FINANCIAL EVALUATION OF FLEXIBLE PROTECTION SYSTEMS FOR NATURAL HAZARD MITIGATION IN MODERN RAILROAD APPLICATIONS

Transcription

1 FINANCIAL EVALUATION OF FLEXIBLE PROTECTION SYSTEMS FOR NATURAL HAZARD MITIGATION IN MODERN RAILROAD APPLICATIONS Frank Amend, Geobrugg North America LLC, Rocky Mount, NC, USA Abstract Traditional methods of rockfall mitigation on most railroad track systems involve placing a trip wire or slide fence near the track to signal dispatch operators of a possible problem. This low-cost approach requires train dispatchers to have the track surveyed to verify train safety won t be compromised before the train can operate through that section at normal speeds. However, is this method s total operating costs considered when comparing to a more modern hazard mitigation program which spans preventing the rocks from becoming loose on a rock face/slope to absorbing their energy/catching them as they near a pre-selected area at the slope s bottom? This paper will explore current railroad mitigation programs, identify the often hidden costs associated with these solutions, identify the future direction of railroad operating requirements, present modern rockfall mitigation systems, and present a total cost analysis between the traditional and modern methods. Introduction Rockfall, falling trees, and debris flow problems along railroad tracks, highways, or other structures can result in damage to those structures, costly shutdowns or slowdowns, high routine maintenance costs, and even personal injury. These problems can be addressed in a variety of manners including slope treatments, catchment ditches, walls, signal fences, or catchment barriers to name a few. One approach widely used by railroads has historically been the use of signal (slide) fences (Figure 1).

2 Figure 1 - Typical Slide Fence Slide Fences Traditional slide fences used by railroads are designed to signal train traffic when a rockfall, debris flow or snow avalanche event has occurred. The systems typically are comprised of conducting cables hung horizontally on 6 inch to 12 inch centers or a heavy gauge wire fabric with 6 inch to 12 inch opening; both methods strung between vertical poles or beams. These wires are in a circuit carrying a current. At the end of the fenceline, this circuit is attached to a warning device circuit using a relay drop. When a rock of sufficient size, debris flow or snow avalanche passes the plane of the fence, one or more wires will be broken, interrupting the fence circuit and activating the alarm. Various warning devices used for these fences include shunt activation of the track light system, phone dialers with recorded messages to the dispatcher, radio messages from trackside robotic radio broadcasts to trains, or warning lights. As evidenced by the wide use of these slide fences, they have performed adequately for the intended purpose. However, there are several shortcomings with the use of these barriers. First, the barriers

3 do not stop material from crossing or impacting the tracks. Thus, whenever a wire is broken by any means, a signal is activated and a warning and consequential train stoppage or slowdown is necessary until the track can be inspected. For significant rockfall events each alarm will affect train traffic, require a track inspection, clearing of the debris, and possible repairs to the track and/or rail bed. Second, many of these warnings will be false alarms since even the smallest rock could break a wire if it impacts a wire. Even with these insignificant events, train traffic is affected and track inspection is required. Third, each time a signal is activated, whether from a significant or an insignificant event, the fence must be repaired. Lastly an accurate account of the labor and material costs for responding to activations, repairing the barriers, train delays, and clearing the tracks has not really been tabulated. The prescient question must therefore be asked: In some cases is there a better way to mitigate rockfall in railroad applications then slide fences? Total Cost of Activation With customers utilizing rail transport more frequently for their shipment of subcomponents involved in their manufacturing processes, just in time delivery has become a very important criterion when deciding upon a shipping method. This poses a large risk to the railroad operators for retaining customers affected by inordinate track delays. This paper will attempt to quantify the direct labor costs (Table 1) associated with the activation of the slide fences along portions of the Appalachian region s rail beds from data provided by a national railroad operating company for only one (1) quarter this past winter. However, it must be noted that these labor costs are only a portion and could be an insignificant part of the total cost for rockfall mitigation (as one should really account for the costs of material, equipment, lost production, etc. as well). Please be aware that over time even the cumulative cost of this direct labor will be very significant. It must be noted that responding to activations already heavily taxes and/or depletes the existing manpower available in most cases creating scheduling havoc for twelve (12) up to twentyfour (24) hours after an occurrence. Added to this is the fact that the rail industry is in a labor streamlining mode and it s quite clear that there isn t the available manpower available to check

