Many urban infrastructures are being moved underground as a way to create new green areas on the surface and improve quality of life. TBMs have recently jumped an astonishing step in this field allowing safe excavation in soft ground even below the water table. But not only round tunnels are needed to fit operational requirements. Caverns, underground stations, bifurcations, cross passages, etc., have to be built under congested urban areas at usually low overburden packed with utilities.

Metro systems, sewage channels, cable networks and others can be easily transferred below the ground and in most of the cases, due to the relevant length of those infrastructures TBMs have been used for this purpose.

The now so-called sequential excavation method (SEM) has proved to be the most flexible tool to construct complex underground structures adapted to the required geometry and clearance for its final use. Most cities today restrict blasting therefore improved massive roadheaders are taking over providing reasonable production rates combined with low noise and vibration levels. None or minimal utility and traffic relocation is required nor construction of temporary access to buildings or garages.

EPBMs and hydroshields allow excavation under pressure in order to compensate the ground deformation and avoid surface settlements, dramatically increasing the capabilities for tunnelling in soft ground. But pressurised TBMs aren’t the only choice in cases where the ground isn’t poor or the alignment isn’t below the water table, or if the tunnel length isn’t very long.

Additionally operational issues usually require changes in the tunnel shape and size. In the case of sound ground and hard rock, many cities restrict excavation with explosives due to security and safety concerns. In these cases in which the limited length of the infrastructure as well as the variable geometry is required, SEM has been proved as the most valuable tool to build the underground space cheaper and faster. Depending on ground conditions, hard to soft, excavation can be done by controlled blasting if allowed, drill and split, roadheader or digger.

Construction alternatives
The two main alternatives to SEM are TBM and surface methods, such as cut and cover or open cut technology. Both should be considered not competitors but construction methods to be applicable in those areas in which they are more attractive in terms of cost and schedule. But, in general, they can be compatible and applicable in the same project in accordance with the design criteria and local circumstances.

It is true that surface methods are simple and cheap in areas without any interference with local utilities and only in the cases of shallow infrastructure. This is usually not the case in urban environments where the excavation of an underground space requires the relocation of a substantial number of utilities. Often these utilities are owned by different organisations, and the removal and relocation could take years — especially because the contractor cannot always manage it from an early stage and has no ability to suggest other possibilities. That means the project completion is in a third party’s hands and not always with a strong interest in the completion time. This can be enough to reject cut and cover as the selected working method in a congested urban environment situation.

Also limits on noise and vibration are more and more a contractual requirement and consequently all activities executed above ground are restricted in time and resources. Adding to this, the stakeholders’ rejection to any construction activity in front of their own business or residence makes cut and cover a reasonable method to be applied in free areas but not in urban areas.

Even in the case of very poor ground below the water table a careful analysis of alternatives between cut and cover and excavation by SEM with pre-excavation grouting should be considered before making the final decision. TBMs are a solution for linear tunnels with fixed geometry and they are suitable for all kinds of ground.

The key point is to properly select the TBM in accordance with the knowledge of the existing ground. Very short tunnels that do not deserve the investment in a TBM, as well as additional infrastructure required for operational purpose like maintenance rooms, cross passages, etc., can be easily constructed by SEM as well.

Advantages of SEM
Invisible SEM does not disrupt citizens’ activities, nor cause disturbances during tunnel construction. A tunnel portal can be positioned outside of the congested area or even out of the contractual alignment in order to minimise this. The excavation of additional adits to connect the site portal or shaft with the required infrastructure is an affordable cost compensated by the advantage of not relocating utilities, setting up traffic diversions and other construction disruptions. The most important requirements or rules of SEM are:

The cross section should always be an ovoid shape.

Installation of immediate and continuous smooth support around its perimeter (and, if required, smooth support at the face) is a significant factor in minimising initial movement in the surrounding ground.

It is also essential to structurally close the supporting ring (shotcrete) as quickly as possible within one tunnel diameter of the advancing excavation face.

The 3D stress redistribution around the tunnel depends on geometry and time. This must be carefully considered particularly where multiple openings are planned. It will govern the progress of tunnelling with respect to stress redistribution interaction and the hardening of the shotcrete support.

