Council archive · source document
Appendix E - Geotechnical site classification report · MCU25/0022
PROJECT NO.: 23/8894 19th JULY 2023
NORTH COAST GEOMECHANICAL CONSULTANTS
1/36 Maud Street
MAROOCHYDORE QLD 4558
Phone 07 5443 2988
Fax 07 5443 5157
[email protected]
SITE CLASSIFICATION REPORT
PROPOSED RESIDENCE AT
193 SANKEYS ROAD
FEDERAL
FOR LACHLAN ROBINS
TESTING AUTHORITY: CIVIL QUALITY ASSURANCE PTY. LTD.
P.O. BOX 1225, NAMBOUR, QLD. 4560
8894-193 Sankeys Rd-FEDERAL Page 1 of 15 19/07/2023
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PROJECT NO.: 23/8894 19th JULY 2023
NORTH COAST GEOMECHANICAL CONSULTANTS
1/36 Maud Street
MAROOCHYDORE QLD 4558
Phone 07 5443 2988
Fax 07 5443 5157
[email protected]
Site Classification Report for,
Proposed Residence at,
193 Sankeys Road, FEDERAL.
OUR REFERENCE: 23/8894
This report refers to the inspection of the above site and field investigations to determine soil types
and their characteristics to give a Site Classification in terms of Australian Standard 2870 – 2011,
“Residential Slabs and Footings – Construction”. The Inspection and Field and/or Laboratory
Testing was carried out by Civil Quality Assurance Pty.Ltd.
Details are as follows: -
1. SITE: The allotment is situated off the western side of Sankeys Road, FEDERAL. It is a
large heavily timbered allotment described as Lot.6 RP216727 with an area of 6.041 hectares.
Refer locality on front cover of this report. The house site will be situated close to the eastern
road frontage. The natural land surface falls to the south/south west at about 15%. An
excavated platform has been created near the eastern road frontage. The site shows no signs
of surface erosion or instability. Testing was carried out across the platform area and the
locations are shown on the attached site plan.
8894-193 Sankeys Rd-FEDERAL Page 2 of 15 19/07/2023
Page 3
PROJECT NO.: 23/8894 19th JULY 2023
2. SUB-SURFACE INVESTIGATIONS: Boreholes carried out revealed silty clay
topsoils of medium to high plasticity overlying layered and fractured siltstone of Kin Kin
Beds Geological origin. The soils are classified as “CI-CH” and “XW” respectively in
accordance with Table A1 of A.S. 1726 “Geotechnical Site Investigations”. No ground water
was encountered. Detailed borelogs are attached.
Testing was carried out to determine soil-bearing capacities using a 9-kg Dynamic Cone
Penetrometer (DCP) at borehole location 2 and 3. The results of these tests are attached. The
consistency of the soil was found to be firm to stiff silty clays over dense highly weathered
siltstone. Soil bearing capacities of 100 KPa were encountered below 0.3 metres depth at the
test sites.
A sample of the Silty Clay was taken and tested in the laboratory.
The results showed a Liquid Limit of 38.5% and a Linear Shrinkage of 14.5%. These clays
were assessed to be moderately to highly reactive to soil moisture variations and therefore
ground movements (Ys) of between 20mm to 40mm are likely.
3. SITE CLASSIFICATION: From the test results viewed, we would classify the site as
equivalent to: - CLASS “M”
-in terms of The Australian Standard 2870, “Residential Slabs and Footings”.
Should site earthworks substantially alter the subgrade from that shown on the
attached borelogs, this classification may vary.
4. FOUNDATION RECOMMENDATIONS:
All site earthworks carried out must comply with A.S.3798 “Guidelines on Earthworks for
Commercial and Residential Development”, to validate this site classification.
All topsoils containing grass, tree roots and other organic material shall be removed from the
building footprint prior to construction commencing.
Structural Filling should be placed and compacted to a minimum dry density ratio of 95%
Standard. Field density testing should be carried out to assess the standard of compaction
achieved to be considered as “Controlled Fill” as specified in AS2870. Any filling should be
keyed into the natural slope (by benched key interface) and battered no steeper than 2.5H:1V
unless filling is supported by an engineer designed Retaining Structure.
