Chilliwack Pipeline Crossings: Combining Pilot Tube Method and Pipe Ramming

A utility owner required the replacement of 600 m of an existing 762 mm (30-in.) OD mainline to meet class location changes under CSA Z662-11 Pipeline code. The existing right of way (ROW) and temporary work space will be used for construction. The replacement pipe section crosses through commercial land and the adjacent 914-mm (36-in.) OD mainline loop will be in service during construction. The unique location and proximity of the existing line created a challenging work environment.

Horizontal directional drilling (HDD) and microtunnelling were considered; however, the geological conditions and construction space requirements made these two options unrealistic. The pipeline replacement sections are located in Chilliwack, B.C., through the parking lots of commercial shopping areas and major arterial roads within Chilliwack. Kamloops Augering & Boring Ltd. (KAB) proposed to use a pilot tube system to install 914 mm (36-in.) steel casings for the replacement pipeline. The trenchless portion was broken up into three sections:

1. 83m (272 ft) Luckakuck Way East
2. 138m (453 ft) Vedder Road
3. 147m (482 ft) Luckakuck Way West

KAB was working with a pipeline contractor that was responsible for the open-cut sections and installing the 30in. pipeline.

Because of the two existing high-pressure, large-diameter gas pipelines and other underground utilities within the right of way, the geotechnical investigation could only be carried out with hydro-vac excavation from surface to 6 m deep. Using a hydro-vac for geotechnical analysis leaves room for interpretation due to the added water during the process and no method of determining the density of the material. All that was known was that it is silty sand and gravel with cobbles, loose to medium dense.

The crossing lengths were longer than typically designed and accepted for the pilot tube method of trenchless construction, in fact the two longer crossings were the longest ever installed to date in Canada. The subsurface conditions, as per the geotechnical data report provided, included loose to medium dense sand to sand and gravel soils with varying depths of ground water. Because of KAB’s newly acquired Akkerman 4815 guided boring machine, the company was comfortable tackling the long crossing requirements with this pilot tube system. These conditions seemed to be favorable for a pilot tube followed by an auger bore installation.

KAB made use of slide rail trench shoring systems for our jacking and receiving pits. The slide rail system is ideal for long open spans with no cross members to accommodate installing 40-ft sections of casing. The depths of the crossings were around 4.5 m. The dewatering for the slide rail systems was very successful in controlling the ground water, so much so that the ground became very hard.

After a successful pilot tube installation, KAB was approximately 5 m in on the first crossing with the auger bore machine when it was noted that jacking pressures had spiked and pilot tubes had stopped moving on the exit side. The transition weld-on adapter from 18 in. to 36 in. had failed. The 36-in. pipe began to engulf the lead transition piece of 18-in. pipe. The casing was cleaned out and confined space procedures were implemented to investigate and repair the apparent problem.

KAB had to remove the 36-in. pipe to replace the transition adapter to a different style of 36-in. weld-on ream head. To do so, KAB had to ram a larger diameter pipe over the existing pipe to remove the damaged transition piece and the entire 36-in. pipe previously installed. A new lead pipe was prepared and pushed into place and re-connected to the pilot tubes.

The continued installation procedure with the new weld-on head did not provide the anticipated relief in jacking pressures that was desired. Work continued until it became apparent that the auger boring methodology would not advance the casing. Because of KAB’s large inventory of trenchless equipment, crews were able to switch to pipe ramming without delay and this would now become the preferred method of installation.

The guided auger boring execution plan had to be revised to incorporate the pipe ramming method to install the casing pipe due to the ground conditions encountered. KAB proposed to use TT Technologies’ pneumatic pipe ramming system as it can accommodate a wide variety of soil types (cobble, soft rock, shale, compacted gravels, etc.). The pipe ramming process with the weld-on reaming head would allow for the larger rocks to enter the casing (“swallowed up”) as it moves forward. The revised plan had to be approved by the National Energy Board. With the approvals in place, the first crossing was then completed using KAB’s Taurus hammer.

An intermediate jacking pit slide rail system that was 4 m wide x 17 m long x 4.5 m deep would be implemented as a common pit that KAB used to install the casings in both directions along the proposed alignment. This is where KAB would complete the two longest shots. However, during the installation of this larger slide rail system, KAB encountered wood during the excavation procedures. The wood debris was approximately 450 mm (18 in.) in diameter and 1 m (3 ft) in length. Some wood remained at the end of the pit as crews were unable to remove it. This wood served as a reminder of what could potentially be encountered during pilot tube installation, as any wood obstruction could potentially stop the pilot tube installation. Old existing helical pipeline anchors were also excavated and noted as potential obstructions.

Fortunately, the pilot tube installation was very successful and KAB’s operator successfully set a new in-house record for length of a guided bore. It should be noted that at about the 120-m mark the optics for a pilot tube crossing typically becomes the limiting factor. With the line and grade issues resolved, it was time to continue with the casing installation.

With approximately 48 m of casing installed on the second crossing, the rate of advancement started to recede and the Taurus hammer was deemed inadequate. Again, because of KAB’s large inventory of equipment, it was able to bring in an Apollo pipe rammer without delay. This is the largest available pipe rammer manufactured by TT Technologies. This provided temporary relief but at approximately the 88 m mark the casing was beginning to slow down again even with the large hammer. KAB proceeded to auger out the casing to investigate the face of the bore and found that several of the old pipeline anchors were being dragged. KAB believed that in addition to anchors being dragged, some wood debris was attached to the weld-on head. KAB was instructed to continue to ram, and was able to successfully install the casing until completion. When the casing pipe exited the ground in the receiving pit it was apparent that there were anchors and woody debris wrapped around the head, which is what caused the slow penetration.

The third and final crossing was very difficult to pilot tube. The guided bore machine was at maximum torque and thrust and almost impossible to complete. This indicated to KAB that the ground had a SPT ≥ 50. The difficulty of the previous crossing coupled with the increased length was a cause for concern. The Apollo hammer was able to advance the casing to approximately to the 85 m without incident. However, the continued rate of advancement was hindered to the point that KAB had to investigate the weld-on head.

The casing was cleaned out and confined space procedures/permits were executed to see what was impeding the casing installation. Four pipeline anchors were wrapped around the casing pipe. KAB removed them and the casing was advanced to approximately the 100 m mark. There were significant concerns about potential damage to the Apollo hammer with the minimal advancement of the casing. The pipeline contractor was able to hydro-vac a 1-m slot trench to provide relief. Once the trench was excavated, the pipe was rammed into place, this slow process was continued until the crossing was completed. This was not the preferred method, but was the only remaining option to complete the crossing.

The use of the combined technologies of pilot tubing and pipe ramming proved successful and could change design considerations for pipeline installations in highly congested areas.

Harry Dickinson, AScT, is project coordinator for Kamloops Augering & Boring Ltd., which is based in Kamloops, B.C.