Since the 1990s, The Sprayed Concrete Lining (SCL) method of tunnel construction has been commonly used in the UK and the technologies associated with SCL have been constantly developed. For instance, due to health and safety and other issues on a large scale project like Crossrail, lattice girders and traditional mesh reinforcement were eliminated (Pickett, 2013). A double shell fibre reinforced sprayed lining sandwiching a sprayed waterproofing membrane became a standard lining system. And still, in many projects, the primary lining is completely sacrificial and is only supposed to take temporary loads.

The tunnelling industry is pushing for a more efficient use of SCL in soft ground by utilising the recent advancement and improvement in materials and construction plant (Smith, 2016). Design methodologies should push to catch up with the recent developments in SCL construction technology, and benefit from a durable primary lining and the high quality concrete which potentially can take temporary loads as well as permanent loads due to high concrete strength in both early age and long term (Su and Thomas, 2013). Relatively few studies have been carried out to investigate SCL lining behaviour and the interaction between the primary and secondary lining where the primary lining participates in the permanent bearing structure. For instance, Marcher et al. (2011) addressed the design challenges of lining systems consisting of a dual or multiple layers and proposed a two dimensional method to study the load sharing between the lining compositions. Su (2016) investigated the impact of composite action on the behaviour of SCL lining using a two dimensional numerical modelling.

He concluded that the load sharing is a complicated issue and depends on many factors, however, a design assumption of no bond or composite action is not necessarily conservative particularly for serviceability. In the Bank Station Capacity Upgrade (BSCU) project, which is similar to the Crossrail design, both primary and secondary linings carry the permanent loads assuming a combined interaction between the lining components rather than a composite action. However, unlike Crossrail, the proportion of the long term ground load taken by the primary lining is not prescribed; the load share between the primary and secondary linings is the outcome of a three dimensional numerical analysis. The methodology of analysis, design principles and considerations to be taken into account for a combined lining system are presented in this paper.

1 – BSCU project and ground conditions

London Underground (LU) awarded a design and build contract to Dragados for the BSCU. This project improves capacity of Bank station and reduces journey times and congestion on the Northern line (NL), Central line (CL) and DLR areas and interchange routes within the Bank-Monument station complex (Figure 1). It includes the provision of a new passenger entrance with lifts and escalator connections; a new NL passenger concourse using the existing southbound platform tunnel; a new NL southbound running and platform tunnel; and new internal passenger connections between the NL, the DLR and the CL. The scheme ensures that the station can maintain acceptable passenger flow conditions, including a margin for forecast demand growth, for at least a further 60 years. Dr Sauer and Partners (DSP) was commissioned to provide design of the SCL structures and assess the impact of the new works on the LU Civil Assets within the Bank and Monument stations (see Haig and Feiersinger, 2016 for more information).

The new SCL structures lie within the typical London Clay, underlying 8m of River Terrace Deposits and made ground. The axis of the new tunnels at CL, NL and DLR levels is nearly 20, 30 and 40m under the subsoil surface, respectively. The groundwater table is assumed 8m below the surface.

2 – SCL design principles

As addressed in the introduction, there are challenges in design of a multilayer sprayed concrete lining system whose primary layers lie outside of the sprayed waterproofing membrane but are considered as part of the permanent support.

The challenges include: the load share between the lining compositions, the interface behaviour (e.g., bonded/unbonded, ability to transfer shear) and what would be the implication of those design assumptions on the lining performance? In order to make such a design concept into success, at the conceptual design stage a set of design principles were agreed with the client to ensure that the optimised combined lining system is able to fulfil the project requirements; the principles are:

  1. Despite being located outside the waterproofing membrane, the primary lining is considered to be durable. Using modern admixtures, mix designs and proficient application, the durability of sprayed concrete is considered to be similar to the durability of regular cast concrete underground. It is not exposed to freeze-thaw temperature variations or wet-dry cycles normally associated with concrete deterioration and once the waterproofing membrane and secondary linings are installed, will be in an anaerobic environment. Satisfactory results from the ground investigation shows the environment is in line with previous conditions for durable concrete in London Clay, including ground and groundwater chemistry and groundwater migration.
  2. A double bonded spray applied waterproofing membrane is located between the primary and the secondary lining to provide a watertight lining system that prevents the migration of water along lining interfaces. The waterproofing membrane is relatively flexible and capable of bridging cracks in both linings up to 3mm if applied to a sufficient thickness.
  3. As per LU-S1055 the design life for all Deep Tube Tunnel assets is a nominal 120 years. The design life for the primary lining acting alone (i.e., prior to construction of secondary concrete lining) is five years.

