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Be very careful of
AS2870-2011
Scott Warner
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OBJECTIVES:
 Site Classification
 Footing Selection
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Part 1
Is it a soil classification?
Or
Is it a site classification?
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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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2.1.3 Classification of other sites
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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2.1.3 Classification of other sites
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)
The site contains uncontrolled or controlled fill as identified in Clause 2.5.3;
d)
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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1.3.2 Normal sites
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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1.3.3 Abnormal moisture conditions
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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1.3.3 Abnormal Moisture Conditions
 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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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 Geotech Engineers’s Handover:
“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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Which lot is yours?
Confirm the lot numbers in the
geotechnical report
are the same as the lot numbers
on the current DP.
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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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What is a “Normal” 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:
1.
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,
2.
Provide the technical information sufficient for the design engineer
to select or determine a suitable footing system.
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Part 1 Summary:
 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
What have they done to my
slab design?
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How much movement
do I design for?
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Design Procedure
1. Calculate Ys.
2. Modify Ys to account for any cutting and/or
filling of the site.
3. Calculate Yt.
4. Either select a standard deemed-to-comply
design, or
5. Use engineering principles.
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Newcastle Site
 Natural Site,
ys = 35 – 40mm
(Class M)
 Cut & Controlled Fill
ys = 35 – 60mm
(Class H1)
 Cut & Uncontrolled Fill
ys = 25 – 60mm
(Class H1)
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Geographical Effects:
 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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Changes to AS2870-2011:
 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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Effect of Trees
 AS2870-2011 Appendix H
 New requirements
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H4 DESIGN PROCEDURE
(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 Dt / HT <= 0.5, Then yt = ytmax
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Trees to Remain
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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Trees Removed Before Construction
“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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Newcastle Site
Tree to Remain
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
I eff
I req
I eff
M*
fMu
M*
fMu
1.5Mcr
Mu
1.5Mcr
Mu
I req
I eff
I req
I eff
M*
fMu
M*
fMu
1.5Mcr
Mu
1.5Mcr
Mu
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If the tree remains:
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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Newcastle Site
Tree is Removed
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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Summary of Tree Designs
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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Conclusion & Comments
 “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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