Application of Underwater Demolition Using Hydrodemolition to Help Repair 

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Underwater hydrodemolition is a demolition work technique that uses a high-pressure water jet (usually mixed with an abrasive material) to achieve erosion. The most common application for hydrodemolition is to remove concrete around embedded material elements such as rebars, reinforced steel, and shear connectors (or studs). Unlike jackhammers, hydrodemolition does not produce as much vibration when used, and the lack of vibration can work for repairing structures sensitive to vibration, such as bridges. The resulting surface over the metal framework can provide a good surface for concrete re-coating. Hydrodemolition techniques can also work best for removing asphalt or concrete overlay during road repairs. Another advantage of using this technique is that it can be remotely controlled, which means concrete components of a submerged structure can be worked on, such as piers, wharves, and bridge columns. 


The essential components of hydrodemolition equipment are the gun, the pump, the supply tank, and connecting tubes. Abrasive materials to aid in the demolition and surface treating tasks enter through the supply tank. Modifications are available depending on the job. The operation can be done remotely by mounting the gun on robotic arms or vehicles or, in the case of underwater demolition, divers can carry and direct the gun at the target. Because of the inherent advantages of using hydrodemolition techniques, this technique offers the flexibility of using specialized attachments for specific tasks. One of these tasks is for repairing the stilling basin floor of a dam.


The Canyon Ferry Dam, located at Helena, Montana, started an extensive repair program in 2004. One of the most challenging tasks is for repairing and resurfacing the dam stilling floor. The dam stilling floor is the part of the dam located at the bottom of the spillway whose purpose is to dissipate the energy of the water falling from the spillway. This part of the dam erodes over time. The challenge presented by this particular project is that the provisions for dewatering the basin will not be in place. That is, the job will be under 50 feet of water. Aside from the demolition and surface job on the basin floor, removing debris accumulated on the basin floor is a part of the preparation work. Guidewires were placed over the area where the work will be done to assure precision and guides to aid divers. 


The demolition work itself was done by Global Diving and Storage, a Seattle-based company. The task for the surface demolition itself will be accomplished by a combination of systems placed on the surface supporting the hydrodemolition gun itself, which will be operated underwater. A specialized frame is constructed to mount the gun, and the frame will be hydraulically operated to control the X-Y movement required to aim at the work itself. The hydrodemolition pumps stay on the surface while the hydraulic control unit is mounted on a barge that will float over the work area. Divers will be helping with directing the aim of the gun itself to assure the precision of the work.  


Divers were oriented on the safety requirements for the job. Specialized diving equipment included a wet suit and a helmet that will receive a nitrox-air mixture. Communication equipment and video cameras are included in the helmet to allow for constant verbal and visual communication with the team positioned on the surface. Shields are not needed for the diver as the water itself can mitigate the trajectory of debris that will reach the diver. However, the resulting turbulence makes visibility difficult, making it impossible to achieve a live feed of the surface work itself.


The parameters controlled during the cutting process are:

  1. the speed of traverse
  2. the number of traverse times before stepping forward (or away from the surface being worked on)
  3. The length of the step
  4. The speed of rotation

These are controlled by an operator located at the barge floating over the surface. Test cuts were done to fine-tune the parameters before proceeding to the cutting process itself. 

The procedure did not compromise the integrity of the existing construction joints in the original stilling basin floor.  Having a new surface allowed for the installation of a new concrete slab which will be tied to the existing floor using rock anchors.


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