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HomeMy WebLinkAboutCT 82-18; BATIQUITOS POINTE; GEOLOGIC STABILITY; 1983-04-21rim C.-T52- _t 9 3467 Kurtz Street San Diego, California 92110 (619)224-2911 Woodward-Clyde Consultants April 21, 1983 Project.No. 53147K-SIO1 Sammis Properties 5030 Camino de la Siesta San Diego, California 92108 Attention: Mr. Fred Vaughn GEOLOGIC STABILITY OF BLUFF TOP DEVELOPMENT BATIQUITOS POINTE CARLSBAD, CALIFORNIA Gentlemen: In accordance with your request, we have made an evaluation of the geologic stability of the existing bluff on the subject site and of the potential effect of the proposed project development on the bluff. The purpose of the evaluation is to address the requirements of the 1976 Coastal Act. To satisfy these requirements, bluff and cliff developments must be sited and designed to assure stability and structural integrity for their expected economic life span while minimizing alteration of natural landforms. This letter presents the results of our evaluation, made in conjunction with our geotechnical investigation for the proposed project. The subjects considered are presented below in the order suggested on page 3 of a memorandum entitled "Geologic Stability of Bluff top Development," adopted by the Coastal Commission and dated May 3,1977. We understand that the proposed project will include subdividing the parcel into 3 lots and an open space easement. Present plans indicate that site grading and development for this phase will only be performed on Lot 2. Site grading will create cut and fill slopes, constructed at maximum inclinations of 2:1 (horizontal to vertical), to heights of less than 15 feet. Proposed fill slopes in the bluff top area are generally less than 5 feet high. No development on or alteration to the bluff is planned. A siltation basin will be constructed in the open space-easement located along the south-central portion of the western boundary. Site improvements will consist of nine building pads that will contain 2 and/or 3 story structures of woodframe and masonry construction and one level of underground parking. Swimming Consuliing Engineers. Geologists and Environmental Scientists Offices in Other Principal Cites • Sammis Properties Woodward-Clyde Consultants Project No. 53147K-SIO1 April 21, 1983 Page pools and lined shallow ponds are also planned. The planned location and layout of the major project structures adjacent to the bluff top are shown on the Site Plan (Figure 1). Geologic Setting and Surface Conditions The subject bluff, or slope, is the western end of the slope. which forms the north boundary of Batiquitos Lagoon. The relatively flat surface north of the slope is part of a broad, longshore marine 'transgressive terrace at approximate elevation 50 feet (MSLD). 'The relatively horizontally bedded terrace materials, characteristic of the Lindavista Formation of Quaternary age, are deposited on a wavecut platform on the underlying B member of the Tertiary age Santiago Formation. The relatively flat terrace surface at the top of the slope is currently under cultivation for flowers and is subject to daily irrigation. Two buried telephone trunk cables and a' 12 3/4 inch gas line extend down the slope and across the lagoon from north to south. The A.T.&S.F. railroad occupies a slot cut along the eastern side of the proposed subject project. (1) Cliff Geometry The natural occurring slope essentially forms the southern and southwestern boundary of Lot 2. It extends from the railroad cut at the southeastern corner of Lot 2 around to Old Highway 101. The elevation of the top of the slope varies from about 40' to 50 feet and the slope height varies from about 25 to 40 feet. The slope inclination varies from about 0.75:1 to 3.3:1' (horizontal to vertical). The slope is generally covered with a fairly heavy vegetative growth. The Coastal Commission criteria defines the area of demonstration for any slope greater than 10 feet high as the slope itself and any ground at the top of the slope lying between the slope top and a line projected upward from the toe of the slope at a 20 degree angle; or the slope plus .a minimum distance of 50 feet back from the top of slope, whichever is greater. For the subject development, the proposed fill slope and about 5 to 10 feet of the southern end of Buildings 5 and 6 fall within the defined area of demonstration based on the 50-foot minimum distance from top of slope. Figure 1 shows the subject slope in plan view. Figure 2 presents cross sections through the south and southwesterly facing portions of the slope. The existing natural slope inclination, the designated 20 degree slope line, the 50-foot minimum distance from top of slope, geologic formational Sarnmis Properties Woodward-Clyde Consultants Project No. 53147K-SIO1 April 21, 1983 