Harker Underground Construction wasestablished in 1986. From the outset, the company focused primarily on pipejacking, using hand techniques to install short lines under railways and mainroads. But by 1998, Harker had begun using mechanical means to install thoselines. Recently, the company provided another update to its operations, focusingon a contract awarded by the Wellington City Council in New Zealand.
Today,Harker has six tunnel boring machines, five of them built by its sister company,Tunneling Technologies, which was set up in 2001 with the specific purpose ofbuilding TBMs and underground equipment.
The original tender for the Harris Street contract called for the trenchlessinstallation of some 630 m of 1,350 mm concrete pipe at depths of 3 to 5 m,almost entirely through old reclamations completed between 1880 and 1964. Theroute was to extend from Wellington Harbor, through busy downtown Wellingtonstreets to low-lying areas in the town center. Borehole information indicatedthat the reclamation varied in depth between 5 and 16 m, but the majority of itwas roughly 5 m thick. Beneath the reclamation was the old beach, then marinesediments, sandy silts and some gravely sands. Further into town, the sedimentswere underlain by moderately weathered Greywacke rock.
During the tender consolidation period and after further groundinvestigations, Harker proposed a deeper route beneath the reclamations and oldbeach, through what was hoped would be more consistent and reliable groundconditions. It was known that the area had been developed and changed over theyears, with photographic information and historical reports indicating that oldwharfs and building foundations could be in the area and old ships werereportedly buried in the reclamations. Consequently, a new, deep alignment wasagreed with Wellington City Council (WCC) that would be up to 16 m beneath thesurface and 14 m below sea level. Over most of the line, the ground wassaturated and tidal in a range of 1.4 to 3 m below ground level. Hydrostaticwater pressures would therefore be of concern during the machine design and itwould have to be designed to withstand 2 bars of external pressure.
WCC asked Harker to take on the entire risk for natural ground conditionswhile City Council would retain the risk for unnatural conditions. The verticalalignment was defined within a 2 m envelope within which Harker could change thelevels, depending on the actual conditions encountered during the shaftconstructions. Council also agreed to allow a major alignment change during theconstruction to eliminate some short drives of less than 50 m if a cost-benefitcould be demonstrated and ground conditions permitted it. This eventually led toa new alignment, called the Bond Street variation.
Also during the tender consolidation period, the machine was being consideredas more ground information came available and the most desirable machine typewas selected. This turned out to be a closed-face slurry machine, as this typeof machine offers the most certainty in varied ground conditions. Furthermore,Harker already had a separation and guidance system from an earlier machine andthese could be modified to suit a new machine. Harker was also aware of an oldIseki machine that had been sitting in a yard for 10 years and was able to bepurchased at a reasonable price. This machine was totally unsuitable, as itfeatured old technology and was grossly underpowered, but it had a good shelland face. These were the only salvageable parts and the rest was completelyrefitted by Tunneling Technologies. The size was 1,500 mm ID so a proposal wasmade to WCC to increase the pipe size at no extra cost.
The first drive was a 100-m, hand-dug pipe jack up Bond Street through marinesediments and, for the latter 30 m, Greywacke sandstone rock and progress ratesof about 3 m per day were achieved.
Once the hand drives had been completed in Bond and Princess Streets,attention was then focused on preparation for the first machine drive — adistance of 180 m between MH4 at the top of Harris Street and MH5 in BondStreet. Service relocations were tagged out of the Harker tender submission, asthis had become a challenge for Council, with many unidentified, incorrectlymarked and poor condition services. These were all in the unstable, saturatedtidal zone, meaning constant and significant pumping was required to keep theexcavations dry.
The method of shaft construction employed was to sheetpile through the upperweaker layers into more competent ground that would seal around the bottom ofthe sheet piles. This worked, but not without some considerable effort. Thelower sections of the shafts had to be sealed with concrete walls and cast tothe ground, enveloping the lower few meters of the sheets. Excavation in theseconditions was slow and the concrete sealing walls had to be cast in sections asthe ground permitted. The MH4 shaft took two months to construct and was 9 mdeep.
Once the shaft was completed, an entry ring was installed to launch themachine and the new machine was installed. Where there was meant to be marinesediments, Harker encountered an open graded stretch of cobbles. As the groundwas porous, large quantities of slurry were lost into the ground and themachine’s automatic systems compensated by increasing the flows. Harkereventually overcame this problem by putting additives in the slurry, constantlytopping up the slurry and installing a booster pump in the delivery line toincrease the flows.
Harker then started getting fragments of concrete over the shaker screens.For a short period, the machine jammed and it took a day to get it moving again.The line passed within 5 m of the library building. Once the machine had passedthe building, a lot of timber was encountered.
Things took a turn for the worse when steel strapping started coming throughthe system and the machine jammed. It was released after a day, but damage tothe machine itself was suspected. The final 30 m of the line encountered stillmore timber and steel and it was suspected timber had jammed onto the side ofthe pipe. This caused the jacking loads to spike and reach the maximum allowablefor the pipes (670 tonnes). By this stage, the installation had turned into a24-hour operation; the average production over the line was 6 m per day, with abest rate of 24 m.
Once the machine was recovered, the head was found to bealmost choked with timber and steel. One large lump of steel was found in thecrusher. Bearing end plays were checked and the machine was found to havesustained damage to the main bearing and gearbox. Consequently, the machine hadto be sent back to Auckland, New Zealand, to be stripped down and repaired whilethe next drive
During that time, the largest and deepest shaft of the project wasconstructed in Harris Street. By now council had become nervous aboutencountering steel and started to fully appreciate the implications of having aTBM stuck 16 m beneath congested downtown streets beneath major infrastructureservices. Consequently, extensive additional ground investigation work wasundertaken from the Harris Street shaft using directional drilling and sonicGPR.
Two machine drives remained to complete the project, the first being 113 mthrough mostly weathered rock, up Harris Street to MH4. Eight directional drillsand three radar surveys of the line were done. Anomalies were found, but nothingconclusive. It was decided that since the anomalies were in rock, findingconditions similar to the previous drive would be unlikely, so the machine wasinstalled.
The first section of the drive went well, but unfortunately a seal betweenthe second and third cans blew when Harker was only half way. This led to aspectacular flooding of the line; this was brought under control by the siteteam and the seal was replaced underground and the seal was redesigned.
Once this drive was completed, attention then focused on the last and mostdifficult drives to the outlet. Council carried out more directional drillingand GPR, finding four areas of significant concern. Harker proposed that thefinal drive be completed by hand, as the ground conditions at the lower depthswere found to be better than expected and sealed out the groundwater. The drivecommenced in a 24-hour continuous operation. The drive was 140 m long and tookfive weeks to complete. The entire project was completed in November 2004.