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Report on Geotechnical Investigation Proposed Residential Development Arcos Project: 240572
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Attention: Sealand Architects 29 August 2024
Arcos Project: 240572
Report 01 Rev A
Report on Geotechnical Investigation
Proposed Residential Development
2/27 Ross Crescent, Sunshine Beach
Dear Team,
With reference to the above project, please find enclosed our geotechnical report for the project at the above
location.
Use of this report is in accordance with our terms and conditions.
If you have any questions, please do not hesitate to contact on the below signed.
Kind Regards,
Engineering Geologist Senior Geotechnical Engineer
Bradley Munro B.Sc (Geo), PMEG Andre Dos Santos B.Eng (Civ), MIEAust, RPEQ,
CPEng, NER
[email protected] [email protected]
For and On Behalf of Arcos Group For and on Behalf of Arcos Group
Document Control
Version Date Details Prepared Checked Approved
Rev A 29 August 2024 Report Issue BM ADS ADS
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Contents
1 INTRODUCTION ........................................................................................................................................ 4
2 SITE DESCRIPTION .................................................................................................................................. 4
3 FIELDWORK METHODS ........................................................................................................................... 6
4 FIELDWORK RESULTS ............................................................................................................................ 6
5 COMMENTS ............................................................................................................................................... 6
5.1 EARTHWORKS COMMENTS .............................................................................................................. 6
5.2 BATTERED EXCAVATIONS ................................................................................................................ 7
5.3 SITE CLASSIFICATION ....................................................................................................................... 7
5.4 FOUNDATIONS.................................................................................................................................... 7
5.5 RETAINING WALLS ............................................................................................................................. 8
5.6 STABILITY AND LANDSLIP MAPPING ............................................................................................... 9
6 LIMITATIONS ........................................................................................................................................... 10
Appendix A – Explanatory Notes ..................................................................................................................... 11
Appendix B – Drawings ................................................................................................................................... 12
Appendix C – Field Work Results .................................................................................................................... 13
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1 INTRODUCTION
Arcos Unit Group Pty Ltd (Arcos) was commissioned to undertake a geotechnical investigation for the
proposed residential development located 2/27 Ross Crescent, Sunshine Beach.
It is understood that the proposed development will comprise a new garage and guest pavilion (two-storey) as
well as a standalone home office. The two-storey addition will also include a lift. Bulk earthworks comprising
cuts of up to approximately 1.8 m are anticipated as part of the works.
This report outlines the results of the investigation, analysis, and interpretive reporting on the following items:
• All factual information resulting from the investigation (investigation methodologies, desktop review, test
location plans, bore logs);
• Summary of subsurface conditions and adopted subsurface conditions including groundwater;
• Earthworks and site preparation comments;
• Foundation soil reactivity in accordance with AS2870;
• Foundation options and design parameters;
• Geotechnical retaining wall design parameters;
• Slope stability assessment in accordance with AGS Guidelines; and
• Construction constraints and risks (if any) from a geotechnical context.
This report can only be used and relied upon under our terms and conditions of engagement. The report cannot
be separated for any reason or redistributed without our written authority.
2 SITE DESCRIPTION
The site is located at 2/27 Ross Crescent, Sunshine Beach and is described as Lot 2 on GTP100987. The site
is broadly rectangular in shape and is occupied by an existing terraced dwelling. The terraces are supported
by a series of rock grouted and boulder retaining walls. The remainder of the site is moderately vegetated with
shrubbery and semi-mature trees measuring up to approximately 5-10 m high. Topographically, the site falls
towards the coastline in the east at grades at between 20˚ and 30˚.
FIGURE 1: OVERVIEW OF SITE, LOOKING EAST FROM SOUTHWESTERN CORNER OF SITE
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FIGURE 2: OVERVIEW OF SITE, LOOKING NORTH-WEST
At the time of investigation, there was no evidence of distress in the existing residence or retaining walls that
would normally be attributed to slope instability. In addition, the existing slope (ie. sand dune) at the site did
not show any signs of disturbed ground, tension cracks or other signs which are normally associated with slope
instability.
The regional geology overlay from available geology sources is given in Figure 3. The mapping indicates that
the site is underlain by by Pleistocene aged dune sand. The subsurface conditions encountered in the bores
comprised sand, inferred to be Aeolian deposits (ie. wind deposited sand dunes).
