Table 5/5 - Classification of Soil and Groundwater Corrosion Properties
UNDERGROUND STRUCTURES
8.1 GENERAL
8.1.1 Scope
Design requirements for depressed or underground structures constructed by cut-and-cover methods and associated temporary works are covered in this Chapter.
Underground drainage culverts and canals beneath roadways and railways shall be designed using this Chapter. However, for these structures, the waterproofing and 0.2mm crack width requirements do not apply.
8.1.2 General Principles
The following general principles shall be followed:
(a) The Permanent Works shall be of reinforced concrete construction, unless otherwise accepted by the Engineer.
Prestressed concrete will in general not be acceptable.
(b) The design, and selection of construction method, of cut and cover structures shall take into account at least the following:
(i) The geology.
(ii) The hydrogeology and strata permeabilities in the vicinity of the excavation.
(iii) The degree of lateral movement and settlement which would be expected. In this context the location of the works in relation to existing structures shall be considered.
(iv) The depth of construction.
(v) Any particular difficulties that special plant might face with respect to access, clearances and working space.
(vi) The noise levels and environmental pollution produced.
(vii) Control over heave and instability of the base of the
(viii) The methods by which the completed structure shall be secured against flotation.
(ix) The method for waterproofing the completed structure.
(c) In the case of railways no movement joints are allowed across the track. For vehicular underpasses and depressed carriageways the number of movement joints shall be minimised as far as possible. The underground works shall be designed as far as possible to be structurally continuous and the ground pressure distribution, long term ground / structure interaction, total and differential settlement shall be considered accordingly.
Where it is necessary to use movement joints they shall be heavy duty with positive reinforcement connections to the structure.
Where applicable, they shall be able to resist the uplift pressure.
They shall be designed to be easily maintained and replaceable.
Irrespective of whether or not the movement joints are located beneath a roadway, the vertical deflection of any unit (or section) of the structure under the application of primary live loads at the serviceability limit state must be less than 0.015H, where H is the height of cover above the structure.
(d) The underground structure shall be completely waterproofed as described in this Chapter. However, the structure shall be capable of withstanding aggressive soil and water conditions that may be present without having to rely on the waterproofing.
(e) The method and sequence of construction, including installation and removal of Temporary Works, shall be considered in the design and be clearly defined in the design drawings. Possible imperfections in fabrication and erection shall be considered in the design and the structurally acceptable margins of tolerance shall be clearly specified for critical members and operations.
Any constraints that the design may place on the construction sequence shall be identified and clearly specified in the design drawings. The design of structures in which the permanent walls and slabs are also used to carry temporary construction loads must be fully compatible with the method of construction to be adopted.
(f) Requirements for ground instrumentation, monitoring and contingency plans for modification of construction methods shall be evaluated having proper regard to the uncertainties inherent in the design. These requirements shall be fully described in the drawings.
(g) The design of temporary / permanent walls, and of dewatering methods within both permanent and temporary walls shall, as far as possible, avoid lowering of the water table outside the Works and shall ensure sufficient cut-off to minimise the reduction in piezometric pressure in the adjacent soils.
(h) Structures shall be designed in such a manner that settlement and heave during all stages of construction and throughout the service life of the structure is kept to a minimum.
(i) All underground structures below roads shall have a ground cover of 2 metres minimum, measured from top of waterproofing to the lowest point on the carriageway unless shown otherwise on the Authority’s Drawings.
(j) Unless agree otherwise in writing by the Authority, a water retaining structure shall be provided at each Underground Structure entry and exit between the face of the Underground Structure portal and where the top of the bottom slab, across its full width, is wholly above existing ground level or such higher level as the Authority or other authorities may require. A transition slab shall be provided at the end of each such water-retaining structures as specified in Chapter 6.
8.1.3 General Requirements for Trainways in Cut-and-Cover Tunnels and Stations
8.1.3.1 Size of Tunnel
With the exception of cells containing sidings per Clause 8.1.3.6, the size of each cell of a cut-and-cover tunnel shall accommodate the various items listed in Clause 7.2.1. Items (f) to (i) can be accommodated in spaces 300mm wide on the walkway side of the tunnel and 350mm wide on the opposite side of the tunnel as shown on the Authority’s Drawings.
8.1.3.2 Cross Passageways
Cross passageways between two independent single-bound cut-and-cover tunnels shall conform to Clauses 7.8.1 and 7.8.2. The door shall be of sliding type if necessary.
8.1.3.3 Drainage Sumps
The requirements of Clause 11.2.2 shall apply for sumps within the lengths of cut-and-cover tunnel
8.1.3.4 Ventilation
In track crossover areas the tunnel ventilation regime may be maintained by the use of jet fans mounted in the roof of the cut-and-cover tunnel. Vertical enlargements in the structure shall be provided
8.1.3.5 Separation of Trainways
With the exception of tracks within the Depot boundaries, all trainways whether for mainline tracks, centre sidings, test tracks or any other purpose, shall be separated from adjacent trainways or other operational areas (for example plantrooms in stations) by continuous reinforced concrete dividing walls, which extend as far as possible consistent with provision of openings for crossovers, cross passages, or other operational requirements, to the acceptance of the Engineer.
