Propuesta de sistema

Opposite page Kilve Chantry. The Chantry at Kilve, Somerset, UK is a group of buildings of the 15th century need-ing further investigation but at substantial risk of deterioration and local collapse. The illustration shows an applied restraint to the lean of the gable wall as a means of retaining it in position in anticipation of later masonry consolidation.

● Thermal stresses and frost

● Vegetation and root damage

● Rodent activity

● Wind erosion.

The two lists are subjective, and will vary between countries and locations. It is far more common to have a combination of elements involved rather than one individual cause. The sad thing about the first group is that, apart from education, there is little that can be done to change human behaviour.

Thus, leaving aside the human aspects, which have been discussed elsewhere, the effects of nature itself are now examined with appropriate engineering actions that can be taken to modify and ameliorate them, thereby ensuring a degree of stability to assist the long-term structural survival.

Planning for cyclic conditions

Stability is not found in nature, which runs its course in a series of cycles, and which we need to recognise and allow for in advance. These natural cycles affect our weather in the seasons of the year to a variable extent, but the years themselves run into a series of cycles of flood and drought, extremes of heat and extremes of cold. The records show that even today’s changes in climate pattern have had their precedents in the historical past. Recognition of the cyclical

nature of climate and its consequences on ruins and complete buildings alike is the first key to planning for stability. The forces of nature cannot be coun-tered, as these are too major, but mitigation plans can be made and potentially exacerbating interventions can be avoided. It is common-sense policy to assess each site with the same care we would expect to give to that for a new structure, and so far as circumstances allow, to install appropriate control measures.

The role of rain and water

In almost all climates, by far the greatest single factor in distress to structures is water in one form or another. Figure 2.1 illustrates the role of water and the various ways it constitutes a degradation regime of its own in a simple gable wall end. It is good to remem-ber that rain is usually a weak cocktail of acids, some due to fixation of atmospheric elements and others due to pollution in the atmosphere. Rain can fall with appreciable force due to wind action and act to scour out previously softened or weakened pockets of lime mortar. This phenomenon becomes more apparent on stone such as basalt than on an alkaline masonry, such as limestone or marble.

Of course, the building solution to these prob-lems would be to provide a properly maintained roof, incorporating collection and discharge features, 12 Conservation of Ruins

Figure 2.1 Rain degradation regime: a typical case.

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which would protect the structure below. Many illustrations in this book show how rapidly the loss of adequate roofing leads to a ruin situation. The nat-ural inclination of engineers is to encourage repair or re-installation of proper roofing, and thereby pro-tect the structure from further ravage. Where this is not possible, or desirable for philosophical reasons, buildings with broken wall tops or of very high cul-tural value or high sensitivity, ruins are best conserved by provision of a free-standing modern roof. A simple example made from readily available materials is shown in Figure 2.2, but numerous alternative con-figurations are possible and can be adapted to suit local conditions (see Chapter 5).

Even in the case of well advanced deterioration, it is possible to intervene in saving structural ruins by a combination of techniques shown in Figure 2.3 but critical to their ongoing survival is control of water, not only the roof but raindrop bounce, standing water and drenched vegetation. A critical zone exists which extends up 450 –500 mm above ground level around the wall perimeter. This is where the com-bined effects of direct rain, seepage, drift down the face, raindrop bounce, wind and often frapping from vegetation, all act together to denude the lower courses of mortar. Unfortunately, it is this zone which

is most critical to the lateral stability of the wall as a whole, analogous to a bottom hinge. Left unattended for long enough this denuded zone causes a lean out-ward of the whole wall, and is often accompanied by a curl of the wall face as mortar is dissolved within the external faces of the bedding joints. These two effects combine to cause a lateral shift at the top of the wall, which when a roof exists, is then resisted by the wall plate attachment. Unless a capping course or top per-imeter beam has been provided, the opposing forces initiate a crack at the top of the wall running into the wall core, which progressively loosens. Frequently this is accompanied by rain penetration at the eaves level as the geometry of the top of the wall distorts, and water trickles into the loosened core carrying particles lower down into the wall body, blocking the natural aeration of the core open-rubble work. Local-ised moisture build-up becomes evident internally, and eventually the build-up of pressure, acting on weakened bed joints, forces bulges to develop in the external face.

A typical reaction by engineers unfamiliar with historic construction is to grasp at underpinning as a solution to the problem. There are cases where this will be necessary, but normally addressing the repoint-ing of the wall from below ground level, progressively

Figure 2.2 Example of simple free-standing roof.

14 Conservation of Ruins

Figure 2.3 Wall alignment by jacking.

Figure 2.4 Schematic of conservation principle on end gable wall.

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to the top of the wall, may be all that is necessary and is the correct solution. Where it is feasible and prac-tical to do so, the wall can sometimes be levered back into place using adjustable props in the procedure shown in Figure 2.3. This may sometimes need to be accompanied by grouting of the core, as is described in Chapter 4. Small deformations evident in the realigned wall surface do not need to be corrected.

