CHAPTER 4
DAMAGE TO EARTH BUILDINGS �
4.1 INTRODUCTION
Many people perceive earth buildings as being very delicate, and requiring a sympathetic climate and frequent maintenance to preserve their
appearance and structural integrity. This chapter outlines the actions by which earth buildings fail, and looks in particular at aspects that are specific to these materials. Damage to buildings, and their failure, are usually the result of changes that have occurred since construction. This chapter looks at structural issues, such as movement of the ground causing distortion of the building, and at issues with structural elements of the building being damaged. It then focuses on the effects of water on earth buildings, and shows how water in earth buildings can cause damage and failure of the building elements. Finally we discuss the presence of organic matter in earth walls, and damage caused by abrasion.
This book deals mainly with buildings that are predominantly constructed in earth, and therefore the failures are due to issues with the earth as a construction material. Half-timbered construction and other buildings where earth is not the main structural material are not covered here, although aspects of the discussion may be relevant to those construction types. The behaviour of earth buildings in earthquakes is complex and not fully understood, and damage caused by earthquakes is not specifically
discussed.
4.2 STRUCTURAL
This section deals with those structural issues that cause particular problems for earth buildings. For a full assessment of a structure, a competent engineer should be appointed. This section describes issues with ground movement, cracking, and structural element problems that present structural problems for earth buildings.
4.2.1 Ground movement
Ground movement can cause structural damage to buildings. Any non-uniform movement of the ground on which a building is constructed will be transferred through the foundations and cause distortion. This distortion of the structure may lead to cracking of brittle earthen materials. At one extreme, cracking may be visually disturbing but structurally safe, and at the other extreme may cause collapse of the building. In the terminology of modern civil engineering construction, the first can be regarded as reaching a serviceability limit state, whereas the second is an ultimate limit state.
Ground surface movement is caused by a change in the volume of the supporting soil. Some reasons for this are given in Figure 4.1. Where this occurs beneath a structure, it is likely to cause damage unless it is uniform across the plan areas of the structure (for example from large-scale mining subsidence). Differential settlement is of most
concern to any building, particularly earth buildings.
Surface movement may result from a change in water conditions, caused for example by raising or lowering the groundwater, or by the diversion of a watercourse.
The volume may also change through variations in loading of the ground, for example by increasing the number of storeys on a building, or by
construction or demolition of adjacent structures.
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4.2.2 Cracking
Where ground movement occurs, a building will attempt to change shape, and cracking is likely because of the soils’ inability to sustain tensile
stresses. Visual inspection of crack patterns allows the nature of a building’s movement to be determined, considering parts of the building, such as facades, to act as simple beams. If we consider three points along the base of a building (usually each end and a point in-between), then if the central point moves below the ends, the movement is known as sagging. If the centre is raised in relation to the ends, then the building is said to be in hogging.
There are different types of cracking, and simple descriptions of shear, bending and extension cracking patterns are given in Figure 4.2. Bending cracking occurs when the extension side of a wall (the bottom in sagging and the top in hogging)
Figure 4.1: Issues causing ground movement problems is unable to sustain tension, and thus cracks
Change in groundwater conditions
Joint opening Wall lean
Consolidation of fill material Undermining of foundations Settlement or subsidence of foundations
Differential settlement
Cracking pattern
Shear
Bending
Extension
Example structure Sagging Hogging
Differential settlement
Figure 4.2: Building-scale crack patterns expected for different patterns of differential settlement and movement
DAMAGE TO EARTH BUILDINGS 39
(Figure 4.3). Shear cracking is distinguished by diagonal cracks (Figures 4.4 and Figure 4.5), best thought of as a result of lengthening the diagonal when transforming a square to a parallelogram (see Figure 4.6). Bending crack patterns can be distinguished by cracks that are open at the top or bottom and closed at the centre height of the wall, and are longest over the point of maximum curvature (usually the centre of the façade, as shown in Figure 4.3[65]. Direct extension is unusual in practice, but would lead to the cracking pattern shown in Figure 4.2, with equal-width cracks
perpendicular to the direction of extension. Bending cracking patterns are more likely when there is no tie between perpendicular walls, and shear cracks are more likely with a more rigid connection to perpendicular walls, which prevents rotation of the ends of the wall.
