Alternatives
Package TU-03 Tunnel Lining, three design options assessed against the baseline
Baseline
C50/60 OPC precast
Embodied carbon
78,410
tCO2e
Reference case
Whole life cost, NPV
$212.6M
capital $184.2M
Reference case
Module D-4,180 tCO2eBiogenic470 tCO2e
Alt 1
50% GGBS blend
Embodied carbon
61,240
tCO2e
21.9% vs baseline
Whole life cost, NPV
$215.8M
capital $189.7M
1.5% vs baseline
Module D-6,640 tCO2eBiogenic630 tCO2e
Alt 2
30% GGBS plus optimised segment geometry
Embodied carbon
54,880
tCO2e
30.0% vs baseline
Whole life cost, NPV
$212.4M
capital $187.1M
0.1% vs baseline
Module D-5,720 tCO2eBiogenic700 tCO2e
Carbon by life cycle stage, package TU-03
BaselineAlt 1Alt 2
Cumulative carbon over the 60 year study period
BaselineAlt 1Alt 2
Baseline upfront share: 79%Alt 1 upfront share: 75%Alt 2 upfront share: 74%
The curves never cross, so there is no carbon payback period to wait out: Alt 2 is lower from day one. 92% of its saving is banked at practical completion, before the asset carries any traffic. Operational carbon accrues on the declining grid factor, which is why the curves flatten after year 30 rather than continuing straight.
| Indicator | Unit | Baseline | Alt 1 | Alt 2 |
|---|---|---|---|---|
| GWP total | tCO2e | 78,410 | 61,240 | 54,880 |
| GWP fossil | tCO2e | 77,940 | 60,610 | 54,180 |
| GWP biogenic | tCO2e | 470 | 630 | 700 |
| Module D, beyond boundary | tCO2e | -4,180 | -6,640 | -5,720 |
| Acidification | kg SO2e | 184,200 | 151,880 | 138,410 |
| Eutrophication | kg Ne | 12,840 | 10,970 | 9,880 |
| Capital cost | USD | $184.2M | $189.7M | $187.1M |
| Maintenance cost, 60 year | USD | $28.4M | $26.1M | $25.3M |
| NPV | USD, 3.5% real | $212.6M | $215.8M | $212.4M |
| Buildability score | 0 to 10 | 8.4 | 7.1 | 7.6 |
Recommendation
Alt 2 delivers a 30.0% carbon reduction for a 0.1% whole life cost saving, at $89 per tonne CO2e avoided across 23,530 tonnes. This sits below the program shadow carbon price of $145 per tonne.
Percentages are stated on the NPV basis shown in the table above. Abatement cost is read from the marginal abatement cost table, so the scenario builder, the MAC ranking and this recommendation cannot diverge.
NEXT STEP
Weigh carbon against cost
Pareto frontier and abatement cost curve