GB2358417A

GB2358417A - A method for construction and operation of subaqueous tunnels

Info

Publication number: GB2358417A
Application number: GB0001017A
Authority: GB
Inventors: Allan Cassells Sharp
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-01-18
Filing date: 2000-01-18
Publication date: 2001-07-25
Anticipated expiration: 2020-01-18
Also published as: GB0001017D0; JP2001220988A; GB2358417B; JP3908464B2

Abstract

A method for accelerated construction and assisted subsequent operation of a subaqueous transport tunnel connecting two land masses separated by a sea comprises tunnelling from each of the two land masses and from a position at sea intermediate the two land masses, the latter being effected by setting a bottom liner on the seabed at a location beneath which, at a suitable depth, the tunnel is to be constructed; landing a concrete or concrete/steel structure having a bore extending the full length of the structure over the bottom liner; running a tieback liner through the structure bore to the top of the bottom liner; evacuating soil and water from the bottom liner and tieback liner combination; drilling a shaft 20 from the bottom of the bottom liner to the required depth of the tunnel; the bore, bottom liner and shaft 20 being of a diameter of sufficient size to permit passage of drilling and tunnelling equipment; drilling out a cavern at the bottom of the shaft; assembling tunnelling equipment in the cavern; and constructing a tunnel emanating from the cavern. The structure serves as a support facility for tunnel operations and providing access to and egress from the tunnel for personnel and passengers, in extremis, and further as a site for equipment providing power, ventilation, pumping and maintenance over an optimised length of tunnel.

Description

2358417

SPECIFICATION

TO ALL WHOM IT MAY CONCERN BE IT KNOWN that 1, Allan C Sharp, a citizen of the United Kingdom of Great Britain and Northern Ireland, residing in the town of Arbroath, Scotland, have invented new and useful improvements in a METHOD AND APPARATUS FOR CONSTRUCTION AND OPERATION OF SUBAQUEOUS TUNNELS of which the following is a specification.

2 1. Field of the Invention

The present invention relates to the use of concrete/steel structures being landed on c at an area of seabed above the planned route of a subaqueous transport tunnel system wi a dry shaft then being constracted from said structure to that depth beneath the sa ed where divergent tunnelling faces may be established.

The technology described herein is adaptable for the purpose of mining in Slit ea locations.

2. Description of the Prior Art.

The construction of long, subaqueous drilled tunnels takes several years and is g MY dependent upon the geology encountered on the tunnel route. In the case of the Ch x el Tunnel linking England and France, tunnel boring machines (TBM) were employ(d o drill through a layer of impermeable chalk marl - a favourable geology - in seven y..-;, s.

s By contrast, the Seikan Tunnel linking the Japanese islands of Honshu and Hokka 0 under a stretch of water of similar distance to the English Channel, though of gre r depth, took seventeen years to complete greatly owing to the difficult geolo., al conditions encountered - heavily faulted rock formations. These tunnels are the lon st subaqueous tunnels constructed to date and were both drilled by workfaces from s rt positions at each of the land segments to be linked namely, England and France, d Honshu and Hokkaido. In each case, the workfaces met at the approximate midpoins 0 provide final through-routes.

I I I I 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 I depicts a subaqueous tunnel system with its dedicated offshore structures in position.

Figure 2 depicts a concrete/steel foundation set in a seabed of rock Figure 3 depicts an offshore structure being towed to its landing position above the preinstalled foundation block.

Figure 4 depicts the offshore structure landed on its foundation block.

Figure 5 depicts the installation operations of a concrete/steel liner being set into a seabed of deep soil.

Figure 6 depicts an offshore structure of concrete/steel landed over the liner with a central tieback liner installed within the bore of said structure to provide a dry access to the bearing stratum.

Figure 7 depicts a shaft and tunnel completed from the structure.

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

4 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The construction of long subaqueous tunnels requires detailed surveying of local geology order that the most appropriate method(s) of face drilling may be ascertained prior o commencement of operations. This invention additionally requires detailed survey of t le seabed in the vicinity of each concrete structure to be employed as the means by whi(,; h a working shaft, sunk to tunnelling depth beneath the seabed, shall provide additIOT I workfaces for tunnel construction and permanent facility for provision of supporting servidis during operation.

The condition of the seabed at each site of landing of a concrete structure is the m 1i IT determinant of the method by which a seabed-tostructure interface is selected and instalk d. Seabed shall be of either rock with little or no soil cover, or of medium to deep soil cd-" e:r down to a stratum suitable for bearing.

