329.1. General 1. The Contractor shall submit to the Engineer for agreement a detailed method statement for instrumentation and monitoring, including instrumentation layout, trigger, design and allowable values and the procedures for evaluating the monitored data.
2. The Contractor shall appoint within his site team an experi-enced Monitoring Engineer who shall lead the Contractor’s monitoring team. The Monitoring Engineer shall present the results of the previous day’s monitoring in the daily monitoring meeting with the Engineer where they shall be presented to the Engineer by the Monitoring Engineer (see Section 329.3).
3. The frequency of such review meeting may be increased if requested by the Engineer.
4. The Contractor’s Site Manager shall attend monitoring review meetings if requested by the Engineer.
5. The accuracy and precision of the measurement required will depend on the purpose of the monitoring.
6. Assessments shall be carried out to establish the zone of influence due to tunnelling works and to determine the likely damage that will occur to existing above-ground and subsur-face infrastructure.
7. The outcome of the assessments shall determine the type and amount of monitoring that will be required.
8. Early preconstruction instrumentation requirements shall be determined so that baseline measurements can be taken, for an appropriate period, to establish the stability of the monitor-ing system and any possible effects of any underlymonitor-ing environ-mental trends that could be attributed to the Works.
329.2. Ground
movement monitoring
1. Unless otherwise provided for in the contract, the Contractor shall monitor the effects of tunnel construction at the surface, including all ground movements and the effects on all struc-tures, including the Works. Where specifically requested, the subsurface effects, including movements of the water table, shall also be monitored.
2. Unless otherwise provided for in the contract, monitoring equipment and instruments shall be provided by the Contractor to enable the response of structures to be determined. Equip-ment and instruEquip-ments shall be installed to the manufacturer’s instructions and shall be calibrated and tested as appropriate.
Monitoring pins and devices shall be securely fixed in position.
Due regard shall be given to the construction of the structure to be monitored and the layout of its primary support.
3. Monitoring shall be referenced to stable survey stations located outside the zone of influence of the Works and not subject to ground movement. Such benchmarks and coordinated stations shall be established and agreed with the Engineer before any ground is excavated and before any ground treatment or dewatering takes place. They shall be checked at intervals during the duration of the Works.
4. The Contractor shall observe, record and analyse the readings to establish trends in movement and reconcile movements measured with those predicted. He shall provide a copy of all recorded results to the Engineer. He shall make available results to the Engineer in accordance with an agreed pro-gramme; however, movement greater than predicted shall be reported to the engineer immediately.
5. Prior to construction Works commencing, a defect survey shall be carried out of all structures within the zone of influence and a schedule of defects shall be prepared. This schedule shall be agreed by the Contractor and the owner of the structure, or his representative, prior to the start of construction. Existing pipelines, tunnels and services shall be regarded as structures.
6. During the execution of the Works, defects which have been scheduled shall be inspected and monitored as necessary.
Defects which arise during the course of the Works shall be recorded. The Contractor shall keep records of such inspections and a copy shall be available to the Engineer.
7. Monitoring of settlement, scheduled defects and defects arising during the course of the works shall continue at agreed intervals for a period of at least 6 months after completion.
329.3. Monitoring of tunnel excavation
1. The Contractor shall survey, monitor and record tunnel and shaft construction as it proceeds, to form a record of the work. Monitoring shall generally be per unit of advance and include line, level, cross-sectional accuracy, shift advance, total advance.
2. Where shields and machines are employed the Contractor shall monitor per unit advance, the attitude of the shield or machine.
Information to be recorded includes rate of advance, line, level, square, plumb, roll. Where applicable, face pressure, slurry den-sity, slurry viscoden-sity, slurry level, slurry flow, cutter speed, rota-tion direcrota-tion, torque screw conveyor speed, air pressure, and to the degree of accuracy applicable, volume of material excavated.
3. When pipe jacking, the Contractor shall also monitor and record the jacking forces and quantity, type, consumption and pressure of injected lubricants.
4. Where grouting is carried out, the type, volume and pressure of grout shall be recorded.
5. All information recorded by the Contractor shall be provided to the Engineer on a daily basis unless another interval has been agreed.
6. Where the Contractor considers that any corrective action he may take will exceed the tolerances in the Contract, he shall so inform the Engineer and obtain his agreement.
7. The strata exposed in the tunnel face shall be mapped and recorded where possible, and the nature of the excavated material shall be noted in all cases.
8. All significant groundwater ingress shall be recorded and monitored.
9. All atmospheric testing shall be recorded and monitoring for all gases carried out in accordance with BS 6164.
10. The Contractor shall keep copies of all recent face records at the workface for the information of supervisory personnel.
329.4. Tunnel and shaft linings
1. When constructing segmental lining, the Contractor shall undertake survey checks pertinent to the accurate erection and position of segments during each ring build. The relative attitudes of the lining and the tunnelling machine/shield shall be recorded.
329.5. Daily review meeting (DRM)
1. The monitoring instrumentation shall be read on a regular basis – as per Drawings and monitoring plan – and the results made available for a daily review meeting (DRM) attended by the senior members of the Contractor’s and the Engineer’s staff.
Input into the meeting shall also include current geotechnical investigations, face logs, tunnel boring machine (TBM) reports and any recent non-conformance reports relating to the tunnel construction.
2. This DRM shall be held daily during the excavation of the tunnels unless otherwise agreed by the Contractor and the Engineer.
3. At the meeting the Contractor shall present the current results of monitoring of the tunnels, together with trends in these results and comparison with the deformations predicted by the calculations.
