The invention relates to the construction of a diaphragm
wall in a substrate. Such a diaphragm wall can fulfil various functions. A diaphragm
wall which is used for the building of a quay wall is mentioned as an example. A
diaphragm wall of this type is excavated on one side in such a way that a harbour
basin can be formed. Another example is a diaphragm wall which is used in the building
of basements and the like, such as underground car parks.
The diaphragm wall is constructed by digging a trench in
a substrate, which trench is filled with, for example, bentonite in order to prevent
the trench walls from collapsing. Reinforcement is then placed in the trench thus
produced, after which the concrete mix is poured that is then left to set. It is
highly important in this regard that the reinforcement, which is usually made from
steel, is thoroughly encased in the set concrete. As is known, steel is subject
to intense rusting whenever it is exposed to oxygen, which must be prevented at
all times. Affected parts in the reinforcement seriously weaken the concrete construction,
in the present case the diaphragm wall.
With the aim of ensuring that the steel reinforcement is
at all times incorporated sufficiently deeply in the concrete, spacers are fastened
to the reinforcement. These may be plastic rings which are braced against the formwork
within which the concrete is poured. The problem in the construction of diaphragm
walls is that it is not possible for such plastic rings to be braced sufficiently
in the trench. After all, there is no formwork in the trench, so the plastic rings
would have to bear against the fairly soft trench wall. Under the effect of the
heavy weight of the reinforcement, the plastic rings could easily be pressed into
the trench wall, as a result of which the reinforcement still ends up on the surface
of the finished diaphragm wall.
The cause of this pressing of the plastic rings into the
trench wall is the high contact pressure. It would therefore be desirable to reduce
said contact pressure by enlarging the bearing surface of the plastic rings. However,
this is not feasible, because this would weaken the concrete construction.
The aim of the invention is therefore to provide a method
of the aforementioned type wherein it can reliably be ensured that the reinforcement
is incorporated in the set concrete in a sufficiently protected manner. This aim
is achieved by means of a method for the construction of a diaphragm wall in a substrate,
comprising the following steps:
- digging a trench in the substrate,
- filling the trench with a support liquid, such as bentonite,
- introducing reinforcement into the trench,
- introducing a spacer into the trench in such a way that said spacer is located
between the reinforcement and a trench wall,
- pouring concrete mix into the trench,
- removing the spacer from the trench before the concrete mix has set.
The method according to the invention provides the advantage
that the reinforcement is centred in the trench, as required, by the spacers. Said
spacers can have a fairly large surface area: after all, they are removed before
the concrete mix has set, so they do not detract from the cohesion of the finished
concrete construction. On the other hand, said large contact surfaces prevent the
spacers from being pressed into the soft trench wall. Although the spacers are removed
before the concrete mix has set, this does not lead to problems, since by that stage
the centred reinforcement, a portion of which has already been incorporated in the
poured concrete mix, has achieved a fairly stable position.
As stated above, the spacers can be removed from the trench
during the filling thereof with concrete mix. Preferably, in this respect a procedure
is adopted in which the concrete mix is poured to a level in the trench such that
the bottom end of the spacer(s) does not dip into the concrete mix, after which
the pouring of concrete mix and the removal of the spacer(s) from the trench are
then coordinated in such a way that the bottom end of the spacer(s) is continually
kept from dipping into the concrete mix.
Such a procedure has the advantage that the concrete mix
is able completely to fill the space in the trench around the reinforcement in a
reliable manner. In fact it has to be borne in mind that the concrete mix has only
moderate flow characteristics. Furthermore, the concrete mix starts to harden somewhat
after it has been poured into the trench, which further hinders the reliable filling
of the trench. If the bottom of the spacers were then to dip into the freshly poured
concrete mix, there would be a risk that the entire space would not be completely
filled. In this case, pockets of bentonite would remain, which pockets would lead
to holes in the finished construction. By removing the spacers in such a way that
they do not dip into the freshly poured concrete mix, the risk of pockets of bentonite
The method according to the invention is particularly suitable
for the construction of fairly deep diaphragm walls. In this case, the method includes
the step of assembling the spacer(s) from successive spacer parts. This assembly
is done during installation of the spacers in the trench. Conversely, the spacers
are disassembled into the separate spacer parts during removal thereof from the
The method according to the invention can further include
installing support rails along the top edges of the trench, as well as suspension
of the spacers from said support rails. As already stated above, in the case of
a fairly deep diaphragm wall, the spacer will be assembled from separate spacer
parts having a limited length. In this case, the already finished portion of the
spacer has to be temporarily supported every time so as then to allow the following
spacer part to be fitted or else removed.
