Mechanical Behaviour of Drystone Walls

Throughout the United Kingdom (UK) distinct types and styles of walling can be observed. The underlying methods can be grouped as either horizontal, vertical or random construction as presented in figure 1 below (Drystone Conservancy, 2015). Every type used has its uniqueness that led to its acquisition in the area depending on the nature of the rock laid at the construction site. Observing the construction works of the drystone masonry, horizontal types are often found to consist more blocky stones like limestone. It is due to the ability of limestone to mound conventionally; at times stones are worked to provide the walls with a better fit. In the UK drystone masonry, horizontal construction is the most dominant version. Vertical type is in most cases associated with slate stones that allow for tight packing that may be less functional in horizontal constructions. The random building is primarily incorporated into more granitic rock types; they are hard to construct with since they possess irregular shapes and difficult to shape. These types of construction at times can be combined in a single building with the same kind of stones applied such as the herringbone construction developed in parts of Cornwall. The essay is set to depict on the scale and organisation of construction process as demonstrated in the traditional retaining walls in drystone masonry (Brochs).

It can be argued that the distinct construction styles of the drystone retaining walls serve in the same manner; as every wall is gravity retaining relying on the frictional forces generated between the stones to gear stability of the building (Tufnell, 2012). However, the distinct mechanical property is applied to different styles to achieve the coherence required. The movement and failures attached to horizontally built walls are documented and can easily be understood when compared to vertical and random construction styles. The picture below demonstrates the mechanical difference between the construction styles of the drystone masonry walls.  

Figure 1:

Demonstrates horizontal, vertical and random construction styles of the drystone masonry walls (Bailey, 2008).

Horizontal Construction

The numerical modelling and experimental testing by (Mundell et al., 2009, p.205; McCombie et al., 2012, p.45; Villemus et al., 2007, p.2125 and Colas et al., 2010, p.1218) depicts that the overturning moment by the walls due to the earth pressure is often resisted by the vertical part hitting in downward shear on the backward sides of the walls.  With a well-constructed wall, the action leads to slight deformations leaving the structure standing still in its position as aligned. With consistent loading, deformation of the walls can become considerable due to the exceeded pressure showing the classic bulge walls adapting the formulated shapes resulting from the higher forces of the applied load as demonstrated in figure 2. Ultimately, the walls can overturn and incase smooth stones used to part of the construction or whole may typically slide forward. Nonetheless, if the walls are well-connected with overlapping rocks, then it is likely to develop a tensile strength along the lengths which significantly helps in redistribution of the asserted load giving the walls a substantial ductility. Moreover, the behaviour mentioned above is primarily dependent on the horizontal construction styles.  

A less valuable consequence generated by the horizontal building technique is the reliance on through stones to manage the coursed stone layers at the front and back walls caused across an infill of rubble.  The applied stones in these constructions are relatively more significant than those encompassed in the vertical and random structures and cannot assure consistent behaviour off by the walls giving chances for failure resulting from movements. 

Figure 2:

test wall of the broch of Mousa created in Scotland in 100 BC standing 42 feet high (Georgievska, 2017); demonstrates a classic bulged profile horizontally laid by use of limestone stones.

Vertical Construction

There has been little research evidence on vertical and random construction styles. However, some indications on how the techniques preform can be rendered from the walls appearances in relations to the behavioural actions of the horizontal constructions. Looking at vertical styles in development it is necessary to expound from figure 3 displayed below.

Figure 3:

Model test demonstrating partially rotated concrete block and wedged inwards against the sides of the test box.

Discussions on vertical construction styles with the constructors in Cornwall and Somerset indicates that vertical model is reliant on pre-stressing compressional forces. With vertical alignment in constructions, all stones tightly packed across the entire wall depth or penetrate the width entirely efficiently forming a continuous cross section. During the establishment of these walls, pre-stressing is encompassed throughout the wedging of the stones at regular intervals to produce a compressional force when enforced with overall confinement attached to the walls. Broch walls are backfilled the remaining gaps filled with the materials at the back of the walls generating further strengthening of the wedges. The evidence to this is displayed in figure 4 the works completed by Mundell (2009) and Bailey (2008). According to Bailey (2008), backfill wedged between the aligned blocks making it more difficult to remove the smaller individual blocks from the nesting box. The test was a nuisance though giving useful information about the underlying mechanical issues in wedging of the other types of walls.

Figure 4:

The broch walls enforced with overall confinement backfilled with materials.

