Microtunneling, HDD Used to Replace Shallow Utility Mainlines
By Robin Dill, Ken Watson and Eduardo A. Vega
The Miami-Dade Water and Sewer Department (M-DWASD) currently owns and operates a 20-in. ductile iron water main (WM) and a 54-in. force main (FM) constructed with prestressed concrete cylinder pipe that traverse below the Fisherman’s and Government Cut Channels. The WM conveys finished, treated water from the mainland to Fisher Island and the Port of Miami (POM) for distribution. The FM carries sanitary sewage collected from Miami Beach and locations north to the Central District Wastewater Treatment Plant (CDWWTP) on Virginia Key for treatment, crossing under Fisher Island. Fisher Island is an exclusive residential community that includes a golf course and marina.
The existing WM and FM were originally installed by the cut-and-cover method. They pass under the navigable sections of Government Cut and Fisherman’s Channel with burial cover depths ranging from about 5 to 20 ft below mudline. These two pipelines are in conflict with the planned Miami Harbor Dredging Program that will deepen and widen the main channels by up to about 20 ft to accommodate the Post-Panamax “Super” Ships. Installation of the replacement pipelines must be completed and the pipelines placed in service by the end of 2012 in advance of the official opening of the expanded Panama Canal. Due to the aggressive schedule, M-DWASD is using design-build delivery for the pipeline construction.
M-DWASD retained AECOM in 2009 to assist in collecting the necessary data, developing the conceptual design, acquiring the necessary permits and easements, and in procuring a design-build team to complete the work. AECOM planned and executed an intensive field investigation program including bathymetric, benthic and geophysical surveys; along with geotechnical borings in the Government Cut waters on Fishers Island and within the POM.
The subsurface conditions along both the FM and WM alignments were found to be quite similar, consisting of the Fort Thompson formation, a very porous coralline limestone with inclusions and layers of loose to medium sand. Up to six distinct layers were found with significant variability in the vertical direction. The limestone bedrock layers of this formation are characterized as karstic, exhibiting vugs and solution cavities of various sizes.
The Fort Thompson Formation bedrock layers exhibit extremely variable intact and rock mass properties. The intact core samples can have UCS values ranging from as little as 50 psi to as much as a 12,000 psi, although typical strengths are below 4,000 psi. The Fort Thompson formation is part of the Biscayne aquifer, a surficial aquifer system in southeastern Florida, and is classified as one of the most permeable aquifers in the world due to a combination of solution pipes, vugs and matrix porosity. In the project area, the maximum in-situ permeability of the Fort Thompson bedrock was found to be on the order of 5 cm/s, making this ground some of the most challenging geology in the world for microtunneling.
Early in the data collection phase of the work, it was intended that the FM would be constructed in a 12 ft ID utility tunnel, just over 1.5 miles in length from CDWWTP to a location adjacent to the existing 54-in. FM in the City of Miami Beach. At that time, pressurized face soft ground tunneling methods were contemplated as being necessary to deal with the extremely pervious ground, as is being used for the much larger POM tunnel project located about 1 mile to the northwest.
However, after several public information meetings were held with the residents of the City of Miami Beach, it was apparent that the land acquisition for easements would be met with extreme opposition, and could potentially lengthen the time needed for the necessary approvals. Therefore, it was necessary to locate a retrieval shaft in the Government Cut waterways, outside of the navigable channel, and immediately south of the City of Miami Beach. A suitable location with minimal environmental impact was found for the retrieval shaft and subsequent underwater connection to the existing main. As the subsurface lands below waterways are state owned, and the shaft site is within M-DWASD’s easement for the connection, land acquisition from the City is not necessary.
The concept put forth by AECOM in the design-build design criteria documents, involves the use of microtunneling and jacking a steel casing from a launch shaft site to a retrieval shaft site, both immediately adjacent to the existing pipelines. Jacking lengths for the WM and FM are about 850 and 1,175 LF, respectively, with casing diameters of 60 in. and 84 in. The new pipelines are to be installed and grouted in the casings, with risers within the shafts, and connections to the existing pipelines adjacent to the shaft locations. The existing pipelines must remain in service until final connections are made.
