FOOTING CONSTRUCTION REPORT - orroroo.sa.gov.au

FOOTING CONSTRUCTION REPORT

BUILDER/AGENT: GP Architects

OWNER: DC Orroroo - Carrieton

SITE: 17 Second Street, Orroroo

JOB NO: 13037

DATE: 17/05/2021

SUPERSTRUCTURE: Single Storey Articulated Hebel Panel Extension

ENCLOSURES:

FOOTING REPORT

FOOTING LAYOUT PLAN

CONSTRUCTION DETAILS

SPECIFICATION FOR FOOTING CONSTRUCTION

GENERAL NOTES FOR FOOTING CONSTRUCTION,

SITE PREPARATION AND SITE MANAGEMENT

CSIRO SHEET BTF 18

FR 1 of 3

FOOTING REPORT

SITE: 17 Second Street, Orroroo

JOB NO: 13037

DATE: 17/05/2021

SUPERSTRUCTURE: Single Storey Articulated Hebel Panel Extension

This is a construction report giving recommendations for the specific superstructure noted above.

Changes to design or construction shall not be made without further written advice from the

engineer. This report shall be read in conjunction with all listed enclosures. Refer to Footing Plan for footing layout and further details.

SITE CLASSIFICATION Soils on this site are classified as class "H1-D" in accordance with AS 2870 – 2011, based on

previous soil testing in Orroroo township.

FOOTING DETAILS FOOTING TYPE: Reinforced concrete raft footing

BEAMS: External E1 300 mm wide x 500 mm deep

3 - N12 top, 3 - N12 bottom

Internal I1 300 mm wide x 500 mm deep

3 - N12 top, 3 - N12 bottom

LIGATURES: W6 @ 900 mm centres

SLAB: 100 mm thick, SL72 top

FILL: Footing design based on fill under slab in excess of 200 mm deep

being controlled fill in accordance with AS2870. If uncontrolled

fill is in excess of 200 mm deep contact this office for revised

Footing Layout Plan.

CONCRETE: Concrete Strength N25.

FR 2 of 3

Job No. 13037

- 2 -

CONSTRUCTION DETAILS:

1. Footing beams shall be fully trenched into firm natural soil.

2. Where piers are present they shall be trenched into firm natural soil. Footing beams

between piers shall be bedded on compacted fill.

3. Articulation joints are required to accommodate footing movements, and are located as

detailed on the footing layout plan.

4. Where brittle floor coverings are to be used eg. large tiled areas, it is recommended that

the reinforcement in the slab should be increased to SL92 mesh or use a flexible adhesive

such as “Resiflex” or similar, in accordance with AS2870-2011 section 5.3.7(a).

5. The specific requirements detailed on the Footing Layout Plan should be noted together

with the requirements detailed in the Specification and General Notes.

6. The Footing Design has not taken into account the effect of trees and/or shrubs. Trees

and/or shrubs having a significant effect on the design of the proposed footings were not

noted on this or adjacent sites.

7. This site has been assessed as Class “H1-D” therefore flexible plumbing fittings will be

required.

8. An equipotential bonding conductor (N12 earthing rod) shall be installed in accordance

with AS/NZS 3000, Clause 5.6.3.

9. Polished concrete floors and embedded heating pipes have not been allowed for in the

footing design. Contact Mace Engineering Services for further specifications regarding

these items if they are required.

10. Site Inspections during construction are MANDATORY at the following stages:-

• After excavation of the footing beams and piers prior to placement of the damp-

proof membrane.

• After placing of reinforcement and prior to pouring of any concrete.

Work cannot be certified unless it is inspected.

Each inspection will incur an additional charge. 24 hours notice is required when

booking inspections.

FR 3 of 3

Job No. 13037

- 3 -

SAFETY IN DESIGN:

Mace Engineering Services has a strong focus on Work Health and Safety (WH&S), including

Safety in Design. Safety in Design, is the consideration of the health and safety of all users of

the infrastructure, from construction, operation, demolition and decommissioning has been

considered in this design, in accordance with the Work Health and Safety Act 2012 (SA).

Eliminating hazards improves Work Health and Safety outcomes and potentially reduces the

long-term cost implications of remediating design oversights.

The construction of residential footing systems, if undertaken by a licensed contractor and to

‘industry standard’ techniques, is considered to be a low risk operation, and the subject site

does not pose any unusual hazards. Shoring, benching or other excavation stability measures

shall be used where excavation depths exceed 1500mm.

It is recommended that the contractor is licensed and experienced in footing construction to

further minimize construction hazards. Further advice should be sought during construction on

the any aspects of the footing construction report if required.

Considering the above, and providing all other parties associated with the design and

construction undertake their duties in accordance with WH&S and other legislative

requirements, to a professional and industry standard level, we cannot foresee any significant

WH&S implications or hazards that can be avoided by design.

Michelle C Verco FIEAust, CPEng NER

DIRECTOR

Mace Engineering Services Footing Construction Specification Page 1

SPECIFICATION FOR FOOTING CONSTRUCTION

1. GENERAL

All materials and workmanship shall be in accordance with AS2870-2011

Residential Slabs and Footings Code and AS3600 Concrete Structures Code and

all other relevant Australian Standards, including current amendments.

2. MATERIALS AND WORKMANSHIP

2.1 Reinforcement

Reinforcement shall be of the following types:-

N - hot rolled high strength deformed bars designated D500N

("Tempcore" or equivalent) to AS4671

S - structural grade plain round bars to AS4671

L - hard drawn steel wire reinforcing fabric to AS4671

W - hard drawn steel wire for ligatures to AS4671

Sizes of bars, ligatures and fabric shall be as detailed in the Footing Construction

Report.

Reinforcing bars and fabric shall be straight, clean and free from dirt, oil, grease

or loose rust. Reinforcement shall be adequately tied in position and supported

on chairs or suspended to ensure that the required cover and alignment are

maintained during concrete placement.

Fabric shall be supported on blocks or chairs spaced at not more than 800 mm

centres in both directions.

Cover to reinforcement, including ligatures shall be:-

(i) 50 mm at the base and sides of beams where a waterproof

membrane is present

(ii) 65 mm where concrete is in direct contact with the ground

(iii) 40 mm at the tops of beams

(iv) 20 mm to the top slab fabric

(v) 30 mm to the bottom slab fabric where bottom fabric is specified


Where laps in reinforcement bars are required, they shall be not less than

500 mm for N12 bars & 700 mm for N16 bars. Refer to construction Detail CD1

for lap lengths. Wherever possible, laps shall be staggered so that not more than

50% of bars are lapped at the same location. Reinforcing bars shall be lapped to

cross one another for the full width of the footing at all T & L junctions. Steel

fabric shall be lapped 300 mm.

