The IM-Rebirth solution successfully overcame some unexpected difficulties to reconstruct approximately 1.2 km of sewer pipeline, the first pipe-eating project of this scale to be carried out in Japan.

The sewers to be reconstructed were located at the end of the downstream, immediately before the Monzenmachi wastewater management center. The pipeline was buried below National Route 249 at a depth of 4.2 metres down to 6.3 metres. The Hakkagawa River runs along the road and there were many cracks observed at the river wall. It was predicted that the whole area was affected from the ground deformation due to the earthquake. Many existing manholes had been dislocated due to the water pressure. The type of soil was water-bearing stratum, which seemed to have increased the warping of the ground. Even from above ground, it was clear that the sewers had suffered displacements.

Choosing the right technology After considering the level of sagging and displacements, CIPP was deemed an inappropriate method to restore the sewers, therefore the choice was made to reconstruct the whole pipe. There were two options for reconstruction; open cut and trenchless. Open cut was rejected as the pipeline was too deep to excavate. Moreover, taking into account the ground water level and economic efficiency, the trenchless method was selected as the optimum solution. An alternative was to install a pipeline next to the damaged pipe using the standard microtunnelling method. However, microtunnelling needs disposal treatment of the existing sewers including removing manholes up to the 1.5 metre level. Considering all of these factors the pipe eating method was selected. The types of existing sewers were;

1. Humes pipe for open cut construction 2. Humes pipe for microtunnelling construction – including stainless steel collar 3. A Humes pipe as a casing pipe – PVC pipe of 200 mm diameter was placed inside.

Pipe eating method A, from the current reconstruction method listed in the guideline issued by Japan Microtunnelling Association, was chosen to crush and remove the existing damaged pipeline (pictured). The IM-Rebirth method

The IM-Rebirth method is based on traditional machines, such as Slim-Ark TA50 and the Iron Mole series, which can excavate hard cobbles and rocks. The difference between these machines and the IM-Rebirth method is the cutting head with a special cutter, spiral or gear type, which can crush the existing reinforced concrete pipes and reinforcing steels bars. This method not only crushes the existing pipe but also removes the pieces. This results in a newly installed pipe. The cutter head can also crush railroad ties and stainless collar used between the pipes.

The jacking head torque is high enough to crush the hard cobbles and rocks located around the pipes. The starting shaft can be very small. For pipe diameters of 250 mm up to 300 mm only a 2 metre shaft is required, and for pipe diameters of 350 mm up to 500 mm a 2.5 metre shaft is required.

The IM-Rebirth method was selected because of the cutter head's ability to crush reinforced pipes and SUS (stainless) collars. However, as the project progressed, it was revealed that a spacer of 5 mm thickness was used inside of the casing pipe for placing VU 200 mm. To negotiate this problem, an excavation test was carried out before the actual construction commenced. The test showed that although the jacking speed had slowed down greatly and the cutting face was abraded a little from the excavation, the machine can drive forward despite the presence of the spacer.

Some parts of sewer were completely out of service and temporary bypassing was carried out. Several drives were progressing at the same time; therefore a well-planned bypassing was necessary. Bypassing of the current live sewers was carried out by using the proven technique for long-distance sewers called Mr. Auto-bypass. The Mr. Auto-bypass has working records of 17.5 km and out of this record, 3 – 5 km were bypassed simultaneously while constructing several, mainly open-cut drives.

Shaft construction

The existing manholes were damaged from liquefaction and needed to be replaced. Therefore, the spaces for replacing manholes were used as departing and arrival shafts. In 300 mm diameter pipes, a shaft diameter of 2 metres was used and for 350 – 400 mm pipes a shaft diameter of 2.5 metres was used, which contributed to reduced construction costs.

Infilling existing pipes

The existing pipes suffered displacement, sagging as well as some infiltration of waters and surrounding soils. It was anticipated that the area around the machine excavation could be a void and cave-ins might occur after the construction. To avoid this problem, infilling of existing pipes was carried out in order to prevent cave-ins. The filling material was a cement-mortar.

Removal of pit-head

The waterproofing equipment and sheet piles used for the first installation, buried in place, needed to be removed. The open-face jacking of 1,000 mm diameter was used to remove this equipment at the beginning of the drive and then removed manually.

The cutter head

The cutting head, equipped with the special cutter to crush reinforced pipes, progressed smoothly along the planned paths. At the peak, seven jacking machines were driving simultaneously. The crushed pipes, taken in by the jacking machine, consisted of materials such as reinforced steel bars, SUS collars, waterproofing rubbers and PVC pieces. Railway ties placed below the pipes were also found in small pieces. The most difficult section of excavating the 400 mm casing Humes pipe and 200 mm VU pipe plus spacers was carried out successfully, although the driving speed did not reach the planned rate. The discharged soil was treated as industrial waste since it contained steel pieces, concrete, VU and others.

In total, 21 drives were carried out and there were no serious problems requiring the driving be halted. The special cutter equipped at the cutting face had some ablation from cutting spacers, but was able to continue the drive.

The pipe eating [and jacking] method is normally used to replace aged pipes with new pipes. However, this construction was carried out to replace existing pipes damaged greatly from the earthquake. Many of the damaged pipes had suffered displacements and sagging and some sewers had lost function completely. Komatsu’s first pipe-replacing technology was achieved in 1995, the time of Great Hanshin-Awaji earthquake, proving that this method could be used for replacing pipes that have experienced large displacements. Smooth bypassing had also contributed a great deal in making the whole reconstruction successful. Much research has been conducted on this method. The record length achieved by this method stands at 1.2 km. The Wajima City construction case has proven the effectiveness of this method, showing the ability to replace greatly damaged pipes using trenchless solutions.

This method can also be an optimum method for solving combined sewer problems or a new installation due to redesigning of the pipe diameter, changing the inclination, and more. Komatsu continues to conduct research in order to solve minor problems found in this construction case and hopes that this method will be widely recognised as an optimum method for replacing reinforced pipe.