144 DESTRUCTIVEINVESTIGATIVE TECHNIQUES
Even these measures may be inadequate for the most
fragile objects such as textiles, leather and other very
friable materials. These are best recovered with some
of their surrounding sediments, usually by transferring
them into a suitable container. This also avoids the object
collapsing under its own weight, which would be the
likely result should it be lifted from water into air. The
whole procedure takes practice and if possible should
be done by someone who is familiar with the type of
object being recovered.
Some material will not need to be recovered after it
has been exposed and recorded (e.g. ship’s structure).
Provision must be made to protect it in the long term.
This may involve reburial, usually involving lining the
trench with a protective membrane, held in place by
sand-bags. The trench is then ‘backfilled’, either with
some of the sediment removed during the excavation or,
alternatively, with a sediment of specific characteristics
brought in for the purpose.
Underwater excavation, just as on land, consists of
two distinct procedures, each of which has its associated
tools: 1) the actual digging and 2) the removal of ‘spoil’ (the
unwanted sediments loosened in the process of digging).
Although excavation can often be carried out by the same
tools used on land, spoil-removal under water is very
different. Occasionally the current alone is sufficient but
normally some form of suction device is used, adapted
from those first developed for industry. One of the
mistakes of early excavators was to use these suction devices
as the means of digging, in effect conflating the two activ-
ities. If control is the most important aspect of respons-
ible excavation, then maintaining this distinction is vital.
The act of excavation is one of constant decision mak-
ing – how deep to dig, what to cut through, where to cut
to, etc. The by-product of this activity is loose sediment.
Some of this may be recovered with an object or it
may be recovered as a sample. The rest is ‘spoil’, having
been judged not to contain sufficient information to
keep. This judgement is not only the province of the
individual excavator but of the trench supervisor and
ultimately the site director. Once created, spoil needs to
be removed. Just as spoil produced by the use of a trowel
on land can be removed by shovel, bucket, wheelbarrow
or conveyor belt, so various tools are used for the same
purpose under water.
The airlift and the water- or induction-dredge are
suction devices originally invented for other industries
but which have become ubiquitous in underwater archae-
ology. In the past, they have been described as the under-
water equivalent of a shovel, but their real function is to
move unwanted excavated material (spoil) away from the
excavation like a wheelbarrow or bucket. In this sense,
they are one of the advantages of underwater excavation
because once in operation they are virtually automatic.
Airlift
The airlift is a simple device consisting of a rigid tube into
which air is injected at its lower end, usually from a
compressor on the surface (figure 15.10). As the air rises
towards the surface, it lifts the tube to near vertical and
creates a suction effect at the bottom. Water and any loose
materials are pulled in and up. The power of the suction
is dependent on the difference in depth-related pressure
between the top and bottom of the tube, and the amount
of air injected.
Airlifts can be run from any source of compressed
air but are best powered by the sort of road compressors
used for pneumatic tools such as jackhammers and rock
drills. These provide large volumes of air at low pressure
(between 7 and 10 bars (100 –150 p.s.i.)). Neither high-
nor low-pressure diving compressors provide sufficient
volume. The size of compressor required depends on the
depth of the site and the size and numbers of airlifts
that will be used simultaneously. The smallest air-tool
compressors deliver 2 cubic metres (275 ft^3 ) of air per
minute and this is adequate to power two airlifts. In no
circumstances should such a compressor (or any other
air source) be used to supply air to divers and airlifts at
the same time.
The limitation of these compressors is their weight; hence
they need a sufficiently large boat or floating platform
unless the compressor can be sited on land nearby.
Another limiting factor is often the cost of such units
although, for short periods, hire charges are not excessive,
especially in relation to the total costs of excavation.
On small operations, the hose can lead direct from the
compressor to the airlift. However, it may be necessary
to secure the air hose to somewhere convenient on the
sea-bed so that there is no additional pull on the airlift
once the hose fills with air and becomes buoyant. OnFigure 15.10 Excavation using airlift. (Drawing by Graham
Scott)