Page 3 contacts, and proposed final grades and locations of structures are indicated on the sections. (2) Historic, Current, and Foreseeable Cliff Erosion Aerial photographs made in 1953 and in 1964 by Cartwright Aerial Surveys, Inc. for the U.S. Department of Agriculture were reviewed and compared to existing conditions. No significant changes in the subject bluff and slope were noted since 1953. The overall slope configuration and the extent of vegetative cover appears to be approximately the same. It appears that no significant erosion of the slope has occurred during the past 30 years. Our visual examination of the area indicates that the current grading for agricultural purposes has resulted in a concentration of surface runoff at several locations. This has resulted in a minor amount of gullying at the top ofthe slope. In our opinion, there should not be any significant erosion of the slope during the foreseeable future. (3). Geologic Conditions The terrace surface at the top of the slope is underlain by from 2 to 4 feet of slightly porous loose to medium dense, brown silty medium to fine sandy topsoil. The topsoil is underlain by 12 to 20 feet of dense to very dense light brown silty sands of the Lindavista Formation of Quaternary age. The upper half of this interval contains some clay and may be moderately expansive. The Lindavista Formation is underlain by the Santiago Formation of Tertiary age. These very dense light gray silty to clayey fine sands were encountered in our test borings at depths of from 15 to 20 feet below the existing ground surface, except in test boring 8, where the contact is lower, possibly due to a filled channel or other erosional irregularity in the Santiago Formation. The slopes are generally covered with vegetation and a thin veneer of surface soils. The south-facing portion of the slope (shown by cross-section A-A') is covered by 0-4 feet of slopewash sands. The southwest facing portion of the slope (shown by cross-section B-B') has an accumulation of slopewash soils on the lower 3 to 5 feet where it flattens. Exposures of Quaternary and Tertiary age sediments on and near the site exhibit horizontal or nearly horizontal bedding attitudes, however, accurate dip and strike, measurements are difficult to make due to cross-bedding in these units. No • Sammis Properties Woodward-Clyde Consultants Project No. 53147K-SIOI April 21, 1983 Page evidence of faulting was observed on or near the site during our field work. No evidence of jointing was observed 'in test boring samples taken in either formation, however, some minor surface jointing was noted in exposures of the Santiago Formation. (4) Evidence of Past or Potential Landslide Conditions No evidence of 'past or potential landslide conditions 'was revealed by our investigation on or near the subject slope. (5). Impact of Construction Activity As mentioned previously, about 5 to 10 feet of Buildings 5 and 6 and a proposed fill slope will be located within the defined area of demonstration. There are also two proposed storm drain lines shown within this area. The proposed fill slope has a maximum height of about 4 feet. This fill slope was considered when evaluating the stability of the existing slope, as discussed 'below under Items 9 and 10. We understand that Buildings 5 and 6 will have a basement level for vehicle parking. These basements will be founded about '8 to 12 feet below existing grade in soils of the Linda Vista formation. Our analyses indicates that the buildings should have no impact on slope stability. The proposed drop inlets and storm drain lines located between the buildings and the top of slope should decrease the surface runoff that reaches the slope. In our opinion', the anticipated grading and construction activities for the proposed project will have no significant impact on the stability of the subject slope. (6) Ground and Surface Water Conditions and Variations As mentioned previously, surface drainage in the slope area is generally from north to south so that water runoff presently goes over the slope. Agricultural grading has resulted in a concentration of surface runoff at several locations which appears to have caused a minor amount of gullying at the top of the slope. Our geologic reconnaissance of the area and, evaluation of water level measurements made during two days of test drilling indicates that the water observed in the borings probably results from daily irrigation of the flower crop on the surface. This water appears to percolate down through the relatively pervious Lindavista Formation to the relatively impervious contact with the underlying Santiago Formation', where it forms a "perched" water table at approximate elevation Sammis Properties Woodward-Clyde Consuftants. Project No. 53147K-SIO1 April 21, 1983 Page 33 to 36 