Known as the Tianqingyuan Residential Community Sewage Reconstruction (TRCSR) project, the work comprised the installation of 171 m of 450 mm diameter PE pipe to form an on-line and on-grade combined rainwater and foul-water municipal sewer beneath the Tianqingyuan Residential Community, which is situated on the South Fangzhuang Road, Beijing.

The new sewer runs in an easterly direction under civil premises and roads, from its starting point in Sink Well No. 11, which is located to the west of the community. The pipeline is designed to have a 0.2 per cent slope towards the downstream end of the route with an average design depth of some 5 m.

Being the first trenchless sewage duct installation of its kind in Beijing, it was also the first project on which BMMCT used its NAVIGATOR® HDD machine, manufactured by Vermeer. The machine was purchased through Balama Prima - Vermeer (China) of Hong Kong, one of Vermeer’s independent and authorised equipment distribution dealerships in Southeast Asia.

Challenging installation

New installation planning was always going to be a challenge, as existing information revealed a web of cables, natural gas pipelines, water supply pipelines, and sewage pipes already running along and across the proposed drilling route. There was also a network of high-voltage cables running both over and under the bore path, which had the potential to significantly influence the function of the Digital Control Eclipse Locator that was to be used as the guidance system for the D55x100A drilling rig. This, of course, was in addition to the demands of the bore itself in terms of on-grade performance and accuracy requirements.

The location and influence of existing services were not the only cause for concern during the planning process. Ground conditions along the bore route were also known to be extremely complex, running through stratum at Weizikeng on South Fangzhuang Road comprising backfilled garbage and soil as well as building waste, which ran from the surface down to approximately 5 m depth. Beneath the made-up ground was a flowing sand horizon. Ground water was also known to exist in some zones along the bore route.

However after careful examination of the ground conditions, existing services, and the pipeline specification, it was believed that, despite the challenges, the HDD solution offered by BMMCT could achieve the outcome the project required.

Installation

Installation of the pipe along a route to the east of the Tianqingyuan Residential Community required it to pass beneath a block of civil housing yards, a retail shop, and adjacent residential communities. Preliminary work, including final ground investigation surveys to pinpoint underground obstacles, equipment mobilisation, and temporary facilities construction. Just three days were then needed to complete the pipeline installation, with a further two days for equipment removal and site restoration.

Work started on 15 June 2005 with the equipment mobilisation and other preliminary preparation works. With completion of the preparation work, pilot hole boring started on 18 June. After just 60 m of the 171 m run, however, the pilot bore intersected an unknown gallery beneath one of the civil housing yards. The signal from the drill head was shielded within the gallery and so control over both the elevation and the direction of the bore was lost. The drill rods had to be retracted from the bore. It was decided to start a new pilot bore some 1.5 m to the north of the original route.

In order to ensure that the new bore did not intersect the same gallery as the first bore, engineers from Balama worked closely with BMMCT’s operators to re-plan the route. A decision was made to increase the overall bore depth by 200 mm. Commenting on the change of plan, Jian-Qiu Guo, chief engineer of BMMCT, said “It is extremely difficult to control the duct grade at a depth of 5.2 m, which is only 200 mm above the 5.4 m depth of the flowing sand horizon. Thanks to the close cooperation of Balama’s engineers and excellent instructions, the contractor’s careful piloting, close coordination with the rig operator, and experience gained from the first attempt, the new pilot bore was successfully completed in just ten hours on 19 June.”

Even after the late finish, early the next day the construction crew was back on site again to complete the installation work, despite daytime weather conditions as hot as 39°C.

Three back-reaming passes were used to increase the pilot bore diameter to that required for installation of the 450 mm diameter product pipe. Reams of 300 mm, 400 mm, and 600 mm respectively were completed with each pass. The backreaming operation was completed on 20 June including a cleaning pass with a 600 mm diameter clearance bit. The pipe pullback formally commenced at 10.20 p.m. the same day and in the early hours of 21 June, the 171 m length of PE pipe was in place and the work was accepted by the client, marking the successful completion of the project.

The pipe, supplied by Jin De Pipeline Group of Beijing was delivered to site. The 171 m length required for the installation was laid out on the pipe side of the bore with sections being butt-fusion welded.

Bore support

Each night installation work had to stop after 10 p.m. to avoid disturbing the local residents. Drilling fluid was used throughout the drilling operation but, with respect for the back-reaming operations, Balama advised increased use of bentonite mud at a rate of about 4 to 5 packs per 1,000 gallons of water. The drilling fluid was prepared using a Vermeer MX250 mixing system.

By using this amount of bentonite any potential for collapse of the borehole, particularly during the overnight downtime period would be avoided. In this way the bore shape was maintained.

The skilled application of the drilling system by the drill rig operators from BMMCT and its guidance team, and the support and back-up supplied by Balama engineers, ultimately delivered the first HDD-based trenchless installation of a new sewage pipeline in Beijing, to both a very acceptable quality and efficiency. Using a drilling rig new to the contractor and in very challenging ground conditions a long-term problem of how to supply a sewer service to the Tianqingyuan Community has been overcome.

With the installation experience gained from this project, BMMCT has achieved its first major success in its trenchless installation business.