RAILROAD DAMAGE FROM THE OCTOBER 16, 1999 HECTOR MINE EARTHQUAKE

Transcription

1 RAILROAD DAMAGE FROM THE OCTOBER 16, 1999 HECTOR MINE EARTHQUAKE By: William G. Byers, P.E. Burlington Northern and Santa Fe Railway 4515 Kansas Avenue Kansas City, Kansas Phone (913) Fax (913)

2 ABSTRACT On October 16, 1999 a magnitude 7.1 earthquake southwest of Ludlow, CA damaged the double track main line of the Burlington Northern and Santa Fe Railway. A westbound Amtrak passenger train, operating within 20 miles of the epicenter at the time of the earthquake, had 22 of its 24 cars derailed. Locomotive units were not derailed. Post earthquake serviceability inspections were completed in less than 8 hours after the event. Track damage included displacement of ballast from cribs and disturbances to track geometry. Bridge damage did not require removal of any bridges from service but six bridges required minor repairs. Track and bridge damage was all within 15 miles of the epicenter. Signal system operation was adversely affected over a greater length of route but not at significantly greater distances from the epicenter. KEY WORDS Earthquakes, Earthquake Damage, Derailments, Bridge Damage, Track Damage INTRODUCTION At 2:46:45 PDT, October 16, 1999 a shallow earthquake with a moment magnitude of 7.1 occurred southwest of Ludlow, CA. The epicenter location was initially reported as N W (1), later corrected to 34.59N W (2). The surface ruptured along a length of 45 km (28 miles) with right-lateral displacements as great as 2.8 to 4.7

3 meters (9 to 15 ft.) (2). The earthquake occurred on a steep surface dipping between 70 and 85 degrees and is described as a complex earthquake beginning with a tiny precursor followed by major events about 4 and 7 seconds after the onset (2). The optimal fault plane solution developed by seismologists with the U.S. Geological Survey, the Southern California Earthquake Center and the California Division of Mines and Geology (3) has a strike of N 29 o W, a dip of 77 o to the east, and a centroid depth of 13.5 km with pure right-lateral motion and moment of Nm. Within the following 5 days, over 50 aftershocks with significant magnitudes had occurred. Aftershocks exceeding magnitude 5.0 occurred three times on October 16 and twice on October 20. The locations of railroads and seismograph stations in the vicinity of the earthquake and contours of peak ground acceleration are shown in Figure 1. RAILROAD INVOLVEMENT The double track main line of the Burlington Northern and Santa Fe Railway (BNSF) is in the vicinity of the fault rupture and was damaged by the earthquake. The Union Pacific Railroad (UP), which has a double track line in the area, but located over twice the distance from the epicenter and about 20 times the distance from the surface rupture, was not damaged. The shortest distance from the UP to the epicenter is slightly over 30 miles. The damaged portions of the BNSF are, with the exception of signal system damage, within 13 miles of the epicenter. The end of the surface rupture is slightly over 20 miles from the nearest point on the UP. The shortest distance between the surface rupture and the BNSF is about one mile.

4 EFFECTS ON RAILROAD OPERATIONS Post earthquake inspections to determine serviceability of the railroad were completed on the BNSF in less than 8 hours after the event. This included notification time for inspection personnel and travel time from their homes. More detailed inspections of bridges for specific repair planning were conducted the following day. Inspections were completed and normal operation restored on the UP in about 4 hours. The affected portion of the BNSF has automatic block signals and operation is double track, controlled by track warrants. Authorized speed is 79 mph for passenger trains and 55 mph for freight trains, except that freight trains satisfying certain specified requirements can be operated at 70 mph. Both tracks have 136 lb. continuous welded rail. Ties in the affected area are wood ties renewed in 1987, and A westbound Amtrak passenger train running on BNSF Track No. 1 (the north track) had cars derailed at MP as a result of ground motions caused by the earthquake. Four passengers and one crew member received minor injuries. All cars remained coupled and upright and were re-railed or cleared by 2:10 PM (14:10 PDT). Track No. 2 (the south track) was opened to traffic at 2:35 PM (14:35 PDT) and Track No. 1 was returned to service at 11:15 PM (23:15 PDT), October 16, both at 25 mph. The initial 25 mph speed restriction was raised to 40 mph on October 21. All speed restrictions were removed on November 5.

5 TRACK, BRIDGE AND SIGNAL DAMAGE Track and bridge damage was limited to an area within 13 miles of the epicenter and within 9 miles of the surface rupture as shown in Figures 1 and 2. Signal system operation was adversely affected over a greater length of route and at greater distances from the epicenter. All damage was within 25 miles of the epicenter and within 15 miles of the surface rupture. Track damage included buckled track, other disturbances to track alignment and surface and displacement of ballast from cribs. Track alignment was disturbed at several locations near MP Track at the end of Bridge No , which was virtually skeletonized by displacement of ballast and/or vertical movement of the track, is shown in Figure 3. There were no pulled apart joints. Due to the short distances to the fault rupture, vertical accelerations may have made a significant contribution to track damage, including displacement of ballast from cribs and apparent upward displacement of ties relative to ballast on bridges. Data on bridges in the affected area, including type, distances from fault trace and from epicenter and estimated peak ground acceleration at each bridge are summarized in Table 1. Bridge damage, summarized in Table 2, was not extensive enough to require removal of any bridges from service but six bridges suffered minor damage, some of which required repair. One damaged bridge is located near the site of the derailment but

