tunneling through cobbles in sacramento california

907

Chapter 74

TUNNELING THROUGH COBBLES IN SACRAMENTO, CALIFORNIA

Rafael Castro

Formally with Underground Construction Managers, Inc.

Ross Webb

Affholder Incorporated

Joel Nonnweiler

Sacramento Regional County Sanitary District

ABSTRACT

The three thousand foot long 66-inch diameter Folsom East Interceptor Section2B was completed accident free, on time and under budget with no measurablesurface settlement. The tunnel passed through nested cobbles, highly abrasive sandsand gravels, and abandoned mine workings with less than five-feet of cover at times.Preventing settlement was critical since the tunnel passed under a heavily traveledstate highway. A pilot project to test microtunneling and pipejacking methods in thesame ground conditions abandoned both drives before completing the planned drivelengths. This paper discusses performance of the conventional tunneling methodselected to excavate the ground and the effectiveness of the ground modificationtechniques employed.

PROJECT BACKGROUND

The Folsom East Interceptor Section 2B (FE2B) project is one component of theSacramento Regional County Sanitary District’s (District) plan to increase the capacityof its existing 95 miles of interceptor system. The project was located in FolsomCalifornia, approximately 18 miles east of California’s Capital, Sacramento. Refer toFigure 1 for the project alignment. FE2B parallels the northern side of Folsom Blvd.and extends from just west of U.S. Highway 50 (US 50) toward the Folsom PumpStation No. 1 to approximately Iron Point Road. Bound on either side by Section 2A ofthe proposed Folsom East Interceptor system, the FE2B alignment is approximately3,000 feet long.

FE2B was designed for and constructed using tunneling to minimize impacts toUS 50 a four-lane highway that was being widened to six lanes concurrently with theFE2B project. Also adjacent to the US50 were protected seasonal wetlands in aCalifornia State Park. A two-pass system was used. Ten-foot O.D. continuoustemporary ground support was designed by the Contractor and consisted of steel ribs

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Figure 1. Project alignment

TUNNELING THROUGH COBBLES IN SACRAMENTO 909

and steel lagging which were expanded against the excavated ground. The final carrierpipe consisted of 66-inch I.D. reinforced concrete pipe installed within the initialsupport and the annulus filled with low-density cellular concrete.

Two tunnel drives were completed from the midpoint of the alignment (manhole 23).The mid point of the alignment was the only area where wetland was not present andenough space was available to stage the tunneling operations. First, the State ParkTunnel was excavated toward the Iron Point Road end of the alignment (manhole 26).The State Park tunnel was excavated through a designated test area 150 feet in length,where the Contractor demonstrated the surface settlement limitations set forth in thecontract documents would not be exceeded using the selected means and methods.After completing of the State Park Tunnel drive, the TBM was lifted out of manhole 26 exitshaft and moved to the main access shaft and began the US 50 Tunnel drive in theopposite direction to manhole 20. Figure 1 shows the alignment and the tunnelsegments.

The Contract was awarded to Affholder Incorporated for $7.14M. Work began inearly June 1999 and was completed in early January 2000.

SUBSURFACE CONDITIONS

Ground encountered in the FE2B tunnel consisted primarily of two units: analluvial channel deposit (Alluvium) overlying a tuffaceous mudstone and welded tuff(Merhten Formation). Except for approximately 1000 feet of the alignment the interfacebetween these two formations was for the most part below the invert of the excavatedtunnel. In the portion of the alignment that the Merhten formation was present withinthe tunnel, the interface undulated quite rapidly in elevation from just near the invert toover the tunnel crown. Of course, this mixed face condition occurred when tunnelingbelow US 50. The ground water table was located below the excavated tunnel invert.Detailed description and ground behavior of these formations is discussed below.

