4.3.2.1 This subsection sets out the specific requirements for the design of cut and cover structures such as tunnels and ancillary buildings of this form.
4.3.2.2 The preferred method of construction is conventional, ‘bottom-up’ within excavations. ‘Top-down’ construction shall only be used where it can be shown to have significant programme and whole-life cost benefits, and where control of ground movement is critical.
4.3.2.3 Where top-down construction is shown to provide sufficient benefits to warrant its use, subject to the approval of the Corporation, particular consideration shall be given to the following:
i) the design and detailing of practical slab to diaphragm wall connections;
ii) the possibility of top-down walls being constructed out of plumb by up to 1 in 80. All tolerances must be allowed for in the effective length of the slab spans and due allowance must be made to overcome the loss of clearance to the structure gauge;
iii) a rebate shall be provided in the wall section at all slab/wall interfaces to ensure an adequate seating arrangement;
iv) watertightness of external joints. At wall/slab joints, a hydrophilic strip shall be glued with resin mortar on the surface of the rebate nearest the ground, at a minimum distance of 40 mm from all reinforcement, before casting the slab; and
v) a drainage channel and hydrophilic strip shall be provided above the slab/wall connections. A typical detail is shown in Fig. 4.9.2.F1. vi) Notwithstanding the above, the wall joints shall always be considered
a source of leakage. The wall shall be designed such that leakage is stemmed or controlled to prevent corrosion of reinforcement.
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Compatibility with Method of Construction
4.3.2.4 The design of tunnels and underground structures in which the permanent walls and slabs are also used as temporary supports to excavations shall be fully compatible with the method of construction likely to be adopted.
4.3.2.5 Permissible methods of construction, including contiguous bored pile, secant piles, packed-in-place piles (PIP), and diaphragm walling shall utilise ground treatment where necessary to control ground-water flows which may affect stability. In shallower excavations alternative methods may be used. The use of ground treatment to control water flows by reducing permeability in these shallower excavations shall be kept to a minimum.
4.3.2.6 Different methods of construction, including form and sequence, type of plant and programme, will produce different design requirements. Full details of the proposed methods of analyses for the permanent walls when these are used as retaining walls in the temporary condition shall be submitted in the Design Statement (DES). Well-proven methods shall be used and these shall take full account of the construction methods proposed, the relative rigidity of the structure, any relevant structure-soil interaction and adjacent ground movement limitations.
4.3.2.7 The method and sequence of construction assumed in the design shall be clearly defined. Any constraints that the design may place on the construction sequence shall also be identified and clearly marked on the Tender and Contract drawings. Minimum support load, pre-load, stiffness, and permissible deflection limitations shall be defined in the design and/or tender and/or contract documents as appropriate. Details of the required presentation style of construction sequence drawings are given in the CADD Manual.
4.3.2.8 If any cut and cover structures are constructed within the harbour or open sea areas then the requirements as specified for immersed tube tunnel buoyancy detailed in Subsection 4.3.4 shall additionally apply to the cut and cover structures.
4.3.3 UNDERGROUND OPENINGS
Definitions
4.3.3.1 Underground openings are defined as tunnels, caverns, shafts, adits and all other excavations formed below ground that are not basements, cut and cover tunnels or immersed tube tunnels and the like.
4.3.3.2 The following definitions for support and lining shall be used in this section. i) Temporary or
Initial Support
- Support installed to stabilise an underground excavation by controlling deformations due to soil, groundwater pressures, rock mass in situ stresses, rock loosening or adjacent underground openings. Normally installed close to the working face or within
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the rear of the tunnel shield. Typically comprise steel rib supports or lattice arch girders with or without lagging; dowels; nails or rock bolts; plain or reinforced shotcrete, canopy tube and forepoling spiles, or any combination of these.
ii) Primary Lining - Permanent support constructed either close to the working face or within the rear of the tunnel shield or tunnel boring machine and comprise either segmental linings of precast concrete, spheroidal-graphite cast- iron (SGI) or steel; or plain or reinforced, cast in situ or sprayed concrete. Primary linings may also control deformations of the opening due to ground and/or groundwater pressures.
iii) Secondary Lining
- Permanent support typically comprising plain or reinforced poured in-situ concrete or plain or fibre reinforced sprayed concrete which are constructed at a distance from the working face or from the rear of a shield where deformation of the opening has been stabilised by temporary support and/or primary lining. General
4.3.3.3 This subsection sets out the specific requirements for the design of permanent underground openings.
4.3.3.4 All underground openings shall be designed and constructed in accordance with BS 6164: “Code of Practice for Safety in Tunnelling in the Construction Industry” and the NWDSM.
