Collaborative research being undertaken by City University London and its industry partners is partly driven by major projects, but the developments will help with delivery of the next generation of schemes
Necessity is the mother of invention and many individual projects help to develop new techniques, but other advances come from longer term research with industry collaborating closely with academia.
City University London’s centrifuge facility has a history of supporting the civil engineering industry with physical modelling capabilities and many research projects are currently underway to help improve geotechnical knowledge. This research update aims to widen knowledge about the themes and focus of current research by the university’s Geotechnical Engineering Research Group.
Tunnels are increasingly being seen as the solution to urban problems but working below existing infrastructure calls for a clear understanding of potential ground movements.
One study being undertaken is looking at the impact of using twin bore tunnels for rapid mass transit schemes. A series of relatively complex centrifuge tests were conducted varying the centre-tocentre spacing of the tunnels and the results showed a greater magnitude of settlement for the second tunnel, contrary to current industry predictions. The focus is now on developing an understanding of the reasons for these differences.
Tunnel face stability is another theme that is being explored. Face stability in unfavourable ground is often overcome by jet grouting or using umbrella arches. A series of centrifuge tests were carried out varying the arrangement of these grout bars around the annulus with a resin mix installed either around the crown, either side of the axis level and below the invert during a simulated tunnel collapse. The tunnel stability pressure was recorded throughout to give valuable new insight as to which arrangement was most effective.
The extent of disaggregation of weak rocks and soils as they pass through a slurry tunnel boring machine is another focus of the university’s research. A large amount of high-cost equipment is required in order to remove the excavated material from the recyclable slurry and is driving research to understand the soil parameters which determine the breakdown. Current research is focused on developing a test procedure and method for laboratory classification of the soil breakdown.
A good example of the university’s collaboration with industry is the field trials by Expanded Piling and the numerical analysis by Arup Geotechnics to look at the addition of ribs to the shaft of a pile, which is estimated to increase the capacity by 30-40%.
Centrifuge testing was then commissioned to investigate a variety of rib spacing and arrangements. The results showed an optimum arrangement which coincided with the analysis. Moreover, the results showed a ribbed pile could perform better than an equivalent pile of diameter equal to the outer diameter of the rib.
Centrifuge testing is also being used to increase knowledge of compensation grouting behind retaining walls and look at how the grouting will influence the subsequent behaviour and movements of a retaining wall and the soil. A series of relatively complex centrifuge tests have been designed to investigate the magnitude and extent of movements.
Not all the research focuses on new construction – demand for modern office buildings means that the first deep piled foundation buildings that were built in the 1970s and 1980s are now being redeveloped, but the existing piles often cannot meet the new loading demands. As a result, techniques have been developed to remove piles and the university has developed a technique through laboratory model testing known as the “cheese grater” to achieve this efficiently. This technique allows for the removal of piles by over excavation and removing of the soil from the pile annulus using a specially developed auger.
The university is also collaborating with Balfour Beatty and Cementation Skanska to investigate the forces required to remove steel casings during pile construction. Current practice uses the rig to “pull” the casing from the ground but the force needed is unknown and unpredictable. Tests have been conducted to develop an understanding of the magnitude of forces required for a given diameter of casing. This will enable contractors to more efficiently plan equipment requirements and improve safety practices.
Safety of the rigs in general operation is an area where the university is working with the Federation of Piling Specialists and Building Research Establishment to improve industry standards. Several piling rig incidents have led to the investigation of the design of working platforms and a series of centrifuge tests were commissioned to determine the effect of particle size to plate size to find a more economical plate test for platform design purposes.
Different sized model plate bearing tests carried out on large, coarse grained Devonian Limestone have shown at small strains there is little influence of the scaling effect.
The topics of research currently underway at the university are varied but all are helping the ground engineering industry to develop a better understanding of soil/ structure interactions that will be vital in delivering the next generation of geotechnical solutions.