Foundations for construction on isolated columns should be supported on mass concrete
bored piers taken into underlying highly weathered siltstone having a minimum bearing
pressure of 100KPa. Depths should also be adequate for the resistance to uplift and racking
forces.
For a proposed lightweight structure with slab on ground construction, we recommend
foundations and slab be designed as a stiffened raft slab designed in accordance with Figure
3.1 of A.S. 2870 for a Class M site. All footings shall be founded through any fill into
underlying dense weathered siltstone.
8894-193 Sankeys Rd-FEDERAL Page 3 of 15 19/07/2023
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PROJECT NO.: 23/8894 19th JULY 2023
4. FOUNDATION RECOMMENDATIONS (Cont.):
Where slabs are constructed over any Uncontrolled Fill, the slab foundations will need to
assessed by the design engineer with all foundations supported through the fill on piers into
the underlying dense weathered siltstone
All foundations shall be designed by an experienced registered professional engineer.
Attention should also be given to erosion protection of cut and filled batters.
It is recommended that all batters incorporate crest and toe drainage. The batters should also
be covered with topsoil and vegetation to provide long term erosion protection.
Before construction proceeds the locations and depths of any surrounding underground
services should be determined to ensure they do not interfere with the proposed
foundations.
5. GENERAL: The test sites have been located to be reasonably representative of foundation
conditions. Should any variation of foundation conditions be encountered during
construction, the Engineer should be notified before proceeding, so any necessary changes to
construction can be made. Footings excavation and reinforcement placement should be
checked before the placement of concrete.
Based on the assessment of the Field Testing, the site is considered suitable for the proposed
development provided the recommendations set out in this report are adhered to.
Yours faithfully,
NORTH COAST GEOMECHANICAL CONSULTANTS
KEN CROCKER R.P.E.Q. No. 638
8894-193 Sankeys Rd-FEDERAL Page 4 of 15 19/07/2023
Page 5
SITE PLAN
TEST LOCATIONS
SANKEYS ROAD 193 SANKEYS ROAD, FEDERAL
NORTH
SS E
ACC
BH2
P1
BH3
BH1 P2
Cut Bank
BH4
193
8894-193 Sankeys Rd-FEDERAL Page 5 of 15 19/07/2023
Page 6
Civil Quality Assurance T/A _ ABN 21 613397934 Civil Quality Assurance Pty Ltd P.O. Box 1225, Nambour Qld 4560 For all Civil Testing PH 0429 674334 Servicing South East Queensland Project:__ 193 Sankeys Rd Requested By:__L.Robins Federal Date Tested:__30-6-23 Client: _ Lachlan Robins Report No.:__A191 Drilling Method: Log of cutting Bore Hole No.:1 Depth Description Comment Mm 00 Silty CLAY, medium to high plasticity, brown, some fine gravel/sand and some organics present, moist, soft to firm (CI-CH) 300 SHALE STONE, fractured, light red/brown, quartz present, moist, weak 1500 Hole Terminated Tests and sampling results relate only to the items tested or sampled. results applies to the sample received/( abnormalities may contribute to the report being compromised) Reports shall not be reproduced except in full 6 Mill Lane Nambour CQA Laboratory ABN 21 61339 7934 Form R26 July 2021 8894-193 Sankeys Rd-FEDERAL Approved Person: C. —— C. Templeton Date: 10-7-23 Page 6 of 15 19/07/2023
Page 7
Civil Quality Assurance T/A _ ABN 21 613397934 Civil Quality Assurance Pty Ltd PH 0429 674334 Project:__193 Sankeys Rd Federal Client: __ Lachlan Robins P.O. Box 1225, Nambour Qld 4560 For all Civil Testing Servicing South East Queensland Requested By:__L.Robins Date Tested:__30-6-23 Report No.:__A191 Drilling Method: Hand Augured bore hole Bore Hole No.:2 Depth Description Comment Mm 00 Silty CLAY, medium to high plasticity, brown, some fine gravel/sand and organics present, 300 500 moist, soft to firm (CI-CH) SHALE STONE, fractured, light red/brown, quartz present, moist, weak Hole Terminated in shale stone ‘Tests and sampling results relate only to the items tested or sampled. results applies to the sample received/( abnormalities may contribute to the report being 6 Mill Lane Nambour CQA Laboratory ABN 21 61339 7934 Form R26 July 2021 compromised) Reports shall not be reproduced except in full Co — Approved Person: of C. Templeton Date: 10-7-23 Page 7 of 15 19/07/2023 8894-193 Sankeys Rd-FEDERAL
