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.

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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

Building On Rock

Based on the Soil Classification rock is considered one of the best surfaces to build a house foundation.

It comes under the Classification Class A.

That doesn’t mean its going to be cheap especially for a conventional raft or waffle pod slab.

Any excavation such as leveling the site and excavation for sewerage and drainage trenches is going to be be expensive. This is due to it requiring a heavy excavator and rock breaking equipment.

Keeping Costs Down

If you have a site that has rock close to the surface the most economic base construction is likely to be Piers for Lightweight Construction. (including Pole Houses and Queenslanders)

If you want a Masonry House (such as Brick Veneer or Double Brick) then you need to minimise the amount of excavation by going for suspended floors.

Also see Provisional Sums to find out the issues of finding rock during the construction.

What Sort of Plan?

Buying Land…….There are several sorts of Plans/ Surveys of your Block to think about before you can start building a house.

Here is a guide to the various plans based on the same block:

Title Plan

Title Plans are the legal document that describes the land. Initially they are usually the only plan that is available They are limited to:

Showing the dimensions and bearings of the boundaries.
Showing the locations of Easement.

The drawing below is a typical title plan

Feature Survey

The Feature Survey is an accurate representation of the block prepared by a surveyor. Typically the information shown is:

Boundaries.
Site Contours.
Locations of trees.
Any other above ground features such as driveway crossovers and manholes.

If you buy direct from a project builder they will normally arrange the feature survey. If you want to get quotes from several builders it can be worth getting a feature survey done first.

Geo-Technical Plan

The Geo-Technical Plan is more in the form of a sketch than an accurate representation of the site. The main purpose is to indicate the approximate location of boreholes. Other information that may be included is:

Indication of location of trees as these may affect the Geotechnical Recomendations.
Any rock outcrops.
Direction of fall.
Ponds / swampy areas.

See Blocks for more information on buying land.

Waffle Pod Raft Slab

A ‘Waffle Pod’ slab is now probably the most common Concrete ‘Raft’ House base. Its generally taken over from the Conventional Raft Foundation.

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

This photo shows the pods, with the reinforcement placed over them, prior to placing the concrete.

Advantages

Cost
No trenches resulting in:
- Simpler excavation; and
- Flat site means site is easier to keep dry than trenches that collect water.
Waffle Pods bring house further out of ground. In an area that is wet the higher the better.
Better insulation from the ground meaning the floor will warm up quickly from cold. (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 not be as resistant to point loads such as jacking a car. Some people specify 100mm thickness for 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 for your new house to reveal fill materials on top of the more stable clay or rock.

Even if there is no fill the builder may need to level a sloping site using ‘Cut and Fill’ methods, or the top layers of soil are generally poor.

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

These are basically a hole that is excavated through the fill 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 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 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 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 so many metres 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.

Category: Ground Conditions