improvements to the overflow and spillway required keeping the
reservoir close to top water level for the duration of the works
in order to maintain supplies
spillway downstream view - Courtesy of Dŵr Cymru Welsh Water
Teifi Reservoir is situated in a remote area on the western
slopes of the Cambrian Mountains in Ceredigion, west Wales, and
was created in the early 1960s by raising a natural lake. It is
impounded behind six embankments constructed across natural low
spots along its south-western margin, covering an area of just
over 25 hectares and with a capacity at top water level of
around 848,000m3. The main embankment is an earth
embankment with a central concrete core, 160 m long and with a
maximum height of 5m. It includes the main overflow and
Overflow and spillway
The overflow comprises a
concrete gravity dam to the north end of the main embankment,
formed between abutment walls and with a crest length of just
under 19m. The draw-off tower containing the piped raw water
supply and scour outlets is integral with the spillway structure
and located immediately to its right (north).
The spillway is divided
into three monoliths, the central one being slightly wider than
the outer two. The downstream spillway face clearly shows that
each of the monoliths was constructed in a number of separate
lifts but the record drawings provide no details of either
horizontal or vertical joint arrangements.
The original overflow
comprised an ogee-type profile with a crest level 5 feet (1.52
m) below nominal embankment crest level and its vertical
upstream face stepped back by 6 inches over its upper section.
This was raised in 1997 through the addition of a simple
ogee-type cast in situ extension doweled into the original
crest. The extension was cast in three sections with the
movement joints lined up with contraction joints below.
A Statutory Inspection
under Section 10 of the Reservoirs Act 1975 was carried out on
the 11 September 2012. In the report of that inspection, the
following recommendations (among others) were made as to
measures to be taken:
Remedial works are
undertaken to the concrete gravity section to control seepage
paths, which are likely to be contributing to deterioration of
the concrete and reduction in stability. As part of the
investigation of options for this, the condition of the body of
the concrete should be ascertained to determine whether any
other works are required to arrest the deterioration of the
In addition to
controlling the seepage through the structure further works
should be undertaken to improve the stability of the concrete
The spillway has suffered
significant seepage through various joints and defects with
various attempts having been made to rectify the situation over
the years. In 1984, the reservoir had been drained to allow
refurbishment of both the upstream and downstream joint faces.
However, by 2004, when Black & Veatch carried out an inspection,
it was reported that “when water levels are high, the top two
horizontal construction joints (about 400mm and 700mm below the
underside of the recent crest raising structure) each show
evidence of a direct seepage path, with spouts of water emerging
during heavy wave action. Even with the water level 1.15m down
during this inspection there was evidence of seepage from both
of these joints”.
The Black & Veatch report
of 2005 also noted that ‘the exposed downstream face of the
spillway has evidently deteriorated over the years, presumably
due to the combined effects of seepage and surface weathering
aggravated by the acidic water. There is no evidence on the main
face of leaks self-healing through calcite deposition which
indicates that the concrete matrix is in poor condition’. The
2012 Statutory Inspection report recorded that ‘The concrete of
the downstream surface of the overflow gravity section is
reported by DCWW staff to be quite soft and in places can be
gouged with keys’.
coverage on the downstream spillway face identifying
Courtesy of Dŵr Cymru Welsh Water
of both the continued seepage and joint deterioration
particularly at the connection of the raised overflow
section with the abutments (February 2016)
Courtesy of MWH
The recommendation to
further improve stability of the concrete gravity section was
made on the basis of stability analyses carried out by Black &
Veatch in 2005. These had concluded that although ‘major
repairs are not required at this stage’ and ‘the factor of
safety against sliding meets the acceptance criteria for all
load combinations considered (except the sensitivity test where
cohesion is reduced or disregarded)’:
The factor of safety
against overturning does not meet the acceptance criteria for
Normal, Unusual and Extreme load combinations.
Factors of safety against
overturning are lower for the lower lift joints (e.g. varying
from 1.17 for the first lift joint below the crest to 0.99 at
the level of the stilling basin for the Unusual load condition).
In July 2014, Dŵr Cymru
Welsh Water appointed MWH to assess options to satisfy the
recommendations made in the Section 10 report and, in
particular, to investigate the feasibility of installing a
proprietary Carpi Tech geocomposite membrane on the upstream
face of the structure to reduce both leakage and uplift forces.