Referring to Figure 1, a tunnel system of one or more major throughroutes and suppoat safety and auxiliary tunnels I provides dry passage between two sections of land 2 separl A by a stretch of water 3. Concrete/steel structures 4 for offshore use are each landed on a foundation block 5 of concrete/steel set on a rock seabed 6 which has preferably ten'i excavated to provide seating for the foundation block 5. The concrete structure has principal bore or shaft 7 of sufficient size to permit passage of drilling and tunnelliir equipment, and has topside facilities to support drilling and tunnelling operations, iz i I facilities to include provision of power, ventilation, lifting, and conveyance, treatment a i I disposal of cuttings from the workfaces. Accommodation for tunnelling personnel may, 1, be part of the topside infrastructure. It is possible to use additional legs to support t ie facilities of the structure but the primary body of concrete steel with principal bore if in essential element of this invention.

A drilled shaft 20 supported by concrete/steel reinforcement is fashioned through t supporting formation from structure shaft 7 to tunnel depth at which shall be constructed a cavern (not shown) wherein tunnelling machinery shall be assembled. Tunnelling in oppc s te directions on the tunnel route provides two additional workfaces for each instance where tl is invention is used. It is clear therefrom that the tunnel of Figure 1, where two concr structures are installed, would be completed in approximately one-third of the time taken C) drill the tunnel in the manner hitherto employed for the long subaqueous tunnels previou Y cited simply because there three times as many workfaces available hence three times gre:r !a r drilling rate.

The foundation block is of particular importance as it provides not only a flat plane for t landing of the concrete structure but also; a static fixation to the seabed; guidance a id location for the concrete structure owing to the steel or concrete/steel spigot sited preferat ly centrally on the upper surface of the forming framework into which concrete shall, )e pumped; a seal between the foundation block and the seabed resulting from the solidifica ion of the concrete and its adhesion to the seabed; and a bore, sited preferably centrally, throu, Yh the spigot continuing through the form to the seabed.

Refering to Figure 2, the procedure for the forming of the foundation block on a seaboi f rock is as follows. A form 8, preferably of steel, is fabricated from section and plate, It s preferable that the form have an optimised porting system, preferably operated by valve:;, iq n order to ensure efficient displacement of entrained seawater within the volume bounded y the form 8, the seabed 9, and the underslung skirting 10 at the edges of the form and central bore 11. The central bore, which has a protrusion above the form functioning as a locating spigot 12 for the subsequently landed structure, is of a size which would permit subsequent passage of drilling and tunneling equipment. The seabed 9 shall be of rock and should preferably be pre-dredged to remove any soil cover and to excavate a seat for the forthcoming concrete. The form is run from a surface vessel and landed at the selected position. Skirting 10 beneath the form 8 shall ensure retention of cement slurry within the form on completion of pumping operations. In order to ensure that the form is level prior to, and during, pumping operations, mechanical or hydraulic jacks appropriately attached to and sited preferably beneath the form shall adjust the elevation as required. Within the form 8 is sited a matrix of reinforcement bars, said bars being either attached to the form or landed beforehand on the seabed. A suitable cement is prepared at surface and pumped into the form preferably through flexible riser pipe and displaces the seawater residing within the form through both the valve-operated porting and under the skirting. Excess volume of cement should be pumped to ensure complete displacement of seawater from the interior of the form. A flat, level block of reinforced concrete conforming exactly to the topography of the seabed within the form results from solidification of the cement slurry. The form may be left in situ as its presence would have little effect on the subsequently landed concrete structure - all loading is borne by the block but any wastage of the form can be minimised by appropriate coating, cathodic protection, and grouting. By the minor design adjustments of making the form detachable from the spigot 12 and reinforcement bars, the form may be retrieved to surface if required. More efficient displacement of water may be achieved by use of compressed air to expel water within the form prior to introduction of cement, then venting off the air as cement enters - this may require additional ballast on the form to hold it to the seabed. Furthermore, the foundation may be of such a size that the form would require to be split into compartments, each to be cemented in sequence, or simultaneously if there is sufficient cement mixing and pumping equipment available.

An alternative to excavating a hole for seating the concrete block is to drill and grout a plurality of posts into the seabed of rock within the locus of the subsequently installed form. There shall remain a sufficient length of each post protruding above the seabed into the space within the form in order that, when the cement pumped into the forrn solidifies, the resultant block shall be fixed to the posts and thereby to the seabed.

