Archives: Projects

Plateau Creek

Project Overview

The Plateau Creek Pipeline was built to replace a pre-existing pipeline that supplied fresh water to rural and urban areas of Mesa County, Colorado, USA. The new tunnel provides water to 70,000 customers and has four times the hydraulic capacity of the old pipeline, which was unable to meet the District’s peak water demands.

The project owner, UTE Water Conservancy District, awarded the construction contract for the entire 21 km (13 mi) pipeline to the Barnard-Affholder Joint Venture. Affholder was solely responsible for the two sections of tunnel that were bored by TBM. The contractor used a 3.3 m (10.8 ft) diameter Robbins Main Beam TBM to bore two tunnel lengths of 1.0 km (0.6 mi) and 3.1 km (1.9 mi).

Geology

The geology consists of sandstone, shale and siltstone with an Unconfined Compressive Strength (UCS) of 69 – 172 MPa (10 – 25 ksi). The rock required immediate support including rock bolts, wire mesh, and shotcrete.

TBM

Robbins rebuilt and modified the Affholder-owned Robbins Main Beam TBM specifically for the project. The machine had been a workhorse for Affholder, who has used the machine on nine different projects totaling over 30 km (19 mi) since purchasing the machine in 1993.

Modifications included a new cutterhead, cutterhead power increase by 25%, a new high capacity main bearing and a thrust increase to allow a loading of 267 kN (60,000 lb) per cutter. The machine was capable of up to 4,893 kN (1,100,000 lb) of cutterhead thrust and could generate up to 455,738 N-m (336,135 lb-ft) of torque at the cutterhead.

Tunnel Excavation

Boring began on the 1 km (0.6 mi) long tunnel in June 2000 and the machine holed through in August, just 1 month later. The machine was then disassembled and re-launched in only 2 weeks to bore the longer 3.1 km (1.9 mi) tunnel.

Excavation on the second tunnel began in September 2000. The machine set a world record in its size class on September 14th when it advanced 67 m (219 ft) in a single eight-hour shift. The TBM holed through ahead of schedule on March 16, 2001.

---

Meråker

Project Overview

The Meråker Project consists of a system of power stations and underground tunnels that increase the electrical output supplying a local smelter and surrounding areas. Three lakes and several rivers supply 44 km (27.3 mi) of tunnels that feed into a generating facility in Tevla. From Tevla, a combined headrace/tailrace tunnel carries the water to the power station at Meråker.

Merkraft, a joint venture of Eeg Henriksem Anlegg a/s and a/s Veidekke, chose a Robbins 3.5 m (11.5 ft) diameter Main Beam tunnel boring machine (TBM) to bore 10 km (6.2 mi) of tunnel at Meråker. The remaining sections of tunnel were excavated by drill and blast.

Geology

Rocks along the tunnel path consist of Cambrian and Ordovician metamorphic sediments with meta gabbro intrusions. In all, six different rock types exist along the tunnel route. These rocks include relatively soft phyllite; mixed face rocks such as greywacke and sandstone; and hard meta gabbro.

TBM

Robbins built the Main Beam TBM to successfully bore through varying rock conditions—from hard to mixed face. The machine featured four 335 kW (459 hp) motors and 25 front-loading disc cutters 19 inches (483 mm) in diameter. The TBM had cutterhead power of 1,340 kW (1,836 hp) and a maximum cutterhead thrust of 10,291 kN (2,313,531 lb).

The High-Power TBM was a stronger, more durable TBM than its predecessors. It was one of the first TBMs of a line that Robbins developed to withstand higher loads. These machines ushered in the new generation of TBMs with features such as triple axle main bearings and cutterhead modifications to improve boring efficiency at high thrust.

Tunnel Excavation

The TBM began boring at an impressive rate in September 1991. In its first full month of boring, the TBM excavated 1,029 m (3,376 ft). Soon after, the machine set a Norwegian tunneling record of 395 m (1,295 ft) during the week of October 28-November 3, 1991. During the course of the drive, the TBM reached rates of penetration up to 10 m (32.8 ft) per hour. Its average rate of penetration was 6 m (21 ft) per hour or 253 m (830 ft) per week. The machine’s best day was 100 m (329 ft), its best week was 427 m (1,400 ft), and its best month was 1,358 m (4,455 ft). The TBM accomplished these high advance rates despite Norwegian regulations limiting underground construction projects to 100 shifts per week.