Alluvium

The Geotechnical Baseline Report (GBR) predicted the Alluvium to be thepredominant sediments encountered in the FE2B tunnel. The alluvium consistedlargely of brown, well-graded GRAVEL with sand (GW); to COBBLES in a matrix ofgravel with sand. The alluvium generally contained a minor fines fraction, typically lessthan 10% by weight, although also present were “clean” (less than five percent) sandsand gravels. Where gravel was predominant, cobbles were generally common ornumerous. The gravel and sand appeared to consist chiefly of granitic material.

Small boulders (< 20 inches in maximum dimension) were also common ornumerous within much of the alluvium as indicated on the Geologic Profile. Many largeboulders (> 20 inches in maximum dimension) were encountered along approximately117 feet of the Highway 50 Tunnel. The maximum dimension of the large bouldersencountered was typically 24 to 30 inches. These larger boulders typically requiredstopping the TBM and rotating the head to feed them onto the conveyor. Occasionallylaborers had to enter the plenum to lift them into the TBM. One boulder, at least 35" inmaximum dimension, had to be broken up with a jackhammer for removal. Bouldersand cobbles within the alluvium were typically composed of very hard, mafic volcanicrock (andesite and basalt). Occasionally quartz boulders or cobbles wereencountered.

Although the fines content of the alluvium was typically less than 5 to 10%, theygenerally provided sufficient cohesion to classify the alluvium as slow to fast ravelingground, as anticipated in the GBR (which referenced Tunnelman’s Classification).

910 2001 RETC PROCEEDINGS

During the advance of the tunnel directly beneath US50, some ground lossoccurred in the alluvium above the TBM. This was an area of mixed face conditionswith only 12 to 24-inches of loose gravel with sand and numerous cobbles (alluvium)exposed at the crown of the tunnel face, overlying the much denser Merhten.

The contact between the base of the alluvium and the underlying Merhten wascharacteristically irregular, with undulations of two feet or more readily apparent withinthe 10-foot width of the tunnel at some locations.

Merhten Formation

The Geotechnical Baseline Report (GBR) predicted 200 feet of MerhtenFormation would be encountered in the FE2B tunnel. This formation consisted of a thinlayer (6-inches to 3-feet thick) of hard welded tuff overlying a softer tuffaceousmudstone. The welded tuff consisted of hard to very hard, fresh to moderatelyweathered, very coarse to fine grained WELDED LITHIC LAPILLI TUFF toTUFFACEOUS SANDSTONE. As the name indicates this rock was composed ofcrystalline rock fragments and ash, highly indurated into a coherent mass. Includedrock fragments were angular to sub-rounded, and predominantly rhyolitic in lithology.The differential hardness of this unit, contributed significantly to bit wear, and in allprobability to shearing off of ripper teeth. This unit was notably harder and moreresistant than the alluvium above or the underlying dark gray volcanic mudstone(described below). Apparently as a result of its hardness and the geometry of itsoccurrence, it tended to break into larger cobble and boulder sized fragments.Although the upper and lower surface of this unit undulated substantially, it generallytransected the face, nearly horizontally, at or above the spring line for over 750 feet.

Below the welded tuff layer was the softer Merhten, which consisted of very hardto soft, gray, fresh to slightly weathered, very coarse to fine grained TUFFACEOUSMUDSTONE (LAHAR). Although it was not readily apparent, it appears that it waspresent just above or below the tunnel invert along the entire alignment of the US 50tunnel. Lahars are volcanic mudflows formed by rapid mixing of loose volcanic debrisand water that move down river systems to form thick, chaotic deposits of large andsmall fragments in a fine-grained matrix. As a result, lahars typically containpyroclastic volcanic debris with intermixed fluvial material, and frequently fragments ofwood. Most of the larger fragments observed in the gray tuffaceous mudstoneencountered at the site consisted predominantly of very hard, rounded to sub-rounded(water worn), dark gray to black, andesitic to basaltic, cobbles and small boulders.Occasional clasts two to three feet in maximum dimension were encountered. Theangularity of the individual clasts generally appeared to increase as the grain sizedecreased. The fine-grained matrix appeared to consist primarily of volcanic ash. Inthe tunnel muck, the origin of rounded alluvial cobbles and boulders was often difficultto determine under mixed face conditions. Very hard cobbles and boulders embeddedin the volcanic mudstone, however, clearly contributed to increased bit wear.

