Recent Articles

A Bogus Journey

Categories

In the Northeastern tip of Minnesota, just a few miles from the Canadian border, there's a lake so small it's Bogus. That's where I have my own piece of the northwoods, a conifer and maple forest overlooking Lake Superior.

At 80 acres, Bogus Lake is certainly small, but it's a lot bigger than some of the drainage ponds they used to call "lakes" when I lived in central Illinois. It's also not the only Bogus Lake in Minnesota--there's another one near Lake Itasca in the central part of the state. There are also Bogus Lakes in Wisconsin, Oregon, and Ontario, among other places.

I bought a parcel of undeveloped land on Bogus Lake in 1991, when the combination of a recession, the collapse of the logging industry in that area, and the fact that Grand Marais hadn't been discovered as a tourist destination conspired to drive land prices to absurdly low levels. I was in college at the time, and scraped together enough money to buy the 158 acre lot. At the time it had been for sale for something like two years with barely any interested buyers.

The land lies at the top of a ridge 1,200 feet above Lake Superior. When the leaves are off the trees you can see the big lake from a number of places around the property. There's a massive beaver pond and stream, and many signs of moose activity.

In my college days, I made a couple of attempts at cabin building with the idea of having a warm place to stay. Unfortunately my ambition ran ahead of my time and abilities--one of the two structures is slowly collapsing, and the other has been in a permanently unfinished state for over 15 years. When I moved out of state for graduate school my visits to Bogus Lake slowed to the barest trickle: first because it was too far, then because I was too busy, and then because it was too hard to camp with the kids.

This year I've been thinking more and more about my place at Bogus Lake. I think this was really sparked by the cousin of She Who Puts Up With Me, who aspires to be the next Thoreau. Over the winter he asked us if he could build a small one-room cabin on our property, and we agreed (it's not like things are getting crowded up there).

The cabin is scheduled to be built starting around the end of May, and this past week I spent three days in Grand Marais hiking all around the property scouting sites. I went further into the property and saw more of it than I ever have in the past, and this has rekindled my enthusiasm for eventually building a vacation (or even retirement) home up there.

I've also hired a surveyor to locate and mark the boundary of the property. Only about 1/3 of the property line has ever been surveyed, so our current understanding of where our land ends is based on taking measurements off topographic maps and punching them into a handheld GPS. This could easily be off by 100 feet or more.

Within the property, there are places where (if it weren't for the trees) you could get stunning 270-degree vistas of Lake Superior and the surrounding hills. There are other places which feel like the shore of a remote alpine lake, and places where the horizon only extends to the first row of trees. When we build a home at Bogus Lake, we will have our pick of places.

In the meanwhile, I've decided that it's high time to start getting up there more often.

Nothing Was Going to Stop Me, Anyway

Categories

When the original iPhone came out, my first reaction was, "Cool, I want one!"

She Who Puts Up With Me was less enthusiastic, viewing it as "an expensive toy and we already have two perfectly good phones so why do we need this?"

Despite these objections, my old phone just happened to fall apart within days of the availability of the iPhone (no, really, it was an old Treo and the screws kept falling out and it was being held together with one screw and a piece of Scotch tape, and besides the web browser was a piece of junk and I couldn't get it to work right with my e-mail anyway). So it came to pass that just a week after they went on sale, I came home with two brand-new iPhones.

For the record, my wife has become a true believer and now plans to upgrade her iPhone (still the first generation one, now starting to lose its battery life) this summer when the next generation comes out.

So it should come as no surprise that when the iPad was announced, my first reaction was, "Cool, I want one!"

She Who Puts Up With Me responded with "it's an expensive toy and we already have five perfectly good computers so why do we need this?"

Let's not fool ourselves. I've basically not grown up past the "give me the shiny new toy!" stage which for most people ends at about three years old. That makes me the perfect target consumer for an iPad--I just needed to find some way to distract the rational side of my brain from the price tag. The mental equivalent of pointing off in the distance and and shouting "Look over there! What in the world can that be?"

