Microtunneling in Hawaii
The Emergency Beachwalk Force Main Construction Bypass Line project involved the construction of a temporary contingency by-pass force main between the Beachwalk WWPS in Waikiki and a 69-in. diameter trunk line that runs parallel to the large Ala Moana Shopping Center.
Phase I of the project, which was completed on July 31, 2006,used a 48-in.,SDR17 high-density polyethylene (HDPE) pipeline that intercepts gravity flows to the Beachwalk WWPS and portable diesel pumps along the Ala Wai Canal to move the wastewater more than a mile to the gravity sewer line that begins just inside the Ala Moana Beach Park.
Phase II is in progress and involves a 1,180-ft river crossing underneath the Ala Wai Canal and under Kaiolu Street, with two parallel 46-in. Steel casings installed using microtunnel methods. The first of the dual microtunneling crossing was completed in early March 2007. Hobas centrifugally cast glass fiber reinforced pipes (CCGFRP) were slipped into the host steel casing (Permalok) and,when connected to the Beachwalk pump station, will serve as part of the active force main pipeline conduit. Currently,grouting of the annulus space and testing of the installed pipeline is in progress. The second crossing was scheduled to begin in April 2007.
In May 2006 and within days after the City and County of Honolulu decided to install the emergency by-pass line, the design-build project team was formed. Leading the team is the Department of Design and Construction (DDC), in collaboration with the Department of Environmental Services (ENV). The prime civil/environmental engineer and planner is R.M. Towill Corp. (RMT), supported by geotechnical and trenchless engineers from Yogi Kwong Engineers LLC (YKE), structural engineers from KAI Hawaii (KAI), electrical consultant Ronald Ho & Associates and Underground Services Inc. for wet well and pipeline CCTV inspection. The construction manager is M&E Pacific Inc. (M&E). The general contractor is Hawaiian Dredging Co. (HDCC), supported by microtunneling/jet grouting contractor Frank Coluccio Construction Co. (FCCC). Healy Tibbitts Builders drove the sheet piles for the microtunneling launch shaft and laid the underwater pipeline for Phase I.
The dual microtunnel canal crossings begin at the north end of the Ala Wai Canal near the Ala Wai Park Community Gardens,traverse under Kaiolu Street and the extremely busy Kuhio Avenue intersection and end at a receiving and valves connection shaft built adjacent to the Beachwalk WWPS. The emergency temporary Beachwalk force main will be in place until the remainder of the permanent reconstructed Beachwalk WWPS force main is constructed and serviceable.
The initial challenges faced by the project team that had to be resolved almost immediately included:
- Microtunneling alignment and profile
- Single force main pipeline vs. a dual force main pipeline for the microtunnel segment
- Use of a jacked casing or direct jacking of sewer force main pipes
- Shaft construction methods, exact locations, sizes and depths
- Groundwater control methods, shaft wall and bottom stability
- Potential impact to marginally stable historic canal walls
- Microtunneling machine requirements and availability
- Locating the existing underground utilities along the proposed route
Based on available existing boring data and past project experience in Waikiki,YKE developed an engineering geologic profile to aid in the determination of the above design and construction requirements. The initial engineering geologic profile was verified and refined by concurrent geotechnical exploration. The microtunneling alignment was selected to provide as much set back as possible from the only high-voltage underground electrical duct line that supplies electricity to Waikiki, and at the same time, stays within the City’s right of way and existing available easements. The microtunneling profile was selected to avoid any need for ground improvement work to be performed in the canal, which would have triggered substantial environmental protection and permitting requirements, and to avoid performing jet grouting work near the canal walls and on the busy Ala Wai Boulevard.
The lagoonal deposits are typically very soft or very loose to soft in this area,and the microtunneling boring machine (MTBM) steering in these highly compressible deposits would be problematic. To stabilize the soils along the selected alignment, a minimum of 3-ft diameter, dual overlapping jet grouted columns spaced 10 ft apart were prescribed. Continuously overlapping jet grout columns were not specified to reduce project costs. Instead, the Hobas force main carrier pipe (with a 50-year warranty) was required to be designed and manufactured to bridge between the jet grout columns, assuming that the steel casing will eventually be severely corroded over time.
YKE recommended the launch shaft be supported by interlocking steel sheet piles. Pre-drilling and the use of a non-vibratory driving hammer were prescribed to minimize potential pile driving vibration that could impact the nearby historic canal wall built on very soft alluvial silts, and the key electric transmission duct supporting Waikiki, located only a few feet away from the shaft.
Based on the preliminary borings, it was initially hoped that the medium stiff to stiff weathered tuffaceous sediments, consisting mainly of low plasticity clays, silts and cinder sands and river pebbles inter-beds would be sufficient to provide a stable bottom groundwater cutoff for the shaft. However, as force main pipe fitting requirements developed, the shaft depths increased to approximately 40 ft below ground surface and 36 ft below mean sea and canal level. Finite element analysis performed by YKE indicated the sheet pile embedment depths and jet grout perimeter walls would deform excessively under the conditions considered. Therefore, a jet grout bottom plug was prescribed to provide ground water cutoff and to improve the shaft bottom stability.
Jet grouting of the interbedded tuffaceous sediments was anticipated to be a difficult process. Pre-drilling was prescribed for the perimeter jet grout columns, and then later extended to other parts of the shaft bottom. In areas where pre-drilling was not performed or not practical, a test program was designed to establish pre-cutting and jet grouting parameters and nozzle configuration, both in size and numbers to install the 36-to 40-in. Diameter jet grout columns in medium stiff to stiff sediments using the single fluid method.
To minimize the potential adverse impacts on the existing active wastewater pump station and old force main on Kaiolu Street, a combination of drilled piers and jet grout/micro-piles were prescribed to support the reception shaft wall. Within the shaft, a jet-grouted bottom plug was required and completed. Pre-drilling was not required at this location due to the presence of highly compressible lagoonal deposits in a significant portion of the jet grout zone.
The initial design and construction planning and ordering of pipes and sheet piles began in early June 2006. Mobilization of FCCC’s Iseki MTBM began in July 2007. Jacking shaft construction started in September 2006 after the completion of the Phase I work and was completed in November 2006.The reception shaft for the first drive was completed in January 2007, and the first drive was launched in early February 2007 and completed in early March. The maximum jacking load recorded during the first 1,180-ft drive was approximately 200 tons. The automated bentonite lubrication system supplied by Microtunneling Inc. seemed effective, and became particularly important when a steel obstruction was encountered by the MTBM about 600 ft from the launch shaft. The jacking operation and MTBM advance was stopped for several days while the steel obstruction was removed. During this stoppage, FCCC’s microtunneling crew periodically injected bentonite slurry outside the jacked pipe segments while the obstruction was being removed.
At the end of March, the Hobas carrier pipe was jacked inside of the steel casing and the annulus grouting was completed.