6 did not contribute to the derailment. The most prevalent damage was to unreinforced or lightly reinforced wing walls, primarily separations at cold joints, preexisting cracks or inadequately reinforced corners. Examples of the more severe damage are shown in Figures 4 and 5. One bridge, shown in Figures 6 and 7, with prestressed concrete slab spans and substructure built from precast concrete elements had damage concentrated at locations where the substructure elements were joined together. The substructure consisted of two column bents under each track, precast dump planks and columns supporting the dump planks beyond the ends of the caps. Individual spread footings were cast monolithically with the columns. The tops of the columns were grouted into 10 inch deep sockets in the 20 inch deep caps. Cracking originated in the relatively thin portion of the caps adjacent to the sockets. There was minor spalling where dump planks contacted precast columns. The surface under the bridge was paved for a roadway and erosion protection. The paving was cracked at the base of several columns. In the segment between the first and last damaged bridges, 13 bridges with independent spans under each track and 14 culverts were undamaged. All bridges, both damaged and undamaged, are ballast deck. For reference and comparison, damage to an abandoned concrete building located near the tracks in Ludlow is shown in Figure 8. Damage to highway bridges was reported to be less than in other Southern California earthquakes of comparable or lesser magnitude (4).

7 Signal problems included wrapped and broken wires and overturned relays. DERAILMENT The Amtrak passenger train, running at about 60 mph on Track No. 1 at the time of the earthquake, had 22 of the 24 cars and one wheel set each on the first and 14 th cars derailed at MP as a result of ground motions caused by the earthquake. The engineer s observation of the track moving ahead of the train and the fact that none of the three locomotive units was derailed strongly indicate that the derailment was caused by ground movement under the train, not by earthquake damage to the track ahead of the train. Based on the locations of the epicenter and the surface rupture shown in Figure 2 of Reference (3) and in Figure 2 of this report, the derailment site is nearly in line with the surface rupture and probably on the hanging wall side of the fault. It is about 13 miles from the epicenter and only about 2 miles beyond the north end of the surface rupture. The derailment location would definitely have been subject to near-fault effects including a relatively long pulse of strong acceleration in the fault strike-normal direction. At this location, the angle between the track and the strike of the fault is not greater than 20 degrees. Other instances where trains or cars on tracks near to and making small angles with the fault rupture were derailed or overturned include the 1906 San Francisco and 1995 Kobe earthquakes.

8 A record of the accelerations obtained 4.5 miles from the fault rupture during the 1994 Northridge earthquake shows a large acceleration pulse in the strike-normal direction with a 0.3 second duration of near-maximum acceleration (5). A similar pulse would have resulted in strong lateral force of the rail against wheel flanges while the train, moving at about 60 mph, traveled between 20 and 30 feet. This distance greatly exceeds the 2 to 5 feet required for wheel climb. The fault involved in the Northridge earthquake is a blind thrust fault. A strike-normal velocity record containing a segment indicating nearly uniform high acceleration for a period in the order of one second is shown in Figure 2 of reference (5). It was obtained at the Lucerne station, located near the fault and about 30 miles from the epicenter, during the 1992 Landers earthquake on strike-slip faults. This pulse was caused by rupture propagation toward the station and was not observed at Joshua Tree near the epicenter (5). A similar relatively long pulse of strike-normal acceleration is highly probable at the derailment site, located approximately 12 miles from the epicenter and less than one mile from a line through the epicenter parallel to the trend of the surface rupture. At the Hector seismometer station, the east-west component of acceleration, the only component measured, was g. The locations of this station and other nearby seismometer stations are shown in Figure 9 together with the components of acceleration and velocity at these stations. The Hector station is nearly twice as far from the fault rupture as the derailment site is. For an acceleration pulse duration similar to the Northridge pulse, sustained acceleration normal to the track at the derailment site is estimated to be in the 0.35g to 0.40g range which would produce lateral force to vertical

9 force (L/V) ratios of 0.7 to 0.8. Such L/V ratios are large enough to cause wheel climb on worn rail. The rail in Track No. 1 was laid in 1986 and had carried over 600 million gross tons at the time of the earthquake. The fact that the 14 th car had only one wheel set derailed indicates that the rail was not overturned under the first 13 cars although it was apparently overturned at some point behind the 14 th car, probably as a result of the derailment. Spectral accelerations at the natural frequencies of the cars could possibly be a contributing factor. Figure 10 shows values derived from 0.3 second, 1 second and 3 second pseudo-acceleration maps together with roll frequency ranges for equipment similar to the types derailed. Frequencies for loaded mail cars would probably be lower than for empty cars. Only the first five cars were passenger cars. SUMMARY A large (M=7.1), shallow, strike-slip earthquake derailed a passenger train operating within 2 miles of the fault rupture and caused minor damage to track and bridges within 13 miles of the epicenter and to signal equipment within 25 miles of the epicenter. Bridge damage occurred at joints between precast elements, inadequately reinforced corners of wing walls, cold joints and preexisting cracks. The derailment was probably caused by wheel climb produced by lateral forces resulting from the earthquake. The BNSF line was out of service for approximately 12 hours while the derailment was being cleared and operated with speed restrictions of 25 mph for 5 days and 40 mph for an

