The answer to whether or not slurry walls should be designed as part of the final structure seems obvious at first. If you are building slurry walls for an excavation then, clearly, they should be used as part of the tunnel or underground building structure. After all, such a design avoids the need for additional, cast-in-place walls inside the slurry walls, the excavation can usually be narrower because double sets of walls are not needed and, therefore, the overall cost of the structure would be reduced.

In practice, however, the answer is not that obvious. We must first look at analytical and design considerations,
construction challenges, and determine whether or not a slurry wall is even necessary.

Analysis and Design Considerations

In recent times, we have harnessed the great capabilities of computers to learn things about slurry wall analysis that we never knew before. For example, when slurry walls are to be connected to the tunnel or building structure, it is now standard practice to do a staged construction analysis—the slurry walls are modeled for construction stages, where excavation proceeds to a certain depth, then tiebacks or cross lot braces are installed. The staged excavation is non-linear in that each stage adds moments and forces to subsequent stages.

Staged construction analysis is very different and more complex than the traditional excavation support analysis, which consisted basically of a final loading condition, an assumed soil and water pressure, and strut/tieback support points. Staged construction analysis also leads to the following problems:

• The moments and forces predicted by the model often can be much greater than the traditional final condition model. There is limited verification in the literature, however, that these critical temporary conditions actually occur during the wall construction. The result can be a conservative design that adds to the cost of the slurry wall.
• The design is dependent upon assumed construction staging sequences. In typical design/bid/build contracts, it is necessary for the designer to design the slurry walls because they are part of the final structure. The wall cannot be simply specified, then designed by the contractor. If, during construction, the contractor needs to adjust the strut or tieback arrangement or sequence, the wall needs to be redesigned and the client ends up paying for two wall designs.

Construction Considerations

Slurry wall construction must deal with some additional challenges, particularly when the wall will be used as part of the final structure. For example, slurry walls may be difficult to build in areas where there are many underground obstructions. These can be natural obstacles, such as glacial boulders, or man-made objects, such as buried foundations from old buildings. In cast-in-place construction, the obstructions are still there, but they are easier to see and remove. Slurry wall construction is more prone to delay and claims than cast-in-place construction.

Another construction challenge is that the field connections of slabs to slurry walls are more difficult than cast-in-place construction. Problems include:

• Crushed blockouts
• Rebars in the wrong place in the wall
• Bars that are difficult to bend
• Leakage because the blind side of the wall (next to the soil) can not be waterproofed or because imperfections in slurry wall concrete (pervious zones) permit groundwater seepage through the wall.

A third challenge is that the construction tolerance for a slurry wall is greater than for cast-in-place construction. Although slurry walls can be built close to where they are supposed to be, it is impossible to get the walls perfectly in position—there will always be a few panels that are out of alignment. In some extreme cases, panels may be installed so far out of alignment that the contractor will be responsible to design special work around connection details. The concerns are magnified when dealing with the tolerance of the reinforcement because rebar inserts can end up in the wrong place and soldier piles in SPTC walls can be out of line and twisted.

Do You Really Need a Slurry Wall?

If there is an obvious benefit to using slurry walls as part of the structure, then it can become tempting to design slurry walls in cases where they may not be needed. Slurry walls offer design and construction benefits, but at a price. The construction is specialized and usually much more expensive than other excavation support walls or a sloped excavation if you can get away with it. Slurry wall construction also requires more space for the slurry plant and reinforcement staging, and it is messier.

Consider two examples from the CA/T project:

• I-93 mainline cut-and-cover tunnels, where slurry walls are part of the permanent structure
• Leverett Circle ramp cut-and-cover tunnels, where slurry walls, where used, are not part of the permanent
  structure.

I-93. It was decided to also use the slurry walls as part of the permanent tunnel walls because, in part, the width of the excavation would be 3 m to 4.6 m (10 feet to 15 feet) narrower than if separate, cast-in-place walls were used. The excavation width was a key design consideration because adding just a few more feet would directly
impact existing building foundations. (See “Using SPTC Walls as Permanent Tunnel Walls” by Das et al.)

Leverett Circle. The connecting ramps feature tunnels that are not as wide or deep as the mainline I-93 tunnels and that will be built, for the most part, relatively far from existing buildings. Slurry walls are designed in some limited locations, however, such as beneath existing railroad tracks. The reasoning for not using the walls as part of the permanent structure was that the Leverett Circle ramps are designed to “float” on a mat foundation. Connecting to slurry walls at one location would introduce a hard point, which meant that either:

• A joint would be needed between the floating mat section and the section connected to slurry wall.
• The entire tunnel alignment would have to be built on deep foundations.

Using slurry walls as part of the final structure can make sense in many cases, but it should not be an automatic
decision. The designer should first consider slurry walls where needed, then weigh other factors, as described above, before incorporating the walls in the final structure.