Figure 4.4: Shear cracking caused by insufficient lintel and columns at opening. Jharkot, Nepal
Figure 4.3: Bending cracking caused by loss of stiffness of base of wall due to increased water content (darker region). El Badi Palace, Marrakech, Morocco
Figure 4.5: Shear cracking between gable and side walls. Jharkot, Nepal
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Shortened
Lengthened
Cracks open
Figure 4.6: Shear crack patterns
Figure 4.7: Structural crack enlarged by water
Earth buildings present an additional complication when identifying cracks and their linkage to
movement, because structure cracks may be increased in size by the flow of water (Figure 4.7).
It is therefore important to differentiate between structural and water-based cracks, and also to differentiate between structural cracks that have been extended by the action of water.
4.2.3 Structural element problems
Problems occur in an earth building when its structural elements do not act as designed.
This type of problem affects roof elements, beam and wall connections, perpendicular wall connections, openings, and wall-to-foundation connections (Figure 4.8).
Lintel
Tie at perpendicular walls Horizontal thrust from roof Beam loading
without wall plate
Tie between foundations and wall
1 – Opening/movement of joint
2 - Cracking
Lift joint Vertical joint
1 1 1 1
2
2
1
Ground movement leading to wall lean
Failure of horizontal member
Flexible joint
Beam bowing Surface spalling
Figure 4.8: Failure of structural elements
Roof elements
An ineffective roof structure may impose excessive horizontal thrust on the top of a supporting wall. This can occur when the joint between the angled and horizontal members of a roof truss is not sufficiently rigid, or if a horizontal tie member is ineffective or missing. This causes the angled members to exert a horizontal thrust on the top of the wall. This can cause the head of the wall to move outwards, and such deformation may cause subsequent problems, such as tension crack opening on the inside of the building, or lean or rotation of the wall. Where the wall moves, the vertical component of the roof load is then placed on a smaller section of wall, leading to increased stress on that section.
DAMAGE TO EARTH BUILDINGS 41
Perpendicular walls
In some earth structures there is a lack of connection between perpendicular walls. This is particularly the case where a different construction material is used at the corner (for example brick to rammed earth) with no tie between the two materials. Where there is no tie, any structural movement is not restrained by the perpendicular walls, and allows the joint between the two materials to open (Figure 4.9). In earthen buildings this is a particular problem, because it allows water ingress.
Figure 4.9: Opening of joint between dissimilar materials. Brick corners and rammed earth walls.
Villafeliche chapel, Spain
Rammed earth joints
Rammed earth is traditionally constructed in discrete formwork boxes, and the vertical joint between these blocks generally lacks structural continuity. If structural movement occurs, the butt joint between two rammed earth blocks may open
(Figure 4.10).
Rammed earth is formed by compacting earth in layers within formwork. When a
formwork box is filled, it may be moved vertically to produce another rammed earth lift . Insufficient compaction of each layer may lead to density banding. This appears at the face of the wall as a concentration of highly compacted soil at the top of a compaction layer, with a gradient of decreasing density through to the base of the layer. The banding is caused by the passage of the
Figure 4.10: Separation of two rammed earth blocks.
Villafeliche, Spain
impact wave through the body of the soil, which spreads out as it passes down through the soil.
As the wave spreads out, the compaction effort is spread over a larger area, and thus the energy imparted per unit area decreases. These areas of lower density present a potentially weaker region of the wall. Rammed earth is usually constructed by moving the formwork horizontally before moving up to ram the next lift . There is therefore a period of time between each vertical lift for the lower rammed earth to dry. When the next lift is placed, the different moisture contents and densities of the two adjacent lifts mean that there is little mechanical key between them, and thus the opportunity for transfer of shear stress is reduced, leading to a weaker plane between lifts (Figure 4.11).
