agreed with the Authority following the completion of the immediate works (section
s1.7).
Temporary backfill materials could be used to form immediate reinstatements, although
the use of other forms of temporary backfill has also been considered in this study.
4.3
4.3.1
Potential uses of temporary backfill
Loose, bulk or bagged materials
Possible loose, bulk or bagged materials for temporary backfill include Class A and
Class B materials, natural sand and gravel.
The material that is most used by
respondents to the questionnaire is Type 1 (as defined in clause 803 of the Specification
for Highway Works), but sand and single size aggregate are also used.
Possible uses of loose, bulk or bagged materials are shown in Figure 4.1 to Figure 4.8.
Figure 4.1 shows an immediate reinstatement with the apparatus, surround and
permanent backfill in place. This use of temporary backfill would allow trafficking before
the bound layers are fully excavated and when the old backfill, surround and apparatus
are in place. Temporary backfill could be used in a similar way when there is a delay in
completing a permanent reinstatement. It could be placed over the permanent backfill
before the bound layers are placed. Both these uses require a relatively small volume of
temporary backfill. Thus, installation and removal times should be relatively quick.
Temporary
bound material
Temporary
backfill
Permanent
backfill
Surround and
apparatus
Figure 4.1 – Use of temporary backfill with apparatus, surround and permanent
backfill in place
Temporary backfill could be used to make an immediate reinstatement before the
apparatus is positioned (Figure 4.2), although much material could be required to
completely fill an excavation. Some material could be placed in bags, possibly making it
easier to place and remove, but loose materials would be required to fill the remainder of
the trench.
Temporary backfill could also be used after the apparatus and surround have been placed
(as is common practice to prevent damage to the apparatus in service and future
excavations) but before some or all of the permanent backfill is placed (Figure 4.3).
However, this is not recommended because it would appear to require the unnecessary
handling of material. Generally, once the surround has been placed and there is no need
for further access to the apparatus, permanent rather than temporary backfill should be
placed.
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Temporary
bound
material
Temporary
backfill
Figure 4.2 – Use of temporary backfill before new apparatus has been placed
Temporary
bound
material
Temporary
backfill
Surround
Figure 4.3 – Use of temporary backfill with apparatus and surround in place but
no permanent backfill (although not recommended)
Plate
Temporary
backfill
Figure 4.4 – Use of temporary backfill without surround but with road plate
Figure 4.4 to Figure 4.8 show uses of temporary backfill with a plate rather than an
asphalt layer. One utility and one contractor that answered the questionnaire said that a
plate is used over temporary backfill, but most of the highway authorities that responded
do not use plates. However, it appears that when used, plates are recessed.
Temporary backfill could be used to completely fill an excavation before the apparatus is
positioned and be covered with a plate rather than a temporary bound material (Figure
4.4, c.f. Figure 4.2).
Less material would be required if temporary backfill was used to fill an excavation after
the permanent backfill has been placed but before the placement of the bound layers
(Figure 4.5, c.f. Figure 4.1).
For all five cases described above, it was assumed that shoring would not be required
because the temporary backfill would resist movement of the sides of the trench under
trafficking. Loose compacted Class A and Class B materials should provide sufficient
lateral restraint, but this may not be the case if bagged materials are used and the loose
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material used to fill the voids cannot be compacted well. Also the load applied to the
apparatus may be higher if bagged rather than loose materials are used.
Plate
Temporary
backfill
Permanent
backfill
Surround
Figure 4.5 –Use of temporary backfill over permanent backfill/bound material
with road plate
Plate
Shoring
Temporary
backfill
Figure 4.6 – Use of temporary backfill without surround but with shoring and
road plate
When shoring has been positioned to provide safe conditions during the works, it may be
possible to leave the shoring in position when the temporary backfill is placed. If the
shoring is reduced in height so it does not extend above the road surface, barriers and
kick boards will be needed to prevent accidents. It may be possible to leave the bracing
for the shoring in place when the temporary backfill is positioned providing it is not
damaged by trafficking.
4.3.2
Block and pneumatic elements
Figure 4.7 and Figure 4.8 show how block elements could be used as temporary backfill,
although it would appear that they have a number of disadvantages when compared to
loose (or bagged) materials, as follows:
• A plate would always be required
• Different sized elements would be required to ensure that the plate would always be
supported.
• Movement of the blocks as a result of trafficking and traffic induced vibration would
need to be prevented.
• Wheel loads would be transmitted through the plate and the block elements to the
unbound material at the base of the trench or the surround. There would be a more
direct transfer of wheel loads than through an immediate or permanent
reinstatement. Because the unbound material at the base of a trench is normally
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•
weak, it is likely that it would deform under wheel loading to the point where the
plate would not be fully supported.
Care would be needed to arrange the block elements and the shoring so they could
be placed appropriately in the trench, which may take a long time. Also, the
apparatus may limit the space available.
Because of these reasons, it is concluded that the use of block elements would be
impractical as temporary backfill for most trenches.
Although pneumatic elements would be quick to inflate, their use would involve a lot of
preparation and they are likely to prove more troublesome than block elements. They
would need to be used with road plates.
Pneumatic elements would require support at the base of the excavation and may need
to be used in conjunction with block elements or with a surround in place. They would
need to be inflated to pressures similar to tyre pressures in order to be load bearing. It
would be difficult to find elements of the appropriate size for most trenches that filled the
space available right up to the underside of the road plate when inflated to the required
pressure.
Plate
Shoring
Temporary
backfill in
block form
Figure 4.7 – Use of temporary backfill in block form without surround, but with
shoring and road plate
Plate
Shoring
Temporary
backfill in
block form
Surround
Figure 4.8 – Use of temporary backfill in block form with surround, shoring and
road plate
4.3.3
Preferred types of temporary backfill
It is concluded that temporary backfill is best used for immediate reinstatements.