4 tracks like there was even ten years ago. Future trends indicate that this labor streamlining will probably never be reversed and may in fact increase if not accelerate. In this environment it is prudent to investigate whether a different approach to rockfall mitigation is warranted. Rockfall Mitigation Methods The following four mitigation approaches will be studied and their material cost outlined as possible techniques to reduce instances of rockfall caused track closure: use of a high tensile wire mesh on the outside of the existing slide fence, use of a high-tensile wire mesh as a rockfall drape to control the energy of the falling rock on the slope, use of flexible rockfall barriers placed typically at the toe of slopes; and flexible barriers to keep debris material from clogging the existing track ways and drainage culverts. Figure 2 High Tensile Wire Mesh High-Tensile Mesh Mounted on Slide Fence: Geobrugg s G65, 3mm TECCO mesh is a high-tensile wire mesh that is fabricated from 256 ksi wire material and the resultant mesh as depicted in Figure 2 has a tensile strength of 10.6 kips/ft while a comparable double-twisted wire mesh commonly used in rockfall today has a tensile strength of 3.1 kips/ft. The use of the high-tensile wire mesh will provide a greater puncture resistance than an ordinary double-twisted wire mesh or the mesh commonly used on the slide fences. If placed loosely on the outside of the slide fence as shown in Figure 3, this mesh should block spurious, small (less than two (2) feet in diameter) rockfall from falling into the rail bed. The rail company s maintenance

5 Incident DATE SLIDE FENCE ACTIVATIONS/ DECEMBER MARCH 2010 Approx. STRAIGHT OVERTIME Labor TRAIN TIME HRS HRS Cost DELAY EXPLANATION 1 12/3/ $ - N SLIDE FENCE ACTIVATED ON ACCOUNT OF LOG FALLING INTO FENCE 4 SPANS, 6 TIERS, 1 BROKEN POST ON ACCOUNT OF SLIDE. USED 2 12/4/ $ 2,640 Y MATERIAL HANDLING TRUCK & 4 MEN TO CLEAN UP 3 12/4/ $ 550 N 1 TON DIRT SLIDE. USED 2 B&B MEN TO CLEAN UP. KEYSTONE COAL OPERATION SLIDE FENCE ACTIVATION ON ACCOUNT OF ROCK FALL. 5 TIERS, 2 POSTS,TRIGGER POST (USED 4 12/9/ $ 2,070 Y BUCKET TRUCK) 5 12/11/ $ 1,250 Y ICE DISLODGED FROM ABOVE CUT, BREAKING TRIGGERING MECHANISM 6 12/19/ $ 16,225 Y 12/19-12/23 SNOW STORM, NUMEROUS SLIDE FENCE ACTIVATIONS FROM FALLEN TREES AND ROCKS. SNOW ACCUMULATION WAS 6-16 INCHES ON 12/19-12/ /23/ $ 1,420 Y 35 TON ROCK FELL, FOULING M-1 & M /26/ $ 3,470 Y 4 SPANS DOWN ON ACCOUNT OF FALLEN TREES & 75 TON MATERIAL 9 1/16/ $ 870 N SLIDE FENCE DAMAGE ON ACCOUNT OF ICE RAIN FALL RESULTED IN 25 TON MATERIAL TO SLIDE INTO FENCE (3 10 1/20/ $ 1,520 Y TIERS). GROUP USED CLAMSHELL TO CLEAR. 11 1/20/ $ 2,300 Y 35 TON OF ROCK BLOCKS FELL, DAMAGING FENCE. CLEARED FOR TRAFFIC AT 8:30 (2 HOURS TO CLEAR ROCK). REMOVED DAMAGED FENCE AND HAD TO BE FLAGGED THROUGH THE NIGHT. 12 1/20/ $ 2,820 Y 8'X8'X20' ROCKS FELL & SEVERAL 3'X4'X4' (LITTLE ROCKS) CONTACTING SLIDE FENCE. A TOTAL OF 36 OVERTIME HOURS WERE WORKED TO CLEAR. 13 1/20/ $ 4,290 Y 150 TONS OF ROCK FELL. TRAIN 177 GOT INTO SLIDE, NO SIGNIFICANT DAMAGE - CLEARED SLIDE FENCE ACTIVATION ON ACCOUNT OF TREE LIMB AND ROCKS/ 14 2/5/ $ 825 Y 3 TIERS, 4 SPANS 15 2/6/ $ 1,780 Y SLIDE BUCK MAIN 16 2/15/ $ 480 N 17 2/15/ $ 1,930 Y SLIDE FENCE ACTIVATION (4 SPANS/3 TIERS) 18 2/15/ $ 1,400 Y SLIDE FENCE ACTIVATION (1 SPAN/ 1 TIER) 19 2/15/ $ 960 N SLIDE FENCE ACTIVATION (1 SPAN/ 3 TIERS) 20 2/19/ $ 1,300 Y SLIDE FENCE ACTIVATION ON ACCOUNT OF ICE 21 2/19/ $ 900 Y SLIDE FENCE ACTIVATION ON ACCOUNT OF ICE 22 2/21/ $ 1,180 Y SLIDE FENCE ACTIVATION 23 2/23/ $ 1,040 N 6'X4'X2' ROCK FELL, KNOCKING TRACK OUT OF ALIGHNMENT (ROUGH TRACK REPORT) 24 2/23/ $ 1,660 Y 20 TON SLIDE FENCE 25 3/15/ $ 960 Y 26 3/15/ $ 3,500 Y SLIDE FENCE CONTACTED BY LARGE ROCK - 2 TIERS/9 SPANS/ BROKE SPRING HANGER AND TRIGGER HANGER. WORK PERFORMED CONSISTED OF 3 M3N/ 1 TRUCK/ 8 HOURS OF WORK PERFORMED. SLIDE FENCE CONTACTED BY 800 TONS OF LOOSE MATERIAL. WORK PERFORMED CONSISTED OF 3 MEN & 4 HOURS. SLIDE FENCE ACTIVATION/ 50 TON MATERIAL FOULING MAIN TRACK 27 3/29/ $ 1,235 N $ 720 N 15 TON MATERIAL AT SLIDE FENCE TOTALS: $ 59,295 Table 1 Activation Data (provided by a national railroad operating company)

