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

Influence of geometry on the bearing capacity of sheet piled foundations

Bored concrete piles are commonly used to support moderate loads from buildings in urban areas. At the end of their 25-30 year lifespan these structures are decommissioned but their foundations are left in place. These cannot be inspected hence the bearing capacity cannot be accurately verified. A hybrid foun- dation comprising sheet piles and a pilecap to mobilise shaft friction and end bearing was demonstrated to be a feasible and sustainable alternative to cast in-situ concrete piles. This research investigated the influence of sheet pile geometry on ultimate bearing capacity. A centrifuge test at 50 g was performed in over- consolidated clay where a square hybrid sheet pile group was ax ially loaded and vertical settlements recorded. Results indicated a square sheet pile group offers 70% greater capacity than a circular sheet pile group of similar surface area and 24% improved performance over the solid p ile loaded in the same test. Analysis of results suggested that the ultimate bearing capacity of the square sheet pile group compared with a solid pile of equivalent base area were within 0.2%, emphasising the importance of shape on capacity and the feasibility of the hybrid system as a viable foundation solution

Molecular accretion in the core of the galaxy cluster 2A 0335+096

We present adaptive optics-assisted K-band integral field spectroscopy of the central cluster galaxy in 2A 0335+096 (z= 0.0349). The H2 v=1–0 S(1) emission is concentrated in two peaks within 600 pc of the nucleus and fainter but kinematically active emission extends towards the nucleus. The H2 is in a rotating structure which aligns with, and appears to have been accreted from, a stream of Hα emission extending over 14 kpc towards a companion galaxy. The projected rotation axis aligns with the 5 GHz radio lobes. This H2 traces the known 1.2 × 109 M⊙ CO-emitting reservoir; limits on the Brγ emission confirm that the H2 emission is not excited by star formation, which occurs at a rate of less than 1 M⊙ yr−1 in this gas. If its accretion on to the black hole can be regulated whilst star formation remains suppressed, the reservoir could last for at least 1 Gyr; the simultaneous accretion of just ∼5 per cent of the gas could drive a series of active galactic nucleus (AGN) outbursts which offset X-ray cooling in the cluster core for the full ∼1 Gyr. Alternatively, if the regulation is ineffective and the bulk of the H2 accretes within a few orbital periods (25–100 Myr), the resulting 1062 erg outburst would be among the most powerful cluster AGN outbursts known. In either case, these observations further support cold feedback scenarios for AGN heating

Plate bearing tests for working platforms

During piling and other construction works, a working platform is often constructed across the site. These platforms comprise aggregate material placed and compacted to a designed thickness. Satisfactory performance of the platform may be confirmed by a plate bearing test. Current guidance given on plate bearing testing of granular soils suggests that the plate be at least five times the nominal size of the coarsest material. For a working platform this may be large and the reaction load required from plant and resources to carry out the bearing test may become excessively high. The aim of the research presented in this paper was to investigate the effect of particle to plate size ratios to establish if the use of a smaller plate would still allow a reliable test to be performed on site. Plate bearing tests were carried out in a centrifuge using a large, coarse grained limestone. The limestone was graded to a scale representation of 6F2 material, a commonly specified particle size distribution for working platforms. The size of plate was varied and the load displacement response recorded. The measured bearing capacity was correlated with the ratio of particle to plate size

Bearing Capacity of Sheet Piled Foundations

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 obstructions 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 use 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 pile group 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 loaded axially whilst vertical displacements were measured. Equivalent cast in place piles were similarly tested alongside the sheet pile groups by way of comparison

Design and development of a large shear box for testing working platform material

On large construction projects where deep foundations are to be installed, a working platform is placed across the entire site. This is comprised of a layer of aggregate (often crushed construction waste) usually with a particle size ranging from 120 mm downwards. Deep foundations are installed using heavy and tall drilling rigs and the working platform is thus safety critical to reduce the risk of machinery sinking and/or toppling which would lead to accidents and often serious injury to workers. Currently available design guidance is felt to result in conservative designs and there are many benefits to be gained from a greater understanding of the behaviour of working platform material. The design of these platforms is primarily governed by the angle of friction of the platform material. The measurement of friction angle for geomaterials that have large particle sizes is problematic (due to reasons of scale) and is often addressed by scaling down the material's grading curve prior to testing in small to medium size direct shear apparatus. The work presented here details the design of and the rationale for a large scale direct shear apparatus suitable for testing geomaterials with particle size distributions of the type that would be utilised in working platforms