Page 8
Civil Quality Assurance T/A _ ABN 21 613397934 P.O. Box 1225, Nambour Qld 4560 Civil Quality Assurance Pty Ltd For all Civil Testing PH 0429 674334 Servicing South East Queensland Project:__193 Sankeys Rd Requested By:__L.Robins Federal Date Tested:__30-6-23 Client: __ Lachlan Robins Report No.:__A191 Drilling Method: Hand Augured bore hole Bore Hole No.:3 Depth Description Comment Mm 00 Silty CLAY, medium to high plasticity, brown, some fine gravel/sand and organics present, moist, soft to firm (CI-CH) 400 SHALE STONE, fractured, light red/brown, quartz present, moist, weak 500 Hole Terminated in shale stone Tests and sampling results relate only to the items tested or sampled. results applies to the sample received/( abnormalities may contribute to the report being compromised) Reports shall not be reproduced except in full > FT Approved Person: Gurl 6 Mill Lane Nambour C. Templeton CQA Laboratory ABN 21 61339 7934 Date: 10-7-23 Form R26 July 2021 8894-193 Sankeys Rd-FEDERAL Page 8 of 15 19/07/2023
Page 9
Civil Quality Assurance T/A ABN 21 613397934 P.O. Box 1225, Nambour Qld 4560 Civil Quality Assurance Pty Ltd For all Civil Testing PH 0429 674334 Servicing South East Queensland Project:__193 Sankeys Rd Requested By:__L.Robins Federal Date Tested:__30-6-23 Client: __Lachlan Robins Report No.:__A191 Drilling Method: Hand Augured bore hole Bore Hole No.:4 Depth Description Comment Mm 00 Silty CLAY, medium to high plasticity, brown, some fine gravel/sand and organics present, moist, soft to firm (CI-CH) 500 SHALE STONE, fractured, light red/brown, quartz present, moist, weak 600 Hole Terminated in shale stone Tests and sampling results relate only to the items tested or sampled. results applies to the sample received/( abnormalities may contribute to the report being compromised) Reports shall not be reproduced except in full " me " ° . Approved Person: Ce Em 6 Mill Lane Nambour C. Templeton CQA Laboratory ABN 21 61339 7934 Date: 10-7-23 Form R26 July 2021 8894-193 Sankeys Rd-FEDERAL Page 9 of 15 19/07/2023
Page 10
ABN 21 613397934 CQA Laboratory P.O. Box 1225, Nambour Qld 4560 6 Mill Lane Nambour Civil Quality Assurance Pty Ltd T/A For all Geotechnical Engineering and Civil Testing Ph 0429674334 Servicing Queensland Test Report CQA Laboratory Accredited for compliance with ISO/IEC 17025 - testin: Dynamic Cone Penetrometer AS1289 6.3.2 Project:- 193 Sankeys Rd Requested By:L. Robins Federal Date Tested: 30-6-23 Client: Lachlan Robins Report No.: A191 Laboratory Test Procedure: AS1289 6.3.2 Reduced ground level at test site(mm): 00 Moisture Condition: moist Sample/ Material Description: See attached Water Table(mm): na Depth below surface at commencement of Penetration(mm): 0.0 Depth Location of test (mm) Blows/100mm Location | Pl at Approx. P2 at Approx. Approx. Approx. Approx. KPA KPA KPA KPA KPA BH2 equivalent BH3 equivalent equivalent equivalent equivalent 00 00 00 100 3 75 3 75 200 3 75 3 75 300 4 100 3 75 400 5 125 4 100 500 8 200 6 150 600 12 300 10 250 700 12 300 10 250 800 12 300 12 300 900 a = 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 Tests and sampling results relate only to the items tested or sampled. results applies to the sample received/( abnormalities may contribute to the report being compromised) Reports shall not be reproduced except in full. KPa have been developed in = CQA Laboratory ABN 21 613397934 Approved Person: Cof 6 Mill Lane Nambour C. Templeton Form R5 July 2021 Date Reported: 10-7-23 8894-193 Sankeys Rd-FEDERAL Page 10 of 15 19/07/2023
Page 11
