Dam Concerned Citizens Inc.

Anchor A-59 Monolith 13

October 2006

The New York City Dept. of Environmental Protection announced on October 5, 2006 that one of the inclined post-tensioned tie down anchors (Anchor No. A-59) experienced failure of 8 of 58 cable strands that comprise the anchor. The communication of this failure was made to the Schoharie County Board of Supervisor´s Chairman Earl Van Wormer III. Each strand is 5/8" in diameter with a design load tensile strength of approximately 17 tons so the loss of 8 strands is a loss of approximately 136 tons of tensile strength. Anchor No. A-59 is one of 7 anchors located within critical monolith 13. This monolith is one of three 76 feet wide individually cast sections of the Gilboa Dam considered most at risk of failure in the event of an extreme flood. The combined tie down strength of the 7 anchors in place at monolith 13 was to be approximately 7,140 tons therefore the loss of 136 tons represents a loss of less than 2 percent of the total tie down force at monolith 13.

The principle purpose of Post Tensioned Anchoring is to exert downward mechanical pressure to hold a dam in place in time of extreme flooding. Dam failure can occur by sliding of the dam due to hydraulic pressure on the upstream face and uplift pressures on the dam foundation or base. Post tensioned anchoring of aging dams to bedrock is a well accepted practice that has been used successfully for over 30 years. The tie down anchors develop tensile strength through bonding to bedrock at the base of the anchor with the use of Portland cement grout. The bond zone or area in the bedrock where the actual anchoring occurs is over a length of roughly 40 feet of borehole at Gilboa. Portland cement is a very strong adhesive and is capable of holding anchor strands firmly in place within the bedrock below a dam.

It is important to understand that prior to the bonding of the anchor to bedrock the installation process first involves corrosion protection which includes sealing each anchor borehole with cement grout, grouting a plastic borehole liner (i.e., corrugated plastic pipe) that encases the entire bundle of 58 cable strands in the case of Anchor A-59, grouting the cable strands within the plastic encasement, and securing the anchor strands within an anchor head assembly. Each strand has a bare length that extends through the 40 feet of bond zone and sheathed length which extends upward above the bond zone to the anchor head and is referred to as the free stress length. It is our understanding that the borehole sealing process at Anchor No. A-59 required several grouting and re-drilling efforts to meet the project specifications for proper borehole sealing.

The failure of the 8 strands in anchor No. A-59 occurred during the installation process known as performance testing. During performance testing the tie down anchors are temporarily loaded to 133 percent of the anchors design load (i.e., At Anchor A-59 the performance test load is 1040 tons times 1.33). The force is applied using a large hydraulic jack that is instrumented and monitored electronically to measure stretching and possible failure. The testing is observed in "real time" on a computer screen connected to the tensioning instrumentation. The point of strand breakage occurred within a few feet of the top of the anchor. There is no indication that of strands within the bond zone have failed. The retesting of Anchor A-59 as a 50-strand anchor passed the performance test and the anchor has been locked off at approximately 110 percent of the design load for a 50-strand anchor. The cause of the strand failure continues to be the subject of investigation. We have not been informed of any conclusions as to its cause at this time. The members of the DCC that took the tour of the dam on October 11th observed Anchor No. A-59 and the failed strands while the anchor was being re-tensioned as a 50-strand anchor.

The failure of anchor strands at Anchor No. A-59 raises some questions. The gravity of this matter is increased by where the failure occurred on the Gilboa Dam.

Some questions that need answering are:

1. As the breakage occurred within a few feet of the top of the bore hole, why wasn´t the tension head mechanism reattached below the break and the whole cable bundle re-tensioned?

2. Since the failure occurred in a critical monolith, wouldn´t it be best for ALL entities concerned to conduct a forensic analysis of the anchor to see what actually went wrong?

3. DCC understands that re-tensionable anchor heads for post tensioned anchors are now in use. These allow for post installation monitoring and re-tensioning, if necessary, of the anchoring system in years subsequent to anchor installation. These are NOT the kind being used at the Gilboa Dam. The "anchor heads" at the Gilboa Dam are buried under concrete after testing out satisfactorily and are not re-tensionable, should they loose tension. Can the DEP retrofit some of the anchors with instrumentation or a means of testing for the purpose of measuring anchor tension during the service life of selected anchors actually on the dam?

4. Understanding that the tie down force exerted on monolith 13 has been reduced doesn´t this failure raise the level of uncertainty in design and underscore the need to design to the higher FOS under the NYSDEC guidelines?

It is the belief of DCC that the "news from Gilboa" has been for the most part encouraging. The failure of the anchor to achieve full capacity at monolith 13 bears mute testimony to the validity of "Murphy´s Law"; "whatever can go wrong, will go wrong". Since its inception, DCC has advocated vigilance and oversight by the public at the Gilboa Dam. Problems and "bumps in the road" such as the recent anchor strand failure may happen again. An ability to think "outside the box" and a willingness to add redundancy to design to achieve higher levels of safety will lead to a successful rehabilitation of the 80 year old Gilboa Dam.