After providing the basic design parameters, the required thickness of support (shotcrete) for a given tunnel/shaft diameter and overburden is basically a reversed function of the internal angle of friction of the formation. Calculated bending moments in ovoid shaped tunnels have little to do with reality. Heavy reinforcement in the lining only weakens the shotcrete support capacity due to shadowing and, therefore, produces an inconsistent lining with a questionable shotcrete quality. Proper structural models applied with skill and experience result in minimum bending moments and therefore require less reinforcement. Continuous, and if applicable symmetric, excavation of (multiple) drifts avoids heterogeneities in stiffness and smoothes the stress redistribution in both the lining and the surrounding ground.

In relation to the construction itself, SEM has multiple advantages. The excavation can accommodate the required geometry for operational reasons and in accordance with ground conditions, and excavation stages and support can be easily adjusted. The well-known toolbox is the key of success of this method and any kind of ground condition can be handled safely by following the incremental excavation/support or pre-support approach together with observational measures to be implemented during the excavation. Geometry and progress may vary from full face to subdivision of the excavation face in top heading, bench, side wall drift, or multiple drifts, etc. The reduction of the length of round or, if necessary, continuing excavation and support flow around the clock in extremely soft ground, influences the progress. Geometry and progress can be adapted as necessary.

Pre-support may start with dewatering of the excavation area and or spiling with various elements. The barrel vault method is probably today’s most effective pre-support over longer stretches. Horizontal jet grouting also improves the ground ahead of the excavation line as does conventional grouting of the surrounding ground. Face support may consist of only the existing ground (earth wedge) or additional shotcrete, or even face bolting. Pocket excavation or multiple drifts are another safe and effective face support technique.

Special methods (e.g., ground freezing, barrel vault method or doorframe slab method) provide a variety and enormous number of possible combinations to tackle any kind of ground. It allows mining in all ground conditions and through all obstructions, including buried foundations without compromising geometrical requirements. This is also true for excavation of large caverns with minimal cover.

In urban areas surface deformation (settlements) causes more and more environmental problems. It cannot be avoided but can be minimised and/or compensated by grout injection adjacent to the underground structure. This compensation grouting has successfully been applied in a number of cities worldwide: London, Barcelona and Toronto, among others.

Criticism and misconceptions
In tunnelling conferences and publications it is not uncommon to hear concerns about the safety of SEM mostly in urban tunnels. SEM must be regarded as a 3D concept to understand its way to control and distribute the ground loads on these complex shapes that are the underground structures.

The success execution of SEM is based on four premises:

  • Thoughtful design by an experienced engineering team
  • Execution by a skilled contractor
  • Competent supervision
  • Interpretation of monitoring results

SEM is an observational method (R. Peck), which means that monitoring (in-situ-measurements) of deformation in the ground and stress in the initial lining (shotcrete) is essential to the actual support means. A weak design or poor performing contractor can always be improved by proper supervision and interpretation, avoiding collapses and damages to third parties.

Figure 1 shows a substantial amount of tunnels built by SEM with low overburden successfully.

Even seismic activities do not pose a serious risk to completed tunnels. The physical principle behind this is simple: The sequential excavation produces a stress relieved (relaxed) zone around the tunnel while immediately sprayed smooth shotcrete support forms a composite structure with the surrounding ground.

This cushion effect and its shell structure provide flexible resistance against shock and shear waves (for example the earthquake in Friuli, Northern Italy, 1976).

In contrast, cut and cover structures are prone to adverse stress peaks impacts, which can lead to their total destruction (for example the earthquake in Kobe, Japan, 1995).

Recommended Excavation Equipment
In sound ground without urban restrictions, drill and blast seems to be the most economical way to build tunnels and can be adapted to the variable geometry of the structure. Controlled blasting can be a tool to deal with urban restrictions. But the ground isn’t always so grateful nor explosives allowed to build the tunnels, and consequently different excavation methods and equipment must be chosen.

In the same way that TBMs have experienced a huge development in the last decades, conventional equipment is more and more powerful and capable to deal with a large range of ground conditions. Based in previous experiences, Figure 2 shows the limits of state-of-the art tunnelling equipment to our knowledge. The distribution is related to the rock compress strength and the rock quality designation (RQD). Assuming that a lower RQD allows excavating with the same equipment at higher compression strength of the rock.

