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Using Bender Elements to Measure Soil Stiffness at Small Strains in Laboratory Samples
Over the last two decades improved laboratory techniques have shown that soil stress-strain responses are highly non-linear over the range of strains and stresses that are relevant around foundations and excavations. Accurate predictions of settlement and of the stresses in the ground depend on knowing this non-linear stress-strain response. One of the parameters needed to describe this response is the stiffness at very small strains. A variety of techniques are available to measure this stiffness. In this paper the bender element method is described. One of the attractions of using bender elements is that they can be easily incorporated into existing equipment, and at least in principle, the test is very simple to conduct. A high frequency pulse is applied to one bender element and the time for this to be detected at the other end of the test specimen is recorded. It is shown how bender elements can be incorporated into existing tests with minimal hardware requirements. A method of automating the test interpretation is presented which should enable reliable stiffness measurements to be obtained from commercial laboratory environments. This method makes use of cross correlation between input and output signals. It is argued that the approach is easy to integrate into existing test control programs, and it is shown how the technique may be implemented at relatively low cost.
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Engineering properties of Coober Pedy rocks
This paper presents the results from a series of tests carried out on rocks from shallow depths at which dugouts and underground structures are constructed in Coober Pedy. The tests include moisture content, porosity, specific gravity, density, uniaxial, tensile, point-load and triaxial strength, durability, deformability, suction and Schmidt hammer. The test results will provide a useful reference for Engineers who wish to apply rational methods to the design of underground structures in the township of Coober Pedy.
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New Approaches to Driven Pile Acceptance
Dr Julian Seidel
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NCIG CET3 Project – A geotechnical perspective
This paper presents an overview of the complex geotechnical challenges faced during the design development of the Coal Export Terminal 3 (CET3) on Kooragang Island, NSW. The project encompasses the construction of coal export facilities which includes a new approximately 4.3 km railway loop leading to a coal unloading station before reaching a coal stockyard and wharf facilities of 30 Mtpa capacity. The works also include dredging in the southern arm of the Hunter River. The site is generally underlain by soft compressible alluvial soil strata of variable thickness.
A summary of geotechnical investigations completed during the design development including test pits, boreholes, cone penetration tests and vibrocores is presented. A trial embankment was constructed to test the performance of the ground versus predicted movements and details of its construction and preliminary results are summarised. Geotechnical instrumentation was implemented to complement and monitor the construction activities. The paper also presents an overview of the geotechnical design philosophy addressing issues associated with railway embankment stability, ground improvement for the railway loop and stockyard areas.
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The Use Of The DRA Technique And Porosimetry For Estimating The Maximum In-situ Stress In Rock From Core
Deformation Rate Analysis (DRA) and porosimetry have been used to demonstrate the “stress memory” effect in rock and to estimate the vertical in-situ stress in rock. The results of this study indicate that the stresses predicted using both methods are similar to those determined from depth and density of the overburden rocks even when the core had been sampled 18 months prior to testing. The results confirm that any laboratory having standard test equipment could apply the DRA method. Further work is required to validate the use of porosimetry for determining in-situ stress however if the method should prove to be valid, this parameter could be estimated using this routine test.
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Real-time monitoring and wireless data transmission to predict rain-induced landslides in critical slopes
Real-time landslide monitoring is an effective technique to minimise landslide risks, especially in circumstances where the potential for structural countermeasures is limited. Rainfall infiltration is considered as one of the most significant factors triggering slope instability. Hence real-time monitoring of parameters: rainfall, volumetric water content and surface deformations/displacements in the soil, enable the early detection of landslides, thus reducing the adverse impacts of landslides. This study involves low cost and simple to install miniature ground inclinometers equipped with MEMS (Micro Electro Mechanical Systems) tilt sensors, volumetric water content sensors, temperature sensors, a rain gauge and a wireless data transmission unit (DTU) for the prior identification of possible slope failure. The DTU receives data from sensor units via radio signal transmission at a higher data acquisition frequency and automatically transmits them via the mobile network to an internet server, and updates in an online web interface for the determination of slope instability. The monitoring programme in operation for more than two years in the Lake Baroon Catchment, Maleny plateau, Australia, accurately captured both creep movement of the slope with wetting and drying cycles and mass movements triggered by rainfall. The current study analysed the surface deformation and rainfall data produced by the real-time monitoring system and validated results using published study outcomes. Herein, a combination of rainfall data, I-D threshold equations and ground tilting rate was identified as a more suitable measure to detect possible slope failure in advance. Further, the issue of a ‘precaution’ at a tilting rate 0.01˚/hr, and a ‘warning’ at 0.1˚/hr is recommended by this study along with the consideration of rainfall data.