This eighth entry was published originally on the 20th July 2013 and received 1,138 views on the old forum
Time for another update. In the middle of the biggest heatwave we’ve seen for years, we’re pressing on with the build of the concrete retaining wall. It’s a bit tedious, and none too green, as the compromise we’ve had to adopt is for a vertical, steel reinforced, concrete block wall. It’s overkill in terms of the ground it’s actually got to hold back, as that seems to stay up even with a vertical sided 2.5m face, but it does reassure the neighbour that the end of his garden won’t collapse into our hole.
Last weekend we left the site with the concrete wall foundation having been poured and a mass of 12mm rebar sticking up vertically at the base of the cliff. Our neighbour to the north is an elderly gentleman who’s fond of the shrubs in his garden, some of which are close to the edge of the cliff we’ve dug out. I woke up in the early hours earlier this week having had a dream that he’d been attending to his shrubs and fallen over the edge, impaling himself on the rebar spikes, rather like a punji pit. This photo was taken yesterday, after the lower few courses of blocks were laid, but there are still spikes sticking up in the air, ready to catch unwary prey that falls over the edge:
Much of the week has been taken up with building this wall, with a few other smaller jobs having been done, like filling in the service trenches and pulling cables through ducts. On Tuesday I went out to double check that the soil pipe was coming up in the right place, and lay out where the water, phone and electricity supply ducts needed to come up inside the house. These are fairly critical, as they have to be aligned so that they pop up just inside the inner face of the wall of the house, and, because of the foundation system we’re using (more on this below) we don’t have the luxury of having footings trenches to guide us as to where things need to go. Because the soil pipe went in when the treatment plant was installed, right at the start of work, the position of the soil pipe was worked out by measuring out from the reference peg in the north east corner of the plot (now excavated away) and running out along a horizontal radial distance and dropping down to the excavated ground level with a vertical pole. We’d anticipated having to shift this pipe around a bit to finalise the position, so were a bit amazed to find that it was within 15mm of where it was supposed to be.
To save having to pay two lots of charges for an electricity supply, one for a temporary contractors supply and another for the permanent metered connection, I came up with the cunning plan of fitting the meter box in a timber fence, that will eventually form the screen for the mandatory recycling/refuse bin area. This is easily accessible for the meter reader, yet means we don’t have the house defaced with a surface mount meter box carbuncle (we couldn’t have an inset flush mount meter box as it would severely compromise the insulation). This means we will just have the meter installed early and use the supply as a contractors supply for the build, and then just hook it up to the house when we’re ready. This short fence (which is planked on both sides) also houses the telephone junction box for the incoming line and a water standpipe from the borehole. The net result is that at it’s base we have a Medusa’s head of bits of ducting poking up at the moment. Hopefully we’ll find a way to hide this lot somehow, maybe with a small rockery.
The main visible change on site this week has been The Wall. It’s been pretty hot work for the guys, and not much fun heaving around big high density concrete blocks. Still, we have around a third of the wall up, after about three days of wall construction, which isn’t bad going. The wall is two (225mm wide) blocks thick at the base, but only a single 225mm block thick for the upper half, so thing should go quicker as it gets higher. It can only go up around four or five courses before they have to stop and pour concrete into the holes in the blocks, creating solid reinforced concrete internal vertical beams to hold the wall together. Everyone who’s looked at it thinks it’s a massive overkill, but best to be safe. At least I won’t be waking up in the early hours having had nasty dreams of the whole lot collapsing on the back of our house………….
The wall will be topped with a 700mm high reconstituted stone wall on the neighbours side and a layer of 100mm blocks on our side (which will be rendered, like the rest of the wall). That will then have a low a timber fence, infilled with trellis panels, built on top. Much of this wall is over 2m high to the existing ground level above, so when topped with another 700mm we’re going to have around 2.5m plus of rendered wall running along much of the north boundary. As the guys have been finding out, we have created something of a sun trap here. We’re already thinking about what we might be able to grow up this wall – my guess is that we could grow some pretty exotic fruit trees (as espaliers) or perhaps climbers, up it. It’ll be completely sheltered from the east wind by the house and fairly well sheltered from the west by the garage, so should be OK for sun loving plants.
The foundation system is a bit unusual, in that it sits at the level the ground is at the moment. There will be a 150mm deep hole dug out, a bit bigger all around than the house footprint, and this will be filled with compacted crushed stone. On top of this will be laid a 300mm thick polystyrene insulation layer, that includes a preformed ring beam former around the edge. The damp proof membrane will then be laid into the big polystyrene tray, steel reinforcement and the under floor heating pipes will be laid, and then it’ll be filled with concrete. The whole weight of the house will be supported entirely by the thick layer of polystyrene foam, which will form a very high performance insulated slab. The load is distributed into the ground by the compacted crushed stone base. This system works even on poor soil, as the bearing pressure it exerts is very low, especially with a timber frame house like ours. The walls will be constructed directly off the reinforced concrete ring beam that sits on top of the foam, which is stiff enough to make sure that everything stays square and true. Because the insulation is wrapped up around the edge of the slab, there’s no problem with cold bridging at the floor/wall junction, as there is with many conventional insulated floor systems. In effect, we will have a continuous layer of insulation running all around, over and under the house.
The system we’re using is the one that our builder, MBC Timberframe Ltd, include in their package build and is the Kore Passive Slab. There are other similar systems around, but they don’t seem to be in common use yet.