Typical Broch Failure Modes
Brochs can fail through many mechanisms, each complex in isolation and more so when in combination with others. A number of typical failure modes found in brochs are outlined below.
1. Destruction by Humans
Reduction in Height:
If the upper walls of a broch tower started to become unsafe and unstable with stones falling into the broch, its inhabitants may have dismantled it to a safe height. This is the case at Dun Mor Vaul where there is evidence to suggest that the roof was taken off and the upper part and debris was carefully and systematically removed after stones fell into the court. (MacKie 1974)
Erosion:
Broch walls are in danger of further collapse by continuing erosion from visitors and animals scrambling over the broch to reach the wall head etc. (Barber 2012)
Destruction:
Brochs may have been destroyed intentionally by humans. MacKie presents the idea that the sudden ending of Dun Mor Vaul may have been due to the exploits of Labraid Loinseach, the legendary Irish hero who “smote eight towers on Tiree and eight strongholds of the men on Skye.” (MacKie 1974)
1. Destruction by Humans
Reduction in Height:
If the upper walls of a broch tower started to become unsafe and unstable with stones falling into the broch, its inhabitants may have dismantled it to a safe height. This is the case at Dun Mor Vaul where there is evidence to suggest that the roof was taken off and the upper part and debris was carefully and systematically removed after stones fell into the court. (MacKie 1974)
Erosion:
Broch walls are in danger of further collapse by continuing erosion from visitors and animals scrambling over the broch to reach the wall head etc. (Barber 2012)
Destruction:
Brochs may have been destroyed intentionally by humans. MacKie presents the idea that the sudden ending of Dun Mor Vaul may have been due to the exploits of Labraid Loinseach, the legendary Irish hero who “smote eight towers on Tiree and eight strongholds of the men on Skye.” (MacKie 1974)
2. Falling in of Roof and Internal Timber platforms
Due to the conoid shape of brochs, their outer wall may be directly vertical over the inner walls foundation at the apex of the broch, as is the case at Carloway. Harding suggests that the construction is dependent on the circular geometry maintained by balance of the outward thrust of the roof. If the roof was to fail due to poor maintenance, decay or was removed due to alteration, the outward thrust would be lost resulting in structural collapse. (Harding 2004) Many brochs have scarcements or an interior ledge around the middle of the inner wall to sustain an interior floor/platform. (Harding 2004) The figure opposite shows a theorised internal broch structure where there are two scarecement platforms. If the timber supports fail and fall into broch, the pull out of the beam in the scarcement would produce a lateral load possibly resulting in instability of the structure and potential collapse. |
3. Settlement due to poor foundations
Brochs are often built on problematic ground (Thew et al 2012) which can result in settlement of part of the structure. Settlement can cause the stones to slip if the settlement angle is large enough, crack due to uneven supports and can lead to the loss of pinnings and infill stones.
Examples:
Dun Bharabhat is an Atlantic Roundhouse build partly on outcropping rock, partly on less secure underlying material as shown in the adjacent figure. The inadequate foundations caused collapse through subsidence; the walls not built on the outcropping rock slumped and collapsed. (Harding and Dixon 2000) Dun Vulan was constructed partly on sand and partly on rock, creating a fundamental instability in the structure. The results of the instability is most obvious in the steps which were snapped in half due to differential settlement. The effect is also shown at the outer wall face on the south side which had slumped dramatically.(Pearson et al 1999) |
4. Water Action
The distribution of brochs throughout Scotland is primarily along the North and West coastlines (Theodossopoulos 2012). A number of these brochs were built close to the coast where they have been subjected to storms and wave actions for the last 2000 years, eroding both the broch and the ground around the broch. Erosion of the broch itself leads to loss of friction among stones allowing for stones to slip. Erosion of the ground around the broch will create subsidence and settlement problems described above.
The West side of Clachtoll has been entirely lost due to coastal erosion, creating ragged cross sections through the wall which are in danger of continuing erosion. For further information on the coastal erosion at Clachtoll, see the Clachtoll page.
This phenomena is also seen, although less extreme, at Dun Vulan where the West side of the broch is heavily scoured, presumably by water action. (Pearson et al 1999)
5. Bulging of Broch wall
Collapse of the upper parts leading to infilling of the broch can create lateral pressures on the remaining inner wall. This pressure will create a large force on the inside of the inner wall, forcing the inner wall to curve outwards into a gallery space, creating a bulge. If this bulge is large enough it can push the courses of the outer wall outwards also, creating a bulge in both walls.
Bulging can also occur due to toppling of the upper wall parts creating eccentric vertical loading in the wall.
The second phenomenon is exhibited to the left of the Clachtoll Broch entrance, the theorised progression of which is given below.
- The outer wall collapsed to a reduced level
- Outer face of outer wall was crudely reconstructed, thought to be by the Picts.
- Outer wall toppled outwards, the reconstructed outer face peeling away.
- Toppling caused eccentric loading, leading to bulging of the broch wall.
See Clachtoll for further information.
The distribution of brochs throughout Scotland is primarily along the North and West coastlines (Theodossopoulos 2012). A number of these brochs were built close to the coast where they have been subjected to storms and wave actions for the last 2000 years, eroding both the broch and the ground around the broch. Erosion of the broch itself leads to loss of friction among stones allowing for stones to slip. Erosion of the ground around the broch will create subsidence and settlement problems described above.
The West side of Clachtoll has been entirely lost due to coastal erosion, creating ragged cross sections through the wall which are in danger of continuing erosion. For further information on the coastal erosion at Clachtoll, see the Clachtoll page.
This phenomena is also seen, although less extreme, at Dun Vulan where the West side of the broch is heavily scoured, presumably by water action. (Pearson et al 1999)
5. Bulging of Broch wall
Collapse of the upper parts leading to infilling of the broch can create lateral pressures on the remaining inner wall. This pressure will create a large force on the inside of the inner wall, forcing the inner wall to curve outwards into a gallery space, creating a bulge. If this bulge is large enough it can push the courses of the outer wall outwards also, creating a bulge in both walls.
Bulging can also occur due to toppling of the upper wall parts creating eccentric vertical loading in the wall.
The second phenomenon is exhibited to the left of the Clachtoll Broch entrance, the theorised progression of which is given below.
- The outer wall collapsed to a reduced level
- Outer face of outer wall was crudely reconstructed, thought to be by the Picts.
- Outer wall toppled outwards, the reconstructed outer face peeling away.
- Toppling caused eccentric loading, leading to bulging of the broch wall.
See Clachtoll for further information.
6. Too Great Diameters
Mousa: Photo sourced from Dr, Theodossopoulos
If the diameter of a broch is very large, the curvature is too small which will not permit the equalisation of stresses about the circle. The broch wall will behave as a straight wall, allowing the stresses to build at a weak point resulting in the broch wall collapsing outward.
Mousa, situated in Shetland, is possibly the tallest standing broch today at over 13m tall. Mousa has a small inner diameter and a very small external diameter compared to other brochs. Its Percentage Wall Base is very large and it can be argued to bear a direct relationship to the brochs stability.
Therefore if the diameters are small and the wall thickness is average – the stability of the broch could be greater.
(Fojut 1981)
Mousa, situated in Shetland, is possibly the tallest standing broch today at over 13m tall. Mousa has a small inner diameter and a very small external diameter compared to other brochs. Its Percentage Wall Base is very large and it can be argued to bear a direct relationship to the brochs stability.
Therefore if the diameters are small and the wall thickness is average – the stability of the broch could be greater.
(Fojut 1981)