Observations and modeling of moisture and decay patterns in stone monoliths in Southern England

Stone deteriorates as a result of a range of natural and human-induced processes, and such deterioration can be unsightly and costly, especially where it affects important stone monuments such as gravestones. Moisture is a fundamental influence on stone deterioration as it provides a medium for transport and reactions (chemical weathering), as an essential factor for micro-organism growth (biological weathering) and as cyclical changes of state (liquid/solid/gas) exert a key control on many physical weathering processes. Despite this importance, little is known about the patterns of moisture distribution and movement within stone, how they vary over time and how they may be correlated with the nature and severity of deterioration.
   Commonwealth War Grave (CWG) stones are found widely across the UK and Europe, dating largely from the early to mid 20^th century and provide a natural test of the variation of stone deterioration under different climatic and environmental conditions involving single blocks (monoliths) which are more amenable to modeling. We report here on results from a linked field experiment, modeling, and field survey-based study to investigate the nature and causes of moisture and decay patterns in relation to microclimatic and environmental conditions at two areas in southern England (Dorset and Oxfordshire). At each field experimental site 8 gravestones and similar sized Portland stone monoliths have been erected, and climatic and environmental monitoring equipment emplaced (automatic met stations, evaporation gauges, soil moisture probes, piezometers). A suite of novel non-invasive and non-destructive methods to investigate moisture regimes and the early stages of deterioration has been developed, including 2D resistivity surveys, hand-held moisture meters, Equotip hardness testing and time-lapse photography. Such a field experiment approach provides an ideal way of testing links between climatic and environmental variables and stone deterioration and validating output from numerical modelling approaches (such as Hydrus, which has been used in this project). Further observations of moisture regimes and decay features have been made from CWG stones within cemeteries near our experimental sites.
   Hand held resistivity and capacitance probe surveys, in conjunction with electrical resistance tomography, provide detailed, spatially-resolved data on moisture distributions which can be compared with mapping of the nature and severity of decay and monitoring of surface water patterns from time-lapse photography. Here we show direct evidence of damp conditions (both surficial and deep-seated) at the base and top of gravestones, associated with two types of deterioration, with a drier central area characterised by less weathered stone. Decay surveys indicate extensive surface damage within the upper parts of the gravestones, and considerably less damage below this.