In this connection, the method according to the invention
further includes the step of straddling a frame across the trench, as well as supporting
the spacers on said frame during introduction of the spacers into the trench. In
particular, said frame can be used in the assembly of the spacers from spacer parts,
the supporting of each top spacer part on the frame, as well as the fastening of
an additional spacer part to the top of the supported spacer part. In this case,
the frame can be supported on the support rails.
As described above, the reinforcement is preferably introduced
once the spacers have been introduced. However, this order could also be reversed.
The invention further relates to a spacer for use in the
method as described above, having at least one essentially flat bearing surface
which is intended for bracing against at least one of the trench walls. Said spacer
may further have a guide surface which faces away from the bearing surface and which
is intended to guide the reinforcement along it during the introduction thereof.
The two faces are parallel to one another, in such a way that the spacers can easily
be introduced into the trench.
The spacer is preferably assembled from a series of spacer
parts coupled one after the other. The top end of the spacer can be provided with
suspension hooks, whereas the bottom end, viewed in longitudinal section, is bevelled
so that it becomes narrower or is made so that it recedes. The bottom end is therefore
somewhat pointed, as a result of which said end is prevented from damaging the trench
wall during the introduction of the spacer. For this reason, the longitudinal edges,
viewed in cross-section, can also be bevelled so that they become narrower or are
made so that they recede.
The invention further relates to a spacer part for a spacer
as described above, having a bearing plate which forms part of the bearing surface
of the spacer. Said spacer part can further have a guide plate which forms part
of the guide surface of the spacer. With regard to coupling of the spacer parts
together, coupling means are provided on the two opposing transverse edges thereof.
On the longitudinal edges of the spacer parts, suspension
means can be provided for the temporary suspension of said spacer parts on the frame
described above. These can, for example, be notches in the longitudinal edges.
The invention further relates to a frame for use in the
method described above, having support means for supporting a spacer. Said frame
can, in particular, have at least two support plates located opposite each other
which can be rotated between a release position, in which the spacer can be moved
up and down in the trench, and a locking position, in which the spacer and the relevant
spacer part are suspended from the frame. The support plates engage in the notches
in the spacer part in such a way that reliable suspension is ensured.
The invention will be explained in more detail below with
reference to the drawings.
- Figure 1 is a view of a trench with reinforcement and spacers according to the
invention installed therein.
- Figure 2 is a cross-section along II-II in Figure 1.
- Figure 3 is a plan view of the trench according to Figure 1.
- Figure 4 shows an enlarged detail of the plan view in Figure 3.
- Figure 5 is a side view of Figure 4.
- Figure 6 is the side view of Figure 5 during the introduction and removal of
a spacer, respectively.
- Figure 7 is a front view of a spacer part.
- Figure 8 is a side view of the spacer part in Figure 7.
- Figure 9 is a plan view of the spacer part in Figure 7.
- Figure 10 is a side view of a top spacer part.
- Figure 11 is a side view of a bottom spacer part.
