Density Digs

Clayton had his basement filled in by Messier, the same company that lifted his house. The Messier crew walked a vibrating compactor over each layer of fill to make it as dense as possible. As the engineer, I had the final say as to whether the fill was sufficiently compacted. So how did I do that?

The most convenient way to test the density of a fill material in place is to take a sample. For starters, I excavated down 8 inches, because the surface doesn’t compact much at all – the vibrating device actually loosens surface fill while it compacts the fill below. Then I scooped out a sample from that depth, with a volume of about two cups. I was very careful to ensure every particle I scooped out made it into my storage container.

Compacted fill sample.

Compacted fill sample.

Density equals mass divided by volume, and I determined the mass of my sample easily on-site with a kitchen scale. To find the volume, I used the sand cone test, which looks like this:

The sand cone apparatus.

The sand cone apparatus.

The translucent white container is filled with sand. When I open a valve, the sand pours out through the metal cone and into the hole I scooped out. When the sand stops moving, I know the hole and the cone are full, so I close the valve and lift up the apparatus:

Sand cone left in the hole after the test. (No need to remove the sand - it gets buried.)

Sand cone left in the hole after the test. (No need to remove the sand – it gets buried.)

Now with just a few extra bits of information – the density of the sand (which I know ahead of time), the volume of the cone (standardized), and the mass of the apparatus before and after the test (use the scale on-site) – I can calculate the density of the fill I scooped out.

Field notes from my compaction testing.

Field notes from my compaction testing.

I’m not done yet. Proper compaction depends not on fresh-out-of-the-ground density but on DRY density. And the most convenient way to dry a sample quickly is with… wait for it… a microwave oven. I borrowed Clayton’s for the task, checking the decreasing mass of my sample after each minute until the mass stabilized. Finally, I calculated the moisture content of the original sample by comparing the fresh-out-of-the-ground density with the dry density.

As it turned out, Clayton’s compacted fill had a dry density well above the minimum density I specified. (Great work, Messier!) The moisture content however was a little low at first, meaning the fill was too dry. Fortunately, the remedy for this compaction issue is simple: just add water. Sandra spent a long morning spraying the entire excavation with a hose, and my last few samples had a moisture content exactly where I wanted.

Sandra on spray duty aims for a perfect moisture content.

Sandra on spray duty aims for a perfect moisture content.

How to Pick Up Your House

As I mentioned last week, my newest client, Clayton, is lifting his house off its foundation to get above Base Flood Elevation, or BFE. The lift is underway! Clayton hired Messier House Moving & Construction to jack up the house to its new height 7 feet 2 inches above the old level (and 1 foot 1 inch above BFE). I got to watch Messier work today and I am nothing short of impressed.

Clayton prepared the house by cutting notches out of his foundation wall on all sides. Messier slid steel beams through the notches – not an easy task, as the beams weigh 1500 to 6000 pounds apiece – so that beams running transversely (the short way) support beams running longitudinally (the long way), which in turn support the house’s existing floor joists. Next, the crew set up a hydraulic jack at each end of each steel beam. They connected all the jacks back to a central control, which ensures the jacks move at equal speeds and lift the entire building uniformly.

Notches in the foundation wall - see how the steel beams fit before lifting the house.

Notches in the foundation wall – see how the steel beams fit before lifting the house.

After putting a slight upward pressure on the beams, Messier cut the anchor bolts around the perimeter, disconnecting the house from its foundation. Now, think of all the careful finish work inside your home and imagine what might happen to it when you pick up the house. Broken tiles? Cracking walls? Crooked cabinets? Artwork crashing to the floor?

None of that. Clayton reported one caulk line that popped in the bathroom; otherwise the interior was undisturbed. These guys are good.

Cribbing towers support the ends of the steel beams.

Cribbing towers support the ends of the steel beams.

Messier lifted the house 14 inches at a time, with each interval taking about 90 seconds. Between intervals, the crew built up neat stacks of lumber called cribbing around each jack, then reset and moved each jack to sit atop the cribbing. They eventually lifted the house nearly 9 feet above its original position. That’s higher than the 7 feet 2 inches Clayton wants, and next month Messier will lower the house back to this height, but right now they need the extra clearance to drive a Bobcat into the basement. Next step: fill and compaction!

Woodshop: Installing Drywall

Drywall is a magical material. It covers walls, it hides plumbing and electricals and HVAC, it accepts any kind of paint job, it provides enough strength for a variety of wall hangings, and it’s easily patched and repaired. And all that in a lightweight, easy-to-cut package. But while drywall is easy to work with, its goal – perfectly plumb finish walls free of bumps and visible joints – is not at all easy to achieve.

The best advice I can give you about installing drywall, if you want it to look good, is to hire a professional. And not just any old building professional, but a drywall specialist. Nobody else has the experience and skill to cut the sheets right, align them right, screw them in the right distance, and give you that perfect finish.

Still want to install drywall yourself? OK. Start with the ceiling, then do the walls from top to bottom. It’s fine to leave an inch gap between the bottommost sheet and the floor, because later you’ll install baseboard to cover the gap. Make sure all your joints are factory edge to factory edge – they might taper to make space for the tape and mud that will eventually obscure the seams. Your outer sheets are the ones you’ll cut to fit the shape of your wall.

Scoring a line with a utility knife and straightedge.

Scoring a line with a utility knife and straightedge.

Straight cuts are quite literally a snap. Measure and draw on your sheet the line you want to cut. Then, using a utility knife, score the gypsum board’s paper coating. (You only need to score one side.) Now imagine the drywall is a book and your score line is the spine, and apply force to close the book. Your sheet will break along the score line with a very satisfying SNAP. Just slice through the paper coating on the other side, and your cut is complete. Repeat as often as necessary to get the shape you need… and remember, once you snap a piece off, you can’t put it back.

Drywall sheets are usually screwed into the studs, not nailed, because the impact of a hammered nail can crack the gypsum. Drywall screws hold great, but a screw head can easily penetrate the outer paper coating, making the connection useless. You’ll need a special drill bit with a gauge that stops the drill as it reaches the wall surface. You’ll also want to run a straightedge across the studs to confirm they’re all in the same plane.

Screwing a drywall sheet to the studs behind it.

Screwing a drywall sheet to the studs behind it.

Here we go. Hold up your cut (or whole) sheet of drywall so the seams are flush with all the seams surrounding it, tack-screw it in a couple places, check alignment, and finish screwing. Don’t leave any part of any screw head sticking out beyond the wall surface, or you’ll be cursing when you go to paint. Also, make sure every screw catches on a stud behind, or it’ll get stuck partway embedded. Any mistake, you’ll need to pull out a screw and then fill in the hole later when you tape and mud.

I told you it wasn’t easy.