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Design Groundwater Levels
Successful design of structures located within or below the groundwater table requires assessment of the variation of groundwater levels over the design life. Currently the methods used for prediction of future water levels are poorly developed usually relying on extrapolation of a limited period of on-site monitoring or reliance on other historical records that may bear little relevance to the site.
Adoption of overly conservative water levels can have a very significant impact on design and construction costs. Therefore, an accurate assessment is required to achieve a design that achieves the optimal balance between risk and cost.
This paper discusses the pitfalls associated with these methods and presents some examples of failure to select appropriate design groundwater levels. Other methods are discussed than may be employed to provide an alternate and potentially more accurate assessment of design water levels so that the risk of adopting poor design levels may be reduced in the future.
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Reliability analysis of upheaval buckling of offshore pipelines
Offshore pipelines are commonly buried in seabed for protection against damage, for better insulation and to prevent upheaval buckling induced by thermal and pressure loadings. The uplift resistance provided by the backfill soil is an important design parameter when determining the correct burial depth for a given pipeline. In this paper, the effect of variability in soil backfill stiffness and operation conditions on the performance of the pipeline upheaval behaviour is investigated. Variations in the soil backfill stiffness, pipe properties and the operational factors such as temperature and pressure are considered to assess the safety of the pipeline probabilistically. An optimized Latin Hyper Cube (LHC) sampling technique is used to draw the sample of soil stiffness, pipe properties and operational conditions from preassigned probabilistic distribution for each variable. Pipeline behaviour was simulated using elastic model, and the interaction was modelled using pipe-soil interaction elements using ABAQUS. The response surface method was used to establish approximate functional relationship between the input parameters and the output response. Reliability analysis of pipeline was performed using first order reliability method and simulation method. The results presented are useful to better understand the performance of offshore pipeline and probabilistic upheaval buckling assessment.
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Effective stress versus total stress analysis of undrained problems in geotechnical engineering
Stability and strength analysis in geotechnical engineering can be carried out in terms of either effective or total stresses. Given fundamental knowledge of soil mechanics and clear understandings of numerical modelling, the numerical simulations should result in consistent outcomes from both approaches. Undrained excavations were modelled in Abaqus, a software application for finite element analysis. The Extended Modified Cam Clay model was used to characterise the soil behaviour in the Effective Stress Analysis (ESA) and the Tresca model was used in the Total Stress Analysis (TSA).
For the comparison between ESA and TSA to be valid, it is critical for both analyses to represent identical soil conditions and characteristics. Therefore, the fundamental part of the procedure was to derive the values of total stress parameters from the effective stress parameters and numerical outputs from ESA. In order to confirm the precise match of soil conditions between ESA and TSA, initial stress distributions and initial values of K0 were compared.
The shape of yield surface in ESA was modified to minimise the difference in the yield surface between ESA and TSA. The values of su were also adjusted to reflect the shear strength in the plane strain problem. While those modifications improved results, most of the numerical outputs showed inconsistencies between ESA and TSA. By comparing the maximum values of forces and moments of structural elements, neither method produced results that were consistently greater than the other method throughout all excavation scenarios. It was justified that the differences in the structural forces and moments were mainly due to the differences in the passive stress on the retaining wall between ESA and TSA. The observations on the stress paths of passive soil elements revealed that the passive soil for all ESAs did not reach the critical failure state, and for TSAs the soil reached the failure for cantilever problems, but not for propped excavations.
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Limiting force profile for laterally loaded piles in clay
Response of a laterally loaded pile is normally dominated by limiting force mobilised along the pile. The limiting force profile (LFP) varies from pile to pile at different sites. In order to provide guidelines for constructing the LFP, in this paper, an extensive back-estimation has been made against measured responses of 32 piles tested in situ. It was done using a spreadsheet program called GASLFP, which in turn was based on closed-form solutions. The solutions and GASLFP were developed by the first author in 2001 and 2002 respectively. Parameters obtained through the backestimation are presented herein for each pile. They indicate remarkably lower resistance than that derived from the conventional Matlock LFP for 18 piles; provide an average (slip) depth of 7.2d (d = pile diameter); offer an average ratio of modulus of subgrade reaction over shear modulus of 3.0 and an average ratio of the shear modulus over undrained shear strength of 92.3. These values may be directly used to design laterally loaded free-head piles and only the soil within the slip depth may need to be carefully investigated or improved.
