Soil and Foundation Challenges

Engineers, and, more specifically, geotechnical engineers, classify soils according to their tactile and engineering properties and as they relate to their 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 the soil’s engineering properties and behaviours.

In the case of soil improvement and foundation construction, each soil type poses its own specific challenges−challenges GeoSolv ground engineers are well versed in and address with a variety of proven solutions.

At GeoSolv, we can help you with the worst soil conditions. No other ground improvement contractor has more experience improving the worst soil conditions−peat, organics, soft clay, and undocumented fill in Ontario.

Soil Classification Systems

The most common engineering classification system for natural soils in North America is the Unified Soil Classification System (USCS).

The USCS has three major classification groups

  • (1) Coarse-grained soils (sands and gravel)
  • (2) Fine-grained soils (silts and clays)
  • (3) Highly organic soils (referred to as “peat”)

Man-made, or fill soil, which can contain soil in one or more of the above groups, tends therefore,  to receive its own classification.

There are three main types of fill soils:

When considering a site for Innovative Foundation Solutions, there are six main types of soil

  • Gravel (G)
  • Sand (S)
  • Silt (M)
  • Clay (C)
  • Peat and Organics (O)
  • Fill (F)

Clay Soil

  • Feels lumpy and sticky when very wet
  • Often rock-hard when dry
  • Drains poorly
  • Few air spaces

Foundation Challenge

When soft to firm, clay can exhibit large settlements under 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, and 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 remolded 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 as much as 20.

Aside from the Ottawa area, soft-to-firm clays that do not generally exhibit swelling or sensitivity exist in certain areas of Ontario.  The main challenge with any soft-to-firm clay is the total settlements they 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.

GeoSolv’s suite of Ground Improvement, Rigid Inclusion, and Specialty Piling Systems were specifically designed for the clay soils of Ontario and across the nation.  Superior stiffness, massive lateral stress
build-up, and drainage characteristics of the Geopier Rammed Aggregate Pier systems make them a cost-effective, high-capacity solution for clays.  No ground improvement contractor has more experience with clay soil in Ontario than GeoSolv.

Sandy Soil

  • Free-draining soil
  • Gritty to the touch
  • Dries out rapidly
  • Less stable in excavations

Foundation Challenge

Sandy soil has naturally shifting characteristics; 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 in order to provide sufficient settlement control and bearing for new structures.

Clean sands (sands with fewer fine particles mixed in) under water 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 liquid.  Once the excitation stops and pore pressure dissipates, the sand becomes frictional again and the settlement stops.  By then, the damage has been done.

GeoSolv’s suite of Ground Improvement, Rigid Inclusion, and Specialty Piling Systems were designed specifically for sandy soils for both settlement control and liquefaction mitigation. Superior stiffness, vertical ramming and crowd pressure set our systems apart from stone columns and also other rigid inclusion approaches.

Silty Soil

  • Smooth and soapy to the touch
  • Can behave like clay OR sand
  • Retains moisture
  • Soil structure is often weak

Foundation Challenge

Silt can be a challenging soil to characterize as it can behave more like sand when the majority 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 Ontario 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.

Since they are very similar in appearance to the eye, silts are often mistaken for clay, often with unfortunate results.  Certain field tests can help to broadly differentiate the clays from silts, however often a hydrometer test must be conducted to properly define the particle size fraction of a silt-clay.   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 Ontario.  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).  Silts can therefore create significant challenges in construction and the dilatancy also makes them more difficult to improve.

Silt soils tend to hold their 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.

GeoSolv has many interesting ground improvement, rigid inclusion, and piling system projects in silts and silty clays.  We work with the geotechnical engineer to properly identify these soils to ensure that we treat the soil according to its engineering properties.

Peat and Organic Soil

  • High organic content
  • Usually dark in colour
  • Often very high water content

Foundation Challenge

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.  Peat and highly organic soils are therefore typically removed and replaced with engineered fills.

GeoSolv is one of the few ground improvement contractors that attacks peat head-on and provides unique ground improvement and rigid inclusion approaches that can provide for reliable engineered properties in peat and organic soils.

Fill Soil

  • Man-made
  • Does not exhibit consistent or reliable engineering properties
  • Heterogeneous by nature
  • Fill can often have environmental legacy issues (contamination)

Foundation Challenge

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 get 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 replacement or disposal.

There are a few types of fill soils:  Engineered Fill, Dumped (undocumented fill) or hydraulically deposited fill.

Engineered fill is of course 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-engineeered, replace it with approved engineered fill soils.

Hydraulic fill is placed typically 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 included organics and deleterious materials.  Old hydraulic fill deposits are sometimes treated as native soil when reasonable compressibility characteristics are expected.

Over ¾ of GeoSolv’s ground improvement projects to date have involved some sort of poor fill soils.  There is no ground improvement contractor in Ontario with more experience providing robust solutions for fill soils than we have.

The attributes and behaviours of soils can be complex and reliance on the Geotechnical Engineer for soil properties is important. GeoSolv’s Ground Experts have a clear understanding of the characteristics of varied soil types and can work collaboratively with the design and construction team to determine the right approach on your problem soil project to reduce risk and save time and money. We are always happy to provide an obligation-free feasibility study or proposal for your problem soil projects.