Which Type Of Foundation Is Best For Sandy Soil?

Even though it is a cliché, it is still true that you should build your house on a solid foundation, particularly the cornerstone, for the easiest and most durable building. Sometimes circumstances are forced on the potential owner builder, and the foundational material may be less than optimal.

The best foundation for building in the sand will depend on the consistency of the sand, the depth of the frost layer, and the forces the building will impose on the foundations. While conventional shallow foundations can be used for gravel, Helical Piers are the preferred method for finer sand.

Modern engineering technology makes building a structure on almost any foundation soil type possible. The only downside is that the build costs will vary substantially. It means that building on sand is no longer impossible; however, dependent on the consistency of the sand; it may present various challenges.

Building On Sand

Sandy soil originated from an area covered in water and is prone to expanding when it comes into contact with water. It makes it an imperfect material on which to build a structure.

Not all sands are the same, so it is essential first to identify what sand you are dealing with before designing the foundation of the new structure.

Sand ranges in a material with bigger grains and behaves more like gravel through to sand with very fine particles like silt. The categories of sand are.

  • Fine sand.
  • Medium sand.
  • Coarse sand
  • A mix of sand, clay soil, and loam soil.
  • A mix of silt.

A house cannot be built directly on the sand. The reasons for this are.

  • Fine sand will not compact, and because of this, it will always have some movement that is unsuitable for heavy construction on top.
  • If the water table is high, and there is a good chance that the sand will be flooded, it will expand. If this happens, it takes on some of the characteristics of clay, and the potential for cracking of the walls is very high.

To overcome this, when designing a house to be built over sand, engineers will typically use concrete piers (piles), which are drilled down to cohesive soil under the sand. Ideally, they will pile into the underlying bedrock.

Under these circumstances, you will have to find the more cohesive soil under the sand that’s harder and less shifty. In this example, the type of foundation is also a very important decision.

Another consideration when building on sand is that vibrations must be kept to a minimum.

What Are Foundation Types Suitable For Sand?

Four factors must be considered when building on top of the sand.

  • The type of sand.
  • The height of the water table.
  • The frost layer.
  • The nature of the structure.
  • The loads exerted by the structure.

The Type Of Sand

If the consistency of sand is close to that of gravel, it may be possible to build foundations in the sand. However, this must be checked with the engineer overseeing the project.

If there is any doubt about the type of sand at the site, a conservative approach must be employed, and stronger foundational types should be designed.

Building On Course Sand

As long as the engineer determines that the ground has an adequate bearing capacity, course sand subsoils may only require strip foundations that are 0.7m deep.

When course sand is compacted and moist, it holds together fairly well. Compacted correctly, it is a good support for a foundation because of its inability to retain water (as long as the water table is below the foundation level).

When sand is wet, the particles lose their friction and will often be washed away, leaving gaps beneath the foundation.

Sand generally holds together well when it is damp, compacted, and uniform, but the sidewalls of the trenches could collapse. Sheet piling is sometimes used to retain the ground in trenches until the concrete is poured.

Building On Medium Sand

Depending on the frost level, shallow foundations can be used for a building where the foundation is made of medium sand.

After a definitive soil sample has been analyzed, you must consult with the engineer.

Building On Fine Sand

If the site is made up of fine sand, alternative forms of foundations will need to be explored.

While traditional piling can be considered and may be very viable Helical piers can be considered.

Helical Piers

The most exciting possibility is to use helical piers.

Helical piers are anchoring systems used for building deep foundations. They are steel foundation pins that hydraulic machinery drives into the soil to a depth below the frost line.

The helical pier has one or several metal discs (called bearing plates) that are installed and attached by weld in a spiral pattern around the manufactures steel center core.

A brickmaker names Alexander Mitchell invented  Helical piers in 1830.

Mitchell first used helical piers to build lighthouses, bridges, and other structures established on unstable soils.

Over the years, helical piers have been developed and are now recommended by engineers to repair and stabilize all types of commercial and residential foundations.

How Do Helical Piers Work?

The helical pier handles load transfer in the following sequence of events.

  • The foundation load is passed from a connection point to the shaft.
  • It is then passed to the metal discs.
  • The discs transfer the load to the surrounding soil.

In addition to how the individual piers handle the load transfer within its structure, they also share the load between the other helical piers in the foundation.

How Are Helical Piers Installed?

Helical piers are installed using a hydraulic device that drills them into the ground layer. As the shaft is drilled deep below the surface,  the installers can add additional extension segments.

These are coupled into the bolted couplers and are drilled into the ground layer. The result is a single shaft capable of being placed hundreds of feet deep into the ground.

