The Features of Stone and Brick-built Arch Bridges
Stone Built Bridges
Materials
1. Stone
As far as possible, local stone would have been used for the construction of the bridges. Transport of stone from quarry to building site was not only difficult and time consuming but came at a significant cost. Access to local sources of suitable stone (mainly Jurassic limestone) was easier in the southern section of the canal rather than the north, and as a result the use of stone as a bridge building material is more common in the south.
The availability and choice of stone would have a profound affect on the appearance and design of the bridges it was used to build. The local limestone in the southern Cherwell valley is well bedded (layered) and tends to naturally split into flat blocks when broken. This produces stone which is well suited for rubble walling, but gave far fewer large blocks for finer ashlar masonry.
Materials
1. Stone
As far as possible, local stone would have been used for the construction of the bridges. Transport of stone from quarry to building site was not only difficult and time consuming but came at a significant cost. Access to local sources of suitable stone (mainly Jurassic limestone) was easier in the southern section of the canal rather than the north, and as a result the use of stone as a bridge building material is more common in the south.
The availability and choice of stone would have a profound affect on the appearance and design of the bridges it was used to build. The local limestone in the southern Cherwell valley is well bedded (layered) and tends to naturally split into flat blocks when broken. This produces stone which is well suited for rubble walling, but gave far fewer large blocks for finer ashlar masonry.
Parapet wall (Bridge 227) : Coursed Rubble Walling
Typical coursed rubble walling using local limestone from the lower Cherwell valley. The stone naturally breaks into flat pieces and is bedded in lime mortar in rough courses. The size and thickness of the stones vary, with the occasional larger block incorporated (probably a discarded piece of ashlar masonry)
(Image: Tony Prothero/ CCAG archive)
Larger masonry blocks with careful shaping and a fine surface finish (Ashlar masonry) were commonly only used where structural integrity or strength were paramount, or where its aesthetic qualities were important. This tended to be reserved for the arch voussoir and imposts, string courses, quoins and some coping.
The stone for the rubble masonry and ashlar work sometimes came from different quarries, but a single source was preferred.
The stone for the rubble masonry and ashlar work sometimes came from different quarries, but a single source was preferred.
Arch voussoir (Bridge 223) : Ashlar Masonry
The finely shaped and finished ashlar blocks of the arch contrast with the coursed rubble walling of the parapet above. This combination of stone finish is typical of the southern Oxford canal bridges and not only provides structural integrity to the arch, but gives a visually pleasing, but unfussy, appearance.
(Image: Tony Prothero/CCAG archive)
2. Mortar
Rubble walling was usually laid with lime mortar, which was a mixture of either quick lime or slaked lime, sand and water. Quick lime was produced by roasting crushed limestone in lime kilns. The resulting quicklime was then hydrated by adding water to give slaked lime.
Ashlar blocks were laid with lime putty. This was a mix of high quality slaked lime, water and linseed oil. Powdered chalk was sometimes added if an accentuated white joint line was desired.
Lime mortar has the property of drawing moisture out of the porous stonework, such that it evaporates from the surface of the mortar rather than from the surface of the stone. This protects the stonework from spalling (where the stone surface flakes off) and efflorescence (staining of the surface due to the deposition of soluble salts).
3. Puddled Clay
Puddled clay was often used as a waterproofing layer on top of the arch voussoir and barrel. This prevented water penetrating from the deck surface above and over time damaging the arch soffit stonework.
Rubble walling was usually laid with lime mortar, which was a mixture of either quick lime or slaked lime, sand and water. Quick lime was produced by roasting crushed limestone in lime kilns. The resulting quicklime was then hydrated by adding water to give slaked lime.
Ashlar blocks were laid with lime putty. This was a mix of high quality slaked lime, water and linseed oil. Powdered chalk was sometimes added if an accentuated white joint line was desired.
Lime mortar has the property of drawing moisture out of the porous stonework, such that it evaporates from the surface of the mortar rather than from the surface of the stone. This protects the stonework from spalling (where the stone surface flakes off) and efflorescence (staining of the surface due to the deposition of soluble salts).
3. Puddled Clay
Puddled clay was often used as a waterproofing layer on top of the arch voussoir and barrel. This prevented water penetrating from the deck surface above and over time damaging the arch soffit stonework.
Construction
The site for the bridge was surveyed and marked out by a skilled surveyor and his assistants. If the bridge was to carry a road across the canal, the roadway might have to be temporarily diverted.
Foundations for the abutments would be laid using rammed rubble, perhaps underpinned by wooden piles if the ground was particularly wet or unstable.
The abutments would then be raised to the height of the imposts on either side. This would involve building the abutment side walls, to retain the fill material and to provide support for the subsequent arch masonry.
