Site Investigation

As part of the design and approval process of building your new house you will require a ‘Site Investigation Report’ sometimes referred to as a ‘Geotechnical Report’.

For a typical suburban subdivision for a one or two storey house on a standard block around 800m2. these cost in the order of $1,500 – $2,000,

What They Do

The site investigation company will normally send out a truck mounted drilling rig to site.

The rig operator will have a look at the site for any potential problems and drill three holes to a depth of about 2 – 3m, or until they hit rock.

At least one of these holes will include any problem areas identified from the visual inspection such as wet areas and disturbed ground.

For larger blocks, larger houses, and houses with more complex structures (e.g.underground garages) more boreholes, deeper bore holes, and more testing will be required which will all add to the cost.

Samples of the different materials encountered will be classified to be able to assist in assessing the strength of the material

The Results

A report will then be prepared based on the tests, and any previous information about soils in the locality.

The Report will give recommendations about foundations for structures on the site.

The key part of  the recommendation is usually a foundation classification  as this can adds tens of thousands of dollars to the cost of the build

Limitations on Report

A couple of things to remember about these reports are:

  • The recommendations assumes that the boreholes accurately reflect the condition for the whole site.
  • The ground can be exceptionally variable and the borehole are testing much less than a hundreth of one percent of the soil that the house will be standing on. (During excavation the builder may encounter worse conditions that need a stronger foundation with additional costs)
  • If the site has a slope which requires cut and fill the foundation may need to be to a higher classification than the report recommendations . . .  at an additional cost.


For more posts related to land see Blocks


Soil Classification

Before you can build your new house you need to know what sort of foundation is needed, which is based on the ‘Soil classification’

Geotechnical Investigation

A Geotechnical investigations is required to provide a report stating the soil classification.

The Investigation,and the Report should be in accordance with the following Australian Standards;

  • AS2870-1996: “Residential Slabs and Footings – Construction”
  • AS1726-1993: “Geotechnical site investigations”

Standard Classifications

The classification of the site is based on the expected movement of the foundation soils – generally related to the capacity of the soil to shrink or swell.

Your site should be in one of the following classifications:

Class A
Mostly sand and rock sites, with little or no ground movement expected.(see these links: Sand & Rock)

Class S
Slightly reactive clay sites. Only slight ground movement from moisture changes expected.

Class M
Moderately reactive Clay or Silt sites which can experience moderate ground movement from moisture changes (See this link: Building on Clay).

Class H
Highly reactive clay sites. Can experience high ground movement from moisture changes.

Class E
Extremely reactive sites. Can experience extreme ground movement from moisture changes.

Class P
A problem site. This can includes soft soils, such as soft clay or silt, varying depths of Fill (see this link: Fill), loose sands, landslips, mine subsistance, collapsing soils, soils subject to erosion, reactive sites subject to abnormal moisture conditions, or sites which cannot be classified otherwise.

Added category ‘D’

Soil types M, H, and E may also have an added classification of ‘D’. This indicates deep seasonal moisture variation which can mean significant expansion and contraction.
For example, from a dry to a saturated state

  • Class M-D may move up to 40mm,
  • Class H-D 40mm to 70mm
  • Class E-D can move more than 70mm, (up to 250mm has been found in some cases)

Why the report may increase the cost

If you are looking at a Project Builders its worth being aware that their standard price will be based on either an S or M class foundation.

Usually you will be lucky if the foundation is in this range………so you could be up for additional costs.

See the following link to understand why a classification from the developer may be better than the one from the builder: Different Soil Classification Results


 Lots more information in the anewhouse Guide to Buying a Block for only $4

For more posts related to land see Blocks


Saline Ground Conditions

“Valley of Salt” – Salinity in the Western Australian wheatbelt near Bruce Rock, WA. photo by CSIRO

Salinity doesn’t have to be as bad as in this photograph to cause problems in new homes.

It’s also quite common for land that has not got salinity issues to develop salinity over several years.

Building Problems Due To Salinity

Problems can range from cosmetic through to significant structural issues.

Efflorecence This is the white powdery deposit sometimes seen on brickwork or concrete. Sometimes it is caused by salts in the materials themselves, (See this link Efflorecence)

Breaking Up Materials If salt is carried into the wall be water and then dries it will form crystals inside the bricks, mortar, or concrete. These crystals can form internal pressure on the materials causing the external surface to crumble away.

