Skinning the Cat (or any good plan is flexible)

Designers make their best efforts when planning our projects, but complete and perfect information - even if possible - would come at a high cost and invite the paralysis of analysis. Minor changes should be expected in every job. Occasionally though, such large problems arise that demand unusual countermeasures to achieve a successful conclusion.

#ONE ROCK TUNNEL
A pair of large river water conveyances had to cross beneath an old rock tunnel interceptor. After the crossing was made the tunnel was rebuilt with a combination of direct bury pipe and pipe within a vault. Over time, significant infiltration into the pipe and rock tunnel segments developed, approaching 7 mgd. From the mid-1980s to the early 2000s a series of projects to reduce the infiltration were pursued. The measures included internal pipe seals, external pipe seals, chemical grouting from within, and cementitious grouting from the surface. The aggregate measures had limited success.

#Another attempt was made in the 2010s, but instead of repeating the past repair approaches this designer proposed abandoning the stretch by utilizing a parallel concrete pipe interceptor. Hydraulic modeling of the modified layout confirmed the acceptability of the concept. The project enlarged upstream and downstream crossovers while six reinforced concrete bulkheads were constructed to isolate the leaky tunnel from the collection system and WRRF.

#ANOTHER ROCK TUNNEL
A subsequent project in a tunnel tributary involved slip lining to exclude infiltration and prevent a structural collapse. Most of the distance had liner pipe installed within failing tile-lined pipe. The final leg anticipated pipe installation through an unlined rock tunnel, terminating at a junction chamber.

#The contractor was very wary about the condition of the tunnel within which his forces were to work, so wary that he proposed using open cut from the surface to install the last stretch of pipe. Instead of insisting on the original method, the owner accepted the no cost change order and the installation was completed, safely and without major issues.

#YET ANOTHER ROCK TUNNEL
During the late 1990s a 1937 rock tunnel interceptor began exhibiting obstructed behavior in the form of persistent surcharging and extended overflow following wet weather events. A lack of access made inspections difficult. Extraordinary measures that included ground penetrating radar and sub bottom profiling sonar indicated a partial collapse, enabling the long-term accumulation of sediment and debris. A cleaning project to physically remove the debris was planned and commenced. Partway into the job it became apparent that the extent of the debris accumulation was underestimated. Deterioration of the rock ceiling and walls created a larger cavity which contained much more debris. The effort was suspended due to the scope change and the associated costs.

#Temporary mitigation of the overflow came in the form of a pumping system installed in a tributary to the main tunnel. Twin pumps intercepted shaft surcharge and conveyed the excess flow through an overland pipeline around the remaining obstructions. This measure bought time for planning a completion project. The follow up project envisioned having a sufficient quantity of debris cleaned from the tunnel to the extent that a liner pipe could be installed. The completed pipe would then be grouted in place, entombing the remaining debris along the sidewalls and providing protection from continued ceiling deterioration. Unfortunately, the bids for this approach well exceeded the revised project budget.

#A rebid was based on continued cleaning of the expanded cross section for the full length, which did fit the project budget and was successful. Once cleaned, the contractor installed rock bolts and ceiling fencing to catch fractured rock, avoiding a repeat of sediment entrapment.

##NOW, A ROCK SHAFT
Sometimes a rock tunnel doesn't present a direct infiltration problem despite having a groundwater seam above it. Access shafts, though, may penetrate the seam as efficiently as a water production well. This was the case at one location where a seam was delivering 1.8 mgd into the shaft and the tunnel below. A mitigation project was based upon drilling and injecting grout to form a circular curtain around the shaft, choking off the flow.

#The contractor followed this approach, using various admixtures to fill the seam and seal it. However, the velocity of the groundwater flow was washing away the grout before it could set. Repeated attempts with different additives were unsuccessful.

#The contractor proposed a change whereby he would excavate a portion of the shaft manhole and erect scaffolding within the shaft. This would give his crews internal access to pack the seam with expanding filler material, slowing the flow in one area for the grout to set, then moving on to the next area. A payment formula was developed to cover their set up costs, thereafter making incremental payments for incremental inflow reduction. Using this performance-based approach the inflow was ultimately reduced down to 25 gpm.

#LESSONS LEARNED
Several lessons learned include building allowances into bid documents to manage unexpected changes expeditiously, owners right to terminate for convenience, use of temporary measures to buy time for reconsidering alternatives or financing continuation, and keeping an open and collaborative environment with the contractor for mutual benefits.