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Home My WebLink AboutCT 75-9; Stagecoach Park Geotechnical Investigation; Geotechnical Investigation Stagecoach Park; 1986-05-143467 Kurtz street • • • . San Diego, California 92110 WOOuWarCl-Clyde COnSUltailtS (619)224-2911 May 14, 1986 Project No. 56150E-SI01 City of Carlsbad 1200 Elm Avenue Carlsbad, California 92008 Attention: Mr. Richard Allen GEOTECHNICAL INVESTIGATION STAGECOACH PARK CARLSBAD, CALIFORNIA Gentlemen: In accordance with our proposal, dated April 4, 1986 and your letter agreement dated April 18, 1986, we have completed a geotechnical inves- tigation at the northeast corner of Stagecoach Park. The park is located easterly of Rancho Santa Fe Road, adjacent to the proposed Mision Estancia Road alignment. The park area has been graded and is currently being developed. INTRODUCTION AND PURPOSE We understand that a landslide was identified by Eberhart and Stone, Inc. during grading operation for the subdivision (Carlsbad Tract 75-9) located easterly and adjacent to Stagecoach Park. The purpose of our investigation was to evaluate the extent of landsliding into the northeast corner of the park and, if present, to provide our opinions as to the landslide conditions. - Our investigation was made within the general area shown on Figure 1. SCOPE OF WORK Our work consisted of the following tasks: ° Review reports prepared by others (listed below under Background Information) of the landslide conditions at Stagecoach Park, Mision Estacia, and the proposed subdivision to the south. ° Conduct a geologic reconnaissance of the site. ° Excavate three borings in the landslide area, log the borings, and collect soil samples. Analyze findings, compare them to the previous studies and prepare a report of our opinion of the landslide condition. Consulting Engineers, Geologists and Environmental Scientists Offices in Othier Principal Cities City of Carlsbad ^'r4,'^8f Wbodward-ayde Consultants Page 2 ° Present findings in a meeting. BACKGROUND INFORMATION We have discussed the project with Mr. Allen and we have been provided with a site plan of the park and subdivision, and a geologic cross-section through both the subdivision ahd the park areas. This cross-section, prepared by Eberhart and Stone, Inc. depicts a relatively deep-seated landslide that extends westerly from the subdivision, below Mision Estancia, and includes a portion of the northeasterly corner of Stagecoach Psirk • To aid us in our investigation, we have also reviewed the following: ° "Supplemental Geotechnical Investigation for Stagecoach Park, Carlsbad, California," prepared by GEOCON, Inc., dated April 18, 1986. ° "Geotechnical Investigation, Santa Fe Knolls, C. T. 75-9 (B) and 85-5 (B), Carlsbad, California," prepared by Eberhart and Stone, Inc., dated July 2, and August 22, 1985, ° Stereographic aerial photographs flown in 1964 (Series AXN-4DD6 thru 8). ° "Pleistocene and Eocene Geology of the Encintas and Rancho Santa Fe Quadrangles", geologic mapping by Leonard Eisenberg (1983). The results of our investigation were presented to the city staff at a meeting on May 6, 1986. GENERAL SITE CONDITIONS Our investigation was made at the northeasterly portion of Stagecoach Park which generally consists of an essentially flat graded area at an approxi- mate elevation of 226 feet (MSL Datum). The flat area is bordered by existing cut slopes along the northerly limits of the park (adjacent to an undeveloped area) and adjacent to Mision Estancia (Figure 1). The slope extending up from the park to Mision Estancia is inclined at approximately 2:1 (horizontal to vertical) is approximately 30 to 35 feet in height. The bottom portion is a cut slope and the top 5 to 8 feet is a fiU slope. The easterly side of Mision Estancia is bordered by an approximately 15 feet high slope inclined: at approximately 2:1, which we understand from City Staff, was constructed as a buttress fill slope. At the time of our investigation, the park site had been graded, road grading for Mision Estancia was completed in the vicinity of the park, and grading operations were in progress on the adjacent subdivision track to the southeast. City of Carlsbad Project No. 56150E-SI01 j j j •» * May 14, 1986 Woodward-Clyde Consultants Page 3 FILL AND SUBSURFACE INVESTIGATION Our field investigation was conducted between April 24 and 28, 1986 and included making a geologic reconnaissance of the general area shown on Figure 1 and the drilling of test borings. Three large diameter test borings were drilled and down-hole logged by an engineering geologist from our firm. Geologists from Geocon, Inc. and Eberhart and Stone, Inc. also observed the test borings. The locations of the test borings are shown on Figure 1 and a generalized geologic cross-section is presented on Figure 2. A Key to Logs is presented on Figure 3, and logs of the test borings are included as Figures 4 through 10. Disturbed soil samples were obtained from the test borings and brought to our laboratory for visual classification. GEOLOGIC CONDITIONS General Geologic Setting The study area is underlain by the Delmar Formation which is comprised predominantly of claystone, siltstone, and lesser sandstone. The Delmar Formation is underlain at depth by metavolcanic rock of