First wall prep

We’re happy enough with the rammed earth test cylinders we made last week. After a day drying they went rock hard, and are certainly sturdy enough for the first job we want to used the rammed earth for.

We posted the idea a little while back, and given that we don’t need planning permission for such a small wall, we can just go ahead and do it. Here’s the vision:


The picture is not entirely accurate: the cladded wall at the rear is a little further back, the rammed earth wall is a little closer to the house and not so wide, and it doesn’t extend beyond the side of the house just visible on the right hand side of the picture.

The first thing is to remove the garden bed that’s currently in that spot.


The lavenders haven’t looked any good for ages, and the bulbs are done flowering. They won’t like being transplanted, but they’ll be back next August guaranteed.


We need to bring the base of the wall down to the undisturbed dirt level, which will be under those bricks. So we dig up all the dirt and cart it down the hill. It’s not consistent clay like you’d find deeper down, so we can’t use it for the wall itself.


We also took out all the red gum sleepers. We wanted to save them, but there was only one or two that can be re-used. The rest were half rotten, which makes me a tad nervous about the rest of the retaining around the yard. Some of it is starting to fall to bits, and I’m guessing that we’ll have to get onto these new walls with a bit more speed than what we’ve applied to the project so far.

The garden bed was cleared out in an afternoon, after the morning was spent procuring a post hole digger. The following morning was spent cutting the grass, since rain was expected in the evening and it’s a lot easier to do when it’s dry. Once the maintenance jobs were out of the way we could get back to measuring out the wall.

The deck is going to be 1.6m around this and the adjacent side of the house. This width marries up with the width of the stairs we’ll be putting in to the lower garden, a couple of which are already in place. So we put some stakes in the ground 1.6m away from the house.

We’ll make this wall 30cm thick, which is way too much to carry the load of half a deck and whatever we might put on it, but which is a good balance between ensuring load carrying capacity (a thicker wall) and reducing the amount of material we have to prepare to put into it (a thinner wall). More stakes 1.3m away from the house, requiring us to remove some of the pavers.

We line it up with the end of the house and the nail we put into the house to mark the end of the garage all that time ago. Total length is 5.2m. We set some string down the middle and measure out spots for 5 piles. The piles will go down into the ground a short distance to anchor the wall against the soil and water pressure that will be exerted on it from the driveway. It’s unlikely that they’re actually required – I haven’t done the engineering for this little wall – but it’s all about practicing our techniques at this stage and a bit of over-engineering here is not going to be wasted effort.


Finally all that’s left is to calculate how much dirt we need. We set the laser level up on the floor inside and shone it out the window. That allowed us to calculate a height from the ground to the top of the floor inside, which was 47.5cm. The top of the deck needs to match that. We take off 10cm for the concrete foundation, 7.5cm for joist bits that sit on top of the wall and the thickness of the decking, leaving a 30cm rammed earth wall. The volume of the wall is then 520x30x30cm or 468,000cm3. That’s 468L. From our cylinder experiments we know that compaction around about halves the volume of uncompacted dirt, so we need double the amount in the compacted wall of uncompacted material –  936L. That’s pretty close to 1m3. Our big dirt-carrying tubs are 42L when full, so it’s a little over 22 tubs in total. Or 11 wheelbarrows, since 2 tubs fit in the wheelbarrow. Which is pretty close to a ute-load (our mulch-carting exploits provided the information that the ute holds around 12-13 wheelbarrow loads of mulch).

The 5 piles will be 15cm in diameter, and go down the length of a standard bit of rebar minus 5cm which will extend into the foundation. That depth is 55cm, so each pile will contain ?x7.5×7.5x75cm or 9,719cm3. That’s 48.6L in all 5 piles. The foundation slab will be 520x30x10cm or 156L, not counting that the rebar inside it will reduce the amount of concrete we need to make. One part in 7 of that amount is 22L, which when added to the same proportion in the piles gives us 29L, the amount of cement we need to buy for the foundation. That’s a little bit bigger than a standard bag of potting mix. Interestingly, cement is sold in bags of a specific weight, not volume, despite requirements for cement being pretty much universally volume-driven. Boral helpfully tells us that 108 20kg bags give us 1m3, which means that we need just over 3 bags for our 29L. If we also put a bit into the rammed earth part (2% of 936L), we need 5.2 bags.