PILOT PROJECT

A Construction Methods Proving Project (Pilot Project) was completed by theDistrict to assess the feasibility of two construction methods being considered for theFE2B project. This project was completed under a separate contract during the designphase of FE2B. The site for the Pilot Project was located several hundred feet west ofthe FE2B alignment where ground conditions were believed to be similar in nature.Two drives were attempted with two different types of microtunneling-pipe jackingmachines. One machine was an open face with rotating cutter bar type and the second


was a slurry machine with a rock crusher. Neither machine was capable of excavatingthe ground effectively causing larges voids, surface settlement, and significant damageto both the pipes being jacked and the machines themselves. The reader is referred tothe referenced report for complete description of the Pilot Project. Based on the resultsof this Pilot Project, the District decided to exclude microtunneling or pipejacking as anoption for the FE2B project. Conventional tunneling with and earth pressure balancemachine was specified.

CONSTRUCTION DETAILS AND PROGRESS

This section describes the construction process, with details of the equipment andmethods used to complete the tunneling. Refer to Figure 4 and Table 1 for productionrates and TBM availability.

Shaft Sinking

The project included seven shafts all at manhole locations. Of the seven one wasused for a work shaft and two others were used for retrieval shafts. Shallow depth,minimal groundwater, and good standup time made shaft sinking fast and uneventful.The liner plate and steel rib work shaft was 32' diameter by 37' deep. The upper 20'were excavated with a Cat 330 excavator. The remainder was completed using a Cat307 excavator in the bottom filling a muck box to be lifted and dumped by crane. Allother shafts were sunk with an excavator working from the surface. Shoring for the 22'diameter upstream retrieval shaft was also steel liner plate. The remaining shafts wereshored with trench boxes and steel plates.

Table 1. Tunnel boring machine availability and utilization

No. of Shifts (During

Working Days Only)

TBM On Site

[1] :

81 7/10/99 to 11/1/99 (inclusive)

TBM Prep Time

[2] :

18 7/9/99 to 7/26/99 State Park Tunnel & 8/31/99 to 9/14/99 US 50 Tunnel (non inclusive)

TBM Mechanical Delay

[3] :

14.2 TBM related delays such as teeth changes, motor changes, conveyor break downs, etc..

TBM not in Use (

not

due to TBM mech. delay)

[4] :

2.7 Non-TBM related delays such as waiting on train, surveyors, generator problem, etc.

Availability : 60.2% ( [1] - [2] - [3] ) / [1]

Utilization : 94.5% ( [1] - [2] - [3] -[4] ) / ( [1] - [2] - [3] )

Total Footage Tunneled : 2975 ft 3029 ft – [shaft diameters (32 & 22 ft)]

Average Production Per Shift : 36.7 ft/shift

Average Production Per Shift TBM Available :

64.5 ft/shift


Tunneling

Tunnel construction was completed with a Lovat, tunnel boring machine (TBM)using a two-pass lining system. The initial and final liner consisted of steel ribs with fullsteel lagging, and 66" reinforced concrete pipe respectively. The 3025 foot long tunnelwas completed in two drives. The 1416 foot long State Park Tunnel was driven first.The TBM was repositioned and the 1514 foot long U.S. 50 Tunnel was driven. Thestate park section of the tunnel was straight with one deflection point while the U.S. 50section was curved along most of the alignment.

Ground cover to the tunnel crown ranged from 45 under U.S. 50 to less than twofeet at the downstream end. It averaged 25' under the State Park. Water was presentbelow springline throughout much of the tunnel but inflows were minimal and easilyhandled by a ten hp pump in the shaft.

Equipment

A 110-ton American Crawler friction crane on surface handled all mucking andhoisting including setting and retrieving the TBM. In the tunnel all haulage equipmentwas rail bound. Eight-ton Greenburg electric locomotives supplied by MiningEquipment Company were used for muck and material haulage. The locomotive pulleda train consisting of four 7 cubic yard muck boxes and two flatcars. Due to the shortlength of the drives no switches were used in the tunnel.