4.3.3.5 All permanent underground openings shall have a design life of 120 years. 4.3.3.6 The permanent support and lining to the underground opening may comprise
the primary lining, the secondary lining or a combination of the two. The permanent support and lining may also comprise permanent dowels, nails or bolts, fibre or mesh or rebar reinforced or plain shotcrete, cast-in-situ concrete and steel segments.
4.3.3.7 It shall be assumed that the temporary support does not contribute to the capacity of the permanent support or lining in any way, unless agreed otherwise by the Corporation.
4.3.3.8 Generally all underground openings shall be designed to be sealed against all water ingress and the full water pressure head occurring around the opening periphery. The design shall include the provision of a waterproof membrane and a durable structural lining. However, for underground openings in rock (as defined in Subsection 4.6) where the structural invert slab is above the highest astronomical tide level (HAT), or where it is uneconomical or technically impractical to either design the invert permanent lining to withstand the full water pressure, or to provide other means for dissipating the water pressure, then a permanent pressure relief system may be incorporated into the design located outside the permanent lining, subject to the approval of the Corporation, providing that it can be demonstrated that:
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i) water inflows will not cause unacceptable drawdown or settlement; ii) the 120 year design life of the structure will not be affected; and
iii) the associated whole-life maintenance and operating costs are more than compensated by the reduction in construction costs, and access for inspection and maintenance is provided.
4.3.3.9 Generally, all permanent underground openings shall be designed for the full water pressure head around the opening periphery. The minimum water pressure head to be considered shall be equivalent to a water level 10 m above the crown of the underground opening. Where a pressure relieved opening form is accepted by the Corporation the lining shall be designed for the minimum water pressure head as defined above down to axis level, reducing to zero water pressure head at the invert.
4.3.3.10 In all underground openings where a pressure relieved system is used a positive drainage system of collecting pipes shall be designed to drain groundwater or pressure relieved water from behind the permanent lining and, within and from beneath the structural invert slab to the tunnel sump(s). Access holes for maintenance to the drainage system shall be provided through the structural invert slab at a maximum longitudinal spacing of 50 m. Pressure relief holes through the permanent lining of underground openings, including the invert, will not be permitted under any circumstances.
4.3.3.11 In all underground openings, provision shall be made for back grouting (contact grouting) between the permanent lining extrados and any waterproofing membrane.
4.3.3.12 Where possible, consideration should be given to co-locating niches, e.g. sumps, passageways, fan niches and suchlike, to minimise the need for many individual niches.
4.3.3.13 Where it is impractical to design the rock permanent lining to withstand the full loading then, subject to the approval of the Corporation, permanent rock dowels or bolts shall be designed in accordance with the relevant provisions of subsection 4.6.10.
Lining Types
4.3.3.14 All linings to underground openings shall be designed in accordance with the NWDSM and the following:
i) the British Tunnel Society / Institution of Civil Engineers, Tunnel Lining Design Guide; and
ii) “Hong Kong Code of Practice for the Structural Use of Concrete”. 4.3.3.15 All permanent underground openings shall have permanent and durable
structural linings which, depending on the nature of the surrounding ground, shall be formed from one of the following options:
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ii) Segmental precast concrete linings.
A minimum structural lining thickness of 250 mm shall be adopted.
4.3.3.16 Segmental lined underground openings shall possess measures to ensure water tightness. Gaskets shall be purpose-designed EPDM rubber and a hydrophilic strip shall be incorporated in the joints. The concrete mix shall contain river sand and the maximum water: cement ratio shall be limited to 0.35 maximum. Other measures to promote construction quality shall be the use of plastic spacers and welded reinforcement cages.
4.3.3.17 The following will not be accepted as permanent linings unless fully justified to the Corporation and other approving authorities as appropriate:
i) unreinforced concrete segments; ii) fibre reinforced concrete segments;
iii) Spheroidal – graphite cast iron bolted segments.
4.3.3.18 Where it can be demonstrated to the Corporation’s satisfaction that the preferred options in Cl. 4.3.3.15 are technically impractical, the options in Cl. 4.3.3.17 will be considered. Full details of the option shall be submitted to the Corporation for approval before proceeding with the detailed design. These details shall include an explanation of why the preferred options are technically impractical, how the design life will be achieved and how access for long-term monitoring and maintenance will be provided.
