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Shallow Replacement Of Expansive Or Swelling Soils
In the June 2020 issue of NZ Geomechanics News ‘The Shrink Swell Test: A Critical Analysis’, Rogers et al., 2020, was published. This article provided an overview around the use of the Shrink Swell Test and current design practice for foundation design in NZ’s expansive soils in accordance with AS 2870:2011 site soil class.
This paper provides an additional summary of the currently existing methods/ways for identification of expansive soils (defined herein as soils susceptible to both swelling and shrinkage) and presents a discussion on recently observed construction practice comprising shallow 500mm replacement of expansive soils with compacted granular fill as a ‘measure’ to reduce the deformation of the expansive soils and mitigate the risk of structural damage to a slab foundation.
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Lateral behaviour of pile located on top of a slope
The study of a pile behavior on top of the slopes is one of the cases that has been attracted more attention these days. Usually, the behavior of lateral loading piles is studied on the slopes with a range of angles. One of the more important parameters in this field is the lateral bearing capacity of piles. The main aim of this research is to evaluate the lateral bearing capacity of piles under lateral loading. Evaluation of different situation is accomplished by changing the pile length and the pile distance to the slope crest in various slope angles. The pile lateral bearing capacity, horizontal and vertical displacements of the pile head in different slope angles are compared with the flat ground. It is shown that the lateral bearing capacity changes in different slopes are dramatically affected by both the length ratio of the pile to the slope height (L/H) and the distance ratio of the pile from the slope crest (X/D). When the distance ratio of the pile from the slope crest is equal to zero, a great difference is observed from all other cases.
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Geostrap® & Ecostrap™ Reinforcements For MSE Structures. A New Approach To Geosynthetic Soil Reinforcement
This paper discusses the use of new geosynthetic solutions for soil reinforcements reviewing the whole process from design through to the construction stage.
Recent developments in high strength polyester (PET) GeoStrap® and polyvinyl alcohol (PVA-L) EcoStrapTM soil reinforcements, combined with GeoMega® Sleeve, a new method for connecting geosynthetic reinforcements to concrete facing panels, offers durable design solutions for Mechanically Stabilized Earth (MSE) structures located in sea water and aggressive environments.
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“Value-Adding” to Routine Rock Testing for Underground Excavation Projects
A geotechnical investigation will be enhanced by careful consideration, at its earliest stages, of the intended eventual construction methods and outcomes, and the expected time scales. This paper will discuss the range of “routine” laboratory testing procedures for rocks that could be considered. Minor enhancements of standard testing and reporting procedures may yield valuable and subtle insights into potential rock mass behaviour and excavation efficiencies and contribute to beneficial improvements to eventual outcomes.
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Groundwater control around a large basement
Melbourne’s Crown Casino was constructed on a site bordering the Yarra River and underlain by problem soils of the Coode Island Silt Formation. The development needed to provide two levels of basement car park over the entire site. An innovative approach to groundwater control around the excavation was required to avoid depressurisation of adjoining soils, leading to settlements. Analysis showed a conventional bentonite cut-off wall would still allow depressurisation by lateral flow through the Coode Island Silt during the construction period. The high cost and construction difficulty of a very low permeability wall mitigated against it. An hydraulic wall was proposed in conjunction with a conventional cut-off wall. This comprised a curtain of wick drains surrounding the cut-off wall and charged with water. Control of seepage through an underlying aquifer by a cut-off wall was considered, but a more cost-effective method using recharge by wells was adopted when shown necessary. Monitoring of groundwater pressures around the site showed that the maximum change in water pressure was less than 1 m head, the design criterion. Part way through construction, recharge was initiated when monitoring of the deep aquifer showed pressure reduction attributed to vertical leakage through a basalt tongue.
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AGS Townsville 2024 Symposium
Slope Stability
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Permanent Deformation Behaviour Of Two Victorian Subgrade Soils Under Repeated Loading
Mechanistic design methods of pavement structures require the knowledge of permanent deformation behaviour of subgrade soils under anticipated repeated loading. Limiting subgrade deformation up to an acceptable level is one of the main design philosophies of mechanistic pavement design approaches. In the Austroads pavement design guide, a general subgrade failure criterion is given, which was originally derived from CBR pavement design chart. However, for a proper design, one should know the permanent deformation potential of the subgrade that is being dealt with. In this respect, development of predictive equations through dynamic testing gains importance. This paper presents some of the findings of a continuing research project on permanent deformation and resilient characteristics of subgrade soils. Repeated load triaxial tests were performed for two Victorian fine grained soils, in which the level of loading was defined as percentages of the soil static strength. The permanent deformation test results based on various stress- strength ratios were then analysed by using power model. Correlations between model constants and other parameters such as stresses and soil physical properties were investigated. Stress-strength ratio was found to be highly correlated to the logarithm of permanent strain occurring at the first load application. This was because the inclusion of strength parameter into the model which may also indirectly represent the effect of soil physical properties. Model constants of predictive equations were established by means of regression analysis. Predicted permanent strains were compared with measured strains and Austroads subgrade failure criterion.
