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

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

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

Modelling of sediment dynamics in a laboratory-scale experimental catchment

The variability of hillslope form and function is examined experimentally using a simple model catchment in which most landscape development parameters are either known or controlled. It is demonstrated that there is considerable variability in sediment output from similar catchments, subjected to the same hydrological processes, and for which the initial hillslope profiles are the same. The results demonstrate that, in the case of catchments with a linear initial hillslope profile, the sediment output is initially high but reduces through time, whereas for a concave initial profile the sediment output was smaller and relatively constant. Concave hillslope profiles also displayed reduced sediment output when compared with linear slopes with the same overall slope. Using this experimental model catchment data, the SIBERIA landscape evolution model was tested for its ability to predict temporal sediment transport. When calibrated for the rainfall and erodible material, SIBERIA is able to simulate mean temporal sediment output for the experimental catchment over a range of hillslope profiles and rainfall intensities. SIBERIA is also able to match the hillslope profile of the experimental catchments. The results of the study provide confidence in the ability of SIBERIA to predict temporal sediment output. The experimental and modelling data also demonstrate that, even with all geomorphic and hydrological variables being known and/or controlled, there is still a need for long-term stream gauging to obtain reliable assessments of field catchment hydrology and sediment transport

Large scale tests on the expansiveness of resin-injected clay

The effects of resin injection on the expansiveness of a cracked expansive clay are difficult to ascertain. This paper presents the results of a laboratory study in which large scale oedometer tests were conducted on 12 samples, where 6 had been injected with expanding polyurethane resin, and 6 had not. A method of taking large (300mm diameter) samples using a thin walled tube pushed by a CPT truck, is described. Then, the procedure of testing these samples under a load of 25kPa using a large scale oedometer arrangement is described. The results indicate that contrary to expectation, resin injected samples generally seemed to swell less than non injected samples. The trends were not as clear as might be expected due to the inherent variability of the clay soils, but because of the large samples and large number of samples tested, a reliable conclusion can be drawn. Reasons for this unexpected outcome are discussed, and it is generally concluded that the process of injecting hard clays with an expanding resin may create more cracks than it fills

Dunmore Bridge case study: an introduction to geotechnical engineering via finite element analysis

This paper describes the development of a case study based upon the preliminary design of a working platform to support a 1200T crane during the replacement of bridge spans on the historic Dunmore Bridge (Woodville, NSW, Australia). The case study was developed to enrich the teaching of finite element methods to undergraduate students by exposing students to practical aspects of finite element modelling and the use of commercial finite element software. The nature of the project permitted geotechnical engineering topics to be incorporated into the case study and introduced students to CPT testing and interpretation, importance of working platforms and realistic soil profiles. The Dunmore Bridge project was selected for the case study as it has many interesting aspects that are topical, interesting and motivational to the students. Historic records of the bridge construction are available and being a local bridge, the site can readily be visited by students. As the upgrade of the Dunmore Bridge is scheduled to occur within the next 18 months the project is topical and will be covered by the local media. The project is being recorded using time-lapse photography which will provide an additional multi-media resource for use in refinement of the case study. Feedback from students on the case study and how it helped motivate student learning was obtained via an anonymous online discussion forum

Calibration of the Perth sand penetrometer (PSP) for silica sands

Abstract– The blunt tipped “Perth Sand Penetrometer” is used extensively in Australia for characterization of silica sand sites prior to the construction of light weight domestic structures. The hand operated device, which can be readily deployed in fine granular soils at depths up to 5 m, is routinely and successfully used to infer in situ density and internal friction angles in extensive coastal sand deposits. Some calibration charts are presented for a light weight dynamic penetrometer device. The charts presented here are based on the results of 18 calibration chamber tests, performed in a specially constructed chamber, 1m high and 1m in diameter. All tests were performed on silica sand samples were consistently prepared to 3 densities using the sand raining or pluviation technique. The prepared samples were pressurized during testing to simulate overburden effects due to burial at depths up to 5m. Particular account is taken of the effects of rod friction

The effect of a gap between the access tube and the soil during neutron probe measurements

The neutron probe is a tool employed for the measurement of water content in a soil mass. The presence of a gap between the soil and the neutron probe access tube, filled with either air or water, inevitably introduces a systematic error in neutron probe readings. In this study, experimental investigations and numerical analyses were carried out to evaluate the effects of this gap on neutron probe calibration. The numerical model was developed based on the multigroup neutron diffusion equations and the finite element method. The experiments were conducted in a heavy clay soil. The results show that an air gap of 2.5–30 mm between the soil and a 50-mm-diameter aluminium tube could lead to an underestimation of soil water content by 5–45%, but significant underestimation was apparent for air gaps <10 mm. It is also found that the neutron count is significantly overestimated if the gap around the access tube is filled with water rather than air, but this effect is most significant for larger gaps. The results of this research clearly indicate that a gap between the neutron probe access tube and the soil profile should be avoided during field installation, and that if a gap between the access tube and soil develops during service, a systematic error will be introduced into measurements