Site location
FIGURE 3: GEOLOGICAL EXTRACT FROM ARCOS MAPS
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3 FIELDWORK METHODS
The fieldwork for the investigation was undertaken on 19 August 2024 and included the following:
• Three (3) bores drilled and sampled in accessible locations to various depths utilising hand-held auger
equipment; and
• A dynamic cone penetrometer (DCP) test was undertaken at surface level at each bore.
A suitably experienced and qualified geotechnician was present during the fieldwork to carry out site
assessments, perform insitu testing and to log the encountered subsurface conditions through observation of
drilling and excavation techniques, returned cuttings, and anecdotal evidence provided by drilling staff.
4 FIELDWORK RESULTS
The subsurface conditions encountered in the bores are described in detail on the log sheets in Appendix C,
together with accompanying notes in Appendix A which define the classification methods and descriptive terms
used. The depths were measured below existing surface levels at the time of investigation.
The subsurface conditions encountered on site can be broadly categorised in Table 1.
TABLE 1: SUMMARY OF SUBSURFACE CONDITIONS
Material Information
SAND The site can be characterised as comprising dune sand. The sand is very loose
initially, grading medium dense below approximately 2 m depth. The medium dense
sand is the target stratum for footings.
No groundwater seepage was encountered in the bores at the time of investigation; However, given the nature
of the subsurface conditions, it is paramount to give consideration to how groundwater levels can fluctuate
with time and are transient in nature and how they could be perched or how they could be introduced from
foreign sources (such as broken water infrastructure).
5 COMMENTS
5.1 EARTHWORKS COMMENTS
Prior to any construction at the site, any deleterious materials and vegetation, should be stripped from within
building envelopes.
During construction, some loosening of the surface sands in excavations is expected. Therefore, the base of
any excavation should be soaked and re-compacted using a vibratory plate compactor prior to constructing
any footings/slabs. Confirmation of adequate compaction of sands could be obtained by carrying out DCP
tests to achieve the above target value.
If filling is required, it is recommended that sand at this site and imported fill be placed in loose lift thickness of
not more than 200 mm, within 2% of optimum moisture content, and with each layer compacted to achieve not
less than 8 blows per 300 mm rod penetration to a depth of 1.0 m, when using a DCP.
Imported fill, if required, should comprise free draining cohesionless sand with less than 5% by weight of
particles passing a 0.075 mm sieve. The material should be free from organic matter and large particles.
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5.2 BATTERED EXCAVATIONS
The sand will tend to collapse when cut to near vertical. It is suggested that an allowance for battering at
1H:1V of temporary shallow (<1.5 m depth) excavations be made in planning purposes.
5.3 SITE CLASSIFICATION
In strict accordance with AS 2870, due to the presence of trees and very loose sand with an assessed allowable
bearing capacity of less than 100 kPa, the site in its present state would be given a ‘Class P’ (problem site)
classification, requiring design by engineering principles.
It should be noted that the site is likely to be subject to Class S characteristic surface movements (ys) were it
not for the Class P.
The developed site should be maintained in accordance with the CSIRO publication "Foundation Maintenance
and Footing Performance: A Homeowners Guide", a copy of which is included in (see attachments). The guide
describes suggested site maintenance practices aimed at limiting foundation movements and at keeping
cracking within acceptable limits.
5.4 FOUNDATIONS
The use of steel screw piles (raked for lateral support) with a pile cap could be adopted for lightly loaded
structures requiring minimal lateral resistance. Screw piles could be designed using the ultimate values
indicated in Table 2 below.
TABLE 2: DEEP LEVEL FOUNDATION GEOTECHNICAL DESIGN PARAMETERS (ULTIMATE)
Material fb (kPa) fs (kPa)
Medium dense sand (below approx. 2 m) 600 NR
Notes: fb = Ultimate base bearing pressure for compression pile; fS = Ultimate shaft adhesion pressure for compression
or tension pile; NR = Not recommended.
