PARTICLE-SCALE INVESTIGATION OF SEEPAGE INDUCED GEOTECHNICAL INSTABILITY

Lead Research Organisation: University of Sheffield
Department Name: Civil and Structural Engineering

Abstract

Civil engineering works often encounter water flowing through the ground. Examples include embankment dams, flood walls and embankments, excavations beneath the water table, tunnels and deep basements. When considering their design, engineers seek to avoid cases where the buoyancy forces exerted by the seeping water are sufficient to reduce the effective stress in the soil to zero, resulting in heave failure or "quicksand". This critical scenario is identified by considering the soil to be a continuous, but porous material. However soil is made up of individual particles of varying size and shape. Awareness is growing that seepage forces imparted on the particles can preferentially erode the smaller particles in sandy soils. There can be significant internal erosion of the soil under scenarios that are considered safe according to the classical continuum calculations used in engineering practice; this phenomenon is called internal instability.

We will improve understanding of internal instability and thereby our knowledge of how to design and assess infrastructure safely, by studying the fundamental, particle scale mechanisms involved. Internationally, several research groups are undertaking relatively large experiments of this problem. The particle-scale emphasis in the proposed research will complement, rather than supplement, these studies. The specific research direction originates from our prior research, recently published experimental data from other groups, and consideration of recently published design guidelines (e.g. the International Levee Handbook) and the proposed modifications to the hydraulic failure guidelines in the Eurocode EC7 for geotechnical design.

This cross-institutional proposal will combine experimental expertise in testing transparent soil at the University of Sheffield (UoS) with skills in discrete element modelling (DEM) at Imperial College London (IC). We will use these techniques in their most advanced form. At UoS testing facilities equipped with a laser light source will be developed to enable visualization of particle movement inside soil samples while also using tracer particles to observe fluid flow. The development of a triaxial stress path apparatus where the confining and deviatoric stress can be controlled while making these observations will be a particularly novel aspect of this research. IC will continue to champion the use of high performance computing to enable geomechanics DEM simulations and the project will exploit recent work that was carried out in the Department of Mechanical Engineering to enable DEM simulations to be coupled with computational fluid dynamics (CFD) modelling of the fluid flow. Both UoS and IC have been working independently to examine the problem of internal instability and so this proposal marks a timely collaboration to unify their complementary skill sets. For example the DEM model can provide information about particle stresses that cannot be measured in the laboratory, while instability can be directly observed for real materials in the physical tests without any of the idealizations and assumptions which are inherent in any numerical model.

The research will clarify:

(i) Which materials are initially susceptible to internal instability with volume change and the conditions whereby a material that initially erodes at a constant volume (i.e. settlement or collapse of the particle structure), transitions to having volume change.

(ii) Whether seepage velocity or hydraulic pressure gradient correlates better with the initiation of erosion.

(iii) How the stress level influences susceptibility; particularly considering stress anisotropy and the relation between principal stress orientation and seepage direction.
 
Description Fluid tracking within a granular system has begun to enable local fluid velocities to be determined experimentally. Initial focus has been on monodisperse systems (one conference paper has proceeded from this) and is being extended to polydisperse systems. In addition a triaxial permeameter has been developed but has yet to be used in transparent soil research.
Exploitation Route Research is ongoing.
Sectors Aerospace, Defence and Marine,Chemicals,Construction,Energy,Pharmaceuticals and Medical Biotechnology

 
Description Collaboration between Imperial College and Sheffield 
Organisation University of Sheffield
Country United Kingdom 
Sector Academic/University 
PI Contribution The collaboration between Dr. E. Bowman and Dr. C O'Sullivan is central to this project.
Collaborator Contribution Dr. E. Bowman and her team will provide the experimental data for this project.
Impact This is not a multi-disciplinary collaboration. We co-organized a kick off workshop for the project. Regular discussions are ongoing.
Start Year 2017
 
Description Collaboration between Imperial College and the University of Glasgow 
Organisation University of Glasgow
Country United Kingdom 
Sector Academic/University 
PI Contribution Dr. Tom Shire is a Co-I on this project.
Collaborator Contribution Dr. Shire is advising on the DEM simulations.
Impact This is not a multi-disciplinary collaboration. The ongoing discussion will inform the research outputs.
Start Year 2017
 
Description Collaboration with the University of British Columbia 
Organisation University of British Columbia
Country Canada 
Sector Academic/University 
PI Contribution Prof. J. Fannin is advising on the direction of the project.
Collaborator Contribution Prof. Fannin is discussing the detailed nature of the research based on his track record of experimental research.
Impact The collaboration is not multi-disciplinary. Discussions with Prof. Fannin are informing the direction of the research.
Start Year 2017
 
Description Workshop on Seepage 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact On August 31 and Sept 1 a workshop on seepage induced instabilities was held at Imperial College London. The workshop was organized in conjunction with the British Geotechnical Association and the British Dam Society. The workshop was also supported by three technical committees of the International Society for Soil Mechanics and Geotechnical Engineering: TC 105 Geo-Mechanics from Micro to Macro, TC 201 Dikes and Levees, and TC 213 Scour and Erosion. The workshop was organized to launch the EPSRC funded collaboration between Imperial College London, the University of Sheffield entitled "Particle-Scale Investigation of Seepage Induced Geotechnical Instability", and the EPSRC provided financial support for the workshop. There were 48 participants including representatives from consulting engineers, dam owners and academics.

