3 Description of Barrier Fencing
This fence is composed of surplus carbon steel landing mats, 12-ft long, 20-in. wide, 1/4-in. thick (dimensions approximate). For each mile, 3,080 panels are used. Landing mats are welded to 3 to 6 in. (various) steel well-casing pipes buried to a depth of 8 ft every 6 ft along the fence. Landing mats are stored surplus and are available without cost to the project. Currently, roughly 14 miles of this fence is installed along the border in the San Diego District, with enough panels in surplus to construct an additional 60 to 90 miles of fencing. Some areas also include an anti-tunneling measure to prevent entry underneath the fence. The measure is subsurface steel panels (often damaged landing mat unsuitable for fencing) set in a concrete matrix along the fence line to a depth of 2 to 3 ft. The U.S. Army Reserves are responsible for the construction of new fence, and the U.S. Border Patrol is responsible for its M&R. Terrain/topo-graphic concerns are minimized because of ease of installation.
The landing mat fence (shown in Figure 1) is one of the few designs where significant long-term experience has been gained for its application. Although its installation was an impressive effort in response to the mission needs of the Border Patrol, the benefit of functional experience offers some lessons learned. An initial lack of detailed design, construction procedures, and performance requirements has resulted in problems. These problems include:
ˇ inadequate footings for the terrain, loads, and soil conditions involved
ˇ lack of provision for thermal expansion
ˇ inadequate corrosion protection
ˇ lack of dig-under prevention
ˇ inadequate resistance to mechanical attack
ˇ inadequate barrier to quick re-entry into Mexico to avoid apprehension (a Border Patrol operational requirement).
Figure 1. Landing mat design barrier.
Appendix B describes a detailed engineering inspection of the 14 miles of fencing at the San Diego/Tijuana border. This fencing is predominantly of the landing mat design.
Concrete panels 15-ft high, 12-ft long, 4-in. wide are held in place by 4-in. carbon steel I-beams mounted in 16-in. concrete piers to a depth of 8 ft. Each panel weighs approximately 10,000 lb and is manufactured offsite at a central processing location. Similar fences have been used as sound and visual barriers along U.S. highways nationwide. Carbon steel I-beams (improved active degradation resistance) measuring 8 in. can be substituted for an approximate $15,000 additional per mile. Terrain and topographic concerns may cause difficulty. For steep hill regions, fence panels would require stepping or terracing to accommodate I-beam design constraints. This consequence necessarily enhances construction difficulty because of increased ground preparation time. Dig-under protection can be added without price increase by simply burying 2 ft of the panel below grade, leaving a 13-ft high barrier above grade.
Figure 2 shows 12-in.-diameter concrete bollard fence staggered every 5 in. along the fence line. Poles are 12-ft high, buried to a depth of 2 ft, and anchored in concrete. Certain areas of fence are also equipped with 48-in. steel outriggers and wire mesh mounted atop bollard poles. The concrete bollard fence requires surveying of the installation site and concrete forms for installation. Any damage to the concrete requires forms to hold the concrete in place until cured. Approximately 1,000 ft of bollard fence (bare concrete) is being used as a secondary barrier on a trial basis.
The concrete bollard design has been modified to address the concern that bare concrete poles would be destroyed by active degradation at an unacceptable rate. The modification encases the bollard poles in steel sheaths (drainage culvert pipe) to prevent chipping and connects the tops of the poles in series to prevent breakage between adjacent poles from pressure applied by automobile jacks. This arrangement alleviates the need for concrete forms used during construction of the poles. However, forms are still needed for constructing the base for the poles. A proposal is under consideration to construct approximately 4,000 ft of bollard fence (steel reinforced) as a primary barrier on a trial basis.
Figure 2. Bare bollard design barrier.
Both of the bollard designs (i.e., bare concrete and steel cased) being considered for service as a secondary barrier are identical to those being considered for service as a primary barrier. For a full description see the Primary Barriers section earlier in this chapter.
This angled two-piece fence (shown in Figure 3) is intended to prevent climbing by using gravity and the weight of the trespasser. Posts on 10-ft centers support the fencing. All clamps and bolts are corrosion-resistant galvanized steel.
This patented fence design (shown in Figure 4) is used in many detention centers. The curved fence design and small gauge mesh of the fence hamper climbing. Posts are centered every 10 ft for support.
Figure 3. Sandia Fence design barrier.
Figure 4. First DeFenceŽ design barrier.
This standard steel cyclone fence is made of 9 gauge wire mesh.
This standard steel cyclone fence is also made of 9 gauge wire mesh.