6 Figure 3 TECCO Mesh Placement on Existing Slide Fence crew could weld ½ bolts or ½ wire rope clips (Crosby clips) on the outside of the rails as shown in the drawing and the mesh could be fitted over and secured by the bolts with a lashing of bailing wire around the post and mesh or just put the saddle on the u-bolt in the case of the wire rope clips. To make one contiguous sheet, the top and bottom layer of mesh would be connected together using T3 connection clips. Then rocks falling from the adjacent slope would hit the mesh and bounce backward away from the track. In the event that a larger boulder would strike the TECCO mesh and plunge through to the track, the slide fence would be activated and the signal would stop traffic as it does now. The cost for a solution of this type would be $2.50/ft 2 for the mesh (which comes in rolls that are 1,130 ft 2 ) and the ½ bolts and the labor would be borne by the division s maintenance department. When compared to some of the activations listed in Table 1, this solution would have an immediate pay-back.

7 High-Tensile Mesh Drape: In applications with persistent rockfall occurrence at specific locations along a steep slope with a small catchment area between the slope and the track, a rockfall drape can be secured to the slope as shown in Figure 4 to prevent the rockfall from entering into the rail bed. Utilizing 3 mm TECCO mesh, 4 mm TECCO mesh, and/or SPIDER mesh depending upon the rock size and energies involved, the drapes would be secured to the top of the slope with ¾ wire rope anchors, a ¾ top support wire rope, and other mounting hardware. Examples are shown in Figures 5 and 6 below. Figure 4 Rockfall Drape

8 The attractiveness of a rockfall drape is its relatively low cost a 4mm, G65 TECCO drape total installed cost (materials, labor, etc.) is typically $6.00 to $8.00 per square foot depending upon installation conditions and access. Again, another appealing low-cost rockfall mitigation solution. Figure 5 TECCO Mesh Drape Figure 6 SPIDER Mesh Drape Flexible Catchment Barriers: Alternatively, properly dimensioned flexible catchment barriers will stop rocks and grown trees (Figures 7 & 8) before they can reach the tracks. In all but extreme cases this results in the elimination of necessary repairs to track and rail bed, elimination of false alarms and consequent negative effects to train traffic, and greatly reduced repair needs to the fence, thereby reducing overall costs over time. The installed cost of these barriers for low energy rockfall sites can be comparable to signal fences, with virtual elimination of maintenance costs. Though for higher energy rockfall sites the total installed cost may be more, the cost over time will be regained due to the reduction or elimination of maintenance costs. Tests have been conducted on these flexible catchment barriers fitted with alarm systems, to provide warning of unusually large impacts to the barriers by rockfall, debris flow or snow avalanche. Catchment barriers incorporating strength and flexibility represent the most effective and long-term cost efficient rockfall mitigation alternative. The flexibility and strength of these systems allow large

