The project consists of a new rail yard for a private owner on private land; although, it will connect to existing lines outside of the property. The rail yard is roughly 600,000 square feet. The railcars will be carrying highly flammable material.
The mine workings were originally "room and pillar" which were partially extracted according to the detailed mine map. The air-rotary borings drilled across the site encountered mainly broken bedrock at the mine level with 100% air loss indicating small voids throughout. A down-hole camera was only able to observe an approximate 1-foot void in one of the borings, but could not get down to mine level in the rest of the borings due to collapsing after drilling at the mine level. 1 boring encountered intact coal, which could mean full height voids (up to 13 feet) in the area because of the pillars remaining and roof rock possibly not collapsed. Cover is only approximately 75 feet, so a moderate to high risk was assigned based on the depth to the mine and current condition of the void space.
Grouting of the mine is currently recommended unless a more detailed investigation shows that no full height voids exist and if the estimated settlement from subscidence in partially collapsed mine workings is within tolerable levels.
I am just trying to find out if the client has no choice but to stabilize the mine based on any railroad regulations. Thanks for your interest and any help you can provide is appreciated.
Suggest you contact the connecting railroad(s) in question directly. Most have their own policies (and lawyers). Many subscribe to AREMA guidance, but this is an area that AREMA may not explicitly address.
Spread the topside rail-to-rail distance apart a bit further than the standard in-yard separation; then use the increased distance between every two adjacent rails (and the switches/turnouts) together with a thicker, more resilent grade or underlayment to reduce the average weight-per-sq-yard of the railroad system.
So, rather than fill the old mine shafts up with concrete+ grout to increase the average ground strength, reduce the average surface loading of the rail cars above ground so the 74 feet of overburden can manage the average load on each rail.
Alt: manage the cars so no two loaded trains are ever parked side-by-side of each other. (Probably not possible, but its only to bring up in a meeting so the client can smile for a few minutes.)
Looked in our AREMA books already, and couldn't find anything on mine stabilization. I'll find out who owns the connecting lines and see what they say then. Thanks.
Loading is not an issue when it comes to potential mine subscidence; unless it was some astronomical surcharge. When the mine collapses, it's merely the mine level materials wearing away until failure.
I doubt you will much in the way of regulations for this. Unless you have observed subsidence, you are just speculating that it may occur. Is there any evidence of subsidence? Perhaps you can get aerial photos or topography that can show if the site has settled any in the past, relative to the surrounding area. Are there any sinkholes in the area? Are there any reports from previous landowners of settlement? There will be some risk of settlement if you construct the railyard. That risk is likely very small. Why not just construct and then closely monitor? Develop a risk management plan that calls for monitoring and then mitigation if anything develops.
Based on the depth of the mine and what we've encountered so far, the risk of subsidence is moderate to high according to PADEP criteria. The property used to be farmland so topography does not show any evidence of past settlement from continually regrading. No past information exists from previous land owners experiencing any subsidence issues. Monitoring could be very dangerous given the highly flammable materials because severe subsidence can occur rather quickly without signs of distress.
The problem I am having is not with what to do about the mine. Grouting appears to be the option to go with at the moment; although, stabilization may not even be required based on further investigations and theoretical settlement calculations if no large voids are present. I am merely looking for guidance on railway regulations on the subject of mine stabilization. I know railway regulations are usually over-conservative and thought no matter what the size of the void or condition at mine level, it has to be stabilized. I also posted this question in the railroad engr section, where no reply has been given so far.
Thanks again for the continued interest in my issue.
I don't know about railroad regulations, but I know there has been a fair amount of research and case histories using FLAC and other FEA methods on subsidence from underground mines.
For what it might be worth, considering cost per square foot or other method for improving the site, I'd bet there are many alternative that are less cost than grouting.
This somewhat reminds me of folks worrying about the loads from truck traffic on highway fills. In most of those cases the weight of the trucks has very insignificant effect on settlements. The same probably would go for your RR yard loads. Your weights you might more likely need to think about are those soil and rock weights above the former mine workings. I've experienced measuring the effect of loaded freight cars on culverts under the ties and there is significant spread of load under loaded freight cars and locomotives are even lighter (than the cars I worked with).
Time also is important. How long ago was that mine worked? The more time involved, the less likely there will be future RR yard problems.
What does it cost to re-set track in case it tilts or has a roller coaster vertical profile? You could bury this cost in annual maintenance expenses. RR guys easily can drag a whole long section of track, with ties attached, off an area, re-grade it and drag it back. That would be in a real bad case of settlement only.
If you are still concerned and want to do something, consider running a rolling surcharge of earth across the site. I'd think about a "windrow" maybe 10 feet high and 20 feet wide on the top (any length) being shoved across the site with a dozer or a backhoe sitting on top. You could monitor the effect with simple settlement platforms. Moving fill this way is cheap, probably much cheaper than the grouting option. It would put much more load (pressure) on that mine than any bunch of loaded freight cars.
Dig a hole at one corner of the site to get this surcharge, move it over the site in a "U" pattern, bringing the earth back to its source after traversing the site. You would not have to wait at any location, since this is something like compacting earth with a roller.
