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A rational procedure for the evaluation of soil design parameters for use in Adelaide
This paper is concerned with site investigation practices that are used as part of the design of foundations for civil engineering infrastructure in Adelaide. The semi-arid conditions in Adelaide mean that the results of any soil testing need to be viewed within the context of soil moisture deficiency, and the soil parameters adopted for design need to take into account the critical moisture profile likely to be encountered over the service life of the structure. It is argued that guidelines should be developed based on the soil moisture conditions in order to ensure that realistic soil parameters are used in analyses and designs. The paper presents a procedure for selecting shear strength and compressibility design values. The variations in total soil suction profiles due to construction activities and over the service life of the foundation are highlighted.
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Laboratory Investigation On The Use Of Vertical Drains To Mitigate Mud Pumping Under Rail Tracks
The build-up of excess pore water pressure (EPWP) in undrained soft subgrade under repeated rail loads is the key mechanism causing soil to fluidise, consequently yielding slurry tracks (i.e., mud pumping). This issue has substantially reduced transport efficiency associated with immense cost for track maintenance though considerable effort has been made over the past years. Therefore, this study is carried out to investigate how prefabricated vertical drains (PVDs) can be used to mitigate the accumulated EPWP and associated mud pumping. A series of cyclic triaxial tests including undrained (i.e., without PVDs) and PVD-assisted drained soils are conducted, and their results are compared to evaluate the effect of PVDs on cyclic soil behaviour. In this investigation, subgrade soil collected from a mud pumping site is used while loading parameters including the frequency, confining pressure and cyclic stress ratio (CSR) are considered with respect to heavy rail load condition in the field. The results show that PVDs can help dissipate effectively the accumulated EPWP, thus mitigating soil fluidisation. The current study shows that for undrained condition, lower frequency loading (i.e., slower trains) takes a smaller number of cycles to cause soil failure, whereas for drained cases (i.e., PVDs-assisted specimens), an opposite trend is observed. The study proves that installing PVDs into shallow layer (i.e., 3-5 m depth) is an effective approach to stabilise soft subgrade soil under rail tracks.
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A Study Of The Effects Of Municipal Landfill Leachate On A Basaltic Clay Soil
The performance of a landfill clay liner is generally evaluated using the hydraulic conductivity values obtained from laboratory tests during the design stage. Laboratory tests for the determination of hydraulic conductivity are frequently carried out either using water as the permeating liquid or some times using a chemical permeant to represent leachate. However, any investigations incorporating time as a variable in analysing the effects of leachate on various other soil properties that can influence the hydraulic conductivity are very limited. This study is aimed at investigating the effects of landfill leachate on the performance of a compacted basaltic clay soil, over a period of time. For this purpose, a typical Melbourne basaltic clay with varying percentages of montmorillonite clay was selected and a synthetic leachate was developed based on the composition of typical municipal waste landfill leachate reported in the literature. The clay – leachate interactions were allowed take place under controlled anaerobic laboratory conditions. Samples were then tested at different time periods to identify possible variations of engineering properties such as volume change, consistency and grain size distribution due to the effect of leachate over time, since variation of these soil properties can affect the hydraulic conductivity of a clay soil. The analysis of test results suggests that the behaviour of a basaltic clay liner could be significantly affected by clay leachate interactions over time, due to possible alterations to physical and mineralogical properties of the clay.
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Factor of safety in AS4678: Earth retaining structures
Factor of safety is used to provide safety margin over the theoretical design capacity to allow for uncertainties in loading, material strength and design process. Design of earth retaining structures has traditionally been based on the overall factor of safety method. However, the current Australian Standard for Earth Retaining Structures, AS4678-2002, is based on partial factors of safety method. In this paper, cantilever retaining walls and embedded sheet pile walls have been designed based on the recommendations of AS4678-2002 to examine the overall factor of safety inherent in the standard. Various wall heights and soil parameters are used in the designs. The overall factor of safety is then back-calculated for each wall based on its designed dimensions. The results of analysis are presented in the form of the overall factor of safety associated with the dimension of the walls and soil properties. The overall factor of safety of walls in cohesionless soils varies between 1.7 and 2.3; shorter walls have higher factor of safety. However, when the backfill soil has some cohesion, the overall factor of safety is generally higher than 2 and becomes more than 5 for soil cohesion greater than 30 kPa. For embedded sheet pile walls in cohesionless soils, the factor of safety remains constant for one particular type of soil, regardless of the height of the wall. The results of analyses of these walls in cohesionless soils also show that the factor of safety increases slightly as the friction angle of the soil increases. For the walls embedded in cohesive soils, the overall factor of safety is higher compared to those in cohesionless soils and this behavior is consistent with the one observed in cantilever retaining walls.
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Woolgoolga To Ballina Pacific Highway Upgrade – Reliability Assessment Of Soft Ground Treatment Design
Eleven road sections with an approximately 20 km total length out of the 155 km Pacific Highway Upgrade project between Woolgoolga and Ballina (W2B), NSW traverse areas having significant depths of soft soils. At Maclean Interchange or Clarence River Interchange, soft soil thickness was up to 20 – 25 m under the road alignment. Soft ground treatment design for the identified soft soil areas was undertaken in 2014. The main objectives of the soft ground treatment were to provide certainty of delivery of the highway upgrade within a given time during the main contract with a satisfactory long-term pavement performance.
The highway section between Whytes Lane and Pimlico Road of approximately 3.85 km is one of the longest road sections underlain by up to 8 m thick soft soil that required ground treatment. Due to the significantly length of the soft ground treatment for this road section, one of the main objectives was to reduce or optimise the cost of soft ground treatment.
