Design Optimization of the Tunnel Lining

By Jane Hampton, Enrica Vardaro, Anna Simic and Jose Flors

On the coronary heart of the capital, the most important challenge within the U.Okay. water trade is underway. Tideway is a brand new mixed storage switch system, commissioned to offer London’s sewage community with further capability and stop air pollution of the River Thames. The colossal scheme consists of a 16 mile-long TBM-bored tunnel with quite a few shafts, related outfalls and floor constructions alongside its route. The contract for the Central part, the longest and most difficult of the three, consists of seven.5 miles of tunnel and eight worksites with deep inlet shafts and ancillary constructions. The Central contract tunnel main lining has an inner diameter of 25.5 ft and has been constructed utilizing primarily metal fiber strengthened precast segmental lining. A fiber strengthened concrete secondary lining reduces the completed inner diameter right down to 24 ft . The tunnel main lining is required to maintain excessive inner surge pressures and to restrict water ingress and egress by the joints to a negligible quantity with out the contribution of the secondary lining. The largest design problem was to supply a single design answer for the first lining crossing completely different floor circumstances.

Historical past

London’s present sewer system is over 150 years previous and has reached its capability, with mixed sewer outflow (CSO) discharges into the River Thames turning into a frequent occasion. Because the sewage system was designed by Joseph Bazalgette within the 1860s, London’s inhabitants has greater than doubled and the realm of permeable surfaces decreased. Consequently, wastewater and floor water run-off has elevated dramatically, and the present sewer system is unable to manage. In truth, simply 2 mm of rainfall can lead to uncooked sewage being discharged into the River Thames through mixed sewer overflows (CSOs).

In an effort to improve the sewage community, and to make sure the standard of the River Thames complies with the European City Wastewater Remedy Directive (EU 1991), the Tideway challenge was commissioned.

Tideway Central Part

The contract for the Central part concerned the design of 9 shafts together with related outfalls and floor constructions alongside the route of the 25.5 ft ID TBM bored tunnel. The primary tunnel has been constructed in two drives; westbound with an approximate size of three miles, and eastbound with an approximate size of 5 miles. The launch of the TBMs for the Central part in the direction of east and west occurred from the Kirtling Road Shaft inside two SCL adits attributable to house constraints within the shaft.

Geology

The primary tunnel of Tideway Central contract of the Central part within the London Basin consists of Made Floor, Alluvium, River Terrace Deposits, London Clay Formation, Harwich Formation, Lambeth Group, Thanet Sand Formation and Chalk.

Over the course of the Central part, from the Carnwarth Street website to the Chambers Wharf website, the primary tunnel encounters main adjustments within the geological stratigraphy.

The Tideway Central foremost tunnel lies inside the geological province of the London basin and covers a spread of various floor circumstances. The geological stratigraphy that the primary tunnel encountered throughout its excavation, from west to east, transitions from London Clay into Lambeth Group with interlayers of cohesive and granular strata, by the Thanet Sands with interlayers of sand and silty deposits earlier than reaching the Chalk strata at Tideway East’s Chambers Wharf website. Groundwater is current within the floor superficial deposits of alluvium and the River Terrace Deposits, supported by the underlying, low-permeability London Clay Formation. That is known as the “higher aquifer” and it’s tidally influenced by the River Thames.

Groundwater can be current within the Harwich Formation on the base of the London Clay, in granular models within the higher part of the Lambeth Group and notably within the Channel Sands. These variable water-bearing strata are collectively termed the “intermediate aquifer”.

The primary water-bearing sequence is named the ‘decrease aquifer’ and contains of the Chalk, the overlying Thanet Sand Formation, the granular Upnor Formation and Decrease Mottled Beds which kind the basal models of the Lambeth Group.

Essential Tunnel Efficiency Necessities

Tideway’s design specification for tunnels, shafts and junctions states that the primary tunnel shall be supplied with a main and secondary lining to make sure that, all through the required 120 yr design lifetime of the works, the minimal efficiency necessities are met. The minimal efficiency necessities for the finished tunnel embody:

• Stand up to all moderately foreseeable exterior hundreds
• Stand up to inner hydraulic pressures
• Meet water tightness standards
• Meet sturdiness standards
• Stop extreme floor actions
• Stop aquifer contamination

One Tunnel Lining Resolution for the Entire Central Part Drive

One of many foremost challenges encountered through the design of the primary tunnel lining was to offer a singular answer for the liner that might fulfill structural and minimal watertightness efficiency necessities with out the contribution of the secondary lining to enhance time and price effectivity. Inside loading circumstances various from empty case to a most inner surge stress and along with an in depth variability within the exterior floor circumstances they might impression the tunnel lining in many alternative situations.

