There are two Thames Water egg shaped brick built sewers which are no longer in service and require capping off. These two Thames Water sewers need capping to enable the continuation of the building of the prestigious One Aldgate Place in London.
Sewer inverts depths are 5m and 4.8m. Deep excavation is required to reach the inverts of the Thames Water sewers and allow a good working space all around the sewer – max excavation depth of 6.2m was needed.
There was a 600mm diameter ductile iron Thames Water main in close proximity to the capping works required on the Leman Street sewer. The exact location of the TW main was plotted to ensure a 5m control zone could be established from the 600mm ductile iron TW main as advised.
During the capping works vibration monitoring was conducted in accordance with Thames Water requirements. A vibration monitor was set up in an agreed location and readings were taken (minimum of 2 per day for 10 min) to ensure the agreed level is not exceed in the vicinity of the main (10mms/s) if so the works were to be ceased immediately. SB Civil Engineering anticipated vibrations will be much lower than 10mm/s as a hand held pneumatic breaker was to be used to break the old sewer.
During cutting of the Thames Water sewer and sewer capping works – High pressure stoppers could not be placed in the sewers during capping works for the following reasons;
- There is no access to the sewer from the Leman Street side and therefore the stopper cannot be retrieved once the sewer has been capped.
- There is a ladder in the manhole shaft and this would prevent the stopper sealing the shaft below the invert of sewer pipe which is to be capped.
A handheld air compressed pneumatic breaker was used to carefully break out the brick work. Great care was taken to ensure no fracture occurs within the section of brick sewer which is to be capped. Once the first third of the sewer is broken out the same method was used until the whole of the sewer pipe was neatly cut and the redundant section broken out in its entirety within the manhole box.
The bottom third of the sewer was cut from inside the brick sewer. The sewer face had the first course of brick work carefully cut out and removed with hammer and bolster creating a toothed finish to the brickwork. Class B engineering bricks with a minimum 40N/mm2 were cut where necessary and laid across the face of the sewer. The new brickwork was tied into the toothed section of the exiting thames water sewer.
With completion of the sewer face brickwork, Sikadur31 waterproof render was applied to the whole face of the sewer and left for sufficient time to cure before the next stage of capping works commence. On completion of the waterproof render application to the sewer capping timber formwork was used to create the shutter around the sewer allowing the specified dimensions for the concrete capping. Concrete was placed around the end of the sewer pipe using concrete skips suspended from a 21 tonne excavator.
When the concrete has cured for the sewer capping the timber formwork will be removed and shaft back filled. This completed the the Thames Water sewer capping works in London.