H. Huisman

Under Pressure: A Laboratory Investigation into the Effects of Mechanical Loading on Charred Organic Matter in Archaeological Sites

The present publication investigates what happens to archaeological sites when they are built over. Focus is put on the degradation of charred organic materials by static loading. It is assumed that materials lose archaeological value if their fragments become too small to be recovered, or too distorted to be classified at species level. Several charred ecofacts of a few millimetres in size (wood fragments, hazelnut shells, and seeds) have been selected and subjected to individual particle strength tests. Assemblages of these particles have also been compressed one-dimensionally and scanned at several stages of testing using laboratory based X-ray microtomography. Microscopic damage by splitting or crushing is found to be limited at the macroscopic yield stress. It initiated at stresses less than 80 kPa for the weakest assemblages, and in all cases at stresses below 320 kPa. (80 kPa represents the load of a 6 m high sand embankment on soft soil that has half-settled underneath the groundwater table, while 320 kPa corresponds to stresses applied beneath the pile foundation level of high-rise buildings.) Sand seeded with charred particles has also been tested to illustrate the beneficial effect of embedment of charred particles in sand during static one dimensional loading.

Resistance of Buried Archeological Site to One-dimensional Mechanical Loading

Our article focuses on wetland archeological sites that are subjected to one dimensional compression by the placement of a soil body for the construction of a line infrastructure or a landfill. We study the resistance to mechanical loading of ecofacts that are often investigated in archeological prospection works: charred and non charred plant remains and shells. We conducted one dimensional compression tests on assemblies of ecofacts, sand samples seeded manually with ecofacts and natural soils rich in ecofacts and used X-ray micro-tomography to evaluate the integrity of the ecofacts as function of loading. We assumed that fragmentation of ecofacts results in a loss of archeological value if particles become too small to be recovered. The ecofacts tested so far are unlikely to get crushed when included in an archeological soil above which a sand embankment of 1 to 10 m height is constructed. Some might however be deformed, flattened and re-aligned.