1. Ground Water
So far, our reviews of publicly available information have indicated that the approximate ground water levels are lower than the depth of excavation. Where the ground water levels are lower than the excavation level, neither dewatering or exclusion walls are required. However, seasonal fluctuations are possible, so we have discussed temporary and permanent exclusion below.
Temporary exclusion or “dewatering” is often used during construction, to lower the water table in the immediate area with subsoil drainage or wellpoints and pumps. Once these measures are stopped, the ground water levels return to their original level. Granular soils like the coastal sand in this area lend themselves to dewatering as water moves easily through the gaps between each piece of sand (pore spaces).
Permanent exclusion requires a low permeability or impermeable barrier to stop the flow of water within the ground, such as slurry walls, sheet piling, and pile walls. It is likely that the sandy conditions would require the walls of the cutting to be made from such walls, thereby providing exclusion walls by default (see CFA pile wall pictures below).
2. Pile Walls
If the diagram to the left is not clear, there are some great youtube clips showing how CFA piles are contructed:
These are some of the piles that were used on the McKinnon/Ormond grade separation. A pile wall was also used on the Springvale grade separation, with the same “water table” issues. The specific pile and foundation design needed will be a matter for the detailed design team – these notes are intended as general explanation. We note that CFA piles are particularly suited to wet sand, with minimal noise and vibration. The installation of piles occurs before any excavation takes place – this allows the work to proceed with minimal ground impact until piling is completed.
Once completed, the piles form a wall which can be safely excavated to the desired depth, although the pile length will go past the excavation depth for design reasons. A base slab (if required) will then be poured to support the rail, and tie-into the walls. The wall face can then be finished as appropriate with pre-cast panels, shotcrete or cast in place concrete.
There are some great youtube clips about this kind of grade separation being constructed, in “high water level” areas:
- Vicroads timelapse of the construction of Springvale Grade Separation
- Alliance video of the challenges and victories of the Springvale Grade Separation
And so you can imagine what the non-elevated option might look like here are some other “rail under road” options from the LXRA, in the days before “elevated rail” became the main option being marketed.
- Vicroads timelapse of Mitcham level crossing removal
- LXRA Package 2: Blackburn Road, Blackburn; Main Road, St Albans; Furlong Road, St Albans and Heatherdale Road, Mitcham.
3. Flood Water
The track area would be a catchment area requiring drainage for heavy rainstorms. Drainage would be provided by a sump at the low point and possibly intermediate cut-off drains and sump, with sump pumps, pumping to the public drainage system after oil removal. (This is a typical arrangement in high rise basements for instance).
There could also be an opportunity for filtration, storage and re-use of this water for landscaping.