The principles defined above provide the following potential benefits:

  1. Durable primary lining and unaggressive groundwater condition allows that the primary lining can be considered part of the permanent support.
  2. A permanent primary lining which can contribute to the long-term load capacity of the lining system. In the temporary condition (i.e., before the secondary lining is installed) the primary lining can be designed with a lower load partial factor adjusted for the design life of five years. For instance, the adjusted load partial factor of 1.2 is used for the ground loads (Spyridis, 2014). However, once the secondary lining is installed, the combination of the primary and secondary lining system can be used to resist the long-term load conditions including full partial factors compatible with a 120-year design life. In this case ground load partial factor of 1.4 is used. This potentially results in a thinner primary lining.
  3. The interface should be considered as bonded assuming no shear stress is transferred between the primary and secondary layers. However, realistically a slight shear stress may be transferred at the interface. This system is called combined lining which is not a double shell nor composite lining (for definition of double shell/composite systems see Marcher et al. 2011). The permanent loads carried by the combined lining are shared between the primary and secondary shells proportionally to their stiffness. The shared load between the two components of the lining is determined through three dimensional Finite Element Analyses (3D FEA). Considering the fact that in a case of a fully saturated primary lining and the waterproofing membrane, the characteristics of the bond strength between the primary and secondary lining may degrade considerably, worst case scenarios for the design of the secondary lining to be taken into account where no bond and no shear transfer are presumed in the primary and secondary lining interface. This load case and FEA method of analysis is discussed later in this paper.
  4. Serviceability criteria of the linings were confirmed through assessment of durability, function and water tightness requirements. As a bonded waterproof membrane will be used and the public areas are fully clad, serviceability compliance can be confirmed without SLS analysis and therefore only ULS analysis were performed. Thus, the designer is less concerned about the crack width size at the secondary lining intrados in case where shear stress is transferred considerably at the waterproofing interface. This concern has been raised by Su (2016) that the design assumption of no shear transfer at the interface may not be conservative in terms of serviceability.

It should be noted that the beforementioned design assumptions/principles were also adopted by the appointed independent checker (a so-called CAT III checker) that is part of a standard assurance process in the UK. Otherwise all the attempts in optimisation of the lining thickness will not be approved.

3 – Lining System and Design Requirements

The SCL lining system is a double shell lining (primary and secondary linings) with both linings considered part of the permanent load bearing structure. A double bonded spray applied waterproofing membrane is sandwiched between the linings (Figure 2).

3.1 – Minimum thickness for secondary lining

In order to prevent introduction of a potential degradation path for the secondary linings, no fixings installed from inside the completed tunnel structure shall penetrate the waterproofing membrane. In order to comply with this design principle the secondary lining has been considered with a minimum thickness of 150mm; this is not necessarily a structural requirement, but a minimum considered sufficient to prevent penetration of the membrane.

The minimum secondary lining thickness of 150mm does not include the secondary lining regulating layer, used in tunnels with a sprayed secondary lining to cover protruding fibres.

3.2 – Fire resistance Requirement

There is no fire load/temperature curve specified in the contract for the structural design of the tunnels at Bank. LU Standard S1055 requires that the design meet the requirements for passive fire resistance to support the fire safety strategy and also all materials used in construction, decoration, equipping, and furnishing LU premises be selected so as to maintain the fire loading as low as reasonably practicable. The BSCU Project Fire Strategy notes that structural fire resistance will comply with the Building Regulations (2010), British Standards, and LU Standards. The fire strategy does not propose including a sprinkler system, so considering the tunnels as a basement structure of more than 10m depth the minimum period of fire resistance is taken as 90 minutes (Building Regulations 2010, Category 4 Table A2). Using the worst case value in Table 5.4 of EC2 (2008) and considering the tunnel linings as structural load bearing solid walls, the wall thickness must be greater than 140mm. The required value is less than the 150mm minimum thickness chosen for the secondary lining.

3.3 – Design Criteria

Steel fibre reinforced concrete (SFRC) is used for both primary and the secondary linings. The design approach uses an idealised linear-elastic-perfectly-plastic behaviour for SFRC. Figure 3 illustrates the material model employed in the design. In compression the material acts elastically only. In tension, the material behaves perfectly plastically when the stresses reach the design value of the residual tensile strength.