Acid attack Acids can be formed that will increase the porosity of concrete and reduce its strength.

Increased Steel Corrosion A wet saline solution is likely to result in corrosion of concrete reinforcement and Brick Ties.


  • Provide a Damp Proof Membrane under the slab.
  • Make sure the Damp Proof Course detail is effective; and is an appropriate height above ground level.
  • Check the land slopes away from the house.
  • High quality concrete including:
    • Low Water/Cement Ratio, with no added water on site.
    • Proper Curing procedures.
    • Sulphate resistant cement and/or higher strength concrete.
    • Concrete well vibrated to remove entrapped air and ensure high density in and around the reinforcing and formwork.
    • Increased concrete cover to steel reinforcement.

Potential Acidic Sulphate Soils

Look to buy a block on a coastal plain, alongside a river, or a reclaimed wetland and you may see the area described as Potential Acidic Sulphate Soil (PASS).

Well PASS doesn’t mean OK!

Risk of PASS

Why, And Where It Occurs

These types of soil generally occur in areas which were flooded by seawater within the last 10,000 years.

During that time sediments containing iron and organic matter were deposited. Bacteria then living in the sediment was able to produce hydrogen sulfide which then reacted with the iron to form iron sulfide (pyrite).

This Australian Gov’t map shows coastal areas with a risk of PASS.

Acicidic sulphate soils have however been found in inland areas such as along the River Murray.


Draining the Soil

Pyrite is chemically stable unless it is exposed to air; such as when it is drained, or excavation takes place. (or even a prolonged drought)

Exposure to air means the pyrite oxidises and produces sulphuric acid and a range of other chemicals including the toxic hydrogen sulfide gas.

Release of the suphuric acid is likely to cause significant environmental damage over long periods as the strong acid can be released from soil for more than 50 years from the original exposure.

Because of the environmental issues all Australian States have management controls for areas of potential acidic sulphate soils

Damage to Foundations

Acid released from the soil can attack both the concrete, and the steel reinforcement within the concrete weakening the house foundation and the cement within the brick walls

Low Load Bearing Capacity

Many potential acid sulfate soils are weak clays that have not fully consolidated, and are likely to further subside or settle.

Precautions When Building

  • Deep, expensive, piles will normally be required to support your foundations.
  • Sulphate Resisting Cement should be used in all concrete.
  • Extra care will need to be taken to ensure all concrete is protected from groundwater by a heavy duty plastic layer.


Pre-Wetting A New House Block

Reason For Pre-Wetting

If you are Building On Clay that is dry there is a risk of Soil Heave.

This is due to expansion of the clay if the moisture level increases with particular risk factors being:

  • Building after a long period of very dry weather when large cracks in the surface are visible.
  • Knock down and rebuild projects where there is a risk that part of the new foundations are on drier land than other parts.
  • Where there are trees, or trees have recently been removed.

To minimise the risk there is often an Engineers requirement to pre-wet the site to stabilise the moisture content.

Pre-Wetting Requirement

A typical requirement is to run sprinklers for 2 hours a day for 8-10 days which should increase the soil moisture to a depth of approx 1m.

Then the fill/construction pad below the slab should be laid within 2-3 days,

You are aiming for the equivalent of 20-25 mm of rainfall a day (20-25L/m2) The intention is on each day to thoroughly wet the top layer of soil and fill all the cracks with water without actually waterlogging the site.

For maximum effect the best time to do the pre-wetting is the late evening to minimise evaporation.

I have hear of builders saying the pre-wetting isn’t needed. Don’t accept this without soil moisture tests that have been accepted by the engineer who made the original requirement.

Effectiveness of Pre-Wetting

Although pre-wetting will minimise the risk of Soil Heave

  • It doesn’t mean that good foundation design can be neglected.
  • Letting the site moisture content stabilise through a winter, after removal of buildings/trees, is likely to be more effective.


See Ground Conditions for more Posts


Stormwater Discharge Point

One check that is often forgotten when buying a house block in an established suburb is how storm water is removed from the site.

Forty, or more, years ago when land was cheaper, it was not unusual for blocks to be quite large compared with the size of the house.

