FOUNDATION ISSUES

Two distinct issues that may cause a building to deform because of problems with the foundations were outlined in Chapter 4. These are failure of the foundations, which can be remedied by foundation improvement, and changes in soil behaviour,

necessitating ground improvement.

If a building appears to be deformed or cracked, the movement should be thoroughly assessed. This will determine the nature of the movement, and ascertain whether the foundation or the ground is still moving, with the potential to cause further damage. It is essential to take steps to mitigate ground movement prior to any further interventions. If building movement is not halted, then further repairs to the structure (such as crack filling) are likely to be ineffective. Repairs to foundations and ground improvement can be resource intensive and costly, and if not performed correctly may cause further damage to the structure.

Such repairs must be well-designed, and undertaken by professional engineers.

There are many reasons for damage to foundations that necessitate remediation. Some were outlined in Section 4.2.1, but they are usually independent of the type of building construction, and so foundation improvements are only briefly discussed in this book. Foundation improvement may be required when the foundation is not

strong or stiff enough, when the use of the building changes, or when the properties of the soil change.

There are two types of foundation improvement:

replacement and repair.

Foundation improvement is achieved either by increasing the depth of the foundations, so that a stronger soil stratum is reached, or by increasing the width of the foundation, so that the building load is spread over a larger area and thus the bearing stress reduced. Specialist advice should be sought when considering foundation improvement. Measures that may be considered include repairing eroded or undermined foundations, or inserting new piles or extra minipiles. Many historic earth buildings are built on rubble foundations, and if these have been eroded or undermined, the rubble should be rebuilt as originally constructed to ensure that water is not able to move through capillary action into the base of the earth wall.

The ground beneath a structure may be improved by compensation or jet grouting, in which an expansive gel or cement–water mixture is injected into the ground. This expands in the soil, increasing its volume and raising the profile of the ground surface, before setting and leaving a strong foundation. This can be an expensive solution, and is likely to be justified economically only for a significant structure.

 As described in Chapter 2, the mechanical properties of a soil are a function of the water content and the pore water pressure. The properties of the soil beneath a structure can therefore also change if the water content and pore pressure change. This can happen when the groundwater distribution in the vicinity of a structure changes, with resultant effects on the strength and stiffness of the soil underneath it. The groundwater flow may be complex, and must be well understood before any intervention measures are implemented. If a building – or part of a building – has settled because the strength or stiffness of the soil has

CONSERVATION STRATEGIES 57

reduced, this settlement may be arrested by altering the groundwater profile, perhaps by installing drainage, or by diverting or containing a watercourse, or by providing pumps to remove water.

5.5 CRACKS

Cracks form when an earthen material’s tensile or shear strength is exceeded. The reasons for crack formation in earth buildings were given in Section 4.2: they include issues with the ground, problems with structural elements, and damage by water. In earth buildings, water can collect in cracks: this may cause saturation and further loss of soil, and thus increase the size of the crack.

The reasons for cracking should be

determined before undertaking any remediation work, because it is usually a symptom of other problems with the structure that need to be addressed. These may include foundation and ground issues, as described above in Section 5.4, or the repair of structural elements that are not constructed with earth.

Patterns of crack movement should be assessed to determine structural movement and appropriate repair strategies. Cracks should be monitored over a period of time to assess the relative movement of cracked sections of wall, and to determine whether the cracking is continuing, or is historical (Figure 5.3). Many suitable proprietary devices are available for this purpose. Periodic monitoring (for example weekly) should be undertaken at the same time of day, to establish whether the structural movement is permanent, or is merely a result of the thermal expansion and contraction of the structure through the year. Patterns of crack movement should be assessed to determine structural movement and appropriate repair strategies implemented.

Where the cause of cracking has been addressed completely, and no further structural movement will occur, then a crack can confidently be filled (Section 5.5.1). If there is a risk of

structural movement, or stress transfer across the crack is possible, then the cracked area needs to be reinforced and repair techniques such as stitching (Section 5.5.2) may be attempted.

5

1

Figure 5.3: Example crack monitoring, Ambel, Spain. Position of monitoring points and cracks on the inside of the structure: yellow and blue indicate opposite walls; current gap refers to clear distance between gable wall and adjacent floor. Photograph shows external face of the building with some cracks visible.