At corners and T- junctions, one outer bar top and bottom shall be bent and

continued 500 mm. Alternatively a 500 mm long (each way) L bar shall be

provided top and bottom. Refer to construction detail CD4.

The construction of a slab shall achieve the following dimensional tolerances as

per AS2870 - 2011.

(a) The cover to the reinforcement from the surface in contact with the

ground shall be within +40 mm and -20 mm of the specified cover.

(b) The cover to the reinforcement from the internal surface shall be within

+20 mm and -10 mm of the specified cover.

(c) The surface shall be generally within + 10 mm of level.

(d) The surface finishes shall be suitable for the specified floor coverings

without further treatment. In the absence of any specification, a steel

trowelled finish with a tolerance of + 5 mm from a 3 m straight edge

shall be used.

2.2 Concrete

Concrete shall be Grade 25 (F'c = 25 MPa @ 28 days) unless specified otherwise

in the Footing Construction Report. Slump shall be in the range 80 + 15 mm.

Nominal maximum aggregate size to be 20 mm. All concrete shall be ready

mixed concrete complying with AS1379.

The placing of concrete shall be carried out in a continuous operation and

compacted using mechanical vibration without displacing the reinforcement.

Piers of depth greater than 1000 mm are to be poured separately to allow

shrinkage to occur prior to placing footing concrete.

Any water in the trenches in excess of 20 mm deep must be pumped out prior to

concrete placement. Shallower depths may be pushed to a low point by the

concrete mass, but must then be baled out.

Concrete shall not be poured when the forecast maximum temperature is greater

than 32OC, without written instruction from Mace Engineering Services Pty Ltd.

Concrete must be cured by water spray and covered with plastic sheeting,

spraying with a liquid membrane curing compound or ponding of water on the

top surface, in accordance with the Building Code of Australia. The plastic

sheeting and ponding shall remain in place for a minimum period of 7 days.


If the temperature in the shade exceeds or is likely to exceed 34 OC during the

time of finishing the slab, extreme caution must be exercised and the following

conditions should be satisfied:

The concrete must be mixed using chilled mixing water.

The steel reinforcement must be kept cool by light water spray.

The use of the vibrator for slab panels must be supervised by a Licenced

Works Supervisor or the Builders.

Concrete shall be placed and finished early in the day, well before the

maximum forecast temperature is reached.

A continuous polythene membrane with minimum 300mm laps must be

place over the slab immediately it becomes ‘walk dry’. The polythene

membrane must be held down and secured in position so as to remain for

a period of 10 days following the pour.

All slab surfaces shall be adequately and properly cured using as

approved treatment in accordance with AS3600 – Concrete Structures.

2.3 Damp-Proofing Membrane

A damp-proofing membrane shall be provided continuous over the underfloor

area and footing beams and shall extend to the finished ground level on the

outside face of the perimeter footing beam. The damp-proofing membrane shall

be Fortecon (or equivalent) polythene sheeting not less than 200 micron

thickness and shall be branded continuously “AS 2870 concrete underlay, 0.2mm

– high impact resistance”, together with the manufacturer or distributors name,

trademark or code.

All joints shall be lapped 300 mm and sealed with 50mm wide pressure sensitive

tape. Securely flash and seal the damp-proofing membrane around all service

pipes and plumbing to ensure a moisture proof joint is provided. Level pins

puncturing the membrane are permitted in the base of the footing beams but not

in the slab areas.

3. CONSTRUCTION

3.1 General

The footing system shall be constructed in accordance with the Footing

Construction Report, Footing Plan and Footing Details, General Notes, this

specification and instructions issued by Mace Engineering Services.

Footing beams shall not be founded on any filling soil, unless it has been testing

and confirmed as controlled fill, as set out in Table A4 – General Notes for

Footing Construction, Site Preparation & Site Management.

3.2 Site Preparation

Refer "General Notes for Footing Construction and Site Preparation and Site

Management".


3.3 Basecourse

A 100 mm thick base course of 20 mm quarry rubble or other approved granular

filling shall be placed over the building area. It shall be compacted to a density

of 95% of its maximum dry density as determined in the Modified Test

(AS1289-5.2.1).

The selected material shall give a finished surface free of any sharp aggregate

which could damage the damp-proofing membrane.

3.4 Footing Beam Excavations

Footing beams shall be excavated in the locations shown on the Footing Plan and

shall have the widths and depths as specified in the Construction Report taking

into account any variation of founding depth because of filling or hard rock.

Where required, piers shall be drilled or dug in the locations shown on the

Footing Plan with the founding depth to 200mm into firm, natural ground.

Beams, and where specified, piers, shall be excavated to firm bearing soil,

trimmed to size and all loose soil shall be removed from the base of excavations.

Over-excavation shall not be backfilled but must be filled with concrete at the

time of pouring footings.

Where beams are excavated through site filling, the specified depth may be

increased if necessary to allow the required depth of excavation to be achieved.

In the case of integral slab/beam footing systems, ie. standard raft, provide a 100

x 100 mm fillet at the slab/beam junction.

At all times, footing beam excavations shall be so excavated that one corner

(preferably the lowest corner) the trench excavations shall be dug beyond the line

of the house so as to permit drainage of any run-off or seepage water entering the

trenches. The water shall be diverted away from the working area of the site or

to a low spot whereupon it may be removed by pump.

3.5 Curing

A curing membrane shall be applied to the slab as soon as practicable after

finishing of the surface. The recommended method is by covering with

polyethylene sheeting securely and continuously held down at the edges for a

minimum period of 7 days.

Care should be taken if chemical curing methods are used, as some products may

not be compatible with adhesives used to fix floor surfaces to the slab. After

vertical surfaces are stripped of formwork, slab edges must be finished prior to

curing.

The concrete slab must not be loaded with stacked materials or building plant for

a minimum of 7 days after pouring.


3.6 Services

Service trenches must be positioned so that the distance between the trench and

the edge of the footings is not less than the depth of the trench. If this cannot be

achieved the Engineer must be notified BEFORE construction commences so

that amendments can be made to the footing design.

In foundation soil profiles containing reactive clays and where the potential for

surface soil movement (Ys) is greater than 40 mm (H1, H2, E and P sites), the

plumber/drain-layer must provide sufficient approved expansion/contraction

couplings in the sewerage pipework system to allow for the anticipated

movement.

Service penetrations are permitted through footings subject to the following

requirements and in accordance with Construction Detail Drawing No CD-6:

All service penetrations shall be taken through the middle third of the

stiffening beams or taken below the founding level of the beams.