ABN 21 613397934 CQA Laboratory P.O. Box 1225, Nambour QlId 4560 6 Mill Lane Nambour Civil Quality Assurance Pty itd T/A For all Geotechnical Engineering and Civil Testing Ph 0429674334 Servicing South East Queensland Tests and sampling results relate only to the items tested or sampled. results applies to the sample received/( abnormalities may contribute to the report being compromised) Reports shall not be reproduced except in full Test Report CQA Laboratory Accredited for compliance with ISO/IEC 17025 - testing Project:- 193 Sankeys Rd Requested By: L. Robins Federal Date Tested: 30-6-23 Client: Lachlan Robins Report No.: A191 Laboratory Test Procedure: AS1289 3.6.1 soil classification tests-determination of the particle size distribution of a soil-standard method of analysis by sieving, AS1289 3.9.2 Soil classification tests- determination of the cone liquid limit of soil-one point method, AS1289 3.4.1 Soil classification tests-determination of the linear shrinkage of a soil-standard method. Sample Method: AS1289.1.2.1 (6) hand augured bore hole sample method Date Sampled: 30-6-23 Sample/ Material Description: BH2 200-500 Material Source: see attached Bore hole Nominal Size(mm): <0.425mm Liquid Limit (%): 38.5 Linear Shrinkage (%): 14.5 Percentage of material retained on 0.425mm sieve (%): 34.5 Tests and sampling results relate only to the items tested or sampled. results applies to the sample received/( abnormalities may contribute to the report being compromised) Reports shall not be reproduced except in full NATA Accredited facility No.: 20084 —_ COA Laboratory Signatory, C_./ Ca ABN 21 613397934 6 Mill Lane Nambour C. Templeton Form R1 July 2021 Date Reported: 10-7-23 8894-193 Sankeys Rd-FEDERAL Page 11 of 15 19/07/2023
Page 12
Foundation Maintenance
and Footing Performance: BTF 18-2011
replaces
A Homeowner’s Guide Information
Sheet 10/91
Buildings 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 Erosion
The types of soils usually present under the topsoil in land zoned for All soils are prone to erosion, but sandy soil is particularly susceptible
residential buildings can be split into two approximate groups – to being washed away. Even clay with a sand component of say 10%
granular and clay. Quite often, foundation soil is a mixture of both or more can suffer from erosion.
types. The general problems associated with soils having granular
content are usually caused by erosion. Clay soils are subject to Saturation
saturation and swell/shrink problems. This is particularly a problem in clay soils. Saturation creates a bog-
Classifications for a given area can generally be obtained by like suspension of the soil that causes it to lose virtually all of its
application to the local authority, but these are sometimes unreliable bearing capacity. To a lesser degree, sand is affected by saturation
and if there is doubt, a geotechnical report should be commissioned. because saturated sand may undergo a reduction in volume,
As most buildings suffering movement problems are founded on clay particularly imported sand fill for bedding and blinding layers.
soils, there is an emphasis on classification of soils according to the However, this usually occurs as immediate settlement and should
amount of swell and shrinkage they experience with variations of normally be the province of the builder.
water content. The table below is Table 2.1 from AS 2870-2011, the
Residential Slab and Footing Code. Seasonal swelling and shrinkage of soil
All clays react to the presence of water by slowly absorbing it,
Causes of Movement making the soil increase in volume (see table below). The degree of
increase varies considerably between different clays, as does the
Settlement due to construction degree of decrease during the subsequent drying out caused by fair
There are two types of settlement that occur as a result of weather periods. Because of the low absorption and expulsion rate,
construction: this phenomenon will not usually be noticeable unless there are
prolonged rainy or dry periods, usually of weeks or months,
• Immediate settlement occurs when a building is first placed depending on the land and soil characteristics.
on its foundation soil, as a result of compaction of the soil under
the weight of the structure. The cohesive quality of clay soil The swelling of soil creates an upward force on the footings of the
mitigates against this, but granular (particularly sandy) soil is building, and shrinkage creates subsidence that takes away the
support needed by the footing to retain equilibrium.
susceptible.
