PARTICLE-SCALE INVESTIGATION OF SEEPAGE INDUCED GEOTECHNICAL INSTABILITY

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.

Research beneficiaries:

The first non-academic beneficiaries of this research will be practicing geotechnical engineers. Engineers working in civil engineering design consultancies, contractors delivering civil engineering projects, and companies who own critical infrastructure such as large dams. The beneficiaries will also include engineers working in local and central government, those in public sector organizations such as the UK Environment Agency, the US Army Corps of Engineers and the US Bureau of Reclamation (who own a large number of flood embankments and embankment dams), as well as industrial experts who will be involved in updating design codes that relate to the hydraulic limit state.

Society as a whole will benefit particularly from improved safety of dams and flood embankments. The importance of such structures to the quality of life (e.g. clean water supply) and economic advancement (hydroelectric power and storage) and the human consequences of any unintended failures cannot be overstated.

Educational benefits will also arise from the provision of parallel online videos of both the physical and numerical experiments via the Zenodo repository; these will enable a particulate view of granular systems to be understood "at a glance". This will give a tool to science educationalists to excite students about the curiosities (and importance) of granular systems which are central to a host of problems in both the natural and industrial worlds. The British Dam Society will fund two undergraduate researchers to participate in the research in recognition of the ability of this project to enthuse young engineers.

Benefits delivered:

Practising engineers will be able to use the improved understanding of internal instability to develop more reliable dams, flood embankments, cut-off walls, and other geotechnical structures and more accurately assess the risk posed to existing facilities by internal erosion processes.

Ensuring the appropriate parties benefit:

We will work closely with professional organizations including the British Geotechnical Association (BGA) and the British Dam Society who have agreed to advertise initial and concluding workshops associated with the project. We will offer preferential registration rates to members of these bodies to encourage engagement amongst their members. We will carefully target potential beneficiaries of the research when we advertise these workshops, e.g. we will contact all of the participants at the recent Environmental Agency workshop on Reservoir Safety and UK All Reservoir Panel Engineers. We will issue bi-annual 2 page summaries of the research and email them to all of the aforementioned groups. We will also present findings from the research via forums such as the poster session at the annual BGA conference which have good industrial participation. Recognizing that practising engineers often do not read academic journals we aim to write some technical magazine articles for trade magazines e.g. Ground Engineering.

We have a strong track record of engagement with industry; for example O'Sullivan's prior research in this area was funded by ARUP and Kajima. She coordinated the Rankine lecture for three years (the largest event associated with the Institution of Civil Engineers), while Bowman has previously organised a two day workshop on internal erosion with the New Zealand Society on Large Dams (the NZ branch of ICOLD) which was aimed at dissemination of the latest research to practitioners. At the end of EPSRC funded grants, EP/G064954/1 and EP/G064180/1, O'Sullivan co-organized a one day workshop which attracted good industry participation and industrial sponsorship, she made the presentations from this workshop available via the BGA website. We will adopt this partnering approach with professional bodies to disseminate the findings from the proposed research as widely as possible.