4. The outcome of the meeting shall be a report, the Required Excavation and Support Sheet (RESS), agreed by the Contrac-tor and the Engineer, which states that tunnelling may continue as proposed, or gives the requirements for modifications to the tunnelling (e.g. shorter advances, smaller headings, higher face pressure and annulus grouting around the TBM).
5. If no agreed report is available by a specified time each day then the tunnel shall be made safe and tunnelling be stopped.
6. All records from these meetings including face logging and monitoring results shall be kept and be available for inspection until the termination of the contract.
329.6. Key
performance indicators
1. A key performance indicator (KPI) system shall be developed for monitoring movements so that actions can be taken in a
timely manner, thereby ensuring that damage to existing buildings and subsurface infrastructure is within calculated pre-dictions.
2. The KPIs to be used to guide construction shall relate to specific monitoring activities:
(a) in-tunnel convergence monitoring (SCL) (b) spoil reconciliation (TBM)
(c) ground movement monitoring
(d ) monitoring of adjacent and overlying structures.
3. The KPI values specified in the design documentation shall be used to indicate whether or not there is cause for concern during tunnel construction. To ensure that the response is appropriate for any specific concern, certain procedures shall be implemented when a KPI is exceeded. These are summarised below.
(a) A full review of the lining performance shall be conducted for the relevant tunnel section and checked against the KPI values. This includes checks on the ground/soil conditions, the quality of construction and the monitoring results pro-vided by the Contractor.
(b) A comprehensive review of the trends for monitoring data specific to the area of concern shall be carried out by the Contractor and the Engineer.
(c) The Contractor shall assess the extent to which the defor-mations comply with the SCL serviceability and extreme limit conditions.
(d ) Together with the Engineer, the Contractor shall decide whether changes in the SCL excavation sequences or TBM face pressure are required. This is an interactive process that will determine whether it is safe to proceed with construction or, if there is reasonable cause for concern, the extent to which it is necessary to implement additional measures or emergency procedures. These measures will be included in a new RESS.
(e) The Contractor and Engineer shall implement the Action Plan, the emergency response to implement contingency measures. If there is reasonable cause for concern, it is emphasised that the response must be rapid.
( f ) The performance of the tunnel is kept under continuous review until the monitoring data indicate that KPI trends show a stable condition.
4. At least three trigger values shall be established: a green, amber and red limit. The green limit marks the boundary of normal behaviour. The amber marks the boundary of serviceability while the red trigger should be set below the ultimate capacity of the lining. The Contractor’s Action Plan should include pre-planned contingency measures that can be taken if a trigger value is exceeded.
5. If a trigger value is reached, first the site team should check that the reading is correct and consistent with the readings from other instruments. If the trigger has really been breached,
then contingency measures will be instigated, in accordance with a predefined Action Plan and as directed in the DRM.
The contingency measures are designed to correct any anoma-lous behaviour.
329.7. RESS –
Required Excavation and Support Sheet
1. Based on the design and the evaluation of the results of monitoring, a RESS will be issued as the outcome of the Daily Review Meeting (DRM) (see Section 329.5). In the absence of any approved changes, the RESS will reflect exactly what is shown on the relevant design drawings.
2. The RESS shall be prepared and endorsed by the Contractor’s Site Manager responsible for the tunnelling works, the designer and the Engineer on site. Unless all the three signatures are obtained, the proposals indicated on the RESS shall not be implemented.
3. The RESS shall address, but not necessary be limited to, the following matters:
(a) the tunnel section (chainages) to which the RESS is applicable
(b) the support to be installed (c) the excavation sequence
(d ) the method of working related to ground support including staging of application of sprayed-concrete layers and lapping of reinforcement
(e) monitoring to be installed in the tunnel section in question ( f ) measures to be taken during stoppage of works
(g) other instructions relevant to the tunnel section in question (h) reference to relevant Design Drawings
(i) face pressure ( j) soil conditioning
(k) annulus grouting (around TBM).
4. A copy of the RESS will be given to the foreman in charge of the work in the tunnel and shall be kept at the working face.
5. A RESS is required for every metre of the length of the tunnels.
6. If for any reason the approved design method of working is changed, then this will be reviewed prior to the DRM and, sub-ject to acceptance by the Engineer, a new RESS will be issued.
329.8. Contingency measures and
emergency procedures
1. The Contractor shall determine contingency measures to deal with potential hazards that may affect the Works. The Contrac-tor shall submit for approval to the Engineer an Action Plan which shall detail the actions, procedures and contingency measures to be followed in the event that the monitoring system shows unacceptable levels of deformation/movement if potential hazards occur.
2. Hazards to be addressed include:
(a) changing ground conditions (b) excessive movement of the linings (c) excessive ground movement
(d ) excessive settlement of the existing structures (e) unplanned stoppages
( f ) mechanical excavation plant failure (g) insufficient labour resources
(h) failure of services to underground works (air, light, power, etc.)
(i) incidents within underground works ( j) delay in supply of sprayed concrete (SCL) (k) delay in supply of segments (TBM).
3. In underground construction works, changes tend to be progressive with evidence of structure or ground behaviour becoming apparent before failure occurs. For this situation a system of hierarchical trigger levels will be appropriate. This allows proportionate response to adverse indications from monitoring.
4. Trigger levels will be based on the results of assessments of at-risk infrastructure. If the assessment indicates that the at-risk infrastructure is unlikely to be able to tolerate the change due to the Works, then triggers will be set based on the levels of change that will be tolerable.
5. There may be some situations where change is less progressive and monitoring may simply be required to give a yes/no response. In these cases reporting is simple and systems of triggers are not appropriate.