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HomeMy WebLinkAboutCT 72-21; Tanglewood; Soils Reports; 1973-04-16et '?&a"a- Larwin-San Diego 6150 Mission Gorge Road SCT 312022 San Diego, California 92120 Report No. 7 Attention: Mr. Walt Wood SUBJECT: Supplemental Report of Soil and Geologic Investigation, Tanglewood Subdivision, Carlsbad, California. Gentlemen: In accordance with your request, we herewith submit the subject report. The intent of this report is to describe the scope of the engineer- ing and geologic analysis performed by Southern California Testing Laboratory, Inc. prior to submission of prior reports pertaining to with a recent conversation between Mr. Levish of Rurkland and Asso- the subject project. The scope of this report is in accordance ciates, Mr. Dennis Hannan, Project Geologist, and the undersigned. Please do not hesitate to contact the undersigned if you have any questions regarding the contents contained herein. We appreciate the opportunity to be of service. f - DES : j le cc: (4) Submitted Respectfully submitted, SOUTHERN CALIFORNIA TESTING LAB., INC. ;Y, L . Ilannan, Project Geologist SlOlO .2.17 - - INSPECTION TESTING RESEARCH DEVELOPMENT 6280 Riverdale St. San Diego, California 92120 Phone 280-4321 . .- " ________". ~~~~ . . ALL R=CO~TS AII .u.wmm A. 7"s ICOWI~SYIIAL PIOR~TI 0. CUIWIS. AUIMORW.VON ,om PU.LIS~O* or OYI IICOWS, COMCLUSIOY.. om IX-~RACI. 110- om m.wmmwe THSM I. IISIIVID C.*DINO OYI wmm* IPVI~OIAL A. A YVTUIL P~O~CT~ON TO CLICW.. THE W~LIC AW OUISILY~.. SUPPLEMENTAL REPORT OF SOIL AND GEOLOGIC INVESTIGATION TANGLEWOOD SUBDIVISION CARLSBAD, CALIFORNIA SCOPE The scope of this report is intended to provide a discussion of the soil and geologic conditions prevalent within the subject site, the analysis of same by Southern California Testing Laboratory, Inc. and the effect recommendations and site grading have produced on any adverse conditions. The previous reports contained the basic field and laboratory test data; therefore, this phase of the previous reports will not be duplicated. FIELD INVESTIGATION A total of 20 test borings have been extended within the subject site at the locations shown on attached Plate No. 1A. The test borinqs were conducted on three separate occasions. Reports presenting our findings, laboratory tests, analysis and recommendations were submitted upon com- pletion of each series of test borings. The soil conditions revealed by the test borinqs were confirmed by down hole logging of numerous test borings. The lack of consistency in strati- fication observed in the site grading, completed to date, was noted in our field investigation and subsequent reports. SCT 312022 April 16, 1973 Page Two Down hole logging of Boring Nos. 8 and 9 revealed stratas of intensely fractured silts and clays. Slickensided surfaces were observed in the strata. The characteristics and orientation of the slickensided surfaces were more indicative of those found in areas of differential rebound of over-consolidated silts and clays than that which would be indicative of existing landslides. Although the conditions encountered in Boring No. 8 did not present conclusive evidence of an existing slide contact, recommendations were made to the design engineer to reorient the proposed cut in this area. The final grading plan indicates a cut slope which was reduced to a maximum height in the order of 27 feet and reoriented to generally a north-south cut slope. Based on the information available at this time, it is our opinion that this was a prudent revision to the preliminary grading plan. This revision eliminated the need. for additional detailed investigation and stability analysis in the area of Boring No. 8. The strata of intensely fractured silts and clays encountered in Boring No. 9 did not reveal sufficient consistency to indicate the presence of an existing slide contact in this area. Samples were obtained from the fractured and slickensided stratas for determination of residual shear strength as described in this report. LABORATORY TESTING Laboratory tests were performed on samples obtained from the test borings in accordance with A.S.T.M. standards to determine the general physical - SCT 312022 April 16, 1973 Page Three - characteristics of the prevailing soils. Residual shear tests were performed on samples obtained from Boring Nos. 8 and 9. The results of these tests indicate a residual soil strength parameter of P, = 22.0a, c = 110 psf and # = 18.5", c = 220 psf, respectively. ." - Detailed results of our laboratory tests are presented in Report No. 2. DurinrJ site grading, a sample was hand cut from an existing slide contact located approximately 200 feet east of Boring No. 16. The existing slide debris, which represents less than 3,000 cubic yards of material, will be removed or buttressed during site grading. Additional investigation will determine the corrective measures best suited for this condition. The sample would be classified as a CH and possess a substantially higher degree of slickensided surfaces than the samples encountered in Boring Nos. 8 and 9. This sample, when tested for residual shear, produced residual strength parameters of # = loo, c = 100 psf. A review of the site grading completed to date indicates that the highly fractured silts and clays encountered in the area of Boring Nos. 8 and 9 are not - similar to the fractured and slickensided soils encountered in the area of the above described existing slide. ENGINEERING ANALYSIS The fact that the general grading concept was established prior to comple- tion of our Report No. 2 presented a condition whereby only three ma'jor slopes to be constructed during site grading required slope stability analysis. This does not include the proposed cut slope in the area of Boring No. 8 which was revised as described above. These three major slopes consisted of the following: SCT 312022 April 16, 1973 Page Four 1) The fill slope located along the northeast boundary of the site, northerly of Boring No. 4. 2) A cut slope on the north side of Elm Street located easterly of Boring No. 14. 3) A cut slope on the south side of Elm Street immediately east of El Camino Real. The above slopes and prevailing soil conditions were analyzed through the use of a Burrows 5700 computer. The computer program does analyze a minimum of 27 potential failure planes for any given slope and soil condi- tion. The computer analysis is continued until the center of radius Of the failure circle producing the least factor of safety falls within a predetermined grid. The computer printout presents the factor of safety without a seismic accelleration factor of 0.19. The above de- scribed analysis produced the following conclusions on the three major exposed slopes: Fill Slope Along the Northeast Boundary The computer analysis produced a factor of safety of 1.70 without seismic forces and 1.42 with seismic forces, based on the slope as designed, utilizing the remolded soil strength parameters of 6 = 33O, c = 250 psf, = 138 pcf. Cut Slope North of Elm Street A detailed analysis of this slope showed that an increased factor of safety could be obtained by increasing the proposed bench width in lieu of the standard reduction in slope ratio. Therefore, recommendations were presented in our Report No. 2A dated September 15, 1972 for the design of this slope. The f I SCT 312022 April 16, 1973 Page Five computer analysis produced a factor of safety of 1.41 without seismic forces. Utilizing direct shear results and a seismic accelleration of O.lg, the factor of safety will be in excess of 1.5. The residual soil strength parameters of P, = 22', c = 110 psf , = 137 pcf were utilized in these calculations. 3) Cut Slope South of Elm Street Based on the absence of continuous bedding or jointing planes and slickensided surfaces encountered in this cut, the stability analysis was based on direct shear results with the recommendation that detailed geologic inspection be performed during site grading. Based on the inspection completed to date, no evidence of exist- ing slide contact or continuous bedding and/or jointing planes requiring the utilization of residual shear parameters, have been observed in the slope. In the event a major bedding or jointing plane system is observed with an out-of-slope dip, re- commendations will be submitted for any corrected measures required. The cdmputer analysis produced a factor of safety of 1.53 based on soil strength parameters of 6 = 29O, c = 225 psf, - - 137 pcf. In consideration of the distance from the exposed slope to proposed structures, a seismic accelleration factor. was not utilized in these calculations. The trace of an inactive fault was observed southwest of the subject site during the initial stages of our investigation as shown on OUK previous reports. During site grading, the evidence of this fault was observed within the subject site. The geology section of this report contains a detailed discussion of seismic activity in the area of the site. i SCT 312022 April 16, 1973 Page Six An inspection of this discontinuity from the soil engineering standpoint showed that the prevailing soils were very dense