9 amounts of kinetic energy to be dissipated rather than resisted, and allow retention of large static loads. Such barriers keep rocks and debris off the track, reduce the possibility of impacts to trains, and diminish the possibility of damage to the rail or rail bed. Such systems are often the most costefficient option in the short term due to design simplicity and ease of installation, and even more so in the long term due to reduced maintenance requirements. Figure 7 Rockfall Barrier Stopping Fallen Trees Figure 8 Rockfall Barrier Stopping Blocks

10 Figure 9 shows a typical flexible barrier design. Each of the system components is designed to emphasize flexibility and energy dissipation. The H-beam columns, however, only serve to support the nets and cables. During a rock impact, kinetic energy is transferred from the nets to the support cables and finally to the anchors. In the case of energies approaching system design limits, the friction braking elements will engage, further dissipating kinetic energy. These friction braking elements will be discussed in more detail later. Figure 9 Typical Barrier Detail After determining the expected kinetic energy and bounce heights of falling rocks at a site, a rockfall protection barrier with appropriate strength characteristics and height can be designed. Kinetic energy can be determined from site analyses, rock rolling tests, and tools such as the Colorado Rockfall Simulation Program (Barrett and Bower, 1989). The design impact load should be defined as that level of kinetic energy at which the system can stop repeated impacts, without requiring immediate maintenance or repair to remain effective other than occasional cleanout of accumulated debris and other minor maintenance. Flexible rockfall protection barriers as shown in Figure 10 have been developed with impact design load energies ranging from 81 kj (30 ft-tons) to a maximum of 1,000 kj (369 ft-tons) (Duffy, 1992). Recently, systems have also been developed with impact design load energies exceeding 5000 kj (1,667 ft-tons). The ultimate impact load energies that these systems can stop vary, but generally significantly exceed the design load capacities. Cost for these barriers varies with the length of system required, barrier height, post spacing, etc. Depending upon

11 its design, it is not unreasonable that the material cost for 200 barrier would be in the $20,000 to $60,000 range with the installation labor provided by the railroad. Figure 10 - Flexible Rockfall Barrier Numerous flexible rockfall barriers have been installed in North America since Most such barriers have been installed along highways to protect motorists, though use along rail routes is steadily increasing. In Europe, flexible rockfall barriers have been commonplace for years. North American railroad installations include: - along BC Rail s Sea to Sky line in British Columbia, - along CP Rail s line in Kicking Horse Canyon, British Columbia, - along BNSF s line east of Steven s Pass, WA at Gaynor Tunnel (Fig. 11), - along BNSF s line at Cajon Pass near Victorville, CA.

12 Figure 11 Gaynor Tunnel Rockfall Catchment Barrier Debris Flow and Shallow Landslide Barriers: There are numerous cases in the mountainous regions where the rail beds were constructed on or adjacent to steep slopes as this was the best land available at that moment to lay the track. Over time these steep slopes either begin to be undercut from erosion caused by river beds / drainage paths and/or the soil becomes saturated due to excessive rain or flooding. In all cases, the slope begins to fail and the material gives way usually down to bedrock. This large mass of moving material will be comprised of mud, rocks, and/or foliage debris and that can clog the tracks and/or drainage culverts. In all cases the debris flows produced will result in severe damage to the rail beds and surrounding areas. A solution for these debris flow occurrences is barrier systems (DeNatale et al, 1996) that are specially designed to mitigate these phenomena. As shown in Figures 12, 13, and 14, these barriers can be installed in the natural channels in which the debris will travel and hold back the material. This will keep the debris from fouling the track and/or prevent storm water drainage from flooding the track and adjacent land. Depending upon its design, it is not unreasonable that the material cost for barrier would be in the $20,000 to $60,000 range with the installation labor provided by the railroad.

