Building with earth - Gernot MINKE (1)

14.53
14.50 Nubian vaults with
support walls and support
arch (side elevations)
14.51 Coordinates of
structurally optimised
domes
14.52 to 14.53 Construction
of a Nubian vault
with support arch
14.52
Nubian vaults
With the Nubian vault technique, used for
centuries in Upper Egypt, vaults can be built
without any formwork by using reclining
arches made of adobe. Illustration 14.49
shows such a vault, which is 3200 years
old and stands within the temple precincts
of Ramses II near Luxor. Such vaults are commonly
constructed of adobes measuring
15 cm in width, 25 cm in length and only
5 to 6 cm in thickness. This means that the
weight of each brick per unit area of mortar
joint is very low, which prevents adobes in
inclined positions from sliding during construction.
The degree of inclination of the
arches is a decisive factor in the construction
process. This should be between 65° to 70°
with the horizontal. As tests have shown, if
the arches are built up at a lower angle, the
lower part of the vault might collapse during
construction, while if the angle is larger, the
adobes might slide off the top.
Nubian vaults need one or two vertical walls
onto which the inclined arches lean (14.50 A
and B). It is also possible to lean the arches
against a central ”supporting arch,“ which
typically has the section of the vault and has
to be made with shuttering (14.50 C and
14.52). The cross-section of the Nubian vault,
which is mainly loaded by its own weight,
should have the form of an inverted catenary,
so that it contains only compressive
stresses.
At the BRL this traditional technique was
refined in two ways: first, instead of using
rectangular formats, a square block measuring
20 x 20 cm, 6 cm thick was used for
the lower part of the vault, and tapered
versions of these blocks were used in the
upper part of the vault, with the lower part
shortened by 1.5 cm. This reduced labour
input and the quantity of mortar required.
It was found that by using an optimum
mortar composition with high binding force,
it is also possible to use adobes with thicknesses
of up to 10 cm. This leads to further
savings in mortar and time.
Second, the shape of the vault was controlled
during construction by stretching
a cord from one support wall to the next
(or to the corresponding scaffolding). It is
essential that this cord passes through an
eyelet on one end and is held taut by a
weight. When deformed by lateral pressure,
the cord will be immediately restored by the
moving weight to the correct position.
When building the reclining arches, it is
advisable that the blocks forming the arch
are held together by keeping them touching
on the inner edge with hardly any mortar in
between, and wedging with a stone chip
on the outer edge if required, so as to display
arch action even before the mortar is
dry (14.53).
A B C
14.50
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Designs of building elements