EPB TBM Design for Boulder Conditions_Craig Bournes

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EPB TUNNEL BORING MACHINE DESIGN FOR BOULDER CONDITIONS

Michael A. DiPonioJay Dee-Michels-Traylor JV

David ChapmanLachel, Felice & Associates

Craig BournesLovat, Inc.

ABSTRACTHistorically, boulders are a frequent source of problems in soft ground tunneling.

During tunnel construction, the need to manually break and remove boulders asobstructions causes delays to the project. Repairs to a tunnel boring machine (TBM)can also be a source of delays.

Managing these problems is difficult since normal soil investigation techniques donot accurately predict the presence or frequency of boulders. This has lead to consid-erable number of claims for extra costs and delays during the construction of softground tunneling projects. These issues are exacerbated in pressurized face tunnelingsystems where there is limited access to the TBM cutterhead for obstruction removaland/or cutterhead maintenance.

The project team that built the Big Walnut Augmentation/Rickenbacker Interceptor(BWARI) Project for the City of Columbus successfully managed the design and con-struction of a 4,267 mm (14 ft) diameter soft ground tunnel with high boulder concen-trations in a complex geologic setting. The management of the boulder issue tookplace during several phases of the project.

BOULDER INVESTIGATIONGeologic research and reconnaissance early in the project indicated that boulders

were a major issue for the project. The project setting is the Till Plains PhysiographicProvince, where thick sheets of glacial till cover deep buried valleys filled with glacialoutwash from earlier glaciations. The till is weathered and oxidized at the ground sur-face, but at tunnel depth it is extremely compact (unit weight of 140145 pcf) with amixture of grain sizes encompassing the entire range of soil texture from clay sizes togravel with cobbles and boulders. The glacial outwash also includes concentrations ofcobbles and boulders, particularly at contact zones atop till sheets. Although notdetected in very many of the standard split spoon borings, boulders excavated fromfields and house cellar holes can be observed guarding driveways at numerous loca-tions in the surrounding countryside.

It was recognized that characterizing the boulder fraction of the ground would bea critical element of the geotechnical investigation and development of the Geotechni-cal Baseline Report and other components of the contract document package, partic-ularly related to risk allocation. In addition, recovery of continuous samples usingrotasonic boring techniques showed the extreme nature of the stratification which is

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216 2007 RETC PROCEEDINGS

not as accurately depicted using standard split spoon borings with the normal 5-footsampling interval and potential loss of samples in wet granular soils or if a gravel par-ticle becomes lodged in the split spoon.

The geotechnical investigation for the project was documented in Frank andChapman, (2001). One special measure taken for the characterization of the coarsefraction included the performance of large diameter borings 914 mm (36 inches) and1,067 mm (42 inches) from which the large cobble fraction (rock retained on a 152 mm(6-inch) grid) was separated for volume determination within each 1.5 m (5-foot) verti-cal interval. Another was provided by the fortuitous presence of a gravel mining opera-tion adjacent to a portion of the tunnel route, which afforded the opportunity to performcounting of boulders from 10 days production of gravel mined with a dragline andscalped over an 457 mm (18-inch) grizzly. These datasets were later used to develop amathematical model for quantification of the boulder fraction for presentation in theGeotechnical Baseline Report (Frank and Chapman, 2005). In addition, rock type wasdetermined for boulders in boulder stockpiles, and boulders were cored to obtain sam-ples from which to determine the range of compressive strengths. As expected, manyof the boulders were of igneous or metamorphic origin and were inferred to have beentransported from the area of the Canadian Shield, that in itself demonstrating theirresistance to destruction. Compressive strengths ranged up to nearly 45,000 psi.

PRESENTATION OF GROUND CONDITIONS IN THE GBRThe boulder quantification described by Frank and Chapman (2005) is shown in

Figure 1. During development of the contract documents, the appropriateness of pre-senting the boulder baselines was debated, since with the planned tunneling methods,it would not be possible to count the boulders or to know if the baseline was exceeded.It was eventually decided to present the information as a means of communicating thepotential magnitude of the boulder fraction to the contractor.