Based on the fib Model Code, the allowable strain level in compression is 3.5‰ on condition that the concrete section is subjected to a bending moment. If the section is fully under compression the maximum allowable compressive strain would be 2‰. The tensile strain limit of SFRC is 10‰ for any part of the lining (e.g., intrados/ extrados) under tension. However, the designer set a tensile strain threshold of 5‰. No cracking width limit was required for serviceability of the secondary lining.

The design flexural tensile strength, fFtud, of the SFRC was set to 410kPa (The tensile yielding stress value in Figure 3). To ensure the real lining performance is complying with the design criteria, the beam test results (BS EN 14651) on the specimens from the site sprayed panels should have a minimum residual flexural strength of fR,1= 2.5MPa and fR,4= 1.5MPa, at CMOD1 = 0.5 mm and CMOD4 = 3.5 mm, respectively (Crack Mouth Opening Displacement). The SFRC lining was designed based on concrete grade C32/40.

4 – Finite Element Modelling

A series of 3D Finite Element Models were created in order to provide the basis for the analysis used for both SCL tunnels design and impact assessment of the BSCU works on the existing LU assets. Figure 4 depicts one of the 3D models made in this project at the location of the CL platform tunnels illustrating the complexity of the tunnels geometry.

In order to design an optimised combined lining system, a simplified numerical modelling methodology taking into account the three dimensional effect of the existing and new tunnels was developed. Detailed information regarding methodology of analysis and modelling are given in Nasekhian and Spyridis (2017), however, main aspects of the modelling are outlined herein. The numerical simulations were carried out using Abaqus 6.13-1 of Dassault System.

The combined lining system of the tunnel structures consisting of primary and secondary linings were simulated by means of 3D shells (skins) in the same surface as their excavation boundary. The shells for the primary and the secondary lining share the same nodes. The shell elements model a laminated lining that are bonded together (i.e., the two lining compositions may not detach from each other) and the shear between the two linings is not transferred, however no slip between the primary/secondary lining at the nodes may take place. For the general ground loading condition this behaviour is expected to be realistic and reasonable. In special load cases, though such as secondary lining self-weight and fixings where a different behaviour (e.g., secondary lining debonded from the primary lining) may be expected a separate FE model was provided, which is explained in the next section.

All the major excavation sequences (e.g., top heading, bench and invert subdivisions) have been considered in the models. The load share between each lining component is obtained from the following two FE modelling phases which consider effect of the long and short term loads experienced by the combined lining through the life cycle of the tunnel structure:

  • Phase 1 All the construction sequences of the SCL tunnels are simulated step by step. In this phase only the primary lining (including initial lining) is in place and surrounding soil layers are in undrained conditions. It is assumed that the design life of primary linings is five years at this stage.
  • Phase 2 While the primary lining is in place (keeping all the stress history from the construction stage) the secondary linings of all structures are installed and the long term loads for the design life of 120 years are applied to both linings. Water pressure is applied on the common nodes and not at the back of the secondary lining only. This is due to the fact that the waterproofing membrane is assumed to be bonded and it is not expected that the ground water can freely travel at the primary/secondary lining interface.

Ground is modelled using a Mohr-coulomb constitutive material model that takes into account the non-linear soil behaviour in undrained and drained conditions. The nonlinear behaviour of both the primary and secondary linings of the SCL structures is also simulated using the “Concrete Damaged Plasticity” model from the Abaqus material library with a tensile force cap equal to the design residual flexural capacity. The compressive behaviour is assumed linearly elastic, so as to avoid concrete compressive overstress in any location of the lining, as this would compromise the structure’s robustness.

As for the concrete stiffness, for Phase 1, the stiffness of the primary lining is adjusted to the hypothetical modulus of elasticity in order to account for the average early age sprayed concrete stiffness and gradual soil-structure convergence.

Mohr-Coulomb model (MC) in combination with total stress analysis and undrained soil parameters of the London Clay subsoil layer have been employed.

This modelling set up has been successfully examined in realistic prediction of the ground movement and SCL structural behaviour verified by in-situ monitoring records in the preceding projects with similar ground conditions. The adequacy of the lining thickness was checked by comparison of the maximum compressive/tensile stresses and strains in the intrados/extrados of the shell elements against the limiting strain criteria for SFRC members as described in section 4.3.

