Ghost of loadings past. The case of a Roman vicus under a 19th century embankment in Vechten, The Netherlands

The ratification of the Malta convention has meant that construction works are commonly preceded by research to establish whether damage to archaeological remains will occur. If this is likely, development plans are adapted to enable preservation in-situ or the developer funds a rescue-excavation. A factor playing in favour of in-situ preservation is the overconsolidation of archaeological layers. A soil is said to be overconsolidated when it has been exposed during its history to vertical stresses higher than current ones. As a result, it is only when stresses applied by new constructions exceed the pre-consolidation pressure that soil de-structuration and mechanical damage to archaeology may occur. Many processes can generate overconsolidation at archaeological sites. Site burial followed by erosion, construction of structures, then demolished, soil trampling by
heavy animals, drainage of topsoil to keep site occupants’ feet dry, increase of ground water level after site abandonment, but also chemical reactions and ageing can cause over-consolidation.
High over-consolidation has been measured in the foundation layers of a high embankment (up to 15 m high) built in the 19th century at Vechten, the Netherlands. The embankment was erected on top of a former Roman vicus established on the natural levee. The Roman level and the 19th century topsoil are rich in fragments of ceramics, bones and metal objects. Over-consolidation pressures are in line with the measured strength and stiffness of the foundation material and the massive soil structures analysed using X-ray micro-computed tomography and micro-morphology. The absence of deflection of the interface between the embankment and its foundation layers indicates that the archaeological layers were already in an over-consolidation state before the construction of the embankment and remained over-consolidated after its construction. In such circumstances, it is only by affecting the groundwater flow and chemistry that the embankment could have degraded underlain archaeology.