Temporary backfill could be overlaid with plates rather that asphalt, but this adds a level
of complexity that removes many of the benefits.
In fact, respondents to the
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questionnaire said that the most common reason for using temporary backfill overlaid
with asphalt rather than plating systems is because of its ease of use and safety. This
would not be the case if it was necessary to overlay temporary backfill with a plate. Also,
there may be no plates available of the required size.
During some works, trafficking at speeds of 50 mph or more is not permitted in a lane
next to an open excavation. Whereas temporary backfill would normally be used in
trafficked areas, another potential use would be to fill excavations in footways,
carriageways or central reserve areas, thereby allowing traffic to use adjacent lanes (see
Section 4.4.2.
It is concluded the only suitable form of temporary backfill for both applications would be
loose (and possibly bagged) materials. They must be easy to place, compact and
remove.
4.4
4.4.1
Potential works for temporary backfill
Trafficked areas
The respondents to the questionnaire only use temporary backfill in relatively small
volumes and not for long sections of trench. It tends to be in place for nearer to 3 to 5
days than for less time. Most users have used it over a weekend when no work was to
be done. However, as explained above, it would be possible to completely fill any small
excavation with temporary backfill during road and street works in order to prevent lane
closures because trafficking next to an open excavation is not permitted. Excavations
during kerbing works or those for the installation of gulley pots, manholes etc. may also
be candidates for temporary backfill. Because small volumes of material would be
required, the time to place and remove the temporary backfill would be relatively quick.
If suitably compacted, the temporary backfill would be able to withstand the wheel
loading of an errant vehicle without the need for an asphalt layer.
As described in Section 3.4.5, at the A406 Henlys Corner site, the daytime off-peak
period was only 5 hours long. If it takes an hour each time the temporary backfill and
plate are placed and removed, the daytime working time would be only 3 hours.
Similarly, the overnight off-peak period was 9 hours and the overnight working time
would be 7 hours if temporary backfill and plates were used for both the morning and
evening peak periods.
Because it is unlikely that more than one or two cubic metres of temporary backfill could
be removed within one hour, temporary backfill would be most feasible for relatively
small trenches or when it can be left in place for a relatively long time. Examples of
longer term usage include:
• During delays encountered during emergency and planned works
• On (some) weekdays, including when there is only weekend working
• On weekends
• For special events
Therefore, the most likely scenarios for temporary backfill would appear to be one, or
some combination of, the following:
1. To allow small trenches to be trafficked in peak periods.
2. After the permanent backfill has been placed and there is a delay in filling the trench.
3. When there is a long time delay during the works, e.g. from the morning to the evening
peak period, weekdays, weekends, special events (e.g. parades, market days).
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A meeting was held with Thames Water to discuss the possible use of temporary backfill
for works to install two 600 mm diameter flow meters in Lane 2 northbound of the A41
Hendon Way in Autumn 2012. A work zone comprising a 50 m length of Lanes 1 and 2
has been requested for the works (using conventional methods), but this would leave
only Lane 3 open to traffic for the planned duration of the works of 20 days. TfL has
requested that at least two lanes be kept open at peak hours and has asked Thames
Water to consider the potential to use road plates or temporary backfill to achieve this
aim.
Thames Water was originally intending to make one excavation of approximate width 2 m
and length 3 m to enable the installation of both flow meters. The depth of the
excavation is to be approximately 3 m. At the meeting, it was proposed that the flow
meters be spaced further apart so two smaller excavations can be made. Whereas these
could be plated during peak hours, this would take time and there may be problems if
the size of the excavation was larger than the opening for which the plates available were
designed. There were road worker safety concerns if the works were carried out during
peak periods because of the problems with access to Lane 2 and traffic management with
traffic in Lanes 1 and 3 at peak times. It was suggested that the works be carried out at
weekends and that the excavations be filled with temporary backfill to enable trafficking
in Lanes 1, 2 and 3 during the week. Because the traffic flow is tidal, work could start
shortly after 20:00 on Friday and could continue until 15:00 on Monday.
Shoring would be required for such a deep excavation and this would need to be trimmed
so it did not extend above the road surface. It may not be appropriate to traffic the
props, but if this is the case, they could be removed in stages as the temporary backfill is
placed. The props could be replaced as the backfill is removed. The amount of
temporary backfill that is required may be limited. As an example, breaking out of the
concrete may take the first weekend so the temporary backfill would only need to replace
the bound layers and shoring may not be required. The excavation could be completed,
the flow meter replaced and the permanent backfill placed during the second weekend.
The permanent reinstatement could be completed with rapid curing concrete and asphalt
on the third weekend. This approach would avoid any closures during peak periods and
reduce the total site occupation days required.
4.4.2
Untrafficked areas
The feasibility of using temporary backfill in areas that would not normally be trafficked
was considered for the highway works at the A406 Henlys Corner (McMahon et al.,
2011). During part of the works, drainage was to be placed in the central reserve. TfL
wanted an extra lane to be open at peak times but the contractor considered it unsafe to
do this with an open trench in the central reserve, in case an errant vehicle left the traffic
lane. After discussion with the contractor it was concluded that the trench would be safe
if the trench was filled to the level of the road surface with fully compacted Type 1
material. It was considered unnecessary to cover the trench with a plate because it
would be subject to very few if any wheel loads. The opportunities for this application
were thought to be limited because of the planned works programme, but times when it
could be trialled were proposed.
4.5
Why use temporary backfill?
Temporary backfill is potentially the most versatile of the three main techniques that are
being considered for reducing congestion. It does not require special materials (in the
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