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.

 

 

Termites

Did you know Australia has around 15 species of termite which can damage the timbers in your new house.

Although some species of timber are resistant to termites none are termite-proof. In practice any structure containing wood can be attacked, unless protective measures are taken.

Even if you have got a steel framed house, or double brick, you will still have timber in things like doors and architraves.

photo from Wickipedia

Know Your Enemy

Termites are more like cockroaches than ants.

Subterranean termites do more damage to timber than either damp wood or dry wood termites.

The termites generally remain within a system of tunnels that can extend 50m, from the central nest, to food sources.

Its not unusual for the termites to build their tunnels round any barriers so no matter what termite protection you use you still have to inspect the barriers regularly.

In order to get to their food source of wood, termites can damage materials they cannot digest such as plastics, rubber, metal or mortar.

Protective Measures

In the past certain areas were identified as at risk of termite attack while others were considered termite free. I thick it is much better to consider all properties at risk.

I’m not a fan of regular spraying of chemicals so for me a permanent barrier is a must.

Basically you need a continuous barrier to prevent termites climbing up through the external wall and individual protection around any pipes and conduits that penetrate the slab.

Options for the barrier in the walls, in order of rising cost include:

  • Exposed Concrete This is cheap and effective as it involves leaving the bare concrete of the slab exposed for a minimum of 75mm. Unfortunately not very attractive,  although you could use a concrete paint to match the brick colour.
  • Barrier Containing Insecticide Probably the most common is  Kordon, which is a  combined DPC and termite protection. It is two layers of plastic sandwiching an insecticide impregnated layer. (Expect to pay around $1,500)
  • Termimesh A fine stainless steel mesh. (expect to pay around $2,000)

Last time I built I used Termimesh as I was concerned about appearance, and preferred not to use chemicals.

 

Decisions on your new home? . .  see  Selection/Pre-Start Guide

Only $4

 

Height Above Ground


For the typical modern house with slab on ground base there are two minimum heights above ground level that need to be considered:

  • Next to the building.
  • Distance of 1m away.

The reason for these minimums is to keep water out of the building, including the structure and foundations.

Next To The House

Floor level above external finished surfaces must be a minimum of:

  • 50 mm above impermeable (covered paved or concreted areas) that slope away from the building
  • 100 mm above the finished ground level in low rainfall intensity areas ( 5 minute intensity of less than 125mm/hour for a recurrence interval of 20 years -see: Rainfall Intensity to check your site), or sandy well-drained areas
  • 150 mm in any other case.

Distance of 1m

The external finished surface surrounding the slab must be sloped away from the building, for the first 1m by a minimum  of the following:

  • 25 mm in low rainfall intensity areas for surfaces that are reasonably impermeable (such as concrete or clay paving)
  • 50 mm  in any other case.

Normally in clay soils I would allow another 15-20mm to allow for any ‘Soil Heave’. (See:Building on Clay Soils)

Overall

To meet the Building Code the range of total heights above ground will vary between 75mm and 200mm depending on circumstances. If you want a smaller step between inside and outside (for reasons such as Better Accessible Design) you will need to talk with your House Designer about a detail which meets the intent of the Building Code (Keeps water out of the house).

 

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

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