with no indication of poten- tially differential in situ soil conditions. Within this zone, the follow- ing corrective .measures have been taken. The natural soils exposed within 3 feet of finished grade have been removed and recompacted to 90 percent of maximum dry density. Further, all foundations which will encroach on an area 15 feet L7:- - wide along the fault trace will be reinforced with four No. 4 bars. c d It is our opinion that the above recommendations will prevent differential movement within the proposed structure, due to dissimilar in situ soil conditions. A review of the final grading plan will show that the adverse soil and geologic conditions encountered within the subject site will be eliminated by the proposed site grading, with the exception of those for which correc- tive measures have been taken. SITE GEOLOGY Mapping of the site has utilized five mappable subdivisions of the mat- erials with regard to their engineering significance, basic soil varia- tions, and sedimentary bedrock conditions. These five engineering mat- erial types have been delineated on Plate No. 1A and are as follows: 1) - Qal - Alluvium consisting of brown to dark brown, medium dense to dense, silty-clayey sands, silts and clays which cover the canyon bottoms. These soils exist in a loose compressible con- dition for a depth of 1.5 feet or more and have required addi- tional compactive effort prior to the placement of structural SCT 312022 April 16, 1973 Page Seven , i fills. In some areas of particularly loose, saturated alluvium, it has been necessary to remove and recompact as much as 4 to 5 feet of material. 2) Qt - - Terrace deposits consisting of interbedded silty-clayey sands, silts and clays are located approximately below elevation 150 feet only in the extreme northwesterly corner of the site. These deposits are undoubtedly a combination of materials which reflect ancient variations in sea level and stream conditions at the head of Buena Vista Lagoon. Construction of building pads in this area will encounter moderate to highly expansive soils. 3) Qls - - Landslide involved materials may exist in these areas. Boring No. 8 encountered several closely spaced slickensided rupture surfaces within fractured olive mudstone material. The dip of these surfaces was noted to be in the order of 20 to 24 degrees in a northerly direction suggesting sliding may have occurred along these planes. It must be noted, however, that the olive silt-clay in which these rupture surfaces were observed characteristically contains an abundance of randomly oriented clay filled joint planes which are not a result of sliding. Similar conditions of jointing have been noted within many of the clay-silt beds during excavation and this condition exists in areas that are not slide involved and which are not slide prone (in excavation beneath the area between Boring Nos. 2 and 3). v r I SCT 312022 April 16, 1973 Page Eight 4) - Tss' - Eocene Santiago Formation consisting primarily of white-pale yellow, dense, coarse to medium grained, massively to cross bedded silty and clayey sands was mapped above approxi- mately elevation 200-210 feet. Included in this unit are widely scattered lenticular olive clay and silt beds whose random dis- tribution does not allow accurate geologic mapping. Further, accurate mapping is hampered without easily recognizable and traceable marker beds. TSS has been delineated as a separate unit from the underlying materials (Tsm) because of a greater percentage of slightly expansive to nonexpansive sands. It is this unit which produces materials with an overall low expan- sive potential that may be utilized to cap finished pads expos- ing moderate to highly expansive soils. Materials from this area would also be suitable for use in buttresses requiring soils with higher strength parameters than those common to on- - site clays and silts. - 5) Tsm - - This unit encompasses a wide variety of interbedded and _- interlensing clayey-silty sands, silts, and clays of the,Eocene Santiago Formation. The white-light gray-pale yellow, dense, medium to fine grained, silty and clayey sands of this unit are massivly bedded. Some faint bedding is noticeable only rarely and has not proven suitable for obtaining reliable bedding " attitudes. Lensing with, and in irregular contact with, the clayey and silty sands are massively bedded pink-olive-yellow, - " - SCT 312022 hard .ts and si1 clay i April 16, 1973 Page Nine 's. These silts and clays are the predominant materials of the Tsm - unit and are of moderate to high expansive potential. The