Sheet pile groups as an alternative foundation solution to cast in-situ concrete piles

Concrete piles have become a common high load bearing foundation solutions providing end bearing and frictional resistance along the shaft. They are typically used for founding commercial or residential blocks with a design life of approximately 50 years. Following this the superstructure is decommissioned and may be demolished. However, piles are difficult to remove and therefore future developers can incur significant expense and programme delays in preparing the site to avoid obstructions. If removed, concrete piles are required to be broken down which is a slow and laborious process. However, a foundation solution has been developed that allows foundations to be installed and extracted with relative ease whilst still achieving a similar, if not improved capacity. This solution has been defined as a hybrid foundation comprising deep sheet piles for shaft resistance and a pile cap as a shallow foundation. The hybrid pile offers significant advantages over concrete piles include ease of installation, extraction, reuse and economy. Axial capacity of individual sheet piles is low, however geometrically arranging sheet piles; was shown to offer comparable or improved capacity over conventional concrete piles. The results from a series of centrifuge tests are presented in this paper

Cold Feedback in Cooling-Flow Galaxy Clusters

We put forward an alternative view to the Bondi-driven feedback between heating and cooling of the intra-cluster medium (ICM) in cooling flow galaxies and clusters. We adopt the popular view that the heating is due to an active galactic nucleus (AGN), i.e. a central black hole accreting mass and launching jets and/or winds. We propose that the feedback occurs with the entire cool inner region (5-30 kpc). A moderate cooling flow does exist here, and non-linear over-dense blobs of gas cool fast and are removed from the ICM before experiencing the next major AGN heating event. Some of these blobs may not accrete on the central black hole, but may form stars and cold molecular clouds. We discuss the conditions under which the dense blobs may cool to low temperatures and feed the black hole.Comment: 6 pages, no figures, to appear in the Proceedings of "Heating vs. Cooling in Galaxies and Clusters of Galaxies", August 2006, Garching (Germany

A comprehensive study of the radio properties of brightest cluster galaxies

We examine the radio properties of the brightest cluster galaxies (BCGs) in a large sample of X-ray selected galaxy clusters comprising the Brightest Cluster Sample (BCS), the extended BCS and ROSAT-ESO Flux Limited X-ray cluster catalogues. We have multifrequency radio observations of the BCG using a variety of data from the Australia Telescope Compact Array, Jansky Very Large Array and Very Long Baseline Array telescopes. The radio spectral energy distributions of these objects are decomposed into a component attributed to on-going accretion by the active galactic nuclei (AGN) that we refer to as ‘the core’, and a more diffuse, ageing component we refer to as the ‘non-core’. These BCGs are matched to previous studies to determine whether they exhibit emission lines (principally Hα), indicative of the presence of a strong cooling cluster core. We consider how the radio properties of the BCGs vary with cluster environmental factors. Line emitting BCGs are shown to generally host more powerful radio sources, exhibiting the presence of a strong, distinguishable core component in about 60 per cent of cases. This core component more strongly correlates with the BCG's [O III] 5007 Å line emission. For BCGs in line emitting clusters, the X-ray cavity power correlates with both the extended and core radio emission, suggestive of steady fuelling of the AGN over bubble-rise time-scales in these clusters

Optical emission line nebulae in galaxy cluster cores 1: the morphological, kinematic and spectral properties of the sample

We present an Integral Field Unit survey of 73 galaxy clusters and groups with the VIsible Multi Object Spectrograph on the Very Large Telescope. We exploit the data to determine the H α gas dynamics on kpc scales to study the feedback processes occurring within the dense cluster cores. We determine the kinematic state of the ionized gas and show that the majority of systems (∼2/3) have relatively ordered velocity fields on kpc scales that are similar to the kinematics of rotating discs and are decoupled from the stellar kinematics of the brightest cluster galaxy. The majority of the H α flux (>50 per cent) is typically associated with these ordered kinematics and most systems show relatively simple morphologies suggesting they have not been disturbed by a recent merger or interaction. Approximately 20 per cent of the sample (13/73) have disturbed morphologies which can typically be attributed to active galactic nuclei activity disrupting the gas. Only one system shows any evidence of an interaction with another cluster member. A spectral analysis of the gas suggests that the ionization of the gas within cluster cores is dominated by non-stellar processes, possibly originating from the intracluster medium itself