A Discontinuity Survey was carried out at the Golwern Quarry and at the
Hendol Mine by Civil Engineering Students from the University of Warwick. The
investigation was carried out as instructed by Mr Clive Richley, the Client.
This Interpretative Report presents results obtained by these students as well
as analyses this data to calculate the unsupported stand-up time of the mine and
estimate the rock stability of each of the four Golwern quarry rock faces.
2.0 Object and Scope of the
The objective of this investigation was to collect the necessary
information about the geology of the Golwern Quarry to predict the cause of
collapse of each of the quarry faces, and sufficient information about the
Hendol Mine to estimate the stand-up time of the undergound cavern. Each of the
4 slopes of the quarry will be modelled on Stereonets to observe which failure
mechanisms could cause the collapse of the Quarry. The Stand-up time of the
mine is estimated through working out the Rock Mass Rating by carrying out a
Discontinuity Survey of the Mine.
Site research such as its location, geology
and its history are shown in section 4. Information about the commissioning for
the site testing can be found in section 5 and section 6 presents the types of
tests carried out by students. Appendices 5 and 6 presents the results for the
site tests and section 7 interprets and analyses these results. Section 8
produces a suitable evaluation for this interpretation and lastly, section 9
focuses on an environmental assessment by describing ways that healthy and
safety can be improved at the site.
4.0 The Site
The Golwern Quarry and Hendol
Mine are located next to each other, in the North of Wales, 0.7 miles from the
nearest Train Station, Fairbourne (15-minute walk). If travelling by car from
the North, you will need to take the A470 through Snowdonia National Park and
then turn right on to the A493 towards Fairbourne. If travelling from the East
take the A458 and the A470 and then turn left onto the A493. If travelling from
the South, take the A487 towards Snowdonia National Park and then turn left on
to the A493.
After reaching the A493, you
should find places to park near where the Fordd Panteinion is on the left. The
site is a 400m walk from where Ford Penteinion starts, to the left of the A493.
The approximate Ordnance
Survey National Grid Reference
for the site is SH 620 121. The latitude and longitude co-ordinates are 52.689069, -4.041753. Figures … and …
(Appendix …) pin point the location on Google Maps.
The Galwern Quarry is a regular
rectangular shape with an area of approximately 3000m². The Hendol mine is a
triangular shape with an area of roughly 62500m². The site is located amongst a
lot of greenery- grassland and plenty of trees. A small country lane- Fordd
Panteinion lies just North of the Hendoll Mine, and the A493 road, just East of the Mine. South and West of the site
is a lot of Greenland.
The site is located on quite a
hilly geology. The mine is on a lower land of 436 ft above sea level, whereas
the Golwern quarry is elevated 579 ft above sea level as can be seen from a
topographical map in figure ….
The site is located just
1.15km East of the Irish Sea, but is not at risk of flooding from the sea or
rivers. The quarry is however, at a high risk of flooding from surface water,
as can be seen from figure….
4.3 Geology and Ground Water
When we arrived at the site,
the walk to the quarry was quite wet and slippery due to the presence of many
streams leading to the Blue Lake. The steep walls that surround the lake were
fairly dry. However, whilst walking to the mine, the route was very slippery.
The mine itself was very wet and slippery as well so care should be taken when
walking through the slate mine. Water is dripping from the mine ceiling constantly,
making it quite haphazard to walk.
Figure .. from Appendix ..
shows a mineral map of Wales and clearly shows that Slate is present in the
quarry as well the presence of multiple historic slate quarries. The map also
states that there is a Lead and Copper Metallic mineral occurrence near the
site. The Galwern Quarry surrounds a unique lake of “blue” water which can be
explained by the presence of dissolved copper minerals.
4.4 A Brief History
Historically, Wales has always
been famous for Slate quarries and world class slate products. Slate has been
mined for centuries in Wales. But how did slate get there in the first place?
Around 550 to 410 million
years ago, volcanic activity and periods of time when seas were shallower meant
that more sandy rocks, clay minerals, and micah were deposited on the sea-bed
pointing in all directions. 400 million years ago, tectonic plate collisions
and large underground pressure lifted seabed forming the Welsh Mountains. The
pressure also compressed the minerals causing the recrystallisation of the
material forming Slate, through metamorphism. 1 This explains the
presence of slate high up in the Welsh Mountains, and in the Golwern Quarry and
Golwern Quarry opened in 1865
for slate miners, however only had an output of 50tons between 4 men in 1882. The
limited output of the quarry meant that the quarry was not cost effective and
so, inevitably shut down in 1915. The quarry was filled with water in the late 1800s
to provide hydro-power for Bournemouth. 2
Figure .. from Appendix …
contains 2 maps of the slate quarries- one from 1892 and the other from 1920.
Both maps show both Hendol Slate Quarry and Golwern Quarry next to each other.
