Be very careful ofAS2870-2011

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 sitesA 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 sitesNormal 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 CLASSIFICATIONThe 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 ReactiveLots 15 – 19 inclusive, Class M, Moderately ReactiveLots 20 – 25 inclusive, Class H, Highly ReactiveLots 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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“Please design slab systemfor Lot 16.

The site is Class M.”

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6.2 SITE CLASSIFICATIONThe 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 ReactiveLots 15 – 19 inclusive, Class M, Moderately ReactiveLots 20 – 25 inclusive, Class H, Highly ReactiveLots 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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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. A Normal Site (& it will remain a Normal Site) or2. 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:

3. 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,

4. 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 movementdo 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, or5. Use engineering principles.

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Newcastle Site

Natural Site,ys = 35 – 40mm (Class

M)

Cut & Controlled Fillys = 35 – 60mm (Class

H1)

Cut & Uncontrolled Fillys = 25 – 60mm (Class

H1)

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Geographical Effects:Newcastle Site

Cut & Controlled Fill, ys = 25-60mmClass H1

Maitland SiteCut & Controlled Fill, ys = 50-80mm

Class E

Muswellbrook SiteCut & 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

Changes to AS2870-2011:

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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 RemainAppendix 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 SiteTree 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 mmEdge Heave: ym = 0.5x55 = 28 mm

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Yst = Ys + Yt/0.7

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1.5Mcr Mu

1.5Mcr Mu Mu

Mu1.5Mcr

1.5Mcr

M* fMu M*

M* M*

fMu

fMu fMu

I req

I req

I req

I req

I eff I eff

I eff I eff

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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 analysisindicates we need:

Class H1 slabspecification

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Newcastle SiteTree 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 mmEdge Heave: ym = 0.5x55 = 28 mm

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If the treeis removed:Newcastle site, with natural ys = 35 to 40mm.

The site will be cut & filled 600mm;A tree is going to be removed;

CORD analysisindicates we need:

Heavier thanClass H2 slabspecification

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Summary of Tree DesignsNewcastle Site: Hs = 1500 mm

Natural Movement: ys = 35 to 40 mm600mm 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.

Class H1 Slab

If Tree is removed:410 mm deep waffle

pod slab with N16 reo.

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