Friday, February 15, 2013

Coal, Trains, Landslides and Long Distance Geology Arm Waving

Dave Petley has had plenty of landslides to post about from his homeland of England of late due to a long stretch of very wet weather even for wet England. He is a clearing house for all sorts of landslide and geology risk information. He recently posted on a rather unique recent landslide:
The Petley post stirred my interest so I had to do some long distance geology investigation.
Uplifted and pushed rail tracks
Aeriel view of slide and rail damage 
The above images sure look like a classic deep-seated rotational slide. The fractures on the slope are the upper part of the slide where the slide surface is dropping. If one projects a curving line down through fractures, the lower part of the slide is being lifted upward as the upper part drops. It is a gigantic balance. Hence, the ground at the rail lines is lifted upward while the top of the slope drops. Early accounts of less severe uplift along the rail line support this interpretation.
The gray pile of soil is a mine waste pile from a near by coal mine. So we have coal, trains and landslides. Just the kind of thing to get folks in Washington excited. But all the coal stuff aside, this slide has really got my interest at several levels as a geologist. For one, there are several deep-seated slides like this along the Salish Sea shorelines with very similar geometry including one in Seattle as well as a couple I have worked on in Jefferson County on the Olympic Peninsula along Hood Canal.  
My first long distance investigation of this slide was pulling up Google Earth.
The slide area is in the center area of the image
Also. you can click the Google Map Link Google Map Aerial View
Besides getting a sense of the land area, I did checked on Google Earth's elevation for the site. The mine waste pile at the time of the Google Earth image was as high as 109 feet. But that was in 2008, and based on the geometry of the pile in the aerial with the fractures, the pile was even higher and  the high area was much broader. The ground elevation just south of the rail line is generally 15 feet. Hence, a pile of mine waste rock piled up on the order of 95 feet.
This is a really flat board plain with a river estuary about 10 miles to the east. (If you want to take a virtual trip to the area go to Google Earth and type in Stainforth, UK)
A street view of the pile from the north, non-failure section has a warning sign that maybe says it all.
North side of the mine waste pile.
And then there is the disconcerting thought of what would this failure would have been like if instead of being on the south side of the pile, it had been on the north.
Homes across the street from the opposite side of the mine waste pile where the failure took place
At this point I know that the pile was really big. I also know that the elevation of the area is very low. Any streams flowing in this area will have very low gradients. That means that recent alluvial sediments underlying the area are likely silts and clays and possibly filled in organic rich soils. I also know that this area has been glaciated and the mass of ice likely pushed the local land surface downward below current sea level. More reasons to suspect silts and clays. But I know very little else and the extent of my geology understanding of England is way less than a little sketchy.
But this slide is way too interesting so I took a crack at trying to find some geology maps of the UK on line. The British Geological Survey has geology viewer similar to out own Washington State Department of Natural Resources geology viewer. I was able to get a 1:50,000 scale map view of the area which given the geology is reasonable: Alluvium - clay, silt, sand and gravel. Perhaps not real helpful, but given the extreme low gradients in the area and low, silts and clays seem probable. I also noted that map indicates glacial lacustrine (lake) deposits. I would suspect that these units may underlie the area and based on my own experience in western Washington, these units are susceptible to deep-seated rotational failures when the slope geometry is right.
The mine waste pile is located in the yellow alluvium area
Dave Petley did the same geology exercise before I finished my long distance investigation the-geology-and-a-possible-mechanism-of-the-hatfield-stainforth-colliery-landslide/, and includes a block diagram of a deep-seated rotational failure.
I also took a crack at trying to find a Soil Survey. Canfield University National Soil Resources Institute has a soils view site that describes the soils as: Naturally wet loamy and clayey flood plain soils.
Soils Map from Canfield University.
A 95-foot deep pile of mine waste placed on soft, unconsolidated silts and clays strikes me as a recipe for soil failure. The zone of influence from this kind of mass would be very deep and would cause settlement and compaction of the underlying soils. I have been involved in projects where we have placed a pile of soil on a development site to compact the soils in somewhat similar flood plain settings. Typically it will take a few months. The pile is removed and the site is then ready for the loads from the building. But the soil piles we have used are never at the scale of the mine waste at this site. I also know that for soft glacial marine silt/clays preloading as described above typically does not work as the tight clays do not compact very rapidly.
A possible cause of the deep-seated failure may have been the loading was too rapid and too large causing a fracture plain to develop through the underlying silts and clays as water was being rapidly squeezed out by the huge load of mine waste. Throw in the added loading on the pile from lots of rain over the past few months and a highly unusual failure resulted.
Perhaps a bit of arm waving theory here, but for an engineering geologist exciting stuff and an opportunity to learn some lessons and think through some possible scenarios. As this failure is further investigated, there may be some interesting engineering lessons for a future text book. Disasters have a way of being teachable moments - at least one hopes. 
There is little doubt in my mind that this is a deep-seated rotational failure. As such there will be no easy fix. Simply removing the uplifted soils and replacing the tracks will create an imbalance along the now established slide failure surface and will very likely lead to more slope movement. A significant amount of the pile will need to be removed in order to prevent additional movement and fixing the rail lines. It may be easier, faster and cheaper to simply reroute the rail lines.
There is another aspect of this failure that may be useful to understanding some of our local western Washington geology, but I'll save that for the next post. 

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