Tunnelling in the Himalayas: risk assessment and

Tunnelling in the Himalayas:

risk assessment and management for tunnelling in extreme

geological conditions

Dr Ian McFeat-Smith, Director

IMS Tunnel Consultancy Ltd

RISK MANAGEMENT FOR TUNNELLING IN

EXTREME GEOLOGICAL CONDITIONS

• Key risks and challenges for tunnelling in the Himalayas

• Geological classification & risks arising from mis-classification

• Water inflow risks & mitigation ( TBM & drill and blast )

• Risks & appropriate measures for drill and blast

• Case History of a TBM in extreme ground conditions in the Philippines

• Code of Practice and Risk Management Plan

• Key risk mitigation measures for tunnelling in the Himalayas

• Risk evaluation

for one party is

not necessarily

the same for

another - it

involves a

considerable

amount of

judgment

• To objectively

evaluate risks

requires a

considerable

level of

expertise





- Summary of key risks

• Access and portal instability

• Risks from variable rock mass quality

• Risks from unfavourable orientation of joints / features

• Risks from weathering effects and tunnelling in major fault gouge zones

• Stress induced risks

- rock bursts

- squeezing / convergence

• Risks from sudden inrushes

- water inflows

- silt / debris inflow

‘The problem is mathematics is black and white

but the real world is grey’ – Albert Einstein



Ground Classification Systems

- overly complex classification systems create risks of selection of inappropriate methodology

- in critical situations we rarely can see, or have time to evaluate the geological features affecting the tunnel

- KIS

Longitudinal Section at Chun Wo tunnels showing

110m long collapse zone of both tunnel bores

Water Inflow Risks & Mitigation

Water Inflow Management - Large inflows from probe holes at full hydrostatic head

Water and Silt Inflows Risks at Parbati &

Dulhaste HEP Headrace Tunnels

At Dulhaste in Kashmir

inflows of over 1,000l/sec innundated an open TBM depositing 4,000m3 of sediments. The project was abandoned for 50 years

Above, Left. Water Inflows at Parbati HEP Headrace tunnel deposited over 4,000m3 of sediments burried a TBM resulting in delays of over 15 months todate

Water Inflows for

IMS Class and

Impact of Inflows

on TBM

Utilization

Performance

Conventionally Excavated Tunnels at 12- 20 m span

Water Inflow Risks - easier access for probing and grouting

Water Inflow Risks for Large Span Tunnels in the Himalayas Mitigation – Selection of appropriate modern grouting materials,

patterns, and plant for local ground conditions

IMS Rock

Class

Grouting

Required

Grout

Materials

Kgs/ MFC/ m

grout hole

1 None or spot targeted MFC; OPC if

joints > 0.5mm

10

2 Limited - continuous MFC 15

3 Continuous MFC 26

4 Continuous MFC 50

5A Continuous, closer

spacing in stages

MFC/UFC 21

5B Ditto UFC 100

Conventionally Excavated Tunnels at 12- 20 m Water inflow risks - Extensive drilling and grouting in Poor Rock

Design of Temporary Support Systems

Based on IMS Rock Classes

IMS

Rock

Class

Description Support System for 12-20m span

Tunnels

1 Massive competent

rock mass

Feature dowels

2 Favourable rock mass Feature dowels with shotcrete in

overbreak

3 Moderately fractured /

weathered rock

Feature dowels 2.4m c/c in roof with

50mm shotcrete

4 Highly fractured or

weathered

Feature dowels 2.2m c/c in roof with

70mm shotcrete

5A Fault zones or

completely weathered

Feature dowels 1.6m c/c with 90mm FR

shotcrete

5B Fault gouge / soils Fully grouted pipe pile roof; Steel

arches at 0.7m c/c; 250mm FR shotcrete

in roof, 150mm in walls

Major Soft Ground / Weathered Zone at Portal Risk Mitigation – Full Face Excavation, Steel Ribs, Fibre

Shotcrete and Face Support designed and installed using IMS

Rock Classification System

Major Fault Gouge Zones with High Cover Risk Mitigation – Plaxis modelling, support analysis, close

supervision of top heading using horizontal pipe piling, steel arch

ribs and FR shotcrete support

Tunnelling in Extreme Geological Conditions

for Umiray Angat Scheme in the Philippines Anticipated Physical and Geological Risks -

• Difficult access - extreme logistical concerns

• 4.88m diam, 13.2 km long tunnel from only one portal

• Variable geological conditions - hard basalt agglomerate intercalated with soft sedimentary rocks

• Cover up to 1,200m in an earthquake zone with potential for severe convergence, major fault zones and major water inflows

Tunnelling for Umiray Angat Scheme in Philippines Risk Mitigation: In order to deal with these extreme conditions

the contractor GLF/SELI elected to:

Transport all plant and

service the first 5kms of

TBM drive and its back

up by helicopter.

