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| Recent paving in the Barn Hill Conservation area - why not Mapesbury? |
A Mapesbury resident who is a retired Civil and Structural Enginer (MIStructE and MICE) has carried out a pavement survey of Dartmouth Road where Brent Council wishes to replace the paving in this Conservation Area road with asphalt.
The survey has been forwarded to the Council and members of the Mapesbury Residents Association.
The works have been temporarily suspended giving time for review and it is hoped that in the light of the Council's Climate Emergency Declaration an assessment will be made of the comparative carbon footprint of renewing broken paving compared with taking paving up and replacing with asphalt.
Dartmouth Road Pavement Survey.
Carbon footprint of proposed replacement of paving slabs by asphalt
Date: 20.11.19
This report is based on a detailed survey of the pavements on both sides of Dartmouth Road:
A: between nos. 103 to 131
B: between nos. 60 – 92
Which is ¼ of the length of Dartmouth Road.
C: between nos. 1 – 24. This last section has recently been repaved and is in excellent order. See appendix for the survey results for this section.
Pavements are on average 2700 mm. wide and are formed using 600 x 750 and 600 x 600 precast concrete paving slabs. The width consists of two of each size staggered:
(2 x 750 ) + ( 2 x 600 ) = 2700.
Each property is approx. 10 m wide and so there are theoretically 16 ½ x 4 = 68 slabs per property. But many properties have vehicle crossovers which reduces the number of slabs. The crossovers are either of concrete or block paving construction or a mix of one of these plus paving slabs. There are also a very few tarmac crossovers.
Where trees occur the paving is extended upto the tree or there is a resin gravel type infill upto the tree, or occasionally tarmac or nothing with the soil visible. Whatever has been installed next to the tree has usually failed in some way and is uneven. These areas have the most trip hazards.
Concrete and concrete block vehicle crossovers have performed best and are often in good serviceable order. Any area of crossovers that is surfaced with paving slabs is in poor condition with on average over 1 / 3 of the paving slabs cracked.
Good condition slabs . | Cracked slabs . | ||||
Between crossovers | At vehicle crossovers | Total | Between crossovers | At crossovers | Total |
1540 | 243 | 1783 | 129 | 72 | 201 |
% of total 129/1540= 8.3 % | % of total 72 / 243= 29.6% | ||||
Whole length of Dartmouth Road pro-rata (i.e. x 132 / 32 ) | |||||
6350 | 1000 | 7350 | |||
It can be seen that paved crossovers contribute nearly 4 times the rate of cracking that occurs in areas between crossovers.
It can also be seen that within crossovers there are 243 – 72 = 171 uncracked slabs which is comfortably more than the 129 cracked slabs within the paved areas between crossovers. So when the defective crossovers are replaced there will be sufficient uncracked slabs recovered to replace all the cracked slabs between the crossovers.
We can extrapolate this detailed survey to the whole of Dartmouth Road because a visual inspection indicates that the area surveyed in detail here is of the same configuration as the whole of Dartmouth Road and the total figures for the whole road are shown in the table.
The plan proposed by Brent Council is to remove all the paving and replace the crossovers with block paving and to infill between the crossovers using asphalt.
Therefore the number of paving slabs to be removed from between crossovers and dumped is 6350 slabs. The slabs are 50mm thick
The average weight of a slab is (.6m x .675m x .05m) x 2000 kg / m cube = 40.5 kg
Therefore the weight of slabs to be removed = 6350 x 40.5 / 1000 = 257 tons. These slabs have to be lifted, piled up and then grab loaded onto a lorry and taken to a dump. Such loading will achieve about 10 tons per typical 8 wheel lorry giving rise to 26 lorry movements. And the dumping will attract landfill tax. More seriously, these slabs will remain in landfill unchanged for centuries.
Asphalt requires a well compacted base layer of stone which will have to be imported because the sand / soil / clay found under the existing slabs when lifted will need improvement.
The area of new asphalt will be the same as the paving removed = 6350 x 0.6 x 0.675 = 2570 m square.
Allow for a restored formation thickness of 75mm, this will require the removal of 75mm of existing soft material ( to maintain existing levels) and reinstatement using 75 mm of stone material. So the volume of material removed and replaced will be 2 (1 removal and one replacement) x 2570 x 0.075 = 385 m cube = 770 tons. Which at 15 tons per lorry for this type of material gives rise to 51 lorry movements.
The amount of asphalt required if 20mm thick will be 2570 x .02 = 51 m cube = 100 tons at least, requiring another 10 lorry movements.
So the total weight of materials being lifted and moved is 257 + 770 + 100 = 1,127 tons requiring 26 +5 1 + 10 = 87 lorry movements.
The lorries will have to travel to a dump outside London and the asphalte and stone will have to be brought in from a quarry, also well outside London. This represents a lot of diesel at 10 mpg! And this is just Dartmouth Road. For the whole of Mapesbury Estate there will be at least 6 x these quantities, i.e nearly 7,000 tons of materials dumped or imported, involving 500 plus lorry movements.
Furthermore, since Brent plans to change all the pavements in the Borough for asphalt, the quantities will become huge.
It is obvious that the proposed use of asphalt to replace existing slab paving is totally unacceptable on carbon foot print grounds for material handling and transport and the energy intensive manufacture of the asphalt itself, which is a heavy crude oil hydrocarbon based product. Levelling the slabs in situ would have minimal carbon foot print and lead to a pleasing durable solution appropriate for a Conservation Area.
Appendix: P
Appendix:The section of Dartmouth Road between Mapesbury Road and Exeter Road has also been surveyed in detail as follows. This section was re-laid using slabs 5 years ago and is in very good condition and shows how successful slab paving can be when well laid.
Survey on Dartmouth Road between numbers 2 and 14 and the opposite odd numbers side which is in equally good condition | |||||
Good condition slabs . | Cracked slabs . | ||||
Between crossovers | At crossovers | Total | Between crossovers | At crossovers | Total |
1043 | No paved crossovers, all are conc. or blocks or tarmac | 1043 | 19 * | No paved crossovers, all are conc. or blocks or tarmac | 19 |
% of total 19/1043 = 1.8 % | % of total 19/1043 = 1.8 % | ||||
About half of these cracked slabs are so finely cracked it is difficult to see the crack because they have not moved having been very well laid.
This table applies to only 15 % of Dartmouth Road and so would not affect the pro-rata totals used above very much and in fact further support the case for retaining all the existing paving in Dartmouth Road.