SOIL AND WATER LOADS

Soils Criteria

4.4.8.1 For the calculation of the weight of backfill, in the absence of site specific tests or data, the bulk unit weights of 20 kN/m3 shall be used. Where more onerous for the element being considered, allowance for the effects of horizontal compaction loads shall be made in the lateral soil coefficient adopted in accordance with Subsection 4.6.

4.4.8.2 Lateral loads on underground structures shall be derived from maximum saturated bulk density or submerged density as applicable and the effective angle of shearing resistance (Ø') using the methods defined in Subsection 4.6 for each soil stratum

Groundwater Criteria

4.4.8.3 For the calculation of groundwater pressures and flotation, a groundwater unit weight of 10 kN/m3 shall be assumed. All groundwater levels used in the design of permanent works shall be determined following a review of the site investigation data relevant to each specific element of the works.

Differential Water Levels Across Structures

4.4.8.4 All underground structures shall be designed to resist a difference in groundwater table level between opposite sides of the completed structure. In the absence of detailed information, a minimum difference in groundwater level table of 5 m shall be used. This exceptional or temporary loadcase is considered to represent a 'burst water pipe' or 'groundwater flow' differential loading condition. The actual site conditions shall be considered and where alternative water level differences are considered appropriate they shall be proposed in the AIP and subject to the approval of the Corporation.

Temporary and Permanent Structures in Soil

4.4.8.5 Lateral loads on underground structures shall be derived from maximum saturated bulk density or submerged density as applicable and the effective angle of shearing resistance (Ø') using the methods defined in Subsection 4.6 for each soil stratum.

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4.4.8.6 Distinction shall be drawn between drained and undrained strengths for temporary and permanent designs. The appropriate water pressures shall be applied.

4.4.8.7 In the absence of more accurate calculation methods, the completed Permanent Works shall be designed to resist the loads resulting from earth pressure 'at rest'. The effects of compaction of backfill and the lateral stiffness of the structure shall be included and considered in the selection of the 'at rest' coefficient of lateral pressure. 'At rest' or 'active' coefficients of earth pressure used to calculate ground loads shall be derived in accordance with Subsection 4.6.

Negative Skin Friction

4.4.8.8 Deep foundations and other structural elements which are constructed in newly reclaimed areas will be subject to negative skin friction (NSF) loads caused by settlement of the fill and underlying soft material. NSF shall be considered as a permanent load on the structural elements and shall be determined from the settlement profile of the relevant strata in accordance with Subsection 4.6. 4.4.8.9 Bitumen, other slip coats or liners applied to structural elements to minimise

negative skin friction or other adverse soil conditions may be considered together with appropriate reduction factors.

Existing Buildings and Future Development Loads

4.4.8.10 To allow for existing buildings or the possibility of future developments being constructed next to underground railway structures, additional vertical and horizontal soil pressures shall be considered in all permanent load combinations. In the absence of site specific data, the pressure shall be 20 kPa acting horizontally on any one side of the basement wall over the entire depth from the ground to the bottom of the base slab. Loads shall be applied in combination or individually, whichever is more onerous for the element being considered.

4.4.8.11 For all underground structures in areas not designated for development or future highway construction, a minimum vertical live loading over the structure of 20 kN/m² shall be assumed in addition to other loading applied.

Highway Surcharges

4.4.8.12 Imposed loads directly borne by the soil behind the retaining structures or slopes shall be allowed in the design. In the absence of more exact calculations, the nominal surcharge loads due to highway loading shall be in accordance with Table 4.4.8.T1.

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Table 4.4.8.T1 Surcharge Loads for Retaining Structures or Slopes

Road Class Type of Live Loading Equivalent

Surcharge Urban Trunk Roads HA+45 units of HB 20kPa Primary Roads,

Construction Access HA+37

1_/

2 units of HB 15kPa

District and Local Distributors, Other Rural Roads, Access Roads, Carparks

HA 10kPa

Footpath, isolated from roads

Play areas - 5kPa

Hydrodynamic Loads

4.4.8.13 Hydrodynamic loads due to current or wave effects shall be considered in accordance with BS6349 - Code of Practice for Maritime Structures and the HKPWDM on all structures where the structure is exposed to them, such as seawalls, piers, IMT's, foundation piles in rivers or waterways. In addition allowance and/or protective measures, such as suitably graded rock blankets, to resist scour around structures due to wave or current effects shall be adopted. When designing for scour particular care shall be taken to adopt suitable current enhancement factors where turbulence due to the structure intrusion in the current flow is predicted to occur.

4.4.9 FLOTATION LOADS

4.4.9.1 Underground structures shall generally be checked for flotation using the minimum water density given in Cl. 4.4.8.3. However, in situations where the groundwater will be influenced by seawater, the most onerous groundwater density unit weight in the range of 9.8 kN/m3 to 10.05 kN/m3 shall be used to allow for the variation in salinity, temperature and sediments content.