A minimum of 50 mm cover shall be provided between the sleeve or pipe and

the reinforcing steel. Where this is not achieved the following must be done:

(i) Provide additional reinforcement so as to match the number of bars in the

beam correctly placed with 50 mm cover and lapping 50 bar diameters

either side of the sleeves.

(ii) Where the sleeve is close to the bottom reinforcement additional

excavation must occur below the pipe and the bottom rods placed and

lapped so as to provide the correct cover to both the base of the trench

and the pipe sleeve.

It is generally unacceptable to have pipes placed between and parallel to

reinforcement along footing beams. Where this cannot be avoided

additional steel as directed by the engineer must be provided.

All plumbing penetrations through slab or beams shall be made using 40mm

foam lag or similar approved closed cell polyethylene lagging material eg.

Compriband or Aeroform.

All sewer trenches both within and outside the perimeter of the house must be

carefully backfilled for the full depth of the service trench using impermeable

clay to minimise the risk of water infiltrating through the trenches and under

the footing. Take care not to damage pipework or fittings during the

compaction process.

Mace Engineering Services General Notes Page 1

GENERAL NOTES FOR FOOTING CONSTRUCTION,

SITE PREPARATION AND SITE MANAGEMENT

1. GENERAL

The details in this report contain advice designed to minimise risk to the

building. It is an important document and should be kept in a safe place. It is

essential that this report be supplied to subsequent owners so that they are aware

of the consequences of making changes to the building and garden. Without this

information, they may institute changes to site management that could jeopardise

the long term serviceability of the building.

The footing design for your building has been based upon Australian Standard

AS 2870, "Residential Slabs and Footings", for the specific building construction

noted.

The client will at all times after receiving the Footing Construction Report

comply and procure compliance in all aspects with all recommendations and

directions contained in such Footing Construction Report including the attached

Notes, Specifications and Appendices.

It is impossible to design a footing system that will totally prevent movement,

particularly so for reactive soils. Some minor aesthetic cracking, whilst

undesirable, will occur in a significant proportion of buildings. It is the intention

of the footing design to prevent cracking exceeding the Category of Damage

classified as slight (Category 1 and 2) as outlined in the Australian Code AS

2870 Table C1 and CSIRO Sheet No. 10-91.

It is important for owners to understand that reactive clays move vertically

because of moisture changes and even relatively stable clays will move

significantly if subjected to extreme moisture changes. It is neither possible nor

economical to design for extreme conditions. The owner is the only person who

can maintain reasonable moisture conditions at the site.

The building owners must familiarise themselves with the attached C.S.I.R.O.

document, Sheet No. 10-91 "Guide to Homeowners on Foundation Maintenance

and Footing Performance" by Dr. P.F. Walsh. This document gives advice on

garden establishment and matters pertaining to site management. The footing


design has been based upon the assumption that you, the owner(s) will observe

the requirements for proper site maintenance as set out in the CSIRO document.

If you do not follow this advice, then building damage may result due to your

neglect. Neither the builder, nor Mace Engineering Services can accept any

responsibility for the performance of your footing system where site maintenance

requirements have not been met or where other relevant instructions noted in the

footing construction report have been disregarded or ignored.

2. SITE INSPECTIONS

A fee, additional to that already charged for the preparation of this report will be

levied for each inspection carried out. The onus for notification of each

inspection, shall at all times remain with the builder or client.

The Engineer shall be engaged to carry out site inspections as follows:

(i) Upon completion of primary earthworks, where the depth of excavation

exceeds 600 mm. The inspection shall be limited to a visual assessment

of the earthworks, and any approval by the engineer to proceed with the

works in accordance with the Construction Footing Report shall be

conditional upon the Client completing the final earthworks to the

correct levels and slopes at a later stage. Where the Engineer considers

that additional testing or investigation is required as a result of the

earthworks, work shall not proceed until the additional services have

been completed. Any such additional testing, investigation and reporting

shall incur additional fees.

(ii) Upon completion of excavation for footings and prior to the placement of

any waterproof membrane or reinforcement. Where footing construction

is completed in stages, an inspection must be carried out at each stage.

(iii) Upon completion of fixing of reinforcement and at or prior to the

commencement of the concrete pour. It shall remain the Builder's or

Client's responsibility to ensure that:

• The correct cover to reinforcement, the concrete quality and quality of

workmanship are maintained.

• Waterproof membranes are not punctured.

• The concrete is finished to correct levels.

Site inspections specifically exclude the checking of levels, layout

dimensions, squareness, relationship to boundaries and matters which

will not affect the structural performance of the building.


3. SOIL BORELOGS

3.1 Site Classification

Foundation soils: All soils are affected by moisture infiltration. Silts are

weakened by water and some sands can settle if heavily watered, but most

problems arise on clay soil foundations. Clay soils swell and shrink due to

changes in moisture content and the potential amount of ground movement is

implied in the site classification in accordance with Australian Standard

AS2870, which is designated as follows:

1. A means Stable (Non-reactive)

2. S means Slightly Reactive.

3. M means Moderately Reactive.

4. H1 and H2 means Highly Reactive

5. E means Extremely Reactive.

6. D – subscript, added to M, H and E, eg (M-D), represents selected sites

where deep seated clays and arid climatic conditions prevail.

7. P – problem sites, eg collapsing soils, uncontrolled fill, trees, reactive

sites subject to abnormal moisture conditions or sites which cannot be

classified otherwise.

The soil profiles, as indicated by the test bores, form the basis of the footing

recommendations contained within this Report. The footings have been selected

on the basis of the recognised characteristics of the soil profile.

Unless otherwise stated these characteristics have been visually assessed and

related to known performance of the soils under optimum conditions of site

development and use. It has been assumed that aspects of site drainage, paving

and landscaping which are described in this Report have been or will be

implemented as soon as possible after completion of construction.

It is not economically possible or practical to determine every sub-surface feature

on a site. Because of this, any variations or discrepancies in soil type, colour, or

horizon depth shall be referred to the Engineer immediately. The Engineer will,

as one of the mandatory inspections, inspect all trenches prior to placement of

the polyethylene moisture barrier for a raft or reinforcement for a strip footing.

3.2 Site Maintenance

3.2.1 Class A and S sites Sands, silts and clays shall be protected from

becoming extremely wet by adequate attention to paving, site drainage

and prompt repair of plumbing leaks.

3.2.2 Class M, H & E Sites classified as M, H1, H2 or E shall be maintained

at essentially stable moisture conditions and extremes of wetting and

drying prevented in so far as this is practically possible. This will require

attention to the following:


(a) Drainage of the site: The area surrounding the building footprint shall be

graded or drained so that water cannot pond against or near the building.

