Any ideas??
To me there seem to be a number of fundamental conflicts that are thematic across the advance sequence during the SCL works that I am eager to hear your collective thoughts on.
Overview
In outline, SCL works in the station caverns focus on widening the 6m tunnels bored by the Tunnel Boring Machine into 10m diameter platform, concourse or cross tunnels. Tunnels are excavated and then rapidly sprayed with concrete to stabilise them and use as the initial lining of the tunnel. See this time lapse to give the idea:
http://www.youtube.com/watch?v=cAWAIwf3nOY
The construction sequence from the method statement is, in basic terms, as follows:
Tunnel Elevation Sketch
1. Excavate top heading to profile, and to 1m chainage. Engineer utilises pre programmed total station to guide plant operator.
2. Initial SCL layer is sprayed to a minimum of 75mm to all exposed ground to seal. Test panel is then sprayed to confirm entry to the exclusion zone in the proximity of freshly sprayed concrete. Using a penetromenter test, a compressive strength of at least 0.5N/mm2 must be verified before personnel enter the exclusion zone.
3. A second top heading of 1m excavated as per point 1. SCL later sprayed in the same way. Critically this means that the engineer, plant operator, nozzlemen and shift geological engineer will move forward into the exclusion zone, and underneath the ‘freshly’ sprayed concrete, albeit not until 0.5N/mm2 is achieved.
4. Spoil in invert removed, and battered to allow access to full profile. Larger tunnels will excavated bench then invert in two stages. SCL sprayed to complete full profile.
NB. This sequence is in basic terms. A number of other activities are conducted pertaining to water management, joint preparation, spoil removal etc. I will not cover these here.
Issues
Three key risks strike me here:
1. Technical.
The spray concrete in the crown of the tunnel is not actually tested for its early age strength. An assumption is made that the concrete on the substrate is behaving as that in the panel. It has been noted by the miners that often the concrete on the wall cures slower than that in the timber test panel (perhaps the clay has a cooling effect, the act of spraying the substrate causes cooling, or indeed the exposed clay is cooler than the ambient temp????)
Penetrometer tests are often conducted in the wall but as it is difficult to reach the crown without getting underneath it, it is not done. As the crown is the last to be sprayed and therefore (probably) the last to cure, this assumption based on the panel strikes mean as not robust?
2. Managerial
Miners are incentivised by a bonus scheme based predominantly on progress. Their base daily salary is around £300 per shift. There is a further £200-£250 bonus if you advance 4m per shift. Any less than that and the bonus is reduced proportionally. Thus the gangs are at full throttle and any reduction in production needs to be a critical requirement or else you will hear about it. Due to the opportunities currently available for qualified miners and nozzlemen, they slightly have the contractor over a barrel here, and if the bonuses were to be reduced or altered, they are likely to go elsewhere.
The shift engineer, from what I can see almost always fits the cliche…young timid graduate who is there to be seen and not heard…but take the rap when it goes wrong. The relationships with the shift pit boss (think grisly SSM) and the lads are limited. The shift patterns are deonflicted meaning that engineers don’t often work with the same gang and therefore their rapport and mutual understanding is limited. It means that it is a significant challenge for the engineer to direct, lead the shift, particularly if he is seeking to slow the rate of advance for say, quality purposes. Its probably fair to say that the engineers are often rushed to keep testing the panel so that the instant 0.5N/mm2 is recorded the advance continues.
3. Health & Safety
Fallout from the crown are relatively regular (very approx 1 per week). Fallout should be reported as a near miss. However, the miners are very wary of reporting these incident, due to its perceived impact on progres for that shift, and any ramifications and blame. Clearly a certain element of discretion has to be exercised, and if the exclusion zone is enforced nobody should be near it. However, it could suggest a wider weakness in the lining.
Solution
I feel that there must be a way of removing the element of assumption from the compressive strength test in the crown, and that the test panel penetrometer need to be augmented with a reliable test that is quick to conduct safely, such that it gets buy in from the guys on site. Having used thermal imaging technology quite extensively in reconnaissance, I though perhaps there is a way of using it here. Having spoken to my line manager it, turns out that there are already products on the market that can test temperature at range, and he was one step ahead of me having arranged a visit from an old mucker of his at Warwick University…a doctor of engineering who has agreed to conduct testing on remote equipment to deliver a TI image of the curing concrete, identify temperature and translate it into an estimate of early age strength in the crown. Im going to get involved in this, with one eye on a TMR, butahead of seeing any results I remain unconvinced. Therefore really keen to hear any inspirational ideas of reducing this risk in the short term that I can cynically take as my own?
The bonus culture, and the fundamental conflict of interest it delivers on site remains an issue. My suggestion of associating the bonus with quality rather than rate of advance was largely dismissed as ‘not what happens…the lads will just find another job’. Not letting this one go…
Keen to hear your thoughts!
