Sunday, December 5, 2010

How to make your own rammed earth walls.

Welcome to the detailed wrap up of the rammed earth wall sections that have recently sprouted on the AltShiftNZ research and development site.  This post will hopefully clarify and solidify one's understanding of compacted earth walls (if it doesn't please ask questions!).   First, and most necessary for this type of earthen building method, is the formwork.  The formwork is composed of 4 main elements:
(1) the 'shutters' which are approximately 4'X8' 2 in. thick pieces of plywood.
(2) 2"x10" wooden planks (a.k.a. 'whalers' and/or trusses) that compress the shutters together
(3) Bulkhead strips (the pieces of wood to complete  the form on the ends of the wall) which determine the width of the wall.
(4) Pony pipe clamps (heavy duty pipes with screw-on clamping ends) which hold it all together.

These 4 main pieces are assembled to produce a form for a section of wall as depicted below:

Desired width of the wall is the first variable you'll want to consider as the footing/foundation will need to match that dimension.  The illustration above (from a 1982 Popular Science article) has a great side perspective detailing of the form's interaction with the foundation in the 'close up' circle located in the lower left.  Ideally you want to have this lip/ledge so that the formwork can have a base to rest upon and be clamped against.  There are two different ways to achieve such a ledge: 
(1) You can screw lengths of wood on to the shutters when pouring the footing in order to displace the concrete on the upper outside edges, leaving a 'knock-out' for the ledge.

(2) To create a better thermal break from the footing (preventing loss of heat via conduction to the ground and cold concrete footing) one can have the wall section rest on Hebel supercrete blocks.  These blocks are aerated in their setting process, and thus have a much higher insulative value than normal concrete due to the air pockets.  These blocks are then cut with a skill saw for the ledge as pictured to the right.

Also important to factor into the footing pour, is to have rebar tie-ins to obtain a vertical reinforcement.   The rebar you see to the right then had 4m long pieces of rebar welded on to them so it would extend through the entire wall.  During the wall compacting horizontal 'stirrups' of rebar can be placed around these vertical pieces at set increments to increase structural integrity (I believe on the AltShift walls stirrups were added every 800mm).  

After the form has been put together there are a few additional measures that need to be taken.  If you want to adjoin another section, a 'key' cavity must be created in the end of the wall so that the next section can have a mechanical connection to slot into.  This is easily done by screwing a 2"x4" (by the height of your wall) piece of wood in the center of the bulkhead(s).   To the left you can see the 2x4 in the end of the wall to be removed for the reception of the next wall .  Another action to be carried out to ensure ease of dismantling the forms, is to place wedges in between the bulkhead pieces and the pipes of the pipe clamps (as shown in the Popular Science illustration above).  This is so you can knock out the wedges after the wall has been done instead of having a massive pressure buildup of the bulkhead on the pipes.

With the form built and ready to fill, there is a lot of room to get creative and explore the possibilities of the placement of forms, relics, and niches into the wall.  For our walls we chose to keep it relatively simple and put in a few Volume Displacement Boxes (aka V.D.B.s) to create functional spaces in the wall.  A lot of people who have chosen to do rammed earth around this area take antiques and artifacts from the gold mining days of the area, and insert them into the surface of the wall.  When viewing a house in Hawea, we saw a wall that had an old mining pick embedded in it.  This is done by affixing the desired object to the inside wall of the form and carefully tamping around it.  Some people put rocks and stones towards the exterior of their walls and create mosaic like patterns, others place 'emergency' boxes about an inch under the surface of the wall to store supplies such as a bottle of moonshine (no joke! some people actually do this) or a satchel of gold coins that can be broken into if needed.  Also in this same line of thought, is the planning and positioning of electrical outlets or other conduit that you want to run through the wall, as after the wall is rammed it becomes extremely difficult to work with.  Rammed earth walls are a very fun medium to play around with, just make sure to take your time in preparing any inserts as you fill the form with the mix,- the next step in the process.

Content with all of the fun fixtures and V.D.B.s to be placed in your wall, you will now start the mixing and filling process.  As previously discussed in an earlier post, the ideal soil mixture is a 20-30% clay & 70-80% sand/aggregate.   Then mixing in a 10% ratio of cement, add water and thoroughly mix with a tiller, cement mixer, or tarp method, and keep the mix relatively dry (the wetter, the greater chance that the forms will stick to the wall on stripping, creating rough texture, or the longer you will have to keep the forms on).  Using either a bucket or tractor (for larger scale applications) transfer the mix into the form and begin tamping.  Layering the un-tamped earth in 100mm/ 4", tamping can be done by hand with a tamper or by a pneumatic tamper powered by an air compressor (the pneumatic tamper takes about a tenth of the time though).  It is then a marathon to the top of the form in the step and repeat filling and tamping process, placing your items as you get to their desired height.

Having compacted the final layer at the top of the form, the shutters and form can be taken apart immediately!!  Again, if the mix was too wet, it is best to leave it for a while, but with the correct water content it can be stripped right away.  Although it can stand on its own instantly, connecting the adjacent section to it should wait a day or two for the material to harden around the key insert.   Moving on to the next section provides one with two options on clamping the form together.  

(1)  The pipe clamps can run through the new/next wall section with PVC sleeves around them so the pipes can be slipped out later (patching the holes created by this method isn't too hard).

(2)  Employing 'strongbacks'-aka very large pieces of timber (around 4"x10")-, whalers are sandwiched to the shutters/form by putting the strongbacks against the outside edge of the whalers and clamping the strongbacks to each other.  Here is a picture to help clarify---->
Taken at the top of the keystone piece, you can see that the strongbacks are clamped to each other at the top (and are angled away from the viewer to achieve the keystone shape).  By using the strongbacks, one still has to put a pipe or two through the new wall, but it is only a couple patch jobs rather than 10-14.  

Lastly I will briefly touch on how the keystone was achieved.  We could have gone for a more standard lintel rectangular look, but decided that the angles would possibly help share the load on the sides, as well as provide a bit of intrigue.   We started off with the two separate walls with the door way in between and then buile a support for the base in the doorway.  Bolting into this wooden support in the doorway as well as the two existing sections, a ledge for the formwork was made, and the shutters were raised into place.  The two receiving walls already had the angles worked out by screwing the bulkheads in at the correct degree (70) and had the key gaps.  The tricky part with this keystone is that we had to lean the strongbacks at the 70 degree angle so the clamps didn't get in the way of tamping.  The strongbacks weigh approximately 200+ lbs each and are quite awkward to deal with as they are 14-15 ft (4.5-5m) in length.  But it all worked out and we ended up with this:

So, that about wraps it up for that recap.  Rammed earth is such a fantastic building method not only for its plentiful ingredients (earth and a little cement), and its creative oppurtunities, but also because the forms are completely reusable and can be employed hundreds of times if taken care of and treated properly.  Bottom line if building your own home (along with a few others-mountain village?), rammed earth is a very economical and solid way to raise walls.  We'll part with a great example of a sweet striated wall as well as the link to a well written Popular Science article by Richard Day all the way back in 1982.  Please don't hesitate to ask any questions that you may have (either to our email or in the comments area below) -we love to hear from you, and we want to get as many people comprehending all of this as possible.  Peace!

Popular Science Article :

1 comment:

  1. Thank you for a well written article with great pictures. We are going to build a small shed as an experiment before a house. This small-scale experiment is the learning curve for our future building so we're trying to get our methods down. The first question we have is, is does the rammed earth wall rest on the footing without a steel or rebar tie?