Cruise Report: CD149 – RRS Charles Darwin 18th July to 6th August, 2003. Spreading-ridge geometry, hydrothermal activity, and the influence of modern and ancient hotspots on the Carlsberg Ridge - northwestern Indian Ocean

Cruise CD149 on board the RRS Charles Darwin aimed to explore the Carlsberg Ridge, Northern Indian Ocean. The cruise recovered multibeam swath bathymetry (EM12), dredge samples from 20 stations, water column profiles from 16 stations and water samples from one station, between 57 and 61.5°E. The initial results were: a discovery of a hydrothermal super plume – a plume signal rising 1200m above the seafloor and extending 30km along the ridge crest (named by the ship’s company as the iGass Plume); recovery of an extinct hydrothermal site with oxidised sulphide chimney fragments; imagery of a megamullion site with recovery of dolerite, flazer gabbro and moderately fresh peridotite from a core complex of lower crust and upper mantle; and fresh basaltic glass samples from 95% of the sites sampled.Until this cruise, the western Carlsberg Ridge was almost unknown, with only two or three poorly located rock samples, no continuous bathymetry, only a few single-track geophysics lines and no exploration for hydrothermal activity. However, the ridge is important since it probably includes the unradiogenic end-member of the Indian Ocean mantle source (at its eastern end), is likely to have a distal influence from the Afar hotspot (at its western end), and has a history of recent changes in spreading geometry reflected in an unusual segmentation pattern. It also represents a distal portion of the midocean ridge system that is connected through its eastern end only, thus having significant implications for the dispersal and colonisation of hydrothermal ecosystems

Cruise Report: CD149 – RRS Charles Darwin 18th July to 6th August, 2003. Spreading-ridge geometry, hydrothermal activity, and the influence of modern and ancient hotspots on the Carlsberg Ridge - northwestern Indian Ocean

Cruise CD149 on board the RRS Charles Darwin aimed to explore the Carlsberg Ridge, Northern Indian Ocean. The cruise recovered multibeam swath bathymetry (EM12), dredge samples from 20 stations, water column profiles from 16 stations and water samples from one station, between 57 and 61.5°E. The initial results were: a discovery of a hydrothermal super plume – a plume signal rising 1200m above the seafloor and extending 30km along the ridge crest (named by the ship’s company as the iGass Plume); recovery of an extinct hydrothermal site with oxidised sulphide chimney fragments; imagery of a megamullion site with recovery of dolerite, flazer gabbro and moderately fresh peridotite from a core complex of lower crust and upper mantle; and fresh basaltic glass samples from 95% of the sites sampled.Until this cruise, the western Carlsberg Ridge was almost unknown, with only two or three poorly located rock samples, no continuous bathymetry, only a few single-track geophysics lines and no exploration for hydrothermal activity. However, the ridge is important since it probably includes the unradiogenic end-member of the Indian Ocean mantle source (at its eastern end), is likely to have a distal influence from the Afar hotspot (at its western end), and has a history of recent changes in spreading geometry reflected in an unusual segmentation pattern. It also represents a distal portion of the midocean ridge system that is connected through its eastern end only, thus having significant implications for the dispersal and colonisation of hydrothermal ecosystems

The influence of enhanced excavation base stiffness on prop loads and ground movements during basement construction

The use of piles installed beneath deep excavations as a means of enhancing the stiffness of the soil and so reducing the spread of movements to the surrounding ground has been investigated. Experimental data were obtained from a series of plane strain centrifuge model tests undertaken at 100g in which three different formation base stiffnesses were modelled. The tests were able to simulate the stress changes that result from the complex propping and excavation sequence associated with top down basement construction. Reductions in horizontal loads of the order of 30% were found when piles were introduced to stiffen the ground beneath excavation formation level. Additionally, significant reductions in heave at the base of the excavation led to overall reductions in both horizontal and vertical ground movements behind the retaining wall. The number of piles was found to have a strong influence on the magnitude of reduction in ground movement, especially with increasing time after completion of the simulated excavation

A study on the reinforcing capabilities of Forepoling Umbrella System in urban tunnelling

Adequate heading stability is crucial to the safe construction of any tunnel. Insufficient support will lead to ground movements which have the potential to cause damage to existing infrastructure. Congested urban environments have led to a requirement to minimise these tunnelling-induced deformations. Forepoling Umbrella Systems (FUS) have proved to be a beneficial soil reinforcement measure for controlling ground movements due to NATM tunnelling in urban areas. However, there is limited understanding of the influence of tunnel geometry and FUS parameters on its reinforcing efficiencies. A series of centrifuge tests has been conducted to investigate the benefits of FUS using different arrangements of steel pipes placed in a model tunnel heading at various depths. The results show the importance of the steel pipes near the tunnel spring-line and the embedded lengths on the reinforcing effects of forepoles. In addition, relative benefits of forepole location and embedded length are shown to vary as the soil cover above the tunnel changes