In addition, the layering detected in the rotasonic borings was interpreted and sev-eral important trends were identified and were also presented as geotechnical fac-tors, determined to be useful in TBM selection and in ensuring application of meansand methods which result in ground and face control by the TBM in order to meet set-tlement requirements. (GBR, May, 2003) Subsurface conditions along the tunnel routewere interpreted and summed to estimate total expected footages and distribution bystationing along the tunnel to which the defined conditions applied. The factors weredivided into TBM Selection Factors and Tunnel Face, Ground, and Settlement Con-trol Factors, and presented graphically as per the excerpt shown in Figure 2. It can beseen that these factors describe such characteristics as extremely coarse grainedsoils; glacial till; mixed face conditions such as flowable granular soils in the tunnelcrown overlying hard, resistant materials lower in the tunnel face; and low groundcover areas. It was reasoned that these descriptions would help the contractor to visu-alize the range of challenging conditions and their extent to aid in selection of tunnelingequipment as well as in planning of tunneling procedures and selection of ground con-ditioning agents.

SPECIFICATION OF TUNNEL BORING MACHINEIn conjunction with the provisions of the Geotechnical Baseline Report, develop-

ment of the specification for the TBM was given careful attention relative to risk manage-ment for the project. The tunnel design engineers recommended purchase of a newtunnel boring machine to ensure the capabilities required to provide the best possible


EPB-TBM DESIGN FOR BOULDER CONDITIONS 217

opportunity to successfully mine through all of the ground conditions. This was based onthe challenges presented by the geologic environment and on the tunnel size, whichmeant that there were few if any existing machines in the required size range that hadthe specified range of capabilities. It was desired to level the playing field among biddersand ensure that the bidding would not be won by virtue of a marginally capable TBM thatcould not be rejected but that would potentially be ineffective under the difficult mining

Figure 1. Boulder and cobble study results

Figure 2. Geotechnical factors by station along centerline



conditions. This recommendation was accepted by the City of Columbus and new TBMswere specified for both Part 1 and Part 2 of the project.

It was considered by most involved with the design that the specifications shouldallow either an earth pressure balance (EPB) or slurry machine. Consideration wasgiven to the provision of differential contingency levels for EPB versus slurry machines.This concept was based on the views of some that this might help to balance thehigher cost of slurry machines and potentially permit slurry machines to be more com-petitive. Limiting the specification to slurry machines was considered as well. Accord-ing to some recognized European standards the geotechnical conditions pointedstrongly to the Slurry TBM approach. These concepts were both ultimately rejectedbased on previous successful completion (in North America) of tunnels in difficult gran-ular conditions with EPB machines, e.g., the South Bay Ocean Outfall Tunnel.

Highlights of the tunnel specifications included: Slurry or EPB TBM allowed Design TBM for 3 bar operating pressure Design, build and maintain the TBM to operate in all ground conditions indicated

in the Contract Documents. Fit the TBM with a compressed air lock(s) and associated compressed air

equipment designed of 3 bars (45 psi) of working air pressure Design the TBM to be capable of excavating through ground containing cob-

bles, small boulders, and obstructions (boulders exceeding 457 mm (18 inches) in dimension) of the number and size and in all conditions indicated in the Con-tract Documents without interruption of the excavation operations

Design the TBM to accommodate both ripper teeth and disc cutters capable of cutting and removing boulders.