13 Figure 12 Site Before Installation of Barrier Figure 13 Same Site with Barrier Installed and Storm Debris Contained

14 Figure 14 Wooded Debris Held by the Barrier Alarm Systems in Flexible Catchment Barriers: Rockfall is sporadic and unpredictable and because of the multiplicity of factors affecting rockfall dynamics, rockfall mitigation is not and cannot be an exact science guaranteeing the safety of individuals and property. In other words, impacts exceeding the expected kinetic energy rating of a system are always possible, resulting in possible damage to the system or passage of the material through the system. Furthermore, extraordinary event such as snow avalanche which can exceed the design capabilities of the barriers may be possible. To address these possibilities, flexible catchment barriers can be fitted with an alarm circuit to allow the owner to inspect the system after the most significant impacts, to ensure the desired protection level is not degraded by impact damage exceeding the design limits of a particular system, or to inspect for material that is on the tracks or has damaged the tracks. In response to this need, Geobrugg has devised and tested an alarm devise for these barriers which is inexpensive, easy to

15 install, compatible with existing railroad track circuitry, easy to repair and maintain, and most importantly effective and reliable without being over-sensitive. As discussed previously, one of the key components of these barriers are the friction braking elements, located in the barrier support cables and upslope tieback ropes (see Figures 15 & 16). Operating essentially as mechanical fuses, the braking elements dissipate energy through friction, when the capacity of the barrier is being neared. For approximately 90% of all impacts to a barrier, there will be no braking element engagement. Figure 15 shows a new unengaged braking element, and Figure 16 shows a fully engaged braking element. Since the support cable runs freely through the steel pipe of the braking element, when the element is engaged, the rope is essentially lengthened. The signal device takes advantage of the performance characteristics of these braking elements. Figure 15 New Braking Element Figure 16 Fully Engaged Braking Element Impact Sentinel provides a specifically adapted device to a flexible rockfall barrier to detect bigger impacts into the system and trigger an alarm to the maintenance department. The Impact Sentinel: The impact sentinel is a small monitoring device (sentinel) with integrated sensors and antenna mounted on the brake ring. A small wire which is fixed on a pin inside the small box is itself fixed on the other side of the brake ring. Once a brake ring is activated (in case of a bigger impact), the wire

16 pulls the pin out of the sentinel and a message is sent to a close by relay station, 650 1,000 feet away. Figure 17 Elements of the Impact Sentinel The sentinel is electrically independent by means of small solar cells located on top of the sentinel box. The sentinel contains also an integrated shock detector - different shock thresholds can be set for an alarm to be sent out to the relay station. The relay station would then send a signal to the maintenance department by the local phone or a radio system. This signal frequency can be adapted to any applicable local standard. Figure 18 - Installed Sentinel in Castelmola, Italy

17 Though simple in design, testing has proven this to be a viable signal alarm alternative. Because the alarm device is associated with engagement of the barrier braking elements which engage only during large impacts approaching the design load capacity of the barrier, the amount and frequency of actual alarms will be very small. The more frequent smaller rockfall events will be fully retained by the barrier without any damage or necessary repair to the barrier and will also not trigger an alarm; thus greatly reducing or nearly eliminating false alarms and alarms for insignificant events. However, after very large events that may or may not cause damage or allow material to pass the barrier, an alarm will be triggered enabling warning to train traffic and inspection of the site. Conclusions Traditionally rail companies have utilized slide fencing as a sort of warning system to alert operations personnel that unsafe rockfall conditions may exist on the rail bed. Devised during the early stages of rail transportation, significant maintenance personnel were employed to make this practice work and survive. Now during this age of just-in-time delivery, expensive labor costs, and the advent of inexpensive rockfall mitigation techniques, it has become prudent to look at better ways to control and mitigate rockfall problems on the tracks. And when taking into account the total cost of the traditional system, it is clear that the newer technologies will provide more cost effective solutions and lead to safer operations.

18 References Barrett, Bob, and T. Bower, Colorado Rockfall Simulation Program. (CDOT-DTD-ED3/CSM- 89-2B), Colorado Department of Transportation, Denver, CO. DeNatale, Jay S., G.L. Fiegel, and G.D. Fisher, Response of the Geobrugg Cable Net System to Debris Flow Loading. Report for Brugg Cable Products of Santa Fe, New Mexico, U.S.A. 68 p.