WMH Corporation, Inc. April 29, 2014 50 West San Fernando Street, Suite 950 Project No.: 2013-105-WCB San Jose, CA 95113 Attention: Mr. Tim Lee Subject: Surface Fault Rupture Displacement Hazard (SFRDH) Analysis for Bridge Structures Widening of Walnut Creek Bridges – Bridge Numbers: 28-0240L/R (CC-4-13.4) (within Alquist-Priolo Earthquake Fault Zone) I-680/SR-4 Interchange Project - Phase 3 (SR-4 Widening), Contra Costa County, California Dear Mr. Tim Lee: As requested, we are providing this memorandum for the “Surface Fault Rupture Displacement Hazard (SFRDH)” analysis for the widening of Walnut Creek Bridges that are part of the “I- 680/SR-4 Interchange Project – Phase 3 (SR-4 Widening)” (Phase 3 Project). SUMMARY Caltrans bridges 28-0240L/R are located approximately 2 miles north of downtown Concord, Contra Costa County (CC-4-13.4; Plate 1). The bridges are within the Alquist-Priolo Earthquake Fault Zone (EFZ) for the Concord fault (Plate 2). The Walnut Creek Bridges straddle the mapped Main Trace of the Concord fault. The mapped East Trace of the Concord fault is located approximately 230 meters (755 feet) east of the Main Trace. The Concord fault is capable of moderate displacements, but the probability of off-fault displacements decreases rapidly with increasing distance from the fault. We expect moderate displacements to occur along the Main Trace of the Concord fault within the bridge footprint. Conservatively, we have assumed that all of the potential slip will occur along the Main Trace of the fault. The potential off-fault horizontal displacements within the bridge footprint are; 0.22 meters (8.7 inches) deterministically and 0.57 meter (22.4 inches) probabilistically. Vertical displacement of approximately 10% of the horizontal value, which is up to 0.06 meters (2.7 inches), should be assumed to occur along with the horizontal displacements. The two bridges are parallel to one another and perpendicular to the Concord fault. Therefore, the results of the analyses are identical and this memorandum presents the results for both bridges. Per Memo to Designers (MTD) 20-8, the larger of the deterministic and probabilistic values or a site-specific value should be used for design. 2360 Qume Drive, Suite A, San Jose, CA 95131 l P (408) 452-9000 l F (408) 452-9004 l www.PARIKHnet.com San Jose ♦ Oakland ♦ Walnut Creek ♦ Sacramento ♦ Fresno WMH Corporation I-680/SR-4 Interchange Project - Phase 3 (SR-4 Widening) Project No: 2013-105-WCB April 29, 2014 Page 2 INTRODUCTION This evaluation was prepared for the Widening of Walnut Creek Bridges as a part of I-680/SR 4 Interchange Project - Phase 3 (SR 4 Widening) in accordance with Caltrans policies outlined in MTD 20-8 and MTD 20-10. Caltrans requires a fault rupture evaluation if a bridge is located within an Alquist-Priolo Earthquake Fault Zone or within 1,000 feet of an un-zoned fault 15,000 years or younger in age. Caltrans policies regarding evaluation of fault rupture hazard at bridges are described in MTD 20-10. The Walnut Creek Bridges cross the mapped Main Trace of the Concord fault at approximately a right angle within the Alquist-Priolo Earthquake Fault Zone (EFZ) for the Concord fault (Plate 2). The mapped East Trace of the Concord fault is located 230 meters (755 feet) east of the mapped Main Trace. That is 174 meters (570 feet) east of the eastern abutments of the bridges. The SR-4 Bridges over Walnut Creek (Bridge Numbers: 28-0240L/R, County Route 4, MP 13.40) were built in 1981. They are 5-span slab bridges on reinforced concrete pile bents with reinforced concrete abutments. Our evaluation of the surface fault rupture displacement hazard at the Walnut Creek Bridges included review of published literature and consultants’ reports, evaluation of topography and air-photos, discussions with experts, and a site reconnaissance. Since the bridges are parallel and are perpendicular to the Concord fault, we performed deterministic and probabilistic calculations of potential off-fault displacements within the footprints of both bridges in compliance with MTD 20-10 utilizing the Excel-based worksheet provided by Caltrans. This memorandum presents the findings and results for both bridges. PUBLISHED LITERATURE AND CONSULTANTS’ REPORTS Poland (1935) Poland (1935) recognized the presence of a fault in the Ygnacio Valley (southern Concord) based upon depth-to-groundwater data that indicated the presence of a linear subsurface barrier to flow of groundwater. He named it the Concord fault and showed it trending N 30 degrees West on the map in his Stanford University Master’s thesis. Sharp (1973) Sharp (1973) mapped the Concord fault based upon surface evidence of creep. Beginning north of Suisun Bay, Sharp projected the Concord fault southeastward across the bay, through the marshes at the mouth of Pacheco Creek and along the base of the west-facing flank of “Tank Farm Hill” east of Buchanan Airfield (coincident with the man-made linear channel of Walnut Creek). Sharp’s map points to evidence of right-lateral (“RL”) displacement on Waterfront Road (approximately 13,400 feet northwest of the subject SR-4 bridges), noting a “bend in bridge on WMH Corporation I-680/SR-4 Interchange Project - Phase 3 (SR-4 Widening) Project No: 2013-105-WCB April 29, 2014 Page 3 Waterfront Road consistent with RL movement near W abutment” and where a Santa Fe Railroad bridge crosses the Walnut Creek channel (approximately 7,000 feet northwest of the subject SR-4 bridges), noting “RL warping of railroad bridge built in 1965”. However, in 1973, where SR-4 crossed Walnut Creek on a bridge built in 1938 (now called the Marsh Road Bridge), Sharp noted “bridge no obviously deformed by RL offset”. South of SR-4, Sharp noted “Possible old SW-facing scarp” where Solano Way parallels the linear channel of Walnut Creek and “Probably natural sag, now destroyed by grading” where the channel bends toward the west (subsequently developed as Stanwell Drive). At Concord Avenue (approximately 9000 feet southeast of the subject SR-4 bridges), Sharp noted the railroad “track not offset” and “small RL offset of curb”. Further southeast, Sharp mapped two parallel fault traces several hundred feet apart with numerous notations of evidence of fault creep. Burkland and Associates (1974) AP-0062 Burkland and Associates conducted a fault investigation of the site of the proposed Deutscher Company Warehouse at the intersection of Solano Way and the Southern Pacific Railroad tracks. (The site is labeled with the number “0062” on the attached Plate 3.) The investigation included several magnetometer and seismic refraction surveys and three trenches. Their findings led Burkland and Associates to conclude that no active faults cross the site. (Consequently, those findings suggest that the Main Trace of the Concord fault is located west of this site.) Burkland and Associates (1973 – 1976) – AP-0001.1 through AP-0001.4 Burkland and Associates conducted fault investigations at several places within the Stanwell Drive Office complex (located between 5,000 and 7,000 feet southeast of the subject SR-4 bridges). (The site is labeled with the number “0001” on the attached Plate 3.) The investigation included trenches that exposed the main trace of the Concord fault within a 30 to 50 foot wide zone trending northwest across the sites and projecting northwestward beneath the linear portion of Walnut Creek toward the subject SR-4 bridges. ENGEO, Incorporated (1977) – AP-0001.5 ENGEO (1977) also trenched across the Main Trace of the Concord fault within a portion of the site located near the north end of Stanwell Drive. (The site is labeled with the number “0001” on the attached Plate 3.) ENGEO exposed the fault in trenches and confirmed the location of the surface trace as previously mapped. Woodward-Clyde Consultants (1979) AP-0001.6 and AP-0001.7 Woodward-Clyde Consultants (1979) trenched across a site located west of Stanwell Drive. (The site is labeled with the number “0001” on the attached Plate 3.) Woodward-Clyde Consultants concluded that the Concord fault does not cross the then proposed building location. WMH Corporation I-680/SR-4 Interchange Project - Phase 3 (SR-4 Widening) Project No: 2013-105-WCB April 29, 2014 Page 4 Harding Lawson Associates (1982) – AP-0001.8 Harding Lawson Associates (1982) also trenched the site at the north end of the Stanwell Drive cul de sac and found the eastern edge of the Main Trace of the Concord fault essentially as previously mapped. ENGEO (1977) – AP-1741 ENGEO conducted a fault investigation on the site of the then proposed Kiewit Office Building located a several hundred feet southwest of the subject SR-4 bridges over Walnut Creek. (The site is labeled with the number “1741” on the attached Plate 3.) ENGEO’s study included geophysical profiling and trenches which showed that the main trace of the Concord fault does not pass through that site and is probably located beneath the Walnut Creek channel east of that site. Peter Kaldveer and Associates (1983) AP-1562 Peter Kaldveer and Associates (1984) conducted a fault investigation of a large site located north of SR-4 and east of Solano Way. (The site is labeled with the number “1562” on the attached Plate 3.) The trenches crossed the mapped eastern trace of the Concord fault (derived from Sims, et al; 1973) and found no evidence for past faulting. Subsequently, Wills and Hart (1992) recommended that the eastern trace and the Special Studies Zone associated with it be eliminated. That fault trace and zone were not included on the 1993 release of the revised maps. Peter Kaldveer and Associates (1984) AP -1712 Peter Kaldveer and Associates (1984) conducted a fault investigation of the seven acre site located immediately east of Walnut Creek, north of SR-4 and south of then Arnold Industrial Way (now Imhoff Dive).