5.5.1 Crack filling

When the reason for crack formation has been addressed, and monitoring suggests that no further movement is taking place, then filling of cracks is appropriate. Two methods are described here.

Cracks narrower than 50 mm should be filled with an earth-based mortar. Earth mortar shrinks as it dries so larger cracks require the placement of dried earth bricks in an earth mortar (the column method), otherwise the filling will shrink away from the crack faces.

Small cracks in earth walls should be filled as follows. Make a mortar using earthen material similar to that of the wall being repaired. Brush loose material from the crack, and use a paintbrush to douse the inside faces of the crack with water.

The earth mortar should be made wet of optimum (see Figure 3.8) so it can be forced into the crack using a plasterer’s trowel, and well compacted. Fill the crack from the base upwards. The mortar will shrink and produce smaller cracks as it dries, and these cracks should be filled with further mortar.

Fill larger cracks using the column method (Figure 5.4). Brush the crack clean, and widen it if necessary so that a column is cut into the wall around the area of the crack. Measure this column, and then construct blocks of the correct size to fit into the column. These blocks should be made of the same material as the wall (for example as

small rammed earth blocks, cob blocks or adobe), to ensure that the filling material is of the same density and has the same mechanical properties as the original wall The blocks should be fully dried before they are inserted in the wall. To lay the blocks, first douse the inside faces of the column with water, and lay an earth mortar into the column. Wet the blocks using a brush, and then place them firmly into the earth mortar. Fill the column with the blocks, from the ground upwards.

When the top of the crack is reached, it should be properly protected (as described in Section 5.7), and if necessary the face of the wall should be rendered (Section 5.8.1).

Crack-filling repairs are designed both to prevent further water ingress, and to improve the appearance of the building. Structural continuity is not restored, so if further movement occurs, the fill material is likely to become dislodged. This soft crack filling, in which no attempt is made to  join the two sides of the crack together, allows

movement of the structure to be monitored.

Other materials such as stone (Figure 5.5) and cement (Figure 5.6) have been used, but earth is recommended. Where other methods to prevent building movement have not been employed, or a ‘belt and braces’ approach is required, then the stronger crack stitching (Section 5.5.2) should be employed.

Figure 5.4: The column method

Figure 5.5: Crack filling using stones. Muktinath, Nepal

1. Original crack

2. Cut back to form chase 3. Manufacture blocks 4. Place blocks in chase

Figure 5.6: The Cracked Tower at the Alcazaba, filled with a cement mortar. Granada, Spain

CONSERVATION STRATEGIES 59

5.5.2 Crack stitching

 A crack should be stitched either when there is a possibility for the requirement of stress transfer across the crack, and sections of wall cannot act independently of each other, or where measures have been taken to prevent further damage from occurring, but a ‘belt and braces’ approach is being adopted. An engineering assessment of the stability of the structure locally should be undertaken to assess the potential forces across the crack to allow the number, size and position of stitches to be determined.

Stitching can be viewed as a crude stapling operation of material across a crack. It will provide some structural continuity using material similar to that of the wall, which means that the material properties such as stiffness and thermal expansion are matched. To create a stitch, cut a horizontal chase into the face of the wall, across the crack (Figure 5.7). Cut a deeper section at each end of the chase to provide hooks into the wall. These sections may need to be propped if they are cut too deeply, and take care to ensure that the stability of the wall is not affected in cutting the chase or the hooks.

Figure 5.7: Crack-stitching repair technique

Crack

Bricks

Cut chase

Vertical wire mesh

Prop 5

Figure 5.8: Crack-stitching repair. Basgo, India

Measure the chase, and manufacture blocks that can be inserted, as with the crack-filling techniques.

Brush the interior of the chase clean to remove loose material, and douse it with water. Place earth mortar at the bottom of the chase, and lay the first layer of blocks onto this. Then add further layers, and if necessary insert vertical mesh reinforcement into the stitch, as shown in Figure 5.7. When the top of the chase is reached, pack a final layer of mortar into the gap. This technique has been successfully employed at the castle of Basgo in northern India (Figures 5.8 and 5.9)^[69].