are gravel-based, but these are often crushed, sorted and graded materials for specific purposes. Engineered angular gravel is the main ingredient of a Rammed Aggregate Pier element. Engineered gravel is used in massive quantities for construction across Canada, either in a quarried crushed form (typically angular to angular particles) or from sand and gravel pits (more rounded and rounded particles, unless further crushed down). Engineered gravel is used as road base, sewer and pipe bedding, infiltration galleries or as foundation drainage layers. 
Mark Tigchelaar is president and founder of GeoSolv Design/Build Inc., an Ontario-based geotechnical contracting firm that specializes in providing a range of innovative foundation solutions for challenging soil sites and provides clients with the options and advice necessary for them to make informed decisions about their projects. For inquiries about this article or ground improvement, reach Tigchelaar at mark@geosolv.ca.