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2023 AGS Sydney Chapter AGM & Debate Night
Sophisticated Numerical Analysis in Geotechnical Design: Is It a Waste of time?
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Geotechnical Testing Refresher: The Triaxial Test
Dr. Michael Munro
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Finite difference modelling of soil-structure interaction for seismic design of moment resisting building frames
The importance of Soil-Structure Interaction (SSI) both for static and dynamic loads has been well established and the related literature spans at least 30 years of computational and analytical approaches for solving soil–structure interaction problems. Since the 1990s, great effort has been made to substitute the classical methods of design by new ones based on the concept of performance-based seismic design. Also, the necessity of estimating the vulnerability of existing structures and assessing reliable methods for their retrofit have greatly attracted the attention of engineering communities in most seismic zones throughout the world. In the present study, in order to draw a clear picture of soil characteristics effects on seismic response of moment resisting building frames, a ten storey moment resisting building frame, resting on shallow foundation, is selected in conjunction with three soil types with shear wave velocities less than 600m/s, representing soil classes Ce, De and Ee, according to Australian Standard AS 1170.4. The structure is modelled considering the three mentioned types of the soil deposits employing Finite Difference approach using FLAC 2D software. Fully nonlinear dynamic analyses under influence of different earthquake records are conducted, and the results of the different cases are compared and discussed. The results indicate that as shear wave velocity and shear modulus of the subsoil decrease, interstorey drifts and subsequently the necessity of considering SSI effects in seismic design of moment resisting building frames increase. In general, by decreasing the subsoil stiffness, the effects of soil-structure interaction become more dominant and detrimental to the seismic behaviour of moment resisting building frames. These effects substantially alter performance level of the building model resting on soil classes De and Ee from life safe to near collapse. Consequently, structural safety for the mentioned building frames could not be ensured by employing the conventional design procedure excluding SSI.
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Seasonal variations of soil suction profiles in the Perth Metropolitan Area
For the purpose of residential slab and footing design, the Thornthwaite Moisture Index (TMI) is used by AS2870 – 2011 to estimate the depth to which the in situ moisture content changes on a seasonal basis in order to classify residential sites based on the soil foundations reactivity. It does so by assigning a TMI value to a given climatic zone, which can then be used to predict the depth of design suction change, Hs, within the soil profile and, hence, calculate the characteristic surface movement. However, the most effective method for determining the potential ground surface movement is to establish seasonal soil suction profiles at the site. Unfortunately, little research has been undertaken in Western Australia to assess these profiles. This paper attempts to address this by comparing soil suction profiles for both wet and dry periods at two locations within the Perth metropolitan area, from which the depth of design suction change was better established. This allowed for a direct comparison of the experimental values to the existing values given in AS2870 – 2011. It was shown that the currently accepted value of 1.8 m given in the standard is possibly too low and that, given the drying climate, a higher value may be necessary. However, more research into soil suction profiles around Perth would help to generate more reliable data that could be used to better understand the seasonal moisture changes occurring in clayey soils of the Perth metropolitan area.
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Drained shear behaviour of sand with inherent transverse isotropy, for various b values
The objective of this paper is to reveal the fundamental characteristics of inherently anisotropic Toyoura sand for constant b values under drained conditions. To this end, monotonic loading tests with fixed principal stress axes and loading tests with rotation of the principal stress axes, were carried out using a large hollow cylindrical apparatus. Through analysis of the experimental results, characteristics related to the hardening law, the principal deviatoric strain increment ratio and the direction of the maximum principal strain increment etc. have been clarified, taking the effect of the b value into consideration.
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Bearing capacity of jacked piles in a layered soil profile
The bearing capacity of large scale jacked piles in layered soils is developed from theoretical considerations and confirmed by project observations. When a dense sand foundation layer overlies a weaker layer, the weaker layer can affect the geotechnical base bearing capacity of the jacked pile. Whether or not a pile can be pushed through a dense sand layer with a force less than the piling rig reaction has been found to depend on the strength of the upper stronger and lower weaker layers, the depth of the weaker layer and the pile size.
The punching shear failure model first developed by Meyerhof and co-workers was used to predict the pile base bearing capacity of a jacked pile founded in a dense sand layer overlying a weaker layer. The H/B ratio, defining the distance between the pile base and the underlying weaker layer for which the weaker layer does not influence the pile base bearing capacity, was found to depend on the strength ratio of the two layers, the friction angle of the stronger layer and the geometric ratio of pile size to depth of the weaker layer. In the case of a weaker layer overlying a stronger layer the pile base bearing capacity can be taken as linearly increasing over a depth increment of 10B, beginning 2B above the stronger layer. Observations of the pile jacking behaviour at a site containing dense sand with weaker layers provided field verification of the developed concepts.
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Low permeability slurry trench groundwater barriers – Recent experiences of design, construction and testing
Slurry trench ground water barriers are increasingly being used to control groundwater flows for geo-environmental or geotechnical purposes. The methodology of construction and material properties are described, with particular reference to case histories of completed projects for differing applications.