The ground immediately adjacent to the building shall be graded to a fall

to suit paving requirements. The subfloor space for houses with

suspended floors shall be graded or drained to prevent ponding.

The site drainage requirements shall be maintained for the life of the

building.

NOTE: On some low and flat sites surface stormwater drainage is not

always possible without either building the site up to achieve gravity

flow of the drainage system or by installing a sump and pump system.

The Owner is advised to discuss with their Builder the consequences of

building the site up or the installation of a sump and pump disposal

system.

The Owner should be aware that surface stormwater disposal shall be in

accordance with the Building Code of Australia and AS3500.3.

(b) Limitations on gardens: The development of gardens shall not interfere

with drainage requirements of the subfloor ventilation and weephole

drainage systems. Garden beds adjacent to the building should be

avoided. Care should be taken to avoid overwatering of gardens close to

footings. Watering must be maintained during droughts and summer-

autumn seasons to prevent excessive drying out of the soil.

(c) Restrictions on trees and shrubs: Planting of trees should be avoided

near building footings or neighbouring buildings on reactive sites as they

can cause damage due to drying of the clay. To reduce, but not

necessarily eliminate, the possibility of damage, tree planting should be

restricted to a distance from the house of:

i. 1 x mature height for a single tree

ii. 1.5 x mature height for a group of trees

iii. 2.0 x mature height for four or more trees in a row

Removal of trees from a site prior to building construction can also cause

damage as the soil moisture may become unbalanced and take some time

to regain equilibrium.

(d) Repair of leaks: Leaks in plumbing, including water supply, stormwater

and sewerage should be repaired promptly.


4. EARTH WORKS

4.1 Materials

Selected approved site materials excluding topsoil or organic bearing soil

may be used for compacted filling. Where site excavated materials are

unsuitable because of their nature or moisture content, quarry rubble or

other 'imported' granular material may be used. Materials of high

plasticity shall not be used as fill.

4.2 Excavation

It is imperative that the Builder, owner and/or his servant or agent

provide sufficient supervision of the cut and fill operation in order to

ensure that the following requirements for satisfactory completion of the

cut and fill and drainage scheme proposal are adhered to.

• Vegetation and roots must be scraped off and removed from the site

prior to cutting and filling.

• The extent of the cut and fill outside the building line should not be

exceeded with respect to the following requirements. Cut or fill on

the boundary should not exceed 600 mm (unless a suitably designed

retaining wall is specified).

• Cut on the Boundary should not undermine any structure that exists

on an adjacent property.

Generally cut or fill within the property (ie. not on boundary) should not

exceed 1000 mm (unless a suitably designed retaining wall is specified).

Where bank heights do not exceed 1.5 m and the natural slope of the site

does not exceed 1 in 15, the batter slopes recommended below may be used.


TABLE A3

DESIRABLE BATTER SLOPES

MATERIAL

Heavy Clay

Soft Clay

Sands and cohesionless

materials

Friable and sandy

cohesionless soils

Weathered rock in

sound condition

Sound rock

SURFACE SLOPE

1 vertical to 1 horizontal

1 vertical to 1.5 horizontal

1 vertical to 2 horizontal



Nearly vertical

Notes:

(1) Embankments should be protected from damage arising from surface erosion or

groundwater flow by the construction of a cut-off drain at the top.

(2) If a retaining wall has been specified the cut or fill should not exceed the design

height of the specified retaining wall.

4.3 Fill on Steep Slopes

Where the surface slope of an area which is to receive filling is steeper

than 1 (vertical) to 8 (horizontal), a series of benched terraces shall be

excavated along the contour over the whole of the area which will

receive filling. This will stabilise the fill against downhill slip. Contact

the Engineer if further information or clarification is required.

4.4 Sewer Drain on Fill Material

Where any sewer drain is to be laid within filling soil, approval of the

appropriate authority (Local Council or SA Water) shall be obtained.


4.5 Compaction

The standard of compaction of filling, shall, for the various material

types given, be not less than that noted in Table A4 below. The specified

standard of compaction shall be provided to an area extending not less

than one metre beyond the perimeter of the building, and shall also be

provided beneath any filled driveways or other trafficked pavements.

The footings specified in the report have been proportioned assuming

that the builder will achieve the specified compaction. Notwithstanding

this, no footing beam shall be founded in the filling unless the Engineer

has checked its compaction standard and given written acceptance of its

compliance with the specification.

To achieve the Table A4 compaction requirements, vibrating

smoothdrum rollers (for granular materials) or vibrating sheepsfoot

rollers (for clays) are recommended. Take care with vibrating rollers,

particularly if there are houses constructed on adjacent allotments.

Contact the Engineer if in doubt regarding this.

If the builder elects to place filling without the use of compaction

equipment, the filling will be assumed to be incapable of supporting any

building load. Accordingly, any concrete slab over such filling will be

increased in thickness and have an additional layer of bottom fabric

reinforcement. In addition, the footing beams must be piered to firm

natural ground, in accordance with the Footing Construction Report.

Refer to the Footing Construction Report for details.

4.5.1 Controlled Fill

It is acceptable for all footing types to be founded on, or into controlled

fill.

Controlled fill is defined in accordance with AS2870-2011 and includes

clean filling soil, not containing organic matter, topsoil, rock fragments,

boulders or any other damaging material such as building debris or

general refuse. The fill must be compacted to a standard not less than set

out in AS2870-2011 Clause 6.4.2 (a). Density tests are required, and

certification of the controlled fill necessary where the controlled fill

depth is greater than 400 mm. The testing requirements for controlled fill

are specified in Section 8.2 of AS 3798. The level of supervision

required during the placement of controlled fill is outlined in Appendix

A of AS 3798.

Compaction testing for controlled fill shall be undertaken by a NATA

accredited Soils Laboratory.


TABLE A4

LOCATION OF FILL

MINIMUM RELATIVE COMPACTION, %

MINIMUM DENSITY RATIO

(AT STANDARD COMPACTION EFFORT)

(COHESIVE SOILS)

(SEE NOTE 1)

MINIMUM DENSITY

INDEX

(COHESIONLESS SOILS)

(SEE NOTE 2)

Residential – lot, fill, house, sites

95

(see Note 3)

70

Commercial – fills to support minor

loadings, including floor loadings of up to

20 kPa and isolated pad or strip footings to

100 kPa

98

(see Note 4)

75

Fill to support pavements (see Note 3)

(a) General fill

(b) Subgrade (to a depth of 0.3 m)

95

98

70

75

NOTES:-

1 Density ratio may be either dry density ratio (see AS 1289.5.4.1) or Hilf density

ratio (see AS 1289.5.7.1) as applicable. These test methods require reporting to

the nearest 0.5 % and this is assumed in these values.

