Category: Foundations

Waffle Pod Raft Slab

A ‘Waffle Pod’ slab is now probably the most common Concrete ‘Raft’ House base.

It has generally taken over from the Conventional Raft Foundation.

It’s an example of composite construction with polystyrene pods forming the voids in the underside of the concrete ‘Waffle’.

This photo shows some of the pods, with the reinforcement fixed over them, prior to placing the concrete.

Advantages

  • Cost
  • No trenches resulting in:
    • Simpler excavation; and
    • Flat sub-base means site is easier to keep dry than trenches that collect water.
  • Waffle Pods bring house further out of ground so in an area that is wet the higher the better.
  • Better insulation from the ground meaning the floor will warm up quickly from cold when you put the heating on. (especially if you go for in slab heating)

Disadvantages

  • More susceptible to bad workmanship – If the pods move during placing of the concrete.
  • More susceptible to bad drainage which can lead to soil heave if building on reactive clay
  • Lower thermal mass so less suited to passive solar design.
  • Thinner slab (typically 75mm rather than 100mm) means may it not be as resistant to point loads such as jacking a car. Some people specify 100mm thickness for the garage floor.
  • A thicker overall slab may be needed if you want Floor Drains (To allow for the dropped floor in those rooms)

Problem Sites

If  your block has difficult ground conditions its still possible to use a waffle pod slab with:

 

Also see Ground Conditions

 

Building On Fill – Concrete Piers

Its not unusual for the Site Investigation Report for your new house to reveal loose fill materials above a more stable clay or rock.

It might also reveal the soil is in poor condition.

To deal with the problem of building on poor ground concrete piers are a commonly used solution.

These are basically a hole that is excavated through the poor material and into the top of good ground. The hole is then backfilled with concrete to the level of the underside of the slab.

When they are finished you have a flat site with piers visible as concrete ‘pads’  (like the photo below), ready for the slab to be constructed

You don’t have to have Concrete piers over the whole site, just the parts where there will be fill, or weaker soil between the bottom of the slab and the stronger soils.

The following diagram shows a typical slab on a cut and fill site with piers excavated through the fill to the good ground.

See the following link for an alternative to Concrete Piers: Screw Piles.

The Slab over the piers is likely to be either a Waffle Pod Raft or a Conventional Raft Slab

When the site investigation indicates fill the builder will often put in a Provisional Sum for the estimated metresdepth of concrete piers.

At the time of construction they will drill down to  good ground in the fill areas and calculate the actual metres you will need to pay for.

If your whole house is on ‘Controlled Fill‘ You may not need to use concrete piers.

 

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

See Ground Conditions for more about your new house foundations.

 

Conventional Raft Slab

A ‘Conventional’ Raft Slab is a concrete base laid directly on a compacted base.

It is strengthened by cutting trenches in the base and adding reinforcement.

These ‘thickened beams’ are formed at the edges and also under loadbearing walls.

This photo shows the base covered in poly , reinforcement placed and edge forms in position ready for placing of concrete.

Advantages

  • Higher thermal mass, as it incorporates the thermal mass of ground  so better suited to passive solar design.
  • Less susceptible to bad workmanship by concretors
  • More resistant to point loads, such as jacking a car.
  • A thicker overall slab makes it easier if you want Floor Drains (To allow for the dropped floor in those rooms)

Disadvantages

  • Generally uses more concrete, than ‘Waffle Pod Slabs‘, with more waste as volume is less predictable
  • Rain can cause construction delays as the trenches for the beams can fill with water
  • More complicated excavation can lead to additional cost

 

Also see 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.

 

Building On Pipeline Easements (Or Close To)

Previous posts have talked about Sewer and Drain Easements but you build close to or over a pipe on an easement?

Permission

You must get permission from the owner of the easement to build on the easement.

Some easement owners won’t permit any building.

However some will allow certain works after a fee is paid.

This fee can amount to several thousand dollars particularly if you want to build right over the sewer.

Costs

The minimum costs are likely to be a CCTV survey of the pipe which could cost over a thousand dollars.

Additional costs may include either exposing the pipe and encasing the whole line in concrete, or re-routing the pipe and paying for the cost of establishing a new easement.

Getting the Design Right.

When building close to a buried pipeline, whether the building is in the easement, or close to it, the designer needs to ensure no loads are placed  on the pipe.

To avoid placing any load on the pipe the base of any foundation should be below the zone of influence of the pipe.

This zone of influence starts at the base of the pipe and rises at a slope of 1 in 1 to ground level.

