Cracks in concrete pipe bells 1

[Bjerregaard]

We are currently installing a 48-inch diameter sewer (Class IV RCP). The depth to flow line is 15 to 20 feet. Bedding for the pipe is 3/8-inch rock vibrated into place using "concrete stingers". Fill above the pipe zone is native material or import compacted to 90% of standard proctor. About 1 foot of 1" rock is placed below the pipe for foundation stabilization. The native soils are a mixture of sands, silts, and clays. The natural water table is 3 to 4 feet below the ground surface and the working area is dewatered prior to pipe installation.

The problem is that about 3% of the RCP bells are cracking. The bottom third of the bell is cracking and displacing downward. Cracks range from hairline to displacements of 1 inch. Even with close observation of the contractor's installation procedures, we are still seeing cracks. Has anybody experienced similar problems with large diameter concrete pipe?

 

[Ron]

Look at QA at the pipe plant. Considering the frequency of occurrence and the mode of failure, it could be a reinforcement issue in the bell.

One other consideration is to look at how the contractor is seating the pipe end into the bell. He could be "overdriving" it, thus cracking the bell. Also check the bell vs. end clearances. If too much clearance, the ends will go too deep into the bell. If too little clearance, the bell gets cracked trying to force pipe in.

Check also the overall deflection/settlement of the pipe. If the bedding is good, this shouldn't be an issue, but if you are getting joint displacement, the cracks could result from this.

 

[ishvaaag]

It also could be related to a too rocky bed. Look in what to the usual standards your typical section differs.

 

[Bjerregaard]

Thanks for your help. The cracks seem to be occuring most often when the native soil beneath the foundation stabilization material is fine sand. I now suspect that when the dewatering equipment is removed, the fine sand beneath the pipe is carried into the foundation stabilization rock beneath the pipe, causing slight settlement of the pipe. When the settlement is uneven, it puts large forces on the pipe joints and causes the cracks. We may use filter fabric to enclose the rock or change to a well graded foundation stabilization material.

 

[Ron]

Bjerregaard ....thanks for posting a result/follow up reply.