TBM DESIGN AND PROCUREMENTDuring the bidding stage for the project, the contractor team of Jay DeeMichels

Traylor Joint Venture considered both Slurry and EPB TBMs. There was a substantialcost advantage to the EPB for the procurement of the TBM and the support systemsfor each tunneling process. Additionally, we were unable to quantify any advantage inthe operational cost of Slurry tunneling vs. EPB tunneling that would offset the higherinitial outlay for the Slurry TBM. As such, EPB was deemed to be the most cost effec-tive. The Slurry TBM alternative would require any soil particles larger than 152 mm(6 inches) to be reduced to this size. In addition to anticipated boulder quantitiesexceeding 7,000 for the project, the number of cobbles larger than 152 mm (6 inches)were projected in the hundreds of thousands. EPB was therefore considered to betechnically superior because of the lesser degree of size reduction of boulders andcobbles required for muck ingestion by the EPB TBM as compared to the Slurry TBM.

The Contractor selected an EPB TBM designed and manufactured by Lovat, Inc.of Toronto, Canada. The main emphasis of the TBM design was building a machinewith high torque, the ability to pass large sized particles and robust cutterhead anddrive system. The diameter of the screw conveyor was maximized to gain the ability topass large sized rocks. A rear discharge screw conveyor was adopted to minimizelocations that a large particle could hung-up. A center stem auger was used instead ofan open center or ribbon auger. Ribbon augers can pass larger sized particles for thesame screw diameter. This design however was not considered to be sufficientlyrobust for this application. Further, there were concerns about this designs ability to



achieve a soil plug that would withstand the hydrostatic head throughout the high per-meability reaches along the alignment.

The Contractor requested and was allowed to deviate from the original ContractDocument requirements for airlocks mounted to the TBM and for the location of thescrew conveyor at bottom of the cutterhead chamber. Instead, airlocks mounted in thetunnel behind the TBM would be used if compressed air was necessary for cutterheadmaintenance. If compressed air was needed for cutterhead entry, it was determinedthat there was substantial risk for sudden air loss through the highly permeable sandand gravel deposits. The small pressure reservoir afforded by the TBM mounted airlockwould subject workers in the pressurized environment to injury from a sudden pres-sure drop during such an event in addition to the threat from water and soil inflows. Theroom afforded by removing the airlock from the TBM allowed for the screw diameter tobe increased by at least 152 mm (6 inches), thereby reducing the number of rocks thatwould have to be broken before they could be ingested by the TBM.

Another deviation from the original Contract Specification involved raising thescrew location off the bottom of the cutterhead chamber. This had several advantages.Locating the screw at the bottom of the cutterhead chamber requires it to be positionedoutside of the cutterhead drive. With the screw conveyor positioned within the interiorof the bearing; a larger, more robust bearing with more drive locations could be incor-porated into the design. Further, it was identified during testing of soil conditioners forthe project that the larger soil fraction (cobbles and boulder fragments) would sink tothe bottom when left undisturbed for a few hours. With the screw conveyor positionedat the bottom of the cutterhead chamber, the screw would get buried under a pile oflarge rocks during a shutdown. This could create a condition with resumption of tunnel-ing that would require considerable effort by the screw to re-mix the separated largefraction with the balance of the soil in the cutterhead chamber. Having the screwlocated up off the bottom allowed for the rotating cutterhead structure to pass beneaththe screw and sweep the pile of rocks up off the bottom so that they could be redistrib-uted throughout the conditioned soil paste.

The cutterhead drive was configured with eight 150 hp electric motors coupledthrough mechanical gearboxes to pinions engage the main gear at eight locations. The1,200 hp drive was an increase from the 900 hp drive that was included in Lovats orig-inal proposal. The 150 hp electric motors were controlled with variable frequencydrives (VFD) such that the cutterhead could be rotated at speeds from 0 to 3.4 rpm.The VFDs also allowed for startup of the electric motors while directly connected tothe cutterhead without a substantial rise in amperage in the electrical system. Thisdrive system was selected over a more common hydraulic drive system due to theincreased efficiency of the electric direct drive. During the course of tunneling a sub-stantial benefit was identified from the contribution of the increased inertia of the rotat-ing cutterhead with the direct coupling of the electric motors to the cutterhead. This willbe discussed later in this paper.

