Instrumentation and monitoring as a tool in tunnel and underground design and analysis

Abstract

Instrumentation and monitoring can be a tool to validate the design assumptions during desk studies by assessing the ground behavior through proper instrumentation during execution. All design process starts from scratch with some basic design assumptions and input parameters as available from previous studies and field investigation. The outputs of the design studies predict certain values i.e., forces, movements, mechanical behavior in both short term and long term. Instrumentation is the tool, which can validate those forces and movements to ensure that design assumptions fit for the structure. Tunneling in urban areas is a considerable challenge in terms of significant volume loss (%) of 0.5-1.0, which can impact ground and structures. Instruments predict behavior of ground along with movement of tunnel boring machine. These instrumentation readings provide both designers and contractors, a confidence on stability of both ground and superstructures. Similarly deep excavation in urban areas is critical in term of stability of retaining structures which have an impact to the structures behind the retaining wall. Temporary structures used to support the excavation were critical to the surrounding structures throughout the execution period. Overall movement and forces, which arising due to deep excavation in retaining structures, temporary steel props, bracings and columns were monitored regularly to ensure stability of the whole system. This paper presents the monitoring records of the instruments installed to monitor deep excavation and tunnelling works in one of the metro projects in India. Further, a case history is also presented to show that validation of the design and necessary changes in the construction methodology can be made based on the instrumentation data recorded for the excavation works in the similar ground conditions.

1. Introduction

Instrumentation monitoring is a vital part of urban underground works. It is being used to identify any unexpected behaviour in the vicinities of tunneling and deep excavation including the ground during or immediately after construction activities. Based on the output, decision can be made regarding any additional support requirement or alteration/modification in the construction approach. The key points of geotechnical instrumentation monitoring were:

• Instrumentation monitoring
• Evaluation and interpretation of the instrumentation and monitoring report

The instrumentation is used for two key purposes: 

• monitoring of surrounding structure to assess the impact of the construction works ?
• Internal monitoring (within the excavation) to validate the design and to ensure the safety of the works.

2. Instrumentation areas

2.1 Bored Tunnel and Cross Passages work

Following instruments were proposed in bored tunneling and cross passage construction for Ahmedabad metro:

• 3D Bi-Reflex targets to monitor movement of finished tunnel section. 
• Standpipe Piezometer – To record ground water table drawdown due to execution activities. 
• Vibrating wire Piezometer - To measure piezometric pressure of ground water levels. 
• Extensometer - Multiple Point Borehole Extensometer/Magnetic Extensometer were installed within the ground proximate to the tunnels to monitor vertical movements in different layers of the ground as tunneling works progress. 
• Inclinometer - Vertical Inclinometers were installed in the ground to monitor the lateral response of the ground to tunneling works. 
• Surface settlement marker – Surface Settlement monitoring points come in a variety of types and were installed in open ground, on pavements etc. for monitoring ground surface movements and structure movements induced by the tunneling works.
• Tiltmeters/Tilt Plates - Tiltmeters were installed on structures to record rotation of the structure elements as adjacent works progress.
• Crack Meter - Crack Meter were used to monitor movement across an existing crack or joint. 
• Building Settlement Markers recorded the building subsidence or heave due to close proximity excavation work 

2.2 Deep excavation works

Following instruments were proposed in any type of deep excavation activities i.e., underground stations, Ramp, Cut and Cover tunnel, shafts construction:

• 3D Bi-Reflex targets to monitor movement of temporary supports. 
• Strain Gauge/Load Cells – Monitor load developed on temporary supports. 
• Standpipe Piezometer – To record ground water table drawdown due to execution activities. 
• Vibrating wire Piezometer - To measure piezometric pressure of ground water levels. 
• Inclinometer - Vertical Inclinometers were installed in the ground to monitor the lateral response of the ground to excavation works. 
• Surface settlement marker – Surface Settlement monitoring points come in a variety of types and were installed in open ground, on pavements etc. for monitoring ground surface movements and structure movements induced by the excavation works. 
• Tiltmeters/Tilt Plates - Tiltmeters were installed on structures to record rotation of the structure elements as adjacent works progress. 
• Crack Meter - Crack Meter were used to monitor movement across an existing crack or joint. 
• Building Settlement Markers record the building subsidence or heave due to close proximity excavation work.

3. Principles of Instrumentation and monitoring

Following principles were followed during planning, design and execution of monitoring activities in site:

• Monitoring section distribution were specified according to actual geological conditions and requirement before any installation 
• All instruments installation were completed prior to the respective construction activities 
• Initial reading was taken, as early as possible, after installation and base reading were fixed at least after recording of three consecutive readings 
• Frequency of reading for every was instrument specified and followed strictly 
• Regular data processing of instrumentation records was made to understand both behavior of ground and structure along with progress of excavation activities.

