Field of the Invention

The present invention relates to the field of tunnelling, and particularly the field of construction of tunnels in waters on the continental shelf wherein concrete/steel structures would be landed on an area of seabed above the intended tunnel, from and through said structures may be fashioned dry shafts to the depths necessary for assembly and launch of tunnel boring machines (TBMs).

It is considered that the possibility of catastrophic ingress of seawater into the shaft is not negligible and must be addressed on a technical basis. Consequently, methods to ensure that the shafts shall seal are provided in this document.

Description of the Prior Art.

Concrete structures have been employed in the North Sea, especially in the Norwegian sector, for the recovery and processing of oil and gas. Multi-hull structures such as Troll and mono-hull structures typified by Draugen are set on the seabed in depths down to 1000 feet (300m).

UK Patent Number GB2358417 by this Inventor describes how the aforementioned concrete structures can be used for accelerated construction of subaqueous tunnels. The present invention describes technical improvements and complements to this art, these being: method with materiel for setting a starter caisson through deep soil into the bearing stratum for commencement of shaft sinking operations in dry conditions, the dry conditions being achieved by ensuring that cement placement is successful and further, that if partially unsuccessful, that remedial placement of cement is feasible; method with materiel for converting the full-bore construction shaft which provided support to and access for tunnel-boring operations to a suite of shafts for essential functions of a completed and working tunnel system, said functions including ventilation and access/egress for personnel and equipment; method with materiel for ensuring that the function shafts as completed and operating can seal against catastrophic ingress of seawater into the main central shaft; method of ventilation for ultra-long tunnels for which several offshore structures are necessarily present to ensure operability after construction.

Brief Summary of the Invention In accordance with the present invention there is provided a method of constructing a tunnel as set out in claim 1 appended hereto. Additionally, preferred or optional features are set out in dependent claims 2 to 15. Further preferred or optional features are set out as follows. Preferably, the one or more concrete structures are deployed with appropriately configured topsides modules. Preferably, the caisson body is formed of concrete, steelwork and steel reinforcement bars.

Preferably, the method includes mounting an integrally-set rail on the caisson body and coupling a series of circumferentially-rotating drilling tools and cuttings removal systems to the rail to drill and ream the bearing formation below the caisson assembly.

Preferably, the preparatory engineering further includes setting the suitably- constructed caisson assembly to the required depth by at least one of jetting, weighting, and excavation to an impermeable stratum and more preferably comprises at least two of jetting, weighting, and excavation and may comprise all three of jetting, weighting, and excavation.

Typically, the method further includes facilitating remedial work on the cement whereby pipe assemblies are installed within a cellular annulus defined between the caisson body and external casing subsequent to reaching cementing depth but prior to commencement of cement placement operations.

Preferably, the concrete segments (those which are specifically formed to facilitate the landing from above of pre-formed blocks which are receptacles for barrier assemblies) are pre-cast reinforced concrete segments. Typically, the said barrier assemblies provide means of sealing against catastrophic influx of seawater.

Typically, when the primary and upper barrier assemblies are closed and tested, they allow removal and replacement of the concrete structure, said barrier assemblies being installed as upper and lower assemblies in receptacles (said receptacles typically being formed at least two of and more preferably all three of concrete, steel and corrosion-resistant alloy), and said barrier assemblies preferably functioning separately and independently of each other. BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of this invention are set against the background of its applications in the detailed description which follows and in conjunction with accompanying drawings wherein:

Figure 1 depicts a section through the seabed with a concrete and steel caisson being set into soil down towards a bearing stratum or bedrock.

Figure 2 shows a sectional plan view of Figure 1 of said caisson.

Figure 3 depicts the caisson being set into bedrock prior to cementing. Figure 4 shows the caisson set into bedrock and cemented.

Figure 5 depicts a concrete/steel structure installed over the caisson. Figure 6 shows a sectional plan view within the original construction shaft as converted and completed to a functional shaft system.

Figure 7 shows a sectional elevation of the completed tunnel and cavern at the interface with the functional shafts.

