Trenchless Technology

NOV 2018

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Page 49 of 68

W W W.T R E N C H L E S S T EC H N O LO GY.C O M 49 es between design elevation and in- stalled elevation could more easily be accommodated (and deviations could be re-designed) on the south side be- cause the open-cut portion could be re-aligned to accommodate. Once the risks associated with the soil and groundwater conditions were identified, design drawings and speci- fications were written to reduce the potential adverse effects of identified risks. There were three primary ways the drawings and specifications at- tempted to mitigate risks and empha- size the importance of grade control and careful construction practices: 1. The drawings indicated that the launch pit be on the north side where the grade control of the gravity lines was most important . 2. The specifications required daily settlement monitoring of the highway. 3. The specifications included language stating that the grade of the two gravity lines is "critical to the hydraulic performance" of the system. 4. The specification required the contractor to either meet the line and grade requirements (within tolerances) or pay for re-design of the piping system ( including bearing all schedule and cost impacts of the re-design). Construction At the start of construction, the contractor requested launch pit place- ment and pipelines installation from the south side of the embankment . After some discussions, the contrac- tor was permitted to install the 36-in. pressure pipeline from the south side; however, the gravity lines were re- quired to be installed from the north side as per the contract drawings. The first casing installed was the 36-in. pressurized intake pipe. Over the course of the crossing , 12 in. of grade was lost . While the lost of grade for the pressure pipeline was concern- ing , it did not impact functionality. However, concerns were raised about grade control for the next two instal - lations of gravity pipelines. The con- tractor elected to move launch opera- tions to the north side of the highway because soils on the south side ap - peared wetter/looser than the soils on the north side. It was hoped that more competent soils in the launch vicinity would provide better grade control . The 30-in. casing lost 12 in. of grade (mostly in the last 40 ft of in- stallation). For the 48-in. casing , the contractor set the pipe at a reverse grade ( i .e. inclined to account for grade loss). Within the first 13 ft after launch, the reverse grade was lost . At this point the contractor requested to switch to auger boring. Vibrations as- sociated with pipe ramming combined with the combined weight of the casing and soft soil within the casing may have caused the grade loss. Auger boring eliminates vibrations and removes the weight of soil from within the casing. There were still over-excavation con- cerns during auger boring , but the con- tractor 's request was allowed. At com- pletion of the auger bore installation for the 48-in. casing , there was grade loss but within the acceptable tolerances. Conclusion A number of lessons were learned from this project . In particular, soil plugs do not always form at the lead- ing edge of a pipe ram. This is espe- cially true in loose, saturated soils that can liquefy and flow under cy- clic dynamic loading . Design consid- erations should be made to account for this possibility and specifications should require the contractor to have a contingency plan. This contingency plan may be as simple as placing sand bags in the pipe prior to ramming . Other lessons learned were to have flexibility during construction, to listen to the contractor, and to be prepared to make changes based on the actual conditions encountered in the field. A design is based on the information available at the time of design. With three pipelines being installed within essentially the same footprint, the best information we get is from the previous installations. The contractor anticipat- ed possible challenges during construc- tion due to the very soft soils and pro- actively set-up the launch pit with the flexibility to switch between ramming and boring. With a good contractor, it is very helpful to be able to discuss options and changes to construction based on soil information and soil be- havior learned during construction. Michelle L. Macauley, P.E., is principal engineer and owner at Macauley Trenchless. Figure 2. Profile View (profile courtesy of MWH Americas)

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