Aerial access platforms are a convenient and usually cost-effective method of close access. Depending on the height of the ruin and the distance which needs to be bridged to reach the ruin (the outreach), machines can either be self-propelled or mounted on a lorry. Larger machines which achieve greater height and outreach are normally lorry mounted (Figure 3.15). Aerial access platforms, even small units which achieve a maximum height of 10 metres, are heavy in weight to counterbalance the load which is lifted. Mobile platforms therefore require a solid level ground (with no risk of subsurface voids) to operate from. It is important to anticipate the risk of voids in the ground surrounding the ruin and preliminary geomatic surveys using ground penetrating radar or resistivity are advisable if the make-up of the site is unclear. The route across the site must also be reasonably level and firm, otherwise a temporary roadway will be required.
The key advantage of aerial access platforms over scaffolding is their flexibility, especially the larger lorry-mounted machines. They can traverse obstructions in front of the ruin and even extend up over the ruin and drop down the other side to provide access across significant areas of vertical and horizontal elevations. They can even telescope down barrel vaults to inspect the underside of the vault, provided there is access for the platform and boom to safely enter. They avoid contact with the ruin but get close enough for the surveyor to touch the surface.
Surveys from aerial access platforms often demand greater discipline when adopting a methodical approach, due to the absence of scaffolding lifts, and horizontal reference points and close proximity to the building face, which can lead to disorientation. To avoid this, it is often useful to divide the building into horizontal bands following the lines of architectural mouldings, such as string courses, band courses, floor levels (defined by a line of windows), plinths, etc. It can be difficult and sometimes impossible to take this approach when dealing with plain masonry elevations, for example rubble walls without architectural detail. In these instances, orange string lines can be temporarily pinned into mortar joints across the elevation to provide horizontal reference lines to work to. When adopting this approach, set out the lines at two-metre spacing so that they approximate the position of scaffold levels for ease of reference for conservators who will need to locate the defects during a later programme of repair. In some cases it will be easier to survey a wall in vertical bands, but the survey data should be translated into horizontal bands in the office when the survey is written up. In this case orange string lines can simply be dropped down the wall face and retained in place by weights at the top and bottom (bricks or stones).
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