• Consolidation settlement is a feature of clay soil and may take Shear failure
place because of the expulsion of moisture from the soil or because
This phenomenon occurs when the foundation soil does not have
of the soil’s lack of resistance to local compressive or shear sufficient strength to support the weight of the footing. There are
stresses. This will usually take place during the first few months two major post-construction causes:
after construction, but has been known to take many years in
exceptional cases. • Significant load increase.
• Reduction of lateral support of the soil under the footing due to
These problems are the province of the builder and should be taken
into consideration as part of the preparation of the site for erosion or excavation.
construction. Building Technology File 19 (BTF 19) deals with these In clay soil, shear failure can be caused by saturation of the soil
problems. 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 changes
Notes
1. 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).
8894-193 Sankeys Rd-FEDERAL Page 12 of 15 19/07/2023
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Tree root growth
Trees 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 Movement
The types of ground movement described above usually occur
unevenly throughout the building’s foundation soil. Settlement due looting settlement
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 external footings. The doming is accentuated and cracking reduces
still. Erosion can undermine a footing that traverses the flow or can or disappears where it occurred because of dishing, but other
create the conditions for shear failure by eroding soil adjacent to a cracks open up. The roof lines may become convex.
footing that runs in the same direction as the flow. Doming and dishing are also affected by weather in other ways. In
Saturation of clay foundation soil may occur where subfloor walls create areas where warm, wet summers and cooler dry winters prevail,
a dam that makes water pond. It can also occur wherever there is a water migration tends to be toward the interior and doming will be
source of water near footings in clay soil. This leads to a severe accentuated, whereas where summers are dry and winters are cold
reduction in the strength of the soil which may create local shear failure. and wet, migration tends to be toward the exterior and the
Seasonal swelling and shrinkage of clay soil affects the perimeter of the underlying propensity is toward dishing.
building first, then gradually spreads to the interior. The swelling Movement caused by tree roots
process will usually begin at the uphill extreme of the building, or on In general, growing roots will exert an upward pressure on footings,
the weather side where the land is flat. Swelling gradually reaches the whereas soil subject to drying because of tree or shrub roots will tend
interior soil as absorption continues. Shrinkage usually begins where to remove support from under footings by inducing shrinkage.
the sun’s heat is greatest.
Effects of Uneven Soil Movement on Structures Complications caused by the structure itself
Most forces that the soil causes to be exerted on structures are
Erosion and saturation vertical – i.e. either up or down. However, because these forces are
Erosion removes the support from under footings, tending to create seldom spread evenly around the footings, and because the building
subsidence of the part of the structure under which it occurs. resists uneven movement because of its rigidity, forces are exerted
Brickwork walls will resist the stress created by this removal of from one part of the building to another. The net result of all these
support by bridging the gap or cantilevering until the bricks or the forces is usually rotational. This resultant force often complicates
mortar bedding fail. Older masonry has little resistance. Evidence of the diagnosis because the visible symptoms do not simply reflect the
failure varies according to circumstances and symptoms may include: original cause. A common symptom is binding of doors on the
• Step cracking in the mortar beds in the body of the wall or vertical member of the frame.
above/ below openings such as doors or windows. Effects on full masonry structures
• Vertical cracking in the bricks (usually but not necessarily in Brickwork will resist cracking where it can. It will attempt to span
line with the vertical beds or perpends). areas that lose support because of subsided foundations or raised
Isolated piers affected by erosion or saturation of foundations will points. It is therefore usual to see cracking at weak points, such as
eventually lose contact with the bearers they support and may tilt or openings for windows or doors.
fall over. The floors that have lost this support will become bouncy, In the event of construction settlement, cracking will usually remain
sometimes rattling ornaments etc. unchanged after the process of settlement has ceased.
With local shear or erosion, cracking will usually continue to develop
Seasonal swelling/shrinkage in clay until the original cause has been remedied, or until the subsidence
Swelling foundation soil due to rainy periods first lifts the most exposed has completely neutralised the affected portion of footing and the
extremities of the footing system, then the remainder of the perimeter structure has stabilised on other footings that remain effective.