The high performance achieved by roadheaders is remarkable — being able to deal with rocks up to 150 MPa and 100 per cent RQD despite achieving low production rates. Also hammers are currently heavier and more powerful but the limitation in their application in tunnelling is related to the associated carrier. For example, a strong 4,000kg breaker requires a 40 to 70t backhoe, too big to be useful in most of the tunnels.

On the highest strength of the rock where roadheaders are not able to excavate with reasonable rates, the alternative is the splitter. Drill and split is a solution in those cases were explosive is not allowed and there isn’t any other equipment capable of dealing with hard rock. Also the production rates use to be quite low, mainly due to the astonishing amount of holes required to be drilled at the face in order to break the block by hydraulic wedges. Additionally a free face is required in this method to initiate the excavation. This starting face can be done by secant holes or with diamond disc cutter.

Cities Using SEM
In Santiago, Chile, 150sq.m station caverns were excavated 7m to 9m below the city streets for the Metro. The cavern’s face is divided into sidewall drifts and a central core section.

Each section utilises a top heading, bench and invert sequence. This approach eliminates the need for forepoling or spiling ahead at full span top heading rounds under shallow cover and allows optimum rotation of medium size equipment and crews between several advancing faces.

The East Side Access project will connect the Long Island Rail Road to Grand Central Terminal in Manhattan. Two TBMs, from Seli and Robbins were assigned to build the running tunnels but substantial amounts of excavation in this project is related to areas with variable and not rounded geometry: bifurcations, cross passages, operation rooms and the two main caverns 344m by 20m by 18m each. For its excavation a combined process of TBMs, roadheader and drill and blast has been applied. Rock bolting plus shotcrete layers have being the temporary support installed.

Seattle, which has the privilege of hosting the world’s largest TBM tunnel for the Alaskan Way Viaduct replacement project, also has experience in SEM. Beacon Hill is a Light Rail Station located 46m beneath the surface. The station complex includes central elevator/access shaft, concourse, opposing platform tunnels and cross passages. Predominant geology is soft ground, over-consolidated glacial clay and till with fractured zones together with intermittent sand and silt layers with perched groundwater.

The San Francisco Municipal Transportation Agency (MUNI) is building a 1.7-mile (2.74km) underground extension to the Third Street Light Rail Project.

The underground section start near Bryant Street, run along Third Street, cross under four BART Tunnels at Market Street and continue under Geary and Stockton Streets to Clay Street. Underground subway stations are located at Moscone Center, Market Street, Union Square and Clay Street in Chinatown.

The geology ranges from soft clay (Bay Clay) to stiff sands (Colma Sands) and competent to highly fractured rock (Franciscan Formation). A considerable part of the tunnel alignment is below the groundwater table. The restrictions on surface have recommended the Chinatown Station to be designed and built by SEM.

London Underground has a lot of experience in SEM. The famous London Clay is a great ground, easy to be excavated and very adequate to design SEM structures. The latest stations extensions awarded or under bid are focused in this method. Also the Crossrail Project, linking the East and West suburbs with London’s core, currently under construction, has envisaged a substantial amount of underground structures to be designed and constructed using the sequential excavation method (Called SCL – sprayed concrete lining).

Conclusions
Every tunnel and underground structure can be built by different methods, all of them with their own pros and cons. During the design stage those cons must be very carefully evaluated in order to overpass them by selecting alternatives.

SEM has most of these advantages and very few disadvantages in the case of urban tunnels. It is flexible, easily adapted to the required geometry and to the ground quality, giving the possibility to apply different support or excavation stages to minimise the disturbance in the surrounding ground and on the surface. SEM gives an additional advantage: the control of the process is in our hands, with our toolbox and proper knowledge of the ground. It allows the excavation by multiple front faces if required to fulfil the schedule.

The initial investment in equipment is much less than in the TBM process and also the required support use to be cheaper than the precast concrete segments installed as final lining.

SEM also minimises the impact on the communities’ daily activities as opposed to the cut and cover approach. Collapses in tunnels happen with all the excavation procedures and are always directly related to poor performance in design, construction or supervision. Professionalism and experience is the key to the construction of underground structures in urban spaces.