It may be further preferred to cast some of the foundation block within the form prior to deployment subsea and then to grout the block as necessary thus requiring a lesser volume of cement and greater assurance of concrete quality.

A hybridised procdure'combining elements of the methods cited in a technically coherent manner as appropriate to seabed conditions would become evident to those skilled in the art. These methods of setting concrete foundation blocks on seabeds of rock or little soil cover to facilitate subsequent dry shaft sinking and tunnelling operations are further applicable to seabeds having a significant slope.

Figure 3 shows an offshore structure 4 of concrete/steel being towed into position above the foundation block in the manner typically employed in the offshore oil and gas industry ballast tanks control the elevation of the structure. The structure has a principal bore 7 of sufficient size to permit passage of drilling and tunnelling equipment and personnel, and installation of conveyance equipment for excavated material.

6 Figure 4 shows the structure 4 landed on the foundation block S. The interface betwor th( structure and foundation block must be grouted to ensure a seal between the bore 7 and th( sea 3 in order that the bore may be subsequently pumped dry. The grouting betweqn th( structure and foundation block may be carried out through dedicated pipework built in -.c th( structure wall to convey grouting fluids from surface to the interface, preferably arOLInd th( spigot. I A different approach must be adopted where soil of a greater thickness than that which c4 oak be dredged or expelled is in place above a bearing stratum. Referring to Figure 5, a botory liner 13 of concrete/steell is being set into soil 14 down to a bearing stratum 15. The liner Ila., internal dimensions which would permit subsequent passage of drilling and tunne]RnE equipment, and a wall thickness which shall resist all loads encountered during instal la Jor andoperation. Not shown is pipework set within the wall of the liner through which j1111t ng mud and fixing/scaling grouts may pass, said fluids pumped from surface typically thJ'C L g11 flexible piping 16 attached to the top of the liner.

At the bottom of the liner is a cutting profile 17, preferably of steel, as typically employe J ir the civil engineering sector. The liner is set to depth by self-weight, jetting, ar d b3 excavation of soil inside the liner as required to reduce wall friction. In the event that d esc measures are insufficient to set the liner at required depth, piledriving equipment attache, I tc the bottom liner through an appropriate interface - the tieback liner to be subsecide, tly installed is such an interface - shall ensure final attainment of setting depth. Upon rea3l. ng the bearing stratum, the lower end of the bottom liner should be grouted to fix the lit, ie in position and to provide a seal between the highly permeable soil and the interior of the li - er. This bottom liner is designed to be of sufficient length to reach the bearing stratum al to have a length protruding above the seabed, said protrusion to function as a guide I.."cr the concrete structure to be subsequently landed.

Figure 6 shows a concrete structure 4 which has been guided by, and landed ovel, he protrusion of the bottom liner 13 at the seabed and set onto the surrounding soil. The I o; rer region of the bottom liner 13 has been fixed by appropriate grout 19 pumped through, c - ed pipework. Preferably fixed to the external surface of the protrusion are centralisers of fie type typically used to centralise tubular strings run downhole in oil well drilling and ca., ng operations; these centralisers shall ensure that the liner and the bore of the structure are;et acceptably centrally to each other. The liner resists both the compressive forces 6n its external surface resulting from soil compression under the structure and the differnlial hydrostatic pressure across the liner wall which shall exist when the dry shaft is constructe d.

A tieback liner 18 of concrete/steel is run through the principal bore of the structure to I ind on the top profile of the bottom liner 13. Mechanical attachment and sealing is effecti-c by the mating profiles of the tieback and bottom liners and, as matching pipework is buili ito the tieback liner, the connection may additionally be sealed by cement if required. ''he tieback liner 18 is of sufficient size to permit passage of drilling and tunnelling equipment ind of sufficient length to terminate at the deck level of the structure; it may have sholer, detachable elements at the upper end for it is widely known within the offshore oil indu - ry that soils underneath structures of the type cited in this invention consolidate over tirle In the event that seepage into the liner shaft is encountered, grout shall be applied as neceSs, Lry. In this particular instance, any settlement of the underlying soil shall result in sinking Of the structure but there shall be no transfer of load to the liner as it is mechanically deco,11 led fi-orn the structure.