Random Construction

Random construction incorporates features with high similarities to those in the vertical and horizontal building. However, the differences in the morphological variables of the stones in use make it null to generalise the assumptions about any further mechanism in construction applied (Walker et al., 2007, p.98). Moreover, it can be observed from the random building that the absence of bonding bars the development of tensile strength in the face of the broch walls. In the random construction model, the stones used must rely match more on the induced sizes to enhance the stability of the structure. The broch of the Mousehole created in the Cornwall in the UK had its geological stones of granite with most of the construction in the building applying the casual style. Nonetheless, the use of these distinct models in the fabrication of the old walls was often depended on the type of the rocks found within the construction sites. The horizontal construction of the broch is usually seen where the nature of the underlying surface is rockier as compared to the other types of buildings. 

According to Le et al. (2016, p.240), the building stones of the horizontal broch are more often stone types encompassed with more fractioned surfaces for instance limestone and are purposed to help the constructed structure transfer more loading with lots of ease occurring within the walls. The vertical model of the broch constructions seems to have used more of slate-type stones. The thin cross-sections observed in the building are evidence of the laminated nature of slate stones incorporated in the development of these prehistoric sites of the broch. The applied mechanism is useful since the slate stones have less fractioned surfaces as the shear forces generated by the earth's pressure travels through the rough covers instead of the smooth ends. Lastly, the random construction is exercised where the stones are difficult to build with such like areas of granite rocks and where a variety of rock types exists. For the random building, all the surrounding stone varieties are most likely involved in the construction as observed with the drystone masonry of the broch in Scotland.

Specific Styles in Construction of the Broch Walls

Although it is depicted that wall construction is much reflective to the kind of stones in the surrounding, sometimes the imaging used reflects the purpose of the building. Probably the best demonstration of this is the walls of the broch and the stair-casing incorporated in the structure as observed in figure five below. Broch walls in the drystone are of significant age up to 1390 dating back. The age and construction period affirms that the walls were completed of specific reasons and thus the type of the stone types applied not necessary depended on the environmental factors such like the rocks surfaces in the place but the quality intended to serve the purpose of the building (Cody and Cody, 1972, p.473).

Figure 5:

Walls at the Mousa broch depicting the staircase and the large blocks in building the structure (Johnson, 2017).

Research by various individuals on the traditional drystone has been carried out including the broch and the harbours by Tufnell Richard in 2012 which he demonstrated in the 13th

International Dry Stone Walling conference held in the UK in 2012.

Understanding Construction of the Broch

Broch like most other present drystone retaining walls was constructed over 100 years ago; however, there is no clear record on how they were created and by who (Harkness et al., 2010, p.8). Even in the contemporary world, such walls are established based on the set rules of thumb incorporated with little or no engineer’s efforts. Surprisingly, many of these structures created have remained firm for quite long and still hold the stable retaining proportion of the United Kingdom as well as used as reservation centres and domestic use. Since most of these buildings are loaded, they are subjected to have increased pressure especially those used in road networks. The combined need to have the old structures maintained from collapsing and ageing by repairing and replacing the walls means that the whole construction of broch stability is increased. Assessment of the broch constructions is very tricky since their designs and building were not formerly built by the directions of engineers unlike the practised trend of in the modern building regulations act.

Assessing the drystone retaining walls in the same manner as done for the modern buildings is inappropriate due to their inherent flexibility and un-mortared nature; as it implies that walls deformation does not necessarily mean they are unsafe for stability. However, much of the present views on the safety of these walls are dependent on the engineers who participated in examining the situation of the structure which determines its condition (Alija et al., 2017, p.476). Where the engineer is informed and has a clue to the drystone retaining walls is most likely to present a reasonable argument about the constructions, unlike the clueless personnel who will take the building's oddities and deformations as signs to failure. With inappropriate assessment of the broch walls, the action of bringing them down can result in replacing them with less quality and unsustainable modern materials. To improve the existence of these structures, further engineering skills are useful in understanding the overall structural behaviours of the broch and the attached effects of individual property elements. Among other things is understanding the dynamism factors such as weathering of the stones through time processes.

It is also essential to understand the generic structures and the walls behaviour to help place a closely related judgment to what precisely the traditional building factored most as compared to the modern building mechanism. Most of the broch have had their structures grouted to bar continuous movements; however, the action has kept to the prevention of correct drainage as the paths through the walls are blocked (Warren et al., 2013). Blocking the drainage paths of these buildings is disadvantaging them from the un-mortared nature they are developed in to allow free drainage. With continuous stay without free drainage, the walls can contain water that will, in turn, lead to increased inner pressure causing them to collapse. Unfortunately, some of the barriers need some severe grout to maintain them from demolish due to aged factor and expanded stretching from the outside pressure and that of the increased loading. The art of grouted walls can be observed from figure 6 below.