The cutterhead was designed with 32 cutter mounts that could be interchanged inthe field from ripper teeth to disc cutters. There were an additional 6 mounts for ripperteeth only. All cutter mounts were designed as rear loading for cutter replacement fromwithin the cutterhead chamber. The 14 outer-most cutter positions were oriented to beredundant such that multiple cutters cut the same kerf.

There has been considerable debate amongst the Contractors team and gener-ally throughout the industry on the proper cutters for dealing with boulders in softground. There has been considerable use of disc cutters in soft ground applications onslurry TBMs. However their use on EPB TBM applications is in dispute. After consider-able consideration, it was determined to configure the TBM with six disc cutterslocated at or near the gauge of the TBM. These disc cutters were redundant to rippers



that cut the same kerf and would operate as back-up cutters to ensure that the gaugewas cut in the event the ripper cutters were damaged from a large boulder that may beonly partially into the tunnel face. The remaining cutter mounts were outfitted with rip-per teeth. A diagram of the cutterhead as configured in shown as Figure 3.

The openings through the cutterhead were equipped with closure doors that couldbe activated to fully breast the face during cutterhead maintenance. The cutterheadwas armored with 19-mm (34-inch) thick chromium carbide plate. The outer-most rippertooth, one of three cutter positions that cut the gauge, was outfitted with a hydraulicwear detection system. A 6 mm (14-inch) diameter hole was drilled to within 25 mm(1 inch) of the end of the ripper. It was then plumbed with a hydraulic line back to theTBM operators controls. The TBM operator could identify when the ripper was worn orbroke to expose the end of this hole by it inability to maintain hydraulic pressure.

The 915 mm (36 inch) diameter screw conveyor had center stem auger that wasable to readily convey rocks up to 305 mm (12 inch) diameter. Three grizzly barsmounted across each of the cutterhead openings prevented passage of rocks largerthan what could pass through the auger (Figure 4).

The TBM was christened Mary Margaret and shipped to the Columbus forassembly and launch.

TBM PERFORMANCEThe Contractor made some minor configuration changes to the TBM during

assembly on site. These included the addition of chromium carbide plating on theinterior surface of the cutterhead chamber and surrounding the EPB sensors. Further,

Figure 3. TBM cutterhead configuration



Two of the three grizzly bars were removed to help prevent rocks from nesting in thecutterhead openings and causing a blockage. (This was later determined to be illadvised in that it led to the screw becoming plugged with an oversized rock on twooccasions.)

The six disc cutters that were mounted on the cutterhead, proved to be useless.They were recessed from the engagement point of the ripper teeth by approximately102 mm (4 inches). This meant that they didnt engage undisturbed soil unless theadjacent ripper teeth were gone. This occurred at one location on the project wherethe wear indicator ripper showed that the gauge rippers needed replacement when wewere approximately 30 m (100 ft) from a manhole location. It was decided to wait untilthe manhole was reached before changing cutters. At that manhole the two discs thatengaged the ground were worn flat. One of these disc cutters is shown in the photo ofthe front of the TBM at manhole #2 as Figure 5.

The ripper cutters wore at a rate that was proportional to the distance that eachcutter traveled. This was indicated by the outside cutters wearing at a faster rate thanthe inside cutters. In order to predict the cutterhead maintenance intervals, the amountof cutter wear was recorded during cutterhead maintenance and this data was plottedagainst the distance that the cutter traveled since it was last replaced. A general rela-tionship was identified by this plot. A representative sample of this data plot is shown inFigure 6. The relationship varied from one maintenance interval to the next. This waspartly due to the varying soil conditions; however, there were several other factors thataffected the cutter wear rate. A large proportion of the ripper cutters were breakingfrom impacts with boulders. After the ripper cutter was broken, the rippers that cut theadjacent kerfs would wear at an advanced rate.