4.3.3.19 Other underground structures (such as portals, head-walls, ventilation or outfall works, sump pits, and invert concrete) which do not form a part of the final lining structure of underground openings may be constructed of reinforced concrete in accordance with the NWDSM only if it can be demonstrated to the satisfaction of the Corporation that the use of plain concrete is impractical.
4.3.3.20 SGI lining, where permitted, shall conform to BS EN 1563: “Founding Spheroidal Graphite Cast Irons”. Depending on ground conditions, methods of construction, and specific location in the works, precast concrete or SGI segment lining may be either a bolted or expanded articulated type and may also be parallel (plain) or tapered.
4.3.3.21 Segmental shafts sunk as caissons shall have choker rings and cutting edges provided.
4.3.3.22 In specific locations, SGI and bolted concrete segment pans may need to be infilled to provide a smooth internal surface to aid air flow. Bolt pockets in which water may possibly accumulate shall be filled with concrete.
4.3.3.23 Where steel bar reinforced precast concrete segments are permitted for permanent lining, the bars shall be electrically connected within each segment and between adjacent segments to allow for the collection of stray
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currents and for the possibility of installing a cathodic protection system in the future; details shall be discussed with and agreed by the Corporation.
Temporary Lining Loads
4.3.3.24 All temporary linings shall be designed to withstand all environmental and applied loadings and effects appropriate to the needs of its design Life without distress. Linings shall be designed for, but not be limited to, the following loadings and situations:
i) superimposed surface loading based on traffic loading and the loading due to existing buildings over and/or adjacent to the underground openings;
ii) appropriate ground loads; iii) groundwater pressures;
iv) the structural requirements for resisting buckling; v) short-term ground yield or squeeze;
vi) unequal grouting pressures; vii) adjacent excavation(s);
viii) openings in or extensions to the lining;
ix) short-term loads induced by the construction procedure including, but not be limited to, localised de-watering, ground freezing, compressed air working, compensation grouting etc;
x) temperature, shrinkage and creep;
xi) handling loads, especially in the case of unreinforced or reinforced segments;
xii) loads (impact and thrust) from construction equipment such as the TBM; and
xiii) installation loads such as bolted segment connections. Permanent Lining Loads
4.3.3.25 The final permanent structural lining shall be designed to withstand all environmental and applied loadings and effects without distress. Linings shall be designed for, but not be limited to, the following loadings and situations: i) superimposed surface loading based on traffic loading and the
loading due to existing buildings over and/or adjacent to the opening, or any specified future loading whichever is the greatest;
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iii) ground and ground water loads (using Ultimate and Serviceability Limit States for deepest and shallowest sections; highest and lowest hydrostatic heads, differing geological conditions etc);
iv) earthquake loading;
v) railway loading including loading due to derailment and construction vehicles;
vi) the structural requirements for resisting buckling; vii) short and long-term ground yield or squeeze; viii) unequal grouting pressures;
ix) adjacent excavation(s) including tunnel to tunnel (or other opening) interaction;
x) openings in or extensions to the lining;
xi) short and long-term loads induced by the construction procedure; xii) temperature, shrinkage and creep;
xiii) segment handling and stacking (including single point or vacuum lifting / erector devices, eccentric segment stacking and allowable stacking heights), especially in the case of unreinforced or reinforced segments;
xiv) loads (impact and thrust) from construction equipment such as the TBM (including eccentric loading of shove ram jacks due to segment misalignment);
xv) installation loads such as bolted segment connections (including the possible effects of shear on the circumferential joint connections from having staggered longitudinal joints);
xvi) imposed distortion caused by lack of circularity; xvii) annulus and subsequent back-grouting;
xviii) poor segment ring build (including analysis of out of plane circumferential loading and bursting stresses due to axial loading caused by ‘birds-mouthing’ and stepping of segment joints);
xix) segment casting inaccuracies;
xx) prevention of floatation (including possibility of ground heave); xxi) temporary and permanent services; and
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xxii) reinforced concrete design (including checks on: hoop stresses, bursting, overall buckling, torsion load from TBM head rotation, compression of gaskets by bolts, circumferential groove capacity, rotation at each segment joint under load etc).
xxiii) Loss of strength due to a fire.