pinkish-olive mudstone encountered in many of the borings is characteristically fissured with randomly oriented slickensides exposed on freshly broken surfaces. The fractured and fissured nature of this mudstone does not appear to be re- stricted to areas of sliding or faulting (south cut of Elm Avenue) and may be due simply to differential rebound of over-consolidated sediments. The shear strength of this soil type has been care- fully considered with respect to slope stability. Topsoils over- lying this unit (Tsm) usually consist of 1.5 to 3 feet of sur- ficial clays ranging in color from dark brown-gray to dark yellow brown and possess a moderate to high expansive potential. These topsoils are subject to slip-outs on steeper slopes (greater than 30°) and numerous examples of this are evident throughout the site. Landsliding: Ancient landsliding referenced above as Qls, or possibly landslide involved sediments, was suggested only in the area of Boring " - __ No. 8. The previously mentioned planes dipping 20 to 24 degrees in a northerly direction, whether due to sliding or jointing, were felt to be a serious enough condition that planning moved a 1.5:l cut slope 20 feet lower in elevation and oriented the slope north-south to avoid inter- setting the unfavorably dipping planes. - - - A recent slide of very limited extent was mapped in the southern portion of the site (between Boring Nos. 8 and 13) and resulted due to ever steep- ening of canyon slopes adjacent to an actively eroding drainage course __ - and the unfavorable distribution of fissured clays. Site grading has SCT 312022 1 April 16, 1973 Page Ten totally removed the slide debris in this area. A slightly larger recent slide was noted just south of the property and east of Boring No. 14; also due to over steepening of canyon slopes. Both these recent slides appear to have failed on slope gradients of approximately 1.5:1 or steeper and for slopes exceeding 25 to 50 feet in height. The proposed configuration of cuts and fills appears to be such that , the canyon and valley portion of the site will be filled and those cuts proposed for areas designated as possibly landslide involved are of limited height and extent. Possible exceptions to this may be in the areas of Boring Nos. 8, 16 and 17 where careful geologic reconnaissance is being conducted during excavation. Our inspection of the cuts in these areas should, in our opinion, provide the necessary information to evaluate possible slide hazards should they exist. During the excavation of a 1.5:l twelve foot high cut slope 200 feet east of Boring No. 16, a slope failure occurred on what appears to be a pre-existing rupture surface or a low angle joint plane dipping out- of-slope. Additional investigation of this area will be undertaken to fully evaluate the existence of slide hazards or adversely dipping fracture planes. Recommendations for stabilizing this area will be forth coming pending the investigation of this failure. Bedding and Jointing: Bedding attitudes have not been obtainable from the Tss-Tam materials due to the massively to cross bedded conditions. In those areas where outcrops were available for inspection contacts .. SCT 312022 April 16, 1973 Page Eleven between silt-clay beds and the sands indicate horizontal bedding. A cut slope excavation at the east end of Cross-Section B-B’ further indicates horizontal bedding. One questionable attitude near Boring No. 19 was obtained during excavation of this area and suggests a northeasterly strike with a slight 0-5 degree dip to the northwest (west of the fault). Inspection of cut slopes in Qt - material just north of the site also indicate horizontal bedding. Jointing of the sands, silts, and clays appears to be very intensive to moderately intensive. Generally, the joint planes have varying strikes but usually have high angle dips exceeding 40 degrees. Occasional low angle dips of several degrees have been noted in the dark olive clays and silts. It is difficult to distinguish between what may appear to be low angle joint planes or partings along bedding planes. Those joint planes in the silts and clays are rarely without clay coatings, slicken- sided and non-slickensided, and the clay coating is often a dark olive. Joint planes in the silty-clayey sand8 are often coated with dark bluish- green clay. Slopes containing th&s.