In 1992 the TBM finished the 10 km (6.2 mi) tunnel six months ahead of schedule. The early completion allowed workers to finish all 44 km (27.3 mi) of tunnel in late 1992.

---

Tampa Bay

Project Overview

The South Central Hillsborough Intertie tunnel passes under the Alafia River in Tampa Bay. The tunnel is part of Master Water Plan Stage B, an ambitious three-stage plan to replenish Florida’s depleted groundwater.

Tampa Bay Water, the project owner, awarded Contract 2 for the South Central Intertie to Kenko Inc. in 2002. The contractor chose an innovative new solution to deal with the difficult ground conditions of the tunnel: a Robbins hybrid EPB shield machine.

Geology

The tunnel travels though the extremely permeable, highly fractured limestone of the Floridian aquifer with over 2.5 bar of hydraulic face pressure. Above the limestone is a layer of very stiff green clay and above the clay is a 4.6 m (15 ft) thick layer of loose, silty fine sand.

EPBM

The contractor chose Robbins because they needed design parameters that encompassed hard rock TBMs, EPBMs, and slurry shields. The EPBM featured eight double and four single backloading 17 inch (432 mm) cutters. The machine was capable of 5,783 kN (1,300,000 lb) of thrust and could generate a torque of 409,457 N-m (302,000 lb-ft) at the cutterhead.

Two face ports at each side of the machine permitted drilling and grouting in difficult conditions. Excavated ground was extracted with a 17 inch (432 mm) diameter invert auger screw. The muck was then conveyed to a mixing chamber to agitate and crush the limestone. The entire excavation system was a closed and pressurized face built to withstand up to 3 bar of hydraulic pressure.

From the mixing chamber, the muck traveled to a slurry pump installed inside the tunnel. The slurry pump transported the muck in a tube to the shaft where it connected with a second pump that brought the muck to the top of the shaft. The slurry pumps directly discharged the cuttings to the surface because the limestone was too porous to form a matrix. Therefore, the sluggish characteristics of the muck made muck transport via screw auger into muck cars too difficult.

Tunnel Excavation

The hybrid EPBM began boring on May 27, 2002. In the early stages of tunnel excavation, the machine operated in usual EPB mode and material was removed with muck cars.

As the machine continued boring, it encountered increasing hydraulic loads of up to 2.5 bar. This anticipated condition was treated with a circular break system that kept the EPBM in place.

However, the high water pressures prevented the excavated material from forming a plug in the screw conveyor. Injection of ground conditioning additives did not improve the situation and water inflow to the tunnel continued. The mucking system was then converted from muck cars to the more efficient slurry system involving slurry pumps.

After the conversion, the EPBM progressed well and broke through on August 22, 2002 only 3 mm (1/8 in) off of target.

---

South Mountain

Project Overview

Arizona’s South Mountain water tunnel is part of a 27.3 km (17 mi) water pipeline that delivers 178 million liters (47 million gallons) of water a day to the city of Phoenix. The pipeline serves the Ahwatukee Foothills area, which has experienced 145% population growth in the last decade.

In 2000, the City of Phoenix awarded the $11 million USD construction contract to Affholder Inc. Affholder chose a refurbished Robbins Double Shield TBM adapted for the variable rock conditions in the tunnel.

Geology

The tunnel passes through three types of rock. The first and last sections of tunnel consist of the hard, coarse-grained rocks granite and gneiss. The middle of the tunnel consists of two infiltrations of alluvial soils. This soil is mixed grain and exhibits secondary calcification. Locally known as caliche, these soils can exhibit behavior similar to rock.

To address the challenge of soil deposits, tunnel design consultants recommended steel ribs with lagging for tunnel support.

TBM

Robbins rebuilt the 2.4 m (7.9 ft) diameter Double Shield TBM for the variable rock conditions in the tunnel. The TBM design included dual propulsion systems, a protective shield, recessed cutters, and the ability to reverse cutterhead direction.

The TBM featured 432 mm (17 in) cutters and a maximum thrust of 4,372 kN (1,006,000 lb). The 60 Metric ton (66 US ton) machine was capable of generating 194,172 N-m (146,000 lb-ft) of torque.

Tunnel Excavation

The TBM began boring the 1.85 km (1.15 mi) long tunnel in December 2000 and encountered its first soil deposits three months later. The soft soil prompted a switch from rock bolts to steel ribs and wood lagging for the tunnel lining. The machine reached the second infiltration of soil after boring through a 360 m (1,181 ft) section of gneiss. The TBM excavated the final sections of the tunnel with no problems and finished on schedule in August 2001.