3.1 Frequency of monitoring

During initial phase of construction for any zone monitoring frequency will be more and will decrease according to the stable behavior of monitoring readings. Table 1 presents monitoring frequency for an underground metro project in India.

3.2 Monitoring Control values

Alert, Action, Alarm values were developed for adjacent buildings and structures within the zone of influence of excavation and monitoring the respective structure constructed. Control levels were specified in terms of ALERT (0.5 times the serviceability limit), ACTION (0.8 times the serviceability limit), ALARM (full serviceability limit) for respective instruments installed. Violation of the limits provided different guidelines in terms of adoption of different mitigation methods to safeguard all structures within influence zone. Table 2 specifies examples of all critical limits for a particular instrument,

3.3 Evaluation, Interpretation and Presentation

Upon completion of daily monitoring activities and pre-processing of monitoring raw data a preliminary evaluation of the monitoring results by a plausibility check were performed. Plausibility checking were performed by the responsible surveyor together with the geotechnical expert. When the database is updated with the actual monitoring results, the geotechnical expert proceeds with the evaluation and interpretation of the monitoring results. To guarantee a quick decision with regard to support requirements and working procedures the database is regularly updated with the daily measurements latest.

4. Instrumentation and monitoring as a tool – case studies on validation of design and execution planning

Instrumentation and monitoring data were used as a tool to validate design assumptions for tunnel design and deep excavation analysis. During design studies there were lots of inputs which were not readily available and considered based on previous experience or similar project experience. monitoring data helps to validate the inputs and verify the design. Following case studies represent several examples supporting the theory.

4.1 Validation of Volume Loss (%) during TBM tunneling

Volume loss (%) is an important parameter considered for settlement trough estimation due to TBM mining impact and impact to superstructures lying on tunnel alignment. The parameter is related to machine performance and varies with stiffness of ground and ground cover lying above TBM. No direct co-relations exist to establish volume loss (%) considering all the parameters. In Ahmedabad Metro TBM drives, based on ground monitoring data as obtained from ground settlement markers established perpendicular to tunnel alignment at certain distance along line which were defined as arrays, and certain arrays were established at every 20m intervals along the tunnel alignment. Max ground movement as recorded at every array provided volume loss at that particular section based on O’ Reilly and New (1982) formula. This provided a pattern of volume loss data along the completed drive which provided an idea of trend of volume loss that was actually encountered during TBM drive.

Figure 1 depicts the max surface movement at that particular section. Similar arrays were considered along the tunnel alignment at defined intervals. The respective maximum movement at a particular array and as per the following formula in figure 2, an approximate idea about TBM tail end volume loss (%) actually encountered by ground during TBM mining were established. The same were repeated for all respective arrays along the tunnel alignment. Table 3 shows the calculation to estimate volume loss (%) from surface monitoring data and provides a comparison between designer’s assumed values and actual values encountered. It further shows that designer’s assumptions were more on the conservative side except at few locations due to insufficient face pressure not maintained at TBM cutter-head during mining.

This is the main advantage of instrumentation and monitoring where parameters like volume loss (%) can be indirectly co-related with measured surface settlement values. The total bored tunnel to be mined was 3.3km for each tube in the respective project and the total tunnel drive were divided in 3 zones. The above exercise caried out in one zone was the basis for the balance 2 zones and planning for future instrumentation were carried out too. Designer was also able to modify his assumptions accordingly as per the results obtained which provided a scope of modification in instrumentation proposals where volume loss (%) has effect on TBM induced movement at ground. Also, the TBM crew got the ideas of the volume loss (%) that may arrive in further mining and further planning of the mining as per situations encountered.

4.2 Validation of monitoring control values

Every monitoring instrument has its own control value in terms of green, amber, red levels to provide alert in terms of consequences that may arise due to execution work and necessary action to be taken to prevent the consequences. The control values were specified by designer in term of analysis output that were verified by monitoring records periodically. The alarm limits as below were specified in terms of its consequence and respective contingency measures to be adopted.

Following two figures depicts the importance of respective limits on forces and movements of structural elements that may arrive during whole execution period in the project. Real values were monitored by respective instruments and compared with respective limits set.

