So why may you have to go deep? The usual reasons are:-
Now, if you have a deep trench, it may be cheaper to fill with concrete - you may wish to calculate the comparative costs of concrete versus laying blocks. You will be able to calculate the cost of concrete from the trench dimensions and costs from the ready mix company.
To calculate the costs of blockwork, assume a cost of £2 per block for the brickies. (Lets make it easy and assume that this figure will also cover the sand and cement for laying the blocks and cement & aggregate for the cavity fill.)
The brickies will prefer trench fill - it is not easy standing in a deep trench laying blocks - especially if you have large feet!
|This is obviously an example of a trench prepared for trench fill. The depth to the top of the batten is 34 inches!|
|Still a lot of concrete required to get to the top of the marker sticks.|
|At the bottom of the trench can be seen a thin seam of coal. Coal can produce an acid with rain water, which will weaken the concrete - a stronger mix to cover this seam was asked for from the Readymix company|
|These foundations are on a sloping site, which necessitated step foundations - the concrete is stepped by securing form work into the trench.|
Walls that are to be rendered and painted will not normally be affected. What is most affected is brick work, especially where the work is in contact with the soil, ground water, exposed to salt air near the sea, and sea water.
So, to minimise the risk, use a sulfacrete cement. Also, keep the bricks dry before they are used as wet bricks will tend to release the sulphates quicker during the time when the cement is setting.