The crew worked in 9-hour shifts and the maximum rate of advance was 26 m (85.3 ft) in a single shift. The TBM average rate of advance was 1.5 m (4.9 ft) per hour.

---

Cleveland Sewer

Project Overview

The Cleveland Heights Interceptor Project is a sewer system improvement project located in the Heights/Hilltop area of Cleveland. The project is one of hundreds projects mandated by the USA Environmental Protection Agency in the wake of the Clean Water Act which required communities to improved the quality of America’s river and lake water. (For more information on the Clean Water Act and its effects, see www.epa.gov/r5water/cwa.htm).

During the torrential rains of spring and summer in the Cleveland area, the water flow overcame the existing sewer system resulting in flooding and the discharge of untreated water into Lake Erie. Larger-diameter pipelines were proposed to help reduce the basement flooding and sewage overflow problems which were common in the crowded urban area.

Geology

The TBM passed mostly through gray Berea sandstone that is fine to medium grained and significantly fractured in discrete zones. The rock has moderate to high abrasiveness as well as a potential for gas and water inflows from fissures and joints.

TBM

Robbins designed the 2.2 m (7.2 ft) diameter TBM to bore through fractured rock with potentials for water inflow. The TBM included a variable speed, hydraulic cutterhead drive and a high-capacity asymmetric main bearing. The machine also included roof and probe drills for rock support and pre-excavation ground investigations respectively. The TBM featured 12 inch (305 mm) cutters that could generate a maximum thrust of 4,472 kN (1,006,000 lb). Maximum cutterhead torque was 194,172 N-m (146,000 lb-ft). The 60-ton, gantry-style back-up system was designed to run efficiently with muck cars at maximum capacity.

Tunnel Excavation

The TBM began boring in July 1998. It experienced few problems and achieved impressive daily advance rates. Between the months of September and December 1998 the machine averaged 42 m (138 ft) per day and achieved a best day of 55 m (182 ft) bored. The machine finished in December 1998 with a monthly average of 442 m (1450 ft). Crews worked in 5-day work weeks with 2 shifts per day and the machine averaged 21 m (69 ft) per shift.

---

Big Sky

Project Overview

The Yellowstone Club, a private resort in Big Sky Montana, includes an 18-hole championship mountain golf course in addition to miles of ski trails. This golf course is irrigated by 318 ft (96.9 m) of pipeline from a nearby 79 million gallon reservoir.

In 2005, the project owner contracted Tunnel Systems Inc. to bore the pipeline. The contractor started out with an Auger Boring Machine with a Christmas tree’ head attachment. However, they ran into problems 59 feet (18 m) into the dig when they began boring through hard rock. The next two days of the bore cleared only 16 feet (4.9 m). For the rest of the dig, Tunnel Systems Inc. leased a Robbins 30 in (762 mm) diameter Small Boring Unit (SBU).

Geology

The section of pipeline through hard rock contained mixed ground conditions including sections of solid rock and mixed rock with soil.

SBU

Tunnel Systems elected to utilize a Robbins SBU because they are designed for bores just like this project — in rock with an Unconfined Compressive Strength (UCS) greater than 24,000 kPa (3.5 ksi). They utilized an SBU without stabilizer feet, available on 30 in (762 mm) and 24 in (609 mm) models. Stabilizer feet are standard on all SBU models 36 in (914 mm) in diameter and above, as well as available on some 30 in (762 mm) models. The SBU featured Robbins’ patented disc cutters to obtain the highest advance rates in hard rock. The SBU’s design is based on the same technology as the large-diameter tunnel boring machines.

Tunnel Excavation

At the jobsite, the SBU was welded to the lead pipe casing. In order for the boring to begin, the SBU received thrust from the pipe casing and torque from the Auger Boring Machine (ABM). Muck was then removed through the auger.

Upon restarting the dig, the SBU achieved impressive advance rates of 43-49 ft (13.1-14.9 m) per day. The ABM and SBU bored through solid rock for approximately 197 ft (60 m) and bored through mixed rock and soil for the final 20 ft (6 m).

The SBU finished the project on time despite some setbacks. Harsh weather conditions on the job site of 19 degrees Fahrenheit (-7 degrees Celsius) and 40 mph (65 km/h) winds made it too difficult to work and the bore was forced to halt until the snowstorm passed. The crew completed the project in just a few hours on the following day.