My excuse is that the iPad looks like the perfect gizmo for a tradeshow booth where you need to do one-on-one demonstrations of a web-based application. It's very portable (saves shipping), you can have several of them in the booth, prospective customers can hold it up and touch the application (better than huddling around a mouse and screen), and the novelty value alone will bring people into the booth.

It just so happens that my company's reporting system is web-based, and we spend a lot of our time when exhibiting at tradeshows doing one-on-one demos.

So I've ordered an iPad--of course this is to "evaluate its suitability for use in our tradeshow demos," but we all know the truth.

iPad First Reactions

Categories
  1. I've seen lots of commentary along the lines of "It's doomed to fail because it doesn't have X" (where X is a camera, flash support, desktop-style OS, multitasking support, a hardware keyboard, HDMI output, an open app store, or any of a dozen other features various people consider "must have"). This is wrong. No gizmo can do everything--the question is whether this one does enough.
  2. The iPad isn't intended as (or even capable of being) your primary computing device. It will succeed if it's a more convenient laptop for casual web surfing. It will fail if it's an iPhone which doesn't fit in your pocket.
  3. iPad is a clunky name, but so is MacBook. If it succeeds, nobody will care about the name.
  4. Nine years ago, when Bill Gates announced that everyone would be using Tablet PC's, I don't think he meant "made by Apple."
  5. I have no idea if the iPad will be useful, but for the price I may be willing to take a chance. I've certainly spent more than this on gadgets which didn't meet my expectations in the past. The $500 price is critical in this decision--if it had been $1,000, I would look at it more like a new laptop than a potentially useful toy.
  6. Apple clearly intends to upend the accepted practices of user interface design (practices which Apple was instrumental in popularizing). On the whole, this is a Good Thing, since the desktop user interface is (or has become) far to complicated and technical for a large population of users to properly manage.

Optimistic Sign #11: Jobless Recovery

Categories

In the past few months, the Conventional Economic Wisdom (CEW) has swung from a recession of indefinite duration (but always lasting at least 18 months longer) to a jobless recovery. This can only mean one thing: job growth in the United States is about to explode.

This is not based on any particular insight I have, just the observation that job growth is a lagging economic indicator, and the CEW is always looking in the rear-view mirror. The CEW saw continued growth in the first half of 2008 after the recession had already started, hard times as far as the eye could see in the first half of 2009 as the economy bottomed out, and now that growth is returning the CEW insists that it isn't really at least not for most people.

So I will once again stake out my contrarian position and claim that the pessimistic CEW is a leading indicator for imminent job growth.

APPENDIX: My contrarianism has actually served me reasonably well. Looking through my blog archives, I find that at the end of 2005 I wrote that there would probably be a recession starting by the end of 2007 (true, but barely). At the beginning of 2008 I wrote that we were already in a recession (before the CEW acknowledged the fact) but that we were close to the bottom (sadly, too optimistic). Then at the beginning of 2009, as the economy was bottoming but the CEW saw nothing but pessimism, I started looking for signs of hope. This time around I could be completely off-base or way too early, but by golly I'm going to stick to my contrarian optimism until I'm right.

Random Thoughts: Syrup Energy

Categories

Instead of Tony the Tiger in the tank, how about Aunt Jemima? Would it be possible to use a simple sugar syrup (about 50% water and 50% sugar) as a vehicle fuel?

Why Syrup?

One of the biggest challenges of large-scale use of biofuels is that refining the fuel is often extremely energy-intensive. Most products of biological processes are water-soluable, since biological process all take place in a water medium. Unfortunately, however, most current internal combustion engines can't run on a fuel+water mixture, so it is necessary to remove the water from the fuel as part of the process of refining the biofuel. This can take almost as much energy as is present in the fuel to begin with.

