FEATURE ARTICLE Seepage Control, Cutoffs Walls Manual: A Progress Report The U.S. Army Corps of Engineers (USACE) has 704 dams in its inventory and over 100,000 miles of levees with a capital value of more than $150 billion. Currently, USACE has over $2 billion of cutoff wall projects under construction and/or design/planning. Given the magnitude of the dam and levee safety issues, we are developing a new engineering manual, Seepage Control Cutoffs for Dams and Levees. USACE has partnered with Deep Foundations Institute (DFI) in preparing the manual to ensure that it is comprehensive and draws from the expertise of the specialty foundation engineering companies that design, manufacture and construct seepage control cutoffs around the world. The manual contains case histories as well as a comprehensive project statistical summary and guide specifications. The goal of USACE was to make the manual a state-of-the-art document for use by USACE districts and the civil engineering profession. Currently, there are 319 dams classified as high hazard, with actionable failure modes that will require structural modifications. The USACE dam inventory has been evaluated and assigned a Dam Safety Action Classification (DSAC) rating from I to V, with I being the most serious. We are also evaluating the levee inventory, using Levee Safety Action Classification (LSAC) ratings assigned from I to V. Cutoff Wall History Slurry wall construction techniques were developed in the late 1940s in Italy by ICOS. The first use of the slurry trench method of construction in the U.S. was by the USACE Memphis District in September 1945, to form a partial cutoff along the Mississippi River levee just below Memphis, Tenn. A soil-bentonite slurry trench cutoff was first used for control of underseepage at a major earth dam at Wanapum Dam on the Columbia River in Washington in 1959. A cutoff wall using cement-bentonite was first utilized in the U.S. at the Tilden Tailings Project to store tailings from the Tilden Mine in Michigan in 1976. And finally the first cement-bentonite cutoff constructed at a dam on a river retaining a reservoir was completed in 1978 at the Elgo Dam (formerly the San Carlos Dam) in Arizona. The USACE has designed and constructed some of the deepest and most complex cutoff walls for dams since the 1970s. The first major concrete panel cutoff wall constructed in a dam in the U.S. was at Wolf Creek Dam by ICOS from 1974-77. Arturo Ressi, who was a trustee of DFI, was president of ICOS at the time. In 1968, about 17 years after first being impounded, wet areas and muddy flows in the tailrace and sinkholes in the downstream toe adjacent to the switchyard occurred. Other major cutoff wall projects include Mud Mountain Dam — one of the deepest walls ever constructed, Beaver Dam, Mississinewa Dam and the Walter F. George Dam. Center Hill Dam is currently under construction. AUTHOR Figure 1. Source of life safety risks for USACE dams USACE Dam and Levee Safety Programs USACE and other federal agencies are using risk-informed decision processes in dam and levee safety programs. The basic elements are: potential failure modes analysis, event trees, load frequency analyses, probabilistic analyses and models, subjective probability and expert elicitation, and consequence evaluation. USACE classifies its dams into one of five Dam Safety Action Classifications (DSAC). We determined that 319 dams were either I, II or III, with failure modes above the Tolerable Risk Guidelines, which will likely require a structural modification. The most prevalent actionable failure mode for dams was seepage and piping that could lead to breach, see Figure 1. The manual we are developing with DFI will be limited to foundation treatments by vertical cutoff-type barriers for dams and levees, and does not discuss horizontal seepage barriers or other upstream sealing alternatives. It contains eight chapters: • Chapter 1 – Introduction • Chapter 2 – Dam Safety Issues • Chapter 3 – Geologic Considerations • Chapter 4 – Engineering Considerations • Chapter 5 – Continuous Trench Cutoff Walls • Chapter 6 – Soil Mix Cutoff Walls (excluding jet grouting) • Chapter 7 – Element Walls (panel and secant) • Chapter 8 – Jet Grouting and Other Methods including Geomembrane, Sheet Pile and Vibratory Beam David B. Paul, Risk Management Center, Lead Engineer, U.S. Army Corps of Engineers, Lakewood, Colo. DEEP FOUNDATIONS • MAR/APR 2014 • 85
