tunnel settlement

Appendix A16.4 Tunnel Settlement Assessment Theory

DART Underground Córas Iompair Éireann

Appendices A16.4- 1

A16.4 Tunnel Settlement Assessment Theory

A16.4.1 Introduction

Tunnelling is accompanied by ground movements both towards the tunnel face as tunnel construction proceeds and around the void opened up to install the final tunnel lining. These ground movements manifest themselves as a settlement trough at the ground surface. The magnitude and shape of the settlement trough is influenced by the tunnelling methodology, the ground conditions and tunnel size and depth.

A16.4.2 Settlement Profile

It is widely accepted that the shape of the surface settlement trough at right angles to a tunnel axis approximates to an inverted Gaussian distribution curve1 symmetrical to the tunnel axis. Figures A16.1(a) and A16.1(b) below show details of the curve.

The shape of the trough can be calculated using the following equation:

w = wmax exp(-y2/2i2) (1)

where:

w = magnitude of settlement at distance y from the tunnel centreline;

wmax = maximum value of w over the tunnel centreline, i.e. y=0.

i = distance from tunnel centreline to the point of inflexion on the settlement trough (this is the limit between the sagging and the hogging region)

From equation (1) the volume of the settlement trough, V, per unit length of tunnel can be calculated using:

V = √(2π) ⋅iwmax = 2.5iwmax (2)

Therefore, if any two of V, i, or wmax are known it is possible to predict the likely settlement trough profile caused by tunnelling operations.

1 This is discussed further in Peck’s paper (1969) “Deep Excavations and Tunnelling in Soft Ground” Proc 7th Int. Conf. Soil Mech. Foundation Eng., which is based upon studies by Schmidt (1967): “Settlement and Ground Movements Associated with Tunnelling in Soil” PhD Thesis University of Illinois, Urbana. USA.


Appendices A16.4- 2

Figure A16.1(a): Transverse settlement profile (Peck: 1969)

Figure A1(b): Transverse displacements and strains (Peck: 1969)

A16.4.3 Trough Width

The trough width parameter, i, represents the horizontal distance from the tunnel centreline to the point of inflexion on the settlement trough. In order to assess the settlement trough profile the value of i must be obtained.

The paper by O’Reilly and New (1982)2 provides the following equations for calculating i for tunnels in predominantly cohesive or granular soils:

2 O’Reilly, M P and New, B M (1982). “Settlements above tunnels in the United Kingdom - their magnitude and prediction”. Tunnelling ‘82, London, pp 173-1 8 1


Appendices A16.4- 3

The value of i varies linearly with depth, i.e.

i = Kz

where:

K = trough width coefficient (empirical constant)

z = depth

Typical ranges of K values for soft ground tunnelling are contained the table shown below:

Table A16.1: Summarised Surface Settlement Trough Data for a Range of UK Soils (provided C/D > 1), after O’Reilly and New (1982) and Yeates (1984)3

Ground Conditions

Tunnelling method

Trough width parameter

constant, K

Volume loss VL

(%) Remarks

Soil fissured clay Shield or none 0.4 to 0.5 0.5 to 3 Considerable data available, losses normally 1÷2%

Glacial deposits

Shield in free air 0.5 to 0.6 2 to 2.5

Shield in compressed air*

0.6 to 0.7 1 to 1.25 *Used to assist in controlling ground

movements

Recent silty clay deposits

(cu = 10÷40 kPa)

Shield in compressed air

2 to 10

Granular material above the water

table 0.2 to 0.3 1 to 5

Granular material below the water

table

Compressed air/slurry/EPB

0.4 to 0.5 1 to 10

A16.4.4 Volume Loss

The volume of the surface settlement trough, V, can be expressed as a percentage of the volume of excavation and is referred to as the percentage volume loss.

For a TBM tunnel the volume loss is the sum of the following four components, the magnitude of which will depend upon the methodology and control of the construction, and the soil type:

Face Loss: This component is due to loss of ground into the face of the excavation. The magnitude of face loss is influenced by the ground type and the support pressure applied to the face.

3 See CIRIA Project Report 30, “Prediction and effects of ground movements caused by tunnelling in soft ground beneath urban areas”.


Appendices A16.4- 4

Figure A16.2(a): Face Loss

Loss Around the Shield: The excavated diameter of the tunnel is typically greater than the diameter of the tail skin of the shield. This leaves an annulus around the shield into which the surrounding ground can move. The magnitude of the resulting volume loss is dependent on the stability of the ground around the tunnel.