(Note that oil-based biofuels, like biodiesel, don't have this problem since the oil will naturally separate from the water. However, oil-producing plants tend to have a much lower yield of oil than sugar-producing plants have of sugar.)

So if you can build an engine capable of running efficiently on a fuel+water mixture, you can get a lot more biofuel for the amount of energy you put into growing and refining the fuel. In addition to making the biofuel much more sustainable, this also makes the economics of producing biofuels much more compelling since it's no longer necessary to buy massive amounts of fuel to separate the fuel from the water.

Once you've decided to use a fuel+water mixture, sugar becomes a much more compelling fuel choice than ethanol. Ethanol production always begins by fermenting sugar anyway (even cellulose-derived ethanol, since that uses enzymes to break the cellulose down into simple sugars), and sugar has a significantly higher energy density than ethanol. Sugar is a lot cheaper, too.

The only reasons to prefer ethanol over sugar are (a) ethanol can be used in existing engines with little or no modification, and (b) ethanol is a liquid, and sugar is a solid, and solid fuels are really hard to deal with in an internal combustion engine. But if we're designing a new engine specifically to run on a fuel+water mixture, we've already decided that compatibility with existing engines doesn't matter; and a sugar syrup is a liquid.

Sugar syrup has some other advantages: it's readily available from a wide variety of sources, it has a low freezing point and high boiling point, and the desired 50% mixture can be achieved fairly readily by removing water from certain plant saps (no need to dry it all the way to granulated sugar). You can even make the stuff at home, cheaply and easily.

Can It Be Done?

I don't know if a syrup-powered engine is possible, but I think it would be.  The challenge is that before the fuel can burn, the water has to boil completely inside the cylinder, since the water boils (even at high pressure) at a lower temperature than the ignition point of the sugar. Boiling the water takes energy and cools the gas inside the cylinder, making it harder for the fuel to ignite.

This isn't an insurmountable problem: you just have to get the cylinder that much hotter to overcome to cooling effect of the water in the mixture. The trick is to design the engine so that the energy used to boil the water can be recovered to help turn the engine. Since the role of the water in the syrup is essentially to vaporize and cool the combustion gasses, the engine has to be designed for a slightly higher volume of slightly cooler gas.

Thinking in terms of modifying an existing engine design, I would think that a diesel engine would be ideal, since it's intended to operate with very high compression and hot cylinders, and fuel which burns as a mist rather than a vapor. Somewhat higher compression (to yield a hot enough gas to ignite the syrup) may be the only change necessary.

One final note: sugar actually is used as a rocket fuel for some model rockets, typically mixed with potassium nitrate (saltpeter), but this is normally done with solid dry sugar, not syrup, since if the mixture has any water in it it becomes difficult to ignite. I did find, however, some YouTube videos of experiments with including sugar syrup in a rocket propellant.

Found on Scooter's Christmas Stocking

Categories

"Dear Santa,

"Please disregard any infractions regarding my behavior this year.

"From, John"

We had thought Scooter, at almost 11 years old, didn't really believe in Santa any more. When Christmas Eve came, however, it looks like he wasn't quite ready to deny Pascal his wager.

How to Remove Snow

Categories

We had our first major snowstorm of the season last night, and as I was shoveling the driveway I was thinking about different ways to remove snow.

Okay, I'll be honest--I was trying to figure out how to justify installing a snow-melting system when we have to replace our driveway in a few years. I still shovel the drive by hand, but I can foresee a time when I won't want to do that any more or will be traveling enough so I can't.

There are four basic ways to remove snow and ice from a driveway: shovel it by hand, clear it with a snowblower, melt it with a heated driveway, or hire a snowplow service. (You could look at a fifth possibility, melt it with chemicals, but that would require so much chemicals as to have serious environmental consequences. Chemicals are best used for stubborn patches of ice which are hard to remove mechanically.)

