Page 264 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

Chapter 9: Excavation 243
unless there is some form of control. However, if a start is made at one end of the site and excavation proceeds forward across the site, with experi- enced staff it is possible to excavate quickly and efficiently. A bar or tape can be used to help keep the excavation moving evenly across the site. The great advantage of this technique is that complex structures can be excavated in one piece.
When excavating in localized areas, a grid or defined trench is essential to confine the extent of the excavation. Excavation proceeds downward, layer by layer. The process is repeated grid by grid. However, it is often difficult to correlate grid square layers, as structures sometimes extend through several squares creating serious problems. On a site with little ver- tical component, the trench system works well.
One ingenious method of excavating a hole in order to make a test pit in loose sand or mud is to use an open-ended 200-L fuel drum as a caisson (Figure 9.7). By operating the airlift inside the drum, the interior base can be excavated away and, as this happens, the caisson drops lower and lower. The caisson keeps the sand from falling into the hole, thus enabling an easy excavation down to approximately 1m. Using this system, it is also
Figure 9.7 Caisson excavation using a 200-L fuel drum open at both ends, Takashima, Japan.