Setts and the City
Cobbles, setts and historic townscape
||The new cobbles being hammered in
The setting of a historic building is
as important as the building itself.
English Heritage and the Commission
for Architecture and the Built Environment
have repeatedly stressed the importance
of understanding and conserving ‘the
complete picture’ of a given site or space.
Yet, very often, the quality of the setting,
clearly visible in old photographs, is now
completely lost. This is particularly true
where public streets form part of the setting
because the materials that make up the road
break up over time and have to be changed.
Original road surfaces have often been replaced
because they failed, or covered over with
new materials because they were considered
to be inadequate for modern conditions.
Likewise, historic footways have usually
been replaced because they have cracked or
broken up completely. Typically, stone paving
slabs that might have been in place for 200 years
only failed when heavy road vehicles became
more common after the Second World War. There are exceptions. Places that are
inaccessible to vehicles often retain their
original paving. Usually these places are
in raised areas, such as at the entrance
to a church beyond a flight of steps, or
completely raised sections of pavement.
There are also many places where
original road surfaces, usually granite setts
(small rectangular paving blocks), are intact
but have been covered up. They are there
to be re-discovered just under the modern
surface. Historic drainage channels and
large granite kerbs likewise often remain.
This is because they are both practical and
robust and, of course, difficult to move.
To identify the original materials in a
particular street, it may be helpful to look at
the adjacent private courts or access roads
as well as old photographs. These courts and
private roads are less frequently changed
so many still have their original granite slab
tracks for wheels and small cobbles at the
centre which served to give the horse traction.
CAUSES OF FAILURE
The most common cause of failure in
historic road and pavement surfaces
lies not in the visible surface but in the
supporting or base layer below it.
Stone is strong in compression, making it
difficult to crush, but weak in tension, allowing
it to crack or split. In road construction it
needs to have a continuous underlying support.
The weight of a vehicle is transferred through
the top layer of stone to the base below. If the base collapses, the stone surface has
insufficient support and then also collapses.
The individual setts in a historic road surface
may still be intact but they will have fallen apart
leaving the whole road surface unusable.
RADCLIFFE SQUARE, OXFORD
|The new cobbles before hoggin dressing
Radcliffe Square is at the centre of historic
Oxford. It is surrounded by Grade I listed
buildings, notably the circular Radcliffe Camera
(c1750). The square is made up of public streets
with a central fenced oval of grass next to the
Camera. There is a low volume of traffic, but
there are occasional heavy goods vehicles
which deliver to the adjoining colleges.
The public street was surfaced in small
round river cobbles set in a compacted hoggin
(a mixture of clay, sand and gravel) and some
areas of granite setts, also in hoggin, with
traditional Yorkstone pavements at most edges.
This surface was laid more than 150 years
ago. Since the 1960s the cobbled surface had
begun to fail. The cobbles themselves were
not damaged, but they had become loose and
large potholes had appeared. These potholes
had been filled either with areas of new cobbles
set in regimented rows in concrete, or with
areas of square setts, or simply with blacktop.
Once an area of cobble was dislodged to
form a pothole, the hoggin became unstable
and the pothole grew. By 2005 the potholes,
both new and patched, covered a wider area
than the surviving original cobbles. The
conservation objective was to reinstate the
historic visual effect of the original surface
while creating a robust road surface. The
solution was to prepare a measured drawing
of all the ground surfaces, take up the
remains of the original surface and then
replace them exactly as the drawing indicated,
but with a base specifically engineered
to withstand heavy vehicle traffic.
Measured drawing of the existing layout at Radcliffe Square, Oxford
From the bottom upwards this
specification (see section diagram, below right) was:
Class 6F1 capping, a selected
fine grade material
150mm type 1 granular sub-base
175 cement bound material
category 3 (CBM3)
70mm SteinTec bedding mortar BM 04
70mm x 100mm river cobbles hammered
into the bedding mortar to the correct level
SteinTec jointing mortar HD
0.2 between cobbles
20mm locally-sourced hoggin top dressing.
