Research Centre for Multi-scale Geotechnical Engineering

Principal Investigator: Dr Sam Divall

Description: The development of urban infrastructure often requires complex engineering solutions to access an underground space. These structures often can take the form of concourse tunnels, passageways, shafts and launch portals which are often erected in areas where space is limited. The construction processes involved in the creation of these structures often have a reasonably high level of uncertainty attached due to the complex soil to structure interaction taking place. City, University of London’s geotechnical centrifuge facility allows for these construction processes to be investigated using well-controlled physical modelling techniques. Model scale observations can be advantages in understanding the uncertainties at the prototype scale.

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Principal Investigator: Dr Richard Goodey

Description: Recycled demolition waste can be used in a number of engineering applications for example, as fill.  This waste generally comprises crushed concrete which would otherwise be sent to landfill.  In order to maximise usage of recycled waste, confidence needs to be assured in the engineering properties of the material.  These properties could include strength (angle of friction), particle strength, compaction and crushing characteristics.  As these types of materials often have large particle sizes, large scale testing would be required but, for practicing engineers, some methods whereby small scale analogues could be tested would be most beneficial.  The large scale testing facilities at City, University of London allow testing of full size particulate materials that would be otherwise be impractical using conventional apparatus.

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Researcher: Eric Ritchie

Supervisors: Dr Richard Goodey and Dr Sam Divall

Description: Centrifuge models are usually made from reformed soil, creating a uniform homogeneous model. However, there is a fundamental disparity between this process and the deposition of natural soils, because natural soil are deposited in layers creating a unique structure. This structure is important for modelling true soil behaviour because some essential soil properties (such as permeability, stiffness and strength) are not identical in all directions, this research will:

• Develop a technique for producing layered soil models
• To perform element tests to determine soil properties
• Undertake well documented centrifuge experiments for comparison with homogeneous models

Outputs: Divall et al. (2018)

Researcher: Ciaran Kennedy

Supervisors: Professor Sarah Stallebrass and Dr Joana Fonseca

Description: This research aims to investigate the mechanical behaviour of a novel mixture of recycled aggregate and shredded rubber from tyres to be used to create a working platform for construction plant.  At present, working platforms commonly comprise crushed construction waste which degrades during use and is usually sent to landfill when no longer required on site.  The rubber is expected to reduce the degradation and can be re-used if screened out of the working platform material.  Consequently, the thickness of the working platform will be reduced creating a saving in material required and hence the lorry movements necessary to supply the material.  There will also be a reduction in material sent to landfill.

The rubber-aggregate mixtures will be investigated at full scale and reduced scale using X-ray tomography, conventional laboratory testing, the large shear-box soon to be commissioned in the Structures Laboratory and field testing. Experimental methods will be combined with numerical analysis including the μFEM analysis method (Nadimi & Fonseca, 2018).

Objectives:

• To investigate the strength of recycled aggregate/rubber shred mixtures and the dependence of strength on the micro mechanical response of the mixtures, at a range of confining stresses consistent with loads from construction plant.
• To investigate the effect of rubber shreds on the degradation of recycled aggregates under cyclic loads from construction plant.
• To establish optimum aggregate/rubber mixtures and size ratios for recycled aggregate/rubber shred mixtures.
• To develop design guidance for working platforms constructed from recycled aggregate/rubber shred mixtures.

Researcher: Hashmi Sohawon

Supervisor: Professor Neil Taylor

Description: This research project assesses the suitability of simple solutions and finite element analyses in the prediction of ground movements caused by tunnelling. Field measurement data and experimental data gathered from previous centrifuge model tests are used to back analyse and evaluate the accuracy of both simple calculations and more complex finite element analyses using the three-surface kinematic hardening constitutive model. A particular feature of the finite element analyses is to explore the effects of recent stress history on the ground movements induced by twin tunnel construction. The research incorporates verification analyses and a numerical parametric study on ABAQUS and the results will assist in the understanding of this complex soil behaviour while normalised design curves will be proposed for use by the industry.

Outputs: Sohawon et al. (2018)

Researcher: Greta Tanghetti

Supervisor: Dr Richard Goodey

Description: The key design criterion for working platform design is whether it can safely support the application of an extremely high monotonic load applied eccentrically through the tracks of plant. Thus, the design criterion is stability. The BRE guide idealises this load case as load acting over an equivalent area, rectangular in plan, which lies on layered ground. The depth of the upper layer, the working platform, is determined by assuming a punch through failure mechanism in this layer and conventional bearing capacity failure in the subgrade.  Designers following this method then need to determine the appropriate angle of friction to be used in the calculation of the resistance of the platform material (construction demolition waste) to punching failure.  The aim of this approach is to provide a conservative design method given that insufficient data exists.

