Holdfast Training Services Trial Blog

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!