![\"The](\"https:\/\/blogs.agu.org\/landslideblog\/files\/2021\/12\/21_12-Spruce-Tree-House-1.jpg\")
The Spruce Tree House alcove complex in Colorado. Image from Wikipedia.<\/a><\/p><\/div>\n .<\/p>\n Unfortunately natural rock arches are, in a geological sense, transient features, meaning that in the long term the safety of this complex is inevitably compromised.\u00a0 Back in 1908 a large crack was documented in the arch and downward deformation of the rock mass was also observed.<\/p>\n The history of subsequent works to try to stabilise the arch is documented in an NPS report (Mason 2016<\/a>) that analyses the stability of the arch.\u00a0 This notes that in the 1920s the site suffered a significant rockfall, and by the 1940s major problems were apparent, most notably a very large crack at the crest of the arch, as the image below shows:-<\/p>\n The large crack at Spruce Tree House, as depicted in the 1940 report. Image from Mason (2016).<\/a><\/p><\/div>\n .<\/p>\n To manage the risk, a drainage ditch had been constructed. Mason (2016) notes the full range of works undertaken in the 1940s:<\/p>\n “…the arch crack was cleaned … of debris and plants and subsequently filled at the ends with emulsified asphalt and a roof structure was built over the wider center sections. Lancaster cleaned the crack along the full length and installed a \u201cwater-proofing\u201d system to prevent surface water from seeping into the alcove. It appears that the technique was ineffective because of numerous attempts at resealing.”<\/em><\/span><\/p>\n In 1960 the problems became serious once again after another rockfall.\u00a0 A further round of works were undertaken in 1962, including the installation of 46 rock bolts and further efforts to seal the crack.<\/p>\n Whilst the site remains intact, a further round of inspections and analyses were initiated after a rockfall in 2015.\u00a0 These have found that there are problems with the rock bolt method that was adopted, and it is likely that the seal in the crack has caused a rerouting of groundwater flow paths, which is probably degrading other parts of the rock mass.\u00a0 And of course the whole rock mass is undergoing natural degradation.<\/p>\n Since then there has been an extensive set of investigations and analyses to determine a solution for the problems at the site that will protect it for the next century.\u00a0 The Durango Herald has a good article that provides some detail:<\/a><\/p>\n “It appears the fix is going to be multi-pronged. The park will likely use rock bolts again, but this time, installed at offset angles, which would increase strength (think of the root structure of a tree). And, new material, likely a type of metal, will be used to fill the crack to stop water from entering.”<\/em><\/p>\n It will be fascinating to see the detailed design for the Spruce Tree House site – this is rock slope engineering at its most challenging given the importance of the location; the rock conditions; the vulnerability of the arch and the archaeology; and the importance of the site to the National Park.<\/p>\n .<\/p>\n![\"The](\"https:\/\/blogs.agu.org\/landslideblog\/files\/2021\/12\/21_12-Spruce-Tree-House-3.jpg\")
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