A guest blog by Steve Roughton
As a Chartered Civil and Structural Engineer, Steve has over 30 years’ experience defying gravity, designing, and investigating structures. After graduation he trained with Ove Arup, later managing his own small consultancy for over 20 years in Stourbridge. He has investigated structures that have sunk, tilted, bent, twisted, risen from the ground, partially collapsed, cracked through over-stress and chemical action. He has designed structures in earthquake zones, above old coal, limestone, salt and ironstone workings, previously infilled gravel and clay pits, and on sites where the deadly Japanese knotweed was prolific and taken account of buildings founded on ground with radon gas, methane and toxic chemicals. He writes here about his investigations of the Crooked House.
During my career I cultivated an interest in the forensic examination of buildings, compiling a list of over 50 British crooked buildings, delving into their history, their mystery and applying my technical knowledge to fully appraise and understand the structure - their character. The Crooked House at Himley was, and despite its sad demise, still is number one on my list. Many years ago, I visited the pub, not socially as many times previously, but on a mission. Explaining my structural interest, the landlord kindly allowed me to inspect the structure, including the basement. Using professional work contacts, I procured the geological logs for the adjacent shafts and inspected the mining maps of the worked seams beneath the building. Combined with the Oak Farm Estate records and maps, I have put forward a proposition to explain the mystery of the severe differential settlement, and the ability of The Crooked House to resist the subsequent severe distress.
Figure 1. Extract from 1840 map of Oak Farm Estate, ‘Shewing the get of the coal’
The Crooked House started life as the corn mill of the Oak Farm estate, powered by the Himley Brook. An 1840 map (figure 1) refers to it as the Old Coppice Mill (the new Coppice Mill being somewhat upstream on the Earl of Dudley’s land). The sinking of the Glynne Arms became a metaphor for the sinking financial crisis of the Oak Farm estate, both of which took place in the mid 1850’s. Mining has always been given as the cause for the dramatic subsidence. From first impressions, the dramatic mode of failure suggests the building subsided into the void of shallow mine workings. Looking at the geological section (figure 2), this is however not the case. The ‘thick coal,’ the significant seam in the Black Country was mined at depth, around 100m from the surface. Some accounts point to the fact that the building was situated on the edge of the Earl of Dudley’s estate and the coal mined up to the boundary resulted in the loss of support at one end. Again, this is not plausible. For deep mining, it is possible for a surface slope to occur at the edge of the subsidence ‘wave.’ However, from an examination of the mining maps, the ‘thick coal’ was mined beneath the building, making this less of a possibility.
Figure 2. Geological section
Changes in the mill pond outline and flooding are recorded on old local maps (1840-1870). The maps indicate, as the thick coal was excavated, the flooded area around the mill increased, which is indicative of general subsidence of the surface. At the commencement of the thick coal mining operations, the 1840 the map in figure 1 shows the mill stream flowing to the southern end of the working mill where it would have powered a waterwheel. The mill started to subside during extraction of the ‘thick coal’ (10m thick), generating extensive subsidence in the area; no doubt precipitating some differential movement. But any initial tilt may well have been exacerbated by the mill wheel weight, the vibration and water turbulence, which could gradually soften up the ground below the wheel, resulting in controlled and gradual slow settlement of the strata below the stream bed; one which the structure could resist.
From the map records, general subsidence of the area was evident in the 1840’s before the building became the Glynne Arms Public House. The mill was still operational in 1851. By 1852 the building was noted as The Glynne arms. Subsidence would have stopped the mill wheel turning, which could have been the reason for the change of use, giving rise to the birth of The Glynne Arms. In 1865, the building was noted in Estate records as ‘very much pulled by mining operations.’
But how did the building stay intact? Unusually, with this amount of tilt, most brick structures would have broken up and been demolished. To understand we must look at the foundations. In The Crooked House, the whole basement storey acts as a stiff, deep ‘raft-like’ structure which can tilt as an entity, supporting the structure over, rather like a ship listing at sea. The basement structure comprises brick arches with cross walls in both directions affording great rigidity (figure 3). This inherent lower-storey strength was responsible for saving the building. Above ground, the windows have distorted greatly but the brickwork remains intact, probably due to the presence of soft and flexible lime mortar, prevalent at the time. Additional support has been added by steel ties – girdles - which have been inserted to compress and hold the walls. To the southern end, three brick buttresses have been added to give lateral stability. Buttresses only function if founded on solid ground, which may have been difficult to achieve. Ironically, if founded on soft ground, buttresses will act as weights rather than their intended restraints, pulling the structure down further, increasing settlement.
Figure 3. The tilting mechanism
Thus to summarise, although the deep mining in the area may have caused general subsidence in the area, the tilt of the Crooked House was most likely due to localised subsidence due to softening of the ground to the southern end by water from the mill race and the vibrations of the water wheel, that allowed that end of the building to sink, whilst the strong basement of the building allowed it to remain intact.