Figure 8 shows a sectional elevation of the upper barrier assembly

Figure 9 shows the lower barrier assembly and its method of installation. Figure 10 shows the sectional elevation of the lower barrier assembly as installed

Figure 11 shows the concrete structure and tunnel interface as completed

It should be noted that these drawings are not to scale and should not be construed as being so.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

It is considered that UK Patent Number GB2358417 by this Inventor provides a basis onto which improvements to the manner of commencement of the construction of the offshore access locations for subsea tunnel construction may be provided within this document, given that there is little indication in the former of how the principal shaft should be completed after construction of the tunnel. The principal shaft, as previously described, is based on a mono-bore offshore concrete structure typified by Draugen in the Norwegian sector of the North Sea but it is noteworthy that the structure may require to be of more than one shaft, such as Troll which has four. Nonetheless, the inventions as described hereafter are applicable to each and any embodiment of concrete structure. The inventions described hereafter are:

1. more technically-robust methods for commencement of the principal shaft as the Inventor perceives technical insufficiency and,

2. the equipment and manner of installation necessary to ensure that the shafts completed within the principal shaft and used for provision of personnel, maintenance equipment and services, and ventilation, can be sealed against severe seawater ingress.

3. the ventilation system as applied to ultra-long underwater tunnels i.e. tunnels exceeding the functional limits of known ventilation systems as applied to the Seikan tunnel (Japan) and the Channel Tunnel (England - France)

1. SHAFT CONSTRUCTION

This invention requires detailed survey of the seabed at the intended location of each concrete structure to be deployed as the means by which a primary working shaft, sunk to a cavern depth beneath the seabed, shall provide additional workfaces for tunnel construction and permanent facility for provision of supporting services during operation.

The condition of the seabed at each site of landing of a concrete structure is the major determinant of the method by which a seabed-to-structure interface is selected and installed. Seabed shall be of either rock with little or no soil cover, or of medium to deep soil cover down to a stratum suitable for supporting tunnel construction. The term 'bedrock' shall be considered synonymous with a stratum suitable for supporting tunnel construction. It is considered that deep topsoil down to a suitable bearing stratum is the most technically challenging case. The term 'mud' shall be used to describe fluids which are necessarily thixotropic to carry out the intended function, typically of bentonite as a component.

At each location selected as a site for a concrete structure, the seabed must be surveyed in order to establish which method shall be used for the installation of the base. For the case where the seabed is of a deep topsoil down to bedrock, the sequence of events commencing as depicted in Figure 1 is now described. Figure 1 shows a section through the seabed with a concrete and steel caisson 1 being set into soil 2 down towards a bearing stratum or bedrock 3. The caisson 1 has internal dimensions which would permit subsequent passage of drilling and tunnelling equipment and removal of the spoil, and a wall thickness which shall resist all loads encountered during installation and operation. Within the wall of the caisson 1 is incorporated pipework 4 through which jetting mud, slurrifying fluids and fixing/sealing grouts may pass, said fluids pumped from surface vessels through flexible piping attached to the top of the caisson 1. It is noted that the caisson 1 shall require means of centralisation in the excavated and drilled recess in the bedrock 3 in order that sealing cements shall be successfully placed. A stabilising / guiding template 50 of welded structural steel section shall be set on and fixed to the seabed, either by piling or by drilling and grouting of fixing posts, for the caisson to be run through in a supported and stable manner. Located on this template 50 shall be clamping equipment which can be used to suspend the caisson 1 during drilling and under-reaming operations. This template is omitted in further drawings as the other features of the invention assume greater importance. Note that the template would be removed prior to the operations depicted in Figure 5.

At the bottom of the caisson 1 is a leading edge 5, preferably of steel, as typically employed in the civil engineering sector. The caisson 1 penetrates the soil 2 by a combination of self-weight, jetting, and by excavation of soil inside the caisson 1 as required to reduce skin friction. In the event that these measures are insufficient to set the caisson 1 at required depth, pile-driving equipment attached to the caisson 1 through an appropriate interface - the tieback liner to be subsequently installed is such an interface - shall ensure final attainment of setting depth.

For very deep soils where a single caisson element is impractical then, as the leading caisson element penetrates into the soil, caisson elements of similar diameter, wall thickness, and integral pipe pattern can be landed onto and connected by a locking system to the lead element with the interface being sealed by grouting. This will provide the necessary weight to penetrate soil to the bedrock or the relatively impermeable formation suitable for tunnel construction.

Figure 2 shows a sectional plan view of the caisson.

