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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.
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Effects of internal stress state on performance of embankments on soft soils
Analysing the behaviour of soft soil under embankment loads is a challenging task for geotechnical engineers. This numerical study revisits the case study of an embankment constructed on Boston Blue Clay, considering the model incorporating the influence of soil creep on the ground lateral deformation and pore water pressure. Then a numerical parametric study is conducted to investigate the influence of the lateral earth pressure coefficient at rest (K0) calculated based on the different available correlations on the vertical and horizontal displacements, pore water pressures, and the factor of safety of the embankment. The results indicate that although the value of K0 influences the predicted horizontal and vertical displacements notably, it has minor effects on the predicted pore water pressures. The lateral earth pressure coefficient influences the long term stability of the embankment and thus the effects of the initial stress field on the stability of embankments should be taken into consideration, while assessing the performance of embankments constructed on soft soils. It is recommended to determine the in situ horizontal stresses using accurate methods such as self-boring pressuremeter to predict the behaviour of embankments on the soft ground precisely
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Shaft design for large-diameter microtunnelling in soft ground
This paper examines parametric studies in the geometrical dimensioning, configuration and spatial efficiencies of large diameter shafts. Structural design aspects, construction solutions for excavation stability and parametric studies on ground improvement for deep shafts are also discussed.
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Site characterisation by the HVS Technique
A geotechnical characterisation study of a proposed development site has been conducted using the non-invasive passive ambient noise HVSR technique. The simplicity of the HVSR technique is manifested in having a short setup time (relative to other methods), and precluding the need for any active excitation sources and external cabling. In the study a lightweight battery operated sensor was utilised to capture the prevailing ambient noise (microtremors) reflecting the structure in the ground. Microtremor measurements were taken at regularly spaced stations at the proposed development site, with each recording at a station taking no more than 10 minutes. Thus the entire site was measured expeditiously within hours. The site characterisation involved (1) estimating the site fundamental resonance frequency and amplification, parameters of interest in the study of a site’s susceptibility to seismic activities (2) interpreting the measured HVSR curves for a preliminary assessment of the soil layering and (3) fitting the HVSR curves to a theoretical model to estimate the shear wave velocity (Vs) profile of the ground. Results from the HVSR technique have been verified against data from classical invasive methods (viz. borehole data and SPT). Further verification has also been made against the results from the array based MASW technique. The study suggests that the HVSR technique could be used for characterising a site in combination with a reduced number of mechanical in situ tests, and especially to fill in the gaps of the soil stratigraphy at the locations not covered by the mechanical tests.
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Case study: Compaction of coal ash fill by impact rolling as a method of subgrade improvement beneath a heavy duty pavement
An intermodal logistics centre for bulk distribution of shipping containers by both road and rail is currently under construction at a site near Enfield in SW Sydney. The 70 ha project site was used as a rail maintenance facility until the early 1990’s, when it was largely dismantled. A container terminal yard will occupy approximately one-third of the project site, which will require construction of a heavy duty pavement to support the operational loads. The design of the terminal area is based on construction of a uniform earthworks platform, requiring both cut and fill to attain design levels, to act as a stable subgrade for the pavement layers. Previous studies revealed the presence of poorly compacted fills across the site that vary significantly both in composition and thickness. Of particular interest was the presence of a significant volume of coal ash fill, which was the thickest fill deposit across a large part of the site. The coal ash was found to be poorly compacted, but laboratory testing indicated it could achieve high strength and CBR index values when compacted to a dense state. The coal ash could thus be treated in place and incorporated into the subgrade earthworks or it could be mined and re-used as general fill in other parts of the site. A programme of field compaction trials was implemented, using conventional vibratory rollers and also impact rollers to determine the optimum compaction method for the coal ash and other fill materials. The field trials showed that compaction of the coal ash significantly reduced its void ratio by approximately 30% with corresponding settlement on compaction. The outcome of these field trials enabled the contractor to develop a ground treatment methodology tailored to the fill material type, which also conformed to the design specification for the subgrade layers.
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Modified Asphalt By Coffee Cup Fibres: An Optimum Mix Design Using Response Surface Method
Stone Mastic Asphalt uses fibres to stabilise high binder contents. The fibres are typically made from natural cellulose, and prevent binder draindown. This study investigates the use of post-consumer coffee cups (liquid paperboard) as a recycled fibre alternative. Coffee cups (CC) use laminated polyethylene as a liquid barrier, making them challenging to recycle in existing paper and plastic recycling systems; approximately one billion coffee cups are landfilled annually in Australia. In this research, coffee cup fibres are used up to 0.6% by total weight of the asphalt mixture across fourteen mix designs. The mixes are optimised by the response surface methodology using the Design-Expert software. Models are created based on Marshall and draindown test results. This study finds that shredded coffee cups effectively reduce the draindown, reduce flow of the asphalt mixture, increase stability, and increase the air voids. The LDPE content slightly increased the viscosity and the softening point of the asphalt binder and dropped its penetration value. Overall, the asphalt mixture containing 0.4% coffee cups (by total weight of the mixture) met all specifications in Australian Standards and performed similarly to the commercial cellulose fibre. This paper also provides practical insights from field trials conducted in Western Sydney using the optimised parameters identified. The pavements will continue to be monitored to develop long term performance comparison between CC and traditional fibres in SMA applications.
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Geotechnical Stability Analysis
51st Rankine Lecture
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Ground Improvement By High Impact Energy Dynamic Consolidation Approach
Several ground improvement approaches can be utilised when constructing over poor ground conditions. Where soft grounds are encountered near ground surface, consolidation approaches such as surcharge with Prefabricated Vertical Drains (PVD) are commonly adopted. It is known that surcharge fill placed over soft clayey ground pressurises the porewater within the soft clay, creating an excess pore pressure. This excess pore pressure drives the pore water towards the PVD which forms a drainage medium driving the excess pore fluid towards a horizontal drainage blanket.
In situations where surcharge fill is difficult to obtain near the site, or costly to be brought to site, an alternative method that utilises dynamic compaction energy to pressurise the pore water can be applied. This approach has proven to be capable of developing excess porewater pressures in soft saturated clays. When this method is applied together with PVD, the drainage of the ‘locked-in’ excess pore fluid can be expedited.
A method of dynamic compaction using rolling impact compaction approach known as High Impact Energy Dynamic Compaction (HIEDYC) is described in the paper, a process known as dynamic consolidation. Two case studies of applications of this approach in soft ground improvement, together with back analyses, are described in this paper. These case studies showed that dynamic consolidation can offer several benefits and produce better or equivalent performance as conventional treatment, including higher bearing capacity, cost saving, and shorter treatment duration.