Holdfast Training Services Trial Blog 
1) Presumably the test pannel 0.5 N/mm2 value has been established in the knowledge of the methodolgy and geology i.e. the specifier will have known the factors at play and set a value accordingly – Your concern indicates a professional awareness of a risk that you do not know to have been mitigated/considered. You should check that the specifier of the requirment has not used unreasonable caveats and if you remain concerned ask the question so that they cannot deny knowledge and responsibility – this is not an umberalla exercise in abrogation but a deliberate act which usually makes people check thier specification
2) I would hope that recent events would make most folks think about their desire to go home at the end of the day. It is after all thieir safety that is at risk and if there is a colapse there will be a stop order in place PDQ so 4m will not happen for longer than they might want to think about. All you can do is make sure that anyone you are responsible for is aware that they carry a heavy burden of responsibility and that it’s not about reaching a good number and a bonus, it’s about not getting killed or having someone else’s death on your conscience.
3) If there is a crown fall reported it enables a good picture of safe mixes and conditions and more risky condictions to be built up. This not only enables probably dangerous situations to be identified earlier but could also potentialy allows progress to be permitted when presently it is not. Greater knowledge and understanding = safer and potentially better for all. Not to report is negligent and there is no such thing as discretion when falsification of records could lead to incorrect data being gathered resulting parameters, adjusted in the light of experience, being unsafe!
You want to work in a tunnel that might fall in? Use a false liner that will give you time to get out before it gives up the ghost. It reduces working space but might save a life or two.
look forwards to the thermal curing results – I think there’s going to be quite a lot of variation according to the temperature of the retained material that will confound heat of hydration and curing calculations. Watch out for witch craft and be ready to ask apparently stupid questions to really understand the thought process that is being applied I think it might be interesting.
1. It seems I’ve uncovered a lack of knowledge as to the origins of the 0.5N/mm2 figure. In my naivety as to how the SCL tunnel works in engineering terms, I scribbled an engineering model to try to put some figures to it myself.
Assumptions:
a. Crown of the tunnel sits on average 25m below the surface. (Datum)
b. GWL sits approx 5m below datum
c. Dont consider self weight of concrete or surcharge from the urban environment
I calculate the vertical total stress = 485kN/m2
Modelling the crown as a beam 1m in length by the minimum thickness of SCL 75mm thick, I worked out a ball park figure, and that due to the curvature of the tunnel lining, assume that the beam supports act at 45degrees
This model suggests a horizontal component of stress = 0.46N/mm2
Given my assumptions, this figure is clearly quite low and therefore the compressive strength criteria for the initial sealing layer would be unacceptable.
Therefore, after some reading, I discovered;
The art of tunneling is to utilise the inherent strength of the ground in order to minimise loading on the actual structure, by three dimensional soil structure interaction. Therefore much of this stress is redistributed, and the remaining load is carried by the composite structure of lining and ground. Which I think means that the risk is local rather than global.
So the lining is designed to stabilise the ground , preventing deformation and localised stresses as the ground moves.
After a bit more digging, it seems that the 0.5N/mm2 figure is designed with view of supporting a pre-determined dimension of soil that may be loose in the ground. Apparently there is a phenomenon known as ‘greasy back’ which can create a failure plane in the soil surrounding the tunnel, causing a point load on the lining? Not massively convinced of the explanation I received, and there was no dimensions attached to this phantom floating lumpm of earth. More digging to come.
Its also worth noting that this critical safety measure which defines the hold point in the advance sequence is not standardised across sites. Uniquely at Farringdon, they use 0.3N/mm2. Again no-one seems to know why…
2. Agreed. This presents an interesting managerial challenge, however, given the wider acceptance of the incentivisation. The unspoken culture is that programme is the priority, but after the recent events the focus has been entirely on safety and quality to learn lessons, perhaps at the expense of the elephant in the room/tunnel…
3. The discretion I alluded to was with regard to the spectrum of fall outs which exist. One mans rebound is another mans near miss. I agree that a greater understanding of the engineering could allow more efficient and safe decision making process on site, but its finding a decent level which caters safely for the common denominator
4. Literally received my thermal imaging cameras. 4 x FLIR E40bx at a total cost of £12k!!! Im digging through the technical specs now to write a How To guide, which we can use for the trials. Meeting with a contact of my line managers this afternoon…Dr Benoit Jones from Warwick University. Amongst other things he delivers the Tunneling and Underground Space MSc, and has some experience with using these things. Ill let you know
Ryan
Thanks for that Ryan. Sorry to have missed you last week, hope it is all going well on site and that tesiting is interesting. Look forward to an update or close out on this later. R 🙂