The Physics Perspective

The most obvious way to look at the problem of How to Remove Snow is to compare the energy required to melt snow vs. move it.  I measured our driveway and found that it is about 1,200 square feet (I'm going to use English rather than metric units because they're probably more familiar to my readers).

If we get a heavy snowfall of a foot, which translates to an inch of equivalent rainfall (Minnesota's snow tends to have one inch of rainfall equivalent for every 8-15 inches of snow), that's about 6,000 pounds of ice on the driveway which needs to be melted (which will yield about 750 gallons of water, if you're keeping track).  It takes 144 BTU to melt a pound of ice, so it will take about 850,000 BTU to melt all the snow.

In addition to melting the snow, you also have to heat the driveway itself. If there's three inches of brick over the 1,200 square foot driveway, that's about 40,000 pounds of brick. In the worst-case scenario, that brick needs to be warmed by about 100 degrees F, which will take about another 900,000 BTU. Normally a snow-melting installation includes a layer of insulation underneath the driveway, so we don't need to heat the ground underneath the driveway. In total, then, we need about 1.75 million BTU to melt a foot of snow from the driveway on a very cold day.

Calculating the energy it takes to move the snow isn't quite as straightforward since it depends on whether you push the snow (with a plow), lift the snow (with a shovel), or launch the snow (with a snowblower). Hard-to-measure factors like friction and ice adhering to the surface can matter a lot. The simplest case is the snowblower, which essentially fires the snow out a chute. If we assume that the snowblower shoots the snow out fast enough to launch it about 30 feet straight up, then it will take about 300 BTU to clear all the snow.

This is a rather lopsided result: it takes about 5,800 times as much energy to melt the snow as to clear it with a snowblower. This is not a helpful result in my quest to justify a snow melting system. It's not the end of the story, though: a snowblower turns out to be much less efficient.

It turns out to be fairly easy to convert chemical energy from natural gas into heat. Our on-demand hot water heater (which would likely be pressed into service to drive any snow-melting system) claims to be 98% efficient, and the required plumbing would have only minimal loss, so over 90% of the energy of the natural gas would be available to heat the driveway. Delivering our 1.75 million BTU to the driveway will require just a little over 1.75 million BTU of natural gas.

Small gasoline engines, like the ones used to drive snowblowers, are not very efficient. Only about 10% of the energy content of the gasoline is actually converted into mechanical energy in the driveshaft of the engine. What's more, the snowblower has a lot of internal friction, idle time, and other losses. It's probably reasonable to assume that only 10% of the output of the engine actually gets converted into flying snow. Realistically, then, it probably takes about 30,000 BTU of gasoline (or about 1/8 of a gallon) to clear the driveway.

Even accounting for the relative efficiency of melting vs. moving snow, it still takes 58 times more energy to melt the snow. This is still not a helpful result, but there's one more wrinkle: a foot of snow on a very cold day is a worst-case scenario for the snow melting system, and melting less snow on a warmer day leads to a direct reduction in the energy required. The snowblower, on the other hand, is likely to use about the same two cups of gasoline no matter how little snow fell or how warm the weather, because most of the energy is going into friction and the important factor is how long it takes to walk the machine across the entire driveway. With only an inch of snow on a warmish sunny day, the snow-melt system might require only 2-3 times as much energy as the snowblower.

The Fuel Perspective

Another way to look at the problem is to estimate the amount of fuel consumed by the different ways to remove snow. For our foot of snow, the snow-melt system will consume about 18 therms of natural gas, or about $13 of gas at recent prices from our gas company. The two cups of gasoline the snowblower consumes is about $0.30 of fuel these days.

The amount of gasoline consumed by the snowplowing service is harder to estimate because they likely burn more gas getting to and from our driveway than they use in actually clearing the snow. Plow services tend to drive big four-wheel-drive trucks which get poor mileage (especially with a giant plow rig attached to the front), so it seems reasonable to assume they burn about 1/2 gallon (or $1.20) getting to and from each client on the route.

