Category: Engineering

Design Considerations

Updated 27/05/2016 – Water and Noise

(Updated 2/6/16 Spelling only)

Our comments on the issues and constraints affecting the 8 sites still in planning:

1. Water

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 sand section, and CFA pile pictures below).

 

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.
cfawall
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.

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. Sand and 3. Disruptions

More information coming – Please look again later.

4. Utilities

Say NO To Sky Rail (Skyrail) - Frankston Line - Engineering
Mc Kinnon Station – Old Service Ducts

Relocation of utilities is a consideration on every large construction project and is business as usual – not insurmountable.  We are currently unaware of any large utility easements of concern.

Given the long lead times before construction begins, we would expect that LXRA and the Utility companies would be communicating any concerns.

5. Noise 

We note that any form of grade separation is likely to reduce some existing rail noise as the need for boom gate bells, some train braking and some mandatory train horns are eliminated or reduced by the grade separation. Since all options would provide the same savings of this kind of rail noise, most of the discussion below is in relation to other rail noise (vibrations of the carriages, tracks, freight engines etc.) across the various options.

Does Government Policy Protect Us?

Maybe.  The good news is that a grade separation will mean changes to the existing rail infrastructure and would require compliance with the Passenger Rail Infrastructure Noise Policy (State of Victoria 2013).

The Passenger Rail Infrastructure Noise Policy (State of Victoria 2013) is available here.

The policy comes into effect if noise levels following the project’s completion is predicted to exceed:
– 65dB(A) during the day (6am-10pm), OR
– 60dB(A) at night (10pm-6am): OR,
– A maximum level of 85dB(A) (at any time).

For reference: 60dB is the noise of a conversation, 70dB is the noise of a shower or dishwasher. 85dB is the noise of a passing diesel truck – hopefully no one has that noise level in their homes from rail now!

Sadly, the bad news is that this policy only requires that the authorities to consider options to avoid, minimize and mitigate, not actually do so. To quote from page 6 (underlines are ours):

If an assessment shows that the investigation thresholds will be exceeded, noise impacts should be considered a primary matter. This means that transport bodies and planning authorities should consider options for avoiding, minimising and mitigating rail noise by applying the policy principles set out in Attachment 3 as a set. Transport bodies and planning authorities may find that there are no appropriate options in some cases.

Transport bodies and planning authorities should seek the views of the Minister for Public Transport and the Minister for Planning.

In accordance with section 21 of the Transport Integration Act 2010, transport bodies and planning authorities should consider publishing a report demonstrating how the principles have been applied. (Page 6, Passenger Rail Infrastructure Noise Policy, State of Victoria 2013). 

Elevated Rail Bridge Noise

Anyone who has every stood near an existing rail bridge in use (near the Melbourne Aquarium for instance) can attest that it is certainly no quieter than the current at-grade rail we already experience.

But currently with the at-grade tracks, the houses immediately on each side of the tracks are the most impacted by noise, as the noise is partially filtered to the houses further back by other houses and vegetation.  This is slightly less obvious where roads occur.  The State government policy noted above mentions this on Page 15 of “Considerations for applying policy principles”: “Shielding provided by other buildings”.

Assuming that the elevated rail bridge produces the same rail noises (as the carriages move over the ends of rails, vibrations etc,), the noise from an elevated rail bridge would be noticeably increased for those houses not currently experiencing the full impact. Based on our conversations with LXRA, each one of the grade separations will be around 1km long in total, so a large area would be affected.

In other words,  the neighbourhoods who currently walk to the station would now start to hear the trains in a way they have not heard them before.

The LXRA document “Understanding rail noise and vibration” claims the proposed elevated structure would reduce noise and vibrations through:

  • walls and screens to mitigate noise transfer
  • new high-quality, continuous smooth tracks
  • purpose-built resilient fastenings to attach the new tracks directly to the structure
  • rubber insulators under the track to dampen vibrations.

We did not find any claim that all noise impacts would be mitigated. Walls and screens do help reduce noise, but would add cost and visual impact – the height of the walls/screens is not noted but these would surely be a new goal for taggers.  Continuous tracks could reduce the “clack clack” noises, but could also be used in a below ground option.  The use of fastenings and rubber insulators are not new to the rail space, and could equally be applied to the below ground option.

As engineers, our concern is how well these mitigations will be working in 5, 10, or 50 years of constant use.

Below Grade Rail Noise (In Ground Rail Design)

It is perhaps stating the obvious to point out that an in-ground rail line (the top of the train below surrounding ground level) would significantly and noticeably reduce rail noise compared to current “at-grade” noise levels.

Other than for those very close by, noise would be largely contained as it was generated within two side walls.

But I heard on the news that a LXRA report for the Dandenong Line which says there will be no extra noise from an elevated rail?

Yes, we get lots of questions about this.  For more discussion on the LXRA’s issued noise reports, please read the noise blog at the “Noise” box down a bit on the engineering main page (click here).  It contains the content above plus new content related to this LXRA report.  We have done this so we can update at one location only.

More Information Coming:

We are continuing our work and will publish more information soon – Please look again later.