4.6.5.1 This subsection addresses the design of the following main surface excavations types.
i) unsupported excavations where stability is achieved by providing stable
slopes; and
ii) supported excavations where the steep or (often vertical) sides are supported by structure.
Unsupported Excavations Buried Services in Slopes
4.6.5.2 Buried services (particularly water carrying services) shall not be permitted in slopes unless there is no practical alternative. For guidance on providing services in slopes, reference shall be made to PNAP APP-76 and Code of Practice on Inspection and Maintenance of Water Carrying Services Affecting Slopes.
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4.6.5.3 Modifications to an existing slope occasioned by new works will mean that the modified slope shall be treated as part of the new works and as a result subject to the minimum factors of safety defined in the Geotechnical Manual for Slopes. 4.6.5.4 Only those soil slope stability analysis computer programs which have been
validated and subsequently approved by the Corporation or Buildings Department may be used. Other slope stability analysis computer programs shall not be used unless approved either by the Corporation or by the Buildings Department.
Groundwater - Soil Slopes
4.6.5.5 For design at feasibility stage and in the absence of site specific data, the minimum wetting band thickness shall be taken as 1.5 m located anywhere in the soil profile which results in the most critical conditions for slope stability. Surface Protection and Support - Soil Slopes
4.6.5.6 Vegetation shall be considered as the principal method of covering for newly formed or regraded soil slopes. Where necessary consideration shall be given to the use of biodegradable or geosynthetic matting or other forms of protection (e.g., geoweb) to ensure that erosion is minimised.
Groundwater - Rock Slopes
4.6.5.7 If rock slopes or portals are to be retained for the Design Life of the Railway, then a groundwater drainage system shall be designed to ensure that the various groundwater levels assumed in the design of the rock slopes and portals will be maintained throughout the Design Life of the Railway.
4.6.5.8 The effect of blasting for rock excavation on the permeability of the rock mass and the impact on water inflows into the excavation shall be included in the tunnel seepage analysis.
Stabilisation - Rock Slopes
4.6.5.9 On the basis of the rock slope stability analyses, the design and associated stabilisation measures shall be proposed to ensure that during and after construction no rock blocks are released from the rock slope. The evaluation of rockfall and required control shall be carried out in accordance with the Geotechnical Manual for Slopes.
4.6.5.10 The design of dowels, rock bolts and anchors shall be in accordance with subsection 4.6.10.
Tunnel Portals - Soil and Rock
4.6.5.11 Design of tunnel portal slopes will have to be allowed for blasting effect should blasting be adopted as the excavation means.
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Supported Excavations Design of Excavation Support Systems
4.6.5.12 Where excavations are adjacent to EBS sensitive to movement, computer modelling of the excavations and support systems shall be carried out using validated either finite difference or finite element based computer programs approved by the Corporation. The results obtained by numerical modelling shall always be verified with other established empirical design procedures. Design Parameters
4.6.5.13 The design parameters, in particular stiffness, assigned to regions of the computer model shall be selected with due regard made to the magnitude and direction of stresses and strains likely to be experienced by that region of the model.
4.6.5.14 When carrying out computer modelling, soil parameters back-calculated from similar existing structures may be used. However, the back-calculation of parameters shall be carried out using the same computer program that will be used in the analysis for the new works. The use of back-calculated soil parameters shall be subject to the approval of the Corporation.
Design Loads
4.6.5.15 Loads shall be determined in accordance with Subsection 4.4. In addition, a nominal uniformly distributed construction loading of 10 kPa shall be applied at the ground surface immediately behind the excavation support for a distance, at least equivalent to the depth of the excavation or all the area within the failure zone, whichever is greater.
Groundwater Levels
4.6.5.16 The design groundwater levels shall be in accordance with Subsection 4.4. The impact of the proposed excavation works on groundwater levels shall be limited in accordance with Subsection 4.3.
Vertical and Lateral Movements
4.6.5.17 In areas where movements caused by the proposed excavation will have no significant impact on adjoining areas (e.g., green field sites) then such movements may be predicted using empirical or semi-empirical methods. Where the proposed excavations are in movement sensitive areas (e.g., adjacent to sensitive EBS) then detailed methods such as finite element analysis shall be used to predict the resulting movements.
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4.6.6 UNDERGROUND EXCAVATIONS
Scope
4.6.6.1 This subsection addresses the design of underground excavations in soil and rock. Refer to subsection 4.3 for details on design requirements for underground openings.
Definitions
4.6.6.2 Refer to subsection 4.3.3 for definitions of Temporary or Initial Support, Primary Lining, and Secondary Lining.
Rock Material and Rock Mass Properties
4.6.6.3 The rock conditions at the Site shall be categorized by the Norwegian Geotechnical Institute (NGI) Q-system in accordance with Grimstad & Barton, 1993 where Q is the rock tunnelling quality index.
Loadings
4.6.6.4 As mentioned in subsection 4.3.3, 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.
Temporary Support Design
4.6.6.5 The preliminary design of rock temporary support may be based on empirical methods using the Q rock mass classification system. Temporary support may also be designed based on the predicted extent of the failure zone surrounding the excavation as determined from analytical methods. However, sufficient allowance shall be made for monitoring of movement and installation of additional support if dictated by conditions and/or monitoring records.
4.6.6.6 Where stability of rock underground excavation is likely to be controlled by the geological structure in the rock mass discrete element methods of analysis shall be used. Proposed computer programs shall be submitted for approval to the Corporation.
Permanent Support Design – General
4.6.6.7 The permanent support and lining to the underground excavation 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 of bolts, fibre reinforced or plain shotcrete, cast-in-situ concrete and steel segments.
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4.6.6.8 Where open face methods of excavation are adopted, it shall be assumed that the temporary support does not contribute to be capacity of the permanent support or lining in any way.
Permanent Support Design – Soil
4.6.6.9 The permanent support/lining constructed in soil in conjunction with shield excavations may comprise a primary lining followed by secondary lining. The primary lining shall be designed for the effects of the shield jacking loads, calculated in accordance with Szechy, 1966 and as summarised in Whittaker & Frith, 1990. A minimum factor of safety of 1.5 shall be applied to the shield shove jack load when determining the structural adequacy of the primary lining. Permanent Support – Rock
4.6.6.10 Permanent linings for underground excavations in rock shall be designed to withstand rock loading defined by:
i) an equivalent permanent support pressure applied uniformly to the full span of the opening and taken as:
Proof = 2 Jn0.5 Q-0.33 / 3Jr (Kg/cm2)
(after Grimstad & Barton, 1993) Where, Jn = Joint set number (Geoguide 4)
Q = Tunnelling Quality Index (Geoguide 4) Jr = Joint Roughness Number (Geoguide 4)
and;
ii) the weight of potentially unstable wedges applied as a distributed load over the exposed area of the wedge as determined from structurally controlled instability analysed by hand or by validated and approved computer games.
4.6.6.11 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 of bolts shall be designed in accordance with the relevant provisions of subsection 4.6.10.
4.6.7 RETAINING STRUCTURES