-condition have also been inspected during grading. Our inspection during grading has recognized and mapped many of the joint planes and we find that, in addition to generally steep attitudes, there are many conchoidal fracture planes within the more dense, hard silts and clays, whereas the joint planes in the silty and clayey sands are more consistent. Out-of-slope dips on fracture planes exceeding 10-20 degrees are being carefully analyzed where noted in the dark olive clays and silts. SCT 312022 April 16, 1973 Page Twelve Faulting: Due to the serious lack of previous mapping in the area of Carlsbad and Oceanside (also the area south of Carlsbad to Del Mar), the availability of fault information is poor. Student mapping by California State University, San Diego geology department has been strictly reconnais- sance in nature and has not proven to be reliable. The Santa Ana Sheet, Geologic Map of California, 1965, snows several northeasterly trending faults in the Camp Pendleton area some 15 miles northwest of the site. Mapping completed in late 1972 by Ken Wilson (personal communication) shows the presence of northeasterly trending faults in areas east of the subject site and south of Aqua Hedionda Creek 3 miles southeast of the site. These faults do not intersect the subject project. A fault was noted southwest of the subject site several hundred feet south of the intersection of Elm Avenue and El Camino Real. An approximate attitude at this location and subsequent attitudes during grading suggest an approximate northeast strike of between 10-20 degrees and an easterly- southeasterly dip of between 43 and 13 degrees. Faulting along this trend appears to be restricted to a zone approximately 15 feet wide in which dark olive clay gouge and slickensided surfaces may be observed. Due to the lack of conclusive stratigraphic marker beds, the amount of throw has not been calculated and no estimate of offset is feasible at this time. The direction of slickensides does indicate either normal or reverse dip slip. The fault trace does not exhibit the physiographic features such as sag ponds, offset drainages, springs, and fault scarps that are common to proven active faults. A southwest extension of the fault does not appear - SCT 312022 April 16, 1973 Page Thirteen - to demonstrate noticeable offset of the Pleistocene terrace materials (reddish sands and gravels) and most importantly, the general surrounding area does not contain any mapped earthquake epicenters (Natural Resources Inventory Fault Map, 1972, San Diego County). It might also be noted that the tectonically active faults in southern California are northwesterly faults, whereas faulting in the Oceanside-Carlsbad area is northeasterly and may, with regard to the lack of seismic activity, be regarded as probably inactive. C. R. Allen stipulates that current displacements on faults with high degrees of activity in the recent geologic past (last 10,000 years) are the most likely candidates for future activity (Allen, C. R., St. Amand, P., Richter, C. F., and Nordquist, J.J., 1965, “Rela- tionship between seismicity and geologic structure in the southern Cali- fornia region:, Bull. Seismological SOC. America, vol. 55, p. 761). Work conducted for Southern California Edison and San Diego Gas and Electric Company’s San Onofre nuclear generating plant by Western Geophysical Company of America, Houston, Texas, consisted of a geophysical survey of the offshore region opposite of San Onofre and south to San Diego. Special emphasis was placed on the location of faults and the age of cessation of their movements. Seismic reflection and refraction surveys conducted on three different horizons comprised of the sea floor sediments, the upper Miocene, and the effective acoustic basement rocks (locally the San Onofre Breccia or top volcanics) resulted in a comparison of the faults on these horizons. Western Geophysical‘s conclusion stated that activity has significantly diminished through geologic time, and the activity has been minor since upper Miocene. They suggest that the .. .. SCT 312022 April 16, 1973 Page Fourteen region is relatively stable by comparison to adjacent regions of southern California (Final Report, Southern California Edison and San Diego Gas and Electric Company, San Onofre Offshore Investigations, Western Geophysical Company of America, Houston, Texas, March 7, 1972, in San Onofre Nuclear Generating Station Units 2 and 3, Amendment No. 11 to Preliminary Safety Analysis Report, p. 43). It is our opinion that within the present geologic framework of the Oceanside- Carlsbad area and through the use of available information, the discontinuity noted crossing the subject site shows no features which would suggest recent seismic activity. / Geology By: Dennis 1,. Aannan