The present invention relates to an apparatus for sealing joints in
diaphragm walls, and in particular to a multi-part waterbar incorporating a hydrophilic
material which swells upon contact with water.
A diaphragm wall is made by casting a series of concrete panels, which
may be reinforced, in excavated trenches as described, for example, in EP 0 101
350 and EP 0 402 247. In some cases, alternate 'primary' panels are constructed
first, followed by infill (i.e. closing) 'secondary' panels. The installation sequence
would be, for example, panels 1, 3, 5, 7, 9, 11 etc. followed by panels 2, 4, 6,
8, 10 etc. In other cases, only a few 'primary' panels are first constructed, for
example panels 1, 10 and 20. Following this, a series of 'continuity' panels 2,
11, 3, 12 etc. are installed, with the diaphragm wall being completed by 'closing'
panels 9 and 19. All primary panels require the use of shutters at each edge of
their respective trenches in order to provide well-defined edges to each panel so
as to ensure that the joins between adjacent panels may be made watertight. Continuity
panels, in contrast, require only one shutter at the edge of the trench furthest
away from the previously cast panel. No shutters are required for closing panels.
The shutters are conventionally known as 'stop-ends', and provide the concrete at
each vertical edge of the panels with a predetermined shape.
In order to reduce water leakage across the joints between panels,
it is possible (with some types of stop-end) to install a waterbar between adjacent
panels. A waterbar comprises a strip of suitable material, for example rubber, PVC
or steel etc., which has one longitudinal edge embedded in the edge of one cast
panel and the other longitudinal edge embedded in the adjacent panel. Preferably,
the waterbar extends over substantially the entire height of the diaphragm wall.
Such a waterbar may be installed by employing a stop-end provided with a slot in
its face into which the waterbar may be fitted, with about one half of its width
remaining exposed. When concrete is poured into the trench on this side of the stop-end
and allowed to set, the stop-end may subsequently be removed so as to leave approximately
half the waterbar embedded in the resulting concrete panel. When the next panel
is cast, the remaining exposed portion of the waterbar will become embedded in concrete,
thereby resulting in a seal between the two adjacent panels. Typical waterbars have
beaded longitudinal edges, giving the waterbar a dumb-bell shaped cross-section,
with an optional central bulb.
This type of waterbar, however, does not necessarily guarantee watertightness
at panel joints. This is because the known varieties of waterbar rely on the fact
that concrete shrinks slightly upon setting. An element which is completely surrounded
by concrete will be gripped all round as the concrete shrinks, but the beaded edges
of the known varieties of waterbar are not completely surrounded by concrete due
to the presence of the central part of the waterbar. Accordingly, there is a potential
risk of water leakage. Furthermore, installation conditions are in practice far
from perfect, partly because the waterbar is installed "blind" under a bentonite
mud containing suspended sand and the like, and partly because the concrete is cast
without vibration, which means that the bentonite and/or sand and the like may not
be fully removed from the edges of the waterbar.
European Patent Application Number EP-A-0580926 discloses a waterbar
comprising two parts, used within diaphragm walls formed by concrete panels, in
which the two parts are cast within neighbouring concrete panels and may engage
with each other at an intermediate position between the two panels.
Waterbars made of hydrophilic materials have been used at construction
joints in conventional "above ground" concrete structures. The hydrophilic material
is placed at the joint in dry conditions. If and when water enters the joint, the
hydrophilic material will swell, thereby forming a seal between the two adjacent
concrete members. In contrast to this, the use of hydrophilic waterbars in diaphragm
wall construction presents a number of problems, not least because installation
takes place in an aqueous environment, and the part of the waterbar to be incorporated
into the second panel will swell before the trench for that panel is excavated and
the concrete cast. One way of approaching this problem is to use hydrophilic waterbars
provided with a protective, e.g. sugar coating, which can theoretically give a delay
of several days before swelling occurs. This, however, is often unreliable, mainly
because the coating resembles a sugar glaze which is cracked when the waterbar is
flexed, is easily damaged in handling, and has inherent imperfections in the coating,
all of which will lead to premature swelling of portions of the waterbar. Moreover,
in many installations it is not unusual to leave more time between the casting of
adjacent panels than can be accommodated by the protective coating; indeed, intervals
of up to thirty days are not uncommon.