4.4.9.2 The information on groundwater levels to be considered for each stage in the design life of the structure shall include, but shall not be limited to, the following: i) the current and projected tidal variations in accordance with the

HKPWDMl;

ii) the long-term rise in sea level which, in the absence of any detailed information, shall be taken as 0.5 m over the design life;

iii) the design free surface water levels due to storm, wind surge, and ponding;

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iv) the design groundwater level including the influences of rainfall, surface water run-off and groundwater movement;

v) the damping of seawater tide influences by intervening ground; vi) ground permeability; and

vii) accidental influences, such as water main breakages and dewatering. 4.4.9.3 The following design groundwater levels shall be considered for flotation loads:

i) Construction

The design groundwater levels during Construction stage shall take into account all events during the time-span which represents the construction of the Works. It shall consider all aspects of the proposed Temporary Works and the case of an excavation over or adjacent to the Works at a later date.

ii) Operational

The design groundwater levels during the life of the structures shall take into account the appropriate combinations of conditions given in Cl.4.4.9.2 that may occur over the design life of the structure.

iii) Extreme

The Extreme case reflects the element of uncertainty in assessing groundwater conditions and shall be represented by groundwater levels which would be considered as the 'worst credible' level for the particular location of the scheme. In the absence of supporting documentation, the minimum groundwater level to be assumed for the Extreme case shall be 1 m below general finished ground level.

Backfill Load Constraints

4.4.9.4 The weight of the backfill above the roof slab shall be based on the bulk or saturated unit weights above and submerged unit weights above and below the water table. The depth of the backfill shall be that appropriate to the load combination under consideration.

4.4.9.5 Where the depth of the backfill is less than 0.5 m it shall be neglected in the calculation of the net downward load. Where it is explicitly specified that the backfill over the structure may be re-excavated to allow for future construction, it shall be neglected down to the proposed excavation depth in the calculation of the net downward load. This particular load case shall be considered under a construction combination.

Structural Load Constraints

4.4.9.6 For all stages and structural elements under consideration, the self-weight of the structure shall exclude the loads from:

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i) any separate building above the structure; ii) any live load internal or external to the structure; iii) any E&M equipment/plant loads;

iv) any extensions of the walls or columns above the roof slab.

4.4.9.7 All surface structures above the underground structure under consideration shall be considered likely to be demolished within the design life of the underground structure. However, where it can be demonstrated to and is agreed by the Corporation that surface structures are likely to remain in place for the life of the underground structure, their permanent loads may be considered to contribute to the resistance against flotation. In such circumstances, the requirement/constraint shall be highlighted in the Operations and Maintenance Manual and in the AIP and subject to the approval of the Corporation.

4.4.9.8 The dead weight of the structure may include any extension of the walls below the base of the structure provided that the wall is reinforced sufficiently to carry all forces acting upon it.

Frictional Constraints

4.4.9.9 In calculating the restraining forces, consideration shall be given to the possibility of the introduction of sliding interfaces during construction. These may include, inter alia:

i) a waterproof membrane;

ii) a temporary works interface; and

iii) a bentonite layer on the face of diaphragm walls, barrettes or piles. 4.4.9.10 Frictional resistance effects shall generally be ignored for structures

constructed using the bottom-up technique, unless it can be demonstrated that the resistance force can be effectively mobilised and maintained by elements such as structural outstands. The frictional restraint force derived shall be factored in accordance with Subsection 4.8.5 requirements.

4.4.10 WIND LOADS

4.4.10.1 Wind loading for bridges shall be assessed in accordance with Subsection 2.3 of the HKSDM. For unloaded structures the dynamic pressure head value quoted for 'normal exposure' in Table 3 of the HKSDM shall be used unless it is considered that more onerous conditions exist for particular elements.

4.4.10.2 In addition to considering the overturning effects given in Cl. 5.3.8 of BS5400, Part 2, railway bridge superstructures shall remain stable in the event that a stationary and unloaded train is left out during a typhoon. This check shall be at the serviceability limit-state in which the ratio of the restraining to overturning effects shall be greater than unity. The check shall not be considered in the design of the structural element. Two conditions shall be considered as follows :-

i) To represent the train on its rails, the structure shall be considered as 'loaded', the height of load dL = 3.7 m and the dynamic pressure head,

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q = 1.2 kN/m². The coexisting live load shall be a static vertical load of 18 kN/m applied at the track centre line.

ii) To represent the train toppled by wind where the structure shall be considered as 'unloaded', the train shall be considered to act as part of the structure and will increase the depth, d, with the highest part of the train 3.1 m above rail level. The dynamic pressure head shall be that calculated for the 'unloaded' condition in accordance with Cl. 4.4.10.1. A coexisting additional vertical dead load of 18 kN/m shall be applied on the leeward rail.

4.4.10.3 Wind loads for building structures shall be in accordance with the Codes of Practice on Wind Effects, Hong Kong, published by the Buildings Department.