According to the Post-Tensioning Institute, post-tensioned rock and soil anchors provide a cost-efficient, high-performance solution for projects in need of temporary and permanent stabilization solutions. There are numerous different post-tension applications in the construction industry, but one of the most important areas is in dam stabilization.
Post-Tensioned Anchors
Post-tension anchors are a relatively easy and cost-effective way to address dam safety concerns due to an aging concrete structure, compromised construction joints, the need to reinforce for additional storage capacity, or when planning for new future flood levels.
Post-tensioning in dams provides permanent tie downs to increase resistance to sliding and overturning. These anchors can consist of cable strands, high-strength threaded steel bars, or a combination of both. The size, number, and makeup of the anchor system is largely a design decision, but constructability should also be a factor. It is an excellent idea to consult with a contractor familiar with their installation beforehand.
Cables vs. Bars
An important design consideration is whether to use cable strand anchors or bar anchors. The deeper the anchor goes, the more cost-effective cable stand anchors become, as it is possible to fabricate these strands in a continuous length. Deeper bar anchors require the use of couplers due to manufacturing limitations. Cable strand anchors are less rigid than bar anchors which are not as easy to work with and employ a different grouting sequence. Modern bar anchors can come fabricated with bond breaker materials in place that allows for easy one-step grouting from bottom to the top in one shot rather than primary and then secondary grouting.
Working in tandem with cables or bars is considering which drilling method to utilize. The two most frequently used methods for drilling the anchor hole are core drilling and rock drilling, and each has its pros and cons.
How Should We Drill?
Core Drilling
Core drilling is a very portable operation due to the smaller equipment size which would make it easier to work in tight access or remote places. Core drilling can also tell an owner and engineer much about their structure since you can analyze the core upon removal. However, core drilling is a slower operation which can drive up labor costs. Core drilling is also susceptible to issues during the drilling process, including the drill getting stuck.
Granular bedrocks such as sandstone can also cause problems for core drilling. As a core drill progresses, water is flushed down the inside of the coring bit. This water brings the “cuttings” up and out of the hole along the outside of the drilled core, which is a relatively small annulus space. This is generally not a problem when drilling through concrete or hard bedrock since the “cuttings” are a liquid slurry. In the case of sandstone, the slurry contains granular particles which can become lodged in the annulus space and seize up the drill. There are ways to combat this, but it either involves more time to drill slower or incorporate bentonite in the slurry, both of which are not always desirable.
Rock Drilling
Rock drilling, on the other hand, is a faster and more cost-effective method for drilling. This drilling method pulverizes the concrete and bedrock removed from the hole via air pressure. Modern accessories to rock drills incorporate a misting system to provide dust control, which helps meet OSHA’s new silica standards. Because the rock drills pulverize the rock, they are typically not susceptible to the challenges of drilling deep holes, or in the granular substrate. A downside of this method is that it disallows examination of the structure as a core drill would. Although cameras can be used to examine construction joints, etc. Depending on design considerations regarding hole size and drill depth, rock drills are often less portable than core drills.
No matter which of these two drilling methods occurs each is fully capable of drilling through embedded steel rebar that might appear and both can adjust to achieve angled core holes as required by design.
Making the Right Choice
There are numerous aspects to consider when designing and constructing a post-tension anchor project. Costs, work area, equipment, and material needs all play an important factor. Choosing the right technique for your job is a vital component to ensuring success. Post tension anchors can extend the lifespan of a dam by reestablishing its structural integrity and improving its stability.