The original cobbles were some 50mm in
diameter and were laid on their side and
pressed into the hoggin. To achieve the
same visual effect, larger cobbles were
used, but laid on their ends, so that only
the tips of the cobbles could be seen. When
the original hoggin was re-applied as a
decorative top dressing the overall appearance
was virtually the same as the original. The complex specification was designed to
ensure that the new cobbles were firmly fixed.
Being both round and smooth there was a
danger that they would be pushed out sideways
by the severe lateral pressure of the power-assisted
steering of modern heavy vehicles.
Considerable time and effort were taken
to make sure that the finished appearance
was accurate and pleasing. Three test samples
were undertaken in the workshop and then
another three under normal conditions on site.
One challenge was to avoid the appearance
of the cobbles seeming to float in a sea of
mortar, like icebergs. It was during these
tests that it was decided to fix the new cobbles
vertically, so that only their tips were visible.
A proprietary non-cement mortar was used
because it is stronger than ordinary Portland
cement and does not leave stains. It can also
be washed off during the curing process.
REINSTATING THE SURFACE
||Section showing the construction specification
Areas of square setts, laid out as smooth walking
surfaces across the cobbles, were easier to
re-install. They too were bedded into the non-cement
base and were also pointed up with the
same material. The original layout also included
a series of granite sett drainage channels
across the cobbled area. In practice, as the
cobbles were originally laid in and on hoggin,
surface water percolated through it so that
the drainage channels were more decorative
than functional. With the introduction of a
new solid inflexible base, surface water needed
to be correctly channelled so the setts were
pointed flush with the tops of the setts. Care
was taken to lay them out accurately, even
though the setts were not all exactly the same
size. The three-row drainage channels, for
example, were set out with the aid of a string
line so that the outer edges of the setts were in
a reasonably straight line. Any variations in sett
size were accommodated at the inner edges.
Accessibility for wheelchair users
was considered carefully from the outset.
Fortunately, the original design of the square
included a path of continuous smooth York
stone paving at the outer edge and also around
the perimeter fence of the circular garden.
Admittedly, walking on the round
cobbles is not particularly easy; but then it
never had been. As there is a smooth access
surface in all directions, people who prefer
not to walk on the cobbles are not obliged
to. The result is very satisfactory and a
measure of its visual appeal is its ongoing
popularity as a shooting location for television
productions, both period and contemporary.
VALUABLE LESSONS FOR OTHER HISTORIC STREETS
The most common street-level conservation
work involves footway reinstatement. The
problem of broken slabs occurs frequently. In
almost every case, the damage is caused by
vehicles mounting the footway and damaging
the paving. The damage typically occurs
because the slabs were originally laid without sufficient support or have been lifted and relaid without sufficient support
during the digging of a service trench. Because service pipes and cables
are laid directly below pavements, paving slabs are constantly being
lifted and re-laid. Some authorities count their service openings in the
thousands per year. This is the reason why odd broken slabs are so often
seen next to inspection covers or along the line of a service trench.
|The Radcliffe Camera, Oxford with new cobbles in the foreground
The solution is to ensure that a non-compactable fill is used to
close service trenches. Otherwise the trench will sink as the fill material
gradually compacts. The support for the paving material is eroded
and as soon as any heavy load is applied, the paving slabs break.
In the carriageway, setted surfaces often fail where they form
part of or are adjacent to a flexible surface. Most blacktop surfaces
are sufficiently resilient to withstand some movement caused by
heavy vehicles. Because the setts themselves are completely solid and
cannot flex, the pointing of setts is put under strain. The slightest
movement causes a very small crack, which allows moisture to enter.
The moisture freezes and expands in winter, the crack opens up and
eventually the setts are forced out by the movement of traffic.
Areas of setts at bus pull-ins are very vulnerable. As each bus
turns its front wheels to leave the pull-in, the same small group of
setts is put under sideways pressure, sometimes dozens of times per
day. Repairs on such areas should ensure that the pointing between
the setts is as strong as the setts themselves so that the lateral forces
are transferred from one sett to the next and eventually to the base.
Historic road and footway surfaces can be conserved and
reinstated but they need to be either protected from modern
heavy vehicles or reconstructed to withstand them. This means
uniting engineering skills with an awareness of local character
and sound urban design; in short, urban engineering.