On the outside of the concrete caisson 1 may be attached a rigid steel casing 6 which will run the length of the caisson 1 down to the height at which the caisson 1 is intended to penetrate into the bearing stratum or bedrock 3. The casing 6 will be welded to the rebar pattern of the caisson 1 and will have sufficient bracing plates 7 to ensure that it remains structurally sound as the caisson 1 penetrates the top soil

2. The casing 6 will capture plugs of soil in the cellular annulus gap 8 between it and the caisson 1 and these plugs will be flushed upwards and out of the envelope of the casing 6, and away from the location where cement is to be placed thereby enhancing the quality of cement. The annulus of soil captured as the caisson 1 penetrates into the soil 2 shall be slurrified a short distance above the position of entry into the casing 6 in order to ensure that there is no additional friction which would resist the penetration of the caisson 1 into the soil 2. The slurrification would be done by pumping mud down the pipework 4 to slurrify and remove the soil captured in the annulus, and to render it as a slurry which would have little significant resistance to penetration. The pipework 4 comprises a suite of pipe conduits integral to the caisson 1 which may be considered as two separate groups, these being a) pipes which permit pumping of mud to remove soil in the cellular annulus 8 and which, post-cementing, may be drilled through to support remedial cementing operations below the cutting face and b) pipes which permit pumping of mud to expel soil below the cutting edge in order to facilitate penetration of the caisson 1 into the soil 2 and, subsequent to the setting of the caisson 1 into bedrock

3, to provide passage for cement.

Figure 3 shows the caisson set into bedrock. Upon reaching the bedrock 3, under-reaming drilling shall remove rock immediately below the caisson 1 in the form of a socket to diameters which provide sufficient clearance for the caisson 1 to allow penetration of the caisson 1 into the bedrock 3 to a depth which would facilitate cementing i.e. placement of cement through the caisson pipework 4 which seals the external annulus between caisson 1 and bedrock 3. The under-reamer shall necessarily be mounted on integral circumferential rails sited on the bore of the caisson 1 at a set distance above the cutting profile and may be powered by water or may be an adaptation of the Mitsui 'Aqua-Header' (RTM) drilling system used for large diameter piles. The weight of the caisson 1 can be suspended and controlled during drilling operations, either from the master vessel on the sea above or at and by clamping equipment mounted on the template structure 50 as previously cited in GB2358417. As previously noted, the caisson 1 must be centralised in the excavation of the bedrock 3 in order to ensure successful placement of sealing cement slurry. It is therefore a feature of the design of the caisson 1 that it has centralisers 10 integral to the form of the caisson 1 which shall be set at a position sufficiently above the leading edge 5 to allow placement of a complete and continuous volume of cement but also not so high as to be above the rock into which the centralisers 10 will abut and impose centralisation on the caisson 1.

After achieving the requisite depth, the caisson 1 shall be cemented into place. Sufficient volume of cement shall be pumped to ensure that the bore of the caisson 1 is sealed against external ingress, and that the quality of the resident cement is good. It would require large-scale testing to determine the manner of pumping to achieve the best placement of cement but the volume of cement to be pumped shall be several times greater than that of the cement in the socket below the caisson's leading edge 5. Consideration would be given to whether the cement should flow from the region near the centralisers 10 down the outside and into the bore or vice versa. In order to direct the cement slurry as necessary, a weighted cover 11 which seals to the caisson 1 shall be landed on the caisson 1 just prior to pumping of cement. As pumping commences, the cement shall flow preferentially in the direction desired because the pressure in the internal void 12 can be positively or negatively pressured by a controlling pump and valve system 13 on the weighted cover 11. The excess of cement will fill the conduits, the cellular annulus gaps 8, formed by the caisson 1, bracing plates 7 and casing 6 thus providing a head of cement to counter any inflows of soil or seawater whilst the cement is a liquid. As the cement begins to acquire gel strength, the advantage of the head of cement will diminish.

It is absolutely vital to the integrity of the intended shaft that the cement placement operations be successful and verified as being so. Survey equipment to conduct a cement bond-log may be run inside dedicated profiles within the caisson 1 or set into protective recesses on the caisson 1 in order to verify that cement slurry surrounds the caisson 1 and that there are no zones where cement is not successfully placed in-situ. Figure 4 shows the caisson set into bedrock and cemented.