The specification allowed for the use of either an open face digger shield withbreasting capability or a closed face Lovat or equal TBM. Both machines were requiredto pass boulders up to 20 inches in diameter. Affholder elected to use a Lovat TBMbased on the crew’s substantial experience with Lovat machines and the availability ofa suitable machine in Affholder’s fleet. While Affholder owned smaller diameter Lovatsthe consensus was that without substantial modification the smaller Lovat machineswouldn’t be able to pass a 20" boulder and might prove under powered for the difficultground anticipated.

The TBM selected was a Lovat model 121 PJ/RL series 8600. Refer to Figure 2.This TBM has a total connected horsepower of 350 and a maximum thrust of 850 tons.It is propels itself off the initial support and has a 5.5 foot stroke. The cutter head hadsix openings to allow material to enter the plenum. Initially the TBM was equipped withhydraulically operated doors for these openings but after the first 125 feet of tunnel thecobbles and boulders destroyed the hydraulic door cylinders. The doors were deemedunnecessary to control the ground and were subsequently removed.

Initially, muck removal was facilitated by a 300-degree muck ring, mounted in thecenter of the forward shell, which transfers the muck to the conveyor system. After thefirst several hundred feet of the State Park Tunnel, most of the muck ring had been tornor sheared off by the boulders and cobbles. The TBM tunneled the remainder of theState Park Tunnel and the entire U.S. 50 tunnel without the muck ring. It was found thatthe material was dry and cemented enough to hold a stable vertical face in mostinstances. The TBM performed well given the difficult nature of the ground. Themajority of mechanical downtime was related to changing teeth.

Despite the Lovat’s mechanical reliability nothing really seemed to work well forthe cutting tools. The cobbles and boulders encountered averaged 40,000psi andtended to shatter carbide tips on teeth. The cutterhead wasn’t fitted for disc cutters andeven if it had been in most instances the cobbles and boulders weren’t cemented firmlyenough for discs to be effective. Esco or backhoe teeth worked well until they began tobreak usually after less than about 200'. Ripper teeth also worked but experiencedhigh wear rates. While a combination of Esco and ripper teeth were acceptable for theloosely cemented gravels and cobbles they tended to stall the head in the dense, firmly


cemented Merhten formation. Carbide tipped bullet teeth were the most effective atcutting the Merhten but were easily shattered in the presence of cobbles and boulders.

Ground Support

The use of wood shoring in the tunnel was forbidden by the specifications. One ofthe primary reasons were special orders received from CalOSHA on another project,the Bradshaw Interceptor in Sacramento California for the same owner. The result wasthat Bradshaw project had to install additional fire protection measures during thecourse of the work, which represented an additional cost. That project and the specialorders are covered in detail in Chapter 46 of the 1999 RETC proceedings “BradshawInterceptor Section 5A”.

Ribs and steel lagging were selected over the other option of two pass concretesegments. American Commercial supplied both ribs and lagging. W4x13 steel ribswere placed on five-foot centers and 3" x 6" steel channel lagging were placed in theflanges of the rib with legs pointing into the tunnel. Lagging was placed for the full 360degrees and a steel plate filled the expansion joint. Refer to Figure 3. Despite initialcomplaints the miners were quite successful with this system. Especially after a glovewas found that let the miners grip the damp, often oily steel but resisted cuts from thesharp edges. In fact after the first few weeks miners were standing a full set in lessthan ten minutes. Tunnel production rates in most instances were not hampered by theuse of steel lagging. The notable exceptions being curves and alignment changeswhere adjustments in line are far easier with wood lagging.

