Molonglo Gorge Crossing

The existing track through the Molonglo Gorge is one of the most difficult sections of the entire existing line. It is extremely tight, with radii as small as 240m limiting speed to 60km/h or less.  For this reason, few previous proposals have considered an easterly exit from Canberra. While straightening the existing curves is possible, our desired minimum radius is not achievable; the best we can do is about 750m, which will limit even aggressively tilting trains to about 150-160km/h. This is one section we will have to replace entirely.

You could be forgiven for assuming that the steep terrain of Molonglo Gorge is hopeless for any faster alignments, but you would be wrong. A lucky break of geography means that with a series of short bridges (~100m span) and a new tunnel of well under 1km in length, we can achieve an alignment with minimum radius of 5000m+, allowing speeds of 300km/h or greater. The alignment just stays within the exceptional maximum gradient of 3%.

This is going to be an expensive section, but it will be worth it – a few kilometres up the track, we will emerge onto the Carwoola Plains and be able to use large sections of the existing line with minimal modification; many sections of which are already straight enough to be high-speed grade (though the track itself needs replacing).

Alignment design

Molonglo Gorge crossing – click to enlarge

Molonglo Gorge Crossing - Profile

1 – Gorge entry

Section length 600m, sector start. A slight realignment of the existing railway is required to improve the approach angle for the first Molonglo Bridge; this requires some minor cutting of no greater than 5-7m depth.

  • 250m on existing alignment, 350m on new (slightly modified) alignment
    • 100m cut widening; depth 3m = 1800m3
    • 70m cut; depth 5m = 5950m3
    • 50m cut, depth 3m = 2250m3
    • Total cut = 10,000m3 (yes, exactly!!)
    • Cost @$31/m3 = $310,000
  • Works to entire length:
    • Dual ballast track 600m @$2.25m/km =$1,350,000
    • Chainlink fence 1.2km @ $50,000/km = $60,000
    • Drainage 600m @ $200,000/km = $120,000
    • Balises and communication cable (S&C) $127,000/km = $76,000
  • Total section cost = $1,916,000 ($3.19m/km)

2 – Fourth Molonglo crossing

Section length 440m starting at 600m, consisting of a single bridge which crosses the Molonglo River twice.

  • First option: balanced-cantilever segmental concrete bridge with two 220m spans and a single pylon on the middle spur. Lowest environmental impact (single pylon), but highest cost.
    • Cost function 19541e0.005S = $58,704/track-metre
    • Total cost $51,660,000
  • Second option: Balanced cantilever segmental concrete bridge with pylons just above gorge floor in order to minimise mainspan. 3 x 110m spans over river crossings and central spur, and 2 x 55m half-spans at both approaches. Consider as a single bridge with 110m mainspan. Still a relatively low environmental impact (4 pylons).
    • Cost function 19541e0.005S = $33,869/track-metre
    • Total cost $29,805,000
  • Third option: Twin arch bridges of 110m each, with four 27.5m spans across the centre spur, and 2 x 27.5m approach spans at either end. Lowest cost but highest environmental impact (9 pylons required); this is arguably offset by the highly aesthetic qualities of arch bridges. For this reason the arch bridges will be selected.
    • Arches cost function 27358e0.0032S = $38,900/track-metre
    • Arches section cost $17,116,000
    • Approach spans cost function 11725e0.012S = $16,309/track-metre
    • Approach spans cost $7,176,000
    • Total cost $24,292,000
  • Whole length works
    • Dual ballast track at $2.25m/km = $990,000
    • S&C = $56,000
  • Total section cost = $25,338,000 ($57.59m/km)
Three options for the first Molonglo Gorge bridge (approximately to scale)

Three options for the first Molonglo Gorge bridge (approximately to scale)

3 – Fifth Molonglo crossing

Section length 460m starting at 1040m, consisting of an at-grade approach to a bridge across the Molonglo river to the tunnel approach.

  • 140m at-grade; only minor modifications to existing alignment required, but no track can be re-used.
    • Chainlink fence 280m @ $50,000/km = $14,000
    • Drainage 140m @ 200,000/km = $28,000
  • We will utilise the same basic bridge configuration as before in order to minimise engineering cost: Arch bridge with 110m mainspan, the remainder (210m) comprised of 27.5m approach spans. Cost function will be the same as above.
    • Arch cost function $38,900/track-m
    • Arch section cost $8,558,000
    • Beam cost function $16,309/track-m
    • Beam section cost $6,850,000
    • Total bridge cost $15,408,000
  • New dual track along whole section length
    • Dual ballast track $2.255m/km = $1,035,000
    • S&C @ $127,000/km = $58,000
  • Total section cost = $16,543,000 ($35.96m/km)

4 – Kowen Forest Tunnel

Section length 750m starting at 1500m, consisting of a single-bore, dual track tunnel and a cut approaching each portal.