Of particular importance to the successful implementation of the invention is the ability to conduct remedial cementing operations in the event that the placement of cement into the under-reamed recess by pumping is incomplete or not entirely effective. The pipes of the pipework 4 which are used to provide slurrifying fluids to remove soil in the annulus cells 8 go down to near the end of the caisson 1, and these pipes will remain unfilled by cement after completion of cement pumping operations. Drilling through these pipes to the bottom of the caisson 1 as required selectively by survey reports would allow remedial cement to be pumped to areas at and near the main cutting face 5. Within the annulus between the caisson 1 and the casing 6 structurally supported by bracing plates 7 are the annular cells 8 into which slurrifying mud is pumped to remove entrained soil. Into these cells shall be run pipes 48 which shall be sealed at the lower ends and filled with weighted mud. These pipes would preferably be assemblies shown for information as two pipes with centralising elements 49 set down into each cellular annulus gap 8 after sinking operations but prior to cementing operations. The pipe assemblies would run down the caisson 1 to a final location above the bottom of the casing 6 and would preferably be locked to the caisson 1 by mating profiles at the top of the caisson 1 and the top of the pipe assembly. When cement has been pumped and has set around said pipe assemblies, the pipes 48 would provide starting points in the proximity of the location of the unsatisfactory cement for drilling down in order to place remedial cement. This measure would ensure that remedial cement can be placed in the location below the casing 6. In combination therefore, all locations of the set caisson 1 into which cement has been pumped for essential sealing can be accessed for remedial cement placement. On completion of cementing and after waiting on cement to harden, the integrity of the cement can be tested by pressuring the inside of the caisson 1 through the weight cap 11. The leakage rate will be calibrated to indicate whether losses are due to an inadequate cement placement or to natural loss through the unsealed formation.

Figure 5 shows the concrete/steel structure landed over the caisson.

The concrete structure 14 shall be towed over the caisson 1 and landed on the surrounding soil 2: all soil stresses are resisted by the caisson 1. After landing the structure 14, successive caisson elements 15 shall be landed through the shaft 16 of the structure and cemented to each other until the stacked elements comprise a liner 17 reaching the surface of the structure 14. This embodiment is proposed where soils may consolidate over time and it may not be suitable to seal the structure 14 to the caisson 1 because any subsequent movements of the structure 14 owing to consolidation of soil would transfer loads to the caisson 1 which may cause overload of the caisson 1 and threaten the integrity of the shaft system. Additionally, the completion of the liner 17 as described provides double-barrier integrity. The water within the caisson shaft 16 can now be pumped out and shaft sinking operations inside and below the liner may commence. This configuration is necessary to ensure that no loading is transferred to the sealing caisson 1 from the structure 14 set on the soil 2. It should also be noted that, in the absence of a seal between the structure 14 and liner 17, there is a hydrostatic pressure in the annulus between the structure 14 and the liner 17.

These are important improvements to the method described in UK Patent Number GB2358417, as it is considered that the leading edge of the caisson must penetrate into the bedrock or the bearing stratum in order to provide a shielded target zone for the placement of cement. It is of absolute importance that the bottom of the caisson be sealed against external ingress of seawater as compared with ingress due to the permeability of the underlying formation, because the latter is acceptable and manageable by pumps.

2. SHAFT COMPLETION

When the tunnel is completed and all tunnel construction operations are ended, the principal shaft, as used for construction, must be converted to a series of functional shafts for access/egress of personnel and passengers in extremis, and provision of services and ventilation. The shafts shall include systems of lifts for movement of personnel, passengers and equipment as part of the maintenance and repair program of the tunnel. All shafts must have the ability to be sealed against catastrophic ingress of seawater otherwise the tunnel could be lost to flood waters beyond the capacity of pumps to displace. The barriers against water ingress shall be shown as an embodiment of a rotational barrier system. It is anticipated that other methods such as gates, slabs, and moveable bulkheads may find use. However, where the section of the primary shaft of construction is limited, such methods would be considered inferior to the preferred embodiment particularly as redundancy of function is required. Henceforth and for clarity, the term 'rotational' shall be used to describe the barrier system: this is not intended to be used in a limiting sense as, for example, the item described subsequently as the 'ball' 28 need not be a complete rounded profile as only the part which effects the seal would necessarily be round - a half-ball could fulfil the technical requirements.