Figures 1 - 3 illustrate a specific phase of the method
according to the invention for the construction of a diaphragm wall. In this phase,
a trench 2 is dug in the substrate 1. Two elongated reinforcements 3 are positioned
in the trench 1 so as to reach the bottom 4 of the trench 2. In a subsequent phase,
not shown, concrete mix is poured into the trench 2. It is important in this regard
that the reinforcements 3 are completely encased by this concrete mix. This is necessary
in order reliably to protect the reinforcements 3, which are made from steel, against
In this connection it is necessary to keep the reinforcements
3 at all times at a certain distance from the walls 5 of the trench 2. It will be
clear that this distance from the reinforcements 3 to the trench walls 5 must be
maintained over the entire height. According to the invention this is achieved by
means of the spacers 6, which are placed on either side against the trench walls
5. To that end the spacers 6 are lowered into the trench 2 and the reinforcements
3 are then placed between them.
Once the reinforcements 3 and the spacers 6 have been placed
in the trench 2, the concrete mix can be poured. In this respect the procedure is
firstly to pour an amount of concrete mix into the trench 2. During this procedure
it is preferably ensured that the bottom end of the spacers 6 does not dip into
the concrete mix. During further pouring of concrete mix into the trench 2, the
spacers 6 are raised at regular intervals in such a way that the underside thereof
remains at some distance above the concrete mix. This ensures complete filling of
the trench around the reinforcement without pockets of bentonite being able to be
formed. This does not present any problems for the position of the reinforcements
3, since these are already sufficiently stabilised by the amount of concrete mix
Once the finished trench 2 has been completely filled with
concrete mix in this way, the spacers 6 are removed and the concrete mix can set.
The reinforcements 6 are completely encased in the concrete with this procedure,
since during the pouring thereof they were kept at a distance from the trench walls
5 by means of the spacers 6.
Since the method according to the invention is suitable
for the construction of deep diaphragm walls, it is important that the spacers 6
are each made up of separate spacer parts 7. Such a spacer part 7 is illustrated
in Figures 7 - 9. Each spacer part 7 consists of a bearing plate 8 and a guide plate
9, which form a bearing surface and a guide surface, respectively. At the transverse
edges 10 the spacer parts 7 are each provided with coupling sleeves 11, 12, by means
of which the spacer parts can be fastened to one another. As can be seen in the
plan view of Figure 9, the longitudinal edges of the spacer parts 7 are made somewhat
receding so they become narrower. The purpose of this is to prevent the spacers
6 from damaging the trench walls 5 during introduction into and removal from the
In Figures 1 - 6 it can be seen that support rails 13 are
situated on the upper edges of the trench 2. The spacers 6 can be suspended from
these support rails 13 by means of the hooks 14. As illustrated in Figure 4, the
frame indicated in its entirety by reference numeral 15 can be placed on these support
rails 13. This frame has two interconnected crossmembers 16, 16 on which the rotatable
support plates 17 - in the illustrated embodiment, two pairs thereof - are mounted.
As illustrated in Figure 6, these support plates 17 each
serve to support a spacer part 7. For this purpose the support plates 17 are rotated
into the position illustrated in Figure 6 in such a way that they become situated
in the notches 18 that are located in the longitudinal edges of the spacer part
7. In this situation, an additional spacer part 7 can be linked up or else detached.
For this purpose the locking pins 19 illustrated in Figure 6 are inserted into or
else removed from the coupling sleeves 11, 12.
Each spacer 6 can thus be assembled step by step, the support
plates 17 being used in each case. Once an additional spacer part 7 has been linked
up in the manner described above, the series of spacer parts 7 are raised somewhat
in such a way that the support plates 17 can be rotated to the position illustrated
in Figure 5. The series of spacer parts 7 can then be moved further downwards into
the trench 2 and the then top spacer part 7 can again be supported by the support
plates 17 with regard to the linking up of an additional spacer part 7, etc. During
removal of the spacers 6 from the trench 2, the procedure is reversed.
Figure 10 shows on an enlarged scale the top spacer part
7', on the top of which there is a narrowed or otherwise receding portion 20. In
addition, the hooks 13 are fixed to the guide surface 9 of the top spacer part 7'.
Figure 11 shows on an enlarged scale the bottom spacer
part 7", which has a narrowing or receding portion 21 at the bottom. A shape of
this type is suitable for preventing the trench walls 5 from becoming damaged during
introduction of the spacer into the trench 2.