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A Smart Geotechnical And Geological Approach For Future Building And Transport Infrastructure Projects
There is a rapid and unprecedented scale of infrastructure planning and development across the Sydney region. A SMART approach that captures historical ground investigation and regional geological data is required to support early transport planning by Government. This will allow the refinement of geological and geotechnical knowledge gaps that will be augmented with additional investigation once these corridors are further assessed as the design develops. To allow a SMART approach in infrastructure planning and development, Government Departments and potentially the private sector could integrate their internal geological and geotechnical data as part of a centralised state-wide data collection centre. This will require Government to legislate a registry system for factual geotechnical data for all Departments and Authorities. Consideration would also need to be given to how to release this information from the private sector many of whom would claim this was their intellectual property despite typically being derived (and paid services for) from Government projects.
Consideration should be given to a two-stage process so as not to derail the implementation due to potential delays with the private sector:
- Combine and integrate geological and geotechnical data from historical Government projects including those delivered under corporatised government entities.
- Integration of factual data obtained from the private sector.
Any data compiled under both (i) and (ii) will need to be relied upon without any impact or recourse to the originators. This has been key to the success of similar data sharing mechanisms in the United Kingdom (British Geological Survey) and the Netherlands (Dutch Geological Survey).
A way of making this work successfully in New South Wales, following successful international models such the UK and Netherlands, is to have government allow contracts or documentation to have historic data relied upon. The State will achieve better value for money by way of having significantly more geological and geotechnical data as part of Environmental Impacts Statements to inform approvals and stakeholders as well as for its Request for Proposals (RFP). In all cases with more reliable information a better outcome will be achieved by way of increased certainty and avoiding approval delays, possible injunctions, as well as more informed Request for Tenders (RFTs).
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A case study of deep excavation and retention design for Sydney Metro Northwest
This paper provides an overview of the Sydney Metro Northwest (formerly the North West Rail Link) project and the underground station excavation retention design and construction works, including the key requirements set out in the scope of works and technical criteria (SWTC). Based on the assessment of the geological conditions a soldier piled wall retention system was adopted for all five new underground stations and one of the two services facility shafts for ease and speed of construction. During the Castle Hill Station excavation a new planar wedge instability mechanism was considered to be credible based on the additional geological data, with the original three-dimensional block instability being no longer suitable. This led to redesign of the south wall stabilisation works based on the updated geological model and input parameters. The instrumentation and monitoring plan was also adjusted to ensure the required additional support provided would be adequate for the safety of the station box excavation. The monitored lateral movements at the capping beam and at the inclinometers were within the trigger values, indicating that the retention system constructed was robust.
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A new approach for characterising expansive clay sites
Highly expansive soils are common throughout the world. In Australia more than 20% of the near-surface soils are considered moderately or highly expansive, many are in large cities. The Australian standard (AS2870) recommends 3 methods to ‘characterise’ a building site and in each case some form of regular laboratory testing is required to test the soil indices. To do so this the Standard recommends any of the following: Shrink/Swell, Loaded Core Shrinkage, or Unloaded Core Shrinkage test. The most common of these is the shrink/swell test (S/S); however in this test soil suctions are not measured and instead assume certain parameters. This paper presents a new Conditioned Core Shrinkage test (CCS) with suction measurements. This test is performed in the suction range of 3-7pF and provides the soil shrinkage index values (Iss) along this range to calculate the ‘characteristic surface movement’ (ys). The test can be carried out in 4 -7 days depending on the type of soil and recent climate.
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Effect of depth on results from the Perth sand penetrometer test
The Perth Sand Penetrometer (PSP) is widely used in WA for compaction control, particularly for preparation of sand foundations for domestic and light commercial structures. The relevant Australian Standard (AS1289.6.3.3–1997) specifies that the test is to be performed by first inserting the penetrometer to a depth of 150 mm below the ground surface, and then counting the number of blows required to achieve a further 300 mm penetration (to a total penetration depth of 450 mm). In practice the test can be, and frequently is, continued to greater depths – typically for another 300 mm (to a depth of 750 mm), but sometimes to even greater depths. However, there is contradictory evidence of how the increase in test depth affects the measured blowcounts for a given constant relative density with depth. This paper presents the results of some studies into this effect. It is concluded that there is no definitive universal relationship between blowcounts and depth for any given relative density, and hence site-specific calibration would be required if the test was to be used in this manner for any important project.
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Geotechnical Challenges For Construction Of Diaphragm Walls And Foundation Of Sydney’s Tallest Building, Crown Sydney Hotel Resort
Crown Sydney Hotel Resort is the Stage 1C component of Barangaroo South and is being developed as a single high rise mixed use tower of 72 stories (271 m high), rising over a multi-level podium and a 3 level basement car park (total 75 levels). The Crown Sydney Hotel Resort basement retaining wall comprised 33 diaphragm wall (D-wall) panels and 36 barrettes for the foundation of the main tower and more than 130 bored piles (including bored compression piles, bored tension piles, bored sleeved piles and permanent plunge column piles). AECOM was engaged as designers of the foundation works by Piling Contractors Bauer Australia Joint Venture (PCBAJV) who constructed the foundation works as the D&C foundation contractor. The depth of foundation elements varied from 25 m to 50 m below ground level.
