Analysis And Management Of Risks In Tunnel Construction Essay

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The structures required for the operation of the society is referred to as infrastructure. It includes all forms of public assets that facilitate production. It refers to the physical structures like roads, telecommunication, sewers, water supply, etc. As people expect a higher standard of living, the demand on infrastructure and related service will increase.

Infrastructure systems:

The word 'systems' can be used to denote facilities like drainage systems, highway systems, communication systems, etc. Infrastructure systems include fixed assets, buildings, vehicles required to operate, manage and monitor these systems. Infrastructure and related services can be categorized in seven groups based on their primary function and services. They are as follows,

INFRASTRUCTURE SYSTEMS

Transportation system:

Ground transportation (roads, bridges, tunnels, railroads)

Air transportation (airports, heliports, ground facilities, air-traffic control)

Waterways and ports ( shipping channels, main terminals, dry docks, ports)

Intermodal facilities (rail/airport terminals, truck/rail/port terminals)

Mass transit (subways, bus transit, light rail, monorails, platforms/stations)

Water and waste water system:

Water supply (pumping stations, treatment plants, main water lines, wells, mechanical/electric equipment)

Structures (dams, diversion, tunnels, aqueducts)

Agricultural water distribution (canals, rivers, weir, gates, dikes)

Sewer (main sewer lines, septic tanks, treatment plants, storm water drains)

Some of the other infrastructure systems required for the management of solid waste, nuclear waste, hazardous waste are,

Landfills

Treatment plants

Recycling facilities

Energy production and distribution system:

Electric power production (hydro-electric power stations; gas-, oil-, and coal-fuelled power generation)

Electric power distribution (high-voltage power transmission lines, substations, distribution systems, energy-control center, service and maintenance facilities)

Gas pipeline (gas production, pipeline, computer stations and control centres, storage tanks, service and maintenance facilities)

Petroleum/oil distribution (marine and ground tanker terminals, pipelines, pumping stations, maintenance facilities, and storage tanks)

Nuclear power stations (nuclear reactors, power generation stations, nuclear waste disposal facilities, emergency equipment and facilities)

Communication system:

Telecommunication network (telephone-exchange stations, cable distribution, power supplies, switching and data-processing centres, transmission towers, and repeat stations)

Television/cable network (production stations, transmission facilities, cable distribution, power supply, buildings)

Wireless/satellite network (satellites, ground control centres, communication systems, receivers, buildings, services and maintenance facilities)

Information highway network (computer networks, cable distribution, data-processing hardware/software, on-line and off-line systems, information sources, buildings, backup and recovering mediums)

Buildings:

Tall buildings-residential/commercial (structures, utilities, security, ground access, parking)

Public buildings (schools, hospitals, government offices, police stations, postal offices, parking structures)

Multipurpose complexes (coliseums, convention centres, religious congregation centres)

Sports complexes (indoor, stadiums, golf courses)

Movie theatres (indoor, drive-in)

Manufacturing/warehouse facilities

Hotels/commercial properties

Recreational facilities:

Parks and playgrounds (roads, parking areas, recreational facilities, office buildings, rest rooms, ornamental fountains, picnic areas)

Lake and water sports (roads, parking areas)

Theme parks/casinos (access roads, buildings, security facilities)

Earth monitoring and measurement networks:

Global positioning system

Tidal monitoring networks

Seismometer networks

Meteorological monitoring networks

Spatial data infrastructure

Tunnel construction:

Overview on tunnelling:

Tunnels are very much different from other civil engineering structures. In civil engineering structures like buildings, the materials used for construction are testable, while in case of tunnelling it is not possible. The load for which the structural analysis is carried out is basically unknown. Safety is one of the main concerns in the construction of tunnels because of the uncertainties associated with the ground. The major steps involved in tunnelling are as follows,

PROCESS INVOLVED IN THE CONSTRUCTION OF TUNNELS

Site investigation:

Construction of tunnels is mainly depends on the characteristics of ground. Hence site investigation forms a major part of tunnelling. Construction technique will be normally decided on the basis of ground conditions. Ground investigation is a part of site investigation which is carried out to find out the subsurface conditions. A full 3-dimentional model of the site can be made to assess the possible geotechnical risk. Site investigation includes four steps,

Desk study

Site reconnaissance

Ground investigation

Production of site investigation report

1. Desk Study:

Desk study is carried out to create a conceptual model of the tunnel by using all the available data of that area. It includes all site aspects, current usage of the site, underlying cable lines, adjacent structures, and geological aspects of that site. It can save considerable money and time in the later process of site investigation.

2. Site reconnaissance:

It also called as walkover survey along the entire length of the tunnel alignment. The key objective of the site reconnaissance is to fix the location of the shaft. Photographs, sketches and notes are recorded during the survey. It is also used to identify any unexpected hazards, any evidence of cracks, location and confirmation of any underlying cable lines.

3. Ground investigation:

In order to achieve an economical tunnel design, ground investigation has to be carried out to find out ground stability, soil and rock stability. Ground investigation includes both field investigation as well as laboratory experiments. The technique used for ground investigation has to be decided on the basis of cost and goals required.

Field investigation can be carried by using various methods like,

Non-intrusive methods

1. Seismic methods

2. Borehole geophysical logging

3. Cross-hole seismic techniques

Intrusive exploration

Standard penetration

Cone penetration test

In-situ sampling is conducted and testing is carried out in laboratory.

Uniaxial test

Triaxial test

4. Site investigation report:

To aid tunnel designer, all the reports of site investigation will be presented in written format. Site investigation report will be used for adopting tunnelling method, design of the tunnel, construction period required to finish the project, estimate total project cost and is therefore forms a major role in the success of the project.