The precise setting out of the dividing walls shall be co-ordinated with the System-wide Contractors, and agreed with the Engineer.
8.1.3.6 Cells Containing Sidings
The minimum internal width of cells for sidings shall accommodate:
(a) The Static Load Gauge, increased as necessary for Vehicle Throw
(b) 600 mm minimum clearance between the Static Load Gauge (increased for Vehicle Throw) and service zones, to accommodate access at track level to a stationary train.
(c) 350 mm wide service zones on each side.
8.1.3.7 First Stage Concrete
The first stage concrete is defined as the concrete needed to fill the gap, if any, between the top of the structural slab/tunnel lining and the underside of the track concrete placed by the trackwork contractor. It would therefore not be necessary to provide a separate layer of first stage concrete if the top of structural slab matches the level of the underside of the second stage (track slab) concrete.
The minimum concrete grade for the first stage concrete shall be grade C30. The minimum thickness shall be 300mm.
8.1.3.8 Concrete Finish at interfaces between Trainway Structure, First Stage and Second Stage Concrete
The concrete finish in all trainways at the interface between the trainway structure and the first stage concrete or the second stage concrete (if no first stage), and between the first and second stage concrete (where both are used) shall be an exposed aggregate finish.
This requirement shall be shown on the design drawings.
8.1.4 General Requirements for Vehicular Underpasses and Depressed Carriageways
8.1.4.1 A minimum headroom clearance of 5.4 metres above all roadways shall be maintained across the full width of each carriageway. This applies to both permanent and temporary structures.
8.1.4.2 The underpass shall be designed to allow for positioning and housing of electrical lighting fixtures, mechanical ventilation system if any and openings for cabling works.
8.2 DESIGN APPROACH
8.2.1 The design of underground railway structures (whose function is primarily to serve railways and their passengers) shall comply with SS CP 65, SS CP 73, BS 8002, BS 8004, other relevant codes and the additional requirements herein.
8.2.2 The design of underground roadway structures (whose function is primarily to serve roadways) and other underground structures shall be designed to BS 5400 (Refer Design Criteria Clause 1.2.2), BS 8002, BS 8004, SS CP 73, other relevant codes, United Kingdom Highways Agency Departmental Standards (Refer Design Criteria Clause 1.2.3) and the additional requirements herein.
8.3 ULTIMATE LIMIT STATE 8.3.1 Structural Stability
For each major structure (tunnels, stations, subways, underpasses etc) there shall be a clear statement as to how the stability of the structures under the design loads is to be achieved. This statement shall address, where applicable, loads from future developments or from the construction of future developments.
Underground Structures shall be checked for stability against flotation in accordance with Clauses 1.3.4 and 8.14. They shall also be checked against failure due to base heave in accordance with Clause 8.15.
Structural stability of earth retaining structures against overturning and sliding shall be assessed in accordance with Chapter 6.
8.3.2 Robustness
Refer to Chapter 3 of the Design Criteria for the effects of impact loads
8.4 SERVICEABILITY LIMIT STATE 8.4.1 Settlement
Refer to Chapter 6.
8.4.2 Cracking
All members shall comply with the requirements of the relevant Codes in respect of limitations on crack width. The maximum crack width under any conditions shall not exceed 0.3 mm, or such other smaller value as required by the Codes. For members designed to BS 5400 to
“very severe” exposure conditions, the design crack width may be taken as 0.2 mm, unless the members under consideration are adjacent to seawater when the more stringent limitation in BS 5400 of 0.15 mm would apply.
In addition, in order to promote water-tightness, those members exposed to earth and/or ground water and forming the hull of underground structures (i.e. roof, walls, base slabs etc.), shall be designed such that the calculated maximum crack width on both external and internal faces due to early age thermal cracking or flexure and/or tension arising from applied external service loads does not exceed 0.2mm on a plane at a distance of 40mm from the outermost reinforcement, irrespective of whether any additional protection (for example a waterproofing membrane) is provided. Where the load combinations for the purposes of checking this crack width include surcharge loading, the value of such unfactored live load surcharge shall be taken as 17 kN/m2.
For embedded concrete walls, the load effects (bending moments etc,) for which this crack width of 0.2mm is checked shall include those which are “locked-in” at construction. Where the embedded wall is designed and detailed to be structurally composite with an inner wall, the crack width at the interface with the inner wall may be 0.3mm. For load effects during construction which are not “locked-in”, the crack width shall not exceed 0.3 mm.
Provision shall be made to ensure that calculated crack widths on any face of such members due to early thermal cracking do not exceed 0.2mm.
The widths of cracks caused by applied external service loads need not be added to those caused by early-age thermal cracking and shrinkage.
The width of the cracks referred to above shall be calculated using the formulae in SS CP 73.