AECOM provided an initial concept design followed by a detailed design services and then, during construction, full time on-site geotechnical inspection of the basement diaphragm walls and foundations. This paper will focus on the challenges of geotechnical verification of diaphragm wall panels, barrette and pile foundation construction and how these challenges were met. During fulltime site inspection, hydraulic trench cutter penetration rates in various sandstone rock classes were measured and compared with the borehole data. Rate of penetration of the piling rig into the various sandstone rock classes, rock quality and rock apparent temperature were closely monitored and recorded as part of verification of the socket requirements. Monitoring, data collection and comparing the data with available boreholes allowed AECOM to develop a method to reliably check the rock socket compliance with requirements across the site. Other geotechnical observations and lessons learned during the inspection of the pile, diaphragm wall and barrette construction are also presented in this paper.
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Design and Construction of Plastic Geocellular Rain Water Harvesting/Stormwater Detention Tanks
Plastic voided modular structures (known as geocellular units) were first used in the mid-1980s in Europe below pavements to store stormwater. Its use has since spread to rainwater harvesting and on-site stormwater detention for residential, commercial and industrial developments. It is an environmentally friendly and sustainable solution. However, there are engineering pitfalls associated with the design and construction of plastic geocellular structures. The main pitfalls are associated with creep rupture of plastic structures, potential construction damage and the lack of care in wrapping the cells with filter fabric and backfilling procedure. As the scale and complexity of geocellular structures have significantly increased in recent years, guidance on appropriate design and construction methods has become more essential for these structures to be adopted as safe, yet economic and sustainable solutions.
In this paper, the author will describe his design and construction experience based on research associated with a court case on the damages associated with a major geocellular on-site stormwater detention project (approx. 8.5 Mega litres), and recent conversion of his backyard swimming pool to a 40,000 Litre rainwater harvesting tank. References on design and construction guidance will be described together with the author’s personal opinion on the use of partial factors in the economic design of geocellular structures.
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Ground improvements and supports for embankments and structures of Regional Rail Link – City to Maribyrnong River
This paper discusses ground improvement and foundation support that were adopted in Melbourne’s Regional Rail Link development that was completed in 2015. The development included realignment of rail track within the existing rail corridor and additional bridges, viaducts, and embankments. The project site was a “brownfield” within a relatively complex geological domain of Yarra Delta Sediments including the recent marine deposits locally called Coode Island Silt (CIS) which is a highly compressible and very low strength clay. The presence of CIS and its variable thickness imposed special challenges for design and construction of the rail link and necessitated ground improvement and support works, including: a) preformed driven, bored and CFA piles for bridges, viaducts and underpasses; b) ground support using controlled modulus columns (CMCs) for the embankments with estimated long term settlement ≥ 150 mm; c) preloading and surcharging for embankments with estimated long term settlement <150 mm. The paper will first cover general geotechnical design considerations for ground improvement using CMCs. A case study will then be presented on the detailed design of the North Melbourne Flyover embankment widening.
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Pre-reclamation in situ testing of soft soil
The Changi East Reclamation Project in the Republic of Singapore comprises the ground improvement of marine clay with the installation of prefabricated vertical drains and subsequent preloading. Prior to the commencement of land reclamation works, a series of in situ tests were conducted under marine conditions with the help of various in situ testing equipment. The In Situ Testing Site was located in the northern part of the project where the thickest compressible marine clay existed. The in situ tests carried out were with the field vane, piezocone, flat dilatometer, self-boring pressuremeter and BAT permeameter. In situ tests were conducted to determine the undrained shear strength, overconsolidation ratio, soil stiffness and coefficient of consolidation and permeability of the marine clay. In situ dissipation tests provide a means of evaluating the in situ coefficient of consolidation and hydraulic conductivity due to horizontal flow of soft soil and were used to estimate these properties of Singapore marine clay at Changi.
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A Case Study On The Variability Of The Coefficient Of Consolidation And Its Design Reliability
The consolidation characteristics of cohesive soils are estimated using established relationships between the coefficient of consolidation (cv) and index tests, as well as laboratory oedometer tests. While the design cv is preferred from the field dissipation tests, the conversion from a horizontal to vertical value needs to be considered. A trial load was used to verify the consolidation parameters during a Queensland Road upgrade, which involved both road widening and raising of the existing embankments over compressible soils. Construction was done in 4 stages, and with preloading and surcharging in selected areas. Settlement monitoring and Asaoka plots were used to validate the design, and “moderately conservative” design values were adopted. This case study is used to show the large variability of the cv by the various test methods. While 99% of the site settlement was within the magnitude and time predicted during design, a 25 m length was not consistent with the data and performance of the rest of this site within the flood plain. The back-calculated cv was below the lowest test value and even data from nearby settlement plate monitoring from adjacent stages. In situ tests were located within 25m of this unconforming area and given that stratigraphy was consistent then the cv value adopted may not be representative. The lessons learnt show the various verification and validation process required to envelope risks, but all conditions with a “moderately conservative” design may not be covered.