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HomeMy WebLinkAbout3190; Rancho Santa Fe Rd Alignment; Rancho Santa Fe Rd Alignment - Soils & Geologic; 1989-06-01SOIL AND GEOLOGIC RECONNAISSANCE FOR RANCHO SANTA FE ROAD ALIGNMENT CARLSBAD, CALIFORNIA FOR THE FIELDSTONE COMPANY SAN DIEGO, CALIFORNIA BY GEOCON INCORPORATED SAN DIEGO, CALIFORNIA JUNE, 1989 ^^^^ 0 GEOCON INCORPORATED Geotechnical Engineers and Engineering Geologists File No. D-4393-H01 June 30, 1989 The Fieldstone Company 5465 Morehouse Drive, Suite 250 San Diego, California 92121 Attention: Mr. Brian Milich Subject: RANCHO SANTA FE ROAD ALIGNMENT CARLSBAD, CALIFORNIA SOIL AND GEOLOGIC RECONNAISSANCE Gentlemen: In response to your request and our proposal dated June 15, 1989, our firm has performed a soil and geologic reconnaissance for the subject project. The accompanying report presents our findings and conclusions from our study. If there are any questions, or if we may be of further service, please do not hesitate to contact the undersigned. Very truly yours, GEOCON INCORPORATED H. Tom Kuper' CEG 1137 RCE 225 EliAbeth L. Herbert Staff Geologist ELH:DFL:HTK:dmc (4) addressee 6960 Flanders Drive San Diego, CA 92121-2974 619 558-6900 FAX 619 558-6159 TABLE OF CONTENTS SOIL AND GEOLOGIC RECONNAISSANCE Page Purpose and Scope 1 General Site Conditions and Proposed Development 2 Soil and Geologic Conditions 4 Santiago Peak Volcanics (Jsp) 4 Granitic Rock (Kgr) . 5 Colluvium and Topsoil (unmapped) 5 Alluvium (Qal) . 5 Fill Materials (Qaf) 6 Geologic Hazards 6 Slope Stability and Landslides 6 Faulting and Seismicity .... 7 Liquefaction 8 CONCLUSIONS AND RECOMMENDATIONS General 9 Future Studies 12 LIMITATIONS AND UNIFORMITY OF CONDITIONS Figure 1, Site Plan Figure 2, Major Earthquakes File No. D-4393-H01 June 30, 1989 SOIL AND GEOLOGIC RECONNAISSANCE Purpose and Scope The purpose of this reconnaissance was to address the soil and geologic features of the area which could affect the development of the property as tentatively planned. Aspects of particular interest included potential geologic hazards such as ancient landslides, unstable or weak formations and faults. In addition, engineering concerns such as the relationship of the spillway outflow to the proposed road, the possibility of seepage from the existing La Costa Dam through the proposed cut slopes and the effects of blasting on the La Costa Dam were to be addressed. The scope of our services consisted of field mapping the various geologic site features and a review of aerial photographs of the property (ASDA, 1953 edition) as well as the following maps and reports: o Untitled Map of Project, prepared by Project Design Consultants, undated. Scale 1 inch = 200 feet. o "Field Guide to Selected Dams and Reservoirs, San Diego County, California," prepared for 26th Annual Meeting of Association of Engineering Geologists, 1983. - 1 File No. D-4393-H01 June 30, 1989 General Site Conditions and Proposed Development For the purposes of this report, the area mapped was taken to extend approximately 300 feet from either side of the proposed road alignment and included the area of the existing Stanley Mahr Reservoir (Figure 1). The site is characterized by a series of approximately north-south running ridges and canyons. The ridge slopes are generally moderate to steep and the canyons range from relatively flat-bottomed to V-shaped. Drainage is accomplished by generally southerly trending canyons, however, the San Marcos Creek flows to the west through the northern portion of the proposed alignment. Elevations range from approximately 660 feet Mean Sea Level (MSL) northeast of the existing dam to a low of approximately 330 feet MSL at the northern boundary of the study area. Vegetation generally consists of thick brush and chaparral. The site is generally undeveloped, except for the reservoir and where the existing Rancho Santa Fe Road and truck by-pass traverse the site. Several undocumented canyon fills associated with the construction of the truck by-pass were encountered. Except for abandoned mine workings to the west and industrial parks to the north, the site is surrounded by essentially undeveloped land. 2 - I I I I I I I I I I I I I File No. D-4393-H01 June 30, 1989 The proposed alignment will pass just downstream of the existing reservoir. A review of the referenced report indicated that the empoundment structure associated with the reservoir, also known as the La Costa Dam, was constructed in 1981 and is 80 feet high from the downstream toe to the dam crest. The reservoir is capable of storing approximately 54,000,000 gallons or 166 acre-feet at thei spillway level. The dam is a zoned dam and is composed of an impervious core of clayey materials with an outer shell of granular material, both materials being mined on site or nearby. The dam services as a temporary storage for treated sewage effluent during the rainy seasons and so will be empty or at low levels during about nine