---

Chicago's Tunnel and Reservoir Plan (TARP)

Thirty TBMs participate in Chicago’s Epic Tunnel and Reservoir Plan (TARP)

Project Description

Spanning 20 years and using over 30 TBMs, Chicago’s massive Tunnel and Reservoir Plan (TARP) has been possibly the largest clean water project of the twentieth century.  The TARP was created in 1975 to combat increased flooding and drainage problems that plagued Chicago and surrounding areas. After heavy rains, combined sewage overflows (CSOs) would seep into residential basements, nearby streams and rivers, as well as Lake Michigan – Chicago’s main source for drinking water. TARP was originally divided into two phases but now refers solely to Phase I.

Phase I

Phase I was directed towards pollution control and consisted of tunnels, drop shafts, and dewatering stations to eliminate nearly 85 percent of CSO pollution. Four tunnel systems comprise the first phase: Mainstream, Des Plaines, Calumet and O’Hare, which have a combined length of 176.1 km (109.4 mi) and range in diameter from 2.4 m to 10.8 m (8.0 ft to 35.4 ft).  Nearly all of the tunnels were excavated in the area’s dolomitic limestone, and required the use of TBMs up to 10.8 m in diameter—the largest TBMs that had ever been built at the time.

The Mainstream Tunnel System was composed of 65.2 km (40.5 mi) of tunnel, the Des Plaines System with 41.2 km (25.6 mi), the O’Hare System with 10.6 km (6.6 mi), and the Calumet System of 59.1 km (36.7 mi). A later tunnel for the Little Calumet Leg of the Calumet system was excavated using a Robbins TBM in 2002, setting multiple records in the process including 138 m (452 ft) bored in one day. Immediately upon completion, each tunnel system was put into service and the benefits were seen almost instantaneously. After more than 30 years and over 160 km (100 mi) of tunnels, the entire first phase of the TARP system became operational in 2005.

Phase II

Phase II, now called the Chicago Underflow Plan (CUP), consists of three main reservoirs: the Majewski Reservoir, Thornton Reservoir, and McCook Reservoir with a combined capacity of 69.05 billion liters (18.24 billion gallons). The reservoirs are a joint project of the Water Reclamation District and the U.S. Army Corps of Engineers, built to provide flood relief by storing the water collected and transferred from the TARP tunnels until it can be treated at local reclamation plants.

Excavation and Breakthrough

Construction on the Majewski CUP Reservoir was started in 1990 and finished in 1998, with a capacity of 1.29 billion liters (342 million gallons). The Thornton Reservoir was divided into two stages, including a transitional reservoir completed in 2003, and a permanent CUP reservoir completed in 2015. The permanent CUP reservoir has a capacity of 29.9 billion liters (7.9 billion gallons) and provides an estimated $40 million annually to 15 communities. Finally, the McCook Reservoir, also planned as a two-stage build, will provide storage of up to 38 billion liters (10 billion gallons). The first stage was completed in 2017, while the completion date of the second stage is scheduled for 2029. To date, the reservoirs have yielded hundreds of millions of dollars in flood damage reduction benefits.

The TARP program has won numerous awards and honors over the years from the local and federal EPA, as well as the American Society of Civil Engineers award in 1986 for the most outstanding civil engineering project.   The tunnels and reservoirs have resulted in a dramatic improvement to water quality in Lake Michigan, and have eliminated CSO overflows.  Fish and wildlife have returned in recent years to local rivers and to Lake Michigan, and the waterfront property is becoming more attractive to businesses and the general population alike. 

---

Pahang Selangor Raw Water Tunnel

Trio of TBMs bore Malaysia’s Largest Infrastructure Project

Project Overview

The Pahang Selangor Raw Water Tunnel, for the Malaysian Ministry of Energy, Green Technology, and Water, conveys raw water from the Semantan River in Pahang to the South Klang Valley area of Selangor state. The three tunnels, totaling 44.6 km (27.7 mi), address projected water shortages due to the area’s rapidly growing population. The tunnel transfers 27.6 cubic meters (7,300 gallons) of water per second to a new treatment plant. The drinking water supplies about 7.2 million people.

The SNUI JV, consisting of Shimizu Corporation, Nishimatsu Construction, UEM Builders Bhd, and IJM Construction, chose three Robbins 5.23 m (17.2 ft) diameter Main Beam TBMs to excavate the three sections of the tunnel. The total supply included back-up systems, continuous conveyors, cutters, spares, and field service personnel.