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Settlement characteristics of deep engineered fills
The long-term settlement characteristics of engineered fills are generally not a significant concern where the fill thickness is less than a few metres. With the increasing scarcity of land for urban and commercial development and the availability of large volumes of excavation materials, sites are being considered for development that previously would have been filled with refuse of other uncontrolled material and left undeveloped. For such sites, including backfilled quarries, the long-term settlement characteristics of the fill becomes an important consideration.
In this paper the settlement of engineered fill has been characterised as having four potential components:
- Short-term Settlement, which occurs due to self-weight as the fill is placed and for a relatively short time after fill has reached full height.
- Elastic Settlement, which occurs in the fill when subjected to loads from footings and floor slabs.
- Long-term or Creep Settlement, which occurs over a period of years. In the case of deep fills with light building loads, the creep due to the self-weight of the fill will be the major component of the long-term settlement.
- Hydroconsolidation (Collapse) Settlement, which can occur and is due to saturation of the fill.
This paper presents data on the settlement characteristics of deep fill with emphasis on the characteristics of engineered fill (i.e. fill placed and compacted in relatively thin layers to an engineering specification).
Data derived from laboratory testing and field monitoring is provided for a variety of materials placed as deep engineered fills for a number of projects in the Sydney Region.
Settlement estimates derived from parameters obtained from a desk study of international literature and some data from the Sydney Region are presented and compared with the results of more recent laboratory testing on materials from the Sydney Region.
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Geotechnical modelling of station caverns for the Epping to Chatswood rail line project
The Epping to Chatswood Rail Line project comprises a twin rail tunnel, three new underground stations and the upgrading of one existing station. The station caverns intersect a sequence of horizontally bedded shale and sandstone. The major rock defects consist of bedding planes, bedding plane seams, low angled cross bed partings and sub-vertical joint sets. Local experiences in the Sydney Basin have indicated that the behaviour of the defects and their interaction with the roof support system are critical to the performance of underground excavations. Another key geological feature that can have significant performance impact is the relatively high locked-in in situ lateral stresses.
Each of the new stations comprises a large span platform cavern, an adjoining concourse cavern, associated escalator shafts and service buildings. The design roof support system is cable bolts with cement grouted end anchorage. The interaction between the bolts and the jointed rock mass within the influences of the various facilities is complex and thus rigorous modelling was employed. This modelling included 3D distinct element and boundary element analyses and 2D finite element approach. The complexity of the numerical models varied from homogeneous rock to layered rock with various discontinuities. Furthermore, both end-anchored and fully grouted bolts have been incorporated.
Various parametric studies were undertaken to assess the effects of various model components (rock mass, defects, in situ stresses, sequencing and roof support installations) on cavern performance. Based on the modelling results, a system of rock bolts and construction staging has been adopted to optimise the permanent support system.
This paper is confined to the geotechnical modelling aspects of the project and presents the geological setting and the various analytical procedures undertaken. The results of the parametric studies and their impact on the final selection of the support systems are also outlined.
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Characterisation Of Complex Ground Conditions For The Rozelle Interchange Project
The Rozelle Interchange Project (RIC) in Sydney is an underground motorway interchange connecting multiple underground and surface arterial roads as well as the future Western Harbour Tunnel and Beaches Link. RIC completes the WestConnex program of works and is a complex array of approximately 22 km of multiple level tunnels, all constructed in an area 2.5 km long and 1.5 km wide.
RIC is located within complex ground conditions that include deep soils, regional faults, structural zones and igneous intrusions. Deep natural soils infilling a valley near Rozelle Bay are mostly recent Holocene alluvial, marginal marine and marine deposits. These soils are interlayered, discontinuous, normally to slightly over consolidated and capped by sand and coarse rockfill from 19th century reclamation.