Figure A16.2(b): Loss Around the Shield

Loss behind the shield: As the TBM moves forwards an annular space is left around the tunnel lining. This will be injected with grout to reduce the associated volume loss. The magnitude of volume loss depends on the effectiveness of the grout injection, the grouting pressure and the time for the grout to gain sufficient strength to prevent ground movement.

Figure A16.2(c): Loss Behind the Shield


Appendices A16.4- 5

Consolidation Effects: The volume loss components discussed above are all short-term construction effects that are usually considered to be immediate. Consolidation settlements occur due to changes in pore water pressures around the tunnel and generally occur over a much longer period. The magnitude of this consolidation settlement is difficult to predict, but case histories indicate that they can be greater than the short-term settlements in certain ground conditions. However, where such settlements occur they affect a wide area and are therefore less likely to cause damage to buildings. In the Phase 1 settlement assessment consolidation effects have been ignored on the basis of the following assumptions:

• The tunnels will be constructed by tunnel boring machines operating in closed mode limiting the amount of water inflow at the excavation face.

• The segmental linings will be provided with ethylene-propylene-diene-monomer rubber (EPDM) gaskets, preventing leakage into the tunnel and long term reduction of the adjacent water pressures that cause consolidation settlement.

• Groundwater is probably in continuity with the River Liffey and therefore lowering of the groundwater table and consequential surface settlement is very unlikely to occur.

• The limestone through which the tunnels will generally be driven is effectively incompressible compared with the overconsolidated clays.

A16.4.5 Station and Shaft Settlement Assessment

A16.4.5.1 General

Ground movements produced by constructing stations and shafts have been estimated according to the recommendations given by CIRIA C580 ‘Embedded Retaining Walls – Guidance for Economic Design‘ for surface movements in correspondence of embedded retaining walls. The report provides guidance on how to predict movements both due to the installation of an embedded wall and due to the excavation in front of the wall.

For the Phase 1 settlement assessment the surface settlements were determined using case-history-based empirical curves rather than numerical analysis; this was considered conservative.

It is proposed that excavation within the station boxes and shafts will be carried out using stiff or preloaded temporary supports at high level and at various intermediate levels, or following a top-down construction sequence (where this is feasible). These represent the only viable options for realising major excavations in a highly congested urban environment such as Dublin and justify the assumptions used in the assessment.

For the station and shaft retaining walls keying into the rock, ground movements are assumed to take place only in the overlying soft ground layers, i.e. for calculation purposes the formation level was taken equal to the rock head level.

CIRIA C580 Figure curve 2.11(b), reproduced as Figure A2(a), for ground surface settlement due to excavation in front of walls in stiff clay has been used to estimate movement in the boulder clay. CIRIA Figure 2.12, reproduced as Figure A2(b), for ground surface settlement due to excavation in front of walls in sand has been used for the alluvial layers.


Appendices A16.4- 6

Figure A16.2(a): CIRIA C580 fig. 2.11(b) - Ground Surface Settlement due to Excavation in Front of Wall in Stiff Clay

Figure A16.2(b): CIRIA C580 fig. 2.12 - Ground Surface Settlement due to Excavation in Front of Wall in Sand

A16.4.6 Potential Settlement due to Ground Consolidation

A16.4.6.1 General

Settlement controlled by tunnel volume loss and ground movements associated with station and shaft construction, as discussed above, is short-term and is usually considered to be immediate. Consolidation effects due to either short or long term changes to water levels in the ground have been ignored in the Phase 1 settlement assessment due to the reasons set out below.

Changes to water levels in the ground over time can cause consolidation settlement, though the effects are generally only significant where the affected ground is soft and compressible. As described in Chapter 13, the ground along the route of the scheme generally comprises stiff or dense boulder clay or gravels overlying limestone rock, though alluvial clay does occur close to the River Liffey between Heuston and Christchuch and in the Docklands area. Settlement caused by dewatering is proportional to changes in water levels, and as dewatering in permeable ground generally extends over wide areas, such as when water is abstracted for water supply, settlement effects also extend over a wide area. In these circumstances buildings and infrastructure generally settle with the ground without any ill effects because they are not subjected to potentially damaging differential settlements.


Appendices A16.4- 7

A16.4.6.2 Stations, shafts and tunnel platform enlargements

During construction of stations, shafts and tunnel enlargements, water within excavations will be pumped out so that work can be carried out in the dry. It is not envisaged that this temporary pumping will significantly affect groundwater levels adjacent to excavations because:

• The excavations will be supported and enclosed with effectively impermeable concrete retaining walls.