The pile parameters presented above are ultimate values. Arcos has not been provided with adequate
information to determine:
• Each weighting factor (wi) of Table 4.3.2(A) of AS2159-2009
• Each risk factor (IRR) of Table 4.3.2(B) of AS2159-2009
In the absence of more detailed information, the most conservative selection has been adopted. In accordance
with Table 4.3.2(C) of AS2159-2009, the geotechnical strength reduction factor (Øgb) would be:
• 0.40 for low redundancy piling system; or,
• 0.47 for high redundancy piling system.
A higher value of Øgb may be applied if further information is available to reassess the above items.
The ultimate geotechnical strength (Rd,ug) of steel and bored piles can be calculated using the ultimate shaft
adhesion and end bearing values given in Table 2. The values given in Table 2 will need to be reduced by
applying a suitable geotechnical strength reduction factor (Φg) for limit state.
To achieve the base pressure, it is recommended that piles are embedded at least one pile diameter into the
founding strata.
The nominated pressures assume that the piles are “long” and that the material underlying the base of the
piles equal to a depth of at least four times the footing diameter are as strong or stronger. Where a weaker
material is encountered, the pressure associated to this weaker material must be adopted.
Settlement values are subjected to a variety of factors; however, a properly designed and constructed pile
should have a total settlement value less than a value equal to approximately between 2% and 3% of its’
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diameter, depending on the design method adopted. Detailed settlement modelling should be undertaken as
part of the detailed design.
It is important that the installation of steel screw piles be carefully controlled in the field to ensure the pile does
not “over-screw”. Where over-rotation occurs, the bearing capacity for the helix would be substantially reduced
and/or pile movements incurred.
The actual capacity of steel screw piles depends not only on the soil conditions but also on structural
considerations of the piles such as the strength of the helix and the helix/shaft joint. Measurement of installation
torque should not be relied upon to indicate pile capacity, as it has been documented that significantly
misleading results can be obtained. For this reason, piling contractors would be responsible for assessment
of actual pile capacities for their piles.
Structural capacity of the steel screw pile should be checked, and due allowance made for inclined or eccentric
loads, and possible corrosion effects.
Lateral capacity of steel screw piles could be increased by constructing concrete pile caps or by using
proprietary head attachments which are dragged into the soil providing additional lateral resistance at the pile
head.
Alternatively, cast-in-situ pile construction (ie. bored piles) could be considered but would need to be cased as
the very loose sand will be prone to collapsing in. Bored piles could be designed using the ultimate values
given in Table 2.
If footings are located adjacent to an underground service or other obstruction that is not homogenous with
the founding strata, the footing should be extended a minimum 0.3 m below an imaginary line projected at a
45 degree angle from the lowest point of the service/obstruction. This is depicted graphically in the following
diagram:
5.5 RETAINING WALLS
Retaining walls must be designed by a suitably qualified and experienced engineer and preferably in
accordance with AS 4678. The geotechnical parameters for use in design of either flexible or rigid wall systems
are presented in Table 3. Passive forces should be ignored in areas where disturbance may occur (ie. future
trenching or earthworks processes).
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TABLE 3: RETAINING WALL DESIGN PARAMETERS (UNFACTORED)
Material Ɣ (kN/m3) Ø (°) C’ (kPa)
Very loose to loose sand 17 28 -
Medium dense sand 19 33 -
Notes: Ɣ = unit weight (moist); Ø = drained friction angle; Ka = active pressure; Ko = at-rest pressure; Kp = passive
pressure; NR = not recommended;
Compaction for retaining wall backfill should be undertaken with due care. Layers should be placed thinner
than for normal earthworks processes (say a maximum layer thickness of 100 mm) and compacted using light
weight equipment such as hand controlled compactive equipment to minimise the stresses on the wall. If hand
controlled equipment is not practical, temporary wall propping will be required.
Walls will need to incorporate full height drainage in accordance with good design practice to minimise
hydrostatic pressure.
5.6 STABILITY AND LANDSLIP MAPPING
Reference to the Noosa Council’s steep land mapping indicates that the site falls within the overlay.
The landslip hazard assessment is based on a number of factors which includes but is not limited to
topography, regional geology and evidence of past instability.
With reference to site features, there was no evidence of slope instability (recent or past) observed during the
site investigation.
A qualitative assessment of the likelihood, consequence and risk to the property in its present condition and
also the proposed development has been carried out in general accordance with the methods suggested in
the Australian Geomechanics Society (AGS) Guidelines (2007).
