Sunday, September 8, 2013

Tech Talk - of grouting, ground freezing and answers at Fukushima (and a gentle cough to PBS)

One of the reasons that I started to write blogs was to help folks to understand some of the technical background that fed into corporate decisions. I was watching the PBS Newshour this past week, and they were discussing the change in philosophy at the Fukushima nuclear plant. After trying to seal the flow of groundwater using a grout wall, the new plan is, instead, to spend some $470 million and build an ice wall.

The problem that I had with the broadcast was that the person appearing to explain the change in philosophy seemed to me to be more concerned with spreading fear and confusion, than in explaining the fairly logical engineering decision to change from one technique to another. And so, with some repetition from earlier posts I thought to explain why the change, and why it is logical, and not a highly dramatic concern.

The ground upon which the Fukushima plant sits is permeable, so that groundwater is continually flowing through it from rainfall in the surrounding countryside, which migrates down into the ground, and then flows down to the sea. At the time of the tsunami the basements in the reactor buildings were damaged, to the point that this groundwater can now enter the buildings. Unfortunately the reactors themselves were also damaged, so that water being used to cool the reactors can escape, and flow down into the basements. Here it can mix with the groundwater, contaminating it, and all the downstream regions to the sea.It has been suggested that there is about 100 tons of cooling water, and 300 tons of contaminated ground water currently flowing into the sea every day. It is not clear if this flow is into the immediately adjacent sea, or whether the contamination is getting into layers of bedrock, which don’t come to the seabed until some distance offshore.

The initial plan was just to rely on a chemical grout that would be injected into the ground to seal the passages in the rock/soil around the plant. I described, in an earlier post, how grouting can be used to seal off the water channels within rock, making it impermeable. At the same time, depending on the grout injected, it is also possible to add strength to the rock/soil so that it is better able to withstand loads. (Thus the rock over and around a tunnel might be grouted). To explain a little of that let me quote from the earlier post:

Figure 1. Drilling pattern used to inject grout around a tunnel line.

In a grouting operation, that is the goal. Normally a ring of holes are drilled into the wall of the tunnel so that they fan out around the planned tunnel path, and they are about 40 ft long. At this point the cement is brought to the site, ready to be injected. However, it is not just a case of bringing in say 8 bags of cement from the local hardware store. When dealing with the choice of cement, its physical and chemical contents and the pressure at which it will be pushed into the rock there are a number of factors that have to be established first.

The temperature and water chemistry of the surrounding rock are some of the initial critical factors. Changing either will change how fast the cement sets, or if it will. The object in this case is to get the cement to flow into the cracks around the drilled holes, so that the cement will flow to fill those spaces completely, before it sets. But it has to set in a reasonable time for work to continue on schedule. And since water chemistry in a tunnel, and temperature, change – so the mix has to be altered to accommodate those changes.

Figure 2. Types of grout used to meet different needs.

The next thing that has to be checked depends on the size of the cracks that the cement is being injected into. If the cracks are very thin, and the cement contains particles that are bigger than the crack, then the wall of the opening will act as a filter paper, stopping the cement particles from getting back into the crack and filling it. On the other hand if the particles are too small, then they will not bridge together to block the crack, and stop the fluid flow long enough for the cement to set up. If there are too many large particles then, when they lock together, they leave too large a gap between them, and fluid can still flow, and the rock will remain weak.

Over the years a rough correlation has been developed between how much fluid is flowing though the rock, and the type of rock, and the size of initial particles needed to provide an initial seal of it. But, as with cementing in an oil well, when the first cement injection has been finished, and allowed to set, then the rock is tested to see if the flows have stopped. Very often they have not, though hopefully they have diminished. This is because the first shot into the rock is more aimed at narrowing the flow passages and slowing the flow of fluid through the rock so that when finer particles are used, in secondary grouting, they won’t be carried away into the rock, before they can set up and block the remaining passages.

And so, typically, after the first grouting operation, there will be a second, to further fill the narrower passages in the rock, and those bits not properly sealed by the first injection. The cement grouts also act to give some strength to the rock, since they are filling the spaces within the rock structure with the set-up cement, that has some strength to it.

Figure 3. Rock after grouting (white lines)

However to fill the finer cracks, and to stop the flow cement may be too coarse a material in some of the rocks found. In such a case, then a chemical gel might be injected into the ground to fill those finer cracks. These tend to set up rather more like Jello, and while strong enough to resist water flow, do not usually give any additional strength to the rock.

There is one caution in injecting grouts into the rock that has to be borne in mind (and I know of cases where it wasn’t). Grouting operations force liquid into existing cracks within the rock surface. The liquid hopefully fills those cracks, before it sets, but if it is injected at too high a pressure, then the force on the walls of the crack can cause it/them to grow. At that point the rock will become weaker, instead of stronger, and the section in the tunnel roof/walls that is already open can fall in. Which is not good!

In the main it is not economic to keep injecting more and finer grouts into a rock until the flow is totally stopped. As the flows diminish the costs to stop them rise, and so it is usually the case that the operator accepts a certain small flow rate as the most economic alternative, and makes arrangements to deal with that water as it enters the tunnel. (If not I have seen tunnel floors lifted by the pressure that develops in the water trapped behind them).