The first session of the workshop focussed on UK practice. The first speaker, Rob Gilbert from ARUP, reminded us all how international UK practice is, describing case histories of dams in Myanmar, Mauritius and Sudan. Dr Toby Roberts from WJ Groundwater presented a number of dewatering case histories and discussed some of the issues associated with filter selection for dewatering wells. Dr Chris Menkiti from GCG presented some case histories from the Warsaw Metro. Dr. Menkiti also highlighted the problem of sea borne transport of mineral ores; instabilities associated with this material have resulted in the loss of a number of large ships in recent years.

The second session of the workshop was loosely based around a consideration of design guidance and codes. The first speaker was Dr Brian Simpson from ARUP, who has been involved in a working group looking at seepage related issues in EC7. Dr Philip Smith from Royal Haskoning DHV spoke about his experience with UK levees (flood embankments) and introduced the Levee Handbook. Dr Bernard Odenwald provided a German perspective, while Mr. Rod Bridle introduced the ICOLD Bulletin 164 on Internal Erosion.

The second day of the workshop was research orientated. In the first session, there was a focus on international experimental research, with presentations from Prof Jonathan Fannin (University of British Columbia, Canada), Dr Didier Marot (University of Nantes, France) and Prof Akihiro Takahashi (Tokyo Institute of Technology, Japan). The final session of the workshop outlined UK research, and started with a presentation from Dr Philippe Sentenac (Strathclyde) on field monitoring of internal erosion in flood embankments. Finally, the joint Sheffield - Imperial research project team gave presentations on transparent soil and discrete element modelling.

Key points from the workshop were:

• There was a repeated theme in the industrial presentations of the need to understand both the geology of sites as well as the history of human intervention at sites, whether the site is an existing levee or a new deep excavation in an urban environment. Dr Smith and Dr Odenwald highlighted that failures are also often observed at irregularities (transitions) or junctions with structures intercepting levees.

• There is scope for new technologies to help us better understand site conditions: Dr Philip Smith from Royal Haskoning DHV highlighted the ability of LIDAR to help identify old meanders and old drainage channels. Dr Sentenec discussed the use of geophysics, focussing on electrical resistivity tomography and describing use of this technology on flood embankments in France.

• The risk posed to dams and flood embankments by internal erosion was highlighted. Considering the performance of embankments in England since 2007, Dr Smith highlighted that the particular threats to UK levees are seepage, internal erosion and uplift; however, with extreme floods such as those in the winter of 2015/2016 overtopping is also an issue. He noted that these mechanisms pose a greater risk to levees than slope stability failures.

• Dr Simpson pointed out that factoring water pressure under using an EC7 approach can lead to impossible situations.

• Dr Smith highlighted the fact that despite the significance of levees, in contrast to dams, there is less legislation and inspection. Dr Smith pointed out that these structures are vulnerable as they were typically not designed to particular standards and are often constructed on poor ground; their fragility is typically not apparent. To emphasize how fragile these structures can be, Dr Smith showed a video of a US Corps of Engineers experiment in which a full-scale levee on silty sand failed only 15 minutes after the development of a pipe beneath the levee.

• Mr. Bridle explained that the ICOLD bulletin on internal erosion is very useful to engineers seeking to understand the mechanics of internal erosion; notably they define the four mechanisms, suffusion, backward erosion, contact erosion and concentrated leak erosion. He highlighted that each mechanism can fundamentally be considered as a load - resistance problem. He pointed out that the challenge is to estimate the hydraulic forces causing erosion in vulnerable soils.

• Prof Fannin explained that in seeking to improve understanding of internal erosion in embankment dams there is a need for both data completeness and model completeness.

• Prof Marot proposed that in assessing suffusion (one form of internal erosion) the power expended by the seepage force should be considered.

• Presentations from Dr E. Bowman (Sheffield) and dir. T. Shire (Glasgow) demonstrated the power of transparent soil and discrete element modelling to examine the fundamentals of seepage-induced instabilities.

• Recent guidelines and resources for practicing engineers were highlighted including The International Levee Handbook, ICOLD Bulletin 164 on Internal Erosion, the German BAW Codes of Practice.

Selected presentations from the workshop are available at http://www.imperial.ac.uk/geotechnics/research/research-projects/seepage-induced-geotechnical-instability/seepage-workshop/. For further information, contact Prof Catherine O'Sullivan (cath.osullivan@imperial.ac.uk).
Year(s) Of Engagement Activity 2017
URL http://www.imperial.ac.uk/geotechnics/research/research-projects/seepage-induced-geotechnical-instab...