months of the year. During construction of the dam foundation, it was found that below the overlying topsoil, colluvium and/or very weathered material, the bedrock (Santiago Peak Volcanics) was moderately to slightly weathered and is highly fractured in random directions. Several shear zones were exposed which trend northwest to southeast and dip from 50 to 72 degrees south. The shear zones are highly fractured, but the fractures are filled with a grey-brown clay. It was felt that no zones were found in the dam foundation which would give concern for excessive leakage. I I I I I I I I I I I I I I I I I I I File No. D-4393-H01 June 30, 1989 We understand that ultimate development will consist of the realignment of Rancho Santa Fe Road approximately between the intersections with Melrose Avenue and La Costa Avenue. The project is anticipated to have cut slopes approximately 70 feet high and fill slopes approximately 90 feet high at inclinations of approximately 2:1 (horizontal to vertical). Further refinement of the grading plans will undoubtedly result in changes of which Geocon Incorporated should be notified. Soil and Geologic Conditions The site is underlain by bedrock consisting of Santiago Peak Volcanics and Granitic rock. Surficial deposits include alluvium, colluvium, topsoil and fill material. The soil types will be discussed below in order of decreasing age. Santiago Peak Volcanics (Jsp). The Triasic-Jurassic-aged metasediments and metavolcanic rocks which compose the Santiago Peak Volcanics constitute the majority of the underlying bedrock. In the area of the site, the Santiago Peak Volcanics are composed of dark green to blue, fine grained, metamorphosed volcanic rock with varying amounts of feldspar phenocrysts and tuff fragments. Bedding was observed where possible and in a rock cut at the south end of the site which had an orientation of N70E and dipping 33° North. Generally, the material is massive but highly jointed with the spacing ranging from approximately six I I I File No. D-4393-H01 June 30, 1989 inches to less than one inch. The joints appear to strike in many directions. The surface rocks are very weathered in some locations and are stained with sulfides and oxides. Granitic Rock (Kgr). Cretaceous-aged Granitic rock is exposed at the northern end of the site. Although resistant outcrops are present, abandoned mining operations to the west of the site indicate that the granitic rock may be weathered enough locally for relatively easy excavation. The granitic rock appears to be granodioitic in composition and medium to coarse grained. Colluvium and Topsoil (unmapped). Due to the limited scope of the investigation, precise depths of the colluvium and topsoil are unknown. However, our experience in the area indicates that a thickness of approximately three feet may be anticipated. It can also be expected that the thickness of the colluvium will increase near the base of slopes and on north-facing slopes. The topsoil and colluvium are generally sandy clays and clayey sands. Alluvium (Qal). The alluvium is typically sandy with varying amounts of silt and clay. Again, no excavations were performed during the reconnaissance so the thickness of the alluvium in the site is unknown as is its degree of consolidation. The maximum thickness of alluvium may be File No. D-4393-H01 June 30, 1989 expected to occur in the northern portion of the site where the San Marcos Creek crosses the site. Alluvial thicknesses of at least 10 15 feet can be anticipated based on exploration data from a previous investigation just upstream from the site. Fill Materials (Oaf). Several different generations of fills are present on the site. We anticipate that the small dam just downstream from the La Costa Dam and the fill forming an old road east and parallel to the existing Rancho Santa Fe Road are the earliest undocumented fills, with the fills associated with the truck by-pass constructed within that same general time period. The documented fill materials constituting the La Costa Dam are probably the next generation of fills. As mentioned previously, the La Costa Dam was designed with an impervious clay core and a granular shell. The spillway and some sections of the access roads were constructed from fill materials that were probably generated on-site. Scattered end-dump fills are present on the site, especially along the truck by-pass route. The depth and degree of compaction of the various fills could not be determined due to the limited extent of the investigation. Geologic Hazards Slope Stability and Landslides. No major landslides were noticed during the reconnaissance although small debris flows and slumps may be - 6 - File No. D-4393-H01 June 30, 1989 encountered during grading. Fill slopes composed of granular materials of the Santiago Peak Volcanics and granitic rocks should generally have adequate factors-of-safety against deep-seated rotational failures at inclinations of 2:1 (horizontal to