During the detailed design stage, soft ground treatments using preloading with or without Prefabricated Vertical Drains (PVD) were considered. Due to issues such as sample disturbance during soil sampling and transporting, limitations of the adopted soil testing methods and equipment, limitations of the available geotechnical investigation information, there was a possibility that the actual ground behaviour could be different from the predicted behaviour using the design soil parameters. Reliability analyses were carried out to assess the potential variability of material parameters on embankment settlement and ground treatment requirements.
The reliability assessment provided quantitative confident levels of the ground treatment designs and suitable contingency measures. The reliability assessment provided indication of the cost and risk balancing. The target confidence level was minimum 70% for the soft ground treatment design with the proposed observation method and contingency measures such as placement of additional surcharge or additional preloading time to respond to changes during the preloading period. The reliability assessment also effectively assisted the client’s decision on the preferred soft ground treatments.
The adopted reliability assessment method as described in Duncan (2000) and the assessment results for the soft ground treatment design were presented. The embankment settlement was monitored during the preloading stage and was back analysed. The reliability assessment results, which were analysed in the design stage, and the ground treatment design were reviewed against the actual embankment settlement performance.
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Comparing numerical modelling finite element results with full scale instrumented pile response in weakly to moderately cemented soil
Design of drilled and grouted piles in cemented soils remains one of the challenging geotechnical problems mainly due to the variable degree of cementation in the carbonate soil and poses uncertainties to the design of foundations for offshore wind farms structures. Carbonate soils encountered in offshore Australia have embedded layers of loose and compressible cemented soft rocks to well-cemented calcarenites. The purpose of this paper is to undertake finite element analyses using PLAXIS software to quantify pile-soil interaction and predict pile response and load distribution along the pile length under various loading conditions. The results from full scale instrumented piles in weak to moderately cemented soils are presented to draw the comparison between experimental and numerical results. The results from in situ (cone penetration test, CPT) and advanced laboratory testing (constant normal stiffness, CNS) on variable cemented soils that replicate the loading imposed by waves on offshore wind farm structure are used to demonstrate strength degradation in carbonate materials during cyclic loading.
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Proof Rolling Revisited
Proof rolling has been used as a technique to prove satisfactory foundation strength for earthworks structures for many years. It is a crude test, but effective in identifying obvious weak spots in subgrade soils that are predominantly of adequate strength to support embankments or pavements. However, problems can arise when proof rolling is inappropriately specified or the limitations of the test are not fully understood.
This paper provides an overview of the test method and its limitations. It also reviews some experiences where poor understanding of the test method has resulted in subsequent pavement failure or the construction of pavements that exceed design requirements.
A theoretical analysis of proof rolling deflections is provided for a range of subgrade soil strengths, under a specified proof roll loading nominated in AS3798. This analysis demonstrates the need to specify proof roll loading that is consistent with specific pavement design requirements rather than adopting a “one size fits all” approach.
Issues that need to be considered when specifying and performing proof roll tests are addressed.
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Geotechnical design and construction performance of abutment modification
Many civil engineering structures rely on geotechnical input to provide practical and innovative solutions, often in the face of uncertainty. In a recently completed major roadway widening project in Melbourne, a geotechnical alternative design was proposed to modify an existing bridge spill-through abutment to improve the functionality of the roadway by enabling the construction of two traffic lanes rather than a single lane proposed in the reference design. The solution involved removing the spill-through abutment and slicing through the counterfort buttress retaining wall and its foundations to form a continuous vertical face, transforming the retention system into a monolithic blade wall laterally supported by soil nails and rock bolts. This paper describes the alternative solution that was adopted and identifies the construction risks that had to be managed during construction. The importance of real-time and continuous geotechnical monitoring as a means to control the excavation sequence and verify abutment performance throughout the construction works is emphasised.
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A soil mechanics study into the liquefaction of shipped metallic ores
The transport of iron ore and other metallic ores by sea has been of increasing concern in recent years as several ships, their valuable cargo and around 200 lives have been lost because of liquefaction of the cargo. This has led to extensive soil mechanics based testing and analyses into the behaviour of iron ore fines during shipping transportation. In particular, the use of the Transportable Moisture Limit (TML), the maximum allowable moisture content based on a modified Proctor Fagerberg Test (PFT) at which a material is designated as being at risk of liquefaction when loaded into bulk carriers, has attracted considerable interest. The International Maritime Solid Bulk Cargoes (IMSBC) Code has categorised iron ore fines as a problematic cargo, prone to liquefaction because of the presence of moisture as well as fines in the material. The IMSBC code uses the TML to prevent liquefaction of the cargo, however, the rationale behind using the TML has been questioned.
This study investigates the influence of fines on the mechanics behind the liquefaction of well graded materials with similar gradings to iron ore fines that could be expected to liquefy during shipping transportation. The results so far show that as the fines content is increased, the density achieved during the loading of the material onto the ship decreases. However, results from cyclic triaxial tests suggest that density alone is not a good indicator of how resistant a material is to cyclic liquefaction. Using the state parameter gives a better understanding of the overall behavioural trend of the materials when subjected to cyclic loading conditions. The influence that these findings and results from preliminary unsaturated cyclic triaxial testing has on understanding the liquefaction behaviour of ship cargoes, will also be discussed.
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Two- and three-dimensional undrained bearing capacity of embedded footings
The ability to predict the ultimate bearing capacity of a foundation is one of the most important problems in foundation engineering. To solve this problem, geotechnical engineers routinely use a bearing capacity equation that contains a number of empirical factors to account for foundation shape, depth and inclination. In this paper finite element analysis is used to predict the undrained bearing capacity of strip, square, rectangular and circular footings embedded in clay. From these analyses, rigorous shape and depth factors have been derived and are compared with previous numerical and empirical solutions in the literature. The bearing capacity behaviour is discussed and the bearing capacity factors are given for various cases involving a range of embedment depths and footing shapes.