Identification

The above description of distress caused to the structure may be evidenced by the following phe-nomena (see Figure 2.1):

● Walls are out of plumb and leaning outward, a condition which can often be seen by eye alone

● There is an accompanying curl outward and concavity lessening towards the top

● Transverse walls show near vertical cracks, which widen towards the top

● Open or significantly degraded mortar joints often exist at the base of the wall

● Internal horizontal cracks develop near the top of the wall

● Internal damp spots may match external bulges

● If roofs are present, underpurlins or secondary timbers lodged onto the end walls often show corresponding cracks on the external face.

In addition to the roof question, which may not be appropriate for most care of ruins situations, certain fundamentals of approach require consideration for the site investigated. For instance:

● Ground should slope away from the base of walls for a minimum distance of 1.2 metres, to a fall of at least 50 mm

● On an upslope side, a cut-off drain or a graded swale should be provided, and be able to dis-charge away from the structure

● A clean surface of gravel or maintained turf should be provided to the graded margins

● In flood plain situations, some capacity for the rapid draining away of floodwater needs to be considered, in addition to provision for normal surface flows

● Pointing should permit free surface flow to cap-pings and vertical faces alike

● Hollow core grouting needs to be considered to back up cappings where necessary.

These recommendations clearly have archaeological implications which may make them difficult to imple-ment but may also be ultimately useful in clarifying

original contexts, facilitating legibility and enhancing presentation.

Remedial action

Wherever possible the desirable solution of roofing the ruin or structure should be given active consid-eration. In a reversible ruin situation, where a lim-ited reconstruction is possible and constitutes a valid option, then the following checklist is useful:

● Correction of lean and curl to the wall by the adjustable prop method. Larger structures can be tackled in the same way, using major shores to the walls, from which jacking forces can be applied

● Where static analysis suggests resultant forces may result in minor tensional stresses, provision of a non-obtrusive ring beam, or substantial con-tinuous top plate, should be considered

● Core grouting, in circumstances indicating partial core collapse, should follow the procedures given elsewhere with respect to first washing out sands and fines.

Ground movement

A vast amount of research has been carried out on the subject of soils, and a basic understanding of soils is key to understanding most of the phenomena encountered in the field.

From an engineering standpoint, soils are interest-ing as the foundation medium and are classified into two main groups:

● Non-cohesive or granular soils, such as sands and gravels, which are classified by grain size.

● Cohesive soils or clays, which are sticky and can be moulded. These are classified according to their swelling and shrinkage characteristics.

Apart from certain types of loess and some sediments, limited to arid zones, which can exhibit some strange features such as collapsing under load, granular soils are not overly concerning. Within the range of ruined structures likely to be encountered, problems such as bearing capacity will have been resolved long before.

Clays, on the other hand, are an ongoing source of problems as all clays move to a greater or lesser extent.

This is because of their ability to store water within the cellular structure of the clay particles, categorising them into ‘highly reactive’, ‘medium reactive’ or

‘low reactive’ clays. The clay particles are microscopic platelets, which change shape and dimension as water is stored or lost. The usual analogy is that a dry clay

platelet starts as a breakfast plate and changes to a soup bowl with the uptake of water, and this feature is responsible for the swelling and shrinking of a clay subgrade.

There is a variation in the extent to which various clays shrink and swell, dependent on the chemistry of the original parent rock. For example, clays derived from basalt sources are more reactive than from granitic sources, and the least reactive, for instance, are those derived from the earlier series of mudstones.

Water is lost from a clay subgrade due to evapor-ation to the surface, and to root action through the fine hair-like roots of major plants such as trees. It is necessary here to introduce the last contender in the soil types, namely silts. Silts are very small individual particles of the parent rock of spherical shape, but below the level of individual grain identification, and they behave differently to both clays and sands. Being very small they are carried to the top of the soil profile and normally form a band between the topsoil–

humus layer and the clay below. There is an ongoing transfer of materials from the surface down to the clay layer by worm action, and as part of this process silt gets transferred downwards, resulting in a mixed band of silty clay sitting directly over the clay proper.

These upper profiles provide a blanket to the clay below, protecting and limiting the extent of evapor-ation, and hence shrinkage. Depending on the effect-iveness of this blanket, the clay subgrade swells and shrinks to a greater or lesser degree, following the sea-sonal variation of wetting and drying in a lagging sequence caused by the time taken for water to store within the particles. The effect of both decreases with depth below the clay surface and hence founding deeper into the clay is preferred to shallower found-ing, as can be deduced from Figure 2.5.

It was well understood in antiquity, from an empir-ical basis, that founding should be on the firmest layer within the soil profile and hence finding poorly

founded structures or ruins tends to be unusual, although these do occur.

It is important when investigating a new site, par-ticularly if sited onto clays, to bear in mind the general geology of the area, the level of weathering and the location of the site within the local topography. Sites occupying hill sides or tops, apart from being closer to the underlying rock, will sit onto clays formed by in situ weathering (Figure 2.6), whereas those on val-ley floors would normally sit onto clays, which have shifted downhill under the combined influences of gravity, frost and water transport.