This research project is supported by The Centre of Excellence in Temporary Works and Construction Method Engineering and the Temporary Works Forum (TWf).

Outputs: Tanghetti et al. (2019), Tanghetti (2018) and Tanghetti et al. (2018)

Researchers: Dr Sam Divall, Dr Binh Le, Dr Sadegh Nadimi & Dr Richard Goodey

Description: This project aims to advance the technique of using Close range photogrammtry and image processing to record the surface and subsurface patterns of movements in clay during geotechnical centrifuge modelling.  Systems have been developed which measure three-dimensional (3D) deformations of a soil surface in geotechnical experiments.  Three, 2 megapixel industrial cameras were synchronised and used to capture images of a deforming soil surface. The images were used to reconstruct the observed scene to a high-density, accurate 3D point cloud. In addition, the work developed a new set-up for measuring a two dimensional displacement field using particle image velocimetry (PIV). The system makes use of the texture (intensity of pixel) of images to determine the accurate pattern of pre-failure ground movements.  This study highlights the benefits of new technology and provides guidelines to minimise artefacts in image processing.

Outputs: Le et al. (2018), Le et al. (2017) and Nadimi et al. (2016)

Researchers: Dr Jignasha Panchal, Dr Andrew McNamara & Dr Richard Goodey

Description: A hybrid foundation system using sheet piles and a pile cap was developed with a view to increasing the sustainability within the construction industry.  This new foundation system was tested against a conventional solid circular concrete bored pile using centrifuge modelling techniques.  Comparisons of initial bearing capacity results between the new foundation and the traditional pile illustrated the potential benefits of using a hybrid foundation.

Outputs: Panchal et al. (2016) and Panchal et al. (2018)

Researcher: Dr Binh Le (previously Dr Sam Divall)

Supervisor: Dr Richard Goodey

Description: The primary objective of the research is to establish clear, relatively simple guidelines for the construction of shafts in close proximity to existing structures. The research will centre on addressing current issues of concern and the main aims are:

• To evaluate the source, distribution and extent of ground movements caused by shaft construction in clay.
• To evaluate the effect of these movements on nearby existing infrastructure.
• To investigate the influence of the shaft's cross-sectional shape, particularly the orientation of the major and minor axes of an elliptical shaft relative to existing infrastructure.

This research project is supported by The Leverhulme Trust.

Outputs: Le et al. (2019) and Divall & Goodey (2016)

Researcher: Dr Jignasha Panchal

Principal Investigator: Dr Andrew McNamara

Description: Centrifuge modelling techniques will investigate methods of reducing ground movements in soft clays arising from deep excavations. This project will focus on feasible construction methods that can be implemented on site to reduce the effects of heave and subsequently mitigate movements behind the wall.

Outputs: Panchal (2018), Panchal et al. (2018), Panchal et al. (2017), and

Panchal et al. (2017) Effects of wall embedment on base heave failure arising from deep excavations in soft soils, Proceedings in the 9th International Symposium on Geotechnical Aspects of Underground Construction in Soft Ground, Sao Paulo, Brazil.

Panchal & McNamara (2016) Excavation techniques to reduce ground movements in soft soils, Proceedings in the 14th BGA Young Geotechnical Engineering Symposium, Glasgow, Scotland.

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Researcher: Dr Hitesh Halai

Supervisor: Dr Andrew McNamara

Description: The London Geotechnical Centrifuge is used to model compensation grouting behind a retained 12m deep excavation. The research study examines the effectiveness of the technique in controlling surface ground movements and the impact on the retaining wall during and after an excavation. The results could provide a better understanding of the limitations present when considering its use behind excavations.

Outputs: Halai (2018), Halai & McNamara (2014) and Halai et al. (2012)

Researchers: Dr Jignasha Panchal, Dr Andrew McNamara & Dr Rohit Gorasia

Description: The Federation of Piling Specialists published guidelines for calculating the pullout forces of temporary steel casings used in rotary bored pile construction.  The research was undertaken in collaboration with Balfour Beatty Ground Engineering and Cementation Skanska Ltd which involved field testing and a series of centrifuge tests.  The centrifuge tests were carried out using the geotechnical centrifuge facility at City, University of London.  An assessment has been made of the influence of casing diameter, embedment and overburden stress on pullout forces.

Outputs: Gorasia et al. (2014) and Panchal & McNamara (2017).