footings while gradually permeating inside the building footprint to lift
internal footings. This swelling first tends to create a dish effect, In the case of swell/shrink effects, the brickwork will in some cases
because the external footings are pushed higher than the internal ones. return to its original position after completion of a cycle, however it
is more likely that the rotational effect will not be exactly reversed,
The first noticeable symptom may be that the floor appears slightly and it is also usual that brickwork will settle in its new position and
dished. This is often accompanied by some doors binding on the will resist the forces trying to return it to its original position. This
floor or the door head, together with some cracking of cornice means that in a case where swelling takes place after construction
mitres. In buildings with timber flooring supported by bearers and and cracking occurs, the cracking is likely to at least partly remain
joists, the floor can be bouncy. Externally there may be visible after the shrink segment of the cycle is complete. Thus, each time the
dishing of the hip or ridge lines. cycle is repeated, the likelihood is that the cracking will become
As the moisture absorption process completes its journey to the wider until the sections of brickwork become virtually independent.
innermost areas of the building, the internal footings will rise. If the With repeated cycles, once the cracking is established, if there is no
spread of moisture is roughly even, it may be that the symptoms will other complication, it is normal for the incidence of cracking to
temporarily disappear, but it is more likely that swelling will be stabilise, as the building has the articulation it needs to cope with the
uneven, creating a difference rather than a disappearance in problem. This is by no means always the case, however, and
symptoms. In buildings with timber flooring supported by bearers and monitoring of cracks in walls and floors should always be treated
joists, the isolated piers will rise more easily than the strip footings or seriously.
piers under walls, creating noticeable doming of flooring.
Upheaval caused by growth of tree roots under footings is not a
As the weather pattern changes and the soil begins to dry out, the simple vertical shear stress. There is a tendency for the root to also
external footings will be first affected, beginning with the locations exert lateral forces that attempt to separate sections of brickwork
where the sun’s effect is strongest. This has the effect of lowering the after initial cracking has occurred.
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Page 14
The normal structural arrangement is that the inner leaf of brickwork in Corroded guttering or downpipes can spill water to ground.
the external walls and at least some of the internal walls (depending on the • Downpipes not positively connected to a proper stormwater
roof type) comprise the load-bearing structure on which any upper floors, collection system will direct a concentration of water to soil that
ceilings and the roof are supported. In these cases, it is internally visible is directly adjacent to footings, sometimes causing large-scale
cracking that should be the main focus of attention, however there are a problems such as erosion, saturation and migration of water under
few examples of dwellings whose external leaf of masonry plays some the building.
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 Seriousness of Cracking
walls must be capable of supporting themselves. 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
Effects on framed structures reproduction of Table C1 of AS 2870-2011.
Timber or steel framed buildings are less likely to exhibit cracking due to AS 2870-2011 also publishes figures relating to cracking in concrete
swell/shrink than masonry buildings because of their flexibility. floors, however because wall cracking will usually reach the critical point
Also, the doming/dishing effects tend to be lower because of the lighter significantly earlier than cracking in slabs, this table is not reproduced
weight of walls. The main risks to framed buildings are encountered here.
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 Prevention/Cure
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 Plumbing
or window opening. It is, however, unlikely that framed structures will be Where building movement is caused by water service, roof plumbing,
so stressed as to suffer serious damage without first exhibiting some or all sewer or stormwater failure, the remedy is to repair the problem. It is
of the above symptoms for a considerable period. The same warning prudent, however, to consider also rerouting pipes away from the
period should apply in the case of upheaval. It should be noted, however, building where possible, and relocating taps to positions where any
that where framed buildings are supported by strip footings there is only leakage will not direct water to the building vicinity. Even where gully
one leaf of brickwork and therefore the externally visible walls are the traps are present, there is sometimes sufficient spill to create erosion or
supporting structure for the building. In this case, the subfloor masonry saturation, particularly in modern installations using smaller diameter
walls can be expected to behave as full brickwork walls. PVC fixtures. Indeed, some gully traps are not situated directly under
the taps that are installed to charge them, with the result that water
Effects on brick veneer structures from the tap may enter the backfilled trench that houses the sewer
Because the load-bearing structure of a brick veneer building is the frame piping. If the trench has been poorly backfilled, the water will either
that makes up the interior leaf of the external walls plus perhaps the pond or flow along the bottom of the trench. As these trenches usually
internal walls, depending on the type of roof, the building can be expected run alongside the footings and can be at a similar depth, it is not hard
to behave as a framed structure, except that the external masonry will to see how any water that is thus directed into a trench can easily affect
behave in a similar way to the external leaf of a full masonry structure. the foundation’s ability to support footings or even gain entry to the
subfloor area.