8.5 DURABILITY
The elements of underground structures which form the Permanent Works in contact with earth and / or groundwater shall be considered critical elements with respect to durability. Selection of water bars and joint sealants shall be based on a life cycle cost assessment (see Chapter 1) of the feasible options. Durability provisions in the following clauses are minimum requirements.
8.5.1 Exposure Conditions
The following minimum conditions of exposure per SS CP 65: Part 1 Table 3.2 for underground structures designed to SS CP 65 and Table 13 of BS 5400: Part 4 for underground structures designed to BS 5400 shall be allowed for in the designs:
(a) The external surface of concrete forming the hull of underground structures (i.e. roof, walls, base slabs): very severe
(b) Where an in-situ inner wall is cast against an embedded wall (diaphragm wall, secant pile wall or contiguous bored pile wall), both the inner face of the embedded wall and the outer face of inner wall: severe
(c) The internal surface of concrete forming the hull of underground structures, and the face of all members exposed to trainways (in both cut and cover tunnels and in stations) or roadways: severe (d) Internal members of underground structures other than above:
moderate 8.5.2 Minimum Cover
Notwithstanding the cover derived from the exposure conditions given in Clause 8.5.1 above, the nominal cover to the outermost reinforcement shall be not less than 40mm for the following locations:
(a) external and internal faces of members forming the external hull of underground structures
(b) both faces of in-situ inner walls cast against the walls of external hull members
(c) the faces of members exposed to trainways or roadways 8.5.3 Cement and Water Content
Notwithstanding the cement and water content values derived from SS CP 65 table 3.4 the following maximum and minimum values shall be adhered to.
The minimum cement content of members forming the hull of underground structures and members exposed to trainways or
The minimum cement content for other members shall otherwise be not less than 325kg/m3.
The maximum cement content shall be limited by the need for temperature control when early thermal and drying shrinkage are potential cause of cracking. In the case of OPC, the maximum cement content shall not exceed 400kg/m3.
The maximum free water cement ratio shall not exceed 0.45.
The above represent minimum and maximum typical requirements.
The design shall allow for more onerous exposure conditions and/or aggressive chemical and physical conditions where such conditions are likely to be present.
8.5.4 Shrinkage and Thermal Cracking
Adequate consideration shall be given to the risk of early-age thermal cracking and shrinkage effects. The designer shall ensure that any requirements considered appropriate such as cement chemistry and curing methods are properly addressed in his review of the Materials and Workmanship Specification.
Crack widths due to early-age thermal cracking and shrinkage shall be calculated using SS CP 73. In the case of early-age thermal cracking, reference shall also be made to CIRIA Report 91 [1] and BD 28/87, for structures designed to SS CP 65 and BS 5400, respectively. However the beneficial effects of using cement replacement materials and other techniques such as internal cooling to control the heat of hydration may be taken into account provided that suitable evidence is presented to justify the magnitude of these effects.
8.6 FIRE RESISTANCE
8.6.1 With the exception only of non-loadbearing separation walls, all other elements of underground structures shall be designed and detailed for a 4 hour fire resistance, unless specified otherwise for railways in the Land Transport Authority Standard for Fire Safety in Rapid Transit Systems.
8.6.2 Attention is drawn to the further information on fire in SS CP 65 Part 2.
The structures shall be detailed to ensure that the required fire resistance is met, and in such a way as to avoid spalling, and, as far as possible, also to avoid the need for the use of mesh, whilst complying with the code. When it is necessary to use mesh, the minimum size shall be A252 to BS 4483.
8.6.3 In the case of non-loadbearing separation walls, their fire resistance shall be determined by other requirements, such as fire compartmentation etc.
8.7 INSPECTION OF CONSTRUCTION
Attention is drawn to Clause 2.3 of SS CP 65: Part 1. Construction tolerances shall meet the requirements of the relevant British Standard or those given in the Materials and Workmanship Specifications, whichever is the more onerous.
8.8 LOADS
The general loading requirements are given in Chapter 3, except that when checking crack width, the unfactored live load surcharge in Clause 8.4.2 may be used (and not the higher surcharge specified in Chapter 3).
Additional loading requirements for Underground Structures are given below.
8.8.1 Load Factors for Earth and Water Pressure
8.8.1.1 Requirements for Underground Structures designed to SS CP 65:
(a) The design shall meet the requirements of SS CP 65: Part 1.
(b) In addition, in load cases where the critical stresses in the structure would be produced by maximum earth and water pressure acting in one direction (e.g. on roof slab) in combination with minimum earth and water pressure acting in the other, (e.g. on side walls) and where both water pressures are derived from the same ground water level then the earth and water load factors shall be taken as 1.4 and 1.2 respectively to obtain the maximum and 1.0 and 1.0 respectively to obtain the minimum loads.
8.8.1.2 Requirements for Underground Structures designed to BS 5400:
(a) Partial Safety Factors for Loads
The partial safety factors for loads shall be in accordance with the requirements of BD 37/88, except where modified as follows:
LOAD SLS
γfl
ULS
γfl
COMBINATIONS