Thursday, 12 September 2013

Level Head

With the square clear it was time to dig the foundation trenches. It's a bit more important that the bottom of this trench is flat and level as any undulations or un-evenness will affect the thickness of the concrete footing. So we got the water level out to help us.

A water level is one of the oldest surveying instruments out there, its unbelievably simple to use and understand as well as being incredibly accurate (and cheap).  if the Romans could build the Colosseum using one then surely it will do just fine for my little garage. here's how it works:

The water level is basically two plastic tubes with a scale from -5cm to +5cm marked on them, joined by a length of pipe. You have to fill the pipe up without any air bubbles and then attach the tubes. Hold the the tubes so they are at precisely the same height and keep topping one of them them up little by little until the water reads zero in both tubes. If you lift one the tubes slightly, the water level in that tube will read lower than the other one, if one says +3 the other should say -3 (it isn't 6cm higher it's only 3).

Stake 1
I got two wooden stakes and marked them both with a pencil exactly 5 feet from the top. The building reg's guys had said they would want to see a 7 inch X 2 foot concrete footing so we dug  10 inches down in one of the corners and knocked in one of the stakes up to the five feet mark(stake 1). This hole is essentially the starting point for our footing trench, we set the bottom of it a little deeper than it needs to be so that any undulations in the top surface will not mean the trench ever becomes less than seven inches deep. Still with me?

Stake 2

We zip tied one end of the water level to the top of the stake that we knocked in so the zero mark was exactly aligned with the top. The other end of the level was zip tied to the other (free) stake (stake 2) so the zero mark was aligned with the pencil mark. In theory, the zero mark on stake one is exactly 5 feet from the bottom of our trench and the zero mark on stake two is exactly 5 feet from the bottom of the stake. all we have to do is keep checking the bottom of our trench using stake two to make sure the bottom stays level.

We checked every meter or so until we got back to where we started, we left a small section where we thought the drainage for the old outside toilet might be so when we were finished we had something that looked like this:

I Reckon there is no more than 20mm between the highest and lowest point of the floor of this trench. Not only is it of uniform depth but the fact we used water means we can be sure it is also perfectly level. What's even more important then the bottom of this trench is of course the top of our concrete footing. here's how to make sure you get that absolutely perfect:

Bridges, little wooden bridges, and the water level of course.

knock in a couple more small stakes on opposite sides of the trench, you can now 'bridge' these two with another length of wood. the top of this bridge needs to be exactly 7 inches from the floor of the trench (as specified by building reg's) and perfectly level side to side. We had also modified stakes 1 and 2 by now, another small piece of wood was screwed to the side of the stakes so a flat edge was facing down exactly four feet from the zero mark.

Once you've done the first one you can set as many more as you like to precisely the same height using the water level. The advantage of bridges over just using pegs or stakes is that you can use a piece of timber long enough to span the gap between your bridges to tamp the top of your concrete. This should result in the top of your footing being perfectly flat and level in all planes. I reckon the difference between the highest and lowest bridges is no more than 2mm.

The water level is a masterpiece of simple design. It's truly unbelievable how accurate it is. If you get your foundations wrong your whole building is ruined, it's cost me just 14quid to know that all 28 meters of my footing is exactly 7 inches thick and perfectly flat and level. Good old Romans!

It's probably worth mentioning how we dealt with the old toilet drain.

We carefully dug around it and then smashed it up. Pretty simple really. We've 'shuttered' the two sides of the trench where the pipe entered and left it with some planks for now to prevent the concrete running into it. We will make a more permanent solution for the remaining pipe at a later date but this will allow to pour the concrete as planned this weekend.

The guy from building reg's came round this afternoon to take a look at our trenches and duly signed them off. Also the first load (4 tons) of sand and gravel arrived for the concrete, the second load (another 4 tons) and the cement (nearly a ton and a half!) is due to arrive tomorrow. Finally we're getting somewhere.