Water Service and Drainage
Where a water service pipe, a sewer or stormwater drainage pipe is in the Ground drainage
vicinity of a building, a water leak can cause erosion, swelling or saturation In all soils there is the capacity for water to travel on the surface and
of susceptible soil. Even a minuscule leak can be enough to saturate a clay below it. Surface water flows can be established by inspection during and
foundation. A leaking tap near a building can have the same effect. In after heavy or prolonged rain. If necessary, a grated drain system
addition, trenches containing pipes can become watercourses even though connected to the stormwater collection system is usually an easy solution.
backfilled, particularly where broken rubble is used as fill. Water that It is, however, sometimes necessary when attempting to prevent water
runs along these trenches can be responsible for serious erosion, migration that testing be carried out to establish watertable height and
interstrata seepage into subfloor areas and saturation. subsoil water flows. This subject is referred to in BTF 19 and may
Pipe leakage and trench water flows also encourage tree and shrub roots properly be regarded as an area for an expert consultant.
to the source of water, complicating and exacerbating the problem. Poor Protection of the building perimeter
roof plumbing can result in large volumes of rainwater being concentrated It is essential to remember that the soil that affects footings extends
in a small area of soil: well beyond the actual building line. Watering of garden plants, shrubs
• Incorrect falls in roof guttering may result in overflows, as may and trees causes some of the most serious water problems.
gutters blocked with leaves etc. 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
Approximate crack width Damage
Description of typical damage and required repair limit (see Note 3) 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 5–15 mm (or a number of cracks 3
replaced. Doors and windows stick. Service pipes can fracture. Weathertightness 3 mm or more in one group)
often impaired.
Extensive repair work involving breaking-out and replacing sections of walls, 15–25 mm but also depends on 4
especially over doors and windows. Window and door frames distort. Walls lean number of cracks
or bulge noticeably, some loss of bearing in beams. Service pipes disrupted.
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Page 15
Gardens for a reactive site • 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 garden
The 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.
Excavation
Excavation around footings must be properly engineered. Soil
extend outwards a minimum of 900 mm (more in highly reactive soil) and supporting footings can only be safely excavated at an angle that allows
should have a minimum fall away from the building of 1:60. The finished the soil under the footing to remain stable. This angle is called the angle
paving should be no less than 100 mm below brick vent bases. of repose (or friction) and varies significantly between soil types and
It is prudent to relocate drainage pipes away from this paving, if possible, conditions. Removal of soil within the angle of repose will cause
to avoid complications from future leakage. If this is not practical, subsidence.
earthenware pipes should be replaced by PVC and backfilling should be Remediation
of the same soil type as the surrounding soil and compacted to the same Where erosion has occurred that has washed away soil adjacent to
density. footings, soil of the same classification should be introduced and
Except in areas where freezing of water is an issue, it is wise to remove compacted to the same density. Where footings have been undermined,
taps in the building area and relocate them well away from the building – augmentation or other specialist work may be required. Remediation of
preferably not uphill from it (see BTF 19). footings and foundations is generally the realm of a specialist consultant.
It may be desirable to install a grated drain at the outside edge of the Where isolated footings rise and fall because of swell/shrink effect, the
paving on the uphill side of the building. If subsoil drainage is needed this homeowner may be tempted to alleviate floor bounce by filling the gap
can be installed under the surface drain. 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
Condensation extra blocking will push the floor up into an accentuated dome and may
In buildings with a subfloor void such as where bearers and joists support also cause local shear failure in the soil. If it is necessary to use blocking,
flooring, insufficient ventilation creates ideal conditions for it should be by a pair of fine wedges and monitoring should be carried
condensation, particularly where there is little clearance between the floor out fortnightly.
and the ground. Condensation adds to the moisture already present in the This BTF was prepared by John Lewer FAIB, MIAMA, Partner,
subfloor and significantly slows the process of drying out. Installation of Construction Diagnosis.
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:
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 Locked Bag 10, Clayton South VIC 3169 Tel (03)
9545 8400 1300 788 000 www.publish.csiro.au Email:
[email protected]
© CSIRO 2003. Unauthorised copying of this Building Technology File is prohibited
8894-193 Sankeys Rd-FEDERAL Page 15 of 15 19/07/2023
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