In the diagram above

  • Foundation A is unacceptable The base is inside the zone of influence.
  • Foundation B is acceptable Even though it is the same horizontal distance from the pipe as Foundations A because the base is outside the zone of influence.

The base of the foundation is the lowest point of the foundation, that is the bottom of the slab, In the case of piers the base of the piers.

Building Over the Pipe

Some water authorities do allow building right over a pipe. in that case the base of foundations on both sides of the pipe needs to be outside the zone of influence.

Any beam or slab over the pipe needs to be designed to span between the foundations.

 

To better understand what you can build see

Restrictions in the Blocks section

 

Soil Heave – Protecting the Slab After Construction

Once the house is completed it doesn’t mean that you shouldn’t be concerned about the foundations.

This is particularly the case if your house is Built on Clay as there is a risk of ‘Slab Heave’.

This is the result of moisture getting into the clay under the slab causing the clay to swell pushing the slab upwards.

Protection Against  Soil Heave

In South Australia there is a requirement for a 1m path around the building if there is a risk of ‘Soil Heave’,

That is good practice whichever state you live in.

The diagram below shows a suitable detail to protect the ground under your slab.

The key issues are:

  • Paving falls away from external walls for at least 1m with a minimum fall of 1:20 (50mm of fall in 1m)
  • Where the water will not continue to flow away from the edge of the path an Aggi Drain in a trench backfilled with granular material should be provided.
  • Although the requirement for an aggi drain is not as critical where the ground slopes away from the slab, it is nevertheless good practice to have one.

Got Problems?

Generally there aren’t easy fixes for foundation problems, and the cures aren’t DIY jobs

You really need to get an expert involved like Geotech Built 

 

Also see Agricultural Drains

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.

Under Slab Membranes

This photo shows a plastic membrane underneath a slab. . . . but what sort of membrane?

Vapour Barrier

The Building Code of Australia requires the installation of a Vapour Barrier with the following characteristics

  • Medium Impact Resistant Polyethylene Film 0.2mm thick,  under slab and up sides to ground level.
  • 200mm Overlaps at joints.
  • All penetrations should be taped or sealed with a close fitting sleeve.
  • Any damage should be repaired with an adequate sized patch and tape.

Damp-Proofing Membrane

The South Australian and New South Wales Authorities don’t think a Vapour barrier is enough; particularly if there is the potential for saline groundwater, or Acid Sulphate Soils.

They require the use of a High Impact Resistant Polyethylene Film 0.2mm thick as a damp proofing membrane.

They reason is type of membrane will be more resistant to puncture during construction.

Remember that during construction the workers will be walking on this membrane, and reinforcing steel may be dropped on it.

Some councils also further specify a layer of sand under the membrane to prevent penetration from stones under the membrane and provide a drainage layer.

Cost Difference

Two rolls of 50m x 4m Polyethylene film should be sufficient to protect the typical house slab.

Based on a quick internet search this week (June 2015) I found the following prices

  • 2 rolls of Medium Impact Polyethylene would cost around $150
  • 2 rolls of High Impact Polyethylene should cost around $220

For an extra $70 wouldn’t you prefer a better membrane?

 

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

 

Placing Fill

Of so you want to place some fill. . . .perhaps behind a retaining wall to level a garden . . . So what do you need to understand?

Here are some things to consider.

Final Volume – Loose Volume

Most people underestimate the volume of material they need because they measure the volume they have to fill, and then quote that volume in the order.

The problem is that the material for delivery is measured by the loader bucket, or the truck load, which is when the material is loose.

Typically the loose volume will compact down by around 10% so if you are filling a large volume to advise the supplier you may need more than the measured volume and you will advise them you may need to add to your order as the job proceeds.

 Consolidation and/or Compaction

There are two ways of getting to the final volume:

  • Consolidation This is letting the material compact under its own weight. This can work quite well for sand, especially if it is ‘washed in’. For other materials it takes much too long (0ften many years) which means you will be forever topping up and re-levelling the top surface.
  • Compaction This is making extra effort to pack the fill down. Considerations in compaction are:
    • Compact in layers the thinner the layer the better the compaction. (Layers should be no more than 150mm)
    • Even compaction will give better result (Covering the whole area several times with a vibrating plate or roller will give a much better result  than running a bob cat up and down a few times on each layer)

For DIY jobs you can hire a vibrating compactor for around $60/day.

If you are engaging a contractor to do the fill ask people giving you a price how they intend to compact the fill. (The cheapest price will be to place the whole lot, level, and run the machine over it. . . .a recipe for an area that will remain soft and continually sink)
 

If you are planning to build on the filled area you really need to have ‘Controlled Fill‘ professionally placed and tested.

 

 

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