As the first tunnel lining would act as the first technique of watertightness, intensive evaluation was carried out to evaluate the potential opening of the joints through the most inner surge.
The first lining of the primary tunnel was confirmed to have the ability to meet each the structural and watertightness necessities independently and due to this fact the forged in place fiber concrete secondary lining was solely required to offer further sturdiness assurance in accordance with the Works Data of the challenge which required a two-barrier system to satisfy structural, watertightness and sturdiness necessities.

Main Tunnel Lining

The tunnel was excavated and constructed utilizing an Earth Strain Stability (EPB) Tunnel Boring Machine (TBM). The reducing head diameter was 29 ft. Behind the reducing head the defend diameter was 28.8 ft. This distinction in radius between the lower radius and the diameter of the tail pores and skin creates an annulus of 1+in. which permits for the bottom deformation and settlements to happen.

Through the excavation, the TBM utilized a face stress to steadiness the forces inside the floor and to manage the bottom deformation and settlements. The stress is utilized by regulating the amount of spoil within the plenum and by rams, which allowed the TBM to advance. The working restrict of the TBM most working stress was 5.3 bar (530 kPa). Because the TBM superior, the precast concrete segmental rings have been put in to kind the first lining. Every ring has 5 “extraordinary” segments, two prime segments and a key with a thickness of Ts=13.8in.

After the erection of the ring, the 6 in. annulus between the excavation diameter and the exterior diameter of the liner was grouted by the tail pores and skin. All segments include metal fiber reinforcement, with further conventional rebar reinforcement cages included the place a extra strong ring was required, comparable to on the first 10 rings and final 10 rings of every drive, on the junctions with the connection tunnels, both facet of the Blackfriars Shaft website and the place drift crammed hollows (pingos) would have been encountered.

The segmental tunnel lining has been designed to maintain exterior and inner water pressures of as much as 6 bar related to most surge occasion comparable to an inner hydraulic grade line of 341 ft ATD as per Works Data necessities, limiting water ingress/egress to a negligible quantity. The seal is created by the inclusion of cast-in ethylene propylene diene monomer (EPDM) gaskets in all segments. The gaskets are provided as a steady loop with the joints fashioned to the right angles for every sort of section. Moreover, the joint formation has been designed to attenuate the extra rubber materials forming the joint to keep away from arduous spots within the gaskets. The gaskets have been stress examined to 10 bar stress with 0.4 in. offset and a 1/eighth in. hole between bearing surfaces in a rig to mannequin a cruciform joint association.

Design Loadings

Through the design of the Essential tunnel, two completely different floor water profiles have been assumed for larger and decrease groundwater circumstances accounting for the rising and reducing of the decrease aquifer sooner or later. As well as higher and decrease sure geotechnical parameters have been thought of through the design together with the utmost and minimal earth stress coefficient (Κ0,min, Κ0,max).
All of the load mixtures have been simulated for higher and decrease sure geotechnical parameters and most and minimal groundwater stage and included the next loadings:

• Most inner water stress
• Main and Secondary grouting stress;
• Operational floor surcharge at floor stage;
• Allowance for future growth at floor stage;

Short-term dealing with and stacking hundreds, gantry wheel hundreds, ram forces and the results of steps, lips and ring construct tolerances have been thought of utilizing separate empirical calculations.

Seven attribute geological sections, contemplating full face floor circumstances or blended face floor circumstances, have been investigated alongside the entire path to replicate completely different floor circumstances the place the impact of the above hundreds was assessed.

Numerical Evaluation

In keeping with present finest design apply for any such construction, two completely different numerical modelling packages have been used. The soil-structure interplay modelling was carried out utilizing the geotechnical finite component software program PLAXIS 2D. This software program was used to point the worst design situations for the tunnel lining the place solely the impact of radial joint was taken under consideration.