A number of constraints
were apparent from an early stage. The most significant was the
requirement that all works be carried out with the reservoir at,
or close to, its normal operating level. Llyn Teifi supplies
around two thirds of the raw water to Strata Florida Water
Treatment Works and it was decided that the risks associated
with lowering the reservoir to any significant extent would pose
too high a risk of supply interruptions, particularly if works
were undertaken during the summer. In the winter months in
particular, Llyn Teifi can become almost inaccessible due to
snow and ice cover and at other times, being exposed, often
suffers high rainfall and very strong winds.
Access for construction
plant and equipment would also be difficult at any time of year,
vehicular access being via a 1 km long private metalled track
approximately 2.5m wide which leaves a winding minor public road
around 4.5 km east of the village of Ffair-Rhos. The only access
to the front face of the spillway would be via a steeply
inclined section of rough track.
Together with the River
Teifi, Llyn Teifi and the other Teifi Pools also comprise the
Afon Teifi Special Area of Conservation (SAC). They also fall
within the Elenydd Mallaen Special Protection Area (SPA) and
Elenydd Site of Special Scientific Interest (SSSI). Any
improvement works would therefore need the assent of Natural
A diving survey by
Edwards Diving Services Ltd confirmed that the overall condition
of the upstream face of the spillway was reasonable and
determined a reservoir bed level of around 4.6m below the
overflow weir crest. The bed just upstream of the spillway
comprised of a thin veneer of soft silt typically 30mm thick
overlying a hard bed of gravels and cobbles.
by Geotechnical Engineering Limited comprised a series of nine
100mm diameter cores drilled to a maximum depth of 200mm. These
were distributed evenly across the spillway, with three on each
block positioned at the top, middle and bottom.
Testing was carried out
by ACS Testing Ltd and Sandberg LLP and suggested that, contrary
to earlier suggestions, the concrete was in reasonable condition
with a good level of compaction and no evidence of
cracking/fissures or honeycombing.
The stability analyses by
Black & Veatch in 2005 identified that the factor of safety
against overturning did not meet the acceptance criteria for
Normal/Usual, Unusual and Extreme load combinations as set out
in ‘Design of Small Dams’ published by the United States Bureau
of Reclamation, 1987 and reproduced below:
Normal design reservoir
elevation with appropriate dead loads, uplift, silt, ice
and tailwater. If temperature loads are applicable to
the specific sites, use minimum usual temperatures
occurring at that time.
Maximum design reservoir
elevation with appropriate dead loads, silt, tailwater,
uplift, and minimum usual temperatures occurring at that
time, if applicable.
The usual loading plus
the effect of the maximum credible earthquake (MCE).
Table 1 (above) sets out
the specific parameters adopted.
Table 2 shows selected
results from the analysis of the overturning stability of the
central monolith, noting that the factors of safety against
sliding were all found to be acceptable.
Tables 1 and 2 - Click to enlarge
Tables 3 and 4 - Click to enlarge
MWH updated the 2005
stability analysis with stability checks as described in the
more recent guidance from the US Army Corps of Engineers,
‘Stability Analysis of Concrete Structures’ (EM 1110-2-2100,
Table 3 (bottom of
previous page) sets out selected results of this analysis for
comparison with Table 2 (top).
Table 4 (bottom of
previous page) shows the loading combinations considered.
It can be seen that, using the more recent guidance, sliding
represents the critical case with factors of safety well below
those required for both extreme (flood) and extreme (seismic).
It was recognised from an
early stage that installation of a carefully designed,
waterproof geocomposite membrane to the upstream face of the
Llyn Teifi spillway should address the leakage issues. However,
particularly after the diving survey, it was also recognised
that it would be difficult to take the membrane down to below
the lower lift joints which had the lowest factors of safety
Two main complementary
options were therefore considered:
Option A: The
addition of a concrete wedge to the downstream face of the
spillway: Infilling beneath the overhang of the raised
weir and effectively negating the need to patch repair degraded
areas of concrete to the downstream spillway face.
Post-tensioned anchors: This scenario assumed that
ground anchors would be drilled from 406.5m OD on the downstream
face at an angle of 80° to the horizontal and giving a tension
of 40kN/m run of the structure.