To drive the long tunnel

by double shielded

TBM from a single

portal

To purpose design a

state of the art double

shielded hard rock TBM

with hexagonal

segmental lining


in Umiray Angat Scheme In Philippines Geological profile, actual tunnelling conditions and TBM progress


for Umiray Angat Scheme in the Philippines

• Individual inflows of over

200l/sec and cumulative water

inflows of about 850 l/sec

• Despite these and other

challenges the Contractor managed

to complete the tunnel excavation

and lining in 24 months; averaging

530m/month

• Jamming of the TBM shield in six long sections of tunnel due to severe convergence

• Cave-ins up to 10m high above the TBM cutterhead at fault gouge zones. This was managed by an arrangement and plant for installing short resin grouted bars through the cutterhead

Innovative Double Shielded TBM Design

for Tunnelling through Extreme Geological Conditions

• TBM shield diameters progressively reduced

to better cope with and converging ground

• Short cutterhead (reducing the incidence of

stoppages by blocks and boulders and

minimizing ground disturbance in poor

ground) with rear loading cutters

• Access for probing and drilling from inside

the shield

• Very high cutterhead power and torque

• Variable frequency drive

• Very high main and auxiliary thrust

• The telescopic shield design introduced

new SELI active articulation system

Water Inflows in TBM Tunnel with 700m

Overburden Cover

Tai Po to

Butterfly Valley

- Cumulative water

inflows experienced

in TBM Tunnels at

700m cover.

- Tail-off of inflows

of 37% in 117 days

Risk Assessment and Management

New Code of Practice for Tunnels

• Client responsible for sufficiency of site investigations

• Geotechnical data forms part of contract

• Geotechnical baseline conditions to be drawn up by Client or Tenderer

• Geotechnical baseline conditions used for assessing unexpected geological conditions

• Risk assessment and management at all stages of development of project

• Continuous tracking and mitigation of risks through risk register

• Insurance cover may be suspended or cancelled in event of a breach of code requirements

RISK MATRIX FOR TUNNELLING IN THE HIMALAYAS

Consequence

Insignificant Minor Moderate Major Catastrophic

Description C1 C2 C3 C4 C5

Rare L1 R1 R1 R1 R2 R3

Unlikely L2 R1 R1 R2 R2 R3

Possible L3 R1 R2 R2 R3 R3

Likely L4 R2 R2 R3 R3 R4 Lik

elih

oo

d

Almost certain L5 R2 R3 R3 R4 R4

Legend: Low risk Moderate risk High risk Extreme risk

R1 R2 R3 R4

Risk Management Systems

Risk management systems ensure that:

• Risks are identified for all aspects of the project

• Identified risks are evaluated as a product of their “Frequency” and “Consequences”

• Risk mitigation plans are established and implemented for each risk

• Resources are focussed on the most significant risks

• Risk status is reviewed on a scheduled basis

• Risk management activities driven by senior management

- Risk assessment and Mitigation Measures

Risk Key Mitigation Measures

Access difficulties Logistical planning; additional resources

Slope instability Carry out additional SI and analysis

Unexpected low

rockhead at portal

Advance and vertical probing; close monitoring of

support and ground movements; good

supervision at tunnel face; face support.

Extremely hard,

abrasive quartzites

Conduct appropriate testing as part of contract SI

Unexpected

geological

conditions

Carry out more SI; use better drilling methods

Be prepared to reject lowest tender for contract

Have design & contracts reviewed by experts

Enforce compliance with codes for safety and risk

Ensure contractor has adequate resources

Conduct partnering, VE workshops

Risk assessment and Mitigation Measures Risk Key Mitigation Measures

Severe ground

squeezing

Experienced temporary support designer on site

Close monitoring; and additional support

Consider multiple face headings

Rock bursts; major

overbreak

due to high stress

Protection and safehaven for workers

High awareness of safety issues and control

Toolbox talks; safety incentive schemes

Close monitoring; observational method

Face, roof and wall support to suit conditions

Major inrush of

ground water; silt

flows

Utilize probing equipment for high pressures

State of the art grouting, grout mixes and plant

Draw up Emergency & Contingency Plan before event

Insufficient time in

Programme /

Breakdown of

stakeholder

relationships

Realistic risk management plan by competent engrs

Early consultation between all stakeholders

Non-contractual partnering to ensure co-operation

Clear lines of communication and responsibility

Conduct workshops for information sharing

Typical Emergency and

Contingency Action Plan

Incident - Major Collapse at TBM Cutterhead

• Stop TBM excavation but maintain support pressure

• Check safety of personnel and plant & Inform appropriate authorities

• Check for surface impacts if appropriate; Determine whether collapse has stabilized & secure tunnel if possible

• Investigate nature of geological feature by probing

• Review collapse and potential impacts

• Mobilise remedial measures and stabilise collapse area

• Review operation and recommence asap

• Mobilise for planned permanent reinstatement

Geological Conditions have a Major Impact on

Underground Construction Costs and Programme

Tunnel for Tolo Effluent Export Scheme –

HK Governments first TBM project - 7.5 km long tunnel

completed within budget and 6 months ahead of programme;

using double shielded TBM

Re-measurement

Risk Sharing Contract for Tolo Tunnel: Payment for geological conditions made on re-measurement of IMS

rock classes

Conclusions

Existing Contractual Arrangements

and negative attitudes towards Risk Sharing carries

a Large Price Tag

Mitigating Risks

for Underground Projects

• Well targeted site investigations as an investment to reduce

uncertainty and risks

• Planning - seek specialist advice particularly on risks, opportunities and programmes

• Encourage technical innovation and alternative designs from contractors

• Use Partnering to focus on openness, co-operation and fair payment

• Ensure that real engineering risk assessment and management is implemented and driven by senior management

• Use risk sharing and re-measurement contracts



• Key risks and challenges for tunnelling in the Himalayas

• Geological classification & risks arising from mis-classification

• Water inflow risks & mitigation ( TBM & drill and blast )

• Risks & appropriate measures for drill and blast

• Case History of a TBM in extreme ground conditions in the Philippines

• Code of Practice and Risk Management Plan

• Key risk mitigation measures for tunnelling in the Himalayas

Documents

Tunnelling in the Himalayas: risk assessment and