2 Density index as a means for control of achieved relative compaction may be

difficult to use and interpret. Local correlations with other methods may exist

and can be used where these are well established.

3 Where pavements will be required to carry a significant volume of heavy

vehicles, the minimum compaction criteria for the upper levels of the fill may

need to be reviewed. For all pavements, it is essential that the specification for

compaction of subgrade materials reflect the condition under which tests carried

out for pavement thickness design are conducted.

5. SITE PREPARATION

After completion of primary earthworks, the site must be prepared for footing

construction. Ideally for raft construction, soils beneath the house area should be

kept in as moist a condition as possible. For strip footings with timber floors the

building area should be kept as dry as possible.

For rafts, provide a base course of a minimum compacted thickness of 100 mm

of quarry rubble or other approved material. The finished surface must be free of

any sharp aggregate which could damage the moisture barrier.


The thickness of the base course may be dictated by Local Council requirements

or statutory requirements giving the relationship between finished floor level,

external paving, flood levels and/or the sewer flood gully. This shall be taken

into account when establishing building platform levels.

6. SITE DRAINAGE

Different moisture variation (ie. wetting or drying) is the main cause of vertical

movement in clay soils. Effective drainage is therefore most important. It

reduces the chance of footings having to cope with extremes of soil movement.

The following list covers the common causes of moisture variation.

Wetting Up

• Leaking sewer, water service pipes or downpipes.

• Downpipes discharging too close to the building.

• Sloping sites and inadequate falls away from the building causing water to

pond or collect close to the building.

• Seepage on sloping sites caused by water travelling on the top soil/clay or

soil/rock interface. Cut off drains are required in this situation.

• Garden or lawn watering immediately adjacent to the footings. As a general

rule this is not acceptable and must not be done except with the explicit

written approval of the Engineer.

• Septic tanks with inadequate soakage trenches, particularly in impervious clay

soils of high swelling potential.

• Flooding during house construction.

Drying Out

• The non provision of paving particularly on the sides of the building coupled

with the non establishment of a garden, (particularly from the rear lawns).

• A change from an established garden situation to a native garden coupled

with a substantially reduced level of watering.

• The most common cause of soil drying is trees being planted too close to the

building. Single trees should be planted a distance from the building equal to

the expected mature height of the tree. Where there is more than one tree in a

row or group this distance should be increased to one and a half times the

maximum height of the largest tree in the group.


In order to control the above factors the following work must be undertaken:

• The building area is to be graded away from the building so water drains

away. On the cut side, ensure water can drain to and away along the base of

the cut. Provide a spoon drain if necessary. During construction provide

temporary stormwater drains.

• On steep or large sites where a significant catchment area is present uphill

from the building, a surface drain must also be constructed across the top of

the embankment.

• Water must not pond within surface footing beams or adjacent to perimeter

footings. If this occurs water must be drained out immediately and the above

grading and drainage implemented at once.

• After footings have been completed the site surface adjacent to the footings

must be graded by additional cutting and/or filling to achieve a positive

surface gradient of not less than 1 in 10 away from the footing for a distance

of not less than one metre. The channel so formed must be graded in turn so

as to discharge all run off away from the building areas. Generally any cut

area shall be drained via a surface drain at the base of the cut embankment

discharging to the low side of the site.

• Establish lawns and gardens around the building as soon as possible

following construction and occupation of the building.

• Ensure all roof stormwater is discharged to the street where possible or

alternatively discharged on the low side of the site not less than 6 m away

from the building but not so as to concentrate a flow of water onto

neighbouring property.

Potential seepage or subsoil drainage problems cannot always be recognised

from the results of a domestic site investigation. All the potential problems with

respect to subsurface water flow or seepage may not be evident until after the

site has been occupied for the first winter.

7. BUILDING CONSTRUCTION & ARTICULATION

It must be realised that there are many factors which affect the performance of

the building. Visible cracking can be caused by shrinkage of roof timbers,

crazing of plaster, expansion of brickwork (brick growth), and shrinkage of

concrete, as well as the most commonly attributed cause, of soil movement.

Generally minor cracking is of no significance and will not detract from the

performance or durability of the building. It is uneconomical if not impossible to

eliminate all such imperfections and they must be considered as "settling in"

movements common to most newly completed dwellings.


It is generally recommended that masonry walls (and in areas of very high soil

movement, also timber framed walls) be articulated at all or some openings.

Articulation involves the incorporation of movement joints at doors and

windows. The location of any mandatory control joints will be given in the

Footing Construction Report together with the required details needed for correct

implementation.

It should be noted that significant economies in footing design may be achieved

by using an articulated superstructure.

Similarly, in building extensions, wherever new masonry abuts existing

masonry, full height mastic filled control joints shall be used.

Because it is very difficult to prevent tilting of any extension relative to an

existing building, the extension must be constructed so as to permit relative

movement between the new and the existing building. This provision for

relative movement applies to all work including floors, tiling, wall and ceiling

fixtures, roofing etc. It should also be borne in mind that the environmental

influence caused by construction of an extension to an existing house may cause

some cracking in the original house superstructure. This cannot be prevented.

8. PAVING REQUIREMENTS

• Concrete pavements shall have a thickness of not less than 100 mm. Where

the soil profile is classified as highly reactive or potentially collapsing, it is

recommended that concrete paving be reinforced with SL72 fabric in a single

top layer, placed 30 mm from the finished surface.

• Control joints shall be provided in accordance with the enclosed standard detail.

• Pavements shall be not less than 900 mm wide and preferably 1.2 metre in

width and shall have a crossfall of not less than 35 mm per 1 metre width,

unless noted otherwise.

• Where the estimated free soil swell is in excess of 40 mm it is recommended

that paving be constructed at the end of winter, when the site soils are wet, so

that crossfalls constructed in the paving will not reduce. It is important,

however, that if the house is occupied during a winter period and no paving is

provided, the soil surface around the perimeter of the house is maintained in a

well drained state until such time as paving is installed.

If, on these soils, it is necessary to construct paving at other times of the year eg.

at the end of summer, the crossfall provided should be not less than 70 mm.

• Paving shall be constructed on a firm clean base. Ensure that all building

debris is removed from the perimeter of the building. Provide a compacted

quarry rubble base if necessary to elevate paving and achieve the necessary

crossfall.


• The paving shall not be constructed above any damp-proof course or built-in

damp-proof membrane.

