Don't Overlook the Importance of Watertight Bulkheads

 

A couple of common applications for cellular concrete include annular grouting in sliplining and utility abandonment of gas lines, sewer lines, water mains, etc. The advantages of cellular concrete in these applications include: reduced cost of materials, reduced buoyancy on sliplined liner pipes, long distance pumpability, and low pumping pressure, to name a few.

With proper equipment, foam concentrate, and training on how to produce cellular concrete for these applications, it's a fairly straightforward process. However, as with anything, there are a couple simple things that get commonly overlooked, which can end up costing time and money. Ensuring that bulkheads on both ends of the pipe on an abandonment or annulus grouting project are properly installed is crucial to ensuring the project goes smoothly.

Richway has been on multiple projects, either as consultants, trainers, or contractors themselves where bulkhead deficiencies have caused delays and wasted material. Because of how easily cellular concrete pumps and flows with little pressure, it's able to find its way into the smallest voids and cracks. While this can be advantageous when filling voids and small space that you want to get filled, it can also cause problems with formwork and bulkheads. To ensure there won't be any problems with leaking material, it's best to ensure your bulkheads and forms are watertight. In some situations it may not be feasible, if at all possible, to test for water tightness. In that type of scenario it's best to be overcautious in sealing forms by using calks, spray foam, or other types of sealing methods.

Described below is a project which was near Richway which we acted as the subcontractor. A total of three 16" pipes were abandoned. Between the three pipes approximately 100 yds³ of 30 PCF material was pumped into place. The first pipe shown in figure 1 and figure 2 shows the far end of the pipe, at higher elevation.

 

Figure 1 - Vent pipe on #1 pipe shown with bulkhead in place

      

Figure 2 - Vent pipe on #1 pipe shown with cellular concrete leaking around
the bulkhead

Verification of a full pipe is usually done by seeing that material comes of the vent pipe, however as seen here cellular concrete is escaping from around the sealed bulkhead. Although cellular grout did not actually come out of the vent pipe itself, the inspector on site was satisfied in calling the pipe full.

The second, and longest pipe at approximately 800' in length, had no bulkhead problems. As seen in figure 3, water was being displaced, and pushed out the vent pipe, as it was being filled with cellular concrete. Clear water was pushed out for nearly 45 minutes before turning to gray water, then cellular grout, indicating the pipe was completely full.

Figure 3 - Clear water being displaced through the vent pipe as cellular fills the pipe

Filling of the third pipe presented the worst problems.  Upon initial filling of the pipe, material began leaking immediately from around the bulkhead.  Given the time of day there wasn't enough time to try making the repair to the bulkhead and try another attempt at falling.  This meant an additional mobilization the next day.  Upon trying the second day, again the bulkhead began leaking, nearly immediately.  With this there was no option but to try filling from the top side vent pipe.  Although it's not the preferred method for ensuring pipes are completely full, it worked fairly well.  By using the test tee (see figure 4) as a venting method, we were able to allow trapped air to "burp" out of the pip, every 20 - 25 minutes.  Also by having a pressure gauge on the outlet side of the pump we could have an idea of when it was time to vent the air out, and when the pipe was close to full.  Upon the last venting, there was no air that vented, only cellular grout, indicating that the pipe was likely as full as was going to be achievable.  

                                                               

Figure 4 - The test tee shown here is used for checking material density and in this
project also for venting air on the last pipe being filled



 

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