Before the arch could be built, a temporary framework would have to be constructed to support the voussoir until they were self-supporting. This framework, known as centring, was erected by the carpenters and often dismantled and re-used on subsequent bridges. Once the centring was erected, the voussoir were laid onto it until all the arch stonework was in place and self supporting. The side walls were often also built, to give the arch rigidity, before the centring was removed.
Arch Centring: Constructed of timber, the framework supported the arch stonework until it was self supporting. It was removed and often re-used for the next bridge.
This example is on a larger scale than the canal bridges, but the principle is the same.
(Image: Photographic Collection from Australia - Arch Ring and Falsework, 1932Uploaded by Oxyman, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=22350509)
Once the basic arch structure was completed, the top of the arch barrel was waterproofed with puddled clay and the fill material (a mix of small stones and clay) put in place. Finally any parapet walling and coping would be completed, and the roadway would be surfaced, often with a cobbled finish.
Depending on the bridge profile and height, an approach ramp would also have to be constructed and surfaced.
Like many other aspects of canal construction, bridge building involved a large and varied workforce; from quarrymen, masons and labourers to handle the stonework, carpenters to build the centring framework and navvies for the earthworks.
Masonry Types
Rubble Masonry:
Random Rubble
1. Un-coursed
Stonework of variable size and shape, arranged to fit by eye with no coursing
Random Rubble
1. Un-coursed
Stonework of variable size and shape, arranged to fit by eye with no coursing
2. Built to course
Similar to un-coursed except the stonework is roughly coursed
Square Rubble
1. Un-coursed
Stones have straight beds and sides but are arranged in an irregular pattern
1. Un-coursed
Stones have straight beds and sides but are arranged in an irregular pattern
2. Built to course
Stones have straight beds and sides and are levelled to give courses of
varying heights.
3. Regular course
Stones have straight beds and sides, and are levelled to form courses
of varying depth but consistent height in each course.
Miscellaneous type Rubble
1. Polygonal rubble masonry
The stones are hammer finished to produce an irregular polygonal shape.
This can be rough or close picked work depending on how well the stones fit together.
1. Polygonal rubble masonry
The stones are hammer finished to produce an irregular polygonal shape.
This can be rough or close picked work depending on how well the stones fit together.
2. Flint rubble masonry
Flints or cobbles of varying sizes are used either coursed or built to course.
The strength can be increased by using a lacing course of long thin stones or bricks
at 1 to 2m vertical intervals.
3. Dry rubble masonry
Mortar is not used in the joints, but much skill is required in its construction.
This is only used for non-load bearing walls.
Ashlar Masonry:
In ashlar masonry, square or rectangular stone blocks are dressed and have extremely fine bed and end joints.
Mortar is not used in the joints, but much skill is required in its construction.
This is only used for non-load bearing walls.
Ashlar Masonry:
In ashlar masonry, square or rectangular stone blocks are dressed and have extremely fine bed and end joints.
1. Ashlar fine tooled
The highest quality masonry. The beds, joints and faces of the blocks are chisel
dressed to remove all unevenness and produce perfect vertical and horizontal joints.
2. Ashlar rough tooled
The beds and sides are chisel dressed but the exposed face is rough tooled.
3. Ashlar rock faced
The exposed face of the stone is not dressed, to leave a rough "natural" appearance.
The beds and sides are chisel dressed but the exposed face is rough tooled.
3. Ashlar rock faced
The exposed face of the stone is not dressed, to leave a rough "natural" appearance.
4. Ashlar chamfered
A special type of ashlar rock faced where the strip around the perimeter of the
exposed face is chamfered.
5. Ashlar block in course.
An intermediate type between ashlar and rubble masonry. The faces of each block are
hammer dressed, but the vertical joints are not so straight and fine as ashlar work.
An intermediate type between ashlar and rubble masonry. The faces of each block are
hammer dressed, but the vertical joints are not so straight and fine as ashlar work.
Arch Shapes for Bridges
The arch supports the the bridge deck and superstructure by transferring the load to the abutments.
The bridges on the Oxford canal generally use three types of arch and variants:
1. Semi-circular arch
The arch shape is a semi-circle (180 degrees)
2. Segmental arch
The arch is less than 180 degrees.
3. Elliptical arch.
The arch is half an oval.
4. Stilted arch.
This is a variant of the classical arch shapes, where the arch is raised on vertical supports
thus increasing the headroom. Probably the most common form used in canal bridge
construction.
The Surviving Stone Bridges
Of the 238 current Oxford canal bridges (including aqueducts in the northern section), only 15 are built of stone (excluding modern examples). These are all found within the southern section of the canal, the most northerly being Twyford bridge (177).
