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Home My WebLink About3190; RANCHO SANTA FE RD; PRELIMINARY FINDINGS AGUA DULCE SLOPE FAILURE & BORROW PIT EVAL; 2005-07-20( SOIL & USTIPIG, :He P H 0 N P.O. Box 600627 (619) 280-4321 Sari Diego, CA 92160-0627 _____ (877) 215-4321 6280 Riverdale Street fAx San Diego CA 92120 *e' (619) 280-4717 www.scst.com 0 PRELIMINARY FINDINGS • AGUA DULCE SLOPE FAILURE AND BORROW PIT EVALUATION LA COSTA TOWN SQUARE RANCHO SANTA FE ROAD AND LA COSTA AVENUE CARLSBAD, CALIFORNIA PREPARED FOR: MR. WAYNE GEORGE ASPEN PROPERTIES 8799 BALBOA AVENUE, SUITE 270 SAN DIEGO, CALIFORNIA 92123 PREPARED BY: SOUTHERN CALIFORNIA SOIL & TESTING, INC. 6280 RIVERDALE STREET SAN DIEGO, CALIFORNIA 92120 Providing Professional Engineering Services Since 1959 0 ( SOIL & TESTING, u(. -I LU ( z I - j o July 20, 2005 Mr. Wayne George Aspen Properties 8799 Balboa Avenue, Suite 270 San Diego, California 92123 PHONE I P.O. Box 600627 (619) 280-4321 I San Diego, CA 92160-0627 TO I. I. I P E E 6280 Riverdale Street (877) 215-4321 F A X San Diego, CA 92120 (619) 280-4717 www.scstcDrn SCS&T No. 0511038 Report No. 2 Subject: PRELIMINARY FINDINGS AGUA DULCE SLOPE FAILURE AND BORROW PIT EVALUATION LA COSTA TOWN SQUARE CARLSBAD TRACT NO. C.T. 01-09 RANCHO SANTA FE ROAD AND LA COSTA AVENUE CARLSBAD, CALIFORNIA Dear Mr. George: In accordance with your request, we have prepared this report to present preliminary findings with regard to our evaluation of the recent off-site slope failure and the borrow pit located to the northwest of the subject site. The purpose of our evaluation is to determine the potential impact of the slope failure and borrow pit on the subject project. The approximate location of the slope failure and the borrow pit are depicted on the attached Plate No. 1. 1 DISCUSSION Mass grading for the new alignment of Rancho Santa Fe Road adjacent to the subject sits was completed in early 2003. A borrow pit was excavated beneath a portion of Rancho Sarta Fe Road and a portion of the subject site (see Plate No. 1) without the knowledge or permiss-on of Aspen Properties, the owner of the subject site. The borrow pit was excavated to obtain material suitable for capping and slope construction, as well as to provide a suitable area fcc the placement of rocky materials (rock fills and rock/soil fills). The borrow pit extended to a depth of about 65 feet below the surface of the new Rancho Santa Fe Road, with the deepest point at about elevation 285 feet above mean sea level (mst). The borrow pit had a length of about 600 feet, a width of about 250 feet, and extended about 75 feet into the subject site. In early 2005, a large failure occurred in the northwest-facing fill slope directly to the northwest of the borrow pit. The fill slope was built during the construction of the old Rancho Sant-3 Fe Road alignment. The slope descends about 30 to 35 feet at about a 1.5:1 (horizontal:verical) 0 Aspen Properties July 20, 2005 La Costa Town Square SCS&T No. 0511038-2 Page 2 inclination from the northwestern edge of old Rancho Santa Fe Road to the rear yards of residences adjacent to Agua Dulce Court The failure surface associated with the slope failure is somewhat irregular, but generally occurs within a topsoil layer at the contact between the fill and the underlying natural ground. Operations to stabilize the failure area and reconstruct the slope are currently under way. Major elements of the slope stabilization and slope reconstruction process include the installation of temporary dewatering and shoring systems, and the construction of a new buttress slope with a permanent subdrainsystem. The introduction of groundwater into and along the base of the failed fill mass appears to have played a major role in the slope failure. 2 PRELIMINARY CONCLUSIONS AND RECOMMENDATIONS Based on the information obtained to date, the following preliminary conclusions and recommendations are presented: It appears that the build-up of groundwater within the rocky materials placed in the borrow pit, and the subsequent infiltration of the groundwater within and along the base of the failed fill mass, was the triggering mechanism for the slope failure. This condition was most likely exacerbated by the heavy rainfall received last winter. Other factors, such as improper site preparation during grading of the fill slope, are also likely to have played a role in the slope failure. At its closest point, the current southeast limit of the slope failure is located adjacent to the northwestern property line of the subject site (see Plate No. 1). The nearest proposed settlement-sensitive imprpvements (exterior retaining walls and underground improvements) will be located about 75 feet from the edge of the slope failure. Assuming that the slope stabilization and reconstruction operations currently under way are successful, the slope failure is not expected to have any long-term, slope-stability- related, adverse effect upon the subject project. The, bottom of the borrow pit is at about elevation 285 feet msl. It is our