The prevailing hazard at the site would be downhill soil creep of the upper loose sand; However, as noted
above, there were no signs of instability (including soil creep) observed during this investigation.
Based on the investigation results, and provided that design and construction is undertaken in accordance
with the recommendations given in this report, the risk of slope instability for the proposed development is
considered to be low. AGS suggest that regulators generally accept risk levels of “low” or lower as a level of
acceptable risk.
The following works should be implemented in addition to other items highlighted within this document. These
points are considered to be typical good hillside construction practice as explained in the documentation given
in Appendix A.
• Any retention system should be designed by a suitably qualified RPEQ engineer and should be designed
to achieve a safety factor of 1.5 or greater against global stability.
• All stormwater and surfacer water is to be collected and discharged by an approved stormwater drainage
system such that overall stability is not adversely impacted. Do not allow areas of the site to become
saturated from seepage or surface water.
• Plumbing and services must not be allowed to leak or block, and regular maintenance and inspections
should be undertaken.
• Erosion protection of slopes will be required (ie. maintain vegetation on slopes).
• Overland flow paths will need to be carefully directed away from footings to approved outlets.
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6 LIMITATIONS
Arcos Unit Group Pty Ltd (Arcos) has prepared this document for the project described on the title page for the
exclusive use of the client on the title page. The document is specific to this particular project and for the
purposes as described within the document. The document cannot be used for any other project on the same
site or other sites or relied upon by a third party. Any party relying upon this document beyond its exclusive
use and purpose as stated above, and without the express written consent of Arcos, does so entirely at its own
risk and without recourse to Arcos for any loss or damage. Arcos has necessarily relied upon information
provided by the client and/or their agents.
It is important to understand that ground conditions can be spatially variable over short distances as a result
of geological processes or human interference. These changes can occur after the time of which field work
was undertaken. Given this, the assumed subsurface conditions within this document are only indicative of the
conditions at the testing locations to the depths investigated at the time of the fieldwork.
The advice given in this document is based upon the assumed subsurface conditions. The relevance and
accuracy of advice given is directly affected by variations in ground conditions across the site between testing
locations that has not been captured.
This document must be kept in entirety, read in full and understood. Interpretations or conclusions made by
others unless supported by an expressed written statement, interpretation, outcome or stated within this
document cannot be the responsibility of Arcos.
This document or parts of it should not be used as part of a specification or design for a project without prior
review and agreement by Arcos.
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Appendix A – Explanatory Notes
About Your Geotech Report
Interpretation of the Subsurface Sheets
AGS Practice Note Guidelines
CSIRO Guidelines
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ABOUT YOUR GEOTECH REPORT
Reporting
These notes have been provided to clarify and The report has been prepared and reviewed by
enhance the reader’s understanding of the suitably qualified and experienced persons. The
terminology used throughout the report, and also to persons draw on previous experience, local
identify where limitations exist. All investigations are knowledge, published literature, and otherwise
undertaken based on budgetary constraints which standard engineering approaches and principals. The
impart a level of uncertainty as a result of limiting the context of which the report has been prepared for must
amount of investigation undertaken. be considered and if the context changes, due
consideration should be given to the possibility that the
report is no longer considered relevant and review by
Copyright Arcos Unit Group Pty Ltd must be undertaken.
This report remains the property of Arcos Unit Group Arcos Unit Group Pty Ltd undertakes quality
Pty Ltd at all times unless ownership is transferred in assurance checks in accordance with its’ quality
writing. It cannot be included in any documentation system requirements; however, restrictions on the
without the express written permission of Arcos Unit project outside of Arcos Unit Group Pty Ltd’s control
Group Pty Ltd can result in unanticipated variations, changes in
policy or interpretation of said policies by the relevant
authority, or the actions of contractors subject to
Subsurface Report Sheets (Bores etc) commercial pressure.
The provided subsurface sheets are inherently an
interpretation undertaken by a suitably qualified and
experienced person; however, the reliability of the
Site Variations
provided information will rely on the method of If site variations become evident at the time of
investigation. It is prudent to state that these sheets construction or after the release of a report, Arcos Unit
only represent a very small percentage of the Group Pty Ltd requests immediately notification. This
subsurface conditions across a particular project. notification is proposed in order to assist in
remediation prior to any permanent structures or
It reasons that given the relatively small representation
design procedures are undertaken.
of the subsurface conditions for a project, spacial
variability between testing locations must be taken into
consideration including the orientation of strata Collaboration
intersections.