The tiny platelets comprising the clay particles are randomly organised within the parent rock and weathering in situ leaves the skeletal structure (includ-ing the microstructure) unaltered; hence the swell-ing and shrinkswell-ing forces are randomised. Those clays shifted downhill become remoulded to some extent, and achieve a partial orientation as the platelets slide together. However, clays laid down by freshwater action are fully oriented as shown in Figure 2.7. This is the worst combination from a structural aspect, since all the platelets now lie in the horizontal plane, resulting in vertical swelling and shrinking being maxi-mised. Ancient clays of saltwater deposit do not align as evenly as those laid down in freshwater lakes, as a result of ionic interaction.

16 Conservation of Ruins

Figure 2.5 Typical clay soil and seasonal vertical soil move-ments.

Figure 2.6 Deep in situ residual clay weathering showing residual basalt boulders.

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It follows from all of the above that an adequate blanket of topsoil and silt should be maintained as far as possible.

Trees, however pose a major threat to stability of ruins and standing structures alike, and large, vigor-ous, deciduous trees particularly so. Figure 2.8 shows the dramatic ground movements, which can be caused by the introduction of a large elm tree.

Figures 2.9 and 2.10 both show a simplified rela-tionship between vertical movement and the tree height-to-distance ratio. From these two examples a simple rule has been developed for new structures, that the closest they should be from a large tree is equivalent to its full mature height. Where a group of trees exist, or are intended, then this needs to be increased by 50 per cent, since the tree roots are in competition with each other and reach further in their search for moisture.

To avoid cyclic ground movement affecting ruins or structures founded onto clays, large trees should not be permitted close to them. Since the phenom-enon exists in reverse also, the cutting down of a large tree nearby may cause heave developing over the next three or four years. Any clearing of trees on

a clay subgrade should be carried out well in advance of other remedial or conservation work, or provision made for follow-up work during subsequent years.

The potential conflict here between structural neces-sity (or advisability) and site ecology and presenta-tion is obvious, and must be discussed and assessed by the conservation team.

Identification

A masonry structure protects the ground below from drying out and a reserve of moisture will have built up after a few years. During dry months, or particularly extended periods of drought, the loss of moisture to the atmosphere will cause a clay subgrade to shrink around the structure, which evidences itself first at the corners. This relative movement causes the corners to appear to drop, creating diagonal cracks radiating upward and outward from the mound edges.

Trees will cause a similar but unsymmetrical effect if located close to a corner, or may cause a central panel to drop if central to a wall. Typical cracking patterns associated with these three conditions are shown in Figure 2.8.

Figure 2.7 Clay particle orientation.

Figure 2.8 Distress cracking on clay subgrades.

Silts

Silts have a special characteristic of being dispersive in the presence of water, which, put simply, means that they undergo a sudden and drastic loss of strength.

They also have a second characteristic of being sus-ceptible to laitance when subjected to fairly minor vibration as water makes its way to the surface, this

action being the prelude to the silt soil turning into a thick fluid resembling porridge. The mere presence of silt should ring alarm bells in regard to any form of construction. Silt normally is associated with clays in only thin bands, but in certain circumstances can be found in deeper pockets up to 600 mm deep, sitting above a clay subgrade.

Identification

Correct identification of silt is vital to determining a safe foundation depth, as what appears as hard as mud-stone when desiccated can turn to a soupy consistency during wetter months when a high local water table persists. Most silts contain a proportion of clay par-ticles, and a simple jar test gives a quick indicative result. To do this, a sample is placed into a small glass jar, which is then almost filled with water and shaken.

The silt particles will settle within a few minutes, whereas the colloidal clay particles will stay in suspen-sion for some hours. Unless the silt proportion, which usually is lighter in colour, is only a very small percentage, then deeper founding is necessary.

18 Conservation of Ruins

Figure 2.9 Variation in settlement with distance.

Figure 2.10 Settlement versus distance/height ratio.

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Remedial action

● Provide means of intercepting winter seepage flows by cut-off drains or swales if archaeologically appropriate

● Discharge downpipes well away from buildings or ruins on their downhill side

● Renew footings or underpin to safer strata if ruin is reversible.

Slumping

The folding of soil as it slumps down a hillside is a common feature in steep hilled country, and these folds are normally referred to as ‘sheep tracks’. They are another example of the cyclic wetting up during periods of heavy rain, which, due to the added weight of water, can increase the density of the upper layers of soil by up to 60 per cent. As described above, the upper layers are composed of mainly permeable soils, whereas on clay or rock hillsides an impermeable interface lies below, which creates a slip surface. Slow draining of water causes the upper layers to increase in weight as water is absorbed, and further rain then acts to lubricate the slip surface.

When buildings are sited on the brow of slopes, any change to the drainage at the top of the slope will upset the status quo and can lead to an adverse effect.

When buildings are sited on the brow of slopes, any change to the drainage at the top of the slope will upset the status quo and can lead to an adverse effect.