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Figure 4.11: Failure along the compaction plane between lifts. Basgo, India
Openings
Openings in walls, for doors for example, usually make use of a lintel. The lintel acts in bending to distribute the load from above the opening to the structure on each side of the opening. When a building falls into disrepair, more valuable material such as stone lintels may be removed, or the timber from which a lintel is made may decay. Where a lintel is insufficient or absent, the earth above is forced to act as a beam element, and tension is induced in the bottom face, which may lead to cracking directly
Figure 4.12: Lintels removed and earth acting as an arch. El Badi Palace, Morocco
above the opening. Where the earth is unable to sustain this bending it fails, and usually falls. This causes the earth remaining to form a catenary-type arch acting in compression only (Figure 4.12).
Lack of, or ineffective, wall plate
Wall plates are used to spread roof or floor loads to a larger area of wall, thus reducing the bearing stress on a section. Such wall plates are usually stone or timber, and in historic structures may decay or be removed following abandonment of the building.
Where repairs have taken place, and no wall plate has been added (Figure 4.13), cracking or spalling of the face beneath the loading point can occur (Figure 4.14).
Figure 4.13: Joist loading with lack of wall plate causing in-plane cracking. Convento de San Juan, Ambel, Zaragoza, Spain
DAMAGE TO EARTH BUILDINGS 43
Figure 4.14: Lack of wall plate, leading to spalling of the wall face. Convento de San Juan, Ambel, Zaragoza, Spain
4.3 WATER
The poor resistance of earth buildings to water penetration is highlighted by comments made by a French officer defending rammed earth castles in Morocco in 1956:‘It’s not their guns I’m frightened of, but God help us if they use water pistols’[66].
An increase in the water content of earth buildings has been shown to reduce strength and stiffness, but to increase ductility. In Chapter 3 it was argued that the strength of unstabilised earth buildings is due to the phenomenon of suction, and that the unsaturated nature of air-dry earth causes an increase in strength above that provided through pure interlock.
The water content, and thus the suction, in an earth wall is controlled by the evaporation
and condensation of water vapour, which is a function of the relative humidity. If the relative humidity of the surrounding air is greater than that of the pore air in the wall, water vapour will condense in the wall. Infiltration of water into an earth wall can be determined by considering capillary flow and vapour pressure gradients.
If the rate of flow of water into an earth wall is greater than the rate of evaporation, then the moisture content of the wall will increase. As the water content of a wall increases, the wall loses strength and eventually approaches saturation.
When saturation of the wall is reached, the soil ceases to be unsaturated and behaves purely as a granular material, losing the ability to form a vertical face and forming a slope at its angle of repose (Figure 4.15).
1. Completely dry. Friction between particles. Particles rest
2. Unsaturated. Friction and attractive suction force between
3. Completely saturated. Friction between particles. Particles rest at at angle of friction. particles. Vertical face can be formed. angle of friction.
Figure 4.15: Dry, unsaturated and fully saturated angle of repose
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incised vertical runnels
Figure 4.16: Reasons for water ingress into earth walls
Figure 4.16 shows water-based problems in earth structures. These problems are related to a lack of upkeep of the building, for example the absence of a render coat; to removal of roof; or to a change in the environment in the vicinity of the building, for example where the wall is used to retain soil, or where the groundwater conditions have changed.
These mechanisms lead to an increase in the water content, and will cause increasing saturation if the water is not allowed to escape.
4.3.1 Rainfall onto the surface of a wall Earth buildings generally employ broad eaves to prevent rainwater from impacting on the wall. If these eaves are not present, rainwater can impact the wall, and this can cause erosion.
When water (such as a raindrop) hits an earth wall, it forms a bead on the surface of the wall, and some water is absorbed through capillary action.
The remaining water flows down the face of the wall. These beads can locally saturate the soil of the wall, and thus pick up small particles and transport them downwards. This results in a phenomenon of larger particles protruding from the face of the wall (Figure 4.17). Where such erosion occurs over a long period, the large particles become unsupported, and are then removed either by abrasion or fall under their own self-weight. If larger volumes of water impact the wall, more particles can be picked up, and the rate of erosion increases.