There is a strong contrast in the level of detail between borehole and CPT data. Distilling this to provide a geological and geotechnical model for a project wide interpretive report for designers of multiple structures required a hybrid approach to model presentation. This included providing a simplified graphical model and including details from specific investigations and laboratory testing allowing designers flexibility to adopt appropriate parameters for their specific application.
Similarly, the rock structural model evolved from development of structural domains to identification and inclusion of regional geological structures overprinting the structural model. Regional scale thrust faults, corridors of structural complexity and igneous intrusions were identified and refined prior to and throughout the design process. These were considered in the design by modification of excavation sequencing and changes to tunnel support.
Tunnel excavations encountered these regional features at the locations predicted and with similar character as those described in the model allowing the safe construction of the tunnels.
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Excessive Post-construction Settlement Of Improved Ground โ Case Histories
This paper presents a few case histories where excessive settlements were reported following ground improvement of a number of soft soil sites. The case histories involved different ground improvement techniques such as preloading and surcharging with and without prefabricated vertical drains (PVD), deep soil mixing, concrete injected columns and vacuum consolidation. The ground conditions and the adopted ground improvement designs are discussed. The observed post-construction settlements for the various cases are also presented. Further, the back-analysis works are detailed in order to provide some insight into the possible contributing factors to the measured excessive settlements.
It is clear from these case histories that observational approach by monitoring the ground behaviour during and after ground treatment and construction should be adopted to ensure that the post-construction performance is consistent with design expectation. In addition, the paper demonstrates that selection and design of the ground improvement techniques should be conducted with clear understanding of the theoretical background and limitation of the improvement techniques, regardless of the system adopted. Consideration of construction activities and staging is also important in order to capture the impact of various construction loading on soft soil consolidation and settlement.
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Comparing the embodied carbon and local environmental impact of common geotechnical foundation solutions for the Australian market
In the Australian geotechnical market, foundation solutions for construction projects are commonly assessed against three key criteria; cost, program and quality. Safety and environmental aspects are often not assessed in the process until a geotechnical foundation technique has been selected based on the aforementioned criteria. As climate change has progressed to become one of the largest and most pertinent environmental issues in society today, it has in turn brought sustainability to the forefront of post construction assessments and is now a greater focus for government bodies, private developers and wider stakeholders. Using Kellerโs in-house carbon calculator and a series of environmental metrics, this paper aims to inform decision makers of the carbon emission and social impacts of various geotechnical foundation solutions prior to their selection.Two theoretical projects are used to compare a range of geotechnical solutions: a 60-storey high-rise building requiring heavy foundations, and a low load warehouse building where ground improvement solutions can be used. Each geotechnical solution designed for the two Australian projects is then assessed using a standard cradle-to-gate carbon calculator. In addition, the impact on the local community is assessed according to the noise, traffic and emissions generated by each solution.
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Estimation of Seismic Load for Design of Low Consequence Category Tailings Dams
Seismic design of embankment dams often requires assessment of the earthquake induced ground and dam movements. There are multiple cases where earthquakes have resulted in sliding and lateral spreading of embankments, crest settlement, and in some instances liquefaction and embankment failure. Hence, evaluation of the effects of earthquakes on embankment dams is of paramount importance for the design. A site-specific seismic hazard study is generally the first step to estimate the potential earthquake loads and the results are often presented in the form of site response spectra, earthquake time histories, and/or plots of peak ground acceleration (PGA) for โrock outcrop”. The PGA and earthquake motions which are used in liquefaction, stability and deformation analyses of embankment dams are to be further processed to develop those within or at the top of the structure. Empirical ratios or deconvolution of the input motion are used for this purpose. There are significant uncertainties involved with the former approach whilst for the deconvolution process earthquake time histories are vital, which are not readily available, at least during the early stages of a project. In this study, a series of one-dimensional response analyses of several Australian dam sites were carried out using SHAKE software (GeoMotions, 2012) to investigate the effects of material parameters (i.e. shear wave velocity) and embankment height on amplification ratios. The results are presented and compared in this paper. A general framework is also provided to estimate the PGA for the embankment analyses for low to significant consequence category tailings dams in Australia where the earthquake design load or a site-specific seismic hazard analysis is not available.