Finally, when I shovel the driveway by hand, it takes me about an hour and burns 720 calories according to government exercise tables. That's about three candy bars, which cost about a dollar each at the convenience store, so about $3 worth of "fuel" is required.

Here, too, there's a slight wrinkle. Our geothermal system uses waste heat to warm a storage tank for hot water, and this heat could be available for use in a snow-melt system. This could give us the first 25,000 BTU or so for free each time we run the heated driveway--not very helpful for the foot of snow on a subzero day, but a significant factor in the case where we're trying to remove a small amount of snow or ice on a warmer day. This low-use scenario could wind up costing $0.50 or less.

The Time and Money Perspective

Finally, we can look at the problem from the perspective of how much time and money it takes to clean the driveway. Right now I spend about an hour shoveling the driveway every time we have a significant snowfall, and for bigger storms this sometimes needs to be done twice or more. As already established, this costs about $3 worth of candy bars.

Clearing the driveway with a snowblower takes about a half-hour, and about $0.30 worth of fuel each time. This may seem like a no-brainer (replacing $3 of Snickers with $0.30 of unleaded and taking half the time), but the snowblower itself will cost about $500 and last perhaps five years. If I have to clear the driveway ten times a season, it's clear that buying the snowblower is the most important expense, adding about $10 to the cost of each snowfall.

Hiring a snowplow service is the most expensive option, but it takes me zero time to clear the driveway. We used to hire a service until about 10 years ago, and back then they charged a minimum of $30 every time it snowed with a surcharge for more than three inches of snow. Today it would probably cost $40-$50 for every snowfall, and our foot of snow could cost as much as $75 with surcharges.

The snow-melt system actually starts to look compelling from a time and money perspective. Like the snowplow service, it requires zero effort for snow removal, but the deep snow on a cold day will only cost about $13 in natural gas. I haven't priced the cost of installing the system, but my guess is that it would add between $2,000 and $5,000 to the cost of replacing the driveway (which will have to be done anyway in a few years). Considering that we already have a water heater capable of driving the system, we could well come in at the low end of the range.

The installation price of a snow-melt system is steep, but it should last for the life of the driveway or longer. Over 25 years, the $5,000 spent on the system will cost only $200/year, or $20 for each snowfall if we need it ten times per season. So (rounding off a little), a heavy snowfall will cost about $35 in fuel plus capital expense to melt the snow, as compared to $50-$75 for a plowing service. A light snowfall would cost only about $20 to melt (essentially just the amortized cost of installation), but $40-$50 for a service.

Concluding Thoughts

There's no question that moving snow takes much less energy than trying to melt it, and the cheapest, most efficient way to clean up after a snowstorm is to shovel by hand. I'm happy to keep doing this, but She Who Puts Up With Me has zero interest in hand-clearing our driveway.

At some point, I might not want to keep shoveling, or my business travel schedule may make it likely that I won't be in town when the snow flies. When that time comes, we can hire a service, buy a snowblower, or install a snow-melt system.

Buying a snowblower is the cheapest option, but also the least convenient--it will still require someone to spend a half-hour in the cold and blowing snow. I don't think She Who Puts Up With Me will be too excited about this, though it's still better than hand-shoveling.

That leaves hiring a service or going with the heated driveway.

If we have to choose between those options, the snow-melt system is substantially cheaper, as long as we anticipate using the service for a number of years. If we expect to need a service for only a few years (maybe we expect my travel schedule to change, or move to a different house), then the capital expense of the snow-melt system makes it more expensive.

All this is still dreaming at this point: the time to make a decision about a heated driveway is when we replace the driveway. Our current driveway is 25 years old and in poor shape, so it could be replaced at any time. On the other hand, after the geothermal system this year we're not eager to embark on another major home-improvement project for a couple years.

