Do I need soil testing before building a home?

by | Sep 6, 2023 | Uncategorized

When you start looking at buying land or building a house, it won’t be long before you start hearing that you need to do a soil test. But what is soil testing and why is it an essential part of the preparation process?

We’ve got all the dirt on soil testing, including why they’re important, and how much you can expect to pay. We’ll also explore how understanding your block’s soil profile is so important and how soil conditions are classified. For even more information on soil testing click here!

What are the different types of soil classifications?

One of the key pieces of information being looked for in soil testing is soil classification. Knowing your soil classification is essential to designing the correct foundation,

A soil classification is designed to lay out the following elements about your block of land in an accessible, easy-to-understand way. The key information about your soil a soil test will reveal is:

  • the reactivity.
  • stability/bearing capacity
  • moisture levels,
  • capacity and substrate materials

Soil Classification

The following are the possible classifications you can receive from your soil testing:

  • Class A: Stable, non-reactive soil. This will usually cover most sand and rock sites. This is an ideal result from your soil test, as it suggests there will be little issue building on your block. Class A means there is little or no ground movement is likely from moisture changes.
  • Class S: Slightly reactive clay sites. Often clay sites will receive this rating. This type of site could be vulnerable to ground movement as a result of moisture content and change.
  • Class M: Moderately reactive clay or silt sites. Class M sites can experience moderate ground movement as a result of moisture changes.
  • Class H: Highly reactive clay sites. Those working on this classification could experience high-ground movement from moisture change.
  • Class E: Extremely reactive sites. Sites that receive this classification are susceptible to experiencing extreme amounts of ground movement resulting from the moisture. Building on this type of block will be difficult, but it can be done with clever design. However, it will cost you more money in the long run.
  • Class P: Problem sites. This is the worst result you can get from a soil test and emphasizes why you should get an independent soil test on any land you’re considering buying. Class P site will struggle even to bear a load. Factors that can lead to a class P rating include uncontrolled fill, rock, mine subsidence, landslip, collapse activity, coastal erosion or some other problem associated with the soil. On these sites, moisture change could cause severe ground movement and may make it nearly impossible to build on.

What do these classifications mean for my project?

Class A and S are the easiest classes of site to build on, and will likely require little, if any, need for structural engineering modifications. Other classes may need the help of a structural engineer and may require the following:

  • deeper reinforcing.
  • more intensive drainage systems
  • Adjustments to the design of your home

After reading the above, you can see how important this information is to your builder. If you were to start building on land without knowing its stability, could severely affect the foundations of your building and cause major issues for your house. Prevention is the best strategy, as any foundation issue that occurs after the building commences will cost a bundle to fix.

Why is it important to do soil testing before building a new home?

One of the most important things you and your builders need to know before any construction is a thorough understanding of the land you’re building on. It’s literally the foundation of your future house and no two blocks are quite the same. Even if your neighbour’s block had no issues, that isn’t a guarantee there won’t be something different about your block.

A soil test serves a similar role for your land as a blood test would for a person. It provides essential information about the health and makeup of your soil. Along with soil classification (see above), this includes:

Bearing capacity

When building a new house on a block of land, bearing capacity is an essential component to understand. Basically, bearing capacity is the measurement of how much weight your block of land and safely sustain.

There are established guidelines and rules about foundation design and footings. The more unstable the land, the deeper the footings will need to be and the more design adjustments that will need to be made.

Fill/ scrape recommendations

A soil test will provide your builder with a better idea of the best fill/scrap recommendations for your block of land. Depending on the lot, your builder may need to fill in your lot or level it out to create a flat surface to build your home.

How much does a soil test cost?

When it comes to surveys and tests, a soil test is one of the more affordable necessities. A soil test will usually cost between $250-$400. Factors that could influence the price of your soil test include:

  • Location: If your tester has to travel, it’ll likely cost more.
  • The company you choose.
  • Size of your block.
  • Number of soil tests you require.

If you’re performing a knockdown rebuild, keep in mind that you’ll need at least two soil tests. One before the demolition and one after. This is due to the knockdown potentially affecting the land structure.

How is a soil test conducted?

Usually, a soil test will be performed by a geotechnical engineer or structural engineer who will use specialised equipment to drill to various depths A soil test will have two main components which are:

Field Component

A field soil analysis involves soil collection, data measurements and site investigation. This will be collected with specialised equipment. Different soil samples will be collected from various depths and then taken to the lab.

Lab component

The soil sample will be taken to the laboratory for tests. These tests will try to find out information like soil reactivity, which will be presented in your site classification report.

How does a soil test impact the build of my new home?

The classification of your soil will help your building team understand the stability of your land and the best way to approach your foundation design.

During the land-buying stage, having a soil test performed before you buy could save you from accidentally getting a lemon. If the seller tries to talk you out of having an independent soil test, consider it fishy.

Are there any other tests that should be carried out before a new home is built?

Along with a soil test, we recommend performing a site survey. This site investigation could involve one of all of the following:

  • Wind rating assessment: This provides your designer with an idea of the expected wind classification in the area.
  • Contour Survey: This will help determine the slope of your block
  • Topography Survey: This will provide an understanding of where everything is on your block
  • Boundary Survey: Establish the boundaries of your property.

You should also get a detailed property report from your builder which will provide a range of important information about your property.

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