Researcher: Dr Sadegh Nadimi

Supervisor: Dr Joana Fonseca

Outputs: Nadimi (2017), Nadimi & Fonseca (2017a) and Nadimi & Fonseca (2017b)

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Researcher: Dr Deqiong Kong

Principal Investigator: Dr Joana Fonseca

This is an EPSRC First Grant funded project.

Outputs: Kong & Fonseca (2019), Fonseca et al. (2018), Kong & Fonseca (2018), Nadimi & Fonseca (2017) and Kong & Fonseca (2017)

Principal Investigator: Dr Joana Fonseca

Description: This study investigates the networks of stress transmitting particles based on the geometrical data obtained from micro-CT images. This study contributes with unique insights into the influence of contact and grain morphologies on the process of the stress transmission from grain-to-grain and consequently, on the deformation and macro-scale response of the material.

Outputs: Fonseca et al. (2015)

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Researcher: Dr Binh Le

Supervisor: Professor Neil Taylor

Description: In weak or unstable ground conditions, tunnelling induced movements could be controlled by providing Forepoling Umbrella System ahead of the tunnel face. This research, through centrifuge modelling techniques, aims to obtain the key features affecting the efficiency.

Outputs: Le & Taylor (2018), Le & Taylor (2017), Le & Taylor (2016), Le et al. (2015) and Le & Taylor (2014)

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Researcher: Dr Sam Divall

Co-researchers: Professor Neil Taylor & Dr Ming Xu

Description: Construction techniques are conducted to form a pre-lining in unfavourable ground conditions when tunnelling.  Often this technique can take the form of injected grout steel pipes as forepoling by or the installation of bars around the periphery of the face, typically over the upper quarter or third of the excavated profile.  A series of eight plane strain centrifuge model tests investigating the effect of inserting inclusions around the annulus of a single tunnel in overconsolidated clay has been conducted using the geotechnical centrifuge at City, University of London.

Outputs: Divall et al. (2016)

Researcher: Dr Neil Phillips

Supervisor: Professor Sarah Stallebrass

Description: The research project has designed a laboratory mixing test using a planetary mixer to aid in the prediction of the grading of excavated soil after transportation through the slurry pipeline. To aid in the disaggregation breakdown, soil classification tests were also carried out. Understanding the amount a soil disaggregates during transportation is key when specifying the separation plant.

Outputs: Stallebrass et al. (2015) and Phillips et al. (2014)

Principal Investigator: Dr Joana Fonseca

Description: This project looks at the change in soil microstructure around the probe during cone penetration to investigate the failure mechanism and the processes controlling drainage in silt. It uses image analysis of backscattered electron (EPMA) images of polished thin sections prepared from frozen samples. Understanding the mechanisms of grain reorientation following cone penetration can help explaining the drainage patterns controlling the cone resistance and the development of pore pressures.

Outputs: Paniagua et al. (2015)

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Researcher: Dr Sam Divall

Supervisor: Dr Richard Goodey

Description: Therefore, a series of plane strain centrifuge tests was carried out investigating twin tunnelling-induced settlements in overconsolidated clay. Apparatus necessary to perform these tasks required a significant amount of time to develop and was relatively complex. The main variables were the spacing between the tunnels, both horizontally and vertically, and the magnitude of volume loss. The tests were conducted at 100g where the cavities represented two 4m diameter tunnels at (usually) a depth of 10m at prototype scale. The tests utilised novel apparatus designed during the research to enable the simulation of the construction processes related to volume loss in separate sequential tunnels.

Outputs: Divall & Goodey (2012), Divall et al. (2014), Divall & Goodey (2015), Divall et al. (2017a) & Divall et al. (2017b)

Researcher: Dr Rohit Gorasia

Supervisor: Dr Andrew McNamara

Description: This research concerns the influence of ribs on the ultimate capacity of a bored pile in overconsolidated clay. Ribbed bored piles are known to give increased shaft capacity in comparison to conventional straight shafted bored piles.  Experimental data were obtained from a series of 23 centrifuge model tests undertaken at 50g. The geometry of the model was such that it was possible to test two piles with each test. Of the two piles tested one was always a plain pile, this allowed for direct comparison to the ribbed pile in the same test and hence any inconsistency in the soil sample to be accounted for. The performance of several rib designs and spacings were investigated, whilst the pile inner diameter and length remained constant. A series of datum tests were conducted to verify the accuracy and repeatability of the testing equipment. Four rib types were tested; concentric ribs, helical ribs, tapered ribs and under reamed ribs. The use of ribs was found to always increase the ultimate capacity of a pile. Of all the rib profiles tested the helical profile was shown to be the most effective.

Outputs: McNamara & Gorasia (2016)