5 – Secondary Lining under Special Loadings (Self-Weight, Fixings)

Theoretically, the ultimate potential long term loads will be applied on both linings and will be shared between them. However, it is very likely that the secondary lining will never experience the same degree of the loading predicted in the usual design scenarios, especially at the beginning of its life. For instance, just after installation of the secondary lining no or little hoop force has been mobilised in the lining, since the primary lining has taken the entire short term ground load and deformation in the tunnel has totally ceased. On the other hand, the wet concrete has not yet gained its final strength. Thus, part of the lining around the crown where the tunnel geometry has the flattest curve can cause cracking or, in a worse case, large fallouts. For this loading case, the bond between the waterproof membrane and the secondary lining is assumed negligible.

Moreover, the secondary lining must be able to sustain the fixing loads during the installation of the station fit out on its own because the fixing bolts shall not penetrate the water proofing layer and utilise the primary lining for that purpose.

The secondary lining has lower shear capacity when no hoop force is mobilised in the lining. The modelling approach employed earlier is not appropriate to analyse those load cases. In order to model the unbonded interface contact elements are employed between the secondary lining shell elements and the ground volume elements. One metre slice of ground is modelled. Contact elements automatically detect the delamination of the secondary lining from the ground due to the applied self-weight load. Contact elements do not sustain any tension.

The flattest and heaviest secondary lining arch available in the project under its own weight was analysed. The ground is weightless and has been modelled in order to account for the ground reaction to secondary lining self-weight loading.

The other assumptions are:

  • SCL Lining stiffness = 10GPa
  • Flexural tensile strength of the secondary lining = 410kPa
  • Secondary lining thickness of the platform tunnel = 200mm

Figure 5 portrays the deformed shape of the secondary lining clearly showing the delamination from the ground/primary lining. The model captures the expected behaviour of the secondary lining, assuming that there is no bond between the primary and the secondary lining over a short period of time after installation of the secondary lining.

The maximum displacement of 1.6mm is predicted at the crown. The maximum tensile stress in the intrados/extrados of the secondary lining is 230/136kPa respectively. It can be concluded that the lining can cope with the tensile stresses induced by the lining self-weight in the absence of hoop force resulting from long term ground/water loads.

5.1 – Fixing loads on the secondary lining

In addition to the self-weight of the lining the various types of loads (e.g., point/line load, symmetrical/ asymmetrical) imposed by fixings for installation of the future fittings were considered, assuming a worst case scenario that the secondary lining carries the loads alone and there is no bond between the primary and the secondary lining. Figure 6 illustrates how the contact elements may explain the behaviour of the secondary lining under fixing load applied on a patch of 150×150 at the shoulder of the lining in addition to a smaller load at the crown as one of the temporary loading conditions.

As far as the total stability of the lining is concerned allowable fixing load for a transverse line load, point load at the crown and the shoulder levels were defined by this approach. Those are the maximum theoretical loads that can be applied on the lining that are still meeting the design criteria and acceptable lining performance.

6 – Discussion

SCL tunnels size in the BSCU project ranges from 3x3m to 10.5×9.6m [width, height]. By applying the combined lining system and the design methodology explained above, the SCL linings thickness varies from a minimum of 150 to 600mm for the primary linings and 150 to 250mm for the secondary linings.

A series of 3D Finite Element models, representing the staged construction of the BSCU works, were used for the analysis of the linings so as to allow for more realistic simulation of ground movement during lining installation.

Employing the primary lining as part of a permanent support and the optimisation approach led to significant savings in excavation material, up to 50% reduction of the secondary lining thickness and construction time. This fulfilled the client’s aims with respect to sustainability while working in a high profile urban environment.

The reality in a combined lining system is that the secondary lining will be installed when the primary lining deformations have ceased. The secondary lining is in the unstressed state when the long term loads are gradually being imposed. In ground condition where the soil pressure at rest (K0) is approximately 1.0, the pressure distribution at the back of the primary support is not changed considerably in the long term since the total stress is turned into water pressure and effective soil stress with the same lateral impact on the lining.

The ground stiffness is increased though due to the consolidation. The later factor may deform the primary lining and consequently the secondary lining will be deformed locally while a uniform hoop force still has not been mobilised in the lining. There is lack of evidence that the water pressure can travel along the sprayed waterproofing membrane interface and it is fully/uniformly applied on the secondary lining. This real situation should be understood if the industry intends to move forward in the direction of optimised combined lining.

It is a sensible approach to take into account the low probability of getting long term loads on the secondary lining and separately deal with the consequences of low hoop force in the secondary lining on the serviceability issues. Bank Station Capacity Upgrade project is the first SCL project in the UK that has taken significant design steps towards optimised lining support in soft ground tunneling.