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Technical Committee 207 /
Comité technique 207
2 SPECIFIC CONSTRUCTION STYLES – HARBOUR
WALLS
Although wall construction style is often a reflection of locally
available stone, sometimes the construction style reflects the
purpose of the wall. Probably the best examples of this are the
harbour walls found around the United Kingdom. Many of these
walls are of significant age - parts of the quay in M
to date back to 1390 (Cornwall-online.co
suggesting that they are well suited to their usag
en looking at harbour walls
ndom rubble.
ousehole are
reported
.uk)
e. Mousehole is
also unusual wh
as much of it is
uncut ra
tection from wave action as a
gre
flected or running up and over an
im rmeable concrete wall.
uction and
rbours by Richard Tufnell (2012) which
Figure 4. Wall at Mousehole, Cornwall, containing of large blocks.
It is typical when looking at harbour walls to find vertical
construction as you would find with slate type materials, but
they can be on a far larger scale to that found in a typical wall.
Stones may also be shaped to suit vertical construction even if
they are not usually built in the fashion, or are constructed from
slate sheets far larger than you may expect to find in a typical
vertically constructed wall. This is likely to be linked to the
convenience of transport material by water, allowing larger
stones to be transported. The vertical construction and larger
stone sizes are both advantages in harbour construction. The
verticality of the stone helps to prevent uplift by presenting a
small bottom face for waves to act on, and provides better
drainage for sea water, both during the changing tides and under
wave penetration. Being drystone and hence free draining is
better suited to harbour construction than most other wall types,
in which any water which penetrated the wall under wave
pressure may not flow out quickly under gravity alone, inducing
extra pressures on the rear of the wall. Having a more massive
construction provides better pro
ater force is needed to move individual stones within the
wall. The voided nature of drystone is also likely to help with
wave energy dissipation as waves will break up into the wall on
impact, as opposed to being re
pe
Research has been carried out into the constr
tradition of drystone ha
he presented at the 13th International Dry Stone Walling
Congress in September 2012
3 UNDERSTANDING CONSTRUCTION AND
ASSESSMENT
Many of the current drystone retaining structures were
constructed around 100 years ago with no records of how or
exactly when they were constructed. Even many of the modern
walls are constructed based on rules of thumb, with little or no
input from engineers. Many of these walls have remained stable
for a number of years and still continue to do so, retaining a
significant proportion of various transport infrastructures
throughout the United Kingdom, as well as being used in other
applications such as harbour walls and domestic use.
Since the majority of these structures have been built, the
loadings that they are subjected to have increased, particularly
on the road networks. This combined with the increasing age of
the walls and the need to be able to replace or repair walls
before collapse means that improved assessment drystone
retaining wall stability is increased. The assessment of these
structures by engineers is often tricky due to the lack of formal
engineering input during their design, as well as a lack of
knowledge of failure mechanisms, unlike with more modern
retaining structures. Assessing these structures in the same way
as modern structures is inappropriate due to their un-mortared
nature and inherent flexibility, which means that obvious
deformation within the wall does not automatically mean that
the wall is unsafe. Guidance on assessment can be found in
work by O’Reilly and Perry (2009) and through the various
publications by the dry stone walling association. However
much of the guidance given is qualitative and relies on the
judgement of the engineer assessing the wall as to whether it is
safe or not. Where an engineer is familiar with drystone walls
in general and the walls he is looking at in particular, then a
reasonable assessment is likely to be made. However if an
engineer has little to no experience of drystone walls they may
take the walls’ natural deformations and oddities to be signs of
failure, and hence make an inaccurate or insecure judgement.
This in turn may lead to walls unnecessarily being taken down
and replaced with less sustainable modern alternatives which
are out of keeping with their surroundings. It could also lead to
a failure which should have been prevented
In order to improve assessment of these structures further
engineering knowledge of them is required, both in terms of
overall structural behaviour and the effects of properties of
individual elements. This should include consideration of how
these factors might change with time, such as the weathering of
the stone. By understanding generic wall behaviour and how
different factors affect this new assessment techniques can be
developed that can enable the engineering judgement to be
better informed.
It is also important to note the modifications that are often
made to drystone retaining walls, often in good faith, which can
be detrimental to the wall’s health. For example, it is common
practise for dry stone walls to be grouted or pointed either in an
effort to prevent further movement, or to protect the base of a
wall from salt spray, particularly in limestone areas. However in
doing so the drainage paths through the wall are often blocked,
thus taking away one of the main advantages of drystone
retaining walls, which is that due to their un-mortared nature
they are free draining. This can cause a build-up of water
pressures behind the wall which did not exist before, and
ultimately lead to collapse. The grouting of a wall will probably
reduce its flexibility, which can be detrimental in two ways. If
the wall is less ductile then it is unable to redistribute load
concentrations, or distribute load away from weak areas, which
could result in a local failure leading to a general collapse. On
the other hand, a local crack which might allow a safe
redistribution of load might give serious concern.
Unfortunately as with most retaining type walls a number of
harbour walls are also being grouted. This is potentially
understandable in harbour walls as over time wave action is
likely to have caused some visible damage to the wall, and it
