Before we came to Hana, we knew it rained 80 or 90 inches a year, but we didn't know how often and for how long it rained on any given day. Having worked in Nicaragua, Brazil, and the cloud forests of Veracruz Mexico (not to mention northern California in the winter) we knew building rammed earth in the rainy season was challenging. Soil gets too wet to compact, tractors get stuck in the mud, and in general, wall quality suffers.
For these reasons, we made the decision to build at least the first of our structures under cover of roof. And since you can't ram a wall with rafters and sheeting overhead, we decided to use the hand-thrown method. Actually, I don't know of any other builders who do hand-thrown, but we've been experimenting with the technique off and on for a dozen years. Hand-thrown is a closet cousin of pise, our high-pressure air delivery method of building structural stabilized earth walls. Both techniques utilize a single sided form, a wet mixture impacted against the form and then shaved off to the desired wall thickness. In production pise, volumetric mixing machines and huge air compressors blast stabilized earth at a rate of 8 to 10 cubic yards an hour. We use the technique to build large structures fast. Hand-thrown, on the other hand, is slow, but effective; good for the pace of island building, and with cement stabilization, very resistant to weathering as we have observed over the last seven or eight years. (In fact, I was pressure-washing the house just today to clean off the salty residue that blows in off the ocean and the walls are like basalt.)
We built the walls in panels, corners with three-foot wings, and three-foot in fill panels between the big window walls. Each panel was reinforced with a grid of #4's at 12", with the bars threaded through holes drilled in the galvanized steel support posts. The wall formula was a combination of site soil, mined during the first phase of the excavation, sand and gravel imported from the isthmus near Kahului, and 10% Hawaiian Cement (imported from China). We measured proportions with five-gallon buckets (one sack of cement fills two buckets0, mixed with an Imer three cubic-foot drum mixer (brought over in the shipping container) and delivered in a wheelbarrow. The corners were about one cubic yard, the infill panels about a half yard.
Building a hand-thrown wall is pretty much like the name implies. Wearing gloves, grab a handful of the mix (it should be the consistency of paste) and throw it against the form. Start at the bottom corner and toss one handful after another against the form, working your way first across the bottom the full width of the panel. Keep throwing handfuls working across the panel, building both out to the finished thickness and up against the form. As you build the wall upward, keep the slope of the working surface at about 45 degrees. If the mix is the proper consistency, each handful will bond to the previous and the mud will quickly take an initial set. If the mix is too wet, too low in clay, or you manage to work too fast, you'll get some slumping. Try to avoid slumping.
After you've built the wall out to just beyond the finished thickness and up to about two feet in height, you'll want to shave back the excess to create a plumb, flat wall surface. We use a gunite finishers tool called a rod (or knife), but you can also use a 1x6 with a bevelled edge or a 24" concrete finishers trowel. The longer the cutting tool, the straighter the wall will be. Keep in mind, however, that the cut side of a hand-thrown wall is by nature an imprecise finish. Getting your timing right on cutting the wall will take a little practice.
Once the first "lift" is shaved, go back to throwing. When we start moving up the wall, we use plasterer's mortar tables to work off. Shovel the mud out of the wheelbarrow onto the table. It's much less work to grab a handful off of the table than out of the wheelbarrow. Some of us even prefer the two-hand toss, so you can get more material onto the wall at one time. You don't have to throw hard, just enough to get a good bond and embedment to the steel. Your arms get tired by the end of a panel. We tried to build one corner or two panels a day. Evan was here at the start, then Lucky (a local) did rest of the mixing, Khyber did most of the throwing, all of the cutting, and I helped out a little.
We used the hand-thrown method on only the first of the three main pavilions, switching to traditional rammed earth for the other two. The advantages to hand-thrown are that (1) you can work under a roof, (2) you only need one form face rather than two and a whole lot less bracing, and (3) the wall can be much thinner because the roof is supported on the embedded posts rather than on the wall itself. The disadvantages are that you'll need a higher cement ratio to make up for the lower compaction factor and of course you need to supply and set the posts.
The beauty of the shipwreck research center is that there can be no failures. Everything is a study in island appropriate methods and materials. We think up a technique or a combination of building materials, design a project to implement them, build it, evaluate the process, then watch it respond to the elements.
The photos in this post illustrate the building of the hand-thrown walls, plus I've included a photo of shooting pise on the mainland.
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