Providing a safe means of egress in an emergency is a critical component of highway and transit tunnel design. Most countries have adopted codes for egress requirements for tunnels, including a minimum distance between points of escape to the ground surface. If a project includes two parallel tunnels, cross passages linking both tunnels can be provided so that escape to the non-incident tunnel can be considered in lieu of egress to the surface. In the US, the main standards providing guidance on tunnel egress requirements and cross passage spacing are NFPA 130 and NFPA 152, for railroad and highway tunnels respectively.

Design and construction of the cross passages can be the most challenging element of a tunnel project, and may be a major component of the overall construction cost and schedule. Excavating cross passages through rock, soft ground and mixed face conditions all present very different challenges, and handling groundwater creates additional difficulties.

Excavation of the main running tunnels, having a relatively larger dimension to the cross-passages, usually involves the use of mechanized equipment, such as a TBM, which provides comparably greater access, and can be designed to mitigate impacts of difficult subsurface conditions. For cross passages, access is usually restricted, and excavation uses small equipment, or alternatively, is performed by hand.

One of the most formidable challenges in the design and construction of a cross passage is when the tunnel crosses beneath a major water body. In these conditions, construction of a cross passage may run the risk of causing inundation to the entire tunnel if a hydraulic connection to the overlying body of water is created during excavation.

Since the running tunnels need to be built before construction of the cross passages can be started, they are often on the critical path of the schedule. As such, having an optimal design for cross passage construction can be a critical component for successful delivery of the project.

Cross passage design elements
The approach to design and construction is largely dependent on the subsurface conditions through which the cross passage must be excavated. Creating a cross-passage opening in a larger tunnel that is already built represents one of the greatest challenges to a tunnel designer.

In soft ground, selection of the most appropriate ground stabilization and treatment methods usually takes the greatest consideration during design. There are often opposing dynamics of minimizing construction costs versus mitigating potential risks associated with a selected excavation method to be used in the identified ground conditions. Mixed face conditions are usually the most challenging of all geology to handle, where the approach is often a combination of both rock and soft ground measures. Subaqueous tunnels, particularly in soft ground, further compound the challenges of excavating through the geologic conditions. Figure 1 shows a typical cross passage.

Junction design
It is desirable when developing the geometry for a cross passage to keep the running tunnel/cross passage/junction structure as simple as possible. This generally requires that the cross passage diameter should be significantly smaller than the main running tunnel. The mainfactors that control both spacing and diameter of cross-passages are their operational uses; emergency evacuation needs; the space needed to house systems and associated equipment or storage of emergency response equipment in a strategic way. In transit tunnels there is often very little space in the main running tunnels, so electrical and mechanical equipment is placed in cross passages.

In most projects, the NFPA required spacing usually defines the total number of cross-passages. The space requirements needed to house both systems equipment and provide clear evacuation routes through the cross passage serves to increase the diameter of the cross passage. If the project design allows, consideration should be given to reducing the relative size of the cross passages by introducing additional cross passages for specific needs, for example separating equipment from emergency evacuation requirements.

The configuration of the junction is controlled by a number of factors, all of which require careful evaluation and scrutiny to establish the optimal connection with the running tunnel.

The relative size of the cross passage in relation to the running tunnel is a significant factor in establishing the geometry of the junction. The ideal configuration is to have a cross-passage significantly smaller than the running tunnel diameter and making an axis-to-axis connection of the cross passage at 90 degrees to the running tunnel alignment.

Often cross passages are geometrically constrained and therefore cannot be configured with an ideal geometry. The axis of the cross-passage may occur at a higher position to the running tunnel axis, such as when allowing for the elevation of the emergency walkway. Cross passages may be skewed either in plan or vertically, dependant on the relative positions of each running tunnel.

Where the relative sizes are approaching similar proportions it makes ring confinement of the running tunnel lining difficult to maintain. It also creates a very complex 3D junction design, and is likely to require a significant extra structural element to be introduced into the junction design.

Analysis methods
There are different approaches to the analysis of the structural support elements required for the construction of cross passages. The selection of an appropriate design method must consider: the geometry of the main tunnel and of the cross passage; elevation differences between the main tunnel and cross passage; the angle of connection; proximity of the tunnels being connected; ground and groundwater conditions and the methods of construction.

Empirical and analytical methods are generally developed for a single tunnel opening and do not account for the complexities associated with cross passages. With experience and some engineering judgment, simplifying assumptions can be made that enable the use of these types of design methods to give an indication of the likely ground loads and structural support required. Similarly, the use of 2D models in conjunction with some careful assumptions can provide useful design data. However, it is increasingly common to develop a 3D model to inform the design of the cross passage lining, the connection, and temporary and permanent support requirements within the main tunnel.

