In the first Rankine Lecture on the topic of support fluids, Stephan Jefferis highlighted their unusual and unexpected behaviour from observations to date, as well as the effects of geotechnical activities on soils more generally.
Former Institution of Civil Engineers president Robert Mair Robert Mair introduced the 60th Rankine Lecturer, Environmental Geotechnics director Stephan Jefferis, at the in-person event at Imperial College London last week (16 March). The lecture was on the topic of “The unusual and the unexpected in geotechnical engineering: observation – analogy – experiment”.
Mair noted that Jefferis had established an international reputation as a leading expert on bentonite, bentonite cement and, more recently, polymer slurries.
Jefferis believes that the advent of synthetic polymer fluids has caused the industry to rethink many aspects of excavation fluid support, as they seem to break many of the rules established with bentonite slurries.
“Support fluids are totally embedded in geotechnical practice, whether it be diaphragm walling, cut off walls, piling, tunnelling, pipe jacking, caisson sinking, horizontal directional drilling, core drilling or water well drilling – it’s a long list affecting many of us involved in geotechnical engineering practice,” Mair said.
He added: “A particular feature of Stephan’s work has been the unusual. Many chemical processes in the ground are microbiologically mediated, with results that can be both unusual and unexpected.”
Piling under partially hydrolysed polyacrylamide solution. Photo: Carlos Lam
Referring to the second line of his lecture title “observation – analogy – experiment”, Jefferis said that a lot of his work starts from practical observations and then moves onto “seeing whether we can solve the problem without doing experiments”.
He highlighted analogy as a powerful tool that can save time and money and focus investigations.
“Once you’ve built a hypothesis and you’ve got something you can quickly test, you go in to experiment,” he said.
Jefferis wanted to centre his lecture on support fluids as they are key to many geotechnical processes.
He divides the fluids into:
“The partially hydrolysed polyacrylamide has been the [polymer] workhorse, but we’re going to see new polymers,” Jefferis said. “I’m bringing in a new acronym – the soil dispersion inhibiting polymer, the SDIP – for a class of polymers that don’t like soil and will stop it dispersing.”
With the use of SDIPs, projects no longer need soil-slurry separation plants, you end up with drier spoil and you can save on excavation and concrete due to better shaft friction and lower over-break, Jefferis noted.
The lecture focused in particular on the behaviour of support fluids used in piling, diaphragm walling, slurry trench cut off walls, pipe jacking, tunnelling and horizontal directional drilling.
The role of the support fluid is different in each of these applications, meaning the key properties are not the same for all of them.
The purpose of a support fluid is to create pressure on the walls of the excavation to provide stability. The groundwater and support fluid in the trench must be kept as separate systems.
Traditionally, bentonite slurry would produce a filter cake against which the full hydrostatic pressure acts. It would be markedly denser than the surrounding groundwater.
The present generation of polymer support fluids do not form a filter cake and have near water densities.
“One of the things polymers have forced us to do […], you realise actually that maybe you don’t need to have that action at the trench face, maybe you can let it be some way into the ground; the pressure drops over some distance,” Jefferis pointed out.
“And once you’ve got that in mind, you can begin to think of other things you can do with your excavation fluid.”
Polymer fluids of the current generation stabilise by viscosity, with a high gradient at the soil interface. So, the fluid loss is controlled by the viscosity and not filter cake formation.
Jefferis said he is interested in site data on how much fluid is lost into the ground. He noted that based on site observations, with enhanced polymers, loss of fluid to the ground seems lower than theoretical predictions.
Bentonite slurries and SDIP fluids are fundamentally different classes of material, and they cannot be managed in the same way. However, they are often still managed via tests that were borrowed from the oil industry more than 50 years ago.
Jefferis then considered the impact of geotechnical activities on soils more generally, as microorganisms will be present in all soils geotechnical engineers are likely to encounter. These microbial systems derive their energy from light or oxidation-reduction reactions.
“Any time we change the ground – tunnelling, earthmoving, flooding – we change the situation in the ground,” Jefferis said.
The microbiological activity usually passes unnoticed and causes no harm.
However, Jefferis referred to an earthquake in Ventura County, Los Angeles, US, which was claimed to have released a fungus from the soil that caused an outbreak of valley fever in 1994.
The industry is now also being asked to use recycled materials to improve the sustainability of projects.
“We’re big users and movers of material. It will be written into contracts – things like crushed concrete,” Jefferis said.
While in most cases the materials are acceptable, crushed and finely divided materials react more rapidly.
“They may have got good environmental credentials, but what do we know about their performance in the ground in engineering terms?” he asked.
“We’re taking liability for them; it’s our insurance that’s going to pay for it.
“We shouldn’t be taking on liability for materials that are unproven geotechnically. Crushed concrete I’ve seen reduced to powder by sulphate attack; I’ve seen problems with high pH water leachate coming through it.”
As a conclusion to his lecture, Jefferis stressed that when challenged by the unusual, geotechnical engineers should assemble all the observations, keep an open mind, seek analogies, develop hypotheses, test and confirm them by targeted experiment – and finally enjoy the unexpected.
Robert Mair recounted one of his first encounters with Stephan Jefferis, which was on a project were a “very, very highly unusual acid attack” had led to serious deterioration and cracking on London Underground cast iron tunnel linings at Old Street.
“Stephan provided the explanation of this mysterious phenomenon,” Mair said.
“Sulphuric acid was formed by the oxidation of iron pyrites present in the Lambeth Group, formerly known as the Woolwich and Reading Beds. The cracking of the lining was caused by an expansive reaction between the sulphuric acid and the contact grouts surrounding the lining segments.”
Another “mystery”, Mair recalled Jefferis elegantly solving concerned an unusual generation of ammonia gas in a tunnel in Singapore.
“This caused a nasty atmosphere with a particularly pungent smell,” he noted. “This at first was unfairly attributed to the tunnel construction workers relieving themselves. A jet grouted annulus had been installed to stabilise the very soft marine clay during tunnel construction. Stephan was able to explain how the chemical composition of the jet grout interacted with the organic material in the marine clay to produce copious quantities of ammonia gas.”
Mair also mentioned a case where a tunnelling contractor was having problems with the chemical agents being used to condition the soils being excavated by an earth pressure balance tunnelling machine.
“Stephan looked at the problem and then advised a simple solution: Go to the local store, purchase a packet of Persil washing detergent and add it to the conditioning agent. It did the trick; the problem was solved,” Mair said
The 60th British Geotechnical Association (BGA) Rankine Lecture is viewable in full here.
The BGA has invited John Carter, emeritus professor at the University of Newcastle in Australia, to present the 61st Rankine Lecture in March 2023.
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