Section 4.2 described mechanisms that may cause earth walls to start leaning. They include insufficient structural continuity between different materials, ground movement, failure of the foundations, or other inadequate structural members. Wall lean may be assessed by dropping a plumb line from the head of the wall, by checking the faces of a wall for tension cracks, or by looking for gaps between floors and walls at the upper levels of a building (Figure 5.10).
An engineering assessment should be
undertaken to gain an appreciation of the potential actions on the structure. Although a wall may
Water- Secondary currently be stable (i.e. it has not fallen over),
enlarged crack
crack system additional forces, such as wind or snow loading,
accidental impact or further building works, should be estimated to ensure the structure’s continuing stability. The cause of the initial wall lean must be determined and addressed before considering mitigation strategies. The mitigation methods should be designed so that their size, number and position are appropriate. Where it is impossible to enact some required mitigation strategies, or a ‘belt and braces’ approach is adopted, then the following may be useful.
Methods for the mitigation of wall lean are shown in Figure 5.11. These include building length ties, where tension members are fixed between opposite walls; battens placed into perpendicular walls; and the placing of buttressing or propping against leaning walls. Where such interventions are implemented, the strength of the wall in bending
Figure 5.9: Crack stitch diagram. Basgo, India should also be checked.
Masonry buttressing
Cracking along compaction planes Brick
stitches 13 m
Section removed for additional building construction
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Figure 5.10: Gap between floor and wall, indicating wall lean. Convento de San Juan, Ambel, Zaragoza, Spain. Courtesy of Cynthia Hendy
Spreader pad
Internal tie beam Internal tie rod
Buttressing Propping Spreader
plate C section and wire
Internal batten
Figure 5.11: Wall lean mitigation techniques
5.6.1 Tying of opposite walls
If opposite walls are leaning outwards, a tension member can be inserted between to tie them against each other and prevent further movement.
This is a popular strategy for conserving historic masonry structures. These tension members usually take the form of steel wires or rods, either running through holes in the walls and below ceiling level internal to the structure, or fixed externally. These members protrude from the walls and are fixed to
Figure 5.12: Spreader plates on the face of gable end.
Convento de San Juan, Ambel, Zaragoza, Spain
external plates that transfer the horizontal force of the leaning wall into the tie (Figures 5.12 and 5.13).
The plates must be stiff enough to transfer the load between the tie bar and the wall. They should be fixed into place with an earth mortar, to provide a positive connection to the wall. The forces on the walls must be estimated, and the tie members and spreader plates correctly sized and positioned to prevent either failure of the ties or punching of the plates through the wall.
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Figure 5.13: Spreader plates fixed to steel tension members inside the building. Convento de San Juan, Ambel, Zaragoza, Spain
Figure 5.14: Timber batten embedded at inside corner of a wall. Convento de San Juan, Ambel, Zaragoza, Spain
5.6.2 Corner battens
A popular historic method for mitigating wall lean is to fix a leaning wall to a perpendicular wall using battens at the corners. These battens transfer the horizontal component of the load from the leaning wall to the perpendicular walls. Although battens are potentially less effective than other methods,
they provide a crude yet quick way to prevent wall lean becoming wall collapse.
Timber battens are often seen in historic repairs (Figure 5.14). The timber is embedded inside the wall of a building and fixed to the leaning wall. The effectiveness of such repairs is questionable, because it is difficult to achieve a positive connection between the batten and the internal walls.
5.6.3 Buttressing and propping
Buttresses provide horizontal restraint against the lateral forces of a leaning wall. This is perhaps the most visually intrusive method of wall lean mitigation, but may be the most effective. Earth-building buttresses usually take the form of mass (rather than flying) buttresses, where the additional weight of the buttress acts by friction with the
ground to resist the lateral forces of the wall (Figures 5.15, 5.16 and 5.17). The expected lateral force should be estimated, and the buttresses sized and positioned accordingly.
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Figure 5.15: Buttress to support leaning wall. Aït Ben Haddou, Morocco
Figure 5.17: Adobe buttresses. Mari, Syria. Courtesy of Emma Cunlife
Figure 5.16: Rammed earth buttress on new-build rammed earth. Margaret River, Australia
It may also be possible to prop the wall
(Figure 5.18) if a satisfactory anchor point is present.
Propping involves transferring the horizontal thrust of the wall into the ground through angled members or frame structures. The anchoring member should be made sufficiently stiff to ensure that load is adequately transferred into the ground.
As with other repairs, the cause of the wall lean must be determined before making the decision to place a buttress. Care must be taken to ensure that the ground is not further loaded by the addition of a heavy buttress, because this could lead to bearing failure and further wall lean.
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Figure 5.18: Concrete prop added to mitigate wall lean. This prop has now been incorporated into a lean-to building at the face of the wall. Kasbah Asslim, Morocco
5.7 WATER
The damage caused to earth walls when they move from being unsaturated to being saturated was discussed in Chapter 4. This damage ranges from loss of strength and stiffness, through full saturation of the wall, when the wall behaves as a purely frictional material, to removal of material in a slurry. This section describes repairs that reduce the problems of water at a site, and the repair of earth walls where damage has occurred.
5.7.1 Water at the head of a wall
Water ponding at the head of wall was identified in Section 4.3.2 as a major problem for earth structures. This water can rest in puddles, which locally saturate the soil. Vertical channels are then eroded in the face of the wall where these puddles overflow. Erosion of earth structures is therefore greatly reduced if such puddles can be prevented.