An example output from a 3D finite element model developed using STAADPro (Bentley 2010) for a cross passage in the No. 7 Line subway tunnels in New York City is shown in figure 2. The model consisted of a series of plate elements around 300mm thick that represent the segmental lining, prior to forming the opening. The radial joints and circumferential joints in the segmental lining were not modeled. The combination of the dowel connectors and bolts across circumferential joints and avoidance of cruciform joints (i.e., radial joints aligning in the same orientation in adjacent rings) was considered to provide a connection between segments in each ring and between each ring. While the segmental lining, in reality, is not continuous, a continuous connection was used in the model, and was considered to be a more realistic approximation to represent the behavior of the lining segments in the analysis.

Ground conditions
As with tunneling in general, in developing the design of a cross-passage, geology has a major impact on the most appropriate ground support for excavation, the configuration and type of ground treatment, and the sequence of work.

In rock, while a relatively greater exposure of the face in rock may be permissible, the following considerations are important for determining the configuration of the cross passage design: the need for advanced ground stabilization measures like spiling or canopy tubes; advanced ground treatment through different grouting techniques to control groundwater ingress; and providing appropriate ground support as the excavation advances.

In soft ground, selection of the most appropriate ground stabilization and treatment methods usually takes the greatest consideration during design. There are often opposing dynamics of minimizing construction costs versus mitigating potential risks associated with a selected excavation method to be used in the identified ground conditions. Mixed face conditions are usually the most challenging of all geology to handle, where the approach is often a combination of both rock and soft ground measures.

Tunneling under a large water body, such as rivers, lakes, seas or oceans (subaqueous tunneling), particularly in soft ground, represents the most difficult form of cross passage. It is vital that the subaqueous design address the possibility, if any, of a direct hydraulic connection between the cross passage and the body of water that, if left unabated, could lead to inundation of the entire tunnel. Under such circumstances, determining the optimum ground treatment method is vital for a successful outcome. Key to this is identifying the ground conditions sufficiently during the design phase.

Ground treatment
The selection of the ground treatment method is one of the most important decisions in the design of a cross passage. In soft ground, the type of treatment is entirely dependent on the particle size distribution of the ground, with dewatering and permeation grouting appropriate for coarser granular soils, and chemical grouting for fine grained soils.

The most common type of ground treatment is probably grouting, which can substantially reinforce unstable soil, but access is generally needed from the ground surface. While it is technically feasible to perform horizontal jet grouting from inside a running tunnel, in practice it is seldom used, with ground freezing being a more cost effective and constructible solution.

For cross passages in rock, ground treatment most commonly consists of cement grouting the fractures to minimize the amount of groundwater that will need to be managed during excavation. The locations of cross passages can be pregrouted from the TBM while the running tunnels are being driven. If the rock is in extremely poor condition, ground treatment methods more often associated with soft ground may be necessary.

For subaqueous conditions in soft ground, due to the potentially catastrophic consequences of inundation from a hydraulic connection to the overlying water body, ground freezing is generally the default method of ground improvement, since it provides greater certainty and reliability. Figure 3 shows an example of the layout of the ground freezing required for a project with subaqueous cross passages (Storebaelt). The freeze plant was mounted on the running tunnel walls so that the freezing could be performed while the main running tunnels were being excavated.

Temporary support
The cross passage breakouts involve partial removal of the lining in the running tunnel. There are several alternative methods to provide temporary support to the remaining lining during the process of removal and construction of the cross passage. We will describe the following two methods: design of special opening segments where framing is incorporated into the design of the segments or the installation of a temporary steel ring beam framework inside the running tunnel.

Cross-passage opening sets
Cross-Passage opening sets are a sequence of specially installed rings for a segmentally lined tunnel that are specifically designed to: accommodate required drilling windows for ground treatment; create a pre-defined launch window for the cross-passage; have builtin structural capacity to accommodate the cross-passage opening and originally enable the adjacent running tunnel to run parallel to one another through the reach containing the cross-passage.

Steel ring beam framework
A typical ring beam framework consists of steel rings installed against the intact lining of the running tunnel on each side of the proposed cross passage opening. Tiebeams spanning between the two steel rings are placed longitudinally parallel with the direction of the tunnel at predetermined positions on the circumference of the tunnel bore. When the opening is made in the running tunnel liner, the tie-beams transfer the resulting loading from the left-in-place tunnel liner to the steel rings on either side of the opening. The tie beams are usually bolted to the steel rings through welded end plates. The full load transfer from the tunnel lining to the framework is achieved using a combination of shims, plates and anchors. The critical load case for design of the framework occurs when the running tunnel lining has been cut, but before the permanent cross passage lining has been installed. The steel frame must be designed provide support and thrust transfer for the running tunnel liner that remains in place.

Based on comparison of several projects, we have found that the use of special opening segments is more expensive and less desirable than the ring beam frame. The internal temporary frame with saw-cutting and breaking of regular precast segments seems to provide a more flexible and efficient construction approach.

