I got a comment on my last post about drilling in karst vs. in other places. Most of the environmental drilling done on the east coast is within the top 20 feet of the surface, because that's where the water is. And within that upper zone, you are more likely to encounter soil (overburden) than bedrock.
Bedrock adds all sorts of complications to drilling, but even staying above the bedrock can be troublesome. For example, the heaving sand I alluded to in my last post.
If you're drilling in a valley or another area with a significant thickness of overburden, it's not uncommon to drill through layers of saturated soil which may act as barriers to vertical flow. If you go through those layers, you may encounter material which is slightly pressurized (confined), and when you punch a hole through, you open a space that has significantly less pressure. That water (and the soil) wants to go up. That's fine if you're drilling through material that has a decent amount of silt or clay, or perhaps has lots of gravel. But if you're drilling through nice, relatively clean sand (of pretty much any size), that sand will flow with the water. Especially because the drill rods have been vibrating as they've been chewing their way down. And you end up with flowing/running/heaving sand. That is, when you pause to take a sample or to add another rod, sand flows/runs/heaves upward into the drill stem and binds everything up. Or, you stop for the day and come back the next morning to find that your drill rods been pushed way up out of the hole and you've got 80 feet of steel waving around in the breeze.
There are ways to minimize this. One way is to make sure you always keep the inside of the drill stem full of water, so there's some pressure acting to keep that sand back. You can also add "drilling mud" - a clay or polymer - to your drilling water to hold things back, although often in environmental applications you're not allowed to add anything fancy that may interfere with later samples or well installation. Or you can close your eyes and try and get through those layers as fast as possible, and then seal your casing/drill stem into something that won't flow, like a clay bed or bedrock.
In certain environments, such as coastal areas, heaving sand just comes with the territory. I've had big sites with scores of boreholes drilled, and one particular 15-20 foot zone is just blank with "heaving sand" for every single boring log because every time you stop to get a sample, you just get loose, undifferentiated slop from who-knows-where. Nothing's wrong with the driller or the rig geologist - it's just the way the stratigraphy is.