vertical). Cut slopes in the hard rock cannot be analyzed by conventional methods but are anticipated to be stable to the proposed design heights. However, shallow sloughage of weathered and/or fractured rock may occur in both cut and fill slopes. Further investigation is necessary to determine the stability of cut and fill slopes in the surficial materials. Faulting and Seismicity. No active faults were observed during the field reconnaissance. Figure 2 indicates the locations of major earthquakes and recent active faults in Southern California. The recent off-shore seismic activity near Oceanside demonstrate that small magnitude earthquakes can be generated by the off-shore faults. The offshore faults are located approximately 20 miles west of the site and the Elsinore Fault is approximately 25 miles to the east. The faults do not pose any greater seismic risk to this site than to any of the surrounding developments. The probability of the San Diego area experiencing a Magnitude 6 or greater earthquake would appear to be low based on present knowledge. Earthquakes less than Magnitude 4 have been - 7 - File No. D-4393-H01 June 30, 1989 common in the San Diego region. Such earthquakes rarely result in significant damage to well built structures. Liquefaction. It is not anticipated that liquefaction will constitute a major risk to the proposed development due to the road alignment being situated generally on hard rock. The alluvium that is present is typically in ephemeral streams, thus reducing the risk of liquefaction. However, the northern portion of the alignment will cross over San Marcos Creek, a perennial stream with undetermined properties and depths of alluvium. Further investigation is recommended to establish more precisely the depth, composition and settlement potential of the alluvium. 8 - File No. D-4393-H01 June 30, 1989 CONCLUSIONS AND RECOMMENDATIONS General 1. No soil or geologic conditions were encountered during this geologic reconnaissance which, in our opinion, would preclude the development of the property as presently planned, provided the recommendations of this report are followed. 2. In general, site conditions consist of a very dense bedrock terrain comprised of Santiago Peak Volcanics and Granitic rocks overlain by surficial soil deposits, such as colluvium, topsoil, alluvium and fill materials. Generally, the bedrock materials should exhibit excellent slope stability and bearing characteristics in proposed cut and fill slopes. 3. The clayey topsoils are typically moderately to highly expansive and are not considered suitable for use at finish grade. Rock fragments with diameters greater than 6 inches should not be incorporated within the top three feet of structural fills. 4. Rippable conditions may be present within approximately the top ten feet of the Santiago Peak Volcanics and Granitic rock, and in the areas File No. D-4393-H01 June 30, 1989 with extensive fracturing or weathering. For deeper excavations outside these weathered or fractured zones, blasting will likely be required. 5. Blasting required for excavation near the La Costa Dam will necessarily require consideration relative to the impact on the dam. It is our opinion that blasting in this area can be conducted for roadway construction. However, the blasting program should be designed by a firm familiar with limitations imposed by the presence of the dam. The blasting program should include a monitoring aspect with instrumenting the dam to record levels of vibration and accelerations. In addition, the reservoir owner agency and the original dam design consultants should review the blasting program. During and after roadway construction, inspection and/or reevaluation by the California Division of Safety of Dams may be required. 6. Remedial grading in the form of removal and recompaction of loose colluvium, topsoils, alluvium and fill materials will be required In all areas where such soils would underlie structural fills or other settlement sensitive improvements. 7. Both surface water and marshy areas were encountered during this reconnaissance. However, it is our opinion that groundwater related problems at the site should not be a constraint to site development as 10 - File No. D-4393-H01 June 30, 1989 proposed. Groundwater may also be present in areas underlain by deeper alluvium or in the cut slopes proposed near the La Costa Dam where fracture zones in bedrock materials may occur. However, as areas of seepage may be isolated, such occurences may not be detected until the excavation is performed. Should this condition be encountered, design modifications can be made to reduce any foreseeable drainage problem. 8. For preliminary design purposes, it is recommended that cut and fill slopes be planned at inclinations of 2:1 (horizontal to vertical) or flatter. Such slopes should have a factor of safety in excess of 1.5 for deep seated failure. All fill slopes should be provided with an erosion resistant ground cover and a well designed and maintained irrigation system as soon as practical to reduce the erosion potential. 