Hanson Bath & Portland have tunnelled down to a lower level of their Hartham Park underground quarry to bring Box Ground Bath Stone back into production for the first time in 50 years. Operations Manager Shahram Hakimzadeh explains how the development took place
The most dense and durable of the Bath Stones come from the Combe Down Oolite formation. And the most famous Combe Down Oolite stone is Box Ground Bath Stone. It was used for the construction of Longleat, the home of the Marquis of Bath, Lacock Abbey, owned by the National Trust, and the incomparable Palladian mansion of Witley Court (pictured on these pages), now preserved by English Heritage as the ruin it became following a fire in 1937.
The last quarry producing Box Ground Bath Stone survived the recessions of the 1920s and early ’30s and the Second World War but finally succumbed to the post-War hegemony of Modernism and closed in the 1950s.
Now, with stone having reasserted itself as the building material of choice for discerning clients and their architects, Hanson company Bath & Portland Stone have brought Box Ground Bath Stone back into production from their Hartham Park underground quarry just 200m from one of the original sources of the stone.
The Hartham Park underground quarry is what most people would call a mine but is referred to as an underground quarry by tradition because of its production of blocks of high quality building stone rather than coal or metal ore. The production of such blocks was historically considered a more skilled job than hacking coal or ore out of the ground, so the work was distinguished by calling it quarrying.
Hartham Park was brought back into production by Bath & Portland stone in 1999 for the extraction of Ground Stone from the Bath Oolite. It has been a resounding success with more than 50,000m3 of the stone sold since then for a wide variety of projects, including the prestigious Queen’s Gallery at Buckingham Palace and the redevelopment of Paternoster Square in London.
Hartham Park underground quarry was originally opened in the 1840s. The construction of a railway tunnel through nearby Box Hill by Isambard Kingdom Brunel for the Great Western Railway had identified the presence of Great Oolite stone beds in the Corsham area. Both the Bath Oolite and the Combe Down Oolite are part of the Great Oolite geological horizon laid down in the Middle Jurassic period 150million years ago.
Bath & Portland Stone did investigate the possibility of re-opening Box Quarry, but by then it was a Site of Special Scientific Interest (SSSI) and in any case had stability problems associated with having been over-extracted.
However, the close proximity of Hartham Park to Box Quarry meant the Box Ground stone could be accessed from Hartham Park and this was one of the reasons behind Hanson’s reactivation of Hartham Park.
Extensive site investigations were carried out in 2000 which proved the presence of substantial quantities of Combe Down Oolite stone beds between Hartham Park and Box Quarry. Petrological examination by the Earth Science Institute at the University of Wales using a scanning electron microscope provided the definitive proof that it was Box Ground Stone and a long-term development plan was formulated to bring the historical stone back on to the market.
Fast-forward to 2010 and the decision was made to start the preparatory work to access the Box Ground beds 8m below the existing quarry bed in Hartham Park.
The most suitable location for beginning the work was identified at a point where the quarry floor was 23m below the surface. The orientation, gradient and physical dimensions of the access tunnel all had to be worked out in relation to the water table.
The first cut into the stone floor of the quarry was made by a Fantini GU50 (pictured on the next page) in May last year.
The progress of the tunnel, descending on a gradient of 1:8, was continuously monitored by surveying and correlating back to borehole data to ensure that various marker horizons were intercepted at the correct intervals. It was particularly important because the boreholes had shown 3m intermediate beds between the floor of the quarry and the top of the Box Ground stone.
After about 60m, the top of the Box Ground stone was intercepted on 29 June, a date that will be long remembered at Hartham Park. There was an understandable sense of excitement and achievement among the dedicated team involved in the prospecting.
The tunnelling continued and on 14 July the first block of Box Ground Stone left Hartham Park underground quarry. It was less than a cubic metre. To reach it, 700m3 of rock had been removed.
By the end of August the full face of Box Ground Stone was exposed at a depth of 7.8m below the quarry floor and the process of levelling the access tunnel and undercutting the intermediate beds started. The working face exposed is 3.5m high with some beds as much as 900mm high.
Once the Box Ground Stone was reached, additional samples were sent to BRE in Watford for full physical property tests to be carried out in accordance with European EN standards and to enable the stone to carry the CE mark.
Such was the motivation created among the team members at the prospect of achieving such an historical feat that the whole exercise was completed in 16 weeks with a minimum of disruption, a testimony to the commitment and professionalism of those involved.
With the stone accessed, the next stage was to develop the Box Ground Stone beds on a commercial scale and reintroduce the stone into the market place in 2011.
There are now a hundred cubic metres of Box Ground Stone blocks in stock. Customers who would like to view it would be welcomed at Hartham Park. The stock level is intended to be sufficient to satisfy the initial demand because Bath & Portland Stone did not want to introduce it to the market without being sure they could fulfill orders.