Concrete ring beam
Large roof structure
Small roof structure
Brick capping Render the head of the wall
Figure 5.19: Methods for preventing water from entering the structure at the head of a wall
Methods for preventing water forming puddles at the head of the wall are shown in Figure 5.19.
A cap to the wall must be impermeable to prevent water flow, and shaped to prevent water collecting and potentially overflowing onto the earth wall. The capping should therefore be angled, to allow water to drain under gravity to its edge, and should overhang the wall to allow a drip to fall vertically to the ground. Where no drainage channels are provided, caps should not concentrate the flow, and should allow drips to form freely at their edge. Under no circumstances should caps accelerate the erosion of the face of the wall.
There are three alternative capping strategies:
with a sacrificial material, a permanent solution, or by covering the whole structure with an independent roof structure.
If a site is under conservation management, so that ongoing repairs are possible, then sacrificial
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Figure 5.20: Rendering the head of a rammed earth wall. Muktinath, Nepal
material should be used to cap the wall. This material is usually earthen, and can take the form of render (Figure 5.20) or preconstructed adobe tiles. Although this material will erode over time, its periodic replacement means that the body of the wall will not be damaged. An innovative solution has been used at the World Heritage site of Aït Ben Haddou in Morocco (Figure 5.21).
Here bamboo is fixed to the head of the wall, and earth is then piled on top. The bamboo extends beyond the head of the wall, allowing water to drip from the end of the culm to the ground.
Although continued wetting and drying of the bamboo means that it may eventually decay, it can be periodically replaced to protect the body of the wall.
A more permanent solution to protecting the head of a wall involves using materials that are not damaged by water. Traditional approaches in Mediterranean countries include interlocked semicircular baked clay tiles on roofs and at the head
Figure 5.21: Bamboo and earth matting at the head of a wall. Aït Ben Haddou, Morocco
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Figure 5.22: Protecting the head of the wall using fired clay tiles. Asslim, Morocco
of walls to direct water away from the face of the wall (Figure 5.22), or the use of flat stone such as slates.
A more modern approach used in Spain is to cap the head of historic rammed earth buildings with a concrete ring beam (Figure 5.23). The head of the wall should be brushed clean, and a membrane laid on the head of the earth wall. Reinforcing bars should be embedded into the head of the earth wall before the concrete is poured. The concrete forms should include drainage to ensure that no water reaches the body of the earth wall, and constructed with a drip groove to the outside of the structure to ensure that drips fall vertically to the ground.
In some cases it is not possible to erect a roof on an existing earth structure, either because a roof was never present, or because the current structure is too damaged to support a roof without significant alteration. Although erecting a structure over the whole site is visually intrusive, it may be a viable solution if the site is of significant cultural value.
The most famous example of this is the steel roof
Figure 5.23: Concrete ring beam at the head of a rammed earth wall, and drainage duct. Tower of Biar Castle, Spain
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Figure 5.24: A roof structure over archaeological remains. Mari, Syria. Courtesy of Emma Cunlife
constructed over the adobe Casa Grande National Monument in New Mexico, USA, erected in 1932.
Roof structures can also be temporary, erected over archaeological digs between seasons (Figure 5.24).
These shelters must be designed to ensure that an adverse local microclimate is not developed.
5.7.2 Preventing water damage at the base of walls Water damage at the base of earth walls was
discussed in Section 4.3.3. Damage is caused by an increase in the water content of the earth wall.
This water may come directly from rainfall, from inadequate roof drainage, from flow at the base of a wall, or by rising through capillary action.
Strategies to prevent water damage are shown in Figure 5.25.
To prevent water impact from above, the size or effectiveness of the eaves should be increased.
Eaves are used to protect the face of the wall from rainfall, and by extending their overhang it is possible to prevent rainfall impact at the base of the wall.
Figure 5.25: Methods for preventing water at the base of an earth wall
Increase size of eaves
Remove talus slopes
Reduce internal ground surface
Remove cement
render Diversion of
watercourse Drain
installation Splash base
Protruding roof drains
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Figure 5.26: Improved drainage from the roof. Insertion of shaped timber drains protruding from the head of the wall. Aït Ben Haddou, Morocco
Where water flow against the wall is causing damage, then the water should be diverted, and drainage installed. Any barriers to evaporation at the base of the slope should be immediately removed. These include talus slopes at the base of the wall, which are a consequence of erosion of the wall. Less permeable repairs to the base of a wall, such as those carried out in concrete, should be removed, because these trap water that has risen through capillary action, and lead to an increase in the water content of the wall.
Many types of earth building feature flat roofs.
These are usually highly effective, and are generally fixed with roof drains that protrude from the wall, thus ensuring that rainwater can flow through the roof drain and drop to the ground away from the wall. When a building is abandoned, or falls into disrepair, the roof drains may become ineffective. This may cause water to drain directly from the roof down the face, leading to saturation and removal of material. As a priority these drains should be repaired (Figure 5.26) or parts of the wall protected (Figure 5.27) to prevent further damage, and damage to the walls should be repaired
using the techniques described. Figure 5.27: Improved drainage. PVC pipe inserted and cement protection to the earth wall. Muktinath, Nepal
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