Copyright © University of Cambridge. All rights reserved.

## 'Concrete Calculation' printed from http://nrich.maths.org/

The builders have dug a hole in the ground to be filled with
concrete for the foundations of our garage. The shape is designed
to avoid subsidence that might be caused by the roots of some very
large trees nearby. The hole is perfectly symmetrical with vertical
walls, 1.2 metres high, surrounding a rectangular area measuring 6
metres by 5 metres. A mound of earth in the centre, the shape of
the frustum of a pyramid, has a rectangular base 4.8 metres by 3.8
metres and a horizontal rectangular top 3.8 metres by 2.8 metres.
The height of this mound is 0.9 metres. How many cubic metres of
ready-mix concrete should the builders order to fill this hole to
make the concrete raft for the foundations? Can you explain the
method simply to help the builders to do the calculations for
similar shaped rafts of other dimensions?