September 11, 2016 Peter 2 comments

Updated 11/09/2016

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

CFA Continuous flight auger
Step by step section of CFA auger in wet and sandy soil – 1. Drilling Down 2 and 3. Center of auger fills hole with concrete as auger comes up 4. Reinforcing cage (if used) is pushed into hole before concrete sets up.

If the diagram to the left is not clear, there are some great youtube clips showing how CFA piles are contructed:

Piling contractors

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.

Drill rigs at Ormond
Drill rigs working at Ormond grade separation, reinforcing cages in the foreground
springvale grade separation under construction piled walls
Springvale Grade Separation under construction – showing pile walls exposed after excavation and prior to wall finishing.
Pile wall after excavation, with a soil anchor at bottom
springvale grade separation under construction
Springvale Grade Separation under construction – following completion of walls and base slab





There are some great youtube clips about this kind of grade separation being constructed, in “high water level” areas:

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.

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.

2 Comments on “Water

  1. You have provided a good explanation for temporarily excluding and managing groundwater impacts during construction. But what about the impact that a permanent 800m long, up to 12m deep “barrier” would have on natural regional groundwater flows across the rail cutting zone?

    The station precinct is part of the Edithvale-Seaford wetlands bio-region.
    It’s a Ramsar listed wetlands, that is susceptible to changes in drainage and subsurface conditions. How would a permanent tanked trench impact on the groundwater flows and wetlands water levels in the region?

    Thinking longer term, beyond the 4-8 year promises of the current state government, what would happen when level crossings north and south of Edithvale are subsequently removed? (Which is much more likely to happen than line expansion to 3 or 4 tracks.) If Edithvale was trenched, would that exclude others from being trenched due to the detrimental effect of multiple adjacent 800m long groundwater barriers?

  2. As an old water & sewerage construction engineer I think you have significantly downplayed the difficulties of coping with groundwater. Wellpoint dewatering is not as simple as you perhaps believe and brings with it the problems of noise from pumps, filtration/disposal of groundwater and the likely necessity to recharge groundwater to prevent subsidence. Since the groundwater level in my Aspendale property is about 0.9 m below the surface (personal experience when installing netball posts) then I think it very likely that excavation for railway cuttings would be severely affected by groundwater. In the late 60s and early 70s the sewering of this area was predicated on the rule of thumb of not going deeper than 5-6 metres due to the overall difficulties of construction in waterbearing ground. This led to the network of sewage pump stations as sub-catchment sewers approached 6 m deep. In my opinion you cannot realistically compare excavation around Bentleigh-McKinnon with the Mordialloc-Frankston corridor.
    On a related note, I saw an item in the local paper that people were getting upset that the Mentone station would be altered. I would like someone to consider how anything can be done at the Balcombe Rd crossing without affecting that station building. Elevated rail probably gives a fair chance that it could be partially preserved, but lowered rail would seem to require total removal.
    I am not personally or aesthetically opposed to elevated rail – I can stand at my letterbox and look west to the existing train line and catch glimpses of the trains at house roof level as they run along the (relatively) elevated existing embankment between Mordialloc and Aspendale.

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