Of course, if all else fails, then you can cut a slot into the wall and fill it with cement to completely seal off the excavation – though this is often done with a series of drilled holes, it can also be done with a variety of saw, known as a soil saw.

Figure 4. Grout wall exposed to show the 12-inch thickness and integrity.
In case the company of Fukushima the company chose to use a form of waterglass which forms a gel when exposed to an acid environment. This has the advantage that the fluid is quite mobile pre-gelling and can thus penetrate even the finer crack networks, and then, when it sets, it forms the seal.

Unfortunately there are a couple of problems with the grouting approach, there is no assurance that all the cracks will be intersected, and it is often necessary to re-inject successively finer grout materials into the ground in order to seal off systems of cracks missed in the earlier injections. The other is that the grout does not have a lot of strength, and if it is being injected into ground with a relatively high lateral flow rate, then the grout can be washed away before it can set.

In circumstances such as these, ground freezing has been an alternative that has been used for decades. It was, for example, used in the Dig Dig in Boston, being at the time the largest frozen earth retaining project in history. To describe that process let me quote from another post I wrote on the topic, back in 2010.
So how does it work? There are a number of different ways of going about the process, but I am only going to briefly describe a couple of them. The first is the more conventional approach, using a brine coolant, and the second is more commonly used when, for example, you’re refurbishing a road tunnel, and the roof suddenly collapses all the way to the surface. (The injection of liquid nitrogen).

Figure 5. Collapse of the Blackwall tunnel(Photos courtesy of Mott McDonald)

By completely freezing the gravel and other constituents of the roof and tunnel line, it was possible to restabilize and excavate through the area, putting in new supports at the same time.

Figure 6. RE-excavating the tunnel, showing the columns of frozen ground that stop water flow and form a wall to hold the ground in place.

Ground freezing can effectively form a temporary roof over an excavation, even if very close to the surface. It was used, for example, during the Big Dig, to create a bridge under the railway lines in Boston, while the new tunnel lining was pushed into place beneath it, using hydraulic jacks. It was also used in Vienna, where a subway had to pass relatively closely under existing buildings.

Figure 7. Ice wall as temporary support (image Joe Summers)

The conventional method of freezing involves inserting two sets of concentric pipes into the ground, inside pre-drilled holes. The outer pipe is sealed at the bottom end, so that as the freezing fluid (typically a chilled brine) is pumped down the inner pipe, and then flows back up the outside, it draws heat from the surrounding rock and soil, lowering the temperature until the water freezes. After circulation the brine returns to the refrigeration plant where it is re-cooled and re-circulated.

It is important to know the chemistry of the water in the ground, since with the wrong combination the water may not freeze at the expected temperature. It is also important that the outer pipe be sealed since if there is any leakage, then the brine may spill into the surrounding rock. At that point it can’t be frozen with the system, any longer, and an alternative method has to be used.

It is generally smart also, particularly when digging near the surface, to make sure that there aren’t any pipes (such as water and sewer) that can act as heat sources during the process. There can be embarrassing results if one of these (particularly the latter) is undetected, and the surrounding ground collapses as the shaft is then dug.

Figure 8. Hole in an ice wall (arrow) note that it depends on how the ground is being dug out, how fast this is detected.

Brine operations generally take a considerable time, and there is a project schedule so that you can get an idea (depending on the depth and size of the hole) of what might be involved in such an operation. Remember that the ice wall has to be kept cold during the excavation, but that keeping the central material unfrozen makes it easier to dig out. Cement poured against a frozen wall, if properly designed, will set as planned, since the heat of hydration overcomes the surrounding heat loss.

Figure 9. A ground freezing schedule.

This requires, obviously, a lot of preplanning. Where there is an emergency this is not possible, and thus the use of faster freezing methods, such as the injection of liquid carbon dioxide, or nitrogen, into the ground, in order to freeze and stabilize it more rapidly.

There are two ways of making the injection. There is the dual-pipe approach where the nitrogen is in a closed circuit, and then there is the simpler process where a lance is, simplistically, pushed into the ground and the resulting gas percolates upward from the end. That is a little less precise, given that the gas moves through the ground following the path of least resistance, but it does have the advantage of being quick, and generally effective in stopping an imminent disaster. Relative to the months of a brine installation the work takes a week or so (depending on size).
Copper freeze pipes with a standard diameter of 2” (54 mm) are installed, at an average distance of 2” (54 mm) On the inside, downpipes with diameters of ½” (10–12) mm are installed.

LIN is fed into the pipes through insulated supply lines. The LIN vaporizes, with 1 kg of LIN extracting about 200 kJ of energy from the surrounding soil, cooling and freezing it. The vaporized cold nitrogen (i.e., exhaust gas) extracts another 100 kJ from the ground. After about one week, this process forms a frozen wall with a diameter of about 1 m. This so-called “establishing phase” lasts four to seven days, and about 300 - 500 gal (1,500–2,500 l) of LIN is used to freeze 1 cubic yard (0.75 m³) of soil.

Notice from the photographs how the equipment can be fielded very quickly to provide emergence stability to the structure, and then other treatments can be used once the flow has stopped, and the structure is stable. One could, for example stop the flow using ground freezing, and then – using the Brown and Root Soil Saw (and a head design that vestigial traces of modesty suggests I don’t discuss) – a channel can be cut down through the permeable ground and filled (as cut) with bentonite, as shown in the figure above.


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