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Australian Standard AS 2870 – time for a change: With specific reference to the slab heave problem with waffle rafts
The purpose of this paper is to review the efficacy of Australian Standard AS 2870 [1] to regulate site classification, design, construction and maintenance of residential slabs and footings. This review identifies a number of issues with AS 2870 relating to the slab heave problem with waffle rafts. It demonstrates how AS 2870 may be applied to hold home owners responsible for damage to their new homes, caused by foundation movement as a result of abnormal soil moisture conditions developing after the owners have taken possession of their new homes. It also demonstrates that slab heave, popularly believed to be caused by deficiencies in the stormwater drainage systems of the dwellings, is not the principal cause of damage. A limit state methodology for ultimate strength design of residential slabs is recommended as a cost-effective way to mitigate the slab heave problem with waffle rafts. Graphs are included to address the issues with AS 2870.
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Mechanical Stabilization Of Unbound Layers: Effect Of Geogrids On Low Strain Behaviour Of Granular Materials And Essential Characteristics For Optimum Performance In Permanent Roads
The use of geogrids in mechanically stabilized earth (MSE) structures and for trafficked areas over soft soils is well known. The essential characteristics of geogrids for MSE have well established over many years and are incorporated into national and international codes and standards. However, the mechanisms by which geogrids function in permanent roads and therefore the essential characteristics of geogrids operating at low strains in this application have often been disputed. This paper will present results from several recent research projects that identify the stabilization mechanisms that operate and identifies essential characteristics for geogrids in this function. A comparison is made between the stabilization function of geogrids and tensioned membrane reinforcement function of geogrids and the significance of this functional difference in the selection of essential geogrid characteristics for specification purposes. Such an understanding is essential if specifications are to protect designs that utilise the benefits of mechanical stabilization to increase pavement life.
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Proposed Revisions To The Piling Standard The Effect Of Geotechnical Investigations And Test Loads
Proposed revisions to the current version of the Australian Piling Standard will affect the way that the geotechnical and structural engineering professions and the pile construction industry will consider the geotechnical capacity of piled foundations. Significant changes are to the manner in which geotechnical investigations and test load programmes affect the geotechnical strength reduction factor, which are generally lower than the current standard. The proposed changes also reduce the permissible settlement under test load Special consideration will be required for the structural capacity of piles which are to be test loaded to prove ultimate geotechnical capacity.
This paper discusses some of the proposed changes and compares them to the current requirements. The general emphasis will be for piles for normal industrial, commercial and residential structures. The developers of large infrastructure and heavy industrial projects usually recognize the importance of comprehensive investigation and test programmes and require little incentive to implement them.
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Quantitative Landslide Risk Assessment Of Cairns
A GIS-based quantitative landslide risk assessment was carried out in the Cairns area to provide information to the Cairns City Council on landslide hazard, community vulnerability and risks for planning and emergency management purposes. Magnitude recurrence relations were tentatively established for the two main slope processes: landslides on the hill slopes and large debris flows extending out from the gully systems on to the plains. From the recurrence relations, landslide hazard (H) was estimated as the annual probability of a point being impacted by a landslide. The nature, number (E) and geographic distribution of the elements at risk were obtained by interrogating the GIS, and their vulnerabilities (V) to destruction by the two main landslide slope processes were assessed. From this information, specific risk (= HxV) and total risk (= HxVxE) maps were produced.
Landslide risk may increase as development extends further into the hill slopes. Large debris flows could impact on subdivisions at the base of the slopes. Blockage by landslides of roads and railways could cause isolation of the community. Flash flooding in Freshwater Creek, or debris flows, have the potential to disrupt the Cairns water supply by blocking the intake or destroying sections of the pipeline.