Analyses for the tunnel:

Primary and secondary stresses are calculated in order to find out the stability of the ground and loading likely to be on the tunnel lining. It will be used to decide whether ground improvement methods are necessary. These parameters are required for the modelling of the tunnel.

Ground stability depends on,

Depth of tunnel

Size and geometry of tunnel

Ground layer strength and its thickness

Geological profile of that location

Permeability of the ground

Water pressures

Ground improvement:

Tunnels are of two types. It may be single larger diameter tunnel or twin-tube tunnel. In twin-tube tunnel, the second tube can function like an emergency exit in case of an accident, such as fire. There are many techniques available for ground improvement which may be either applied from ground surface or applied during the construction of the tunnel itself. Ground improvement will slow down the rate of construction if it is carried out during the construction of the tunnel. Techniques used for ground improvement are,

Ground freezing

Lowering of the water table by wellpoints and deep filter wells

Grouting

1. Chemical grouting

2. Jet grouting

3. Compaction grouting

Ground reinforcement

Forepoling

Face dowels

Roof pipe umbrella

Compensation grouting

Pressurized tunnelling

Construction of shaft:

Shafts are normally constructed by segmental lining. Shafts are of two types, they are ventilation shafts and access shafts. Access shafts are the vertical openings located at regular intervals mainly used for supplying personnel, equipment and support system required for the tunnel construction. Normally the size of the shaft is decided on the basis of tunnel diameter and TBM size.

Shaft sinking cycle:

A shaft sinking includes the following cycle of operations:

SHAFT SINKING CYCLE

Shaft sinking operation starts with drilling holes of length 1.5 to 5m. Water resistant explosives are used since the bottom of the shaft usually has water. Mucking is done by loading the muck in the conveyance for its disposal. Shaft lining is done by number of interlocking structural elements. Segmental lining is applicable for both soft and hard ground conditions.

Segments are connected together by bolts and are made of unreinforced concrete. Durability risk assessment of the segments has to be carried out for both internal and external environments in order to reduce the risk. Auxiliary operations include centering of shaft, ventilation and dewatering.

Tunnel boring:

Tunnel boring can be done by various techniques like drilling and blasting, by TBM. TBM (Tunnel Boring Machine) is a circular machine used to bore tunnels. The diameter of the machines varies from very small micro-TBMs to 16m as on date. TBMs are of two different types.

Hard rock TBMs:

Shielded or open type TBMs are used for hard rock. Decision on selection of TBM machine depends on the ground water ingress and the stability of the ground. Hard rock TBMs consist of four sections. They are the,

Boring section

Thrust and clamping section

Muck removal section

Support section

Boring section has cutter heads which has disc cutters in it. Disc cutters produce compressive stress in rocks and made them into chips. These chips are called as muck and are transferred to the conveyor belt for its removal from tunnel. Thrust and clamping section helps the machine to move forward. Hard rock TBM machines will not install concrete lining segments of the tunnel. Instead, the rock left behind has to be supported by rock bolts, shotcrete, wire mesh and ring beams.

Soft rock TBMs:

Two types of TBMs are used for soft ground. They are Earth Pressure balance Machines (EPB) and Slurry Shield (SS). They are closed type TBM machines with single shield. EPB machine operates with pressure less than 7 bars. A balance is maintained between the machine advance and the amount of soil removal so as to maintain a balance between earth and pressure. Bentonite is injected through pipes to stabilize the ground.

Slurry shield TBMs are used if the amount of ground water is so high. Soil is made into a slurry form by mixing with bentonite and carried out of the tunnel through slurry pipe. Slurry separation plant has to be installed to separate slurry and dirt. Operators of Slurry shield machines have to be provided with completely enclosed environment with sufficient oxygen and communication facilities. Normally, eight to ten labours are required inside the shaft to connect slurry pipe, water pipe, bentonite pipe, communication lines, and ventilation pipe.

Monitoring ground movement:

Ground deformation may cause damage to the near-by structures, buildings, pipelines. Ground movement leads to land slip and also differential settlement of the ground. Hence ground movement has to be monitored continuously during and after tunnelling. Instruments like crakmeters, tiltmeters can be used to measure the movement of the ground.

Ground movement due to tunnel construction near tampines

Risk management:

A chance or possibility of danger, loss, injury or other adverse consequences is called risk. Risk can be managed, minimised, shared, transferred, or accepted but it cannot be ignored completely. Risk has been associated with every phase of the project.

PHASES OF THE PROJECT LIFE CYCLE

Risk management process starts with the identification of risk. After identifying, risk assessment has to be carried out both qualitatively and quantitatively. Possible hazards and impacts of risk have to be understood while assessing the risk. Once the risk assessment has been carried out, the risks need to be accepted, eliminated or transferred by applying suitable mitigation technique.

Risk management cycle

Risk assessment levels:

QUANTITATIVE

QUALITATIVE

LEVELS

1

A

Is acceptable, normal, raises no unusual issues

2

B

Is reason for caution, could be significant but thought to be manageable

3

C

Is a serious concern, that would have significant impact should it arise

4

D

Is a very considerable or intolerable

Qualitative and Quantitative assessment:

Qualitative assessment is a word based process and quantitative assessment is a number based assessment. Assessment of risks has to be carried out once the risk has been identified. If it is done, then it will be very obvious that the risk is understood from both qualitative and quantitative perspective. If it is not understood well, then more research has to be carried out to understand the risk. Then possible mitigation techniques must be looked at and cost required for each mitigation technique is calculated. Sometime a combination of mitigation technique will be the best possible solution.

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