There has already been enormous interest shown by those in the conservation sector, including English Heritage and the National Trust. There have also been enquiries from well-known architects in Bath, Oxford, the Midlands and elsewhere who are keen to use this historical stone in their new build projects.
The geology of Box Ground Bath Stone
Geologist and Consultant Dr Tim Palmer ([email protected]) explains the geology behind Box Ground Bath Stone
The buff and honey-coloured oolitic limestones generally known as Bath Stone occur in a belt that runs from just south of the city of Bath north-eastwards for about 12 miles to the vicinity of Corsham.
They have been worked for building stone throughout the last thousand years at least and can still be recognised in many medieval buildings, so the best of them clearly have good durability.
The main period of exploitation of Bath Stone for building was from the late 18th century until the middle of the 20th century. An article in The Builder magazine of 1895 reports visits to more than 40 working quarries.
Particularly productive were the underground quarries between Box and Corsham at the north-east end of the outcrop, in the vicinity of the railway tunnel under Box Hill. Box Ground Stone was the best of these stones.
The quarries that produced Box Ground are now difficult to access but they are still there and still cover a vast underground area. In the 19th century they supplied building projects throughout England and Wales and their easy access to the railway network kept their prices down.
Geologists have known since the 1950s that there are two distinct horizons the Bath stones come from, separated by a thinner horizon of uncommercial stone known as the Twinhoe Beds.
The upper horizon is known as the Bath Oolite. This level produces most of the commercial stone types available today just as it did in the past.
Underneath the Twinhoe Beds is a second, well-developed band of oolitic limestone referred to as the Combe Down Oolite.
It was from this level that the Combe Down Stone was quarried in the 18th Century, when vast quarries were being exploited south of Bath to provide the stone for the rapidly expanding and fashionable spa-town just across the river to the north. It is also this level that produced the famous Box Ground Stone.
When production of all the varieties of these stones was at its height, it was Combe Down – and Box Ground in particular – that were most strongly recommended for their durability and, hence, for use as mouldings in external work.
The reasons lie in the details of the microscopic structure of the stones from the Combe Down Oolite, which are better exemplified by Box Ground Stone than by any other.
When we look at Box Ground stone through a microscope, we clearly see it is made up of three different components. The original grains that accumulated on the sea-floor are spherical ooliths a quarter to half a millimetre in diameter and larger fragments of broken-up shells. The third component is crystalline calcite cement, which fills the spaces between the two grain types.
This calcite cementing material grew after the sediment had become buried and consolidated it into a rock.
Not all of these three components contribute equally to the strength and the durability of the stone. The ooliths are quite soft and crumbly and easily fall out from the surface of the cut stone to leave a network of little holes in the calcite cement. They contribute little to the overall strength of the stone.
The cement, in three dimensions, forms a rigid porous structure that looks rather like an Aero chocolate bar but still has the strength of a denser and more solid material.
The critical third component is the small fragments of broken shell, mostly between a millimetre and a centimetre across, that form ties within the structure of the stone, rather like the headers in a brick wall add strength to the wall surface and tie it back into the masonry behind.
These features are shown in Fig 2, a scanning electron microscope picture of original Box Ground Stone, and Fig 1, a conventional microscope thin section of the new Box Ground from the Hartham quarries.
Other stones from the Bath region have ooliths and crystalline cement but not so much shell debris to bind the stone together. This is more abundant in the stones from the Combe Down Oolite than the Bath Oolite.
In Box Ground Stone, the shelly material may be scattered throughout the stone or it may form bars of millimetre scale that are oblique concentrations of shell within an oolitic matrix.
These bars are a feature of the most durable examples of Box Ground Stone surviving from the 19th century. They are also present in the new Box Ground Stone that has been extracted from the Combe Down Oolite in the Hartham quarries.
Fig 3 shows a sample of Box Ground from a building of 1865 in a highly erosive environment close to a west-facing coast. It is remarkably well preserved with the shell bars having become slightly more prominent and conspicuous over the past 150 years.
Another feature seen in some of the less durable stones is small patches of lime-mud that form bridges between adjacent grains and cause moisture to wick between them into the stone. It lowers the mechanical strength of the stone and promotes weathering decay. But these mud bridges are always absent from Box Ground Stone.
I have described the relationship between the microscopic structure of various English limestone freestones and their architectural performance more fully in Limestone Petrography and Durability in English Jurassic Freestones. A further reference is Doyle, P, Hughes, T G , and Thomas, I (Eds) England’s Heritage in Stone, published by Stone Federation and the English Stone Forum, in which the structure and weathering of Bath Stones gets a section to itself. Copies of this book can be obtained through the English Stone Forum website (www.englishstone.org.uk).