Long Distance Boredom

Categories

A Northwest flight was in the news recently when it overflew its destination by about 150 miles and the pilots didn't respond to air traffic control.  It turned out that the pilots were working on their laptop computers (against airline policy) and got so engrossed that they missed all the attempts to communicate with them.

I don't want to dive into all the hand-wringing over this incident (which ended without damage to anything other than the pilots' professional reputations and credentials). Others far more qualified than I have weighed in on what a terrible lapse of judgement this was.

But this does highlight what I see as potentially an increasing problem in modern aviation: complete and utter boredom.

Over the past 20 years, cockpits have become more and more automated, and modern airliners literally fly themselves with almost no intervention from the crew. Even 4-seat propeller planes of the kind I fly are becoming more automated--it's getting hard to buy a new airplane without a complete digital instrumentation system (aka "glass cockpit") and sophisticated autopilot.

For the most part, this is a good change. Computers are much less likely to make mistakes than people in the routine operations of the aircraft, and can navigate far more precisely. The job of the human pilots is no longer actually flying the airplane, but communicating with the ground and being ready to take over in case something goes wrong (which it almost never does).

The downside is that it leaves the flight crew with very little to do during the cruise phase. If you think it's boring sitting on a 4-hour flight, imagine what it's like for the pilot and co-pilot. They are required to sit in their seats and be alert for hours at a time, but not permitted to sleep, read books, play games, or do much of anything other than talk to each other and (very occasionally) ATC. Even standing up and going to the bathroom is actively discouraged for security reasons.

This sort of enforced inactivity plus alertness is simply not something human beings are good at. The amazing part of this incident is not that the pilots got sucked into some other activity, but that it doesn't happen more often.

Obama: The First Internet Age President

Categories

In much the same way that JFK was the first TV Age President, it is increasingly clear that Obama is the first Internet Age President.

Kennedy was not the first president to have to deal with television during his presidency, but he was the first one to figure out how to use TV to his political advantage. He knew how to look good on TV, and recognized that this was important. The 1960 Kennedy-Nixon debates, where Kennedy looked presidential and Nixon did not, are considered pivotal in the outcome of the election.

Similarly, Obama is the first President to really know how to use the unique dynamics of politics in the Internet Age to his advantage. Consider this sequence of events, which has played out at least a half-dozen times since the beginning of the presidential campaign two years ago:

  1. Obama's opponents raise an issue which they think makes Obama look bad.
  2. Issue starts to be discussed more and more online, and starts to get distorted by the more rabid of Obama's opponents. Obama's spokespeople may downplay the issue, but Obama himself makes no direct statement.
  3. Emboldened, Obama's critics push the issue. It gets distorted more and more, and begins to spill over into the traditional mass media. Looking for a good story, the media reports on the most absurd, extreme versions of the original story.
  4. As the frenzy crescendos, Obama delivers a calm, rational, centrist speech about the issue.
  5. Obama looks Presidential, and his opponents look like rabid morons.

I'm not the first one to notice this repeating pattern. Andrew Sullivan calls it the "Rope-a-Dope," and speculates that Obama manages to subtly bait his opponents into Step 1 above. I'm not so sure about the baiting part--not because I think Obama's above baiting his opponents, but because he doesn't seem to need to. For whatever reason, Obama seems to bring out the absolute worst in his critics.

This strategy is perfectly suited to the Internet Age, where any idea, no matter how kooky, can find a sympathetic audience. It plays perfectly into the 24-hour news cycle where the biggest challenge is finding fresh material to report on. It draws strength from partisan media like Fox News and talk radio where there's always a willingness to push negative stories about Obama, no matter how implausible.

Back when most people got their news from the three major networks and a big-city newspaper, many of these wacky stories never would have gotten off the ground because the mass media would have considered them too fringe. Indeed, back in the Kennedy administration, the media wouldn't even report on JFK's well-known affairs, judging it a personal matter between the President and the women in his life. Good luck with that today. Even if the mass media doesn't want to report a story, the Internet and smaller outlets now are big enough to give them the breathing room to grow to the point where the large outlets feel like they can't ignore the story.