• On reactive soil sites, it may be found that paving separates horizontally from

the perimeter wall of the house. It is important that any such gaps be

immediately sealed with a flexible mastic sealant.

• Termite management to be in accordance with AS3660.1.

- Where perimeter termite management is installed, concrete

exposure shall not be less than 15mm above the paving on sites

classified as H1-D or less. Concrete exposure on H2, E or P sites

should not be less than 75mm.

- Where there is no perimeter termite treatment management, a

minimum exposure of 75mm of concrete footing is required for

termite management to comply with AS3660.1.

Foundation Maintenance and Footing Performance:A Homeowner’s GuideBuildings can and often do move. This movement can be up, down, lateral or rotational. The fundamental cause of movement in buildings can usually be related to one or more problems in the foundation soil. It is important for the homeowner to identify the soil type in order to ascertain the measures that should be put in place in order to ensure that problems in the foundation soil can be prevented, thus protecting against building movement.

This Building Technology File is designed to identify causes of soil-related building movement, and to suggest methods of prevention of resultant cracking in buildings.

Soil Types The types of soils usually present under the topsoil in land zoned for residential buildings can be split into two approximate groups – granular and clay. Quite often, foundation soil is a mixture of both types. The general problems associated with soils having granular content are usually caused by erosion. Clay soils are subject to saturation and swell/shrink problems.Classifications for a given area can generally be obtained by application to the local authority, but these are sometimes unreliable and if there is doubt, a geotechnical report should be commissioned. As most buildings suffering movement problems are founded on clay soils, there is an emphasis on classification of soils according to the amount of swell and shrinkage they experience with variations of water content. The table below is Table 2.1 from AS 2870-2011, the Residential Slab and Footing Code.

Causes of Movement

Settlement due to construction There are two types of settlement that occur as a result of construction: • Immediate settlement occurs when a building is first placed

on its foundation soil, as a result of compaction of the soil under the weight of the structure. The cohesive quality of clay soil mitigates against this, but granular (particularly sandy) soil is susceptible.

• Consolidation settlement is a feature of clay soil and may take place because of the expulsion of moisture from the soil or because of the soil’s lack of resistance to local compressive or shear stresses. This will usually take place during the first few months after construction, but has been known to take many years in exceptional cases.

These problems are the province of the builder and should be taken into consideration as part of the preparation of the site for construction. Building Technology File 19 (BTF 19) deals with these problems.

ErosionAll soils are prone to erosion, but sandy soil is particularly susceptible to being washed away. Even clay with a sand component of say 10% or more can suffer from erosion.

SaturationThis is particularly a problem in clay soils. Saturation creates a bog- like suspension of the soil that causes it to lose virtually all of its bearing capacity. To a lesser degree, sand is affected by saturation because saturated sand may undergo a reduction in volume, particularly imported sand fill for bedding and blinding layers. However, this usually occurs as immediate settlement and should normally be the province of the builder.

Seasonal swelling and shrinkage of soil All clays react to the presence of water by slowly absorbing it, making the soil increase in volume (see table below). The degree of increase varies considerably between different clays, as does the degree of decrease during the subsequent drying out caused by fair weather periods. Because of the low absorption and expulsion rate, this phenomenon will not usually be noticeable unless there are prolonged rainy or dry periods, usually of weeks or months, depending on the land and soil characteristics. The swelling of soil creates an upward force on the footings of the building, and shrinkage creates subsidence that takes away the support needed by the footing to retain equilibrium.

Shear failure This phenomenon occurs when the foundation soil does not have sufficient strength to support the weight of the footing. There are two major post-construction causes:

• Significant load increase. • Reduction of lateral support of the soil under the footing due to

erosion or excavation.

In clay soil, shear failure can be caused by saturation of the soil adjacent to or under the footing.

GENERAL DEFINITIONS OF SITE CLASSES

Class Foundation

A Most sand and rock sites with little or no ground movement from moisture changes

S Slightly reactive clay sites, which may experience only slight ground movement from moisture changes

M Moderately reactive clay or silt sites, which may experience moderate ground movement from moisture changes

H1 Highly reactive clay sites, which may experience high ground movement from moisture changes

H2 Highly reactive clay sites, which may experience very high ground movement from moisture changes

E Extremely reactive sites, which may experience extreme ground movement from moisture changesNotes1. Where controlled fill has been used, the site may be classified A to E according to the type of fill used.2. Filled sites. Class P is used for sites which include soft fills, such as clay or silt or loose sands; landslip; mine subsidence; collapsing soils; soil subject to erosion;

reactive sites subject to abnormal moisture conditions or sites which cannot be classified otherwise.3. Where deep-seated moisture changes exist on sites at depths of 3 m or greater, further classification is needed for Classes M to E (M-D, H1-D, H2-D and E-D).

BTF 18-2011replaces

Information Sheet 10/91

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Tree root growthTrees and shrubs that are allowed to grow in the vicinity of footings can cause foundation soil movement in two ways: • Roots that grow under footings may increase in cross-sectional

size, exerting upward pressure on footings. • Roots in the vicinity of footings will absorb much of the moisture

in the foundation soil, causing shrinkage or subsidence.

Unevenness of MovementThe types of ground movement described above usually occur unevenly throughout the building’s foundation soil. Settlement due to construction tends to be uneven because of: • Differing compaction of foundation soil prior to construction. • Differing moisture content of foundation soil prior to

construction.

Movement due to non-construction causes is usually more uneven still. Erosion can undermine a footing that traverses the flow or can create the conditions for shear failure by eroding soil adjacent to a footing that runs in the same direction as the flow. Saturation of clay foundation soil may occur where subfloor walls create a dam that makes water pond. It can also occur wherever there is a source of water near footings in clay soil. This leads to a severe reduction in the strength of the soil which may create local shear failure. Seasonal swelling and shrinkage of clay soil affects the perimeter of the building first, then gradually spreads to the interior. The swelling process will usually begin at the uphill extreme of the building, or on the weather side where the land is flat. Swelling gradually reaches the interior soil as absorption continues. Shrinkage usually begins where the sun’s heat is greatest.

Effects of Uneven Soil Movement on Structures

Erosion and saturation Erosion removes the support from under footings, tending to create subsidence of the part of the structure under which it occurs. Brickwork walls will resist the stress created by this removal of support by bridging the gap or cantilevering until the bricks or the mortar bedding fail. Older masonry has little resistance. Evidence of failure varies according to circumstances and symptoms may include: • Step cracking in the mortar beds in the body of the wall or above/

below openings such as doors or windows. • Vertical cracking in the bricks (usually but not necessarily in line

with the vertical beds or perpends).