Ground Support and TBM Design

Tunneling took place in high cover conditions, up to 1,200 m (3,900 ft) below the Titiwangsa mountain range. Geology during the initial stages of advance consisted of hard, abrasive granitic rock up to 200 Mpa (29,000 psi) UCS. The tunnels were supported with ring beams, rock bolts, and shotcrete depending on the conditions. If unstable ground was encountered, invert thrust systems could be utilized to avoid gripping against the tunnel walls.

To successfully excavate the hard rock, each High Performance (HP) TBM was fitted with 19-inch (482 mm) back-loading disc cutters- making them the smallest diameter back-loading cutterheads ever provided. The cutters were carefully monitored for wear using remote monitoring systems. The wireless systems allowed the crew to plan cutter changes and keep track of wear by recording several variables on each cutter, including cutter rotation (which is computed to percentage wear), temperature, and vibration.  Each 19-inch face and gage cutter was equipped with a sensor bolted inside the cutter housing, allowing raw data to be sent to a program display in the operator’s cabin.

Tunnel Excavation

The first machine was launched on November 10, 2010, followed shortly after by the second on December 30, 2010. The third machine began boring in March 2011 and all three machines are currently boring as scheduled. All of the machines were assembled outside their particular adits, then “walked” down a 6-10% grade to an NATM-excavated starter tunnel. Two of the machines were launched with a shortened back-up configuration of 10 decks and a temporary transfer conveyor, while the third for logistical reasons utilized trucks for muck removal in the preliminary boring phase. Once the machines had advanced about 100 m (330 ft), the remaining back-up decks and permanent Robbins continuous conveyor were then installed, due to the adit configurations.

During the initial stages of advance, the machines achieved rates of up to 3.5 m (11.5 ft) per hour, leading the three machines to excavate over 1,400m (4,600 ft), 540 m (1,800 ft) and 330 m (1,100 ft), respectively by April 2011.  As each TBM continued on its 11km (6.8 mi) run, the machinery had to overcome adversity including blocky rock, over-break, power outages and water inflows.

The machines maintained excellent advance rates throughout the project despite many challenges. Due to the hot springs the machines were boring under, water ingress at temperatures up to 56 degrees Celsius was recorded.  Maximum rates of 49 m in one day, 198 m in one week, and 657 m in one month were nonetheless achieved.

Among other methods of ground support used during boring, the near-zero rebound fiber mortar (sprayed shotcrete) is the primary method being used during the Pahang Selangor Project.  This marks the first time this method has been used outside of Japan.  The success of this innovative implementation has been proven through reduced project downtime, dust reduction and good bonding.

Breakthrough

The first of three 5.23 m (17.2 ft) Main Beam Robbins TBMs broke through at the Pahang Selangor Raw Water Tunnel on March 22, 2013 to a large ceremony of cheering onlookers. The breakthrough was attended by dignitaries, contractors and honored guests, with everyone very enthusiastic about the machine’s success.

The two remaining 5.23 m (17.2 ft) machines met in the middle of the tunnel in mid-February 2014. It was a moment worthy of celebration; marking the completion of the longest tunnel in Southeast Asia.

---

Pinglu Tunnel

Veteran Double Shield completes one of the World’s Longest Single-Drive Tunnels

Project Overview

The Pinglu Tunnel, part of the Yellow River Water Diversion Project, was undertaken in 2006 by Joint Venture Sino-Austria Hydraulic Engineering Co. Ltd (SAHEC), led by Alpine Bau GmbH. At 24.5 km (15.8 mi), the Pinglu Tunnel marks one of the world’s longest single-drive TBM tunnels ever excavated. The entire scheme will transfer water from the Yellow River to dry regions of Shanxi Province, an area that receives just 400 mm (16 in) of rainfall per year on average.

The completed Pinglu Tunnel will go into operation in October 2011, connecting the North Main Line of the Yellow River Project to transfer water to Pinglu, Shuozhou, and Datong areas. The South Main Line of the Yellow River Water Diversion Project was completed between 1999 and 2001, which encompassed over 100 km (62 mi) of tunnel excavated using five TBMs, four of which were Robbins Double Shield TBMs.

TBM Design

The Robbins Double Shield TBM excavating the Pinglu Tunnel was previously used on the record-breaking 12 km (7 mi) long segment of the Yellow River Diversion in 2000. During that project, the Double Shield set two world records in its size class of 4 to 5 m (13 to 16 ft): best month (1,855 m/6,085 ft) and monthly average (1,352 m/4,435 ft). Both records still stand.

