10 C O N T E X T 1 3 3 : M A R C H 2 0 1 4 surviving for a long time without any specific deterioration of the condition of the structure. In conservation terms we have to consider the materials of construction as ‘flexible structures’.This is because of the lime mortars holding together the building blocks in the wall, which in many cases were themselves of comparatively soft construction. In a stone wall, the walls themselves, even if built of hard stones, retain a degree of flexibility because of the soft mortars. Although Roman cement, a very hard material, was certainly used in Roman buildings to cement soft and hard stones together, this hard, non-flexible form of structure exists generally where they were building directly on to base rock.This explains why so many of the buildings of that period have retained their stability. We need to look at the method of construction adopted for stone and brick walls to start to see why cracks occur, and why walls become distorted and do not remain vertical. The very form of construction that has given our historic buildings a long life gives an explanation of the situation that we now see in some structures where there are cracks and distortions to worry the engineer, who likes to see vertical and horizontal lines that satisfy him as to the stability of the structure.These walls were built on to comparatively soft ground (ancient builders never seemed to worry about looking for solid ground when building), so inevitably the walls moved. An additional factor is the use of soft bricks, and particularly the soft lime mortars, which did not set instantly, unlike modern building mortars. Continuous movement occurred during the building process and probably for up to 10 years. The use of these mortars meant that generally work was carried out only during the times when the weather was satisfactory, allowing the mortar to set, and not building in cold weather. Distortions occurred as the building went higher, with a general limitation, particularly in the church-type structures, of building no more than 10 feet high in any one year.The result of this was that, built on soft ground in soft construction, walls leaned and distorted out of line, with the inevitable result that hair cracks developed. These cracks generally allowed the wall to settle down to a state of equilibriumwithout major movement at the time, while leaving the wall vulnerable to movement for other reasons brought about by changes in ground conditions by water penetration and inevitable lack of maintenance. The result of this flexible construction was that when the builder came back the following year to continue building, he built the next height of wall vertically on a wall that was distorted and leaning.This section of wall then also moved as the materials slowly set, resulting in the distortions seen in many tall walls, such as Lincoln Cathedral’s west wall, and in many smaller churches. Another significant factor is that if a wall is built of solid construction without any window or door openings, there was a reasonable chance that there would be no significant unequal settlement. However, where openings occur the load on the foundation is very much lower than that from the solid mass of wall, and differential settlement occurred with cracks that were initially hair cracks opening up and appearing on the surface of the walls. The damage to this church was so great that there was a suggestion that the chancel arch should be blocked up.The cracks have since simply been made good and the building remains stable.
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