Field monitoring of expansive soil behaviour in the Newcastle-Hunter region

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Massive landsliding in Narrabeen sandstones in the Watagan region

The Narrabeen Group is a thick sequence of bedded Triassic sandstones which occupies the stratigraphic interval between the Permian coal measures and the Hawkesbury Sandstone in the Sydney Basin. In the northern Sydney Basin it is laterally extensive, extending throughout the southern Hunter Valley from the Central Coast to beyond the Great Divide. In the Watagans region, a unique situation arises where a disconformity causes the Narrabeen group to be underlain directly by Permian upper marine sediments. The result is a dramatic occurrence of ancient massive landsliding, leading to steep sandstone slopes that break to an elevated detrital plane, of considerable lateral extent. This paper describes the unusual geomorphic features associated with the landslide mass and interprets a variety of individual mass-movement and rockfall mechanisms which have contributed to these impressive features in the various stages of its development

Temporal-spatial frequency rockfall data from open-pit highwalls using a low-cost monitoring system

In surface mining, rockfall can seriously threaten the safety of personnel located at the base of highwalls and cause serious damage to equipment and machinery. Close-range photogrammetry for the continuous monitoring of rock surfaces represents a valid tool to efficiently assess the potential rockfall hazard and estimate the risk in the affected areas. This work presents an autonomous terrestrial stereo-pair photogrammetric monitoring system developed to observe volumes falling from sub-vertical rock faces located in surface mining environments. The system has the versatility for rapid installation and quick relocation in areas often constrained by accessibility and safety issues and it has the robustness to tolerate the rough environmental conditions typical of mining operations. It allows the collection of synchronised images at different periods with high-sensitivity digital single-lens reflex cameras, producing accurate digital surface models (DSM) of the rock face. Comparisons between successive DSMs can detect detachments and surface movements during defined observation periods. Detailed analysis of the changes in the rock surface, volumes and frequency of the rocks dislodging from the sub-vertical rock surfaces can provide accurate information on event magnitude and return period at very reasonable cost and, therefore, can generate the necessary data for a detailed inventory of the rockfall spatial-temporal occurrence and magnitude. The system was first validated in a trial site, and then applied on a mine site located in NSW (Australia). Results were analysed in terms of multi-temporal data acquired over a period of seven weeks. The excellent detail of the data allowed trends in rockfall event to be correlated to lithology and rainfall events, demonstrating the capability of the system to generate useful data that would otherwise require extended periods of direct observation

Aspects related to the small strain shear modulus behaviour of compacted soils subjected to wetting and drying

The dynamic properties of a soil are routinely investigated to describe its engineering behavior under repeated loading. Although the effect of suction on the dynamic response of soils is significant, there have been limited studies in which the post-compacted changes in suction induced by wetting and drying cycles have been considered. In this paper, aspects related to the dynamic properties with special reference to the small strain shear modulus of compacted soils subjected to wetting and drying are described. Further evidence on the dynamic response in terms of small strain shear modulus (G0) of a compacted soil subjected to wetting-drying is presented and novel insights into small strain behavior in multiple cycles of wetting and drying are shown. Particular emphasis is placed on the hysteric behavior and its dependence on the suction history. The results not only confirm the importance of the current suction ratio (or CSR), but also suggest that subsequent wetting-drying cycles further induce hysteretic changes in relation to the small strain shear modulus, particularly when following the wetting paths

Rock mass stability in the southern New England fold belt, New South Wales, Australia

The Southern New England Fold Belt of New South Wales in Australia is characterised by an arrangement of fault bound blocks of mildly deformed sedimentary rocks and interbedded volcanics. The geological blocks, which vary from hundreds of metres to tens of kilometres in extent, are bounded by faults that vary in width from metres to hundreds of metres. The strata may dip at any angle, but typically, the dips are in the range from 5 to 40 degrees. For strata dipping between 10 and 30 degrees, there are several examples of block sliding slope failures, where movement has occurred on surfaces parallel to bedding, allowing joint bounded blocks to separate to form cracks and chasms. In some cases, the areas affected are hundreds of metres wide, and the chasms are tens of metres deep. This paper describes two case studies of such occurrences. It explores the geological and topographical controls on stability and it identifies the relationships between geology and landform that are necessay to accommodate a kinematically admissible movement mechanism. Joints are found to be of similar importance to bedding in affecting rock mass stability. In particular, it is shown that the role of joints is more to act in combination than in isolation, and that the nature of this interaction may vary with position in order to achieve instability. It is noted that the presence of a greater number of different joint sets increases the likelihood of instability, as it facilitates a greater number of possible joint combinations

The development of a residual soil profile from a mudstone in a temperate climate

In the paper, the development of an expansive residual clay soil profile weathering from a Permian mudstone is described. The characteristics considered include soil mineralogy, soil chemistry, soil texture and engineering parameters including expansive potential. The effects of weathering are shown to be most evident in the development of a series of texturally distinct soil horizons. It is shown that the chemical and mineralogical characteristics of the parent mudstone are very similar to those of its derived expansive clay soils, even though the expansive potential in the clay soil is almost twice as great as in the parent mudstone. The origins of an enhanced expansive potential in the B horizon are explored, and it is found that, for the weathering of a mudstone in a temperate climate, the effects of weathering on structure are more important than the effects of weathering on mineralogy. It is concluded that most of the smectite clays in the weathered clay soils were inherited from the parent mudstone (with little or no transformation) and that these clay components were likely to have been present in the sediments from which the mudstone formed. The enhanced expansive potential in the residual clays is attributed to the physical effects of weathering in breaking down structure and/or bonding in the parent rock, which reduces the potential expansiveness of the clays contained in the parent rock. This proposal is supported by experimental evidence

A review of geological models for east coast Australian estuaries

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