TBM face stability control

Course in Tunnelling and Tunnel Boring MachineKurs w zakresie drążenia tuneli oraz maszyny drążącejTUNNEL FACE STABILITY WITH MECHANIZED TUNNELINGGENERAL CONCEPTSProf. Eng. Daniele PEILA0

TBM face stability controlWhen tunneling with full face machines in urban areas there are some specificaspects that influence the tunnelling risks and that must be taken into accountin the design and the construction proceduresUrban environmentShallow overburdenStructures on ground surfaceForeign objects in groundConstraints for alignmentRestriction for: impossibility of road closure; place of attack, material transport,access to TBM, exploration and carring out of auxiliary measures (i.e. groundreinforcements), high visibility of damage1

TBM face stability controlGeological and hydrogeological conditionsProperties of the recent geological formationsPresence of a man made filling (sometimes of unknown depth)Frequently changing conditionsPresence of ground waterNon – perfect (i.e. not as designed) performance of the tunneling method2

TBM face stability controlAn optimal design must be based on risk analysis and risk management whichmust adddress the decision and chioce to be taken and also influence theconstruction management.In fact this approach permits after the identification of all potentential hazards,to assign to each of them a proability of occurrence and to allocate an index ofgravity-severity to the consequences.With this picture in mind is possible to define:• the solutions to be used to reduce the probability of occurrence of negativeevents;• the measures for reducing the gravity of the consequences of a negativeevent.(Kovari, 2000, 2004; Guglielmenti et al. , 2002; Guglielmetti, Mathab, Xu, 2007)3

TBM face stability controlThe most relevant risk scenarios in urban area when tunnelling with full facemachines inside a soft soil are:Limiting condition for the machine(that is to say a condition for which the machine is not working as designed)• Eccessive wear of the tools• Eccessive use of conditioning agents, grease, ect.• etc.Interferences with the local enviroment and accidents• face collapse which can reach the surface• damage of already existing constructions both on surface orunderground (i.e. subsidence)• Underground water pollution• etc.4

TBM face stability controlIn the present lecture we will analyse the problem of face stability which cancause the following type of accidents- face collapse which can reach the surface- damage of already existing constructions both on surface or underground(i.e. subsidence)TunnelTunnel5

TBM face stability controlFirst option for risk reductionThe first set of choices which permit the activation of countermeasures to beadopted to face these risk scenarios are :1) the choice of a correct machine for the local urban environment and geology;2) the correct management of the excavation process (control of face counterpressure,evaluation of the amount of exctracted material to avoidoverexcavations, correct and proper filling of shield body anulus etc.)6

TBM face stability controlFirst option for risk reductionWith specific reference to the correct choice and management of the full face machinesit is necessary to control the face stability:a) to correctly evaluate the face pressure to be applied to guarantee the stability andcorrectly apply with reference of the local environment;b) to properly treat (condition) the ground to permit the face pressure to be applied bythe machine;c) to control the excavation process to avoid over-excavation (monitoring of the groundand of the machine performances)If these options are not sufficients, it is necessary to use a second set of options forrisk reduction.7

TBM face stability controlDesign aspects1) Definition of the stabilizingpressure to guarantee thestability2) Definition of the failuremechanism8

TBM face stability controlDesign aspectsThe definition of the appropriate EPB face pressure need to be considered on aproject-by-project basis taking into proper account the soil properties, thegroundwater content and the TBM design.Groundwater pressure: in some types of rock such as weak or broken rockthe principal reason for using a closed-face machine is to control groundwater.The operating pressure should be a small margin, perhaps 10%, above theexisting hydrostatic pressure.In water bearing granular material it is necessary to apply operating pressuresto resist both hydrostatic and soil pressure.In both cases it is necessary to ensure that no risk of ground loss from thecrown.(BTS, 2005)9

TBM face stability controlDesign aspectsUnstable ground: in non-water bearing granual material where there is nowater pressure to balance and where the soil pressure may be small it isdifficult to determine the operating pressure.It is necessary to operate the machine at a pressure that ensures the bulkchamber remains full at all the times to guard against loss of ground from thetunnel crown.In soft clays and silts that can flow, it may be appropriate to change fromresisting soil or hydrostatic pressure to a pressure that approximates to fulloverburden pressure(BTS, 2005)10

TBM face stability controlEPB Soil ConditioningModification or conditioning of the excavated spoil is a process for both slurryand EPB tunnelling system.For EPB shields conditioning agents, usually foam or foam/polymer solution,are injected under pressure into the spoil as it is excavated, to assist thetunnelling process in two ways: When mixed with the soil, the conditioning fluid reduces the permeability andthe internal friction of the material which flow trough the bulk chamber and thescrew conveyor for discharge into the muck-haulage skips at the atmosphericpressure. The reduction of permeability of the material enables the creation of the plugin the screw conveyor to form and ensure that earth pressure balance supportof the tunnel face is maintained.11

TBM face stability controlStability ControlHDS – version with SLURRY FACESUPORT for soil with no or lowcohesion (gravel, sand, silt) and forground with hard inclusions – fortunneling under groundwater table.EPB – version with FACE SUPPORT BYPRESSURIZED SOIL for cohesiveclayey soil (loam, clay, marl) v- fortunneling below groundwater table.12

TBM face stability controlStability ControlMSD – version for “DRY”OPERATION, (if necessary withMACHANICAL FACE SUPPORT –for tunneling through all soilformations above groundwatertable.13

plug in the screw conveyor effectTBM face stability controlEPB Soil ConditioningEarth pressure distribution diagramExcavation chamber100%Low pressure gradient95%Screw conveyor90%85%80%Injection70%50%0%DischargeEarthPressurePlugareaWithout injectionWith injection100%80%70%50%0%14

TBM face stability controlMonitoring of buildings and of the surfaceTorino metro – courtesy GTT-Torino15

TBM face stability controlMonitoring of machine performanceTo monitor an EPB machine the following parameter are used:- the pressure inside the bulk chamber;- the weigh of extracted material (after the screw drive)Many pressure sensors are applied inside the bulk chamber to measure andkeep under control the pressure of the spoil.This control is a key parameter for the correct management of the machine andfor the control of tunnel face stability.Guglielmetti et al., 200216

TBM face stability controlMonitoring of machine performanceAverage value of earth pressure at theface and of the weigh of the extractedmaterial.Guglielmetti et al., 200217

TBM face stability controlMonitoring of machine performanceAverage value of earth pressure at theface and of the weigh of the extractedmaterial.Guglielmetti et al., 200218

a) Constructional measures– Ground improvement from the surface– Ground improvement from underground works– Prepared grouted blocks for stopping the machine– Preventive structuresTBM face stability controlSecondary option for risk reductionb) Additional measures– evacuation of buildings– closure of roads19

TBM face stability controlGround improvementTorino Metro20

TBM face stability controlGround improvementfaldaTorino Metro21

TBM face stability controlGround improvementTorino Metro22

Zimmerberg TunnelTBM face stability controlGround improvementZurich; Kovari, 200423

Zimmerberg TunnelTBM face stability controlPreventive structuresmicrotunnellingZurich; Kovari, 200424

TBM face stability controlPrepared grouted blocks for stopping of machinesLongitudinal sectionCross sectiongrouted bodyTunnel25