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Woolgoolga to Ballina Pacific Highway Upgrade – Embankment Performance Monitoring and Back-Analysis
The section of the Pacific Highway Upgrade project between Woolgoolga and Ballina (W2B) traverses areas having significant depths of soft soils. The presence of these soft soils causes significant risks in relation to timely project completion and potential cost blow out. To reduce these risks, soft ground treatment using wick drains and preloading was used in the form of Early Works to de-risk the project and to reduce the post construction settlement. During the embankment construction and preloading period, instruments were installed to monitor the embankment performance. Monitoring was carried out using an innovative web-based system to enable continuous and real time monitoring of embankment performance, technical review and modifications of construction sequence as required.
Monitoring data were back analysed periodically to assess the embankment performance. Through this exercise, additional filling and stability measures were implemented over a number of embankment sections, where the monitored embankment settlements exceeded the design prediction. In addition to the innovative web-based monitoring system, advanced numerical modelling was used to predict future creep settlement using the back-analysed results. It was found that advanced numerical modelling of creep settlement must carefully consider the two-dimensional stress distribution beneath the embankment including toe berms in order to model accurately the stress history and over-consolidation effects on creep after surcharge removal. When the two-dimensional stress distribution is properly taken into account, the results of forward creep prediction matches well with one-dimensional analytical methods such as that proposed by Wong (2010).
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Technical Note: Strength and stiffness properties of defects within the Hawkesbury Sandstone and Ashfield Shale
The laboratory direct shear test results on defects which were undertaken for three recent tunnelling projects in Sydney – the Lane Cove Tunnel, the CBD Metro (which was not built), and the Northwest Rail Link – provide data to assess the strength and stiffness of bedding planes and joints within the Hawkesbury Sandstone and Ashfield Shale.
The results, which are summarised in Tables 1 and 2, comprise:
- 76 tests from the Hawkesbury Sandstone of 13 bedding planes, ten joints, five shears / clay seams and one saw-cut
- 97 tests from the Ashfield Shale of 13 bedding planes, 11 joints and six shears / clay infill.
The results largely represent planar defects as attested by the descriptions of the individual samples and the displacement plots (vertical versus horizontal) in the result sheets (see Figure 1, for an example). In the case of one dataset, the shear and normal stress values were also corrected for dilatancy as recommended by Hencher and Richards (1989).
Hence, the data can be used to estimate the basic friction (ϕb), i.e. “the frictional component of shear strength for a planar or effectively planar discontinuity i.e. independent of any roughness component causing dilation during shear” (Hencher and Richards, 2014).
The author has interpreted the results for shear strength and stiffness.
- The peak shear stress (τ) recorded for the normal stress (σn) applied during each of the direct shear tests was selected.
- The shear stiffness (ks) was estimated by the author as the gradient of the secant line plotted on the elastic portion of the shear stress versus shear displacement graph (see Figure 1). This is in keeping with the approach taken by Bandis et al. (1981).
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Cohesion of compacted unsaturated sandy soils and an equation for predicting cohesion with respect to initial degree of saturation
Embankments which carry highway or railway need firm foundations, for that reason they are generally constructed with compacted sandy soils for which shear strength plays a very important role in stability. However, there is limited available information in existing literature on shear strength behavior of unsaturated sandy soils. Generally the saturation conditions of soils of embankment during raining season are nearer 70%. However, during heavy rains, the soils become more saturated thereby reducing shear strength and consequently failure occurs.
The purposes of this study are to observe the shear strength variation of unsaturated sandy soils with degree of saturation more than 70% and to develop a constitutive equation for predicting shear strength with respect to initial degree of saturation. A series of triaxial shear strength tests conducted on 3 types of sandy soils (taken from Okayama and Hiroshima areas of Japan) with the same void ratio but varying degree of saturation. In all, 9 specimens of each type of soils were considered for undrained triaxial tests with pore-water pressure measurement. The specimens were prepared by static compaction with different initial degree of saturation ranging from 70% to 100% but with the same void ratio for each soil type (0.61, 0.65 and 0.70). The chosen void ratio gives around 90% proctor compaction. Experimental results show that, the shear strength decreases linearly with increase in degree of saturation. Moreover, experimental studies to determine the shear strength parameters of unsaturated soils are generally costly, time-consuming and difficult to conduct; therefore, many equations have been proposed to predict the shear strength of unsaturated soil with respect to matric suction in the last two decades. However, the measurement of matric suction of unsaturated soil is not easy. This paper also proposes an equation to predict cohesion of unsaturated sandy soils with respect to initial degree of saturation based on experimental results.
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Modelling and uncertainty analysis of softening behaviour of sand using disturbed state constitutive model
An evaluation of the predictive ability of a disturbed state constitutive model for softening behaviour of sand is performed. The model is shown to capture the softening behaviour of sand under both drained and undrained monotonic loading conditions. To use a particular constitutive model into a finite element or finite difference numerical procedure to solve real geotechnical problems, it is often desirable and necessary to include the variability in constitutive model parameters to capture the effects of model imperfections and inherent soil variability. Uncertainty in stress strain characteristics of sand due to inherent soil variability and model parameters’ variability are analysed. The sensitivity or the effects of disturbed state constitutive model parameters on stress dilatancy characteristics is stochastically analysed using first order second moment method to include the effects of model error and inherent soil variability. The most influencing parameters for both the deviatoric stress and volumetric strain are identified.
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Designing For Brownfield Sites – A Young Geotechnical Engineer’s Perspective
Upgrading existing road infrastructure is becoming increasingly common in Sydney. The growing population places increasing demands on existing road infrastructure, requiring upgrades to cope with growing traffic volumes. Some of these upgrades require structures such as retaining walls and bridges. Upgrades sometimes require modifications to existing structures, which further complicates design and construction. Working on ‘brownfield’ sites adds further complexity to the design of both new and existing structures. Site constraints such as limited space, low headroom and both buried and overhead utilities are some of the factors that contribute to the complexity of designing on brownfield sites. Furthermore, sites are usually required to remain open to traffic during investigations and construction. This paper focuses on the methodologies the Author has learnt and adopted to address these challenges to geotechnical work during the site investigation, design and construction phases. Case studies are presented to illustrate some of the challenges and solutions.
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Trees: A root cause of rockfall
This paper discusses the general processes by which trees cause damage to rock slopes. It discusses the actions of root elongation, which can cause root jacking, and trunk expansion. It discusses research aimed at establishing a relationship between root diameter and the forces applied by roots. The results of this research are inconclusive and at this stage provide little useful information for local geotechnical design purposes. It is concluded that, in general, trees should be removed from bare rock slopes adjacent to public infrastructure where there is greater than a moderate risk associated with rockfalls unless other measures are taken to reduce the risk (e.g. rockfall fence or mesh).
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2018 GHD Prize in Geomechanics
Presentations from Finalists
Maddison Marquez Brand, James Cudmore and Lewis Gooch