2 Requirements and Degradation Factors
A fence or barrier is used to deter illegal immigration and drug smuggling at locations other than controlled points of entry. To assist the Border Patrol in performing their duties, a three-fence system comprising primary, secondary, and tertiary barriers is being developed for areas of high population density along the southern border. The primary fence is positioned directly at the geographical international border with the secondary and tertiary barriers parallel and fixed distances north of the primary. All three barriers are required to be reasonably safe for those individuals attempting to traverse them. In particular, this requirement means that razor wire, barbed wire, and sharp edges are disallowed. Structural integrity also needs to be assured given that wind loading and other intermittent dynamic forces could cause a dangerous collapse. For this reason, some less humane options such as electrified fencing were not considered suitable for consideration or inclusion in this study.
The fencing that was considered consisted of two major types. The first type consists of an upright support frame with various in-fill options. The frame is various steel supports set in concrete anchors throughout, and the in-fill is, alternatively, steel panels, reinforced concrete panels, and steel mesh (e.g., chain link, expanded metal, and fine mesh). The second broad category of fencing might consist of posts, pickets, or palisades. The essential feature of this fence type is a repeated vertical element set sufficiently close together so as to be impassable.
For all fence systems studied, a number of issues relating to actual field construction were considered. The effect of steep terrain was assumed to add additional costs both for material transport and construction. For some specific designs, sufficiently steep terrain could severely hamper or even prevent construction (e.g., the inability to position a large crane). Where applicable, the need for any specialized equipment is noted. In addition, if an above average or specialized skill level is needed by one or more construction workers to reliably install a fence design, this also is noted.
The individual requirements for the three-fence systems were assumed to vary in severity with the most restrictive needs existing at the border itself. The primary barrier should effectively:
· prevent vehicle drive-through
· deter climbing over to the North
· deter tunneling under (no time limit)
· be resistant to acts of structural vandalism, or active degradation (no time limit)
· prevent easy return to the South (for Border Patrol operational needs).
The secondary barrier should effectively:
· prevent climbing over to the North
· deter tunneling under (within 1 hour)
· be resistant to active degradation (within 1 hour).
The tertiary barrier has the least restrictive requirements of effectively deterring climbing over to the North.
The various potential modes of fence degradation are assumed to be classified as either active or passive. Passive degradation consists of all environmental effects that occur without any human assistance. These effects would include the results of corrosion, local micro-climate (i.e., proximity to the sea), wind, morning dew, temperature, rain, flash flooding, and long-term erosion. In contrast, active degradation includes all acts of vandalism directed at a fence, which typically involves the use of tools. Ramming the primary barrier with a motorized vehicle is one of the more overt examples of active degradation. However, the use of hacksaws, cutting torches, shovels, hammers, picks, fire, crowbars, and all other implements also fall into this category.
Given the relative severity and economic impact of active compared to passive degradation, only reasonable extremes of active degradation were considered in this analysis. An implicit requirement for all the design options is the ability to withstand passive degradation. Given the other needs and requirements, this ability proved to offer no additional constraint. It is worth noting that the nature of active degradation tends to be opportunistic so that vandalism is typically concentrated at weak points in the barrier. In addition, the vandalism often has an innovative and adaptive element to it. As a result any list of expected
countermeasures for a specific barrier design cannot be completed without some construction and operational field experience.
