
CALCULATIONS Rock Wall Calculations
(Gympie Pyramid Maths)
To estimate the construction volumes of the Gympie Pyramid rock walls, we decided to
calculate the volume of rock in the walls themselves, and the volume of material moved to
create the terraces.
We used as a sample, a 5 metre section of wall, and used the average width of the terraces
(5 metres)
The walls were on an average, 1.5 – 2 metres high, and constructed on the drystone
method, but using “rounded” rocks, which takes quite some effort to make the
wall stable.
The rocks are on average 400mm deep, although the base stones may be bigger, and some are
quite huge.
We calculated the volume of a 5m section as follows:
Volume = Length x Height x Depth
therefore:
5 x 1.5 x .4 = 3.00 cubic metres per 5 metre section (allowing for air gaps, we settled on
a figure of 2.5 cubic metres)
Most of the rock is sandstone, although there is also a high percentage of ironstone, and
some conglomerate containing quartz pebbles.
The accepted weight of sandstone is around 2.32 tonnes per cubic metre, so a 5 metre
length of wall contains around 5.8 tonnes of rock.
We calculated the volume of material moved using the simplest “cut and fill”
method. This
method involves removing half of the desired terrace width of material, and using it to
fill the “downhill” portion of the terrace against the lower retaining wall (see
figs. 1 & 2)
Figure 1
Figure 2
We calculated the total volume of a 5 m section by first calculating the volume of a block
2.5m x .75m x 5m and then dividing it by 2
(2.5 x .75 x 5)/2 = 4.68 cubic metres
The accepted weight of sand, loose is 1.44 tonnes per cubic metre which equates to 6.7
tonnes per 5 metre section.
A previous owner stated that when he owned the site, he paced out one intact section of
wall on one of the lower terraces at over 500 metres long.
To construct this section would require:
250 cubic metres (580 tonnes) of rocks (gathered and laid), and the moving of 468 cubic
metres (674 tonnes) of fill…..
There are 7 known terraces (decreasing in length as they rise) and the possible length of
the lower ones is (was) close to 1km. If the length of the top wall was 160 metres, then
we have a total of 4060 metres of wall !!!
That is:
2030 cubic metres (4709 tonnes) of rock
and
3800 cubic metres (5472 tonnes) of fill material
A professional "rock wall builder" advised us that 4 men could erect a 5
metre section of wall in one day (not allowing for the gathering of the rock), so to
construct 4060 metres of wall would take 4 men 812 days.
This also does not allow for the cutting and filling……
One also has to keep in mind that some of the rocks have been estimated
at weighing over a tonne!!!
