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Cultural Stone Stability Index (CSSI)

Adaptive Index for Built Heritage

The same major geomorphologic processes that damage rock art panels also threaten all other forms of natural and cultural stone. Therefore, with only a few terminological changes, the core index that composes RASI was adapted to address the broader research area of cultural stone. Termed the Cultural Stone Stability Index (CSSI), it can be used to assess anything from building façades, arches, and columns to statues bridges, and  gravestones. Usable on basically any stone object, the CSSI functions in the same manner as RASI: practitioners define and sketch the target feature ("panel" for RASI), and then rate each rock decay form/process on the same scale of 0-3, divided into the same six categories, ending with a final score indicating the feature’s geologic stability.

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Adapting RASI to CSSI

Within the index itself, only a few changes are needed to transform RASI into CSSI. These changes are purely terminological and do not detract from the focus on different rock decay processes. They merely broaden broaden the assessment scope from rock art specifically to more general cultural stone subjects. For example, the RASI item “rounding of petroglyph edges” is changed to “rounding and/or blurring of carved edges or inscriptions”—both elements rate loss of detail, but use different terms. Although, there are a few decay elements added specifically for buildings and cultural stone decay completely unrelated to rock art, such as “anthropogenic fissures” in the Site Setting section,  and “anthropogenic joints/jointing” in the Incremental Losses category (both focusing on decay processes involving mortar work), the overall assessment result is the same.

The CSSI also takes into account the different roles rock coatings play in cultural stone vs. rock art. For certain types of rock art, some rock coatings are not only beneficial, but very necessary for their existence. Most petroglyphs, for example, are created by pecking or scraping through rock coatings to reveal the raw stone beneath the surface. The contrast between the coated exterior and newly-exposed interior makes the art possible. Therefore, in RASI, two of the four rock coating elements have negative scores, indicating them as stabilizing agents.

 

Alternatively, stone building façades and most other cultural stone are created with freshly quarried material, so any rock coating accumulation takes place after the stones are already in situ and beginning to decay. Additionally, since historic buildings and cultural stone often exist within cities and populated areas—as compared to the relative isolation of rock art sites—they may experience higher exposure to air pollution and urban traffic exhaust, leading to the development of harmful toxic rock coatings. To accommodate this difference, the CSSI adds two more elements to the Rock Coating section: “carbonate coating” and “oxidation”, each with positive scores reflecting their destabilizing influence. The two Rock Coating elements with negative scores in RASI (“case hardening” and “rock coating present”) still have negative scores in CSSI, but with CSSI the specific coatings associated with pollution and enhanced urban decay (carbonates and oxidation) can be individually evaluated and included in the overall final assessment.

Excerpt of CSSI Analysis: Dry Wash Bridge, Petrified National Park, AZ, USA

From Groom (2011):

"The Dry Wash Bridge in the Petrified Forest National Park is a potential historic site but the current physical condition, in relation to rock decay and erosion, is unknown. It has significant cultural value being built by the Civil Conservation Corps (CCC) during the efforts to end the great depression in the late 1920s. The bridge itself has a collection of signatures from the very volunteers who built it. Debates have been held to decide if the bridge is a historic site, and therefore should be protected, or if preservation of the signatures would inhibit the maintenance of the bridge itself, rendering it unusable. In order to best proceed with management decisions further research was needed.

The northwest support is quite a bit different from the southeast in that it has been modified a few times and contains several “modern” elements like metal supports, concrete coatings, and sealant. Unfortunately, these components are not as stable, or any more stable, than the original construction on the opposite end. Much like the rock-fill addition on the southeast side, there is no indication as to when these renovations occurred but they still offer insight on the internal stability of the bridge.

The most obvious issue with this section is the pitiful condition of the tar sealant holding the new modifications together. It is disintegrating. It is possible that the contractors for the update underestimated the intense heat the area regularly experiences or they simply didn’t have the technology at the time to create a sealant that could really withstand the harsh conditions of the Painted Desert. Either way, the current condition of the sealant is a concern and could cause some structural problems in the near future." 

Excerpt from: Groom, K.M. (2011). Historical Assessment of Rock Decay on Dry Wash Bridge, Petrified Forest National Park. Rocky Mountain Cooperative Ecosystem Study Unit Project Summary Report. National Park Service, with annotated illustrations.