feet, (MSLD). There is some indication from the variation in vegetation that water does seep along the contact and out onto the slope at the corner; however, no such seeps were noted at the time of our investigation. In our opinion the current-. agricultural irrigation will probably be replaced by irrigation for the proposed project landscaping, thus maintaining a perched water table at about the current level. (7) Potential Erodibility of Site The principal effect land development has on the erosion process is the exposing of disturbed soils to precipitation and surface runoff. Grading operations typically remove protective soil and vegetative cover and alter local drainage characteristics. The proposed grading plans for development of the bluff top include appropriate measures to control erosion and sediment. The potential erosion of the subject slope would be in the form of ruling and gullying primarily due to surface runoff. No construction or grading is planned for the slope and the vegetation is fairly well established. The proposed plans for grading and development of the bluff top include provisions for control of surface drainage. It is our opinion that the planned drainage system should minimize the possibility of erosion due to water running over the top of the slope. • It is our opinion that there is a low potential for future erosion of the site, provided surface drainage is controlled. (8) Effects of Marine Erosion on Seacliffs The subject slope is not exposed to ocean erosion. The toe of • the southwesterly facing portion of the slope averages 22 feet above mean sea level. The toe of the southerly facing portion of the slope is at or near mean sea level and is in Batiquitos Lagoon. Although the toe of this slope is subject to a minor degree of tidal action, it shows no evidence of past wind wave erosion from the shallow waters in the lagoon. • (9,10) Slope Stability Under Static and Seismic Conditions We have performed slope stability analysis of the subject bluff under the proposed development loading conditions as presently shown on the available grading plans entitled "Tentative Map of • Batiquitos Pointe" prepared by O'Day Consultants dated July 17, 1982, revised February 7, 1983. Several cross-sections Sammis Properties Woodward-Clyde Consultants Project No. 53147K-S101 . April 21, 1983 Page through the bluff were analyzed for stability using the Janbu method of analysis. The most critical sections for the south and southwesterly facing slopes are shown in Figure 2. The Janbu method utilizes the normal method of slices as its basis and assumes circular arc or toe failure. In our opinion, this method is appropriate due to the fact that no major geologic factors such as outsiope dipping beds, bedding plane faults, remolded clay seams or highly fractured materials were observed during our field explorations. The strength parameters of -the predominate undisturbed formational soils in the bluff were evaluated by considering the results of moisture, density, and grain size analysis performed on selected samples, visual plasticity classifications, the results of slow direct shear tests previously performed on similar soils from nearby areas, and our experience with typical soils in the general site area. The estimated effective soil parameters used in our stability analysis are: = 125 d'.= 30 (deg.) C' = 500 (psf) In our opinion, these soil parameters are relatively conservative for the actual soil and geologic conditions. The results of our analysis on the slope sections shown on Figure 2 in a factor of safety for deep-seated failure on. the order of 1.6 to 2.1 for static conditions. An application of a seismic factor generally reduces the static factor of safety by 15 to 20 percent. Applying this criterion, the estimated factor of safety under seismic conditions would be on the order of 1.3 to 1.7. In our opinion, the probability of the slopes becoming unstable is low. In our opinion, the proposed development has been sited such that the stability and erosion of the slope should not be adversly affected during the expected economic life span. The plans indicate that the slope will not be altered. If you have any questions, please give us a call. Very truly yours, WOODWARD-CLYDE CONSJJLTANTS Buck Buchanan Buchanan R.E. 26876 BB/RJD/BRS/AA/fla (4) Sammis Properties (1) O'Day Engineering Robert J. Dowlen C.E.G. 1011 • • : • • • PROPOSED BUILDING NO. 6 60 PROPOSED FINAL GRADE 40 Q EXISTING SLOPE I Ts 20 IQUO S LAGOON 20 AREA OF DEMONSTRATION — cr -T-lrM A_Al I' 60 (EXISTING SLOPE LEGEND:. PROPOSED SILTATION BASIN Qin - Indicates Lindavista 7 Formation a: PROPERTY LINE' Ts -Indicates Santiago iTs > PROPOSED EARTH DIKE uj AREA OF I -_ W DEMONSTRATION I 0 40 80 SECTION B-B' - . - - GRAPHIC SCALE (Feet) W000WARD-CLYDE CONSULTANTS. 50 .----- --