The wear of the rippers could be predicted and cutter changes could be sched-uled; whereas ripper breakage was unpredictable. During the first 1,829 m (6,000 ft) oftunneling one-third of all the ripper cutters were being broken by boulders.

Another factor affecting the cutter wear rate was the application of soil conditioners.Soil conditioning was applied at the TBM operators discretion to optimize the TBMs per-formance. A major consideration in controlling the soil conditioning was the appearanceof the excavated muck as it discharged from the screw conveyor. The operator wastrained to condition the muck at rate such that the muck at the discharge point wouldclosely resemble fresh concrete as is exits a transit mix truck. After reaching the firstmanhole location where ready access to the cutterhead was available, the soil condition-ing swivel was found to be damaged and soil conditioner was being injected into thechamber behind the cutterhead instead of in front of the cutterhead. The rate of cutter

Figure 4. TBM profile showing screw conveyor position



wear through this reach of the tunnel was 3 times that of the remainder of the tunnel.This proves one of many major benefits of soil conditioning in EPB tunneling; the reduc-tion in the cutter wear rate.

The screw conveyor generally performed well in handling large cobbles and boul-der fragments. There were three instances where considerable downtime was causedby jamming of the screw conveyor. Two of these instances were caused by bouldersthat were too large to pass getting lodged within the screw conveyor and preventingthe auger from rotating. A considerable amount of time was expended in trying toregain rotation of the auger such that the rock could either be pulled through the augeror ejected back into the cutterhead chamber. Eventually inspection doors to the screw

Figure 5. Front of TBM at manhole #2

Figure 6. Ripper cutter wear



were opened, EPB sensor cells removed and ultimately small holes were cut throughthe conveyor casing to locate the rock. Once the first rock was located, a corner of therock was broken off with a rivet buster which allowed it to pass through the screw. Thesecond rock was lodged in the same location within the screw but attempts to break itmechanically failed. It was then drilled and blasted with explosives in its position withinthe screw. The resulting gravel readily passed through the auger.

The third instance of downtime from screw jamming occurred after a prolongedcutterhead maintenance period in wet sand and gravel face conditions. Entry into thecutterhead was in areas of high permeability if the water head was not severe (lessthan 6 m (20 feet) above tunnel crown). In these locations, the face doors would beclosed and the cutterhead chamber depressurized. Personnel would enter the cutter-head and perform cutter replacement on the upper part of the cutterhead as the screwconveyor was operated as an Archimedes pump to control the water level in the cham-ber. The water inflow would typically be 200 to 700 gpm. A considerable amount of finesoil particles would be washed in with this water, leaving only the coarse fractionremaining in front of the TBM. When tunneling was resumed after this event the lack offines caused the screw conveyor to lock up. After a considerable effort to free thescrew with various methods of soil conditioning injection, the cutterhead and screwwere opened up and manually mucked out. The screw was loaded with cobblesnosand or gravel at all. When the jammed screw was freed, 50lb bags of bentonite wereadded to the cutterhead chamber when tunneling resumed to create a soil paste thatcould flow through the muck system without separation.

ADJUSTMENTS MADE DURING TUNNELINGAt the first manhole crossing (S/M #2) all of the grizzly bars were re-installed to

prevent rocks larger than could be handled by the screw from passing through thecutterhead. The screw conveyor performed with-out any lock-ups after this change.