Helical piers can be designed to support can be positioned to 100sof feet deep, which means they can be installed below the first layer and can support weights of up to five hundred tons.

The depth to which the shaft is placed, as well as the diameter and spacing of the bearing plates, is calculated by the engineer, who will use the local soil type (as ascertained for the soil report) and by the specification of the foundation strength which is needed for the structure.

In rocky soils, the discs will be of smaller diameter; however, wider diameter discs will be specified in the sandy soils discussed in this article.

The Benefits Of Helical Piers

The benefits of using helical piers, particularly in the sandy area.

  • Helical piers do not require excavation or the need to preserve the trenches while the concrete is poured.
  • Installing Helical piers produces minimum vibration, which is critical when working with sand.
  • Because they are essentially screwed into the ground, they can be installed quickly with minimal preparation.
  • Helical pier installation produces very little disturbance.
  • The helical piers can be used immediately without waiting for concrete to cure.
  • Because they produce so little disturbance, they are considered environmentally friendly.
  • They are very suitable for use in remote locations.
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Types Of Soils (In Addition To Sand)

The type of soil under the surface of the proposed structure will determine the type of foundation employed in the building.

Rock

Rocks have a high load-bearing capacity and include the following.

  • Limestone.
  • Granite.
  • Sandstone
  • Shale.
  • Hard solid chalk.

If the above is of suitable quality and consistency, i6 may be possible to shave the rock surface and build the structure directly on top without constructing dedicated foundations.

Chalk

If the chalk is hard enough, it will allow for a shallow foundation with widths of 245 mm (for a single-story building)  to be built.

With chalk, the depth of the foundation must be below the frost layer. The foundation must be dug 700 mm deeper than the frost layer.

If the chalk is not hard, it must be completely removed from the foundations.

Gravel

Dry gravel is adequate for a shallow foundation, as long as it is well above the water table. If the gravel layer falls under the water table, its strength (bearing capacity) is cut in half.

A foundation level of 700mm is adequate for a single-story structure built on gravel.

Clay

Four categories of clay are defined by the plasticity of clay in the soil.

  1. Foundations built on clay with high plasticity will need trenches of at least 1m depth.
  2. Foundations built on clay on medium plasticity will need slightly less deep trenches of  0.9 meters deep.
  3. Foundations built on low plasticity clay only need a depth of 0.75m (750mm).

The best foundation used on clay soils is a “raft” foundation because it effectively floats on the surface, so there is no relative movement and less chance of cracking.

If there are trees in the vicinity, foundation depths of up to 3m may be required. The tree species will dictate the depth as types have varying water demands.

Broad leafed trees generally have the greatest impact.

Filled ground

If the foundation’s site has been filled in, it must be dug down to a level below the fill.

Types Of Foundation

The types of foundations which are generally used in building construction are.

Shallow Foundation

Foundations that are less than three feet deep are considered shallow foundations.

Examples of shallow foundations are.

  • Isolated spread (individual)  footing.
  • Combined footing (wall or stripped).
  • Stem Wall Foundation.
  • Cantilever or Strap Footing.
  • Raft or Mat Foundation.

Deep Foundation

Foundations more than three feet deep are referred to as deep foundations.

Examples of deep foundations are

  • Pile foundation (Sheet Piles, Load Bearing Piles, End Bearing Piles, Friction Piles, Soil Compactor Piles)
  • Drilled shafts and caissons (including open caissons, pneumatic caissons, monolithic caissons, sump caissons, box caissons).
  • Helical Piers.

Statistics Of A Typical House

The following are a few typical statistics for an average house to demonstrate the importance of foundations.

  • The average weight of a typical house is 50 tons.
  • On an average house, foundations weigh 7 ½ tons.
  • The average cost of the foundation against the total project cost is 8-15%.

Why Do Foundations Fail?

The main reason for foundation failures are.

  • Inappropriate material has been used in the backfill, including porous materials, such as clay or organic matter holding water which may cause the foundation to crack as the soil freezes and expands.
  • The foundation concrete is cured too fast and never reaches optimal strength. It must be kept damp while it cures.
  • If the foundational floor is not compacted sufficiently and the concrete is poured over, the concrete will settle and crack.
  • The pour was interrupted at some point, and the second batch was added after the first batch had dried. It should never happen because it forms a “cold joint” between the first and second batch, which will be prone to cracking and leaking.

Conclusion

There was a time when building on the sand was not possible because the technologies of the day did not allow for it. With technologies like the various iterations of piling, rafts, and helical piers, it is quite possible and practical to design a structure built on sand.

Whatever the ground looks like, it is essential that a soil report is commissioned and that the site engineer is fully involved in the decision-making process regarding the most suitable foundation method for use when you build on sand.

References


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