Figure 6 shows a sectional plan view of the completed shaft system as in the process of being converted to the service functions. Each intended functional shaft such as personnel and equipment lifts 22 and 23, ventilation 24 and 25, and manual-laddered 26, this disposition of shafts being exemplary only, shall have a bore lined of reinforced concrete sections 27 fitted and inter-linked then grouted in stages to match the progression upwards of the slip- forming and back-filling of the construction shaft 16. The concrete structures shall remain in place in intended permanence after all construction operations because of the necessity to provide ventilation to the tunnel at optimised distances of separation between points of supply/extraction of air, and also to provide locations for access/egress of personnel and, in extremis, for passengers.

Figure 7 shows a sectional elevation of the completed tunnel and cavern at the interface with the functional shafts. The principal shaft 16, as used for construction, will be completed at the interface of the cavern 18 by pre-formed reinforced concrete segments 39 of the strength required to support the commencement of a slip-formed and back-filled column 20, said column becoming self-supporting as cement stages set against re-barred side walls which are the construction shaft segments 21. Horizontally-placed re-bars should be set into the construction shaft segments 21 in order to transfer load through the re-bars. On completion of the slip-forming and back-filling, there shall be a number of functional shafts set in the region of excavated soil and rock, but not above the upper level of the caisson 1, each of which must be capable of being sealed against ingress of seawater. With regard to the section of the back-filled column 20, not all of the void need be back-filled in concrete: at various heights and sections, forms could be inserted to contain air or water thus reducing the potential loading on underlying structures. By such completion shall the upper rotational barrier assembly 41 and the lower rotational barrier assembly 42 be rendered functional as described hereafter.

Figure 8 shows the sectional elevation of the upper rotational barrier assembly. Using the example of the section of a principal lift shaft, it is seen that a ball 28 having a central bore 29 and rotary shaft 30 mounted on a trunnion 31 and seat 32 permits travel through the lift shaft to/from the tunnel. In order to seal the lift shaft, the ball 28 rotates from the open position to the closed position under the action of a motive force provided, for example, by a released eccentric weight 33 and its associated lever arm, said method ensuring closure under conditions of loss of electric and hydraulic power. The seals 34 and 35 mounted in the seat 32 against which the ball 28 is pre-loaded through self-weight and springs 36 ensure that any ingress of seawater into the principal shaft 16 is sealed against entry into the tunnel noting further that the hydrostatic head of seawater provides additional force for the ball/seal contact thereby enhancing the sealing action.

The ball 28 is preferably of cast steel or iron with cladding of a corrosion-resistant alloy applied by welding or thermal spraying. The trunnion 31 is preferably of similar material to the ball 28 whilst the bearing shaft 30 would be of similar material to that used for ship prop-shafts. The seals 34 and 35 shall be of a seawater-resistant elastomeric noting that the contact zone can be kept lubricated by a constant head of oil, and that degradation of the elastomeric by ultra-violet light is improbable in its location.

The upper rotational barrier assembly 41 is located in a pre-formed reinforced concrete housing 37 located within the concrete section of the back-filled principal shaft 16, and the assembly is secured in the housing 37 by a concrete cover 38 bolted to the housing 37.

Lift systems and their layouts are beyond the scope of this document and would be the domain of those who install lift systems, but it is anticipated by this document that there will be a separation in motive systems for lifts travelling from the topsides to the landing area of the back-filled concrete block, and those for the section down to the tunnel. Cabling typical of lifts in buildings would suffice for the former but, for the latter which must travel through the normally-open closure system, a positive-engagement system such as toothed-gears would have to be deployed because the rotating ball would be incapable of cutting the cables whereas positive-engagement systems would be compatible with the rotating ball-seal, and with supporting the lift in emergency. The rotational barrier assembly thus described would be considered as the primary measure against seawater ingress but it would require back-up by a second rotational barrier assembly 42 situated at the bottom of the lift shaft as it coincides with the tunnel. Figure 9 shows the sectional elevation of the lower rotational barrier assembly during installation.