understanding that the current plans for the reconstructed buttress slope include horizontal subdrains at an elevation of about 300 feet msl, extending about 15 to 20 feet into the rocky borrow pit fill. There is a potential that groundwater could pond within the rocky borrow pit fill between elevations of 285 and 300 feet msl, and that this groundwater could migrate to the southeast into the subject site through relatively permeable sandstone beds, fractures or other conduits. Such groundwater migration could increase the risk or groundwater emerging at the surface and creating adverse groundwater conditions Vice Aspen Properties July 20, 2005 La Costa Town Square SCS&T No. 0511038-2 Page 3 within the subject site. It is recommended that several exploratory borings be drilled along the southeastern edge of the borrow pit to obtain additional information regarding this potential adverse condition. Alternatively, consideration may be given to the use of permanent dewatering wells in lieu of, or in conjunction with, the currently proposed horizontal subdrains. The use of permanent dewatenng wells extending to the bottom of the borrow pit would prevent the ponding of groundwater within the lower portion of the rocky borrow pit fill as discussed above. Based upon the surveyed configuration of the borrow pit and the proposed grading as shown on Plate No. 1, it is possible that rocky borrow pit fill will be exposed in portions of the 'planned cut slope along the southeast side of Rancho Santa Fe Road. If this occurs, replacement of the rock fill with a stabilization fill will likely be necessary. It is recommended that several exploratory backhoe trenches be excavated along the alignment of the cut slope in question to further assess this potential adverse condition. A proposal providing details regarding the proposed borings and trenches discussed in Items 3 and 4 above, as well as an estimate of associated costs, will be forthcoming. If you have any questions after reviewing this report, please do not hesitate to contact this office. This opportunity to be of professional service is sincerely appreciated. Respectfully submitted, SOUTHERN CALIFORNISOIL4 TESTING, IN ///eyl.€ L 1 : Mike P. Farr, CEG GCCGST Senior Engineering Geolos ' / -. DAMPF:sd (6) Addressee / - - . " . i, N. SCS&T LEGEND I z Approximate Limits of Slope Failure U •', Approximate Limits 0 00w Pit (Based on Topograph V. *41,/ T Y" v, ell ,#1 '',<' 213 - -- V /} 3_ / — --- -In 00 - '-,> •-•• \ -\ - L—\- SOUTHERN C \LIFORNIA X (3L SOIL & TESTING, INC. PAD- \.. -.>_--\ ,•---- - -- - —-\. - LaCosta Town Square - - Job No 05110382 Pe No 7/19/05 VIDEO-CLIPS ON CD Several video-clips of some of the noteworthy events, during the project, are presented on the attached CD. These video clips require Windows Media Player® and QuickTime® to run on a PC/(Windows®) computer. QuickTime® movie player one can be downloaded free at: http://www.apple.com/quicktime/download/win.html. Video Clip No. 1: On May 20, 2005, after surface communication of the progressive back- cut failure had reached the toe-of-new RSF road slope, an exploratory excavation was performed along the contact. During the excavation, a dramatic release of the groundwater table within the adjacent rock-fill prism is depicted as it is directed down slope. Video Clip No. 2: This video-clip depicts the same event as video-clip No. 1, but viewed from mid-slope. Video Clip No. 3: On February 6, 2006, installation of a horizontal well (HW-1) is commenced from the southern portion of the key-way extending into the new RSF road rock-fill prism at an invert elevation of 288 feet (MSL). The video depicts pulverized, fine granite rock as it is returned through drill steel encasing during the drilling process. Video Clip No. 4: On February 7, 2006, at approximately 3 p.m., water is returned from the rock prism via HW-1 at a horizontal length of 135 feet. This video-clip provides positive evidence that communication has been achieved between the new slope key-way and the rock-fill prism. Video Clip No. 5: On February 7, 2006, at approximately 7 p.m., drilling operations for HW- 1 terminated at a horizontal length of about 151 feet (measured from the key-way entry) which correlates with the median of the new RSF road (in plan) The horizontal drilling extended roughly 50 feet into the rock-fill prism. Video Clip No. 6: On February 16, 2006, HW-1 is completed by installing approximately 140 feet of 4-inch diameter, schedule 80, PVC pipe. The last 50 feet of the pipe was slotted to receive groundwater from the rock-fill prism. The video depicts clear water drainage flowing from the rock-fill rock prism at an approximate rate of 1.3 gallons per minute. Video Clip No. 7: On February 22, 2006, during installation of horizontal well No. 2, air pressure caused ground fissures during percussion drilling, resulting in attendant displacement of sub-surface groundwater along the base of shoring wall No. 1 (i.e. man-made geyser). This condition did not prevent the successful installation of a second horizontal well. 0