Once it was identified that ripper cutter breakage was occurring at a high rate, aremedy to this problem was researched. Several different ripper designs were testedutilizing modified shapes and various high strength/high wear resistant materials.None of these showed good results. It was decided to increase the size of the rippercutters. The original cutters were made of high strength steel with a cross section of64 mm 140 mm (2-12" 5-12"). These rippers were changed to larger rippers having across section of 76 mm 152 mm (3" 6"). These larger rippers required the installa-tion of new adaptor boxes that fit into the mounting for the disc cutter. The outer-most18 of the 32 cutter mounts that are interchangeable with disc cutters were replacedwith adaptor boxes for the larger ripper cutters. The remaining 14 inner-most rippercutters were shortened by 51 mm (2 inches). It was reasoned that the shortened rip-pers would have a lesser tendency to break and the extra wear length was not neces-sary since the travel distance for these teeth is considerably less than the gaugecutters. The remaining disk cutters were replaced with ripper teeth as well. Thischange-over could not be performed from within the cutterhead, so the modificationwas implemented at the second access manhole crossing (S/M #3) which was2,007 m (6,585 ft) into the drive.

The ripper size increase was very successful. Ripper cutter breakage was reducedfrom one third of the rippers breaking to less than one out of twenty rippers breaking.The rate of wear for the outside cutters was reduced with the addition of the ripper cut-ters mounted in the locations that were previously occupied by the disc cutters.

The new adaptor boxes had thinner walls to accommodate the larger rippers.These new adaptor boxes began to crack with one of them failing entirely. Ultimately



after 4,485 m (14,715 ft) into the drive at the fifth manhole crossing (S/M #6), the adaptorboxes were welded directly to the cutterhead structure. This eliminated the possibility ofchanging these rippers cutters to disc cutters while the TBM was in the ground; althoughthis was not likely to happen on this project anyway.

In addition to the configuration changes made to the cutter head, the operation ofthe TBM was adjusted. The TBM operator increased the cutterhead rotation rate from1.7 rpm to 2.3 rpm. This allowed the ripper cutters to take a smaller bite with eachpass. The rippers would now engage about 25 mm (1 inch) of undisturbed soil per rev-olution instead of 38 mm (1-12 inches). The higher rotation rate would increase theamount of cutter and component wear but would lend to the reduction in cutter break-age. It was not identified at the time, but further analysis indicates that the higherspeed added to the momentum of the high inertia cutterhead drive. This increasedmomentum would increase the impact load on the engaged boulders thereby increasethe TBMs capacity to break the boulders. This is discussed later in the paper.

The modifications made during the tunneling drive reduced the amount of down-time required for cutterhead maintenance. The average cutter change frequency wasreduced from approximately one cutterhead maintenance per 152 m (500 ft) to lessthan one cutterhead maintenance per 914 m (3,000ft). A chart showing the cutterheadmaintenance intervals is shown in Figure 7.

A picture of the front of the TBM after hole thru is shown in Figure 8. This pictureshows the excellent condition of the cutterhead after only one cutterhead maintenanceevent while tunneling through more than 1,875 m (6,000 feet) sand and glacial tillincluding cutting through more than 1.2 m (4 feet) of concrete slurry wall.

Of further note, the project locations that possessed the coarsest soils which wereconsidered by many as being well beyond the capabilities of EPB and the basis for rec-ommending the use of slurry methods, were areas that the TBM achieved its best per-formance. While tunneling through the areas with 100% sands and gravels and lessthan 10% fines the TBM was able to advance at its maximum rate with less than 75%of its full torque.

ANALYSIS OF PERFORMANCEAs mentioned earlier, there was a great deal of discussion regarding the appro-

priate dress of cutting tools that should be applied to the cutterhead. Disc cutters arethe standard tool for cutting rock, but it is debatable if boulders can be cut effectivelywith disc cutters. Disc cutters break rock by loading the rock at the contact point suffi-ciently high to create shear stresses on either side of the contact point. Single disccutters are known to perform better than multiple disc cutters for this reason. In order

Figure 7. TBM ripper cutter change locations



for disc cutters to perform effectively in soft ground with boulders the rock has to beheld in place with sufficient security such that the point load from the cutter can mobi-lize the shear stresses in the rock for it to break. In order to stabilize the rock andkeep it from shifting position, multiple disc cutters are frequently used. There less effi-cient cutters are also used to minimize the side loading on the cutter bearings thatcan result from either a glancing impact on a single disc cutter or from the rock shift-ing during the impact. When the disc cutter breaks the rock, it produces relativelysmall fragments or chips. These small fragments can be effectively digested in aslurry mucking system; however, an EPB mucking system does not require the rocksto be broken into chip size fragments. If the rock is not held securely it will either bepushed through the ground until it is either pushed aside or it will eventually be brokenup by repeated impacts from the cutterhead or other rocks.