7 On Figure 7 is depicted the completed scenario wherein the soil and water within the combined liner has been removed by dredging and pumping to provide a dry route to the bearing stratum 15 through which a shaft has been sunk to tunnelling depth, said shaft being supported by concrete/steel as required and of sufficient size to permit passage of drilling and tunnelling equipment. A cavern 21 shall be fashioned at tunnelling depth wherein tunnelling equipment shall be assembled and prepared for boring operations. The liner (13 and 18), as an entity, provides a dry passage of requisite mechanical strength and durability to facilitate construction of the shaft 20 to tunnelling depth and is little affected by subsequent sinking of the concrete structure. A system of tunnels 22 may now be constructed from the workfaces resulting from the deployment of this invention A liner may be used, if required, within concrete structures set on seabeds of rock as previously described, bing tied back from the foundation structure or other position within the shaft to surface.

The invention has significant advantages over methods currently employed to construct and operate long, subaqueous tunnels, It provides additional workfaces at optimal positions on a tunnel route with clearly definable economies of construction time and, through the provision of all facilities and services required to ensure operation, greatly increases the viable length of such tunnels with no intrinsic additional construction time. Furthermore, each tunnel section is being drilled in isolation from all others, this fact further reinforced by the use of bulkheads and emergency shut-in equipment behind each workface near the assembly cavern. In the event of irrecoverable collapse through to the seabed of any given section under construction, only that particular section is lost - the tunnel can still be completed by drilling around the collapsed section. Considering the Seikan tunnel of 53 kilometres length, portal to portal, one single deployment above tunnel midpoint of this invention in the manner described would have provided two additional workface systems in opposite directions thereby halving the construction time from 17 years to 8.5 years - two deployments reduce the construction time to one third of the actual time taken. The relationship between number of deployments n, standard construction time T, and new construction time using this invention t is; t = T / (I + n).

On completion of a major subaqueous tunnel system, and modification of the shaft to include emergency shut-in equipment, the invention may assure permanent sites for separate and independent provision of ventilation, power, maintenance access, emergency access and egress for personnel and passengers - all of which are unavailable within the subaqueous sections of the long tunnel systems constructed to date. The longer a tunnel system is, the greater the ventilation requirement and the associated pressure losses of the circulated air. The circulation system of the Seikan Tunnel is based upon ventilation shafts 23 kilometres apart. It is foreseeable that this system would be deployed at lesser separation intervals based on a greatly accelerated planned construction time and still ensure complete service provision. There is also no longer any inherent limitation on tunnel length. This is best demonstrated by the following example.

South Korea and Japan are separated by the Tsu-shima straits of approximately 200 kilornetres although there are islands centrallysituated. With ten offshore deployments of this invention plus one island used as construction bases for workfaces in addition to each end point, and an average drilling rate of 10 metres/day/workface, there being 24 workfaces in all, each workface must drill 8.33 kilometres for completion. Drilling completion takes 833 days plus equipment mobilisation time of 1. 5 years means a total completion time of a fully maintainable tunnel system of 200 kilometres end-to-end length of 5-6 years.

8 Another area of particular interest is Hokkaido - Sakhalin separated by La Perouse strii oj approximately 35 kilometres. A tunnelled transport system linking these islands is) nc particular interest in itself but, by a logical extension of the route across the Tatar strai, i f kilometres by bridge or tunnel and thence to Komsomolsk-na-Amure, a fully-intqr, ec el connection to the trans-Siberian railway is achieved. Trade between Japan and Europ a, worth $140 thousand million in 1996 of which 90% is entrainable. The shipping time is -6 i weeks but by rail over the route described would take less than one week.

Other routes rendered technically possible or more viable by this invention are Austrz aTasmania; Wales-Ireland; Italy-Sicily; but only a second Channel tunnel approximately fi m Bournemouth to Cherbourg, a tunnel system from Finland to Sweden tying Helsinki ar d t. Petersburg into the Scandinavian railway system, and a politically difficult Taiwan-l a route approach the compulsive economics proffered by the two Japanese schemes.

