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Soil Tests for the Melbourne Residential Market: What Do They Mean, and Why Do You Need to Know?

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What exactly is a soil test?  What does it mean to a person who is intending to buy land and build a new home?  Do you need to know, or can you leave it to the experts?  This article will give an overview of what a soil report is actually reporting, and why it is useful to educate yourself in this regard. 

In order to know what soil you are building your home on, as part of an application for a Building Permit for a new home you will need to obtain a Soil Test (also known as a Geotechnical report.)  This is required to verify that the foundation design on your working drawings is suitable for the conditions relating to your site.

In a nutshell, what your soil report is telling you is how much clay is in the soil.  Think back to playing with mudpies, you dug down and found some clay, and created an ash tray or other masterpiece.  You set it out to dry in the sun, and pretty soon, it cracked.  When clay is wet, it is sticky, and swells.  But when you let it dry in the sun, it shrinks and cracks. 

When there is high clay content in soil, as the soil loses moisture its capacity as a foundation comes into question.  Soil conditions in Melbourne change greatly from summer to winter, so there is a need to modify foundations to compensate for different clay content. 

Melbourne is broken up into specific soil classifications, based on their reaction to moisture.  A and S Class soils have low reactivity.  M Class is Moderately Reactive which means your soil has medium reaction to moisture.  This is sandy loam to clay mixture.  H is Highly Reactive. E is extremely reactive, and P Class = Problem!  P Class cannot be classified without a lot more work.  This reactivity to moisture is essentially telling you how much clay is in the soil. 

 

Site classifications based on soil reactivity

Class A 

Stable, non-reactive. Most sand and rock sites. Little or no ground movement likely as a result of moisture changes.

Class S

Slightly reactive clay sites. May experience slight ground movement as a result of moisture changes.

Class M

Moderately reactive clay or silt sites. May experience moderate ground movement as a result of moisture changes.

Class H

Highly reactive clay sites. May experience a high amount of ground movement as a result of moisture changes.

Class E

Extremely reactive sites. May experience extreme amounts of ground movement as a result of moisture changes.

Class P

Problem sites. The ability of the soil to evenly bear a load is very poor. Sites may be classified as ‘Class P’ as a result of mine subsidence, landslip, collapse activity or coastal erosion. Ground movement as a result of moisture changes may be very severe.  If you are building on a Class P site you will need to consult a structural engineer.

Source: http://www.build.com.au/siteworks/earthworks-preparation/soil-testing-and-geotechnical-reports-0

 

What does all this mean to you as a person who wants to build a new home? 

Well, it comes down to how many experts need to be involved, and how much the foundations will need to compensate for soil reactivity.  Where a soil test is classified A, S or M, the foundation is generic or standard, and can usually be designed by a draughtsman.  Once you get to H E and P it must be done by a structural engineer.  (There are other factors that can make a report come back as P class, for instance if there are a lot of trees on your block this can impact soil classification because they effect the moisture of the soil.)

When people buy a block of land, they generally don’t have a soil report.  If they are in areas where the soil conditions are fairly uniform, such as Frankston, where it is predominantly sand, with a bit of research they should have a good idea of what soil conditions are going to be like.  When you are on the border of soil types, say in Doreen or Cranbourne/Pakenham, until a soil test is done you may not know what you are looking at.

When a test is done, the engineers bore until they hit rock or a solid clay base.  Even when there is sand, they are looking for an underlying solid clay base.  This will dictate how deep the foundations need to be. Fill is far worse than any amount of clay, because it generally has to be removed for the foundations.  There will be a need for trenches filled with concrete.  Imagine that there’s a metre of fill; now the foundations need to be a metre plus 600 mil below that for steel reinforcing.  These metres of trench will soon add up to cubic metres of concrete, and this is expensive. 

Your builder will charge differently depending on your soil report, because he has to.  When a builder charges more in these conditions, they are not trying to have a go at you.  While undoubtedly there are less than scrupulous builders, and you need to make sure you are with a good one, be aware that foundation costs can increase enormously depending on soil condition.  Soil reports are not smoke and mirrors in a builder’s hands: they are done by independent assessors, either a soil engineer or a geologist

How does Rock tie in? 

Whenever someone asks me, “does the contract include rock?” the simple answer is “No” because without X-ray vision, you don’t know what you will be presented with.  In Melbourne, areas such as Epping and Craigieburn and further West and South, all around to Caroline Springs, you are more likely to encounter Rock.  Just drive along the Western highway and see the fences our ancestors built out of the Volcanic Rock around them. 

Where striking rock is highly likely, the builder can use modern construction methods such as Waffle Pad, which is Reinforcement Concrete slab construction above ground out of Polystyrene blocks.  Instead of digging in, the building site is leveled, spread with crushed rock, and foundations are built above ground.  There is some negative press about waffle pad construction, but this is based on the fact that when it was introduced a few instances were poorly implemented.  The issues were resolved, but there remains some misinformation.  Waffle Pad is a very valid method of construction in certain circumstances.   

When in doubt, speak to your builder.  I don’t usually suggest that people get their own soil report.  It is in your interest to go through this process in consultation with a competent custom builder, who is experienced in the terrain where you wish to purchase.  The builder will have a good idea as to what the soil is like in that area, and will also be able to advise you through the process to the best outcome.  Once you find a builder whose integrity and quality you trust, you will be able to work through the possibilities and process based on your particular situation. 

Do you value integrity, communication and superb quality throughout the building process? Do you have a question about your block or dream home?  John Kearney is a Melbourne Building Design and Construction Consultant who has been in the building industry for over 20 years.  He is passionate about giving people a cost effective alternative to building in the areas they love, and specialises in solutions for difficult and sloping blocks. Find out more at http://slopingblocksolutions.com.au/  Tweet with John at SBSBuilder and Like his Facebook page to get tips and updates!

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