Tunnelling

Question 1

What is the observational method in tunnelling?

Answer 1

As per BS EN 1997-1, when prediction of geotechnical behaviour is difficult, it can be appropriate to apply the approach known as “the observational method”, in which the design is reviewed during construction. The following requirements shall be met before construction is started:
• acceptable limits of behaviour shall be established;
• the range of possible behaviour shall be assessed and it shall be shown that there is an acceptable probability that the actual behaviour will be within the acceptable limits;
• a plan of monitoring shall be devised, which will reveal whether the actual behaviour lies within the acceptable limits. The monitoring shall make this clear at a sufficiently early stage, and with sufficiently short intervals to allow contingency actions to be undertaken successfully;
• the response time of the instruments and the procedures for analysing the results shall be sufficiently rapid in relation to the possible evolution of the system;
• a plan of contingency actions shall be devised, which may be adopted if the monitoring reveals behaviour outside acceptable limits.

During construction, the monitoring shall be carried out as planned. The results of the monitoring shall be assessed at appropriate stages and the planned contingency actions shall be put into operation if the limits of behaviour are exceeded. Monitoring equipment shall either be replaced or extended if it fails to supply reliable data of appropriate type or in sufficient quantity.

Question 2

What is the damage assessment process for ground movement, as per LU Standards?

Answer 2

Stage 1
• Scoping: carry out full ground movement assessment and determine which assets fall within the 1mm contour.
Stage 2
• Initial assessment. Designer carries out analysis using simple methods to conservatively determine risk of damage to assets:
¬ Calculations and assessment based on asset drawings and inspections.
¬ Any asset at risk of damage greater than cat 2 or “slight damage” has to have a stage 3
¬ Heritage assessment done at this stage. This checks ability to tolerate movement
Stage 3
• Full detailed assessment, including migrating design and specific monitoring plans. Assessment should determine:
¬ Impact on building and foundations from predicted movement;
¬ Volume loss that would drop damage back down to “slight”
¬ How to achieve this volume loss;
• Specific control measures or alterations to construction;
• Number of iterations on how to reduce the damage the building may experience;
• Physical mitigations required to ensure the building doesn’t get damaged;
• List of specific areas to monitor etc.
Stage 4
• Construction phase
• Contingency plans, and actually doing the monitoring.
• Verifying predictions.
Stage 5
• Completion and close out.
• Close out defect surveys, decommissioning of monitoring and mitigations.
• Details of any remedial works carried out.

Question 3

What is the Joint Code of Practice (JCoP) for risk management of tunnelling works in the UK?

Answer 3

The JCoP:
• Outlines best practice for minimising & managing risks in design & construct of tunnels, that works in conjunction with guidance & standards.
• Contract insurers require compliance of code on projects worth more than £1m, insurers also permitted to “audit” project to ensure compliance with code.
• The insurer can write to the Client and contractor notifying of a breach of the code, and then our insurance case would become invalid.
• Formalised Risk management procedure must be in place, including Risk Register required to assign ownership.
• Client duties:
¬ Must have technical & contract competence over all for stages: 1) Project Development, 2) Design, 3) contract procurement and 4) Construction.
¬ Take full responsibility for info provided by client.

Question 4

What is volume loss? What causes volume loss? What factors determine volume loss?

Answer 4

Volume loss is the area of bowl created by settlement, divided by the cross sectional area of the tunnel. It is stated as a percentage. The causes are:
• Ground lost at the face due to movements in an axial direction (face intrusion or face ‘take’).
• Radial movements around the shield due to an overcut or the use of a bead on the shield, or due to ‘diving’, ‘pitching’, ‘yawing’ or ovalisation of the shield (radial ‘take’).
• Radial movements due to temporary loss of support at the rear of the shield or tunnelling machine during lining construction (only where no ‘tailskin’ is used).
• Closure of the ungrouted annulus around the newly completed ring (non- expanded type of linings).
• Closure of the grouted annular gap due to ‘bleeding’ and curing of the grout, insufficient grout or loss of grout.
• Time dependent and consolidation effects in the ground, as well as subsequent permanent distortion of the lining.

Influencing factors include:
• Size of the tunnel.
• Distance from the crown of the tunnel to the ground.
• Ground conditions.
• Amount of overdigging at the face.
• Time between opening the ground and ring closure.
• Speed of mucking and build which will depend on the type of tunnelling (hand dug, SCL, TBM).
• Ground treatment (e.g. dewatering).
• Proximity to nearby assets (e.g. shaft under construction).