9. It is our experience that the bulking factors for hard granitic and metavolcanic rock, generally exceed 20 percent. An accurate estimate of the bulking factor is not practical since it depends on many factors and ultimately on the slope and size of the rock fragments which result from the blasting. It should be anticipated, however, that significant volumes of oversize rock will require special handling and disposal within deeper fill. - 11 - File No. D-4393-H01 June 30. 1989 Future Studies 10. We recommend that the owner of the La Costa Dam and the original design consultant review the proposed blasting program to determine if the anticipated vibration will affect the stability of the dam. It is suggested that monitors be Installed near the La Costa Dam to record the levels of vibration and acceleration. 11. The La Costa Dam construction should be reviewed by the original design consultants to establish if proposed development will not impact on dam design parameters. In particular, the relationship of the spillway outflow to the proposed road alignment should be examined to ascertain that water from the spillway will not evade the base of proposed fill slopes. 12. We recommend that a more detailed geotechnical investigation be performed prior to final design to evaluate the areal extent, composition and physical characteristics of the fill, alluvium and bedrock (including estimates of rippability). - 12 File No. D-4393-H01 June 30, 1989 LIMITATIONS AND UNIFORMITY OF CONDITIONS 1. The recommendations of this report pertain only to the site investigated and are based upon the assumption that the soil conditions do not deviate from those disclosed in the investigation. If any variations or undesirable conditions are encountered during construction, or if the proposed construction will differ from that anticipated herein, Geocon, Incorporated should be notified so that supplemental recommenda- tions can be given. 2. This report is issued with the understanding that it is the responsibility of the owner, or of his representative, to ensure that the information and recommendations contained herein are brought to the attention of the architect and engineer for the project and incorporated into the plans, and the necessary steps are taken to see that the contractor and subcontractors carry out such recommendations in the field. 3. The findings of this report are valid as of the present date. However, changes in the conditions of a property can occur with the passage of time, whether they be due to natural processes or the works of man on this or adjacent properties. In addition, changes in applicable or appropriate standards may occur, whether they result from legislation or the broadening of knowledge. Accordingly, the findings of this report may be invalidated wholly or partially by changes outside our control. Therefore, this report is subject to review and should not be relied upon after a period of three years. - :5r X I N S S LUIS 0 SIS PO UJIS oeisPO .1 S^mM Hams l*im»4 ASSOCIATION or ENGINCERIN6 GCOLQSISTS 1973 Kl LOUETER S 1947 \ '45.- \ \ R 0 1 N 0 \ I ^^1'^ TwCNTTMNCN^iuji^a I933J I— 1941 MAJOR EARTHQUAKES AND RECENTLY ACTIVE FAULTS IN THE SOUTHERN CAUFORNIA REGION EXPUNATION' ACTIVE FAULTS Toiat lenqm of foult zone thai breoks Holocene deposils or Ihot has had seismic activily. Fault segment wtlh surface rupture during on historic earthquake, or with oseismic fault creep. o Holocene volcanic activity (Amboy, Pi*;oh, Cemj Piielo and Sol ton Suites 1 1932 EARTHQUAKE LOCATIONS Approximate eptcentrol orea of eorlhquokes Ihot occurred 1769-1933. Magnitudes not recorded by instruments prior to 1906 were estimated h-om damage reports assigned on Intensity W (Modified Mercali scale) or greater; ttus is njuqhly equivalent lo Richter M 8.0. 31 moderate" eorthquokes, 7 major ond one great eorthguoke (18571 were reporte<l in Ihe 164-year period 1759-1933. Eorthquoke epictnters since 1933, plotted fiom mprowl nslnjmenis. 29 moderate" and three ma|or earthquakes were recorded n the W-yeor period 1933-1973. • S«« \jkm^, ll«nli<ld, Procn/ Mnm lor sMiliOMl tt91aA4li«H at MV. i b, m« Sl(«lwal CnquKwl Aiujcialifla q4 CclilMiiN rf«liM a i^rol •arll)*«r» al w l*«r iiix, ol 7^ 01 v««ni; o Mtoi lodWoolit 7 10 '^i ; o BOOMOIO nrlMMU 6 lo 7. Codo r IMO • jliuur UoooiloOi I C<jm|).l«ii Dt HiclW'd J Proctor moinl> Irooi publiifted ond unixiiilisntiJ dolo ot llw CaMomn Omsion 0/ Mmes and Geoh^r. Cali/oma Dcparlmail al Water kcsairm Bulltlii //S-i" 119641: stKtnons from OulUlins ol We SecUKjieal ond Senmolajial 5x:tlin oi America; Irom C F. Hicliltr, Elementary 5>/5A0<lo^ ( 1956}; ond tn< NatKuiai Atlas^ p.66. •••1t> BANMtMJ 1918 s SAN OIEGO 1 M P '••£ S I A / L 1913 •SfOt, M«J vr -.^S^-,, \ EMSCMAOA 1936 US.I 1940' -55'- -34' OF CALIFORNIA GEOCON INCORPORATED FILE NO. D-4393-H0I DATE 6 -30- 1989 FK3. 2