In soft ground tunneling, the ability of the soil matrix to hold the rock in place suchthat the rock can be cut may be possible in many instances but it is not common. Boul-ders are usually broken by impact. If you have a boulder on the ground in front of you,would you use knife to cut it or a hammer to break it? When a disc cutter encounters aboulder in a soft ground matrix, the disc will impact the rock as the cutterhead rotates.When a ripper cutter encounters a boulder in a soft ground matrix, it too will impact therock as the cutterhead rotates. Both tools can act as a hammer to break the rock. Thequestion is which style of hammer will readily survive the impact. The use of disc cut-ters in soft ground requires a reduced preload on the bearings in the cutter such thatthe cutter can be readily turned by hand. Otherwise, when engaged against a softground matrix there will not be sufficient frictional forces for the cutter to roll across thatmatrix. In EPB tunneling this issue is worse since the excavated paste that fills thechamber behind the cutter will provide additional rolling resistance than if the cutter-head chamber was empty (as in non-pressurized face TBM) or filled with liquid (as in aslurry TBM). The EPB muck also enhances the possibility that the disc will get soil par-ticles lodges between the sides of the disc and cutterhead structure causing it to stopturning. If the disc doesnt turn in contact with the ground, it will wear flat and fail. Onthis TBM the disc cutters require at least five times the effort to replace over thatrequired for replacement of ripper cutters.

Figure 8. TBM front at Shaft 8



In order for a ripper cutter to survive an impact with a boulder it must havesufficient strength to handle the impact loading. The smaller rippers that were mountedon the TBM when it was originally configured did not have sufficient strength. Whenthese cutters were replaced with the larger cross section ripper cutters the tools wereable to survive the impact.

WHERE ARE THE BOULDERS?During the course of tunneling we awaited a call from the heading that the TBM

had encountered a rock that has stopped the cutterhead and is unable to advance.This call never occurred. There were several instances where the TBM operator wasable to identify that there were substantial rocks being engaged by the cutterhead. Wewere unable to identify the number or size of these rocks. On one of these occasionswe separated the muck from that push and spread the excavated material across theground. An inspection of the muck identified many rock fragments that exhibited whatappeared to be recent fracture surfaces. A sample of the rock fragments is shown in aphotograph as Figure 9. However, we did not find several fragments of identical rockthat could be pieced together to identify its original size.

We reviewed the data log of the TBM drive system on a regular basis to try toidentify torque or amperage spikes that would be an indication of a boulder encounter.We were hoping to find a pattern in the data in the log that would occur during a boul-der encounter that could be used as a boulder signature. There were continual torquevariations during each advance with generally higher torque during pushes that theoperator reported that he was engaging boulders, but there was no identifiable bouldersignature that could be used to identify a boulder encounter.

We were puzzled by the lack of boulder problems in terms of no shut-downs toclear obstructions. We did have a considerable amount of downtime for cutterheadrepairs, but there seemed to be no stopping of this machine. Could there be enoughstored energy in the rotating cutterhead to break the rocks that we were encounteringwithout the need to draw additional power?

We asked the engineers from Lovat to analyze the inertial aspects of the drive.Here is their analysis.