Given the high ground permeability, from a safety perspective, microtunneling was considered to be the most appropriate method because it can be accomplished without man entry. A closed-face MTBM that applies positive pressure to the tunnel face and its perimeter was required for the project. It was anticipated that either an earth pressure balance (EPB) or slurry pressure balance (SPB) machine could be used, although SPB type machines are most prevalent in the marketplace. The adjacent POM tunnel is being driven using an EPB TBM, but at a much larger diameter.
Construction of the FM retrieval shaft was considered to be one of the most challenging aspects of the project. Not only did this shaft serve as a TBM retrieval shaft, but the shaft also had to be enlarged and constructed over the footprint of the existing FM. This would provide sufficient area to excavate and expose the existing FM, followed by tapping into and connecting to it. The existing FM is located with an invert about 20 ft below mudline. This challenging overwater work requires construction of an integrated, impervious excavation support and bottom slab system through pervious sands and limestone without the use of dewatering. This work also has to be accomplished without damaging or impacting the existing FM that has to remain in service.
To best manage the risks associated with microtunneling, it was necessary to impose rigorous experience requirements into the procurement process. Furthermore, the M-DWASD selection criteria could not rely solely on price. With AECOM’s help, the design-build procurement process was structured accordingly, to be a two-step “best value” approach that selected the design-builder on the basis of both technical solution and price. Instead of price being the primary factor in selection, the more important considerations were meeting schedule and mitigating risks through innovative design and construction features. The Request for Proposal (RFP) was prepared to promote innovation through providing submittals of “alternate proposals” from the competing D-B firms, at their option, if proposers also submitted a base proposal in full compliance with the RFP and agreed to perform the contract base proposal if the alternate proposal was either not accepted by M-DWASD or proved infeasible.
The selected team consists of Ric-Man Construction with Michels Corp. for microtunneling. As part of its proposal, the Ric-Man team had proposed the use of HDD for installation of the WM by Mears Group. This alternative was accepted by M-DWASD, in particular because it saved considerable time in not requiring any shaft construction, although it does require a different and larger staging area. Ric-Man reached an agreement for the lands on Fisher Island, and with assistance from M-DWASD, an agreement was also reached for the required area within the POM. It was necessary to move these staging areas farther inland on both Fisher Island as well as on the POM property, in order to assure that the bore paths would clear existing seawalls.
The HDD work for the WM replacement has recently been completed, including a pilot hole and successive bores necessary for installation of the 24-in. ID HDPE carrier pipe. The pilot hole was completed from the POM site and the reaming operations were completed utilizing two drill rigs, one for drilling and the other for pulling. The carrier piping was strung on Fisher Island and pulled into the bore.
The Ric-Man team is in the process of constructing shafts for the FM microtunnel between Fisher Island and the Miami Beach marina area. The contractor is using secant pile wall construction for the launch shaft on Fisher Island, the original concept anticipated in the design criteria documents. The plan is to first grout the soil before auguring a ring of 42-in. diameter secant piles. Once completed, the contractor will excavate in-the-wet inside the secant pile wall. A 21-ft diameter corrugated metal plate liner will also be installed in the excavation in sections and centered within the secant pile structure. A reinforced concrete plug will then be constructed in-the-wet to resist buoyancy forces. Finally, the annular space between the steel liner and secant wall will be grouted to form a composite structure, before the shaft is unwatered and the microtunneling can begin.
Concurrent with launch shaft construction, the contractor is beginning cofferdam construction for the Fisherman’s Channel retrieval shaft. This work involves construction of a 13 ft finished diameter shaft, just large enough to remove the MTBM in two pieces. This shaft will be constructed in the same manner as for the launch shaft. A second sheet pile cofferdam structure will be built over the footprint of the existing FM, to allow the riser piping from the microtunnel to be connected to the existing main. This work has only just begun, but obviously will need to be completed prior to the completion of the microtunnel drive.
As stated earlier, the entire project must be complete by the end of 2012. Based on current progress of construction, it appears to be on course to an on-time completion.
Robin Dill is with AECOM based in Wakefield, Mass., and Ken Watson is with AECOM based in Miami. Eduardo A. Vega is with the Miami-Dade Water and Sewer Department.