The concrete wedge, in
conjunction with the upstream geocomposite membrane, was
selected as the preferred option due to the problems and costs
anticipated in getting construction plant to the working face
for a small number of anchors, the likely need for removal and
replacement of the footbridge and because of the on-going
requirement for inspection and testing of these.
Detailed design of the
concrete wedge was undertaken by MWH with Carpi Tech appointed
as a sub-consultant to develop proposals for the upstream
Reinforcement placement - Courtesy of Kaymac
Design of the wedge was carried out using STAAD Finite Element
Analysis software, in accordance with Eurocode 2, assuming a PMF
top water level of 408.86m and a reduced factor of safety of 1.2
reflecting the extreme nature of such an event.
This required that 25mm
diameter galvanised high tensile steel L-shaped bars be drilled
and fixed into the downstream concrete face of the existing
spillway on a typical 1200mm square grid using Hilti HIT-RE
500-SD chemical anchors with an embedment length of 600mm.
Dowels were also drilled vertically into the base of the
existing spillway and into the abutment walls.
Reinforcement for the
wedge itself was designed to be continuous over the full width
of the concrete spillway and comprised a single layer of B1131
mesh to each face supplemented with B16 bars at 200mm centres. A
500mm wide proprietary drainage layer with perimeter hydrophilic
waterstops was specified to be applied the full height of each
of the existing vertical spillway joints with 100m diameter
drainage pipes running through the base of the new concrete
membrane: The system proposed by Carpi Tech for the
upstream face of the concrete overflow section comprised a
tensioned geocomposite liner (SIBELON® CNT 3750) prefabricated
from a watertight PVC geomembrane 2.5 mm thick, heat-bonded
during extrusion to a non/woven, needle punched geotextile to
provide anti-puncture resistance and drainage. This would be
mechanically fastened to the overflow structure using a Carpi
Tech patented system and lapped and heat-seamed on site in
accordance with the design details and allowing for installation
underwater by divers.
Liner installation over weir crest - Courtesy of Dŵr
Cymru Welsh Water
At Llyn Teifi, the design
included three separate ‘compartments’; a main central
compartment over the full width of the concrete spillway and two
smaller compartments to each of the upstream abutment wing
walls. The right hand abutment wing wall which incorporated a
masonry facing would have additional protection in the form of a
second layer of anti-puncture geotextile.
Each compartment included
a drainage system to allow any water trapped in the interface of
the liner and the existing structures to be separately
discharged via a common outlet point to be drilled through the
base of the concrete spillway structure.
The contract for
construction of the works was awarded to Swansea-based Kaymac
Marine and Civil Engineering Ltd with Carpi Tech as a
subcontractor to install the liner system. Site works commenced
on 12 September 2016 after access road improvements carried out
over the summer by Dŵr Cymru Welsh Water.
dam and modular floating pontoon - Courtesy of Dŵr Cymru
Concrete removal at foot of existing spillway - Courtesy
of Dŵr Cymru Welsh Water
downstream spillway face
Dŵr Cymru Welsh Water
installation to right hand abutment
Dŵr Cymru Welsh Water
Kaymac’s first task was
to core drill a 175mm diameter hole through the base of the
existing spillway structure, install and grout in the gravity
drainage pipes for each of the three liner compartments. To
facilitate this Kaymac designed and fitted a limpet dam to the
upstream face of the spillway using a floating modular pontoon,
thereby allowing all works to be undertaken in the dry.
After breaking out of
some existing concrete at the foot of the spillway and placement
of reinforcement a proprietary formwork system designed by RMD
Kwikform was used for construction of the concrete wedge. This
allowed the works to be completed in three vertical lifts using
a concrete pump situated on ground to the north of the reservoir
some 40m away. Carpi Tech, with divers from their partner,
Lucatelli Diving of Trieste, commenced installation of the
waterproofing liner on 1 November 2016. All construction was
successfully completed before Christmas 2016.
installation to spillway and right abutment - Courtesy
of Dŵr Cymru Welsh Water
Completed concrete wedge and liner installation over
weir - Courtesy of Dŵr Cymru Welsh Water
Completed works - Courtesy of Dŵr Cymru Welsh Water
and publishers would like to thank MWH now part of Stantec,
and Dŵr Cymru Welsh Water, for providing the above article