TUNNELS IN SOFT GROUND .1 Definition of Soft Ground

In document LTA Civil_standards- Civil Design Criteria (Page 109-114)

Table 5/5 - Classification of Soil and Groundwater Corrosion Properties

BORED TUNNELS AND RELATED WORKS

7.3 TUNNELS IN SOFT GROUND .1 Definition of Soft Ground

Soft ground shall include all grounds except G1 and S1 (see Chapter 5).

7.3.2 Design Method

The Contractor shall use a design method for the analyses of the bored tunnel linings in soft ground which shall take into account the interaction between the lining and the ground, the deflection of the lining and the re-distribution of the loading dependent upon the relative flexibility of the lining and compressibility of the ground.

Acceptable methods for homogeneous soil formations include:

(a) Continuum model by AM Muir Wood(1) combined with discussion by DJ Curtis(2)

(b) Bedded beam model as Duddeck and Erdmann(3) (c) Finite element

Where stratified conditions occur finite element modelling may be necessary.

Due account shall be taken of the degree of flexibility of the linings to be used in the soft marine clays and fluvial deposits. The flexibility may have to be reduced in order to maintain acceptable values for the deflection of the lining.

For the very soft marine clays and fluvial deposits, the shear between the lining and the ground will be small and need not be taken into account in the analysis.

The following load combinations, with the value of vertical overburden pressures as indicated, shall be used to identify the design envelopes of lining stress resultants (bending moments, axial forces, etc.) at both the ultimate and serviceability limit states.

(a) Full ground overburden pressure using water table at lowest credible level together with (where more onerous) live load surcharge

(b) Full ground overburden pressure using ground water table at finished Ground Level together with (where more onerous) live load surcharge

(c) Full ground overburden pressure using Maximum Ground Water Load (refer to Chapter 3)

Where any other more onerous load combinations are appropriate, these shall be identified and used.

In determining the design envelope of stress resultants, the critical load combinations shall include, for the ultimate limit states only, the additional effects of a load case for the distortion of the tunnel cross-section of +/-15 mm on any radius caused by potential future development.

7.3.3 Flotation and Heave

7.3.3.1 Where the bored tunnels are relatively shallow, they shall be checked for the possibility of flotation due to differential water pressure by the following method:

Uplift U = γw π D2 - W 4

where γw = specific weight of water W = self weight of tunnel

(See Clause 7.3.3.4 below) D = outside diameter of tunnel Restraining Force R = γ′ D (hw + D - π D ) 2 8

+ γb D (H - hw) + 2S (H + D ) 2 where γ′ = submerged weight of soil

γb = bulk weight of soil

S = average shear resistance along a-a' = cu for cohesive soils

= ½ Ko γ′(H + D/2) tan φ for cohesionless soils

In the above equations for uplift and restraining force, a partial safety factor of 2.0 shall be applied to the average shear resistance of the ground along the planes of failure, and a partial safety factor of 1.15 shall be applied to the average weight of ground above the tunnel, with the exception of soil type E (Estuarine) to which a partial safety factor of 1.35 shall be applied. The resultant overall factor of safety R/U shall be not less than 1.2.

7.3.3.2 The relatively shallow bored tunnels in clay shall also be checked for the possibility of heave due to shear failure of the ground at tunnel invert level by the following method derived from the base heave analysis after Bjerrum and Eide (4).

For the general case:

Fig. 7.2

F = Nc Cu + 2 S(H - D/2 - he)/D____

0.25 ( γb1 πD) - W/D + q + γb2 he where F = overall factor of safety

Nc = bearing capacity factor (see fig. 7.3)

Cu = average shear strength of soil in the zone of the tunnel invert.

γb1 = average bulk density of soil in zone of tunnel.

γb2 = average bulk density of soil over depth he.

H = depth to tunnel invert from normal ground surface.

he = depth of excavation above tunnel (if any)

q = surcharge at ground level beside tunnel = 22.5 kN/m² W = self weight of tunnel (see Clause 7.3.3.4 below) D = external diameter of tunnel.

S = average Cu along a - a'

In the above equation, a partial safety factor of 2.0 shall be applied to the shear strength of the soil and a partial safety factor of 1.15 shall be applied to the bulk density of the soil, with the exception of soil type E to which a partial safety factor of 1.35 shall be applied.

The overall factor of safety shall be not less than 1.0 when surcharge q is applied nor less than 1.2 when surcharge q is not considered.

Fig. 7.3 Bearing Capacity Factor

Note: Nc rectangular = (0.84 + 0.16 D/L) Nc square

where L = length of structure being considered.

7.3.3.3 Deeper tunnels in the very soft clays shall be checked for possible heave where the ground itself produces an uplift force. This check shall be in addition to the checks required under Clause 7.3.3.1 and 7.3.3.2 above and shall be carried out according to the following method: Circular or square B/L = 1.0

Infinitely long B/L = 0.0

Fig. 7.4

Uplift U = γb π D² - W 4

Restraining Force R = D. Nc . Cu

where, by analogy with foundations:

Nc = 8.25 (after Meyerhoff [5])

γb shall be determined as the average value obtained from the site investigation occurring at the tunnel horizon, i.e. over the range of depth from tunnel crown to invert.

Cu shall be the design value corresponding to the depth of the tunnel axis.

Partial safety factors shall be applied in accordance with Clause 7.3.3.1 above. The overall factor of safety R/U shall be not less than 1.0.

7.3.3.4 In all checking for flotation and heave the self weight of the tunnel shall include only the weight of the lining and of the first stage track concrete and shall have a partial factor of safety of 1.05 applied to it.

7.3.4 Longitudinal Stiffness

7.3.4.1 The Contractor’s attention is drawn to the fact that modulus of subgrade reaction of the soft clays that will be encountered in the invert of the tunnel may be so low that excessive deflections of the lining will occur under train loadings unless the lining has adequate longitudinal stiffness to distribute the concentrated axle loadings.

The Contractor shall carry out a detail analysis of this problem and ensure that the following slope and deflection criterion are not exceeded under train

Slope 1:2000 Deflection 3 mm.

In carrying out the analysis due account of the cyclic nature of the loadings shall be made and stress levels established to ensure that the fatigue life of the structure exceeds the specified design life.

7.3.4.2 An analysis of long term movements of the tunnel shall be carried out to ensure that the tolerances and adjustment limits of the track are not exceeded. These are:

Max. allowable change in grade 1 : 1000

Max. adjustment to track: The lining designer shall refer to the Particular specification and liaise with the trackwork designer to establish the limit of movement incorporated in the trackwork design, and shall ensure that this limit is not exceeded.

Special attention shall be made to the junction between tunnel and station and at abrupt changes of ground conditions.

The long term movements shall include for all future construction above and adjacent to the tunnels shown on the Authority’s Drawings.

7.4 TUNNELS IN ROCK

In document LTA Civil_standards- Civil Design Criteria (Page 109-114)