Figure 6:

Grouted walls of broch at Mousehole depicted on the inner view (O’Reilly and Perry, 2009)

In assessing the failed retaining walls is a bit confusing because after the fall of the un-mortared constructions pile of rubbles is what most likely left (Zhang et al., 2004, p.57). It is challenging to analyse the left section of the structure as it cannot be assumed that the remained part gives the accurate imaging of the whole building. According to Claxton et al. (2005, p.385), some judgment has been made about the construction process from the foundations to the mechanic process involved in doing the buildings as well as backfilling back very minimal is completed about the drystone retaining walls of the broch. The broch walls are suitable in appearance and with the present imperative of low carbon present in them is an ideal solution to their long-lasting effect.

First Inhabitants of the Broch

The broch depicts deposits on the inside floor suggesting inhibition through the mass of artefacts and pottery realised to have existed during its establishment. Specifically, the broch looks to have been occupied by those in the refuge as from the findings of globular vessels and clettraval ware (the new everted rim) that appeared first during the creation of the broch. Profusely, the first inhabitants of the broch are likely the original constructors of the building who based much on ornamental things due to the found decorated pottery staffs with incised lines (Addis, 2010). Appearances of the entire value ware vessels indicate that the Aboriginals initially inhabited the tower as the majority communities settled in the place. The aligned inference, in turn, confirms that the broch tower might have been built by qualified engineers from distinct populations settling in the area. Alternatively, it can be argued that the upper-class builders of the broch may have come from the different parts of the Atlantic province controlling the local population to establish the towers is a significant possibility as well resulting from the appraisal of the Jarlshof (Addis, 2010).

There cannot be any doubt that the users of the Clettraval jars closely associated with the building of the drystone retaining walls of the broch (Ochsendorf et al., 2004, p.90). Additionally, the early fort builders may have been assisted by the Iron Age B by the arrival of the warrior’s aristocracy as they were skilled in developing fortified strongholds in establishing broch. However, the issues remain not justified as to who exactly was involved in building the towers instead it is confirmed that human inhabitants settled in the sites. The problem is bound by the social factors in the Western Isle formulated in the first centuries B.C and A.D which stands less likely to be solved as from the site’s excavations (Edinburgh New Town Conservation Committee, 1981).

A mixed farming economy in the broch towers was evident by the well-preserved grain impression and animal bones on sherds implying domestics animals kept, and barley is grown as well. Most of the found bones in the broch were those of the cattle and sheep in the percentages of 29 and 35 respectively. Also, roe and red deer were depicted as the caught and eaten animals at 14 percent; few pig bones emerged and the remains of one bird identified as the parts of the adult shang. Dog bones were found among the domesticated animals primarily used as security guards in the broch towers of the drystone walls. Fire incidents were observed by the patches of red and black ash peat showcased at various places with no central heart identified.  

Other activities in the broch included iron smelting (slag), corn grinding (a rotary quern fragment), bronze working and thread spinning (whorls). The colour of the found deposits inside the broch was contrasted sharply pinkish and light- yellowish brown forming around the layers and the presence of the dark grey layers on the floor (Acland, 1998).

The Primary Function of the Broch

            There is little data collected on the matters of the indented reasons for the towers constructions. However, the reliable information is drawn from the other closely related structures formulating to the drystone retaining walls such as the harbour constructions in the UK (Castro et al., 2017, p.4). It is evident by the researched findings on the drystone walls with broch lying among them as the indented secured homesteads for the communities lived in that era. Moreover, it can be assumed that the broch buildings are primarily the same to the existing huts of the time with the difference being that they are massively secured (Nicholson, 1828). In other words, it is evident that the broch and duns were taken as settlements of the Iron Aged population. The towers excavation managed to complete up to six floors to date namely Vaul itself, Crosskirk in Caithness, Jarlshof and Clickhimin, Hurley Hawkins in Angus and Torwoodlee in Selkirk. Of all the structures mentioned above, only Crosskirk and Vaul were undisturbed during the excavations as the rest had been cleared off in the 19th century.

The research depicts that the other structures to have had its excavations in the recent times were Midhowe and Gurness in Orkney; however, neither of them had its complete execution to the floors of the broch. With this findings, Vaul remains the only structure to gather useful information about the drystone retaining walls of the broch (Yeomans, 1999). To the present, it is not definite and possible to determine the settled population in the towers. The early inhabitants of the broch were refugees resulted from the wars experienced by the more significant people of the farming populations; thus forming the towers as a sporadic occupied fort and not the domestic dome as presented earlier in the text. 

Conclusion

            In conclusion, drystone retaining walls have dominated across the United Kingdom to date. The scientific researchers have tried their best to seek to understand the primary reasons for their constructions with the example of broch as used in the research as the fortified towers. However, the findings mostly demonstrate these structures to have been occupied by initially aboriginals and the communities’ main activities being mixed farming with the evidence of the remains of barley plants and the domestic bones of the sheep, cattle and dogs as well as shag bird. It is though unclear to the reasons to why these population chose to inhabit these places but rather the assumptions made is that they seek refuge from the dangers of their enemies to protect their farm harvests and domesticated animals.

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