Figure 9. Rock fragments



INERTIA FACTOR IN BREAKING ROCK OR CUTTING TOOLSThe total torque (Ttot) delivered to the TBM Cutting Head (CH) during mining is

distributed as follows:Ttot = Tinertia + Tresist + Tcut (1)

where:Tresist = Torque required to overcome friction forces resisting CH rotation,

Tcut = Torque required to overcome CH cutting forces, Tinertia = Torque required to overcome inertia forces of the rotating

components,

In case of the incidental rapid increase of tangential resistance force (Ftan) on oneof cutting tools caused by hard boulders, the angular impulse on the tool (Iang =RiFtandt) is generated by angular momentum AM = JM of all rotating components.where:

Ri = Equivalent radius of the tangential force, JM = rotating parts mass moment of inertia, = angular velocity of rotating mass

The following equation can be used to find forces, moments and/or energy toovercome these resistances:

(2)The right side in equation (2) can be elaborated for each TBM drive relative to the

CH angular speed as shown in the following equations:JMd = (JCH + JMG + Nr1JGB + Nr2JEM)dCH (3)

where:JCH = CH mass moment of inertia

(for RME193SE JCH = 77,023.4 kgm2)CH = angular velocity of CH (for RME193SE nominal CH = 0.16 rad/s

and maximum CH = 0.36 rad/s)JMG = Main Gear mass moment of inertia (for RME193SE JMG

= 13,354 kgm2)JGB = Gearbox equivalent mass moment of inertia (for RME193SE

JGB =12 kgm2)JEM = Electric motor rotor mass moment of inertia (for RME193SE

JEM =5.8 kgm2)N = Number of drives in Main Drive (for RME193SE N = 8)r1 = Main gear ratio (for RME193SE r1 = 9.82)r2 = Total MD gear ratio (for RME193SE r1 = 1,049.8)

JMd = (77,023.4 + 13,354 + 942.7 + 48,710.7)(kgm2)d = 140,030(kgm2)d (3a)

Therefore, equation (2) for RME193SE can be simplified to the following:(4)

Ri F tan tdt 1t 2 JMd=

Ri F tan tdt 1t 2 140,030 (kgm2)d=



In the case of a hydraulically driven cutterhead, the electrical motor inertialcomponent is not present. The resulting angular momentum would be reduced.

JMd = (77,023.4 + 13,354 + 942.7)(kgm2)d= 91,320.1(kgm2)d (3b)

The direct connection of the electric drive increases the angular momentum of thecutterhead drive system by more than 53%.

Considering the operational change of increasing the rotation rate of thecutterhead from 1.7 rpm to 2.3 rpm will further increase the angular momentum.

CONCLUSIONMaximize the particle size that can be ingested by the TBM by maximizing the

screw diameter. This needs to be considered in coordination with other project param-eters, such as the soil permeability and the hydrostatic head in that a large diameterscrew may need to be longer to effect a soil plug that will dissipate the head without ablow-in.

The best tool for breaking rocks is a big hammer. An electric direct drive cutter-head provides the high inertia that a big hammer features when breaking rock. Theteeth should be sized to withstand the cutterhead drive torque plus the impact loadingfrom the cutterhead momentum.

Unless there is bedrock expected in the tunnel face, do not use disc cutters forcutting boulders with an EPB TBM.

ACKNOWLEDGMENTThe support of the authors employers is gratefully acknowledged.

REFERENCESGeotechnical Baseline ReportBig Walnut Augmentation.Rickenbacker Interceptor

Sewer ProjectCity of Columbus, Ohio, CIP No. 491.1, May, 2003.Frank, Glen and Chapman, David; Geotechnical Investigations for Tunneling in Glacial

Soils, 2001 RETC Proceedings, Society for Mining, Metallurgy, and Exploration,Inc., Littleton, Colorado; pages 309324.

Frank, Glen and Chapman, David; A New Model for Characterizing the Cobble andBoulder Fraction for Soft Ground Tunneling, 2005 RETC Proceedings, Society forMining, Metallurgy, and Exploration, Inc., Littleton, Colorado; pages 780791.