  • Cuts on entry and exit to tunnel are nearly identical, 50m in length and ~7m avg. depth.
    • Volume of 6650m3 each, rock
    • Total cost = $412,000
    • Fencing around cuts basically a rounding error; ignored.
    • Drainage 2 x 50m x $200/m = $20,000
  • Tunnel – 650m dual track single bore
    • $75m/km for high-speed (OD 14.6m), therefore cost = $48,750,000
    • $60m/km for medium speed (OD 12.4m), = $40,300,000
    • Select medium speed in line with policy of avoiding the pitfall of overspecification
    • Excavation volume = 78,496m3
  • Whole length works, 750m
    •  Dual ballast track @ $2.25m/km = $1,688,000
    • Balises and communication cable = $95,000
  • Section cost = $42,515,000 ($56.69m/km)

5 – Cut and fill

Section length 600m starting at 2250m. Consists of a fill running between the riverbank and the pine plantation, and then a cut through a small hill on approach to the third bridge. Assume fill has a slope of 1.5 to 1, and cut has a slope of 1:1.

  • Fill – 400m long,  5m high (average)
    • Volume = 39,000m3
    • Cost = $624,000
  • Cut 200m long, 5m deep  (average)
    • Volume = 17000m3
    • Cost = $527,000
  • Whole length works
    • Dual ballast track = $1,350,000
    • Chainlink fencing, both sides = $60,000
    • Drainage = $120,000
    • S&C = $76,000
  • Section cost = $2,757,000 ($4.60m/km)

6 – Sixth Molonglo crossing

Section length 230m starting at 2850m. Comprises This is a slightly simper bridge than the first two, as it crosses the river almost perpendicularly and can therefore be shorter. A preliminary analysis suggests that one mainspan of 80m and five approach spans of 30m might be appropriate. Perhaps make it five approach spans of 27.5m in total so that we keep the same hardware from the previous bridges (we’ll just ignore the 12-and-a-bit extra metres for the purposes of the cost function.

  • Mainspan (80m)
    • Cost function 19541e0.005S = $29,152/track-metre
    • Bridge mainspan cost = $4,664,000
  • Approach spans (150m, 27.5m spans)
    • Cost function $16309/track-m, = $4,893,000
  • Whole length works:
    • Dual ballast track = $518,000
    • S&C = $29,000
  • Section cost = $10,104,000 ($43.93m/km)

7 – Pine Range Cut

Section length 1750m starting at 3080m. Consists of a fill immediately after the bridge, then a very long and quite deep cut. Assume all the cut is through rock. Section finishes when it crosses over the existing track, which it does at approximately the same level.

  • Fill: 200m at an average height of 10m, 1.5:1
    • Vol. 54,000m3, cost = $864,000
  • Cut: 1550m at an average depth of 12.5m, 1:1
    • Vol. 474,687m3, cost = $14,715,000
  • Whole length works
    • Dual ballast track: $3,938,000
    • Chainlink fence (both sides): $175,000
    • Drainage: $350,000
    • S&C: $222,000
  • Section cost = $20,264,000 ($11.58m/km)

8 – Gorge exit

Section length 1290m starting at 4830m. An embankment bridges two shallow valleys and straightens out the Burbong curves. Rejoins the existing alignment a few hundred metres before the section finish at Burbong Road.

  • Fill: 930m at an average depth of 6m
    • Vol. 117,180m3, cost = $1,875,000
    • Dual ballast track cost = $2,093,000
  • Rejoin existing alignment for 360m
    • Upgrade existing track = 67% of new cost
    • Therefore cost of duplicating and upgrading = $1.875m/km
    • =$675,000
  • Widen cut (average depth 2m)
    • Volume = 6x2x360 = 4320m3
    • = $134,000
  • Whole length works
    • Chainlink fence (both sides) = $129,000
    • Drainage (assume all new required) = $258,000
    • S&C = $164,000
  • Section cost = $4,927,000 ($3.82m/km)

Sector Summary

The total cost of the Molonglo Gorge sector comes to $122,958,000 ($20.09m/km). This is considerably lower than the average cost per kilometre of the AECOM13 ($60m), despite a substantial length of the total in tunnel (10.6%) or on bridges (16.2%). Granted, no stations were located in this sector, nor any co-located infrastructure (like switches, for example), but it is still a suspiciously large difference. This can be explained mainly in the difference in cost estimates for the tunnels, although . While the tunnel specified above is a dual-track, single-bore tunnel of OD 12.4m, AECOM13 specifies twin-bore tunnels each with OD of 11.2m. AECOM13’s cost is $150m/km compared to Hot Rails’ $60m/km.

Component,Cost ($m),Percentage
Civil Works,1.3,1.1%
Signalling & Communications,0.8,0.6%

Earthwork balance: +419,363m3. That is a hell of an excess of excavation spoil to get rid of. And we haven’t even dug the very large Kowen Forest Cut yet – that’s coming up in the next section. Possibly it could be used for embankments on the Canberra Exit section, or for rehabilitating the cuts on the existing corridor which will no longer be needed. When the analysis for all sections is complete, we can begin to look at where we can move our excess spoil to, or if we will need to dump it.


  1. Nice work! When’s next post?

    • Thanks! I’m travelling for a couple of weeks, so maybe 3 weeks? Generally, I hope to stick to a post per week or better.

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