A1 CTRL trackbed support : minimisation of waste by the integration of design and construction with maintainability

Nick O’Riordan, Director, Ove Arup and Partners Ground Engineering Manager, Rail Link Engineering, London

Introduction Today’s high speed railway lines are being procured, built and maintained under ever- increasing financial and environmental scrutiny. Certainty of outcome and performance are the twin challenges facing the industry. UIC has a central role in shaping design standards and those standards can have a major impact on the end product. The design and construction of the UK Channel Tunnel Rail Link (CTRL) has highlighted areas, particularly in construction waste minimisation, where greater rigour can provide large financial and environmental dividends. Waste management strategy The following strategy was adopted for material excavated from within the site of the CTRL construction works: Category I: acceptable material shall be used in engineered fills necessary for the structure of the railway and associated highway works. Category II: acceptable material shall be used in mitigation such as earthworks environmental bunds, landscaping, etc. Category III: acceptable material which cannot be incorporated into the Works shall be made available to third parties. Category IV: topsoil and agricultural subsoil from agricultural land of grades 1, 2 and 3A required for restoration which shall be separately stripped and stockpiled. Category V: Waste, that is material that does not fall into the categories above, shall be disposed of:  on sites within the Limit of Land to be Acquired or Used (LLAU); or  local sites; or  to more distant sites within the route corridor, or north of London. This explicit statement is embedded in the documents supporting the planning consent and is incorporated into the performance requirements for all designers and contractors working on the project. As a result the project has the following characteristics: 1) adheres to existing transport corridors 2) runs in tunnel beneath urban areas or where large surpluses of excavated material would otherwise result 3) adopts a low, ground-hugging profile to minimise visual and noise intrusion 4) uses excavated material either within the works or to enable nearby regeneration projects 5) adopts a generally ballasted track construction, coupled with rigorous dynamic analysis of selected structures and earthworks to confirm that the specified performance, including ride quality, will be met 6) makes use of available feedback from maintenance regimes of other high speed railways to refine design and construction practices. Earthworks and trackbed support Section 1 of the project has resulted this breakdown of earthworks volumes, given in million m3:

Contract Cut Railway and Mitigation Fill Highway Embankment Fill 330 3.0 1.0 2.0 350/410 1.0 0.9* 0.1 420 4.3 1.5 2.8 430 2.0 0.7 1.3 440 2.0 0.8 1.2 *includes material exported to A2/M2 road widening contract by others

It can be seen that the majority of the cut volumes went to mitigation fill, for environmental noise and visual screening bunds. Some treatment in the form of lime or cement stabilisation of the embankment fills were needed in order to achieve a viable formation in wet weather. Studies of the in-service maintenance records of TGV Nord (RLE, 1999) revealed that the incidence of tamping and realignment of the running lines was unrelated to the use of lime or cement-stabilisation measures in construction. However, we were unable to build on this experience, for example by reducing Prepared Subgrade (PS) thickness as a result of the strengthening effect. This is because the method of design in UIC 719R (International Union of Railways, 1994) does not permit modification of PS thickness to reflect such changes in soil properties. To illustrate the effect of the restrictive nature of UIC 719R , a change in thickness of 0.15m on the total volume of PS in Section 1 of the CTRL is in the order of 160000m3 or about 20% of the total volume of imported fills that make up the trackbed support. Furthermore, largely because it displays no underlying engineering rationale, UIC 719R contains no guidance on ‘unsuitable soils’, other than to suggest generic forms of ground improvement. An extreme example of where authoritative guidance is needed is found in Holm et al ( 2002) who describe the successful remedial works at the 200 km/hr design speed line at Ledsgard, Sweden. A new railway on embankment comprising 0.5m ballast on just 0.9m of sub-ballast on soft clays and peat, presumably designed in accordance with accepted standards, opened for passenger traffic in 1997. Due to the development of dynamic displacements in the order of 15mm under the X2000 train loading, a speed restriction of 130 km/hr was applied almost immediately. The poor performance of the 1997 embankment was entirely predictable, with knowledge of analytical work by, inter alia, Selig and Waters (1994) and Fryba (1973). A treatment process involving deep dry soil mixing of the soft ground was evaluated by Holm et al (2000) using a bespoke dynamic analysis of the embankment. This enabled the performance under train loading to be predicted, using engineering material properties. O’Riordan and Phear (2001) describe a similar process where the rational engineering design approach for trackbed support of Li and Selig(1998) has been successfully modified using both static and dynamic analysis methods to permit the prediction of ride quality and maintenance outcome for the CTRL. In order to place the UIC 719R guidance into a rational context, analyses have been undertaken to establish the equivalent dynamic wheel load which produces the same thickness of trackbed support layers, over a subgrade fatigue life of 120 years, under Eurostar trains at an average of 8 trains/day. Figure 1 shows an outcome of these studies. UIC guidance seems to conform to a dynamic wheel load of about 350 kN, which can be compared with a Eurostar static axle load of 175 kN, and the AREA (1996) recommended value of 683 kN.