Collar excavation and support
After the running tunnel liner has been demolished, the next step is to excavate a chamber in the ground around the proposed cross passage opening, and install initial support. This will allow later construction of a reinforced concrete collar to connect the cross passage final liner to the running tunnel segments. The design approach for the excavation and initial support is dependent on the type of formation present outside the tunnel. In figure 4, where subsurface conditions would be representative of good rock, the entire opening may be excavated over its full height and width, with patterned rock bolts designed according to empirical methods. If a weaker rock was present, a similar sequence using a heading and bench could be designed, again using empirical methods.

However, if soft ground conditions are present, even after ground treatment measures have been implemented, great care must be taken during the initial excavation. In figure 5 (Storebaelt), an excavation sequence is shown for a subsea cross passage in glacial till. The initial opening is quite small, generally only the width of one segment, 1.5 to 2m and of similar height. The excavation is then enlarged using sequential excavation methods (SEM), with shotcrete layers providing initial support.

Waterproofing
There are numerous techniques to provide a waterproof system to control groundwater ingress within specified limits. There are a number of different elements within a cross-passage design that require specific consideration: whether or not the cross-passage lining can be drained; the waterproofing design at the junction with the running tunnel; the waterproofing design for junctions within the crosspassage such as sumps or intermediate adits; the cross-passage lining and the residual (if any) running tunnel lining.

Waterproofing techniques include: various membranes with differing properties and resistances, spray on membranes, gaskets and water stops. As each tunnel is different in terms of geology, hydrostatic conditions, lining, construction sequence for collar excavation and operational criteria, there is no one catch-all solution. The success of the design is often assessed against the ability to control groundwater ingress within specification regardless of the challenges successfully accomplished for other elements of crosspassage construction.

An example of a staged waterproofing system, is shown in figure 6 (Storebaelt). It comprises: a first stage outer waterproofing along the perimeter of the contact area, consisting of two strips of neoprene either side of the waterproofing membrane, compressed by a stainless steel clamping plate; a second stage waterproofing, based on an injectable, soft, closed cell, neoprene, elastomeric foam strip, glued on to the back of the running tunnel lining dividing the contact area up into closed cells to reduced risk of leakage and a third stage inner waterproofing around the door opening, consisting of a steel plate and a waterproofing elastomeric sealing strip compressing the gasket.

Subaqueous cross passages
The risk of inundation during construction is a paramount consideration when contemplating subaqueous tunneling. The safety of the underground workers cannot be compromised through inappropriate ground treatment, support failure at the heading or encountering unforeseen conditions during excavation. For this reason, understanding the ground conditions and providing a compatible design is essential. Emergency bulkheads to isolate cross passage construction from other areas where there is active work being performed should be implemented. These can take many forms depending on construction logistics and the overall configuration of the tunnel and access and egress shafts or structures. Typically this takes three forms: full bulkheads isolating the running tunnel in which the cross passage is being excavated; full bulkheads isolating the running tunnel at either side of the specific cross passage reach or emergency bulkhead doors that rapidly deploy at the cross-passage opening. Depending on project requirements, such bulkheads may be considered separately or in combination with one another. Sufficient consideration of this aspect should be made by the designer to establish the schedule constraints such measures may impose. The degree to which concurrency of operations for cross passage construction can be performed with ongoing running tunnel excavation may have a major impact on determining the contract period prior to bid.

Conclusions
Having disucussed the various elements that should be considered for cross passage design, and presented alternative approaches to address different conditions. We offer the following conclusions:

•The challenge for successful completion of a cross passage is to identify the optimum methods and appropriate blend of staged support, excavation sequencing, ground stabilization and most importantly ground treatment. A good understanding of the ground conditions is imperative in application and optimization of the design.

•To reduce the degree to which cross passage construction is on the critical path of the construction schedule, selecting a construction method that may in part enable concurrent working with ongoing running tunnel construction is desirable.

•Detailing for waterproofing at the cross passage junction is one of the most vulnerable and hence critical areas in achieving specified water ingress criteria. Close attention should be given to how the control of water ingress is achieved around this often complex geometry.

•The ideal configuration is to have a significantly smaller diameter than the running tunnel, and making an axis-toaxis connection at 90 degrees. Design should aim to locate the collar as far below the running tunnel crown as possible while meeting requirements.

•The design should consider adopting the ring beam concept of supporting and saw-cutting the standard rings of the running tunnel to create the cross passage opening. This provides greater flexibility during construction and achieves a more cost effective approach.

•Subaqueous cross passage design must address the possibility of a direct hydraulic connection developing, and needs to be particularly robust with mitigation measures implemented to provide certainty.


Figure 1, a typical cross passage Figure 2, output from a 3D model of a cross passage junction Figure 3, ground freezing layout Figure 4, initial excavation sequence in good rock Figure 5, and in soft ground Figure 6, waterproofing detail Temporary steel ring beam framework to support segments