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The Karuah Bypass: A case-study in the engineering geology of the Southern New England Fold Belt
The recent construction of the Karuah Bypass provided an excellent opportunity to improve the state of knowledge of a poorly documented geological sequence in the southern New England Fold Belt and to evaluate some of its engineering characteristics. The 2500 m thick, fault bounded, sequence of rocks transected by the Bypass has been previously consigned to the western Myall Block of the Tamworth Belt. This work has established that it spans an interval that begins with the Johnson’s Creek Conglomerate to the west, includes the McInness and Booral Formations and terminates with the Karuah Formation, which is probably truncated by a fault. These predominantly terrestrial formations are dominated by thickly bedded sandstones and conglomerates, with a variable tuffaceous component (mostly siliceous) and minor shales and rare coal seams. They contain several significant volcanic units, which despite having considerable thickness, were generally not encountered in excavations along the selected road alignment. Residual soils derived from these formations are almost exclusively clays, ranging broadly from low to high plasticity. Where encountered, rocks were mostly of high to very high strength, with some units retaining very high strengths in close proximity to the surface. Due to the presence of extensive localised and regional faulting, outcrop in some areas of the alignment was very poor and the depth of mottled, residual clay rock was considerable. Anomalous conditions encountered along the Bypass include pyrite-bearing dacitic volcanics with acid sulphate potential, layers of pedogenic silcrete that impeded pile driving, numerous deeply weathered basaltic dykes and bedding-parallel thrust faults that show small displacement.
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Seasonal variations in the performance of driven piles in sand
This paper explores the performance of model pipe piles driven into a silica sand deposit at the Water Authority Research Station in Perth, Western Australia. The principal aim of the research was to explore pile response over a range of conditions from fully closed-ended to fully open-ended. There has been some considerable debate of the relative capacity of closed and open-ended piles driven into similar deposits of sand. In many design codes, it is customary to assume lower radial stresses (and hence shaft friction) for open-ended piles, than for solid or closed-ended piles. The base capacity is often taken as the same for both pile types, provided it can be demonstrated that the pile will ‘plug’ under static loading conditions. Potentially lower shaft capacity of open-ended piles is compensated for by the ease of installation since, in principle, the base resistance only has to be overcome around the steel annulus, as a plug of soil moves up the inside of the pile. However, in some circumstances open-ended piles may plug during driving, which leads to increased driving resistance and possible refusal before the design penetration has been achieved. This tendency may be alleviated by the use of an appropriate internal driving shoe, which provides stress relief for the soil plug.
The test results indicated a significant effect of season on the performance of the piles. The test programme included both dynamic and static testing of each pile, allowing a comparison between dynamic and static measures of capacity in tension and compression. Those tests carried out in the summer showed greater pile capacities in both tension and compression than those tests carried out in the winter. Equally the tests carried out in the summer showed a stiffer initial response than those carried out in the winter. The static test results were consistent when examined in conjunction with blow-records and the dynamic data. The test results indicated an optimum design for the driving shoe, which maximised soil plug penetration into the pile under hard driving. Of particular interest was the finding that the pile tip condition appeared to have little effect on the shaft capacity, but did influence the rate at which the end-bearing resistance was mobilised (due to compression of the internal soil plug). Thus, where the base resistance was defined at a fixed tip displacement, open-ended piles had a lower resistance than closed-ended piles. The test results are reviewed in the light of current design methods for driven piles in sand, and recommendations made for future design.
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Postscript to the closure to the discussion of “Assessment Of The Coefficient Of Consolidation With Queensland Data”
This paper presents a postscript to the closure to the discussion of “Assessment of the coefficient of consolidation with Queensland data” by Burt G. Look. https://doi.org/10.56295/AGJ5846.
The Discusser thanks the Author for providing a detailed Closure touching on several aspects of soft soil consolidation. However, in contrast to the Discussion the Closure (and Paper before it) had no long-term, independently (or at least able to be) validated case histories supporting the claims made.
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Case Study Of Railway Embankment Stabilisation Works At Moor Park Station With Progressive Slope Failure
Two railway embankments which are located next to Moor Park station, approximately 30 km northwest of London, support the southbound and northbound London Underground Metropolitan Line tracks. The embankments were identified as showing various signs of progressive slope failure and required implementation of stabilisation works. The earthworks originally constructed in the late 19th century were subject to embankment widening works in the 1940s and have had a complex history of construction and modification with evidence of past instability.
For the remedial works, there were a number of geotechnical and construction issues which included highly variable ground conditions, environmentally sensitive sites surrounding the embankments including a man-made river and existing rail structures along the embankments. This paper describes the measures taken to manage the geotechnical risks.
The adopted design criteria, the slope stability assessment and the proposed remedial works which include a bored pile wall with precast concrete capping beam, a discrete bored pile wall, a slope regrade and toe berm are presented in this paper. For the design and slope assessment, modified soil strength parameters for high plasticity index cohesive embankment fill or weathered London Clay identified from soil investigation was used.
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Bearing capacity of shallow footings in sand over clay by the punching shear method
The bearing capacity of shallow strip, square and circular footings by the punching shear model in stronger sand overlying weaker clay is examined by using published computer solutions and experimental test results on model footings. The Meyerhof punching shear coefficient is shown to be dependent on the strength ratio f’bw/fbs and the footing type, and is used to derive the punching shear model equations for the bearing capacity for sand friction angles ranging from 30˚ to 50˚, and for f’bw/fbs ranging from 0 to 1. The equations are used to explain the effect that several variables have on the bearing capacity, including the sand thickness, the clay strength, the sand strength and the surcharge. The application of these concepts to a variety of geotechnical problems is illustrated by six worked examples. Provided f’bw/fbs>0.4 to 0.5, the simple equations derived from the punching shear model provide a very rapid and convenient means of obtaining the bearing capacity of footings in sand over clay in a sufficiently accurate manner, suitable for routine geotechnical practice.
