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Part 1
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SECTION 3 STANDARD DESIGNS
3.1 SELECTION OF FOOTING SYSTEMS
3.1.1 Selection procedure
Standard deemed-to-comply designs shall be in accordance with Clauses 3.2 to 3.6. These designs shall not apply to – a) Class E or Class P sites:
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Sites with inadequate bearing strength or where ground movement may be significantly affected by factors other than reactive soil movements due to normal moisture conditions shall be classified as Class P.
Class P sites include ......... reactive sites subjected to abnormal moisture conditions.....
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A site shall be Class P if – a) The bearing strength is less than that specified in Clause 2.4.5; b) Excessive foundation settlement may occur due to loading on the foundation; c) d)
The site contains uncontrolled or controlled fill as identified in Clause 2.5.3;
The site may be subject to mine subsidence, landslip, collapse activity or coastal erosion; e) The site may be subject to moisture changes due to site conditions more severe than the normal site conditions described in Clause 1.3.2; or f) The site may be subject to other factors resulting in foundation movement beyond the reactive soil movements resulting from moisture changes due to the normal site conditions described in Clause 1.3.2.
The basis for classification shall be recorded on the site classification report together with recommendations for further geotechnical investigation.
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Normal sites are those that are classified as one of
Classes A, S, M, H1, H2 and E in accordance with Section 2 of this standard and where foundation moisture variations are those caused by seasonal and regular climatic effects, effect of the building and subdivision, and normal garden conditions without abnormal moisture conditions.
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Abnormal moisture conditions are those that result in foundation moisture variations beyond those for normal sites.
Buildings constructed on sites subject to abnormal moisture conditions have a higher probability of damage than those described in Clause 1.3.1.
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 Prior to Construction
 During Construction
 After Construction
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Examples of abnormal moisture conditions existing
prior to construction include the following: a) Removal of an existing building or structure likely to have significantly modified the soil moisture conditions under the footprint of the footing system of the building.
b) Removal of trees prior to construction.
c) Presence of trees on the building site or adjacent site.
d) Unusual moisture conditions caused by drains, channels, ponds, dams, swimming pools, effluent disposal areas or tanks, which are to be maintained or removed from the site.
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Examples of abnormal moisture conditions
resulting from construction include the following: a) Failure to provide adequate site drainage.
b) Failure to detail or construct drainage in accordance with this Standard.
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Examples of abnormal moisture conditions developing
after construction include the following: a) The effect of trees too close to a footing.
b) Excessive or irregular watering of gardens adjacent to the building.
c) Failure to maintain site drainage.
d) Failure to repair plumbing leaks.
e) Loss of vegetation from near the building.
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Page 5
6.2 SITE CLASSIFICATION
The results of laboratory testing indicate that the site soils have shrink/swell index (Iss) values ranging from 1.1% to 5.6%. The laboratory test results indicate that the materials encountered at the site are moderately to highly reactive.
On the basis of the soil profiles encountered during field investigations, laboratory testing and preliminary calculations, the allotments in their current condition are classified in accordance with AS2870-1996 as follows:
Lots 1 – 14 inclusive, Class H, Highly Reactive
Lots 15 – 19 inclusive, Class M, Moderately Reactive
Lots 20 – 25 inclusive, Class H, Highly Reactive
Lots 26 – 42 inclusive, Class M, Moderately Reactive
The effects of changes to the soil profile by additional cutting and filling and the effects of past and future trees should be considered in the selection of the design value for differential movement. Footings for the proposed development should be designed and constructed in accordance with the requirements of AS2870.
Where fill is to be placed to raise site levels, the affected allotments will require reclassification once the depth and type of placed fill are known and the level of earthwork control has been established.
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“The effect of changes to the soil profile by additional cutting and filling and the effects of past and future trees should be considered in the selection of the design value for differential movement.”
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Page 5
6.2 SITE CLASSIFICATION
The results of laboratory testing indicate that the site soils have shrink/swell index (Iss) values ranging from 1.1% to 5.6%. The laboratory test results indicate that the materials encountered at the site are moderately to highly reactive.
On the basis of the soil profiles encountered during field investigations, laboratory testing and preliminary calculations, the allotments in their current condition are classified in accordance with AS2870-1996 as follows:
Lots 1 – 14 inclusive, Class H, Highly Reactive
Lots 15 – 19 inclusive, Class M, Moderately Reactive
Lots 20 – 25 inclusive, Class H, Highly Reactive
Lots 26 – 42 inclusive, Class M, Moderately Reactive
The effects of changes to the soil profile by additional cutting and filling and the effects of past and future trees should be considered in the selection of the design value for differential movement. Footings for the proposed development should be designed and constructed in accordance with the requirements of AS2870.
Where fill is to be placed to raise site levels, the affected allotments will require reclassification once the depth and type of placed fill are known and the level of earthwork control has been established.
“
Please design slab system for Lot 16.
The site is Class M.”
