For every underground construction megaproject, there are as many small, typically unsung but critical components as there are large, more newsworthy sections. Yet without these smaller components, the overall project may not be able to proceed. The District of Columbia Water and Sewer Authority’s (DC Water’s) Clean Rivers Project is no exception.
The $2.6 billion infrastructure and support program involves installation of a series of large-diameter CSO tunnels and diversion facilities designed to capture and store overflow during periods of heavy rainfall and transport it to the Blue Plains Advanced Wastewater Treatment Plant for processing before discharge to the local waterways.
The Anacostia River Tunnel is one section of the overall system. The 23-ft diameter tunnel is approximately 12,500 ft long and runs from the Robert F. Kennedy Stadium in Northeast D.C., under the Anacostia River, and on to the Poplar Point Pumping Station in Southeast D.C. Three critical utilities above three separate areas along the proposed tunnel alignment were identified by DC Water and Impregilo-Healy Parsons (IHP), the joint venture design-builder for this project, as being particularly vulnerable to settlement during TBM advancement beneath.
Settlement-induced damage at any of these locations would create an unacceptable interruption to vital city services. IHP therefore specified a program of compensation grouting to be in place ahead of the TBM reaching those parts of the alignment to compensate for unacceptable movements if they were to occur.
Compensation grouting is a general term given to the procedure of replacing soil volume lost during tunneling in order to mitigate the potential settlement of sensitive structures (or utilities in this case) within the influence zone of the tunnel. Compensation grouting is performed by injecting grout in selected soil zones between the tunneling operation and the at-risk structure above and, if necessary, to induce heave within this zone and thus compensate for anticipated or actual settlement. In such cases, a high mobility grout is injected through pre-placed Tube à Manchette (TAM) pipes to hydro-fracture the soil.
Pre-conditioning of the soils prior to actual compensation grouting is a critical first step in the process. The pre-conditioning phase is intended to consolidate and improve the target zone to the point at which additional injection will result in heave/upward movement of the target soil zone. This step is typically performed in advance of the arrival of the TBM to ensure that heave can be induced in a timely manner when settlement is first observed. The pre-conditioning program generally consists of repeat, limited-volume injections of grout paired with extensive monitoring of the target soils and overlying structures to determine the point at which the soil zone has been adequately improved/pre-conditioned and is therefore ready for the compensation phase of the program when the TBM does arrive.
A total of four sensitive utilities were identified in three separate areas – a 108-in. force main, a 48-in. water main and, at the third location, 30- and 42-in. water mains. The utilities all lay within the fill soils overlying the sites and were at relatively shallow depths, with their inverts less than 10 ft below ground surface. The CSO tunnel alignment was located in the Potomac soil group, which consists of interbedded layers of over-consolidated fat and lean clays and layers of silty and clayey sands with N values in the range of 30-60. Compensation grouting was specified to be performed in the upper reaches of the Potomac formation in a 20-ft thick target zone approximately 10 ft above the crown of the proposed tunnel, depending on the location.
The threshold settlement criterion was set by the owner at 0.25 in. Grouting was specified to be initiated when 0.1 in. of movement was observed, as measured by extensometers installed above the crown of the proposed tunnel. Specialty grouting contractor Moretrench’s crews and equipment were required to be on site around the clock whenever the TBM was within approximately 100 ft in either direction of the utility crossing, underscoring the critical nature of the operation.
A trial was conducted at one of the production grouting locations under the direction of IHP to verify the design assumptions and determine the parameters and amount of effort required for pre-conditioning ahead of production compensation grouting. A cluster of production grout pipes was drilled in place surrounding and extending below extensometers installed in the soils above the crown of the proposed CSO tunnel. Pre-conditioning grouting was performed utilizing repeated, limited-volume injections of grout (16 gallons per injection port). Positive displacement pumps injected the grout at a fixed rate while allowing pressure to fluctuate based on the resistance provided by the soil. A maximum pressure criterion of 1,000 psi was utilized for pressure refusal.
The trial program was successful in determining the amount of effort required for pre-conditioning as well as inducing sustained movement on the extensometers within the identified target zone. This demonstrated that the soils would be conducive to compensation grouting.
Compensation Grouting Program
The selected compensation grout mix design was engineered to have a high solids content and provide a low ultimate strength to permit re-injection of the ports as necessary, and consisted of cement-based grout, bentonite as a fluidifier, and an inert filler. The resulting low viscosity material proved ideally suited to the soil conditions and equipment utilized.
Well ahead of the TBM reaching the area, arrays of evenly spaced TAM pipes were installed at all three sites and pre-conditioning was performed. Similar to the trial, production compensation grouting was performed using limited-volume injections of just 16 gallons per port to ensure even distribution of the grout across the target zone and to control the induced heave. Injections were sequenced in a split-spaced pattern along each TAM pipe, with every other port injected as a primary and the remaining ports injected in a secondary pass.
Structural settlement potential is an inherent risk in soft-ground tunneling projects. At the locations identified for this project, even minor settlement would impact the continuous performance of the utilities that are vital to the communities they serve. IHP was proactive in addressing the global picture and specifying compensation grouting early in the planning stage. As a result, movement of the sensitive utilities was limited to well below the threshold settlement criteria at all three locations during TBM mining.