• The base of excavations will generally comprise either relatively impermeable limestone or boulder clay. Where the base of excavations is not limestone or boulder clay, such as at the Docklands Station, either the retaining walls will extend to the limestone or permeation grouting below the excavation will be used to act as a cut off to water flows from the adjacent ground.

• If the limestone is found to be locally permeable due to fractures, these will be grouted to form a less permeably block, preventing significant water flows.

• If temporary dewatering of excavations in the rock does affect the overlying more permeable gravels, recharge from the River Liffey will ensure no significant lowering of the water table, and in any event any lowering will occur over a wide area.

Once construction is complete the stations, shafts, tunnel enlargements will be effectively impermeable and will not change the current groundwater conditions.

A16.4.6.3 Tunnels

Settlement controlled by tunnel volume loss, as discussed above, is short-term and is usually considered to be ‘immediate’. Potential consolidation settlements due to changes to water pressures in the ground around the tunnels due to them effectively acting as a drain would generally occur over a much longer period. The magnitude of this consolidation settlement is difficult to predict, but case histories in London for tunnels driven in over-consolidated clays indicate that they can be greater than the short-term settlements. However, where such settlements occur they also affect a wide area and are therefore less likely to cause damage to buildings. Consolidation effects associated with the tunnels have therefore been ignored on the basis of the following assumptions:

• The tunnels will be constructed by tunnel boring machines operating in closed mode limiting the amount of water inflow at the excavation face;

• The segmental linings will be provided with ethylene-propylene-diene-monomer rubber (EPDM) gaskets, preventing leakage into the tunnel and long term reduction of the adjacent water pressures that cause consolidation settlement.

• Groundwater is probably in continuity with the River Liffey and therefore lowering of the groundwater table and consequential surface settlement is very unlikely to occur.

• The limestone through which the tunnels will generally be driven is effectively incompressible.

A16.4.7 Calculation of Building Strain

A16.4.7.1 General

When a structure deforms due to the formation of a settlement trough, it can experience strains arising from 3 possible sources. These are as follows:

• Horizontal strain, ∈h – due to horizontal ground movements being transferred to the structure.


Appendices A16.4- 8

• Bending strain, ∈b – caused by flexure of the structure as it attempts to follow the profile of the settlement trough. Vertical cracking is most likely to be caused in the tensile face of the structure.

• Shear strain, ∈s – caused by shear stresses within the structure. It is most likely to cause diagonal cracking within the structure.

These are shown diagrammatically in Figure A16.3 below:

Figure A16.3: Cracking of a simple beam in bending and in shear The horizontal ground strain can be calculated using the following equation:

∈h = d/dy {(y/z)w}

Bending and shear strains can be calculated using the following equations:

∂/L = {L/12t + (3I/2tLH) x (E/G)} x ∈b

∂/L = {1 + (HL2/18I) x (G/E)} x ∈s

Where:

∂/L = deflection ratio over the length of the structure;

H = height of the structure;

I = second moment of area of an equivalent beam (H3/12);

t = furthest distance from neutral axis to the edge of the beam;

E/G = Young’s modulus/shear modulus.

For most cases, E/G is taken as being 2.6. This is applicable for isotropic4 beams. However, if specific information is known about the structure then the value of E/G can be varied. The horizontal, bending and shear strains should be analysed for each hogging5 and sagging zone of the settlement trough6.

4 Structures that present the same behaviour regardless the direction in the space that is considered. 5 For definition of hogging and sagging regions refer to figures A1(a) and A1(b) 6 Further information on the calculation of bending and shear strains can be found in Burland, J.B, Broms, B B and De Mello, V F B (1977). “Behaviour of foundations and structures, State of the art report”, Session 2, Proc 9th Int Cons SWE, Tokyo, 2, pp 495-546


Appendices A16.4- 9

A16.4.7.2 Combining Strains

The total strain experienced by a structure due to the formation of a settlement trough can be calculated by combining the horizontal strain with each of the bending and shear strains, in turn. Horizontal and bending strain can be added directly as follows:

∈bt = ∈h + ∈b

where ∈bt is the total bending strain. The critical value of ∈bt will occur in the hogging zone where both ∈h and ∈b are tensile.

Shear strains and horizontal strains can be summed using the following equation:

∈st = 0.35∈h + {(0.65∈h)2 + ∈s2}0.5 (8)

where ∈st is the maximum tensile strain due to shear distortion.

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