Water flow
Figure 4.17: Larger particles protruding from the face of an earth wall. Cob barn, Devon, UK
If the surface of a wall is not fully saturated, and water can flow down the wall, then infiltration through capillarity will be relatively low. It can be shown that for materials such as earth with a low pore radius (of around 0.0016 mm), water will take 1 hour to penetrate 13 mm into a wall and 2 weeks to penetrate 23 cm[49].
If water is drawn into the body of the wall through capillary action, it produces a region of increased relative humidity ahead of the wetting front. This means that the water vapour pressure in the pores in the centre is lower than at the wet face of the wall. Thus water vapour transfers to the centre of the wall, increasing the water content through growth in the size of the liquid bridges.
If this process continues, the water content of the wall will continue to rise, leading to reduced strength and stiffness of the wall, and the processes described above.
Water that flows onto the face of a stabilised earth wall will also be drawn into the body of the wall, causing an increase in the pore air relative humidity, but the cementing matrix of the stabilised earth wall will be unaffected by this increase in humidity. In a cement- or lime-stabilised wall, if there is additional cementing powder that is unreacted, the presence of water may cause a reaction providing further cementing material, and thus increasing the strength of the stabilised earth.
But if the cementing matrix is incomplete, regions of unstabilised earth between the cementing matrix can become saturated, and these will lose strength and stiffness by the mechanisms described above.
DAMAGE TO EARTH BUILDINGS 45
4.3.2 � Water at the head of a wall and flowing down the face
If damage to a roof is only minor, then the flow of water down a wall may be minimal. In this situation, cast-type structures are found (Figures 4.18 and 4.19).
This is material that has first flowed into a slurry, and then dried to leave mounds of fine material on the face of the wall. The presence of these structures reveals the downward movement of material, and thus indicates past water flow. These structures are formed by the slow movement of water down the face of a wall, for example a bead that is able to pick up material and transport it in solution down the wall, until the bead evaporates and the material is returned to the wall. The low volumes and relatively high speed of this water mean that only the smallest particles of soil can be removed from the face.
Where the roof of a building is insufficient or absent, or the wall is retaining soil, then water may pond at the head of the wall. In this situation, water is absorbed into the body of the wall by capillary action and gravity. If the rate of water arriving at the wall is greater than of that being absorbed, then a puddle will form on top of the wall. The wall directly beneath the puddle will become saturated, and the water in the puddle will seek a low point from which to flow away under gravity. Where this water reaches the edge of a wall, incised vertical runnels form (Figures 4.20 and 4.21). Complete erosion of a wall occurs when adjacent runnels join to remove even more material. Where this occurs, the face of the wall is reduced to the angle of repose of the partially saturated material, such as the top section of the wall in Figure 4.22.
Figure 4.18: Small cast structures. Convento de San Juan, Ambel, Zaragoza, Spain
Figure 4.19: Larger cast structures. Agdz Kasbah, Asslim, Morocco
Figure 4.20: Large incised runnel, caused by (recently repaired) defective roof drain. Kagbeni, Nepal
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Figure 4.21: Incised vertical runnels. Basgo, India
Figure 4.22: Heavily eroded face; the top section of the wall is the angle of repose of the partially saturated soil.
Daroca, Spain
4.3.3 Water flow at the base of a wall
If a constant supply of water is provided at the base of the wall, it may become saturated and behave as a frictional material. This means that saturated material is removed from the foot of the wall, which then forms a slope at the angle of repose of the saturated material (Figure 4.15).
This reduces the effective width of the wall, which can lead to overturning or to failure in unconfined compression. Water flow at the base of a wall is caused by damaged (Figure 4.23) or inadequate drainage, such as at the edge of a paved road, where water drains towards the edge of the street
This reduces the effective width of the wall, which can lead to overturning or to failure in unconfined compression. Water flow at the base of a wall is caused by damaged (Figure 4.23) or inadequate drainage, such as at the edge of a paved road, where water drains towards the edge of the street