GBM/Thrust Bore Project Within Port Botany for ELGAS



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Scope of Works

The UEA Trenchless Division was contracted to complete a 120 metre under bore and install a 457mm OD steel case enveloper pipe with a 10mm wall thickness. Following the steel case insertion UEA had to install a secondary enveloper pipe in the form of a 355mm SDR21 PN8 PE100 Pipe on metal centralisers with the annulus between the PE pipe and the outside steel enveloper grouted. Finally UEA had to install the client supplied and welded LPG pipeline on client supplied centralisers within the PE enveloper pipe.

Techniques Used

An Akkerman Laser Guided Boring Machine (GBM) was used to undertake and complete a successful pilot bore on grade and within 25mm tolerance agreed at design stage. The GBM was also used to pull the pilot tubes during the augering and steel insertion process to ensure the pilot tubes were kept under constant tension to avoid sag in the bore hole.

A McLaughlin conventional Auger Boring machine was used to install the steel enveloper and the secondary PE enveloper pipe.

Dewatering Spear Points and sediment tanks were hired to manage the water within the launch and receipt pits.

The Works

This bore presented numerous challenges from the onset – not only was it a grade critical 120 metre case bore in sand but it was also located in water charged tidal affected ground.

In preparation for this bore UEA decided to undertake the pilot bore with the GBM from the normal position i.e. downstream shooting uphill and to set up the auger borer and install the steel case from upstream i.e. bore downhill. As noted above, this technique enabled UEA to guarantee grade accuracy by using the GBM to pull the pilot tubes whilst installing the steel case. The normal process in other ground conditions is to create one launch and receipt pit where the GBM and Auger Borer shoot from the one pit i.e. the launch pit. With this methodology UEA had to excavate a slightly larger receipt pit than normal, however the advantage of using the GBM to pull the pilot tubes outweighed the disadvantage of excavating a larger receipt pit.

UEA’s launch pit for the GBM was 4.5m deep with dewatering spears installed connected to a sediment tank to manage the water and keep the pit reasonably dry.

UEA commenced the pilot bore on the 17th September with the client’s representative on site along with their designers. UEA got out to 16m and hit and non-penetrable obstruction. Upon a full investigation it was found there were 2no. large 1.5m diameter storm drains running down either side of Charlotte Road that was not shown on any of the Dial Before You Dig’s or Designer information. This resulted in UEA being stood down for 2 days for the design to be reviewed and changed.

The revised design changed the grade and involved UEA lowering the GBM launch pit by 2m. This change in depth had a massive impact on the water within the pit. UEA had to double the number of dewatering spears and bring in a second 6” pump and 15,000ltr sediment tank. The water intake was so great for UEA to manage that dewatering spears had to be installed within the pit as well as around its perimeter. When the dewatering system was completely installed UEA had 45 spear points installed, 2no. 6” Pumps and 2no. 15,000ltr sediment tanks and when up and running it was recorded UEA was removing 10ltrs of water a second for the pit..

The Pilot Bore

Once the dewatering system was onto top of the water ingress UEA commenced the pilot bore using the standard cutting head for the GBM designed for this type of ground with a 15mm over cut. When UEA reached 17m which was beneath the first of the two storm drains UEA lost half their target and both rotation and thrust pressure dropped indicating they had entered unstable soft water saturated ground. UEA decided to continue with the pilot bore with half a target because if half a target can be kept for the entirety of the bore they would only lose 12.5mm and complete within the allowed tolerances.

Unfortunately when UEA reached the additional storm drain at 40m the thrust pressures increase dramatically which is a result of entering stiffer/harder compacted sand. In normal circumstances, this is not a real problem and can be managed. The problem UEA faced was when they increased their thrust pressure to get through the harder ground the pilot tubes would bow under strain within the non-supporting ground beneath the first storm drain and UEA wold loose the complete target which prevented them from proceeding. UEA tried 3 different drill heads to resolve the issue and hopefully complete the pilot bore but all attempts failed between 40 & 50m.

To complete the pilot bore UEA used their HDD experience and technology to devise a means to complete the pilot bore. To do so UEA improvised by building a crossover fitting in house which enabled the GBM pilot tubes to be connected to a sonde housing and rail head drilling head and with this installed UEA envisaged they would be able to use a Digitrak walkover device to steer the bore to completion and on grade. This idea and technology worked and UEA completed the pilot bore on the correct alignment and finished 17mm low which was within the allowed and acceptable tolerance.

Steel Case Insertion

When the pilot bore was complete UEA installed the 120 metres of steel case in 3.5 days utilising the GBM as mentioned above. The steel case when installed was on the agreed alignment and finished 18mm low which was accepted by the client.

Completion

UEA completed the remaining works within 5 days which was to install a secondary PE enveloper pipe, grouting the annulus, installation of the client’s product pipe, backfill and compaction of both pits.

Handover

Delighted with UEA’s capabilities to overcome issues, the client was very satisfied with the completed works. The LPG Pipeline installed was commissioned and commenced operation 10th November.