Wave Propagation and Soil Stiffness Workshop

In 2009 the EPSRC awarded funding to a joint University of Bristol-Imperial College London research project entitled “Micromechanics of seismic wave propagation in granular materials” (EPSRC Grants EP/G064180 and EP/G064954). To mark the completion of this research project, a workshop entitled “Wave Propagation and Soil Stiffness” was held in Bristol on March 20 and 21. The dates were chosen to coincide with the BGA Rankine lecture. A total of 71 participants attended the workshop, about 40 of whom are based in the UK, with the remaining participants coming from diverse locations including Argentina, Japan, Portugal, Germany, Italy, the Netherlands, USA, Mexico, and Australia. 20 minute presentations were delivered by 16 invited speakers, with the discussion on March 20 focussing on experimental studies, while the session on March 21 considered theoretical and numerical studies. The workshop was jointly supported by TC101 and TC105 of the International Society of Soil Mechanics and Foundation Engineering, the British Geotechnical Association and the Institution of Civil Engineers.

With one exception, all of the presenters in the experimental session considered aspects of the used of bender element tests to determine the small strain stiffness of soil; this is sometimes called the elastic stiffness. Bender elements are piezo ceramic strips that can move when a voltage is applied to them, and conversely, that generate a voltage when moved. By inserting receiver and transmitter bender elements at opposite sides of a soil sample, one can transmit shear and compression waves at one point and record the arrival of these waves at another point. If the distance between the two elements is known, then the wave velocity and hence stiffness of the material can be determined providing that some continuum media assumptions are made. As highlighted by the session chair, Prof. Matthew Coop of City University, Hong Kong, experimentalists have known for more than a decade that the interpretation of bender element test data is difficult and that the results can be dependent on the particular interpretation approach adopted. Prof. David Airey of the University of Sydney pleaded in favour of use of a cross-correlation, frequency domain approach to interpret the test data, while other speakers advocated use of time-domain methods such as peak to peak. A consensus emerged that there is about a 5-10% level of uncertainty in estimates of stiffness obtained using bender element tests. Prof. Coop highlighted that this is not news, and queried whether the bender element user community is stuck in a “ground hog day”, just like Bill Murray.

One could feel slightly disappointed or disillusioned that there has been no step change in our ability to interpret bender element test data in the past decade, however the session did reveal significant novelty and progression. Prof. Reiko Kuwano of the University of Tokyo reported on a survey by the Japanese Geotechnical Society that found bender element use is now becoming more common in industry as well as in research. This uptake of bender element testing in industry was also evident from the presentation of Mr. Chris Russell from Russell Geotechnical Innovations, who discussed their use from a UK industrial perspective. As Prof. Kuwano informed us, this uptake in bender element testing in industry has motivated the Japanese Geotechnical Society to develop a standard for bender element testing and interpretation. Prof. Carlos Santamarina of Georgia Institute of Technology (Atlanta, US), gave a range of wonderful examples of the application of bender elements testing to study soil freezing, liquefaction, soil skeleton genesis. There have clearly been recent advancements in the testing technology, for example Antonio Viana da Fonseca of the University of Porto, Portugal, gave a wonderful presentation outlining how accelerometers can be embedded within samples to trace the progression of the shear wave. One would expect the Japanese to be pushing the technology forward, and that is indeed the case, as revealed by Prof. Kuwano, who reported on the development of new disc-shaped transducers that can transmit both compression and shear waves across a relatively large surface, thus minimising the potential local sample disturbances. This approach also overcomes the problem of bender element tests being dependant on the number of particles the bender touches; an issue that provoked some debated in the discussion periods.

The UK can also push the boundaries of what is possible, as revealed by Mr. Simon Hamlin from the University of Bristol, who described fully three-dimensional tests in the complex cubical cell at the University of Bristol. Mr. Hamlin also discussed the use of lasers to measure the velocity of the benders themselves as well as the application of dynamic analysis to infer the real motion of a soil embeded bender, given a particular input voltage. There was some discussion on the frequency content of the received signal, with Prof. Jun Yang, of The University of Hong Kong, showing that there is an upper limit to the frequency that can be transmitted through a sample and this depends on particle size. Prof. Xiaoping Jia, ESPCI ParisTech, France, discussed the propagation of high frequency compression waves in dense granular systems.

Following a very pleasant drinks reception and dinner in the wonderful surroundings of the University of Bristol’s Goldney Hall, the participants convened on the morning of March 21 in a room overlooking the Avon Gorge with a clear view of Brunel’s Clifton suspension bridge. Prof. David Muir Wood of the University of Dundee chaired this session. Opening the session Prof. Stefan Luding of the University of Twente drew on his training as a physicist, and presented particle scale simulations of plane compression and shear wave propagation, and showed how detailed analysis of the system in the frequency domain can reveal interesting phenomena associated with wave propagation in granular materials including frequency filtering and trapping of high frequency response close to the wave source. Prof. Matt Evans (Oregan State University, USA), Dr. John O’Donovan (Buro Happold, UK), Dr. Helen Cheng (University College London), and Dr. Xu (University of Cambridge), all followed Prof. Luding’s lead by discussing the use of the discrete element method to simulate wave propagation and to study the particle-scale mechanics. These numerical analysts seemed to agree with the experimentalists in concluding that there is no agreement amongst the various available methods for bender element test interpretation. Dr Xu, and Prof. Evans all seemed to agree with Prof. Kuwano’s idea that disc-shaped transmitters and receivers are preferable to point sources such as bender elements. Dr. O’Donovan showed that DEM simulation data can give good agreement with laboratory test data. This session was not entirely dynamic; Prof. Vanessa Magnanimo (The University of Twente) and Dr. Colin Thornton (University of Birmingham) both showed the insight that can be gained by simulating static probes using DEM. Dr. George Marketos showed us that particle-scale studies need not use DEM; his stiffness matrix approach has the advantage of revealing all the vibration modes in a model soil.

This theoretical session included application examples, with Prof. Evans outlining the application of wave propagation to look at cemented sand, Drs. Cheng and Xu considered the relationship between shear wave velocity and liquefaction resistance and finally, concluding the workshop, Mr. Sacha Emam of Itasca, France, showed how particle scale modelling and wave propagation can be used in rock mechanics analyses associated with mining. During the final discussion participants seemed a bit divided as to whether we are now in a good place; Prof Santamarina reminded us that sample disturbance effects often introduce an error in laboratory testing that significantly exceeds the error associated with bender element testing; while Prof. Muir Wood argued that a bad interpretation of a bad test can never be a good thing. Dr. Marcos Arroyo of UPC, Barcelona, Spain, who was the most vocal participant in the discussion periods, argued provocatively that pollution of a bender element signal is inevitable.
The feedback from all participants was very positive. The workshop started at lunch time on the 20th and ended at lunchtime on the 21st. This minimized the time participants needed to spend away from home, while allowing adequate and quality time for informal discussion and networking. Some thought was put into selecting a range of speakers, all of whom are experienced at presenting; this resulted in very informative, complementary presentations. The workshop was hosted by the University of Bristol, and all participants seemed happy to have the opportunity to travel to a venue outside of London. As organizers we are very grateful to the workshop sponsors, GDS International, Russell Geotechnical Innovations, Itasca, the University of Bristol, the British Geotechnical Association and the Institution of Civil Engineers.

Edited versions of the speakers slides are available here.

Report by Catherine O’Sullivan & Erdin Ibraim