Ground Modification

The Contract called for the Contractor to perform ground modification in two parts.The first was a demonstration program in the State Park area. The second was

Figure 2. 120-inch Lovat TBM head configuration prior to tunneling. Doors, found unnecessary and damaged, were removed after 150 feet of tunneling.


production grouting beneath the two on/off ramps and the main corridor of US 50. Thedemonstration section allowed the contractor to prove the effectiveness of thesubmitted grouting methods. Affholder proposed compaction and compensationgrouting. Pretreatment methods such as chemical grouting weren’t proposed sincegood natural standup and arching characteristics were evident in the adjacent opencut contract.

Demonstration holes were drilled along the alignment both vertically anddiagonally since in the production grouting areas vertical access over the tunnelalignment was limited. These holes were drilled to five feet over the tunnel on 5 footand 8 foot centers with steel casings installed at the compaction grout hole and sleeve-port-pipe in some of the compensation gout holes. Compaction grouting wasattempted at 300 to 500 PSI after the TBM had passed. Six holes were pumped onand there was no grout take. This was not surprising since the observed groundbehavior indicated there was no loosening of ground beyond that excavated.

Compensation grouting involved pumping a wet water-cement mix (pancakebatter consistency) at low pressures of approximately 3 PSI over the initial support justpast the TBM tail shield. Grout takes were minimal on all holes but one. On this hole 2inches of movement five feet over the tunnel was measured approximately 10 feet pastthe TBM tail shield. Grout take was 1/3 of a yard.

Based on both ground observations and data from the demonstration section,Affholder chose compensation grouting as the grout method for the production areas.In order to minimize disturbance of the ground over the tunnel and disruption to traffic,a decision was made to be on stand-by with drill and equipment but grout only if theaction levels set forth in the specifications were reached. Also, Affholder proposed tofirst attempt grouting, if necessary, from within the tunnel. The catalyst to grout wouldbe based on the ground movement data collect and the well-established limits set forthin the specifications.

Figure 3. It took four miners to install the steel ribs and steel lagging to form the initial support


Tunneling Progress

The State park tunnel gave Affholder a chance to fine tune operation of the TBMfor settlement control prior to mining under U.S. 50. The only settlement measured inthis drive was at the first set of extensometers encountered. The ground responsecurves generated by the CM team showed settlement was occurring soon after the tailshield passed. This indicated that the shoring wasn’t being expanded fully or rapidlyenough. Increased pressure when expanding the set and expansion as soon as the ribleft the tail shield provided easy solutions. At this point the TBM wasn’t creating a voidso there was little the ground modification program could improve on. The TBM’s bestproduction rates were achieved on this drive. Refer to Figure 4 for production rates andactual versus scheduled progress.

Tunneling under U.S. 50 produced some surprises but all were dealt with quicklyand effectively thanks to the experience gained on the first drive. As the TBMapproached US 50 the Merhten formation rose from the invert of the tunnel to overspringline. This unpredicted condition resulted in slowed progress and excessive toothwear. Two extensometers with tips located 5 feet over the tunnel beneath U.S. 50dropped 2 plus feet. Muck volumes indicated over-excavation and visual observationsfrom the TBM head behind and over the TBM also confirmed the extensometerreadings were accurate. A void over the TBM and the support system behind the TBMexisted. Compensation grouting was completed immediately from within the tunnel buttrailing gear made it difficult and not very effective. Because the ground showed goodsigns of bulking and arching, and the extensometers located 10 feet over the tunnelnever moved more than

1

⁄

2

-inch, tunneling continued far enough to allow better accessfor grouting from within the tunnel. In the mean time compensation grout was pumpedinto the two extensometers that showed the movement. No additional holes weredrilled from the surface for grouting. Once the TBM advanced far enough from beneaththe median a second phase of grouting was attempted from within the tunnel. A jack-leg drill was used to drill five to eight feet over the tunnel in the areas under US 50where voids were expected. Voids were obvious when the drill lunged forward. Wherevoids were detected compensation grout was pumped. Overall a total of 25 cubicyards of grout were pumped to fill the voids. The grout takes in this area correlated wellwith the additional muck volumes estimated by the tunnel inspectors.