Arcos Unit Group Pty Ltd strongly recommends open
The presence or lack of groundwater is usually stated and collaborative communication about the use and
on the subsurface sheets; however, it must be noted inherent interpretation of Arcos Unit Group Pty Ltd’s
that the measurements undertaken onsite are typically report. Please contact the author of the report to
very inaccurate and if the presence or lack of arrange for an obligation free discussion regarding the
groundwater will have any degree of affect on the potential benefits of having geotechnical consultants.
project, additional groundwater monitoring techniques
should be employed such as groundwater monitoring
wells.
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Interpretation of the Subsurface Sheets
The terminology included on Arcos Unit Group Pty the most important component. Typically, the strength
Ltd’s has been evolved from AS1726:2016. Typically of rock is based on laboratory testing, tactile
a soil’s strength, colour, plasticity, grain size, other assessment and other observations as relevant. The
inclusions, and moisture will be described. A rock’s industry standard method for assessing the strength of
strength, weathering, colour, grain, and other rock in the field is to use the point load strength index
inclusions are typically described. (AS4133.4.1). It is important to note that the point load
strength index does not take into consideration the
degree of fracturing of a rock mass which can both
Soil lower and increase the overall rock strength. The
Soil is typically described by the components that strength of rock is broken down as follows:
make it up by percentage or whichever has a Very Low Strength (VLS) < 2 MPa (UCS)
governing behaviour. The types and sizes reproduced Low Strength (LS) 2 to 6 MPa (UCS)
from AS1726 are: Medium Strength (MS) 6 to 20 MPa (UCS)
Boulder >200 mm High Strength (HS) 20 to 60 MPa (UCS)
Cobble 63 to 200 mm Very High Strength (VHS) 60 to 200 MPa (UCS)
Gravel 2.36 to 63 mm Extremely High Strength (EHS) > 200 MPa (UCS)
Sand 0.075 to 2.36 mm Very Low Strength (VLS) < 0.1 Is(50)
Silty 0.002 to 0.075 mm Low Strength (LS) 0.1 to 0.3 Is(50)
Clay < 0.002 mm Medium Strength (MS) 0.30 to 1.0 Is(50)
The strength of soil is usually assessed by suitably High Strength (HS) 1.0 to 3.0 Is(50)
experienced persons through the use of laboratory Very High Strength (VHS) 3.0 to 10.0 Is(50)
test results, published literature, field assessments or Extremely High Strength (EHS) > 10.0 Is(50)
a combination of the above. The strength of soil is The Is(50) values are based on a correlation between
characterised into either cohesive (clays and silts) or the unconfined compressive strength (UCS) of 20.
cohesionless (sands and gravels) as shown below: This value varies significantly and site specific testing
Cohessive Soil Strength Term must take precedence.
Very Soft < 12.5 kPa Rock weathering is broken down as follows:
Soft 12.5 to 25 kPa Extremely weathered (EW) Behaves as soil but rock structure
Firm 25 to 50 kPa evident
Stiff 50 to 100 kPa Highly weathered (HW) Colour and strength of original fresh rock
is not recognisable. Decomposition of rock evident
Very Stiff 100 to 200 kPa
Moderately weathered (MW) Defects are heavily stained with
Hard > 200 kPa
rock mass showing signs of discolouration
Cohesionless Soils Strength Term (uncorrected Qc) Slightly weathered (SW) Some staining but minimal to no
Very Loose <2 MPa change in strength
Loose 2 to 5 MPa Fresh stained (FS) Minor staining evident
Medium Dense 5 to 15 MPa Fresh rock (FR) No change
Dense 15 to 25 MPa Rock fracturing is broken down as follows and can only
Very Dense > 25 MPa be accurately described where undamaged
continuous sampling has been undertaken:
Fragmented (Fg) Defect spacings < 20 mm
Rock
Highly Fractured (HF) Defect spacings 20 to 50 mm
Rock is typically described by the components that Fractured (Fr) Defect spacings 50 to 200 mm
make it up by percentage or whichever has a Slightly Fractured (SF) Defect spacings 200 to 1000 mm
governing behaviour. The strength of rock is typically Unbroken (Ub) Defect spacings > 1000 mm
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Foundation Maintenance and Footing Performance: A Homeowner's Guide (i) CSIRO BIF 18 replaces 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 identi 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, 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 construc- tion. Building Technology File 19 (BTF 19) deals with these problems. ? Erosion All 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. Saturation This 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 with only slight ground movement from moisture changes M Moderately reactive clay or silt sites, which can experience moderate ground movement from moisture changes H Highly reactive clay sites, which can experience high ground movement from moisture changes E Extremely reactive sites, which can experience extreme ground movement from moisture changes AtoP Filled sites P Sites which include soft soils, such as soft clay or silt or loose sands; landslip; mine subsidence; collapsing soils; soils subject to erosion; reactive sites subject to abnormal moisture conditions or sites which cannot be classified otherwise 3
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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 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 Uneviin Sell Mavieatt cay Biruriires Erosion and saturation Erosion remoyes 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. Roosraally 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. BGO CeCe litt g oy Be EN wR? a Wall cracking due to uneven footing settlement 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, [n 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 rime 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.