9

Claims

CLAIMS 1 claim

1. A method and apparatus for accelerated construction and assisted subsequent operation of subaqueous transport tunnels comprising in combination:

a purpose-built concrete or concrete/steel structure principally characterised by a flill length through-bore or shaft in the primary structure landed on a seabed location beneath which, at a suitable overburden, shall be constructed a tunnel; a bottom liner preset in the soil present beneath the aforementioned structure; and a tieback liner run t hrough the bore of the structure to the top of the bottom liner from which combination the internally resident soils and waters are evacuated a shaft drilled from the bottom of the liner through the bearing stratum to the depth of overburden for tunnel construction a cavern drilled out of the rock at tunnel construction depth in an area at the bottom of the shaft in which tunnelling equipment shall be assembled and prepared for drilling a series of tunnel sections constructed by techniques appropriate to the local geology emanating from each shaft/cavern sited on the tunnel route wherein each of the tunnel sections links with the others to create a complete dry route linking two sections of land.

2. A foundation block of concrete, for use on seabeds of rock with minimal or little soil cover, which serves as a flat base with locating spigot for the landing of the concrete structure of claim 1 and comprising: an excavated area conforming in plan approximately to that of the structure of claim a fabricated form of framework and plate and a central ring of a diameter of sufficient size to permit passage of drilling and tunnelling equipment; a matrix of reinforcement bars within the form landed in the excavated area; a spigot of steel or concrete/steel sited directly above the central ring and of the same internal diameter.

3. The structure of claim 1 having topside facilities for lifting of drilling and tunnelling apparatus from supply vessels preferably onto a landing deck thence further down the central shaft of the structure or the liner within and further characterised by; apparatus for conveyance, treatment and disposal of excavated material; and ballast tanks situated at or near the bottom of the structure to control the landing.

4. The bottom liner of claim 1 constructed of concrete and steel and characterised by":

a steel cutting face at the bottom of the liner to facilitate the embedment procedul-; pipework built into the wall of the liner to provide passage to outlets at appropria te positions in the liner bottom and external surfaces for jetting and grouting fluids is required; an internal diameter of sufficient size to permit passage of drilling and tunnel i equipment; length sufficient to ensure that upon attaining the bearing stratum beneath the sc il a portion remains clear of the seabed in order to provide a guiding spigot for tl kt structure of claim 1 during the installation on the seabed of the latter; a mating profile sited at the top suitable for subsequent mechanical attachment 1 Ill sealing of the tieback liner; steel springs set on the outer profile of the aforementioned guidance spigot to en-lu centralisation of the liner relative to the bore of the structure.

5. The tieback liner of claim 1 constructed of concrete and steel and installed thro rll the bore of the structure to land on the pre-installed bottom liner, said tieback 1 n >,r being characterised by: a bore of sufficient size to permit passage of drilling and tunnelling equipment; a profile at the lower end to permit mechanical attachment and sealing to the mail profile of the pre-installed bottom liner; pipework built into the wall of the liner to provide passage of jetting and groul il g fluids to the sealing area and additionally to the bottom liner pipework as require 1 '1 installation operations.

6. The shaft of claim 1 constructed from the bottom of the liner of claims 1 and 4 or from the foundation block of claim 2 down to the tunnelling depth supported ly concrete and steel and characterised by a sectional area of sufficient size to p(.,rr it passage of drilling and tunnelling equipment.

7. The foundation block of claim 2 constructed by pumping cement slurry into t C internal space(s) created by a forming framework, with a central ring and tl e boundary surface of the excavated area, said cement slurry displacing the wat r within said space and being pumped as an excessive volume to ensure comp le e displacement of seawater entrained within the form.

8. The structure of claim 1 serving as a support facility for tunnel operations al providing access to and egress from the tunnel system for personnel, and passenget in extremis, and further, as a site for equipment providing power, ventilatio 1, pumping and maintenance over an optimised length of tunnel.

11

9. A multiplicity of the systems as described in claim 1, with alternative use as required and as appropriate to seabed conditions of the system of claim 2, used to construct, operate and maintain a group of subaqueous tunnels functioning as an entity with each concrete structure serving as a facilities centre to provide facilities and services as described in claim 8.

10. The foundation block of claim 2 where some of the concrete within the form is cast prior to installation on the seabed with grout subsequently pumped into the remaining void between block and seabed as described in claim 7.

11. The foundation block of claim 2 wherein a plurality of fixing posts are drilled and grouted at suitable positions within the locus of the form, there being a suitable length of post remaining above the seabed and entirely within the subsequently installed form inside which cement shall be pumped, solidify to concrete and be fixed to the seabed by means of said posts in place of, or supplemental to, an excavated area beneath the form.