The surviving stone bridges are:
Of the 238 current Oxford canal bridges (including aqueducts in the northern section), only 15 are built of stone (excluding modern examples). These are all found within the southern section of the canal, the most northerly being Twyford bridge (177).
The surviving stone bridges are:
Bridge Number |
Bridge Name |
Location |
Carrying |
Arch Shape |
228 |
Yarnton Road Bridge |
Kidlington |
Road |
Stilted segmental |
227 |
Buller's Bridge |
Kidlington |
Field access |
Stilted segmental |
223 |
Sparrowgap Bridge |
Kidlington/Thrupp |
Field access |
Stilted segmental |
216 |
Old Enslow Bridge |
Enslow |
Footpath (originally road) |
Stilted segmental |
214 |
Pinsey Bridge |
Kirtlington |
Field access |
Elliptical |
213 |
Pigeon Bridge |
Kirtlington |
Road (Mill access) |
Elliptical |
210 |
Northbrook Bridge |
Northbrook |
Field access |
Stilted segmental |
209 |
Dashwood's Bridge |
Northbrook |
Field access |
Elliptical |
208 |
High Bush Bridge |
Rousham |
Field access |
Stilted segmental |
207 |
Cleves Bridge |
Rousham |
Field access |
Stilted segmental |
206 |
Heyford Wharf Bridge |
Lower Heyford |
Footpath (originally road) |
Stilted segmental |
190 |
Aynho Bridge |
Aynho |
Road |
Stilted segmental |
188 |
Aynho Weir Lock Bridge |
Aynho |
Field access |
Elliptical |
187 |
Nell Bridge |
Aynho |
Road/Towpath |
Stilted semi-circular |
177 |
Twyford Bridge |
Twyford |
Road |
Stilted segmental |
They all have listed building status (Grade II) except for Nell Bridge (187), which has been significantly altered with the upper portion truncated and a modern road bridge built over it.
The majority of the bridges conform to a standard structural form with minor variations in the arch shape and type of rubble walling used. Most have stilted arches which provides greater headroom and are either segmental or elliptical in shape which gives greater width to accommodate the towpath as well as the canal. The exception to this is Nell bridge (187), where the towpath crosses the bridge, and a semi-circular arch shape is sufficient for the canal only (this being the simplest and cheaper option).
The majority of the bridges conform to a standard structural form with minor variations in the arch shape and type of rubble walling used. Most have stilted arches which provides greater headroom and are either segmental or elliptical in shape which gives greater width to accommodate the towpath as well as the canal. The exception to this is Nell bridge (187), where the towpath crosses the bridge, and a semi-circular arch shape is sufficient for the canal only (this being the simplest and cheaper option).
Stilted segmental Arch: Provides the width and height to accommodate both the canal and towpath.
Note the repairs to the parapet and spandrel walls. Image: Tony Prothero/CCAG archive. |
Stilted semi-circular Arch: This narrower arch is wide enough for the canal only.
The upper part of the original bridge has been replaced by a new road bridge. Image: By Chris Jones From http://canalplan.eu/ |
The sidewalls, spandrel and parapet walling is universally formed with rubble stonework. This varies in quality from virtually random rubble to well dressed and squared stonework with regular neat coursing.
Random Rubble (built to course) Walling:
The stone blocks vary in size dictated by the natural fracturing of the limestone and with little if any shaping. They are laid in lime mortar in rough courses. Image: Tony Prothero/CCAG archive |
Squared Rubble (regular course) Walling:
The stone blocks have been shaped and carefully laid to give straight beds of consistent height within a course, but varying height between courses. Image: Tony Prothero/CCAG archive |
All of the stone bridges have ashlar voussoir and imposts (the quality of this does vary) and an arch string course of semi-dressed stone (except 228 where the arch string is of brick and is a 19th century repair). Similarly they all have arch key-stones except for bridges 228 and 227 where there is significant repair work to the upper part of the arch work.
Bridge 228 (Yarnton Lane): Original and repaired features
The side and spandrel walling of coursed random rubble is original, as is the ashlar masonry of the impost and the first part of the arch voussoir .
The arch string course (sometimes referred to as an arch hood or drip course), and the central arch voussoir have been replaced with brickwork (19th Century). The arch stilt below the impost has also been repaired with brickwork.
Image: Tony Prothero/CCAG archive.
Bridge 223 (Sparrowgap) : Original arch features
The finely worked ashlar voussoir contrast with the coarser shaped and faced arch string course. It is likely that the stone for the voussoir has been sourced from a different quarry to give masonry with a finer finish and a contrasting colour.
Note the key stone has the same stone colour/finish as the arch sting course. This is a consistent pattern for all the surviving stone bridges.
Image: Tony Prothero/CCAG archive.