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The interplay of site reactivity, design practice and construction procurement in structural failures on expansive clay sites
The expansion of housing into the Western suburbs of Melbourne has involved, effectively, mass-production of conventional, masonry veneer dwellings on highly reactive sites. The design favoured by constructors for reasons of cost and speed is the “waffle pod” system, typically 385mm deep on highly reactive sites, with the performance of such footings generally predicated on adequate control of soil moisture in the foundations close to the footprint of the building, both during construction and in the permanent condition.
Damage to masonry facades and excessive floor slab deformations, evidently due to serviceability failure of footing systems, have led to recent publicity and litigation. Whilst the underlying geotechnical characteristics may be causative of soil heave, the unique characteristics of waffle pod footing systems and the associated landscaping and drainage provisions stipulated by the designers, along with the method of procurement of the house itself point to the contribution of systemic and contractual factors in footing failures.
A further aspect is the interplay of differing deformation criteria between AS2870 (“the Standard”) and the structural design standards such as AS1684, whereby, again, dissociated procurement and design of building components (footings and roof trusses in this instance) can lead to incompatibilities in structural behaviour and consequent damage to cladding and finishes. This paper examines the interplay of geotechnical, structural, construction detailing and contractual aspects that can lead down the path to poor footing performance, dispute, and litigation. The question is raised as to whether, given these relatively gross uncertainties relating to the input parameters for codified standard designs and design methods, the degree of refinement and accuracy implicit in the Standard is warranted, and whether more robust standardised designs are justified.
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Sydney sandstone and shale parameters for tunnel design
Inherent in any set of rock mass parameters are various assumptions regarding, amongst other things, the depth of cover, the proportion of materials encountered and the scale of the proposed excavations. The factors typically used to define rock mass conditions are:
- the rock type (lithology)
- the strength of the rock substance (intact rock strength)
- the fracturing of the rock mass by defects (bedding, joints, shears, etc.)
- the persistence and spacings of defects
- the number of sets of defects
- the infill material
- the roughness of defects
- the groundwater pressures
- the reactivity of the rock substance to environmental change (shrink/swell, slaking)
The designers’ rock mass parameters communicates their assumptions regarding the ground conditions to those in the field responsible for implementing the designs. It is vital that those in the field are vigilant in observing, documenting and interpreting geological structures that may dominate support requirements at a particular location, and which would render the “average” rock mass class irrelevant and feed back to the designers where conditions are different from those anticipated.
The intention of this paper is to present typical geotechnical characteristics for tunnel design in Sydney’s Hawkesbury Sandstone and Ashfield Shale following the Sydney classification system (Pells et al., 1998). It is suggested that for geotechnical parameters the Mittagong Formation can generally be considered in line with the Hawkesbury Sandstone classes. This paper also presents design parameters that can be used in numerical analyses for tunnel design.
Different sets of material properties may need to be provided to cater for different scales as design parameters are dependent on the scale of assessment. For example:
- Overall tunnel scale – properties to be used in continuum analyses e.g. FLAC, Phase2, PLAXIS, Abaqus. Specific geological structures, one or two at most, can be included in the model.
- Approximately 1m3 scale – properties to be used in discontinuum analyses where numerous geological structures are explicitly modelled, e.g. UDEC.
This paper updates Bertuzzi and Pells (2002) with data from recent tunnelling projects. The database now includes information from the Ocean Outfalls, Sydney Harbour Tunnel, Eastern Distributor, M5 East, Cross City, cables tunnels, Epping-Chatswood, Lane Cove, CBD Metro, M2 and the northwest rail projects. This paper also presents useful methods by which the designers can communicate their views of rock mass conditions, particularly to those in the field. Finally, the paper brings into consideration the rock mass behaviour types recommended by the Austrian Society for Geomechanics (2010). The example of a nominally 6 to 12 m diameter TBM at depths of up to 50 m is used.
It is hoped that practitioners will find this paper useful in their work in Sydney.