12. A method for accelerated construction and assisted subsequent operation of a subaqueous transport tunnel as herein before described with reference to the accompanying drawings.

12. The foundation block of claim 2 constructed by combinations of methods cited in claims 7, 10 and 11.

Amendments to the claims have been filed as f ollo j CLAIMS I claim:

1. A method for accelerated construction and assisted subsequent operatio f a subaqueous transport tunnel connecting two land masses separated b se I comprising tunnelling from each of the two land masses and from a position a se I approximately intermediate of the two land masses, the latter being effec, U setting a bottom liner on the seabed at a location beneath which, at a suita le t1' the tunnel is to be constructed; landing a concrete or concrete / steel structure in a bore extending the full length of the structure over the bottom liner; run i tieback liner through the structure bore to the top of the bottom liner; evacuati 01 and water from the bottom liner and tieback liner combination; drilling a sh-a the bottom of the bottom liner to the required depth of the tunnel; the bore, ri liner and shaft being of a diameter of sufficient size to permit passage of d Jlib tunnelling equipment; drilling out a cavern at the bottom of the shaft; asse. nj tunnelling equipment in the cavern; and constructing a tunnel system emat nc from the cavern.

2 A method for accelerated construction and assisted subsequent operation a subaqueous transport tunnel in which a plurality of shafts and caverns are drill to tunnel depth at appropriate and optimised positions on the tunnel route as effeclie in the manner of claim 1. r 3. A method for accelerated construction and assisted subsequent operation o- a subaqueous transport tunnel as claimed in claims I & 2 in which the struct re is landed on a foundation block of concrete on the seabed, said block being bound d y a fabricated form of framework and plate, and having an internal matri of reinforcement bars.

4. A method for accelerated construction and assisted subsequent operation a subaqueous transport tunnel as claimed in claim 3 wherein cement slurry is pu 11 d into the internal space(s) created by the fabricated form after installation o. e seabed.

5. A method for accelerated construction and assisted subsequent operation j a subaqueous transport tunnel as claimed in claim 4 where some of the concreteir e fabricated form is cast prior to installation on the seabed.

6. A method for accelerated construction and assisted subsequent operation f a subaqueous transport tunnel as claimed in claims 3, 4 and 5 wherein an excav i d area beneath the form is prepared to accept cement slurry for enhanced stabilityla fixation, said area preferably conforming in plan view to that of the structure.

7. A method for accelerated construction and assisted subsequent operation C f subaqueous transport tunnel as claimed in any of claims 4, 5 and 6 wherei plurality of fixing posts are drilled and grouted at suitable positions within the I c s of the form, there being a suitable length of post remaining above the seabed I entirely within the subsequently installed form so that the pumped cement solidi I around the posts to fix the form to the seabed in place of, or supplemental to,j excavated area below the form.

13 8. A method for accelerated construction and assisted subsequent operation of a subaqueous transport tunnel as claimed in any preceding claim in which the drilling and tunnelling equipment is lifted from supply vessels and lowered down the structure bore and drilled shaft by topside facilities on the structure; and the structure is provided with apparatus for conveyance, treatment and disposal of spoil and, at or near the bottom, with ballast tanks to control the landing.

1 A method for accelerated construction and assisted subsequent operation of a subaqueous transport tunnel as claimed in claim I in which the bottom liner is set in the seabed by means of any necessary combination of a steel cutting face, pipework connected to outlets at appropriate positions on the liner for jetting or grouting fluids, and a suitable temporary tieback column to surface pile-driving equipment; the liner having a length sufficient to ensure that upon attaining the bearing stratum 1:1 beneath the soil a portion remains clear of the seabed in order to provide a guiding spigot for the structure, a mating profile at the top for subsequent mechanical attachment and sealing of the tieback liner and steel springs on the profile to centralise the liner relative to the bore of the structure.

10. A method for accelerated construction and assisted subsequent operation of a subaqueous transport tunnel as claimed in claim 9 wherein the tieback liner is provided with pipework built into wall of the liner to provide passage of jetting and groutina fluids to the sealing, area and to the bottom liner pipework.

I I 11. A method for accelerated construction and assisted subsequent operation of a subaqueous transport tunnel as claimed in any preceding claim in which the structure serves as a support facility for tunnel operations and provides access to and egress from the tunnel for personnel and passengers, in extremis, and further as a site for equipment providing power, ventilation, pumping 'and maintenance over an optimised length of tunnel.