Isolated piers affected by erosion or saturation of foundations will eventually lose contact with the bearers they support and may tilt or fall over. The floors that have lost this support will become bouncy, sometimes rattling ornaments etc.

Seasonal swelling/shrinkage in clay Swelling foundation soil due to rainy periods first lifts the most exposed extremities of the footing system, then the remainder of the perimeter footings while gradually permeating inside the building footprint to lift internal footings. This swelling first tends to create a dish effect, because the external footings are pushed higher than the internal ones. The first noticeable symptom may be that the floor appears slightly dished. This is often accompanied by some doors binding on the floor or the door head, together with some cracking of cornice mitres. In buildings with timber flooring supported by bearers and joists, the floor can be bouncy. Externally there may be visible dishing of the hip or ridge lines. As the moisture absorption process completes its journey to the innermost areas of the building, the internal footings will rise. If the spread of moisture is roughly even, it may be that the symptoms will temporarily disappear, but it is more likely that swelling will be uneven, creating a difference rather than a disappearance in symptoms. In buildings with timber flooring supported by bearers and joists, the isolated piers will rise more easily than the strip footings or piers under walls, creating noticeable doming of flooring. As the weather pattern changes and the soil begins to dry out, the external footings will be first affected, beginning with the locations where the sun’s effect is strongest. This has the effect of lowering the

external footings. The doming is accentuated and cracking reduces or disappears where it occurred because of dishing, but other cracks open up. The roof lines may become convex. Doming and dishing are also affected by weather in other ways. In areas where warm, wet summers and cooler dry winters prevail, water migration tends to be toward the interior and doming will be accentuated, whereas where summers are dry and winters are cold and wet, migration tends to be toward the exterior and the underlying propensity is toward dishing.

Movement caused by tree roots In general, growing roots will exert an upward pressure on footings, whereas soil subject to drying because of tree or shrub roots will tend to remove support from under footings by inducing shrinkage.

Complications caused by the structure itself Most forces that the soil causes to be exerted on structures are vertical – i.e. either up or down. However, because these forces are seldom spread evenly around the footings, and because the building resists uneven movement because of its rigidity, forces are exerted from one part of the building to another. The net result of all these forces is usually rotational. This resultant force often complicates the diagnosis because the visible symptoms do not simply reflect the original cause. A common symptom is binding of doors on the vertical member of the frame.

Effects on full masonry structures Brickwork will resist cracking where it can. It will attempt to span areas that lose support because of subsided foundations or raised points. It is therefore usual to see cracking at weak points, such as openings for windows or doors. In the event of construction settlement, cracking will usually remain unchanged after the process of settlement has ceased. With local shear or erosion, cracking will usually continue to develop until the original cause has been remedied, or until the subsidence has completely neutralised the affected portion of footing and the structure has stabilised on other footings that remain effective. In the case of swell/shrink effects, the brickwork will in some cases return to its original position after completion of a cycle, however it is more likely that the rotational effect will not be exactly reversed, and it is also usual that brickwork will settle in its new position and will resist the forces trying to return it to its original position. This means that in a case where swelling takes place after construction and cracking occurs, the cracking is likely to at least partly remain after the shrink segment of the cycle is complete. Thus, each time the cycle is repeated, the likelihood is that the cracking will become wider until the sections of brickwork become virtually independent. With repeated cycles, once the cracking is established, if there is no other complication, it is normal for the incidence of cracking to stabilise, as the building has the articulation it needs to cope with the problem. This is by no means always the case, however, and monitoring of cracks in walls and floors should always be treated seriously. Upheaval caused by growth of tree roots under footings is not a simple vertical shear stress. There is a tendency for the root to also exert lateral forces that attempt to separate sections of brickwork after initial cracking has occurred.

Trees can cause shrinkage and damage

Wall crackingdue to unevenlooting settlement

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The normal structural arrangement is that the inner leaf of brickwork in the external walls and at least some of the internal walls (depending on the roof type) comprise the load-bearing structure on which any upper floors, ceilings and the roof are supported. In these cases, it is internally visible cracking that should be the main focus of attention, however there are a few examples of dwellings whose external leaf of masonry plays some supporting role, so this should be checked if there is any doubt. In any case, externally visible cracking is important as a guide to stresses on the structure generally, and it should also be remembered that the external walls must be capable of supporting themselves.

Effects on framed structures Timber or steel framed buildings are less likely to exhibit cracking due to swell/shrink than masonry buildings because of their flexibility. Also, the doming/dishing effects tend to be lower because of the lighter weight of walls. The main risks to framed buildings are encountered because of the isolated pier footings used under walls. Where erosion or saturation causes a footing to fall away, this can double the span which a wall must bridge. This additional stress can create cracking in wall linings, particularly where there is a weak point in the structure caused by a door or window opening. It is, however, unlikely that framed structures will be so stressed as to suffer serious damage without first exhibiting some or all of the above symptoms for a considerable period. The same warning period should apply in the case of upheaval. It should be noted, however, that where framed buildings are supported by strip footings there is only one leaf of brickwork and therefore the externally visible walls are the supporting structure for the building. In this case, the subfloor masonry walls can be expected to behave as full brickwork walls.

Effects on brick veneer structures Because the load-bearing structure of a brick veneer building is the frame that makes up the interior leaf of the external walls plus perhaps the internal walls, depending on the type of roof, the building can be expected to behave as a framed structure, except that the external masonry will behave in a similar way to the external leaf of a full masonry structure.

Water Service and Drainage Where a water service pipe, a sewer or stormwater drainage pipe is in the vicinity of a building, a water leak can cause erosion, swelling or saturation of susceptible soil. Even a minuscule leak can be enough to saturate a clay foundation. A leaking tap near a building can have the same effect. In addition, trenches containing pipes can become watercourses even though backfilled, particularly where broken rubble is used as fill. Water that runs along these trenches can be responsible for serious erosion, interstrata seepage into subfloor areas and saturation. Pipe leakage and trench water flows also encourage tree and shrub roots to the source of water, complicating and exacerbating the problem. Poor roof plumbing can result in large volumes of rainwater being concentrated in a small area of soil: • Incorrect falls in roof guttering may result in overflows, as may

gutters blocked with leaves etc.

• Corroded guttering or downpipes can spill water to ground. • Downpipes not positively connected to a proper stormwater

collection system will direct a concentration of water to soil that is directly adjacent to footings, sometimes causing large-scale problems such as erosion, saturation and migration of water under the building.

Seriousness of Cracking In general, most cracking found in masonry walls is a cosmetic nuisance only and can be kept in repair or even ignored. The table below is a reproduction of Table C1 of AS 2870-2011. AS 2870-2011 also publishes figures relating to cracking in concrete floors, however because wall cracking will usually reach the critical point significantly earlier than cracking in slabs, this table is not reproduced here.

