The deterioration of the Pueblo Bonito Great House in the Chaco Culture National Historical Park, New Mexico, USA.

Pueblo Bonito is the iconic pre- Columbian structure in Chaco Culture National Historical Park, a World Heritage Site in northwestern New Mexico, USA. The structure, dating to about 850-1150 Current Era, and built of quarried sandstones, wooden timbers and a mud mortar, has been the subject of archaeological investigations for over a century. The present study is based on the examination of historical photographs of Pueblo Bonito dating from 1887 to the 1920s. It is a retrospective assessment to determine if structural damages, depicted on the photographs, could be attributed to identifiable agents that might have been present at the time of Pueblo Bonito occupancy. A likely causal agent of deterioration at Pueblo Bonito was the inability of Ancestral Puebloan engineers to manage the impacts from the annual precipitation, presently measured at about 220 mm. A resulting time-dependent event was rot to wetted roof and ceiling timbers, lintels, and wall support beams which required decades of incubation by wood decay fungi to reduce wood tensile strength to levels leading to roof and wall collapse. Important time- independent events that could occur any time after construction include water action on the mud mortar which resulted in unstable gravity load paths in stone walls, ponding of water in walls which when frozen would lead to the blowout of wall segments, and the occasional flood that disrupted foundations. Pueblo Bonito may have been an occupation site for centuries but the lifetime of individually constructed rooms may have only been decades, resulting in several build- repair- or abandon cycles being part of the history of that Great House.