UC&T/UCA: Tunnelling in the Nation’s Capitol

Professional engineer Bill Edgerton of Jacobs Associates came to the only student chapter of the Underground Construction Association (UCA) in the country to talk about the Combined Sewer Overflow (CSO) tunnel project in Washington, DC. Edgerton is a principal on the D.C. Clean Rivers Project, of which the CSO is a part, and the chair of the UCA.

The Clean Rivers Project is a multi-billion-dollar endeavour, projected to run until 2025, to stop sewer overflow during storms from going into the three rivers that flow through the D.C. area, which is the current destination for sewer overflow for a third of the region. It will also help mitigate extreme flooding from even relatively minor storm events. The slowest-moving of the rivers concerned, the Anacostia, is being tackled first. Four major CSO tunnels are being constructed, at a cost of $3.5 billion, to tie into and divert storm overflow from the city’s sewers. The longest of these, the twenty-seven-thousand-foot-long Northeast Boundary Tunnel, is still in design. Before construction can begin, an environmental impact assessment must be done. This involves more than just “environmental” impacts: the effects of the project on traffic, pollution, and even the local archaeology must be considered. As a result of such an assessment, a smaller tunnel now under construction (and the shortest of the four at only 2700 feet long), the First Street NW Tunnel, is being constructed using a different method from the other three since it is located in a residential area and the equipment used for the conventional construction method was considered too disruptive for the residents. The twelve-thousand-foot-long Anacostia River tunnel crosses directly underneath the presidential helicopter hangar, so its environmental impacts are also of prime importance – especially potential subsidence, which can damage surface structures.

The majority of Edgerton’s talk, however, focused on the Blue Plains Tunnel, which is planned to be twenty-three feet across and twenty-three thousand feet long, and which is the closest to completion of the four CSO tunnels. In order to dig this monumental construction, a massive tunnel boring machine (named Lady Bird after President Lyndon Johnson’s wife) was brought in, weighing over seventy-five thousand tons. Two massive shafts built by the river to screen and de-water the overflow serve as the launch point for Lady Bird; once the shafts were completed, a special crane lowered the boring machine into the hole and set it on its way.

Edgerton discussed two parts of the construction process: the building of the shafts and the preservation of existing structures affected by the Blue Plains CSO, specifically a football-field-sized sewer pumping station that has been operating since 1900 and several of the station’s contemporary sewers, which are of historical as well as practical significance. Because the turn-of-the-century sewers cannot be replaced or shut off, damage to them would have been near catastrophic.

The two shafts – a hundred-foot-diameter pump shaft and a somewhat smaller screening shaft – were originally designed to be separate, connected by a small tunnel. The client, however, preferred a different design, in which the shafts abutted each other, sharing a wall. While this removed the need for the small tunnel, it posed a suite of new problems, as the stresses on two isolated round shafts are quite different from those acting upon adjoining shafts. The method of construction also changed, so that special Y-shaped cement panels must be designed for the shaft junction. A two-hundred-foot-deep trench was dug for the panels, then fifty-foot segments of rebar lowered into the hole and connected, and finally, after a network of pipes were placed, a concrete slurry was pumped into the space. The resulting cement sections abut each other but have no true joint, so it is essential that the design be perfect. For instance, the “slurry panels” must sit exactly flat on the ground or the structure will not hold. After the shaft’s completion, it was discovered that one of the panels did not sit properly. As it could not be re-set, the engineers had to reexamine the stresses, and they determined that stronger concrete would have to be used in the panel to compensate the error. A miscommunication caused the cement plant to send a weaker concrete mix instead, which was disastrous. After much testing and analysis, the project engineers determined that the weak panel would be just barely sufficient, necessitating the installation of considerable monitoring equipment so that any failure of the shaft due to this panel could be detected and mitigated.

Other factors interfered with construction of the shaft. During the pouring of another one of the wall panels, the slurry contaminated the panel cement at the very bottom of the shaft, requiring the entire panel to be re-done; this set the project back several months. Also, the wet conditions in Washington DC meant that the shafts often filled with water. Rather than pump the water out, Jacobs Associates decided to work with the conditions. When the shaft was flooded, the rebar cages were set by divers, and the slurry could still be poured normally. By not de-watering, the engineers reduced possible ground modification from removal of the water from the surrounding soil, and they also reduced potential exterior pressures on the shaft, since the water equalized the pressure inside and out. But to remove it would have meant the water would be pressing in without anything to push against it.

The second engineering problem that affected the project was that of the existing historic structures, specifically the sewers. Where buildings were potentially endangered by ground movement caused by the CSO tunnel’s construction, ground cores were taken and structural profiles of the subsurface made in order to assess the strength of the soil. In one place, a brick-and-concrete masonry sewer over a hundred years old sat only seven feet away from the planned tunnel route. Lab tests of the soil parameters were plugged into a model to predict possible ground subsidence or displacement where the sewer lay. The wall of the sewer was cored and the actual effective strength of the old materials assessed. The result was that, if there were no mitigation efforts made, the damage to the sewer caused by the tunnel would be irreparable. To prevent a catastrophe, stronger soil was packed in around the sewer to replace the existing weak fill; this is called “ground improvement”. The cost was nearly $800,000, but it prevented damage to the sewer which would have incurred far worse costs.

Another sewer from the same era was at similar risks. This one was larger and had been designed to accommodate wet-weather overflow, much like the modern CSO will do. Engineers went into the sewer during dry weather and found large cracks already present in the walls, even tree roots breaking through. It was clear that the subsidence caused by the tunnel would destroy this sewer, so “steel sets” were brought in to stabilize the walls: bolted-in rebar ribs to keep the sewer from collapsing. These had to be carried in by hand through a manhole, which was the only access to the sewer. As the cracks will probably widen in spite of the steel sets, after the CSO tunnel’s construction is complete, the contractor will go into the old sewer and repair the walls permanently.

Edgerton brought up a final point: a phenomenon known as “conservatism” in engineering design. The faulty slurry panel which was found to pass the bill with weak concrete had been designed to withstand far greater stresses than it will likely encounter. An engineer is always under pressure to make things extra-sturdy, since a failure will be blamed on her design. A contractor, however, is under pressure to save time and materials, since the client wants to spend as little as possible. Every project is an interplay between these opposing forces. Ideally a happy medium will be found, allowing the client to save cost while still receiving a quality project which, like the historic sewers of DC, will last for many years to come.