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Supporting Co/urn ns
Engineering philosophy
For historic buildings
Brian Morton discusses three recent cases where
common sense won the day.
Within the last month there have been three occasions where conservation engineering philosophy has either saved, or potentially saved, clients of ours substantial sums. The basic philosophy behind these required an understanding of the way the buildings were built by diagnosis and analysing the problem of making decisions. In two cases, the clients had previously been told that underpinning would be needed and in the third case demolition of cottages had been proposed. A simple philosophy associated with cracking from structural movement of historic buildings is that if there is a problem there must be a cause. It is then a question of analysing the cause.
It is a matter of fact, of course, that if a building suddenly moves and new cracks appear one is looking for recent changes associated with the building. For example, has the roof spread suddenly due to the failure of the connections between rafters and ties in a roof truss, or has the ground suddenly been inundated with water or has it suddenly dried out? These causes for new cracks can be identi fled fairly easily.
An example I was involved in some years ago was where the central isolated chimney in the middle of a Georgian house dropped about 75mm over a period of about one month. This was very puzzling so some trial holes were dug alongside the chimney. Cavities in the clay/sand beneath the chimney were found.
A chance remark by the gardener that a local farmer had recently contracted with the water company for extraction of water on a large scale from the surrounding area resolved the cause. We thus discovered that the water table (standing level of water in the ground) had dropped approximately 700 mm in a very short period. This had allowed the soil to
dry out causing the settlement to occur.
One of the three cases referred to above concerned a very substantial Victorian extension to a Georgian house with a three storey extension. Dry rot had attacked the upper floors and these had been removed by a previous owner. I was asked to look at the structure because the builder who had known the building for some years and who had been retained by the client had said that a particular crack alongside the front door was an indication of settlement, that underpinning was necessary and he was simply about to embark on it. An Architect friend of the owner asked me to look at it and I could see absolutely no reason for this underpinning. My view was the crack, admittedly quite substantial, had almost certainly occurred when the upper storeys had been taken off by literally disturbance to the structure. There were no signs of recent movement.
The photograph shows the elevation of a timber-framed Medieval house which had been fronted by the Georgians basically a flexible timber structure behind with a lime mortar Georgian wall on the face. The building had been left derelict for some years. At the rear the roof had started to collapse pushing the side walls out and clearly parts of the walls of the rear part of the structure were beyond repair.
I was called in to give a second opinion after an engineer’s report and specification had indicated that underpinning was necessary to the whole structure, in simplistic terms, because the building was sitting on fill.
The philosophy of conservation engineering then takes effect. If one simply conceives that when the building was built it was effectively sitting on a very soft material, then it will be
realised that once this sponge like material had compressed there would simply be no reason for it to compress any more. Thus, in situations like this, if there are no signs of recent movement it is safe to assume that any cracks in the structure occurred probably within the first ten years of the life of the building. Indeed, in most cases the cracks occurred during the building period as the soft lime mortar, taking longer than current mortars to dry out, had allowed this movement to take place with hair cracks forming, thus allowing the building to articulate.
As can be seen from the photograph, there are diagonal cracks at either end of the elevation which relate to the heavier loads that occur due to the weight of the gable end brickwork at either end of the building inevitable differential settlement.
This simple situation where I had indicated that underpinning was unnecessary had a sequel in that I wrote to the Local Authority indicating that, in my view, not only did the main building not require underpinning, but where the walls had collapsed at the rear we could rebuild off the existing foundations, they responded by asking their own consulting engineer to look at the situation. I rang the engineer and explained to him my philosophy which he could not simply accept. He said that he would bring a Mackintosh ‘probe’ to site to test the ground.
This Mackintosh probe is simply an iron bar with a bullet shape at its bottom end which is hammered into the ground with a known weight. The number of blows it takes to knock the probe into the ground gives an indication of soil strength. This machine is not reliable in the circumstances of an existing foundation because it is not designed for this purpose. it is designed to be used in virgin ground
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Context 55 September 1 997

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