Required Thick nes s of Granular Laye rs vs. Design Life for Various Dynam ic Loads for Soil Type CH, re se m bling UIC s oil quality clas s QS1 (O'Riordan and Phe ar, 2001) >1.5 >1.5 >1.5 >1.5 1.4

1.2 UIC 719R Dynamic Load = requirement 250kN )

m 1 (

s r Dynamic Load = e y

a 350kN L

r 0.8 a l u

n Dynamic Load = a r G

683kN

f 0.6 o

s Notes s e

n 1) Granular layers are ballst+sub- k c i 0.4 ballast+prepared subgrade h T 2) Maximum plastic strain in soil=2% (approx 25mm settlement over design life) 0.2

0 0 20 40 60 80 100 120 140 Des ign Life (years ) F Figure 1 Variation of thickness of trackbed support with design life.

Piled Slab optimisation Dynamic analysis has also enabled the optimisation of the 7 km long piled slab for the Thames Marsh crossing. The ground conditions comprise up to 12 m of soft clays and peats, and the rail alignment is constrained to run as close as possible to existing marsh level, at about +2 mOD. Figure 2 shows a visualisation of the analysis: the train mass, suspension system, rail, sleeper and ballast system, are modelled, having been calibrated using measurements on existing rail systems.

Figure 2 Visualisation of LS-DYNA analysis of piled slab from Eurostar loading

Figure 3 shows the variation of maximum pile force, which is incorporated in design. Use of UIC 776 (UIC, 1994) would result in a maximum design value of 560 kN/pile from train loading at the serviceability limit state, whereas dynamic analysis produces a worst credible value, for the condition of crossing Heavy Freight trains, of 340kN/pile. As a result the reinforced slab is only 450mm thick, and is founded on 600mm diameter piles in groups of four, at typically 5m intervals, founded at about 14m depth. Such a configuration is broadly equivalent, using sustainability calculations (either embodied energy or CO2 equivalent, after BRE,1999) to a 4m high, locally sourced embankment. Such a high embankment would have been technically equivalent to the piled slab. It would however be unacceptable on visual and airborne noise grounds, requiring extensive mitigation measures, which would tip the sustainability calculations firmly in favour of the piled slab.

Figure 3 Variation of force in pile during passage of Heavy Freight train, at critical speed of 98 km/hr These examples show the result of careful attention to waste minimisation processes and the development of analytical tools that go beyond the often-cautious guidance in specific UIC documents. These new processes enable prediction of performance over the design life. In this way the CTRL project is resulting in a successful integration of design and construction with long term maintenance.

References American Railway Engineering Association, AREA (1996) Manual for Railway Engineering. Vol 1 Washington DC. BRE (1999) Methodology for environmental profiles of construction materials, components and buildings, Report BR370, UK Building Research Establishment. Fryba L(1973) Vibration of solids and structures under moving loads. Noordhoff, Groningen. Holm G, Andreasson B, Bengtsson P-E, Bodare A, Eriksson H (2002) Mitigation of ground vibrations by high speed trains at Ledsgard, Sweden. Swedish Deep Stabilisation Research Centre, Report 10. Swedish Geotechnical Inst, Linkoping Li D and Selig ET (1998) Method of railroad track foundation design . Jnl Geotech and Geo-enviro Engng, ASCE 124 (4) pp 316 to 39. O’Riordan N and Phear A (2001) Design and compaction control of ballasted track formation and subgrade for high speed lines. Railway Engineering 2001, London. RLE(1999) Treatment of earthworks materials using lime and /or cement from long term experience on TGV Nord, France. Rail Link Engineering technical report for Union Rail, London. Selig E T and Waters J(1994) Track geotechnology and substructure management. Thomas Telford London UIC 719R (1994) Earthworks and trackbed layers for railway lines. International Union of Railways UIC 776-1R (1994) Loads to be considered in railway bridge design. International Union of Railways