Page 20
The normal structural arrangement is that the inner leaf of brick- work 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 cause 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 tubble 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. AS 2870 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 Plumbing Where 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 wrench has been poorly backfilled, the water will either pond or flow along the bottom of the trench. As these trenches usually run alongside the footings and can be ata 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 BT'F 19 and may properly be regarded as an area for an expert consultant. Protection of the building perimeter Ie 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 thar an apron of paving be installed around as much of the building perimeter as necessary. This paving CLASSIFICATION OF DAMAGE WITH REFERENCE TO WALLS Description of typical damage and required repair Approximate crack width Damage limit (see Note 3) category Hairline cracks <0.1 mm 0 Fine cracks which do not need repair <| 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 5-15 mm (or a number of cracks 3 to be replaced. Doors and windows stick. Service pipes can fracture. 3 mm or more in one group) Weathertightness often impaired Extensive repair work involving breaking-out and replacing sections of walls, 15-25 mm but also depend 4 especially over doors and windows. Window and door frames distort. Walls lean on number of cracks or bulge noticeably, some loss of bearing in beams. Service pipes disrupted
Page 21
CC Ld Shrubs Clump of trees; height selected for distance from house Drained pathway Garden bed covered with mulch should 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. Condensation In 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 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 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. Remediation Where 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 specialise 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 jn this and other issues in the series was derived from various sources and was believed to be correct when Due iets The information is advisory. It is eieded in Bom faith and not claimed to be an extiausthe treatment of the relevant subject. Barthes professional radvice needs to bel obtained before taking any ection baseu o on the information provided, Distributed by CSIRO PUBLISHING PO Box 1139, Collingwood 3066, Australia Freecall 1800 645 051 Tel (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
Page 22
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CONTACT
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OFFICES
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Appendix B – Drawings
Drawing 1 – Bore Locations
Page 23
Horiz. Datum Client Consultant LOCALITY PLAN LEGEND @ Borehole Location NOTES 1. Imagery from Nearmap 2. Locations are approximate only and are shown with reference to existing and proposed features Vert. Datum Project PROPOSED RESIDENTIAL Drawn TEST LOCATION PLAN Project No. Drawing No. Rev No. Design ea PETER FLAVEL ! A R Cc O S DEVELOPMENT, 5 240572 01 A = : 2/27 ROSS CRESCENT, SUNSHINE [°° RL Datum ‘Approved ‘Approved Date ‘Signed ‘Sheet Not Recorded arco Status FOR INFORMATION see AS
Page 24
ABN 51 640 311 842
CONTACT
arcosgroup.com.au
OFFICES
Sunshine Coast | Brisbane | Gold Coast
Appendix C – Field Work Results
Bore Hole Logs
Page 25
CLIENT: Peter Flavel PROJECT #: 240572 BORE
PROJECT: Proposed Renovations LOGGED: EJ 1
ADDRESS: 2/27 Ross Crescent, Sunshine Beach EASTING: 511039 m TEST DATE:
19/08/2024
DRILL RIG: Hand Auger NORTHING: 7078869 m
Sampling & Testing
Graphic Log
DCP Results
Material Description Results &
Type (blows per 100 mm)
Comments
Water
RL Depth
(m) (m) 5 10 15 20
SAND (SP): very loose, fine to medium sand, pale grey, moist, aeolian deposit 1
0
0
grey mottled pale grey 0
1
0
0
1
0
0
1
0
1
1
1
1
medium dense 2
2
2
3
3
2
3
3
3
4
4
4
4
5
4
4
3
4
5
5
5
5
5
5
6
6
6
4
7
7
7
dense 10
10
4.5 Bore Terminated at 4.5 m.
Limit of Investigation.
