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70 foot deep Caisson Repaired by Pressure Grouting

Earlier this month we had the opportunity to engage in a very unusual job. Voids were discovered in an existing caisson shortly after it was constructed. This was a 70’ by 6’ diameter caisson that will support the 1-17 Freeway overpass at Munds Park. ADOT officials and engineering consultants were tightly overseeing the process to ensure the final product would be a safe and solid structure.

Voids were discovered at the 14’ depth and at the bottom of the caisson at the 70’ depth. Both areas were contaminated with soil and debris that needed to be cleaned out before any actual repair could be undertaken. To complicate matters the caisson was located in an area with a high water table that allowed water to seep in to the void areas bringing soil contaminants with it.

Several core holes were drilled to both the 14’ and 70’ level. One of the PVC inspection tubes was used and the core holes to inject water and air in various sequences including simultaneously to clean the voids. The return water was carefully monitored to measure the amount of silt, and concrete in it. Cameras were lowered into the core holes to examine the void areas, locate the rebar, and ascertain the surface condition of the voids and rebar.

Packers were installed on the various core hole tops to control the return flow of water. By shutting off holes in various sequences, communication could be forced between the voids to clear them out. In addition directional devices were developed to jet water and air to various locations in the voids after determining the need from camera examinations. Care was needed to avoid washing out too much soil on the outside of the shaft.

Once the voids were clean to everyone’s satisfaction, then the grouting sequences were started. The bottom of the caisson was grouted first. Because the caisson was below the water table a grout material requirement was that it needed to include an anti washout material that would not allow the grout to separate when coming into contact with standing water in the voids. In addition the grout still needed to be high strength, non shrink and flowable. Euco Tremie Grout was chosen.

As the grout was injected, the packers again were shut off and on to control communication within the void chambers. The water was the first to flow out of the various ports as it was displaced by the grout. Once the grout started to flow out of the ports, it was then checked for communication and then eventually all of the ports closed and pressured up to a minimum 200psi and closed off. At that point, the middle void was filled using the same procedures. At the middle void, a thinner mix was first injected to penetrate the smaller voids and cracks, followed by the thicker grout with the similar sequencing as the lower void.

Timing of the injection grout was critical. Because the Euco Tremie Grout was Thixotrophic, stopping grout pumping for longer than a few minutes at a time could run the risk of having the grout thickening up in the ports or injection equipment. Several times the pressure needed to be spiked to clear lines.

The project was completed in 3 days including a Saturday because of the critical nature of the schedule. The project is moving forward and is back on track to be completed by fall of 2011.