4.3.3.26 In addition to the loadings noted in Cl. 4.3.3.25, permanent soft ground linings utilising segmental sections shall be designed to accommodate an additional deflection of ± 20mm on diameter to allow for future development. This shall be achieved by amending the horizontal/vertical load as specified in Cl. 4.3.3.25 to produce the aforementioned ± 20mm deflection on diameter. Alternative ways of analysing the effects of additional deflection may be proposed for the acceptance of the Engineer.
Tunnel Excavation and Lining Design Methods
4.3.3.27 The design of underground opening excavations, linings and related works shall take into consideration the method of construction, the required life span, the proposed use, the ground conditions, the sequence and timing of construction and the proximity of adjacent structures.
4.3.3.28 The underground opening shall be of sufficient size to accommodate all operational envelope requirements and provision for services, fittings, plant, walkways, ventilation, drainage etc. In addition, due cognisance should be made to the construction tolerances given within the M&W Specification particularly in relation to TBM drive tolerances.
4.3.3.29 The analysis or design methods for underground openings excavation and linings shall take into account all interactions including, but not be limited to, those detailed in Cl. 4.3.3.24 to Cl.4.3.3.26 above. The Designer shall use a recognised design method. The following are considered to be acceptable methods for analysis:
i) Soft ground (homogenous and stratified soils):
a) Continuum model by Muir Wood A M, 1975, The Circular Tunnel in Elastic Ground, Géotechnique, Vol. 25, combined with the discussion by Curtis D J, 1976, Discussion on Muir Wood, Géotechnique, Vol. 26;
b) Anagnostou G & Kovári K, 1996, Face Stability in Slurry and EPB Shield Tunnelling, Geotechnical Aspects of Underground Construction in Soft Ground, Balkema;
c) Leca E and Dormieux L, 1990, Upper and Lower Bound Solutions for the Face Stability of Shallow Circular Materials in Frictional Material, Géotechnique, Vol. 40;
d) Morgan H D, 1961, A Contribution to the Analysis of Stress in a Circular Tunnel, Géotechnique, Vol. 11;
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e) Duddeck H and Erdmann J (1982) Structural design models for tunnels. Tunnelling 1982 1M1;
f) Terzaghi’s equation (lowerbound) or full overburden (upperbound) taking into account the ground arching effect; and
g) Finite element methods. ii) Hard rock
a) Grimstad E and Barton N et al, Q-System; b) GEO, Guide to Cavern Engineering; and c) Finite element methods.
4.3.3.30 In the event that computer analyses are used for the design, the Designer shall also provide manual check calculations using established engineering and geotechnical principles to verify the results.
Incremental Excavation and Support Techniques
4.3.3.31 Tunnelling methods that involve incremental excavation and support, such as the New Austrian Tunnelling Method (NATM) or the Sprayed Concrete Lining Method (SCLM), require continuous observation of both the ground and support. Such underground openings shall be designed and constructed in accordance with the NWDSM and the following:
i) the Institution of Civil Engineers design and practice guide, Sprayed Concrete Linings (NATM) for Tunnels in Soft Ground;
ii) the UK Health & Safety Executive report, Safety of the New Austrian Tunnelling Method (NATM) tunnels; and
iii) “Hong Kong Code of Practice for the Structural Use of Concrete”. 4.3.3.32 The design and construction methodology shall take account of the following:
i) length of advance;
ii) whether advance should be partial face or full face; iii) inclination of face;
iv) speed of ring closure; v) face support; and
vi) adjacent activities, such as excavations and ground treatment.
4.3.3.33 The design shall be reviewed and modified during and after construction as a result of comprehensive monitoring and interpretation of the following:
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i) the behaviour of the ground at the tunnel face in comparison with the design assumptions;
ii) surface settlements; iii) lining deformations; and
iv) measurement of ground loadings and displacements.
Appropriate contingency plans shall be prepared and implemented to modify the design and construction should the behaviour of the ground or the lining be shown by the monitoring to be substantially different or in excess of those permitted.
Ribs and Lagging
4.3.3.34 The design of a temporary support system using ribs and lagging in rock shall use a recognised design model. The following are considered to be acceptable methods for analyses:
i) Terzaghi K, 1946, Rock Tunnelling with Steel Supports, Section 1, Commercial Shearing and Stamping Co.;
ii) Proctor and White, 1946, Rock Tunnelling with Steel Supports,