The Yazoo National Wildlife Refuge is 13,000 acres of federally owned property located in the delta region of north-central Mississippi. Like many of the federal refuges there, the Yazoo Refuge is a combination of agricultural fields, hardwood forests, and swamps. It is home to an abundance of wildlife, including whitetail deer, turkey, a variety of waterfowl, and other wetland dependent species. Some of the largest stands of cypress trees in the South grow in the swampy areas within the confines of the refuge, providing the setting for an experimental ditching project sponsored by the U.S. Army Corps of Engineers Vicksburg District and performed by U.S. Army Engineer Waterways Experiment Station personnel.
Engineers from the Vicksburg District first contacted personnel from the WES Structures Laboratory in July 1993 about the feasibility of using explosives to create drainage ditches in the Swan Lake wetlands of the Yazoo National Wildlife Refuge. Existing drainage ditches and water control structures were in place at two weirs in the area. The weirs were intended to aid with water flow in the area and to provide a means by which U.S. Fish and Wildlife Service personnel could control water levels to enhance the natural habitat for wintering waterfowl. However, these ditches were not long enough to reach into the natural drainage patterns within the Swan Lake area.
A preliminary site visit revealed that very boggy conditions existed even during the dry summer months. This made the use of mechanical equipment difficult and costly. In addition, the large stands of cypress trees needed to be protected from damage.
Swan Lake was a Mississippi River channel thousands of years ago and the entire area is interlaced with a series of natural drainage patterns. The refuge management, together with Vicksburg District and WES personnel, determined that explosive excavation could effectively connect the natural drainage and the two weirs, and could do so with less disturbance to the environment than mechanical equipment might inflict.
Following the initial site visit, WES personnel established a scope of work for the refuge project. It was determined that four ditches would be required. The ditches would vary in length from 250 feet up to 350 feet, depending upon the location and terrain. The targeted ditch dimensions were 40 inches deep and 10 to 14 feet wide. Calculations and data from prior projects indicated that a series of 4-inch-diameter vertical pipes located 7 feet on centers and driven to a depth of 31 inches would be needed (Fig. 1 (11K)) with 8 pounds of DBA-105P slurry blasting agent per pipe to create the necessary ditch dimensions. The DBA-105P blasting agent was essentially a formulation developed by a private contractor for military use during the late 1970s. During this time period, U.S. Army R&D elements investigated applications of pourable/pumpable bulk explosives that could be used by combat engineers to create obstacles.
Beginning in late August 1993, a team of five WES personnel began clearing brush along the proposed ditch routes. Routes chosen avoided cypress and other hardwood timber and most effectively tied into the natural drainage system. The job was labor intensive as the crew worked in knee deep mud and water to drive the vertical pipes into the ground with a sledge hammer to the proper depth of 31 inches below-grade.
Once the pipes were in place, the crew began pouring pre-weighed slurry charges of 8 pounds into each pipe (Fig. 2 (38K)). The height of each charge within the 4-inch pipes was approximately 12 inches.
The booster for each charge was two ounces of Composition 4 (C-4) molded around a ULI knot on a 50-grain detonating cord down line. After the placement of the detonating cord and booster, a styrofoam disc was placed on top of each poured charge to separate the charge from water poured into the hole for stemming. Each down line was connected to a single line main of 50-grain detonating cord.
To detonate each line main, a single M-6 electric blasting cap and an M-122 remote firing device were used. Due to the burn rate of the detonating cord and the detonating velocity of the slurry, each line of pipes detonated almost as if it were a single charge. The blasts sent mud and debris over 100 feet into the air (Fig. 3 (26K)). and created ditches approximately 15 feet wide and 40 inches deep (Fig. 4 (49K)).
Over the course of two weeks, a total of 175 pipes were emplaced at four locations, filled with explosives and detonated to create nearly 1,200 feet of drainage ditch at Swan Lake. To accomplish the same amount of work through conventional construction/excavation practices would have taken more than twice as long at a significantly higher cost. In addition, conventional construction practices would have had a detrimental impact on the wetland environment at Swan Lake.
Tim Wilkens, Yazoo Refuge manager, stated, "It (the blasting) is more sensitive to the environment, and it looks very natural." The results of the project show that techniques originally developed for military applications can be modified and adapted successfully to a civil works project. Additional benefits of minimized environmental impact, time, and cost were realized. The technique holds promise for similar applications.
Hank McDevitt can be reached at (601) 634-2705; Dan Carleton is at (601) 634-3812.