• Fig 2 depicts that loads in steel struts as temporary struts were within allowable limits so stability of the opening due to excavation is safe
• Fig 3 depicts that retaining wall movements were within allowable limits thus movement of ground behind the wall would be within tolerable limits

It is concluded further, in-situ monitoring data confirms validity of design and excavation sequence as planned. This is very important for all underground works including tunneling and deep excavation where stability of the ground surface and superstructures lying on it were at critical path. Movements in ground and forces arriving on structural supports requires to be monitored regularly to ensure validation of the stepwise excavation sequence as planned. Any prediction which shows values out of allowable limits, necessitates a change in excavation sequence along with linked design studies. Thus, all design studies to be calibrated by monitoring values so that in-situ conditions can be simulated in study which will predict a realistic model for all future works.

4.3 NATM tunnel design and construction based on nearby underground construction monitoring data

Due to unavailability of TBM machines in site, NATM tunnel methodology was planned between Kalupur underground station and End of UG-01 contract for a stretch of 185m in Ahmedabad Metro construction. During preparation of geotechnical baseline report, very less boreholes were conducted in the NATM stretch which depicted that the ground through where tunnel would be driven consists mostly of stiff clayey silt with ground water table much below the ground. The tunnel was planned to be driven beneath buildings of G+2 to G+3 storied lying in busy urban areas.

Though designer proposed to conduct additional investigations along the tunnel alignment specially at the building locations to confirm the type of soil lying below building foundations. But client was reluctant to undergo additional investigation at the mid of execution considering it would push the project deadline considering the time required for field works, desk studies, its approval and further necessary designs to be conducted. Designer proposed to carry out design studies of NATM tunnel based on monitoring records of nearby deep excavation for an underground station already being carried in parallel. monitoring data of the movement of retaining wall and the ground surface depicted the range of stiffness for the ground being considered which was a basis for tunnel design and its excavation sequence planning.

Instrumentation on retaining structures as installed during excavation for underground station, revealed movement 1-2mm of wall even after open excavation of 20m depth. The values of wall movement along depth with time is depicted in fig 5.

During NATM tunnel execution, very stiff ground was observed from face log sheet as recorded by execution team which confirmed ground is quite stable and susceptible to minor movements. monitoring records as planned during NATM tunnel execution were quite close to adjacent deep excavation monitoring predictions. It was a classic example of NATM tunnel design and further planning of execution method based on adjacent site monitoring records. A dedicated instrumentation and monitoring arrangement along the tunnel alignment predicted very less movement to the ground lying above tunnel during execution. Due to stiff ground, very less movements were imposed to the foundations of all buildings lying within excavation influenced zone. Overall, this process saved around 6 months’ time of additional geotechnical baseline report and design studies for the tunnel including its approval from Engineer.

5. Conclusions

Based on the studies above Instrumentation and monitoring is not only a tool to monitor the impact on ground and superstructures due to construction but also a basis for validation of design. The records of monitoring can be used as a tool to validate input ground parameters and calibrate them accordingly. From a safety point, it is a tool to predict any unusual behavior of the ground inside and ensures the safety of the team working in a deep excavation or tunnel/cavern. monitoring also ensures structural behavior is within limits during its life cycle period.

It is further concluded that proper monitoring data can ensure the following.

• A safe design, which can be constructed safely that is safe for personnel involved.
• Validation of planned excavation sequence along with predictions of forces and movements. Further cross checking of all design values and providing confidence that all values lie within tolerable limits.
• Validation of certain design assumptions which can be critical during construction.
• Planning of certain activities based on nearby ground monitoring data where ground parameters were limited.
• Proper monitoring records of previous activities can be a basis of design for all future activities in similar ground.

Acknowledgements

I am grateful to Dr. K K Moza, freelance independent review consultant (India) and Mr. Muhammad Humza, Parsons corporation (Qatar) to review my manuscript and providing valuable guidance on closing reviewers comments. I further like to thank Dr. Bineshian Hoss, Technical Director, Amberg Engg AG for his encouragement and guidance on writing the manuscript.

References

• Loganathan, N. (2011), An Innovative Method for Assessing Tunnelling-Induced Risks to Adjacent Structures: PB 2009 William Barclay Parsons Fellowship Monograph 25
• Jardine, F.M. 2003, Response of buildings to excavation induced ground movements, Proceedings of the international conference held at Imperial College, London, UK, on 17–18 July 2001, CIRIA
• Poulos, H.G., Pile settlement zones above and around tunnelling operations, Australian Geomechanics Vol 41 No 1 March 2006, 81-90
• Boscardin, C., Building Response to excavation induced settlement, Journal of Geotechnical Engineering, Vol 115, No 1, January 1989
• Sinha, R S, Underground Structures Design and Instrumentation, Elsevier Science Publishers

About Author

Debasis Barman

CEng, MICE, New Delhi, India

Geotechnical & Tunneling Professional, Chartered Engineer who has built on a strong understanding of fundamental engineering principles to lead multi-disciplinary teams effectively in order to deliver on project commitments.