This Investigation was awarded
to Civil Engineering Students after a submission of a tender for the task,
designed by Mr Clive Richley’s
advisors for a discontinuity Survey, according to their requirements.
The procedures were carried
out in accordance with the Mr Clive Richley’s wishes, and all results are
present in Appendix ….. and its location in Appendix …. Experiments were
carried out on the 6th of December. The client decided the type of
Discontinuity Survey that was to be carried out.
The results of this
discontinuity survey for both the Mine and Quarry are displayed in this report
in Appendices …. And …. Respectively.
6.0 Field Testing
The different Field Tests were
carried out on the 6th of December 2017 and were decided by Mr Clive
Richley, the client. The tests conform to engineering standards and can be
presented in the following summary:
Tests performed at Quarry
Chainage of discontinuities observed
Compass Clinometer used to measure dips and dip
directions of quarry faces and discontinuities on quarry faces.
Persistence, aperture, and Nature of Infilling
of discontinuities also observed.
Tests performed at Mine
Geological Hammer used to give strength of rock
Span of Chamber observed
RQD of chamber measured
Discontinuity spacing is measured
Condition of joints observed (aperture,
persistence, roughness, weathering, nature of Infilling)
Groundwater flow estimated
7.1 Quarry Photos Analysis
I think Face A is likely to fail by flexural toppling since there is an
overhang of rock mass towards the top, so the centre of mass of the face is
towards the top and front thus may topple. There also appears to be a
possibility of a wedge failure since there appears to be a rock mass sliding
along two interconnecting discontinuities.
Face B appears to be failing by wedge failure since quite a few examples
can be seen in figure … Failure by toppling does not look likely since this
face is not very steep, thus have a low centre of mass, compared to that of
Face C has 2 parts- a small wall at the front and a larger one behind
it. This means that it is quite likely that the wall could fail through
Face D appears to be failing through plane failure, since the dip of the
face is less than the slope plane. The left part of the wall shows evidence of
a plane failure…
7.2 Stereonet Data Analysis
Table 1 is produced through
analysis of Figures…. To …. From Appendix ….
Table 1: Shows the possible ways
each of the quarry faces could fail through stereonet analysis
Peak of 4
peak of 3
peak of 9
peak of 5
peak of 4
peak of 9
peak of 3
Face A shows evidence of both Plane and Wedge, though not toppling
Face B shows evidence of all 3 kinds of failures.
Face C shows evidence of both Wedge and Toppling, though not Plane failure.
Face D shows evidence of both Wedge and Toppling, though not Plane failure.
7.3 Cavern Data Analysis: Bieniawski stand up time
The Rock Mass Rating of the Mine has been calculated in Appendix… in
Figures… and … Since the Direction of the Excavation (or Joint Orientation) is
not known, the only way to work out the RMR is to take the worst and best case
scenarios of the Direction. RMR values for both of these situations have been
calculated as 61 (worst) and 73 (best). Since the Roof span of the mine is 7m,
the Stand up time of the Mine can be estimated from Figure … in Appendix …
For the best case scenario, where the RMR is 73, the stand-up time of
the mine is approximately Hours or approximately a year.
For the Worst case scenario where the RMR is 61, the stand -up time of
the mine is approximately Hours or approximately a month.
The fact that the two different stand-up times of the mine vary by a
factor of 12 means that the stand-up time is very sensitive of the orientation
of the excavation. Therefore, an accurate value of the orientation needs to be
calculated to produce an accurate estimation of the mine’s stand-up time.
Back analyse the stability of the rock faces
8.1 The Hendol Mine
Since this mine is currently open to the public, it’s presence is
particularly dangerous right now since it’s stand-up time, according to the
calculated Rock Mass Rating is at most only 1 year. It is therefore necessary
to add appropriate supports to the underground cavern to increase it
theoretical stand-up time. However, since this excavation site has been
standing for more than 150 years, I doubt that it will collapse within the next
The quarry also shows evidence of failure mechanisms and thus
9.0 Environmental Assessment
9.1 Risk Assessment
Currently around 1 person dies at the quarry by falling every year.
There should therefore be barriers or even just a warning tape around the top
of the quarry to warn members of the public about the dangers of jumping or
falling from this height.
There is also the issue that the rock faces themselves may undergo a
failure mechanism such as toppling, wedge or plane failure and so it should be
warned against standing too close to the edges of the faces.
The Hendol mine is also quite dangerous since it is constantly dripping
with water causing the floor to be slippery all the time. The excavation
ceiling also changed between high and low quite frequently and so it is quite
easy to bump your head on the ceiling. It should therefore be advised to wear
hard hats whilst going into the mine. One theory about how this could happen is
by having volunteers stand at the entrance handing out a limited number of hard
hats, which must be returned when exiting.