According to the present invention, there is provided a waterbar for
use between adjacent panels in a diaphragm wall, comprising a first member in the
form of an elongate strip provided along a first longitudinal edge with a retaining
channel; and a second member in the form of an elongate strip provided along a first
longitudinal edge with a keying projection, wherein the retaining channel of the
first member is shaped so as to slidably retain the keying projection of the second
member, characterised in that:
wherein the hydrophilic material serves, in use, to seal the
joint between the first and second members
- i) both the first and second members are provided, along the length opposite
to the first longitudinal edge, with a hydrophilic material; and
- ii) either the retaining channel or the keying projection is provided along
its length with a hydrophilic material;
When building a diaphragm wall, a first trench is excavated and a
stop-end is placed at each end of the trench. Each stop-end may be provided with
a longitudinal slot into which the first member of the waterbar is placed, such
that the first edge of the member is exposed. It is usual for the first member of
the waterbar to be placed in the slot prior to installation of each stop-end. Concrete
is then poured into the trench so as to form a panel, the first edge of the waterbar
member becoming embedded in the edge of the panel. Bentonite may be pumped through
the retaining channel of the first member of the waterbar so as to keep this clear
of concrete and other materials. Once the concrete has set and an adjacent trench
has been excavated, the stop-end is removed, leaving the retaining channel of the
first member exposed. The keying projection of the second member of the waterbar
is then slidably fitted into the retaining channel of the first member before further
concrete is poured in order to form a second panel. The two panels are then joined
by a two-piece waterbar which is provided with a hydrophilic material at its interfaces
with the panels as well in the region of its interlocking joint. These are the only
locations where water may flow between the adjacent panels of the diaphragm wall.
The presence of a hydrophilic material which swells upon contact with water serves
significantly to reduce the possibility of such water flow.
It is to be understood that, alternatively, the second member of the
waterbar may be installed first, followed by the first member once the first concrete
panel has set.
By providing a two-part waterbar, each part of which need only be
installed at the time its associated concrete panel is cast, the present invention
overcomes the problems encountered by known hydrophilic waterbars. In particular,
because the present invention does not require the hydrophilic components of the
waterbar to be exposed to water for excessive periods during installation, the problems
associated with inefficient protective coatings are obviated.
The first and second members of the waterbar according to the present
invention may be made of any suitable material, such as plastics, rubber or steel
etc. A particularly suitable material is high density polythene (HDPE) or the like.
The hydrophilic material may comprise a mixture of bitumen and dry bentonite or,
more preferably, a polymer such as that sold under the trade mark Hydrotite.
Alternatively, instead of each stop-end being provided with a longitudinal
slot adapted to receive the first member of the waterbar, each stop-end may be provided
with attachment means which allows the first member of the waterbar to be releasably
joined to one face of the stop-end. This attachment means may take the form of a
pair of generally parallel longitudinal bars welded to one face of each stop-end.
In this way, when the stop-end is removed from a cast panel, the first member of
the waterbar is left embedded in the panel such that the retaining channel is located
within the body of the concrete. This may ease the vertical removal of the stop-end
from the concrete panel by way of a vibrator or simply by lifting the stop-end.
For a better understanding of the present invention, and to show how
it may be carried into effect, reference shall now be made by way of example to
the accompanying drawings, in which:
- FIGURE 1 shows in cross-section a prior art dumb-bell-shaped waterbar;
- FIGURE 2 shows in cross-section a prior art centre bulb waterbar;
- FIGURE 3 shows in cross-section a first member of a waterbar according to the
- FIGURE 4 shows in cross-section a second member of a waterbar according to the
- FIGURE 5 shows in cross-section the first member of Figure 3 joined to the second
member of Figure 4; FIGURES 6 to 8 show in cross-section the steps involved when
installing a waterbar according to the present invention; and
- FIGURE 9 shows in cross-section a first member of a waterbar according to the
present invention releasably attached to one face of a stop-end; and
- FIGURE 10 shows in cross-section a completed waterbar incorporating the first
member shown in Figure 9.
Figure 1 shows a known type of waterbar 1, comprising a flat central
section 2 provided with beaded edges 3 which give a dumb-bell-shaped cross-section.
Figure 2 shows another known type of waterbar 1, similar to that shown in Figure
1 but including a central bulb 4. Both these types of waterbar are one-piece articles,
with the consequence that it is difficult to employ a hydrophilic element, since
this will be exposed to water for long periods during the installation process,
thereby resulting in premature swelling.