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Class P Site:
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Class P Site:
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Class P Site:
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Class P Site:
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Class P Site:
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Class P Site:
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Class P Site:
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Class P Site:
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Class P Site:
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Class P Site:
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Class P Site:
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Normal Site (Maybe?):
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Normal Site (Maybe?):
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Site Classification:
The site classification should identify the site as either:
1.
2.
A Normal Site (& it will remain a Normal Site) or
A Class P Site.
A Normal Site, can be classified as Class A, S, M, H1, H2, or E.
The reasons for a Class P need to be stipulated.
For a Class P Site, either:
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2.
Provide advice for an equivalent level of reactivity to satisfy the reasons that the site is Class P. “The site is Class P (because) and we recommend Class H1 slabs and footings..”. Or,
Provide the technical information sufficient for the design engineer to select or determine a suitable footing system.
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 Misleading simplicity of AS2870-2011.
 Standard deemed-to-comply designs only apply to Normal sites.
 Most sites are not Normal, but are Class P.
 Site Classification needs to recognize the factors affecting the site.
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Part 2
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1.
2.
3.
4.
5.
Calculate Ys.
Modify Ys to account for any cutting and/or filling of the site.
Calculate Yt.
Either select a standard deemed-to-comply design, or
Use engineering principles.
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 Natural Site, ys = 35 – 40mm
 Cut & Controlled Fill ys = 35 – 60mm
 Cut & Uncontrolled Fill ys = 25 – 60mm
(Class M)
(Class H1)
(Class H1)
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 Newcastle Site
Cut & Controlled Fill, ys = 25-60mm Class H1
 Maitland Site
Cut & Controlled Fill, ys = 50-80mm Class E
 Muswellbrook Site
Cut & Controlled Fill, ys = 55-100mm Class E-D
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 Hs values have increased and vary with location
 Cutting and Filling will increase ys
 Calculate ys as part of every design
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 AS2870-2011 Appendix H
 New requirements
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(b) In the absence of advice on mature tree heights, the ratio Dt/HT may be taken to be 0.5
In the absence of advice on mature tree heights, single trees with Dt greater than 25m and groups of trees with Dt greater than 50m may be ignored.
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If D t
/ HT <= 0.5, Then y t
= y tmax
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Appendix H requires:
Centre Heave Mound Height: ym = ( 0.7 ys + yt )
“Mu for centre heave case should not be less than 1.5Mcr, as calculated for centre heave bending, and
Mu for edge heave should not be less than
1.5Mcr, as calculated for edge heave bending.”
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Trees Removed
Before Construction
Appendix H requires:
 Centre Heave Mound Height: ym = ( 0.7 ys + yt )
The Commentary recommends yt may be additive to either the centre heave or edge heave cases. However Appendix H has no such requirement for yt & edge heave case.
 Equivalent bending strengths.
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Trees Removed
Before Construction
STRENGTH LIMITS:
“Mu for centre heave should not be less than
1.5Mcr, as calculated for centre heave bending, and
Mu for edge heave bending should not be less than the moment resistance Mu for centre heaves.”
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“Mu for edge heave bending should not be less than the moment resistance Mu for centre heaves.”
Mu (Edge Heave) >= Mu (Centre Heave) i.e. Equal Strength
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From our calculations: ys = 50 to 58 mm, Use ys = 55 mm yt = 12 to 15 mm, Use yt = 14 mm
Centre Heave: ym = 0.7x55 + 14 = 53 mm
Edge Heave: ym = 0.5x55 = 28 mm
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Yst = Ys + Yt/0.7
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I req
M*
1.5Mcr
I req
M*
1.5Mcr
I eff f Mu
Mu
I req
M*
1.5Mcr
I eff f Mu
Mu
I req
M*
1.5Mcr
I eff f Mu
Mu
I eff f Mu
Mu
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Newcastle site, with natural ys = 35 to 40mm.
The site will be cut & filled 600mm;
A tree is going to remain near the slab;
CORD analysis indicates we need:
Class H1 slab specification
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From our calculations: ys = 50 to 58 mm, Use ys = 55 mm yt = 12 to 15 mm, Use yt = 14 mm
Centre Heave: ym = 0.7x55 + 14 = 53 mm
Edge Heave: ym = 0.5x55 = 28 mm
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If the tree is removed:
Newcastle site, with natural ys = 35 to 40mm.
The site will be cut & filled 600mm;
A tree is going to be removed;
CORD analysis indicates we need:
Heavier than
Class H2 slab specification
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Newcastle Site: Hs = 1500 mm
Natural Movement: ys = 35 to 40 mm
600mm Cut, modified ys = 55 mm
Tree Movement: yt = 14 mm, Ht = 2.5 m
If Tree Remains:
385 mm deep waffle pod slab with N12 reo.
If Tree is removed:
410 mm deep waffle pod slab with N16 reo.
Class H1 Slab Heavy Class H2 Slab
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Commentary:
C2.3.2 Instability Index
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Commentary:
CH4 Design Procedure
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 “But its a Class M Site”
 New requirements of AS2870-2011
 Tree design method originated in SA
 Building Code of Australia
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