Pile and Pile Group Capacity: Some Findings from Centrifuge Tests

The London Geotechnical Centrifuge Centre at City University London is a major experimental research facility for the Geotechnical Engineering Research Group. Some findings from two recent doctoral research projects are presented. The projects were aimed at improving our understanding of piled foundations. Pile group behaviour was studied with particular emphasis in determining the efficiency and load capacity of non standard group arrangements. The project was related directly to a development in London UK in which high capacity foundations were constructed using a Perimeter group arrangement of minipiles since that was all that could be constructed given onerous site constraints. The centrifuge research gave new insights into how pile groups carry large loads and demonstrated that Perimeter group arrangements make much more efficient use of piles than Grid groups. The second project was again industry driven and demonstrated that modifying the profile of a pile shaft can give significant increase in capacity with relatively little additional pile material

The effect of a forepole umbrella system on the stability of a tunnel face in clay

A new series of three-dimensional centrifuge model tests using soft clay has been conducted using the geotechnical centrifuge facility at City University London. These tests aim to quantify the reinforcing proficiency of different arrangements of steel pipes in a Forepoling Umbrella System (FUS). The results highlight some interesting effects of the FUS on tunnel stability and the spread of ground movements in the vicinity of the tunnel heading

Apparatus for centrifuge modelling of top down basement construction with heave reducing piles

The construction of deep basements in urban areas is associated with many risks and problems among which is the possible damage to existing structures and services resulting from settlements near the excavation. A number of methods are routinely employed to attempt to control these movements (e.g. top-down construction, use of stiff diaphragm walls). This paper discusses the methodology and practicalities of a series of centrifuge model tests designed to investigate the effect of deep basement construction. Two sets of experimental apparatus are described in detail and their effectiveness in terms of robustness and generation of repeatable data are assessed. It is shown that using relatively simple techniques and equipment it is possible to model many of the features associated with top-down construction

A new approach to investigate the particle size effects in centrifuge modelling

Geotechnical centrifuge modelling provides an opportunity to examine novel and complex events in a well-controlled and repeatable environment. While grain interaction and contact dynamics are considered in centrifuge modelling, the soil is treated as a continuum, consistent with standard geotechnical analysis. In the last four decades, particle size effects have been normally approached by the ratio of median particle diameter to critical dimension of modelled structure. The current study considers the response of a granular medium in a centrifuge model by coupling physical tests and equivalent discrete element simulations. The response of a strip footing on uniformly graded glass ballotini is investigated. This is chosen as the sample characteristics can be accurately replicated in a discrete element simulation. Particle size distribution, gravity and footing width are scaled in the context of model-the-model technique and the sensitivity of the bulk response to rapid increase in stress level is explored. This will help establishing the link between the micro phenomena and the macro response and contribute towards improving geotechnical design. The paper describes the work conducted to overcome challenges related to physical modelling including particle mixing, sample preparation, image analysis, and loading apparatus

Physical modelling of lime stabilisation in soft soils around deep excavations

Bored concrete piles have been used widely on commercial developments in London for about the last 50 years. The life of a commercial building is between 25 – 30 years and, as each building is demolished and rebuilt, the piles from the previous buildings remain in the ground causing obstruct ions to the new foundations. This paper describes a preliminary study to explore the viability of sheet piled foundations as a genuine alternative to cast in situ concrete piles and all of the complications inherent in their construction and the obstruction they create to subsequent foundations. If it is possible to u se steel piles as foundations they can be easily removed, recycled and will not cause obstructions for future developments. However, individual sheet piles have relatively low capacity when axially loaded and it is therefore necessary to consider a sheet p ile grou p in conjunction with a pilecap, which can be considered a hybrid foundation; a combination of shallow (pilecap) and deep (sheet pile). A short series of centrifuge tests is reported in which model sheet pile groups in over - consolidated clay were l oaded axially whilst vertical displacements were measured. Equivalent cast in place piles were similarly tested alongside the sheet pile groups by way of comparison

A study on performance of three-dimensional imaging system for physical models

A study by Le et al. (2017) reported the application of computer vision techniques structure from motion (SfM) and multi-view stereo (MVS) to measure three-dimensional soil displacements at the surface of physical models. However, little information exists on the significance of the camera resolution and the number of images to the measurement performance. This study assesses the measurement performance of the SfMMVS, provided by an open source software Micmac, with input images taken by two different types of camera including DSLR (18Mega-pixel) and mobile phone cameras (12Mega-pixel). Rigorous quantifications were carried out to examine the precision of the image analysis, in measuring vertical and horizontal displacements, over a region of interest of 420x200mm. The measurement precision, achieved by different numbers of images, ranged from 0.06mm to 0.03mm. The results from this paper can be useful for researchers to select appropriate camera that satisfies their measurement requirements