5
DRILLING METHOD: Hand Auger CASING: Nil
GROUNDWATER: No groundwater seepage observed at time of drilling.
REMARKS: Bore Terminated.
LEGEND:
D - Disturbed Sample from Auger SPT - Standard Penetration Test - Groundwater Seepage Level
B - Bulk Sample from Auger Is50 - Point Load Result (MPa) - Standing Groundwater Level
C - Rock Core PP - Pocket Penetrometer (kPa) - Partial Groundwater Loss
U50 - Undisturbed Sample (mm) - Perched Groundwater Level
Page 26
CLIENT: Peter Flavel PROJECT #: 240572 BORE
PROJECT: Proposed Renovations LOGGED: EJ 2
ADDRESS: 2/27 Ross Crescent, Sunshine Beach EASTING: 511043 m TEST DATE:
19/08/2024
DRILL RIG: Hand Auger NORTHING: 7078873 m
Sampling & Testing
Graphic Log
DCP Results
Material Description Results &
Type (blows per 100 mm)
Comments
Water
RL Depth
(m) (m) 5 10 15 20
SAND (SP): very loose, fine to medium sand, pale grey, moist, inferred aeolian deposit 1
0
0
grey mottled pale grey 0
0
1
0
0
0
1
1
0
0
1
1
1
1
1
1
medium dense 2
2
2
2
2
3
3
3
3
4
3
4
4
3
4
4
4
5
5
5
5
5
6
7
4
7
10
10
dense 10
10
4.5 Bore Terminated at 4.5 m.
Limit of Investigation.
5
DRILLING METHOD: Hand Auger CASING: Nil
GROUNDWATER: No groundwater seepage observed at time of drilling.
REMARKS: Bore Terminated.
LEGEND:
D - Disturbed Sample from Auger SPT - Standard Penetration Test - Groundwater Seepage Level
B - Bulk Sample from Auger Is50 - Point Load Result (MPa) - Standing Groundwater Level
C - Rock Core PP - Pocket Penetrometer (kPa) - Partial Groundwater Loss
U50 - Undisturbed Sample (mm) - Perched Groundwater Level
Page 27
CLIENT: Peter Flavel PROJECT #: 240572 BORE
PROJECT: Proposed Renovations LOGGED: EJ 3
ADDRESS: 2/27 Ross Crescent, Sunshine Beach EASTING: 511051 m TEST DATE:
19/08/2024
DRILL RIG: Hand Auger NORTHING: 7078883 m
Sampling & Testing
Graphic Log
DCP Results
Material Description Results &
Type (blows per 100 mm)
Comments
Water
RL Depth
(m) (m) 5 10 15 20
SAND (SP): very loose, fine to medium sand, pale grey, moist, inferred aeolian deposit 1
0
0
grey mottled pale grey 0
1
0
0
0
0
1
1
1
1
2
20
1.5 Bore Terminated at 1.5 m.
Refusal - Possible Footing
2
3
4
5
DRILLING METHOD: Hand Auger CASING: Nil
GROUNDWATER: No groundwater seepage observed at time of drilling.
REMARKS: Bore Terminated.
LEGEND:
D - Disturbed Sample from Auger SPT - Standard Penetration Test - Groundwater Seepage Level
B - Bulk Sample from Auger Is50 - Point Load Result (MPa) - Standing Groundwater Level
C - Rock Core PP - Pocket Penetrometer (kPa) - Partial Groundwater Loss
U50 - Undisturbed Sample (mm) - Perched Groundwater Level
Page 28
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CONTACT
arcosgroup.com.au
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Sunshine Coast | Brisbane | Gold Coast
Contact Us
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@arcosgroup
[email protected]
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