The cause of this void can be associated with the fact that the Merhten Formationencountered in this area rose to nearly a full face with a layer of looser sand, cobblesand boulders over the Merhten. The TBM working on the Merhten ended up over-excavating the looser material above. Vibrations from the TBM cutting on the Merhtenmay have contributed to the loosening of the alluvial deposit above.

Beneath the eastbound US 50 ramp where cover over the crown was less than4 feet the TBM operator was amazed to find he could advance the TBM withoutremoving any muck. This second surprise, though predicted by the GBR, was theChinese diggins tunnels. These unshored tunnels were constructed years earlierby Chinese laborers for drainage and gold mining. Fortunately the invert of theTBM tunnel and the old tunnel were almost the same so grade control was not aproblem. The TBM mined through the opening and a few dry rotted timbers withoutincident. After the state archeologist determined there was no historic value to thetunnel it was filled during the pipe backfilling operation.

Except for the areas already mentioned no settlement was measured on thisdrive. Instruments closer to the surface 10 and 15 feet over the tunnel and groundsurface points never moved even in areas where cover was less than five feet. The

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Figure 4. Tunneling progress and production rates

0

500

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6/18/99 7/2/99 7/16/99 7/30/99 8/13/99 8/27/99 9/10/99 9/24/99 10/8/99 10/22/99 11/5/99

Week Ending (Friday)

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eet)

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100

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300

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Wee

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Remob. Access Shaft & Tail Tunnel State Park Tunnel US 50 Tunnel

Scheduled

Actual

Weekly


TBM was clearly capable of controlling the ground (at least ground as described by theGBR). The nearly full face of Mehrten clearly posed problems of ground control asshown by the movement measured at depth. However, the grouting program proven inthe demonstration section dealt with this area smoothly.

Pipe Installation and Backfill

Hanson Concrete Pipe and Products in Sacramento manufactured the 66"reinforced concrete pipe final liner for the project locally. The pipe specified was Class Vwith Ameron T-Lock cast into the upper 270 degrees of the pipe. Since the tunnel wasmined with an oversized TBM the substantial annular space allowed a bell and spicketpipe to be used as opposed to the flush bell and spicket pipe more commonly seen intunnel applications. A rail mounted pipe carrier designed and manufactured byAffholder (Figure 5) transported and set the 20' joints of pipe in the tunnel. Pipes wereheld in place with hardwood blocks positioned to maintain grade and prevent flotation.Pipe installation began after tunneling was complete and completed within three weeks.

After the pipe was placed and the manholes completed Pacific International GroutCompany backfilled the annular space with low-density cellular concrete (LLDC). Theconcrete which was mixed on site had an in place density of 45 lb/cu ft and a designstrength of 500 psi in 28 days. LDCC placement progressed rapidly with the on sitebatch plant able to produce 100cy of concrete per hour. Holes drilled after placementshowed complete filling of the annular space between the pipe and the lagging. Groutthat migrated to surface on occasion also indicated that any voids outside the laggingwould have been filled as well.

Figure 5. Pipe carrier being prepared for use on the surface


CONCLUSIONS

The project was completed with no impacts to third parties, on schedule and withonly a one percent increase in cost from change orders. The project’s success can, inlarge part, be attributed to the correct configuration and operation of the selected TBM.Additionally, timely communication of tunneling progress with the CaliforniaDepartment of Transportation’s (CalTrans) knowledgeable geotechnical staff proved tobe an aid in completing the projected without any administrative type delays.

REFERENCES

Black & Veatch Construction, Inc., and Haley & Aldrich, Inc., November 1998, FolsomEast Interceptor – Section 2, Geotechnical Baseline Report for the U.S. 50 andState Park Tunnels.

Black & Veatch Construction, Inc., and Bennet and Stehli Engineers, Inc., 1998,Construction Methods Proving Project.

Underground Construction Managers, Inc. and EPC Consultants, Inc., January 15,2000, Folsom Interceptor Section 2B, Contract No. 3322A, Construction Report.

tunneling through cobbles in sacramento california

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