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Piled cantilever retaining wall at Port Hedland – driveability and wall deflection
This paper describes the design, driveability and deflection monitoring results of a piled cantilever retaining wall at Port Hedland, Western Australia. The retaining wall was required to stabilise an existing access road and conveyor foundations to an existing wharf, prior to the dredging operations for a new export facility in the port. By designing the dredging profile (in front of the retaining wall) as an underwater batter, a cantilever retaining type structure made up of steel tubular piles was found to be feasible. The stability and deflection criteria requirements indicated that some of the retaining wall piles were required to be driven to a toe level of -30 mCD, penetrating through approximately 25 m thick very weak to medium strength rock. General experience of driving piles at Port Hedland area is that the piles are very likely to refuse on a 4 m thick medium strength Conglomerate rock layer starting at about -14 mCD. The piles equipped with external and internal shoe thickening were found to be easier to drive. Measured wall deflections were found to be lower than the initially predicted deflection due to difference in the as-built dredging profile and the assumed design dredging profile. The predicted wall deflection was found to be very similar to the measured deflection when a reanalysis was carried out considering the post dredging as-built batter slope profile. Data from static tension load test carried out on a 610 mm OD and a 1050 mm OD piles for wharfs near the retaining wall is also provided.
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Undrained shear strength of variably pre-loaded Launceston Silt
Earth levees were constructed in the 1960’s as flood protection around the suburb of Invermay, located immediately north of Launceston’s CBD. In 2007 an upgrade program was begun by Launceston City Council (LCC) to provide 1 in 200 year flood protection, meaning that about 5km of levee have to be re-aligned and raised, with the new levee alignment partially over virgin ground and partially over pre-consolidated ground. This paper presents the results of testing and analysis to determine the spatially variable undrained shear strength profiles of the compressible Launceston Silt foundation. Relationships between undrained and drained shear strength were used to predict strength increase caused by preconsolidation, with good agreement with in situ measurements. The analysis allowed development of an innovative stability model that has vertical interfaces between foundation zones of different strengths.
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Illustrative sections depicting landslide susceptibility of the Illawarra Escarpment
The challenge that landslides pose to infrastructure and to domestic and commercial development in the Illawarra region has been recognised in land-use planning for decades. The seminal regional mapping undertaken by Bowman (1974) and, before him, the work reported by Shellshear (1890), set the technical benchmarks for others to follow. The challenges presented to development and road and rail infrastructure were recognised by both Wollongong City Council and major infrastructure providers in the NSW Roads and Maritime Services (previously NSW Roads and Traffic Authority) and NSW Rail Corp (previously State Rail Authority). Continuing support from this group has permitted research and work in the field throughout the Illawarra by Flentje since 1993, which has brought together the collectively faced issues into a composite landslide inventory.
The landslide issues are well recognised by those who are familiar with the Illawarra area – in particular, the typically steep terrain of the 45 km long Illawarra Escarpment, the presence of a colluvial mantle draped over the steep terrain, the presence of many sub-horizontal coal seams throughout the stratigraphy, past and present underground coal mining throughout the region and intense rainfall events generated by virtue of its location and also as a consequence of the escarpment’s influence upon local meteorology with the orographic rainfall.
One of the challenges of the application of Bowman (ibid) is the mapping base available at that time. Bowman reported mapping at 8 chains to the inch (1:6,336) but appears to have used a basemap enlarged from a much earlier base (possibly enlarged from 2 inch to the mile, viz: 1:31,680, or 1 inch to the mile, viz: 1:63,360). Bowman also noted the poor edge matching of his basemaps in his work. The NSW Central Mapping Authority 1:4,000 scale 2 m contours generated in the late 1970’s provided a significant enhancement to the mapping base, though draping of the Bowman mapping over this improved basemap faced obvious challenges. Recently, the availability of the contours in electronic format and the rise of both Geographic Information System (GIS) capability and expertise, have greatly facilitated mapping in the Illawarra. Subsequent Airborne Laser Scan (ALS) digital terrain mapping at high resolution has also recently become available, which together with Flentje’s detailed mapping (Flentje, 1998) and access to landslide records of Wollongong City Council (through council’s Geotechnical Engineer, Peter Tobin) has permitted enhanced understanding of the specific conditions and characteristics that influence the Illawarra landscape. This area is also the scene of the Engineering Geology course run biennially on behalf of the Australian Geomechanics Society (2010).
As no doubt is often the case, a simple question triggers a line of action, and the raison d’etre for this paper fits that pattern. In this case, the question asked of themselves by the authors was “Is there a type-section of landslide issues within the Illawarra?” This paper, and the development of the illustrative sections herein, is a response to that somewhat rhetorical question.