Prevention/Cure

PlumbingWhere building movement is caused by water service, roof plumbing, sewer or stormwater failure, the remedy is to repair the problem. It is prudent, however, to consider also rerouting pipes away from the building where possible, and relocating taps to positions where any leakage will not direct water to the building vicinity. Even where gully traps are present, there is sometimes sufficient spill to create erosion or saturation, particularly in modern installations using smaller diameter PVC fixtures. Indeed, some gully traps are not situated directly under the taps that are installed to charge them, with the result that water from the tap may enter the backfilled trench that houses the sewer piping. If the trench has been poorly backfilled, the water will either pond or f low along the bottom of the trench. As these trenches usually run alongside the footings and can be at a similar depth, it is not hard to see how any water that is thus directed into a trench can easily affect the foundation’s ability to support footings or even gain entry to the subfloor area.

Ground drainage In all soils there is the capacity for water to travel on the surface and below it. Surface water flows can be established by inspection during and after heavy or prolonged rain. If necessary, a grated drain system connected to the stormwater collection system is usually an easy solution. It is, however, sometimes necessary when attempting to prevent water migration that testing be carried out to establish watertable height and subsoil water flows. This subject is referred to in BTF 19 and may properly be regarded as an area for an expert consultant.

Protection of the building perimeter It is essential to remember that the soil that affects footings extends well beyond the actual building line. Watering of garden plants, shrubs and trees causes some of the most serious water problems. For this reason, particularly where problems exist or are likely to occur, it is recommended that an apron of paving be installed around as much of the building perimeter as necessary. This paving should

CLASSIFICATION OF DAMAGE WITH REFERENCE TO WALLS

Description of typical damage and required repairApproximate crack width

limit (see Note 3)Damage category

Hairline cracks <0.1 mm 0

Fine cracks which do not need repair <1 mm 1

Cracks noticeable but easily filled. Doors and windows stick slightly. <5 mm 2

Cracks can be repaired and possibly a small amount of wall will need to be replaced. Doors and windows stick. Service pipes can fracture. Weathertightness often impaired.

5–15 mm (or a number of cracks 3 mm or more in one group)

3

Extensive repair work involving breaking-out and replacing sections of walls, especially over doors and windows. Window and door frames distort. Walls lean or bulge noticeably, some loss of bearing in beams. Service pipes disrupted.

15–25 mm but also depends on number of cracks

4

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extend outwards a minimum of 900 mm (more in highly reactive soil) and should have a minimum fall away from the building of 1:60. The finished paving should be no less than 100 mm below brick vent bases. It is prudent to relocate drainage pipes away from this paving, if possible, to avoid complications from future leakage. If this is not practical, earthenware pipes should be replaced by PVC and backfilling should be of the same soil type as the surrounding soil and compacted to the same density. Except in areas where freezing of water is an issue, it is wise to remove taps in the building area and relocate them well away from the building – preferably not uphill from it (see BTF 19). It may be desirable to install a grated drain at the outside edge of the paving on the uphill side of the building. If subsoil drainage is needed this can be installed under the surface drain.

CondensationIn buildings with a subfloor void such as where bearers and joists support flooring, insufficient ventilation creates ideal conditions for condensation, particularly where there is little clearance between the floor and the ground. Condensation adds to the moisture already present in the subfloor and significantly slows the process of drying out. Installation of an adequate subfloor ventilation system, either natural or mechanical, is desirable. Warning: Although this Building Technology File deals with cracking in buildings, it should be said that subfloor moisture can result in the development of other problems, notably:

• Water that is transmitted into masonry, metal or timber building elements causes damage and/or decay to those elements.

• High subfloor humidity and moisture content create an ideal environment for various pests, including termites and spiders.

• Where high moisture levels are transmitted to the flooring and walls, an increase in the dust mite count can ensue within the living areas. Dust mites, as well as dampness in general, can be a health hazard to inhabitants, particularly those who are abnormally susceptible to respiratory ailments.

The gardenThe ideal vegetation layout is to have lawn or plants that require only light watering immediately adjacent to the drainage or paving edge, then more demanding plants, shrubs and trees spread out in that order. Overwatering due to misuse of automatic watering systems is a common cause of saturation and water migration under footings. If it is necessary to use these systems, it is important to remove garden beds to a completely safe distance from buildings.

Existing trees Where a tree is causing a problem of soil drying or there is the existence or threat of upheaval of footings, if the offending roots are subsidiary and their removal will not significantly damage the tree, they should be severed and a concrete or metal barrier placed vertically in the soil to prevent future root growth in the direction of the building. If it is not possible to remove the relevant roots without damage to the tree, an application to remove the tree should be made to the local authority. A prudent plan is to transplant likely offenders before they become a problem.

Information on trees, plants and shrubs State departments overseeing agriculture can give information regarding root patterns, volume of water needed and safe distance from buildings of most species. Botanic gardens are also sources of information. For information on plant roots and drains, see Building Technology File 17.

ExcavationExcavation around footings must be properly engineered. Soil supporting footings can only be safely excavated at an angle that allows the soil under the footing to remain stable. This angle is called the angle of repose (or friction) and varies significantly between soil types and conditions. Removal of soil within the angle of repose will cause subsidence.

RemediationWhere erosion has occurred that has washed away soil adjacent to footings, soil of the same classification should be introduced and compacted to the same density. Where footings have been undermined, augmentation or other specialist work may be required. Remediation of footings and foundations is generally the realm of a specialist consultant. Where isolated footings rise and fall because of swell/shrink effect, the homeowner may be tempted to alleviate floor bounce by filling the gap that has appeared between the bearer and the pier with blocking. The danger here is that when the next swell segment of the cycle occurs, the extra blocking will push the floor up into an accentuated dome and may also cause local shear failure in the soil. If it is necessary to use blocking, it should be by a pair of fine wedges and monitoring should be carried out fortnightly. This BTF was prepared by John Lewer FAIB, MIAMA, Partner, Construction Diagnosis.

The information in this and other issues in the series was derived from various sources and was believed to be correct when published.

The information is advisory. It is provided in good faith and not claimed to be an exhaustive treatment of the relevant subject.

Further professional advice needs to be obtained before taking any action based on the information provided.

Distributed by

CSIRO PUBLISHING PO Box 1139, Collingwood 3066, AustraliaTel (03) 9662 7666 Fax (03) 9662 7555 www.publish.csiro.au

Email: [email protected]

© CSIRO 2003. Unauthorised copying of this Building Technology File is prohibited

Gardens for a reactive site

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