Soil Types and Concrete Slabs (Part 1)

If you are building a modern home, then that house will probably be built on concrete slab.  At the start of the project, the cost of the slab is one of the most variable of the costs for both you and the builder and it is not uncommon for slabs to cost considerably more than first estimated.

Increases in the cost of the slab can be disappointing to home builders, because it viewed as money spent on something that will never be seen.  However, it is extremely important that the slab be engineered correctly for the house you plan to build, in the location you plan to build it.  In this regard, it should be viewed as money well spent.

What is disappointing is the lack of information that is provided by most builders on slab options and their costs.  If slab costs go up, they generally do so once soil tests are performed.  However, for the buyer, this can introduce costs late in the process that, arguably, could have been foreseen by the builder as likely.

Which Type of Slab is Used When?

The construction of the slab itself (its dimensions and thickness) depend on the layout of the house and the materials used in the construction of the house.  Obviously, a two-storey brick veneer house is heavier than a single-storey timber clad house and the engineers will figure all of that out for you.  What matters (at least in terms of time and extra cost) is the type of slab that is needed for your house, which is primarily determined by the type of soil on which the house is going to be built and the gradient (slope) of the land.

What is Reactive Soil?

Soil experts often talk in terms of how "reactive" the soil is, and for house building, soil that is more reactive is more of a problem than soil that is less reactive.  "Reactive" refers to the degree to which the soil will move in terms of expansion when wet, contraction when dry and the likelihood of any sideways movement (i.e. is there a chance it will wash away in a flood).  More reactive soil means there is more movement, and soil movement is what causes most issues with concrete slabs.

Soil Tests

When soil tests are done, they measure a number of things to determine how reactive the soil is. 

• Clay content - clay is the enemy of concrete slabs and the more clay you have in your soil, the more engineering will be required.  The reason for this is that clay absorbs water and expands when it does so.  It also shrinks and hardens when it dries.  Without proper thought and engineering, this continual expansion and contraction of the soil under and around your house slab will cause it to move and (in extreme cases) even crack.
• Depth to bedrock - Knowing how deep you might need to dig to find solid rock is important
• Composition (sand, rocks etc.)
• The presence of chemicals that might be a problem to a slab

 

What Soil Types are there?

Based on the results of the soil tests, a building site is generally classified into one of the following classes, which is used (in part) to determine what kind of foundations and slab need to be laid:

• Class 'A' - Non-Reactive (you lucky beggars!) Your site is basically sand and rock, with negligible movement likely.
• Class 'S' - Slightly Reactive - There is some clay present on the site and slight movement can be expected over time.
• Class 'M' - Moderately Reactive - There is clay and/or silt present that may experience movement over time.
• Class 'H' - Highly Reactive - Moisture changes in the soil will cause a high amount of movement
• Class 'E' - Extremely Reactive - Are you sure you want to build here?
• Class 'P' - Problem

Most builders quote on the basis of an 'M' Class Slab, whether or not it is realistic that 'M' class will be sufficient in your location.  Remembering that most builders have built in most suburbs, they know what the basic geology of the city is and they know (at least broadly) how likely it is that an 'M' Class slab will be sufficient.

We are a 'Problem' Site

When we met with one builder recently, straight off the bat the sales-woman told us the truth about our site - we are a Class-P problem site.  Why?  "Bluntly", she said, all "knock-down sites end up requiring a Class-P slab.  Once you have pulled down the old house, dug out the old house's stumps and footings, pulled out any trees and their root systems and dug holes to cut off water and sewage connections, you have de-stabilised the soil to such an extent that engineering is required to support the slab."

So we are a Class-P site and will need a "Class-P" slab which means Piering - but more on that in a later post.