Figure 3 shows a first member 5 of a waterbar according to the present
invention. The first member 5 comprises a generally flat central section 6 provided
with a strip 7 of hydrophilic material along substantially all of one longitudinal
edge 8. The other longitudinal edge 9 is provided with a retaining channel 10, which
runs substantially along the entire length of the member 5. The retaining channel
10 is provided with a reentrant mouth 11.
Figure 4 shows a second member 12 of a waterbar according to the present
invention. The second member 12 also comprises a generally flat central section
13 provided with a strip 14 of hydrophilic material along substantially all of one
longitudinal edge 15. The other longitudinal edge 16 is provided with a keying projection
17, which runs substantially along the entire length of the member 12. The keying
projection 17 is shown in the form of a flange, although any other suitable configuration
may be used. In the illustrated embodiment, a further strip 18 of hydrophilic material
is provided along the length of the keying projection 17.
The first and second members 5, 12 may be interlocked as shown in
Figure 5. The keying projection 17 of the second member 12 is inserted into the
retaining channel 10 of the first member 5 by sliding the second member 12 into
position once the first member 5 has been installed. It should be noted that, where
the second member 12 is installed before the first member 5, the strip 18 of hydrophilic
material should be disposed in the retaining channel 10 of the first member 5. This
is because the hydrophilic material would otherwise be exposed to water during the
period between the casting of successive concrete panels.
The strips 7,14,18 of hydrophilic material may be attached to the
members 5,12 by way of adhesive, or they may simply be push-fitted into retaining
The first and second members 5,12 typically have a thickness in the
range of 2 to 10mm, preferably 2 to 6mm, and a width in the range of 50 to 150mm,
preferably 70 to 120mm. The length of the first and second members 5,12 is selected
so as to correspond to the depth of the diaphragm wall being built.
The steps involved in the installation of a waterbar according to
the present invention are outlined in Figures 6, 7 and 8. Firstly, after excavation
of a trench section 19, a stop-end 20 is placed in position. The stop-end 20 includes
a longitudinal slot 21 which is adapted to receive the retaining channel 10 of the
first member 5 of the waterbar. Once the stop-end 20 and the first member 5 are
in position, a plastics pipe 23, typically made out of semi-rigid PVC, may advantageously
be placed into the retaining channel 10, ideally to the full length of the first
member 5. Concrete is then poured into the trench 19 on the appropriate side of
the stop-end 20 so as to form a first panel 22 as shown in Figure 7. While the concrete
is being poured and allowed to set, a flushing fluid, e.g. a bentonite suspension,
is pumped through the pipe 23 so as to keep the retaining channel 10 free of grout
and other debris. An adjacent trench section 19' is then excavated on the free side
of the stop-end 20, and the stop-end 20 (and the pipe 23 if fitted) is removed so
as to leave the retaining channel 10 of the first member 5 exposed. The keying projection
17 of the second member 12 is then slid along the full length of the retaining channel
10 of the first member 5 so to complete the waterbar, and concrete is poured into
the trench section 19' in the usual manner so as to form a second panel 24 in which
the second member 12 is embedded, as shown in Figure 8.
In the final, installed state, the waterbar presents three hydrophilic
strips 7,14,18, one at each extremity of the waterbar and one at the interlock.
These are the only locations where water can flow across the diaphragm wall joint,
and the hydrophilic strips 7,14,18 ensure that the presence of water will improve
the integrity of the joint.
Instead of using stop-ends 20 each provided with a longitudinal slot
21, as shown in Figure 6, it is possible to employ a stop-end 25 having retaining
means 26 on one face thereof, as shown in Figure 9. The retaining means 26 may comprise
a pair of longitudinally-extending, generally parallel bars, which are lightly welded
or otherwise affixed to the face of the stop-end 25. A modified first waterbar member
27 is then attached to the face of the stop-end 25 by way of the retaining means
26, and the composite structure lowered into position. A concrete panel is then
cast in the usual manner and allowed at least partially to set. When the stop-end
25 is removed, the first waterbar member 27 is embedded in the concrete panel in
such a way that the mouth of the retaining channel 28 is flush with the face of
the panel. The bars 26 also become embedded in the concrete panel and are separated
from the face of the stop-end 25 as this is removed. A second waterbar member 29
is then attached to the first waterbar member 27 before an adjoining concrete panel
is cast, as shown best in Figure 10.