Along with the PLAXIS analyses, a structural mannequin utilizing LUSAS 3D software program with each radial and circumferential joints modelled explicitly was developed to analyze the affect of the section joints on the conduct of the liner.

The aim of those analyses was to evaluate the conduct of the first lining below all loading circumstances, specifically through the surge mode and the affect of this loading on the liner joint, with out contemplating any contribution of the secondary lining.

2D Analyses in PLAXIS

Two typical soil fashions have been used to simulate the soil conduct: the Mohr-Coulomb (MC) and the Hardening Soil (HS). For the design of the precast tunnel lining, superficial deposits have been modelled with Mohr-Coulomb mannequin which represents soil stiffness within the in-situ stress state. Nonetheless, to account for stress-dependency of the stiffness moduli when the soil stiffness will increase with stress, the soil constitutive mannequin for Thames Group, Lambeth Group, Thanet Sand and Chalk have been based mostly on Hardening Soil mannequin.
The tunnels have been constructed with the usage of an EPBM which offers a help stress to the tunnel face through the excavation to counter steadiness the earth and water stress appearing on the face of the excavation. The offered help face stress minimizes the motion of the encircling soils and subsequently the floor settlements. Nonetheless, an quantity of the soil mass round and forward of the excavation will loosen up inducing a three-dimensional phenomenon. Because of the comfort of the encircling soil mass, a radial displacement across the excavation will happen, resulting in elevated pressure values across the excavation and consequently decreased stiffness of the encircling soil.

In an effort to simulate this three-dimensional impact, a spread of leisure values, based mostly on the answer proposed by Panet (1979) have been thought of for every materials sort and adopted within the 2D finite component (FE) evaluation, contemplating the properties of the encircling soil, the radius of the tunnel and the strategy of building. The tunnel lining was modelled as curved plates and was characterised by linear elastic two-dimensional plate parts with axial, shear and flexural resistances.

The liner was assigned with isotropic stiffness properties and was modelled as a plate with no joints and the impact of the joints between segments of the tunnel lining was modelled by decreasing the efficient stiffness. The efficient lining stiffness was calculated utilizing the strategy proposed by Muir-Wooden (1975).
A second evaluation was carried out with the tunnel lining modelled as quantity parts and was characterised by linear elastic two-dimensional quantity parts with axial, shear, and flexural resistances. The first lining was assigned with isotropic stiffness properties and was modelled with joints between the segments, which have been prescribed with compressive and shear capacities, however zero tensile capability. The goal of this extra evaluation was to simulate the precise opening of the joints between the segments.

3D Analyses in LUSAS

The 3D finite component software program LUSAS was used to create three fashions to analyze the affect of the section joints on the conduct of the liner. For every mannequin, spring helps, with stiffnesses derived from PLAXIS 2D output, have been used to symbolize the interplay between the liner and the encircling floor.

Within the first LUSAS 3D evaluation the tunnel lining was modelled as a steady ring with no joints between the segments however with orthotropic stiffness to include the impact of the joints between the segments as used within the PLAXIS 2D evaluation.

Within the second LUSAS evaluation the tunnel lining was modelled as a shell component introducing joints between the segments with solely compressive, however no tensile capability to simulate the precise opening of the joint when the liner was topic to inner stress.

Within the third LUSAS evaluation, the tunnel lining was modelled as consecutive segmental rings utilizing dowels within the longitudinal course to attach the rings. The consecutive segmental rings have been modelled as shell parts introducing radial joints between the segments with compressive capability however no tensile capability.

Conclusions

For all fashions the stresses within the lining have been inside the capability of the 13.75 in. thick fiber strengthened precast concrete tunnel part below all loading circumstances ignoring any doable contribution from the secondary lining.

With explicit consideration to the inner surge, the place the liner would attempt to increase and opening the joints undermining the watertightness necessities, it was doable to show that the expected joint opening for essentially the most onerous situations was inside the 1/eighth in restrict which the gaskets have been examined for and due to this fact indicated that the first lining alone is ready to present enough watertightness throughout essentially the most excessive surge occasion.

Essential Tunnel Secondary Lining

The secondary lining is fashioned of metal fiber strengthened concrete (SFRC) to assist its capacity to adjust to the stringent sturdiness necessities. Extra benefits of utilizing SFRC contains considerably improved building prices, time, and decreased carbon dioxide emissions.