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Preload Design, Part 2 — An analytical method based on Bjerrum’s time line principle and comparison with other design methods
Following a review of time-dependent settlement behaviour and preload design methods presented in Part 1 of this paper (Wong, 2006a), Part 2 of this paper presents the development of an analytical approach for preload design based on Bjerrum’s (1967; 1972) time line model, or principle of “artificial aging”. This analytical approach can be readily coded using an Excel spreadsheet for assessing the required preload thickness to limit post-construction settlement to a specified value.
A worked example is presented and compared with other methods (Mesri, 1991; Poulos, 2004; PLAXIS Version 8, 2002) to illustrate the importance of geological and stress history on post-preload settlement behaviour.
The dependency of creep on stress level and stress history is used in the analytical approach introduced in this paper, and the results of the worked example clearly show the reasons for the possibility of significant post-preload creep settlement if the amount and/or the degree of consolidation during surcharging were insufficient.
The purpose of this paper is not to suggest any preference for a particular preload design method with respect to either correctness or accuracy in predicting post-construction settlement performance following preloading. Rather, through the worked example, it highlights the dependency of the results on certain assumptions used in the models, and perhaps also highlights the importance of being able to follow the stress path and applying fundamental principles to explain the predictions and their limitations.
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Understanding Liquefaction Triggering Risk – An Australian Geotechnical Design Perspective
Resilience is the ability to quickly recover during an adverse event. Following an earthquake the resilience of a community can be directly related to working infrastructure. Geotechnical resilience design must consider liquefaction from future large earthquakes.
Although Australia is considered a stable continental region with relatively low seismic hazard, earthquakes do occur and where susceptible geological conditions exist, liquefaction can occur. In fact, liquefaction has been documented in Australia on at least three occasions. In 1897, liquefaction was observed during a large (Ms 6.5) earthquake near Beachport, south-eastern South Australia (Collins et al., 2004); in the 1903 Warrnambool, Victoria (Ml 5.3) earthquake (Mitchell and Moore, 2007); and in 1968, numerous “sand blows” were observed following the Ms 6.8 earthquake at Meckering in Western Australia (Collins et al., 2004).
Liquefaction is a credible geohazard considered in current Australian geotechnical engineering practice, and infrastructure planning desk studies in Australia commonly identify liquefaction as a geohazard where susceptible soils exist within the project footprint. Further assessments are required in subsequent feasibility and detailed design phases. Accurately assessing the liquefaction triggering potential is an essential part of geotechnical design considerations.
The low seismicity of Australia creates a situation where liquefaction triggering is marginal at design hazard levels. This low level of seismic hazard makes the liquefaction trigger assessment very sensitive to the derivation of the seismic inputs. The lack of guidance on liquefaction from AS1170.4 requires interpretation of the basis seismic hazard inputs.
This paper explored the sensitivity to seismic inputs in low seismicity hazard Australia, to better understand liquefaction triggering risk in Australian geotechnical design. The components of the seismic hazard inputs are reviewed. A case study is presented showing that for liquefaction assessments in low seismicity regions, liquefaction triggering is sensitive to the selection of design magnitude and the calculation of the ground motions through the soil profile.
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Residual soil properties of South East Queensland
This project sought to investigate parameters of residual soil materials located in South East Queensland (SEQ), as determined from a large number of historical site investigation records. This was undertaken to quantify material parameter variability and to assess the validity of using commonly adopted correlations to estimate “typical” soil parameters for this region. A dataset of in situ and laboratory derived residual soil parameters was constructed and analysed to identify potential correlations that related either to the entire area considered, or to specific residual soils that were derived from a common parent material. The variability of SEQ soil parameters were generally found to be greater than the results of equivalent studies that analysed transported soil dominant datasets. Noteworthy differences in material properties also became evident when residual soils weathered from different parent materials were considered independently. Large variation between the correlations developed for specific soil types was found, which highlighted both the heterogeneity of the studied materials and the incompatibility of generic correlations to residual soils present in SEQ. Region and parent material specific correlations that estimate shear strength from in situ penetration tests have been proposed for the various residual soil types considered.