So what does it take to be an Internet Age President? In the TV Age, the advice was simple: Look Good on Camera. Nixon failed to do this in 1960 and it cost him the election.

In the Internet Age, the key is to Stay Cool No Matter What. This applies both to the candidate and to his or her campaign and supporters. McCain made a whole series of rash decisions during the 2008 campaign, ranging from suspending his campaign during the financial crisis to choosing Sara Palin as his running mate.  It cost him the election.

There will always be kooks and crazies around the margins of politics, and now that they have a bigger voice it's easy to be baited into doing something dumb. Taking the bait achieves nothing but bringing yourself down to their level.

Instead, an Internet Age politician needs to remain visibly above the fray, while looking for opportunities to use the cacophony to his or her own advantage.

Our Geothermal Adventure (Chapter 4): A Hole Lotta Sink

Categories

It's been three months since our geothermal system was installed.  We've made it through the hottest part of the summer, and proved that a heat pump sized for a Minnesota winter does a bang-up job with air conditioning in the summer.

So far we've discovered only one problem: the sinkhole.

When the contractors buried the plumbing for the loop field, they basically excavated a trench about ten feet wide, twenty feet long, and six feet deep.  That's about 45 cubic yards of material removed.  At the bottom of this pit, they connected the six deep wells to a manifold and a pair of pipes which run under the garage into the utility room.  These pipes circulate the antifreeze solution which transfers heat between the ground and the house.

After all the plumbing was done, the geothermal company just pushed the 45 yards of material back into the hole.  They made no attempt to level the ground, nor did we expect them to.  On the contrary, they made it very clear that they would leave the yard a complete mess and it was our responsibility to fix the landscaping.

A week or so after the geothermal guys left, the landscapers arrived.  They used a bobcat to level and grade the ground and plant grass seed on top.

Now, we had a dry spring and summer and for a while things looked pretty good.  If you've had experience with excavation, though, you can probably see where this is going.

A certain amount of settling is always expected when you dig a hole and refill it.  That's because the granules of dirt, sand, and clay don't just drop back into the same compacted configuration they had been before.  Instead, they're fluffed up a little, and it takes some time to unfluff.  A good soaking rain helps, since the water suspends and lubricates the particles.

This August, we got that rain.  When we got that rain, the ground above the excavation settled.  And collapsed into a big sinkhole.

My best guess is that when they pushed all that material back into the hole, they accidentally left a sizable void in one of the corners of the excavation.  This is easy to do when the dirt is dry and lumpy like it was this past spring.  The void sat there quite happily for a couple months, until we got enough rain to actually soak all the way down to the underground air pocket.

Once the water reached the void, it collapsed and created our sinkhole.

The sinkhole is about a cubic yard in volume, which is to say, big enough to look ugly and alarming, but not big enough to actually be dangerous.  Fortunately it's not in a place visible from outside our yard, so I don't feel like it has to be dealt with this instant to keep the neighborhood looking good.

Right now, I'm thinking that the time to deal with the sinkhole will be in the spring, after we've had a complete freeze-thaw cycle and I can be fairly confident that the excavation is mostly done settling.  I would hate to fill it all in, just to have it sink again.

If I had thought of it at the time, I should have taken the garden hose and run it into the rough-filled pit the geothermal guys left before the landscapers arrived. That would have at least uncovered the void and prevented the dramatic sinkhole, even if the ground would still have settled after being regraded.

Update: A few hours after I wrote this entry, I discovered that I was a little too sanguine about the need to immediately fill in the sinkholes. The sinkholes are trapping runoff which would normally flow downhill and away from the house, and with heavy enough rain some of the water is making it into our basement. Not much, but enough to make me want to go get a couple yards of sand and rough-grade the sinkholes before the next big storm.