Since the TBM was used on a prior tunnel for this project and designed for similar geology, only the back-up system was modified. Due to the length of the tunnel, the back-up frame was extended from one stroke to two strokes. This key change allowed the machine to maintain good advance rates despite 70 minute transit times for muck trains from the machine to the tunnel entrance.

Tunnel Excavation

The machine began boring at the remote jobsite on September 30, 2006.  Tunneling was a challenging process, as the geology consisted of 12 m (40 ft) thick coal seams and abrasive sandstone that required intensive monitoring of tunnel air for particulates.  Up to 70% quartzite content made the rock very abrasive. This combination of 70% quartzite and 6% corundum made the rock seven times more abrasive than quartzite—equal to the material that grinding wheels are made of.  This required rigorous maintenance of the cutterhead with a daily 4-hour shift, and replacement of the bucket lips.

Muck removal was by trains of rota-dump muck cars in two tracks using California switches.  The back-up system was equipped with floor chain movers to shunt the muck cars as they filled. Ventilation in the long tunnel was generated at a minimum rate of 5.4 m³/sec (190 ft³/sec) by high-powered fans.  The fans, situated at the portal, deliver fresh air to the tunnel face via 1.4 m (4.6 ft) diameter flexible ducting.

Lining for the Pinglu Tunnel, which consisted of unique hexagonal segments, was produced near the jobsite by Alpine. A crew of nearly 400 people worked at the remote site and segment factory to cast the specialized structures. During excavation, the segments were placed longitudinally in a honeycomb configuration in rings of four elements which allowed high-speed, continuous boring with no downtime while erecting segments. Advance rates topped out at 50 ring sets, or about 70 m (230 ft), per day.

On November 13, 2010, Alpine celebrated the Robbins machine break through with a crowd of more than 500 including Austrian and Chinese guests of honor and the entire tunneling crew.

---

Chengdu Metro Line 2 Lot 18

Robbins EPB sets Record Rates in Chengdu

Project Overview

Chengdu’s Metro Line 2 includes 26 stations and 17.6 km (10.9 mi) of tunnels between Longquandong and Shiniu areas of the city. Seven lines totaling 274 km (170 mi) are planned to be operational by 2035, and will service 13.1 million daily passengers.

The contractor, CRCC Bureau 23, selected a Robbins EPB with a mixed ground cutterhead for the potentially variable conditions, as well as the back-up system, soft ground cutting tools and spares.  The machine was launched in January 2010 to bore two 1.4 km (0.9 mi) sections of parallel tunnel, with a breakthrough at the midway point into an intermediate station. The tunnel alignment allowed the machine to pass 25 m (82 ft) below residential buildings, and included several curves with a minimum 400 m (1,300 ft) radius.

Geology

The tunnels for Lot 18 of Line 2 are located in highly variable, permeable alluvium, stiff sand, and clay, requiring a unique EPB TBM design and careful monitoring for settlement. This complex alluvial geology is unlike that found anywhere else in China. Cobbles averaging from 20 to 80 mm (0.8 to 3.1 in) in diameter were predicted, with diameters of as much as 120 mm (4.7 in) possible.

Machine Design

The mixed ground, spoke type cutterhead was mounted with Tungsten carbide knife-edge bits and seven 17-inch (432 mm) diameter disc cutters around the gauge. A foam injection system was used to stabilize the running ground, allowing each cubic meter of foam mixture to stabilize about 40 rings of ground. Subsidence was intensively monitored and crews were trained to utilize probe drilling and ground consolidation if settlement was detected. Variable frequency (VFD) drives allowed the cutterhead rotation to be kept low (around 1.5 RPM at maximum) to also minimize surface settlement. High advance rates were instead achieved using increased cutterhead torque, which results in a faster rate of penetration. One-liquid type back-filling grout was used to fill the gap between segment lining which consisted of 300 mm (12 in) thick reinforced concrete segments set in a 5+1 arrangement.

Tunnel Excavation

In June 2010, the machine had broken through into the intermediate station, approximately 1,397 m (4,583 ft) into the 2.7 km (1.6 mi) long tunnel. Following scheduled maintenance, the machine was relaunched to bore the remaining section of the tunnel. Cutter wear was very minimal, with only three cutters changed since the start of boring.

By the time of tunnel completion in December 2010, the machine had achieved a project landmark of 129 m (423 ft) in one week, and 459.5 m (1,507 ft) in one month – higher rates than at least 4 other machines working on Line 2 in similar geology.

---