The Brightwater regional wastewater treatment system in King County, Wash. is currently under construction and is slated for completion in 2010. The Brightwater project, the third wastewater treatment system for the greater Seattle area, has been in the planning stages for many years.
The need for a third wastewater treatment facility for the region was predicted as early as the 1950s with forecasted population growth and the resulting demands on the current infrastructure. Follow-up flow estimate studies conducted in the 1990s projected that the current wastewater treatment system would run out of capacity by approximately 2010, thereby spurring the new project into action. Subsequently, the project was officially launched in 2005.
The Brightwater project includes construction of a new 36-mgd treatment plant, 16 miles of pipeline tunneling, plus a marine outfall that discharges effluent from the Brightwater Treatment Plant into Puget Sound. When completed in 2010, the Brightwater facilities will treat the sanitation requirements of residents and businesses in northern King and southern Snohomish counties.
Due to existing soil conditions, it was clear that construction of the conveyance system at multiple sections of the Brightwater project would require extensive ground modifications in order to assist with tunnel boring machine (TBM) operations and hand-mining operations at vertical access shafts. As a direct result of the region’s numerous glacial and non-glacial cycles the soil conditions ranged from sandy alluvium to very dense glacial till, thereby resulting in a complex interbedded stratigraphy with a wide variety of soil properties. Moreover, the required tunnel break-in/break-out depths ranged from about 70 to 85 ft below ground surface at shaft locations founded in granular soils with near surface groundwater tables. Consequently, implementation of a ground improvement scheme in conjunction with carefully sequenced tunneling stages would be imperative to ensure a safe working environment.
Five portal sites along this alignment were identified as locations where the tunnel boring machines would be launched or retrieved for mining of the four major tunnel sections. Two portals — North Creek and North Kenmore — would become the sites of ground modification. It was found that these two portals, though located barely two miles apart from one another, had dramatically different geologic profiles within each tunnel face at the TBM breach points.
Jet Grouting Approach
Based on preliminary evaluation of the soil conditions, it was determined that jet grouting would be the most appropriate ground modification method to satisfy both technical requirements and the project schedule needs. Jet grouting is a versatile ground modification system that creates soilcrete (soil-cement) in-situ by delivering a stabilizing grout mix through high pressure nozzles at the end of a monitor inserted into a borehole. The soilcrete is created by slowly withdrawing and rotating the monitor at a constant rate, thus eroding and mixing the soil with grout and compressed air.
Extensive field investigation and lab analyses would be undertaken as part of the project’s quality control program. This included test sections for each geographically unique area to verify design geometries, vertical continuity and material properties. Two separate jet grout sections along the tunnel alignment were tested in order to confirm that the initial jet grout construction parameters were sufficient.
Employing a SuperJet grouting technique — a modified double-fluid jet grouting procedure that takes advantage of tooling design efficiencies and increased energy to create high-quality, large-diameter soilcrete elements — a proprietary multi-chambered drill rod system designed by Hayward Baker was used to deliver neat cement grout and air to the jetting tool. The high-velocity coaxial grout stream and air combine to erode the in-situ soils. This process is assisted by the use of opposing nozzles and a highly sophisticated jetting monitor which focuses the injection media. Shrouding the grout jet stream in a sheath of air increases the erosional efficiency of the process thus enabling construction of larger diameter columns.
Considering that columns can be constructed up to 16 ft in diameter, jet grout test areas can be a significant portion of the complete ground improvement program. At the Brightwater project, Hayward Baker’s test areas at the North Creek Portal and the North Kenmore Portal were incorporated into the final production work, using careful sequencing of both column installation and test data retrieval to enable rapid determination of jetting performance.
North Creek Portal Test Section
The North Creek Portal jet grout test section consisted of six columns, the first three of which were constructed with a conservative set of “baseline” grouting parameters. Constructed on a triangulated nine ft center-to-center spacing with column tops 10 ft below grade, the columns were excavated to visually verify the achieved column diameters and geometric overlap. Excavation revealed that the intended 11-ft diameter geometric design was well-surpassed and that substantial column overlap had been achieved.
The second phase of the test section continued with three additional test columns that were integrated into the production work. Three different sets of construction parameters were utilized to construct columns 41 ft in length with a bottom depth of 80 ft and a top elevation of 39 ft below existing grade. The bottom third of each column was constructed using the same conservative baseline parameters as the first three test columns. The middle third increased the rotation speed and withdrawal rate, while the top third utilized yet less conservative parameters.
Three days following construction, the interstice of the three columns was cored. The core samples allowed for a visual inspection to gauge continuity with respect to depth in correlation to the three separate construction parameter sets. Furthermore, core samples enabled testing for UCS and hydraulic conductivity. All three parameter sets yielded satisfactory results. Ultimately, it was decided the initial baseline parameters offered the correct degree of conservatism and were the most appropriate parameters to apply in view of the existing soil conditions and construction constraints.
North Kenmore Portal Test Section
The soil conditions at the North Kenmore Portal did not permit the use of shallow test columns for physical inspection through excavation. Furthermore, jet grouting would have to be conducted within a very dense glacial till at depths up to 94 ft below the working surface. For this reason, test columns were constructed using a larger jet nozzle orifice, thus increasing the flow rate and introducing greater mixing energy. The original plan was to verify the column geometry and continuity as well as in-situ soilcrete strength by coring the overlap of three columns and the overlap of two columns at two separate locations after allowing at least seven days of cure. However, the construction schedule demands did not permit the required curing time prior to verification coring and thus it would need to be delayed until after construction of the ground improvement program. As the schedule could not accommodate pre-production verification coring, it was decided to take wet soilcrete samples from the grout return along the drill tool annulus while jetting the test columns in order to more quickly provide information about soilcrete properties. After casting these samples into 2-by-4 and 3-by-6-in. cylinders and allowing them to cure, they underwent UCS and hydraulic conductivity tests, which indicated that the strength and permeability were well within acceptable limits.
Upon completion of production work, final verification coring was performed at three separate column interstice locations to determine the achieved column geometry. The three core holes indicated that the 24-ft tall jet-grouted soilcrete blocks had two lenses, approximately 1-ft thick, of non-grouted clay above the tunnel spring line and crown, respectively. Fully intact, undisturbed non-grouted clay was recovered through coring and subjected to testing. The nongrouted lenses were determined to be very stiff dark lean clay (CL) with a plasticity index of 13 percent and a 1.3 x 10-7 cm/sec. coefficient of permeability. Given the fact that the material had remained intact within a core barrel designed to extract soilcrete, concrete or rock, the conclusion was that this non-grouted interbed inclusion of clay was sound and competent – and thus would not require remedial grouting.
Verification of Test Results
The jet grout ground modifications conducted by Hayward Baker at Brightwater have been verified by probe holes through the tunnel launching and receiving zones, as well as by hand-mining. All UCS test data indicated that the results exceeded the required 300 psi (in some cases by an order of magnitude) within the zones of sandy gravel. Permeability limits were met in the laboratory and in the field. Subsequent mining indicated very minor and controllable groundwater inflow rates. In all, it provided a clear path forward for jet grout operations throughout the entire Brightwater conveyance system.
Richard Hanke is a Project Manager with Hayward Baker Inc. in Seattle, Wash.
Michael Blanding is a Field Engineer with Hayward Baker Inc., Seattle, Wash.