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Performance And Prediction Of Vacuum Combined Surcharge Consolidation At Port Of Brisbane
During the past decade, the application of vacuum preloading for stabilising soft coastal clay and other low-lying estuarine soils has become popular in Australia. The cost-effectiveness is a major factor in most projects in view of the significantly reduced time for achieving a relatively high degree of consolidation. Due to an increase in trade activities at the Port of Brisbane, new facilities on Fisherman Islands at the mouth of the Brisbane River will be constructed on the new outer area (235 ha) adjacent to the existing port facilities via land reclamation. A vacuum assisted surcharge load and conventional surcharge scheme in conjunction with prefabricated vertical drains was selected to reduce the required consolidation time through the deeper subsoil layers. The design of the combined vacuum and surcharge fill system and the construction of the embankment are described in this paper. A comparison of the performance of the vacuum combined surcharge loading system with a standard surcharge fill highlights the clear benefits of vacuum consolidation. Field monitoring data are presented to demonstrate how the embankment performed during construction. The paper also evaluates the relative performance of the two contrasting vacuum preloading systems (i.e. membrane and membraneless systems. An analytical solution for radial consolidation considering both time-dependent surcharge loading and vacuum pressure is proposed to predict the settlements and associated excess pore pressures of the soft Holocene clay deposits.
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Design of Railway Embankment over Soft Ground
This paper discusses the geotechnical design and performance monitoring of a 2.6 km long railway embankment constructed over soft ground at Hexham, located approximately 16 km northwest of Newcastle and 160 km north of Sydney. The rail alignment traverses over soft ground with soft soil thickness of more than 25 m on the southern end and about 12 m on the northern end of the project corridor. There were challenges in relation to the geotechnical design including (a) design of adequate formation to reduce the risk of soft subgrade failure under cyclic loading (b) excessive settlement of soft ground with time, impacting the performance of the railway tracks in the long term and (c) presence of waste material in the site, as between the 1960s and 1990s, the area was the site of a coal washery and loading facility. Hence, the site has already been preloaded with varying thickness of coal reject fill material placed in isolated areas along the proposed rail corridor, causing potential differential settlement of the proposed rail tracks. Therefore, geotechnical design of formation needs to be carried out by adopting appropriate subgrade modifications in order to reduce (a) the rail formation thickness above the existing ground surface and (b) post construction settlement. Selection of appropriate subgrade modifications requires careful consideration of number of factors including ground conditions, available construction time, performance requirements and cost.
A number of subgrade modification methods including remove and replace, rigid/semi-rigid columnar inclusions, and mass stabilisation using cement or lime have been considered to reduce the post construction settlement as well as to provide a stable formation for the railway embankment. Each modification method has its own advantages and limitations. After discussion with construction team and the Australian Rail Track Corporation (ARTC), “remove and replace” has been selected as a suitable method considering (a) relatively low cost and easy to construct; (b) reduction in primary settlement due to removal of material; and (c) significant testing regime and possible installation difficulties through coal reject fill associated with columnar/mass stabilisation methods. The assessed post construction settlement has been provided to ARTC for the development of maintenance regime. An approach of tamping the tracks to maintain the track geometry within ARTC Standards has been adopted.
The rail tracks have been constructed and in operation successfully from late 2014. Settlement monitoring has been carried out during and after construction. The observed settlements are reasonably consistent with the design prediction. In addition, dynamic deflection of the track has been monitored during the operation of tracks and compared with the design prediction. This paper summarises the geotechnical site investigation, development of design parameters, selection of subgrade modification method, formation design, and back calculation of settlement and dynamic deflection to compare with monitoring data.
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Effects of salinity and sand content on liquid limit and hydraulic conductivity
Soil conditions of construction sites have become worse than ever due to the overpopulation in the metropolitan areas throughout the world. Likewise, the prevention of environmental risks due to individual activities is one of the most important subjects in the geo
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Urban salinity scoping study for Greater Launceston area: Part 2 Investigation work
The investigation of a pilot study area in the suburban Greater Launceston Area (GLA) has confirmed that soils at a number of sites in the study area have high levels of salinity, much of the salts being readily soluble. The investigation also found medium to extremely saline groundwater conditions throughout the study area. The types of salt in the soil and groundwater are broadly related to two groupings based on the mapped surface geology: (i) Tertiary and undifferentiated sediments; and (ii) dolerite and dolerite derived sediments. Groundwater levels in unconfined and semi confined aquifers generally parallel the local topography, with localised upward and downward flows possible anywhere on the gentle slopes of the pilot study area. Areas of observed salinity impact at the Casino Golf Course and St Patrick’s School sports grounds are related to intense irrigation, construction of water storage ponds (Casino) and cut and fill slope modification (School).
The study concludes that groundwater recharge and seepage in response to rainfall and irrigation contributed to creating artesian conditions in the Tertiary sediments which mobilize in situ salts in areas with upward flow conditions. Specific geological confining conditions probably have less of an impact.
The study has no temporal data to allow an assessment of the effect of changing land use on groundwater levels and quality, and therefore any predictions of areas at risk from future salinity outbreaks have no ‘calibration’ at this stage.