Disposing of storm water by allowing it to soak into the garden in the garden was acceptable. (This is still effective in sandy areas such as much of Western Australia)

As a result many properties were built without connection to a surface water sewer.

Modern Knock Down and Rebuild Problems

If you buy block  in an older suburb with the intention of subdividing, or demolishing and building a bigger house, you could have problems if you haven’t got access to a storm water sewer.

Disposal to a much smaller garden area probably won’t work.

In order to protect adjoining properties from overflows from your property the council are likely to make discharge to an approved point conditional on any development approval.

Dealing with the attendant problems if you plan to extend or subdivide a block without a stormwater discharge point can add thousands of dollars to your costs.


Possible options are:

    1. Soakwells. If you are lucky enough to have a sandy block soakwells may be a solution. Low cost.(see this link: Soakwells )
    2. Pipe to existing surface water sewer. This may involve negotiating with adjacent property owners. Very difficult to achieve unless the affected neighbours also want to subdivide. Very expensive.
    3. Discharge to Street
      Check with your council to see if this is permitted. A typical street connection will look something like this kerb outlet. Can be a reasonable cost if the block is above the road, or very expensive if the block is below road level Pumps and a detention tank will be needed (See these links Pumps and Detention Tank)


See Guide to Buying a Block for more information


Different Soil Classification Results

When you look at a new house block its always worth asking the Developer’s Agent what the Soil Classification is ……….. Only don’t take what you are told too seriously!

Usually the developers opinion is going to be that the site is going to be better (lower site costs) than your builder.

To understand why you need to consider the differing situations of the Developer and the Builder.

The Developer

  • May have done 20 or 30 tests over the whole development.
  • Is mostly interested in selling blocks
  • Since the tests were taken has had sewers and drains constructed on the site.
  • Probably done some spreading of fill from roadworks construction and site leveling

The Builder

  • Has had 3 tests done on your block.
  • Knows that even with the three tests the information represents less than 0.01% of the soil under your block.
  • Want to be sure the foundations are strong enough.
  • Want to avoid claims, from you, for foundation movement causing structural cracks.

If I got a soil classification from a Developer I would ask the Builder what extra site costs would be required for that soil, and then budget for at least another $5,000.


For lots more information why not get the Guide to Buying a Block only $4 at this link: Buying a Block

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Drop Edge Beams

If you want to build your new home on a slope you may need to have the additional cost of needing drop edge beams on the low side of the slabs.

These are a sort of retaining wall to hold the fill under the house slab as shown in the drawing below.

Here is a photo of a typical drop edge beam before the brickwork has been started.

Costs (2013) are in the order of $500-$700/sqm of beam, plus the cost of the fill under the slab.

Say. . . $12,000 to $18,000 for a 20m long house with a 1.5m drop beam so a significant extra!

The photo below shows the same wall after the brickwork has been completed.

If you are building on a slope you may also want to know about Retaining Wall Fairness

Thanks to Grumblebum54 for the photos

Types of Fill

You will often find a reference to ‘Fill’ in your Geotechnical Report.

Your site may require ‘Cut and Fill’ to level the site.

Here is a bit of information about the various types of fill:

Ordinary Fill

Ordinary fill is normally excavated material from the site or from a unspecified location. After placement the excavator  tracks across it several times (known as Track Rolling) and then levels the top surface.

This fill is cheap but cannot be relied on for house foundations. You will either need to excavate through the fill to the underlying material, or use Concrete Piers.

Controlled Fill

Controlled fill is a known (tested) material either from the site or a specially imported material.

The filled area is constructed as follows:

  • The material is placed in layers, typically 150mm.
  • The water content is optimised, usually by adding water.
  • Each layer is compacted with specialised compaction equipment such as a vibrating roller.
Photo Courtesy of Coates Hire
  • At least 3 satisfactory tests of the compacted density of the filled area are carried out.

Providing the whole slab foundation will be on the controlled fill  your foundations only need to go down the required depth into the controlled fill.

No matter how well the fill was controlled I would not want to build where part of the house was to be on fill, and part on original ground…… In that case I would still like Concrete Piers, or Screw Piles installed through the fill to the original ground.


See Soil Classification for more information