Walt:
As part of the final stretch to finish up various construction projects before winter comes, we're delighted to have completed the last concrete pour on C-Wall, the concrete retaining wall (that's shaped like the letter "C") that will be the backbone for a 20'x60' greenhouse.
Once upon a time, we did our milking in a 16' wide by 52' long tent. After a couple of years, we upgraded to a 20' by 60' hoop structure that lasted until March of '96 when a heavy spring snowfall followed by a steady rain took it down, along with five miles of the road along the river.
One reason the hoop structure came down was that neither long wall was laterally stable, and under the heavy load, the sides bowed out, allowing the roof to collapse. Understanding that, we knew that our next structure would be designed in such a way as to stand up to a heavy snow load. We also knew that a key part of that enhanced load carrying capacity would involve an earth-sheltered, concrete retaining wall. So, we got started on creating one; here are the forms set for the last pour in that process.
One of the challenges inherent in working with large volumes of concrete here involves the delivery charge which is currently running at around $150 to deliver a minimum of four cubic yards at around a hundred dollars per yard so each delivery runs upwards of six hundred dollars. Most of the time when we order in a concrete delivery, we're pouring a slab floor, and that's when you really want to be sure that you order enough concrete, and then some extra just to be sure.
Which raises the question of what to do with the extra concrete. Sometimes we fill plastic buckets a third full to create concrete pads that are useful for a variety of things. For example, when we put a 4x4 post in the ground as part of the roof support for a shed, we'll put one of these concrete pads at the bottom of the hole for the post to stand on. That distributes any downward pressure on the post to a larger area, thereby ensuring that the post doesn't get driven into the ground like a nail into a board come a heavy snow.
But for the most part, we tried to have forms ready so that the extra concrete could go into C-Wall creating a retaining wall designed to replace the structure that failed, and serve as a solar-heated growing space to produce salad greens in late fall and early spring, and from which to be able to harvest winter hardy greens such as kale.
It's not cheaper for us to mix our own concrete; it's something we do to avoid having to pay that hefty delivery fee, and to not mix any more concrete than is specifically needed for a small job. Given the amount of work that's involved in home mixing a cubic yard of concrete, it's strongly in our interest to piggy-back one project onto another as we're doing here. Since this greenhouse retaining wall won't need to be water tight, having seams in the concrete won't be a problem.
A good way to ensure that the forms run straight is to use the design you can see above, forms made from 3/4" plywood screwed to long 2x4's. By staggering the 2x4's, it's easy enough to make a long form that runs true.
The two sides of the forms are joined together using ties, some short and some long depending on the type of form used at any given spot. Forms made from just a sheet of 3/4" plywood use the shorter tie, and forms reinforced with 2x4's use the longer ties.
The forms are joined together using "shoes" that cinch down on the balls on the ends of the ties.
When using a combination of short and long ties, one option is to drill a hole in a section of 4x4 and slide the shoe on to that.
Sometimes there will be an imperfection in the wall that makes it difficult to get the shoe on using a 4x4 spacer. In that case, using two 2x4's will allow the shoe to slip on.
While were able to use other pours to get this retaining wall well underway, there came a point when we needed to wrap up this stage of the work. And so for the past month, every few days we've added a few more mixes to the wall. Each pour added up, and eventually, we were able to strip away the form from the last pour and celebrate the completion of this work well before winter shuts work down for the year.
Here's a view of the completed wall from uphill.