10 additional 13 days. The UP line, which was undamaged, was out of service about 4 hours for inspection. REFERENCES (1) USGS National Earthquake Information Center, QED Earthquake Bulletin, Updated: Tuesday, 1999 October 19 20:54 MDT (2) Person, Waverly J., Seismological Notes September-October 1999, Seismological Research Letters, Vol. 71, No. 5, September/October 2000, pp (3) Preliminary Report on the 16 October 1999 M 7.1 Hector Mine, California, Earthquake, Seismological Research Letters, Vol. 71, No. 1, January/February 2000, pp (4) Simek, Jaro and Murugesh, Ganapathy, Post Earthquake Investigation Team Report Performance of Bridges during the Hector Mine Earthquake of October 16, 1999, Caltrans Engineering Service Center, 1999, 54 pages. (5) Somerville, P. G., The characteristics and quantification of near fault ground motion, Proceedings of the NCEER Workshop on National Representation of Seismic Ground Motion fpr New and Existing Highway Facilities, May 29-30, San Francisco, California, National Center for Earthquake Engineering Research, Buffalo, NY, 1997

11 LIST OF TABLES Table 1. Bridge Data Table 2. Bridge Damage LIST OF FIGURES Figure 1. Railroads with measured and projected peak ground accelerations Figure 2. Fault trace and BNSF Railway track and bridges Figure 3. Disturbed ballast at BNSF Bridge No Figure 4. North side of BNSF Bridge No Figure 5. Wing wall break at preexisting crack and cold joint, Br Figure 6. General view of BNSF Bridge No Figure 7. Cracks and cap details BNSF Bridge No Figure 8. Damaged concrete building at Ludlow Figure 9. Seismometer stations and peak ground acceleration components Figure 10. Pseudo-acceleration in frequency range for derailed cars

12 Table 1. Bridge Data Bridge Distance in Miles from Aprox. Description Damage Number Epicenter Fault Trace PGA g Rail stringers on conc. pier & abuts. None g Prestr. conc. slabs on conc. pier & abuts. Minor g Steel deck girders on stone abuts. None Steel deck girders on conc. abuts g Prestr. conc. on conc. pier & abuts. None Arch g Double 4x3.5 stone arch None Double 4x3.5 conc.arch g Prestr. conc. slabs on conc. pier & abuts. None Reinf. conc. slabs on conc. pier & abuts g Reinf. conc. slabs on conc. pier & abuts. None g Prestr. conc. on H piles None g Prestr. conc. slabs on conc. abuts. None g Prestr. conc. slabs on conc. pier & abuts. None g Prestr. conc. slabs on conc. pier & abuts. Minor Reinf. conc. slabs on conc. pier & abuts g Reinf. conc. slabs on conc. pier & abuts. Minor g Rail stringers on conc. piers & abuts. None g Timber stringers on conc. pier & abuts. Minor Reinf. conc. slabs on conc. pier & abuts g Prestr. conc. slabs on conc. pier & abuts. None Reinf. conc. slabs on conc. pier & abuts g Prestr. conc. slabs on precast conc. substr.* Minor g Prestr. conc. slabs on conc. pier & abuts. None g Rail stringers on conc. abuts. None Reinf. conc. slabs on conc. abuts g Rail stringers on conc. abuts. None Reinf. conc. slabs on conc. abuts g Rail stringers on conc. abuts. None Reinf. conc. slabs on conc. abuts g Rail stringers on conc. abuts. Minor Reinf. conc. slabs on conc. abuts g Rail stringers on conc. abuts. None Reinf. conc. slabs on conc. abuts g Prestr. conc. slabs on conc. pier & abuts. None g Prestr. conc. slabs on conc. abuts. None Bridges through are beyond end of surface rupture * Precast substructure elements consist of integral column-footing units, caps and dump plank

13 Table 2. Bridge Damage Bridge Distance in Miles from Aprox. Damage Number Epicenter Fault Trace PGA g Wing walls on north side separated from abutments Connection between wing walls and parapet walls was not adequately reinforced. Similar wing walls on south side were undamaged g Displacement at cold joints and other cracking in unreinfoced wing walls, spalled concrete around dowels between old and new concrete placed for modification g Cracked wing walls, displaced ballast curb, concrete deck slabs separated, track and ballast disturbed, severe loss of ballast at end of bridge g Appreciable damage to wing walls on south side g Cracks in precast caps at sockets for precast columns, cracks in floor pavement where cast around columns, displacement of precast concrete wingwall dump planks and spalling of precast wingwall support posts g Plain concrete extension to SE wing wall broken.

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