The first lining has been designed to be able to carrying the exterior and inner design a great deal of the primary tunnel with out help from the secondary lining. Nonetheless, the secondary lining will entice a share of those hundreds and has been designed on this foundation.

Shutter and Placing Time

The SFRC secondary lining is forged in place utilizing a full round shutter, eliminating the necessity for radial building joints.

A typical shutter size of 42 ft, travelling alongside the tunnels, was chosen to tie into the size of the first lining segments – 7 No. segmental lining rings of 6 ft size.

In depth evaluation of the early age power growth of the concrete led to a discount within the required placing power of the secondary lining (6.5 MPa cylinder power). In depth testing was undertaken to make sure a strong concrete combine was achieved that delivers the power necessities. By decreasing the placing power requirement of the secondary lining, the time taken from pouring the concrete to eradicating the shutter has been drastically decreased, bettering the manufacturing fee of building.

Design Improvement

A main tunnel lining evaluation was carried out with the forged in place metal fiber strengthened concrete (SFRC) secondary lining designed to satisfy the sturdiness standards, the place the segmental main lining was re-designed to satisfy the structural and water tightness minimal efficiency necessities for the Essential Tunnel alone.

By finishing up in-depth analyses for the first and secondary tunnel lining choices, a extra intensive understanding of the geological medium, floor parameters and soil-structure interplay was established. Primarily based on the information gained by design growth and collaboration with the primary contractor, FLO JV (a three way partnership between Ferrovial Building and Laing O’Rourke) and Tideway, it was doable to attain a number of optimizations to the earlier lining design. These optimizations included reducing the thickness of the secondary lining from 12 in. to 10 in. with a discount within the efficiency power necessities of the SFRC combine. By the worth engineering of the primary tunnel secondary lining, substantial concrete financial savings have been achieved which is able to alleviate the environmental impression of its building and reduce the challenge’s total price.

Numerical Modelling of the Secondary Lining

To make sure design precisely represented the vary of geological strata current, the Central part alignment was break up into a number of sections at areas of anticipated geological change. In depth parametric research for these completely different floor parameters have been carried out utilizing the soil-structure interplay mannequin PLAXIS 2D to establish the worst design situations for the tunnel lining. Along with this, completely different floor leisure elements based mostly on the Panet (1979) answer have been thought of to judge the completely different floor circumstances that the tunnel will encounter alongside its route. Floor parameters thought of embody: overburden stress, floor power, floor stiffness of the soil for various pressure ranges and variation of at relaxation earth stress coefficient.

Structural Evaluation of Essential Tunnel Secondary Lining

There are two principal situations for the primary tunnel secondary lining:

• No inner surge stress
  • Previous to a surge occasion (uncracked part)
  • After 1st surge occasion (cracked part)
• Inside surge stress

For the design situation with out inner surge stress, all secondary lining needs to be in compression with restricted bending second. For the design situation with inner load from a surge occasion, it’s anticipated that tensile stress will happen within the secondary lining which can’t be resisted solely by the metal fiber strengthened concrete and thus, the liner will not contribute as a structural component.

Following this main distinction between the doable situations, the design route was break up into two branches. Topic to the presence of inner surge stress or not, completely different design procedures have been required.

An extra investigation was carried out to look at the extent of load sharing that the optimized secondary lining would take part in. To do that the ring power (Nt) for the first and secondary lining was in contrast for every chainage, contemplating every building stage and the completely different variables mentioned beforehand. The vary of loading skilled by the secondary lining is proven within the following desk. The outcomes point out that the secondary lining will expertise as much as 30% of load sharing with the first lining, depending on the development stage and variables thought of.

Conclusion

The optimizations achieved within the design course of have resulted in important price financial savings for the challenge and elevated the effectivity of its building.

The editor and publishers want to thank Jane Hampton, Enrica Vardaro Civil Engineers and Anna Simic, Technical Director, with AECOM, and Jose Flors Villaverde, Engineering Supervisor, with FLO JV, for getting ready the above article for publication. The authors thank FLO JV and Tideway for his or her collaboration through the design course of and help in getting ready this text. For data, contact Paul Nicholas, VP Tunneling & Trenchless Know-how for AECOM, at [email protected]