Category: Ground Conditions

Building On Sand

Building on sand at first seems to be an issue…… although I have heard it said that sand is actually the easiest ‘soil to build on.

If you live in West Australia there is a good chance that your house will be on sand so there is plenty of experience around .

Raft Foundations


Modern raft foundations actually cope with this any problems of building on sand quite well.

This is because the raft foundation spreads the load of the building over the whole area of your house which matches the weight carrying capacity of the sand very well.

This is much better than concentrating the loads on the wall foundations.

If you are building on sand a waffle pod raft foundation is probably the best way to go.

This is because the waffle pod doesn’t rely on any excavated trenches of the conventional raft slab to form the beams that gives the base its rigidity.

Soil Erosion

The main concern with sand is that it is very erodible so its important to make sure that the building site is flat and there are adequate retaining walls preventing the sand being eroded on the downhill side of the house………and eventually from under the house!

This is particularly the case if you are building a property with a sea view, you wouldn’t like to finish like this *house!

*photo courtesy of Australian Coastal Society

 

See Ground Conditions for more about your new house foundations.

 

Contracts – Provisional Sums

All new house building contracts will have a section for Provisional Sums.

These are used for work that  MAY be required, but the builder can’t provide an accurate estimate at the time of signing of the Contract.

The main area  for provisional sums is related to foundations where the limited information obtained during the Site Investigation may not truly represent the conditions found over the whole foundation.

A coule of examples are:

    • A rock item may be included even if the site is clay. This caters for any large boulders that may need to be removed.
    • An amount may be included for concrete piers if the amount of fill on site is hard to determine.

So the total contract price reflects the cost of building the house the builder will put in his best estimate of the overall cost of the build.

The provisional sum  will reflect the direct cost  of  those items to the builder only. (Overheads are included elsewhere in the contract price)

For example with a typical provisional sum item for removal of rock…… The builder will often include a figure of say $2,000 for excavation, and removal, of rock.

If there is no rock the contract will be reduced by $2,000.

Alternatively if the cost of excavator, labour and transport was $3,000 the final price will be increased by $1200. ($1,000 extra plus overhead percentage on the extra, typically 20%)

The builder is only allowed to add any overhead for profit and administration, to provisional sum expenditure items where they are above the original estimate.

Did you have any issues with Provisional Sums on your build?

See similar posts in Contracts

 

Building On Clay Soils

Clay is a fairly common soil type.

Although it is quite suitable for building one issue is that some types of clays (‘Reactive’ or ‘Desicated’ clay) that shrink or swell depending on the level of moisture.

This can lead to the following problems:

Subsidence

Subsidence or settlement is when the building foundation drops, caused by the loss of support of the site beneath the foundations.

This is usually caused by shrinkage of the clay due to it becoming drier.

Typically this is due to tree roots spreading under the building and taking the moisture out of the clay.

Prolonged drought can also cause soil shrinkage beneath a building.

Soil Heave

Heave is the expansion of the ground beneath part or all of the building.

Expansion pushes the foundations, and thus the building upwards.

This is a photo of a boundary wall with inadequate foundations.

Due to soil heave the right side of the wall has lifted by over 100mm causing cracking of the mortar joints, and a brick.

Although the mortar joints have recently been repointed you can see they are no longer straight.

The most common cause of soil heave is building is when the clay becomes wetter and starts to expand.

Common reasons for the clay to become wetter are:

  • Water seeping down the side of the house (In areas of reactive soil a normal requirement is to have a 1m wide footpath round the house)
  • Poor site drainage (No drainage, blocked drains, or leaking water pipes)
  • Removal of trees (This means the drying effect of the roots has been removed)

A couple of danger signs of future soil heave issues for a new house build are:

  1. If building after several years of drought the soil may be excessively dry and therefore more likely to become wetter when the drought breaks.
  2. Removal of trees immediately before building.

Summary

There is no reason why you shouldn’t build on clay soils but you need to ensure that;

  • You have chosen an appropriate construction method (Brick Veneer is usually better than double brick)
  • There will be minimal soil moisture change when the house is built. (see this link:  Protecting Your Slab)
  • If you need concrete piers see the following link: Building on Fill.

To find out more about the soil on your block click on this link: Soil Classification

 

 Lots more information in the anewhouse at this link: Guide to Buying a Block

To find out what soil is on your block see this link: Site Investigation

 

Buying A Sloping Block.


It doesn’t take much of a slope to mean that costs will increase.

For the last house I built, in 2005, a 0.8m slope over a 26m wide by 32m deep block added $4,800 to the cost.

That’s for much less of a slope than the block in the picture.

The extra cost was for some cut and fill ($2,900) to level the site under the slab, and about 12m of 450mm high timber retaining wall ($1,900).

Much more of a slope and the costs can really shoot up.

Reasons for Additional Costs

As well as cut and fill costs you could have:

  • Drop Edge Beams – A sort of retaining wall as part of the foundation to make sure that the fill stays under the foundation.
  • Concrete Pier, or Screw Piles may be needed because the different depths of fill under the slab will have varying strengths.
  • Higher Strength Slab – Needed to span between the piles.
  • Extensive Retaining Walls – Once Retaining Walls go above 1m high the costs increase significantly.

One advantage of double storey houses on slopes is that the additional cost of foundations will be lower than those for a single storey house of similar total floor area due to the smaller foundation area.

Steeper Sites

For really steep slopes you may need to go for a Split Level Home or even build on ‘Stilts’.

Either way you are definitely going outside the typical project builders territory and probably looking to get a custom builder and architect involved in the house.

So why build on a slope?

Well for many people its a view, and means you may only be overlooked on one side of the house.

Without a view and I would only be looking at building on a slope if the block was significantly cheaper, to offset the additional costs.

For Similar Posts see Choosing Blocks

Or see Understanding Retaining Walls

 

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.

Solutions

  • 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.

Problems

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.

Options

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

 

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