Of great importance to engendering an understanding of the system to those expert in the art is the manner of installation of the lower rotational barrier assembly 42. The inner liner 43, preferably of corrosion-resistant alloy, serves to support the body of the pre-formed housing 44 and the cavity behind the housing is re-barred then filled with cement to complete the shaft by slip-forming as previously described. The recess 17 where the seat 47 will seal shall be finish-machined as necessary to achieve functional sealing. The seat 47 will be fitted to the ball 23 and locked with a sleeve 26. The assembly will then be lifted through the main shaft from above, into its location and, once in place, the roller shafts 21 shall be introduced through the cross-bore of the ball 23 into the bushing 20 and then held in position by the locking ring 22. The roller shafts shall be of the section indicated, this being of hexagonal or square form in the ball mating profile and circular at the bearing location. The ball's mating profile to the roller shaft shall be of a slightly larger principal dimension in the vertical-closed direction to ensure that the ball 23 has sufficient float to transfer load to the supporting bearing structure below. A toothed, shaft-mounted transmission 24 will be fitted and locked onto the roller shaft 21 as indicated: this shall ensure that the motive force to operate the rotational barrier assembly 42 shall function by drive chain 25 as intended.

Figure 10 shows the sectional elevation of the lower rotational barrier assembly as installed.

The function of this secondary rotational barrier assembly 42 would be to seal the shaft whilst the uppermost rotational barrier assembly 41 is being tested or subject to maintenance work, and also to provide back-up for the sealing of the tunnel from an alternative supply of motive force for closure in the event of failure of the primary rotational barrier assembly 41. The design and manner of installation of the secondary rotational barrier assembly 42 are necessarily different to those of the uppermost, primary rotational barrier assembly 41 and the preferred embodiment is detailed hereafter. The seat 47, under the action of seals 45 and 46 when hydrostatic head from water ingress is present, shall move as permitted by design and seal against the ball 23 when in its closed position.

The housing 44 for back-fill completion of the shaft system at the tunnel-to-shaft interface for the operational stage of the tunnel has as its essential features a recess 17 for a seat 47 and a further, larger recess 19 for bearings 48 which are fitted into the recess after concrete completion. The final embodiment includes the roller shafts 21 and locking rings 22, the ball 23, the shaft-mounted transmissions 24, the drive chain 25.

Following assembly of the lower rotational barrier assembly 42 as described, the method of transmission of the motive force for closure and opening must be coupled to each roller shaft 21 of the rotational barrier at the toothed transmission ring 24. Options include a rack and pinion, or a chain-drive as is shown in the preferred embodiment. Methods of locking a toothed ring to a shaft are known and need not be discussed further.

The aforementioned rotational barriers may, in combination with a cap similar to that of weighted cover 11, allow the structure 14 to be removed and replaced if and when necessary as the rotational barriers and cap 11 would provide a complete and functional sealing system in the absence of the structure 14. The cap 11 may be landed whilst the structure 14 is still in place or after the structure 14 has been removed, each option to follow testing and closure of the rotational barrier assemblies 41 and 42.

The upper rotational barrier assembly 41 would be tested by utilising a test section of reinforced concrete of inner diameter large enough to envelope the upper rotational barrier assembly 41 after removal of the protective housing 38. A recess in the floor of the concrete block now in situ will allow the test section to seal. All that need now be done is to close the upper and lower barrier assemblies 41 and 42 and fill the test section with water, and this will test the sealing function of the upper rotational barrier assembly 41. The lower rotational barrier assembly 42 is easier to test simply by closing it and filling the void above with water. Drainage and pumping as designed and built as appropriate shall ensure removal of the water used as the test medium. It is emphasised that a suitable testing regime shall be imposed upon the system as an entirety. The fact of the ability to test the barriers of one shaft at a time ensures that there is always that function available by means of the other function shaft. For example, if there were not two ventilation shafts, testing of the barriers would close down the function, and temporary loss of ventilation is unlikely to be acceptable. It is noted that the technology of this invention is directly applicable to use in offshore mining.

It is envisaged that those routes to which this invention is intended for use (Japan - Korea, Korea mainland - Jeju-Do, China mainland - Taiwan, China - Jeju-Do, China Dalian Straits) shall require several structures to be in place for construction and the subsequent provision of services to the completed tunnel. The essential service of ventilation shall be provided by daisy-chaining the functions of inlet-outlet. For any one structure which is the location of air supply, those on each side shall be extraction. For a location where the structure supports air extraction, those on each side shall be for air supply. Note that portals shall conform to this requirement.

Figure 11 shows a section through a structure-to-tunnel completion. The skilled person will realise that the present invention is not limited just to being used for constructing tunnels below the sea but could also be used for constructing tunnels below the ocean or below an inland lake such as the great lakes in North America.

Modifications and improvements may be made to the embodiments hereinbefore described without departing from the scope of the invention.