the Santiago Peak Volcanic. Below 30 to 60 feet, the Delmar Formation includes a zone of interbedded fragments of the metavolcanic rock. The Delmar Formation, particularly the claystone portion is typically in- ternally fractured; shears, remolded bedding planes, and remolded clay seams are commonly present. This condition was observed in each of the large diameter test borings. Shears and remolded clay seams were generally observed within the upper approximately 15 to 35 feet of the formation (above approximate elevation 215 feet). The orientations of these features are variable and are generally interpreted to reflect the effects of regional tectonic deformation. Apparent Landslide Condition Features are present easterly of the park within the subdivision that apparently delineate the up slope limits (head scarp) of an ancient landslide. The approximately easterly limits of the landslide are shown on Figure 1 and were mapped at the time of our , field work during the rough grading operations within the subdivision. The up slope limits of the ancient landslide exposed during our mapping also appears to generally coincide with landforms observed on predevelopment aerial photographs. Test borings completed by GEOCON, Inc. within the subdivision encoun- tered what was interpreted to be the slip plane of this landslide at elevations of approximately 265 to 275 feet. According to this interpreta- tion, the landslide apparently "daylights" above the elevation of Mision Estancia within the subdivision area. Alternately, borings made by Eberhart and Stone, Inc. apparently delineated a deeper slide plane that City of Carlsbad Project No. 56150E-SI01 ... _i ^ M ^ May 14, 1986 Woodward>Clyde Consultants Page 4 extends westerly into the park area. No logs of those borings were provided to Woodward-Clyde Consultants. Ground Water Groundwater seepage of varying amounts was encountered in each of test borings at the approximately elevations shown on Figure 2. The flow of water into the borings increased from east to west and the amount of water observed in boring 3 suggests it may be related to the regional groundwater table. Seepage zones observed in the test borings occur below the elevation of the park pad and adjacent cut slope area. Seepage and/or wet areas were not observed in the slopes adjacent to the park. DISCUSSIONS, CONCLUSIONS, AND RECOMMENDATIONS The discussions and conclusions presented in this report are based upon the results of our review of work by others, our field observations and our professional judgement. Apparent Extent of Landslide Discrete, well defined surfaces or zones, such as those that typically define a landslide slip plane, were not observed in the three test borings made for our study. All of our borings were to the west of the landslide identified by Geocon in their report of april 18, 1986. Although, shears and remolded clay seams were observed within each of our large diameter test borings (Figure 2), the surfaces have random orientations and in our opinion do not reflect the presence of an old landslide. Alternately such features are not uncommon in the clay stones of the Delmar Formation and should be considered a zone of weakness. In our opinion, the ancient landslide conditions observed in the adjacent subdivision do not underlie the park area. Suggested Evaluation of Slope Stability It is our opinion that the presence of shears and remolded clay seams and groundwater seepage which could increase from future landscape watering and changed drainage patterns suggests the possibility of future slope instability at the subject location. It is therefore our opinion that slope stability analyses of the existing cut slope at the northeast corner of Stagecoach Park would be prudent. The analyses should include consider- ation of the above factors. UNCERTAINTY AND LIMITATIONS Geotechnical engineering and the geologic sciences are characterized by uncertainty. Professional judgements presented herein are based partly on our evaluation of the technical information gathered and partly on our City of Carlsbad Project No. 56150E-SI01 »« -i ^ ^i. ^ u * May 14, 1986 Woodward-Clydo Consultants Page 5 general engineering geologic experience. Our engineering work and judge- ments rendered meet current professional standards; we do not g-uarantee the performance of the project in any respect. We appreciate the opportunity to be of service. If you have any questions or if we can be of further service, please call. Very truly yours, WOODWARD-CLYDE CONSULTANTS Louis J.'^Lee Daryl Streiff R-E 14129 C.E.G. 1033 LJL/DLS/DWS/sar Attachments Location Boring Number Elevation DESCRIPTION DISTURBED SAMPLE LOCATION Obtained by cotlacting th« cutting* In m plastic or cloth bag. SAMPLE LOCATION Obtainecl by downhota cotlectlnQ bulk •ample fci a plaatic or clotli bag. Brown silty sand (SM) with cobbles and qravels The descriptions include soil classification and color. The soil classifications are based on the Unified SoH Classification System. Geologic classification and symbol may be used in addition to the sol classification and unit name. Indicates type of test (if any) and results. SYMBOLS USED FOR GEOLOGIC LOG INCLUDE: A/\AAAA/i Sheta S5P»55£»R Gravels .•vi^Vi'i'v'-" Sand ~SrEr^ SUt or clay N10»W 70*SW Indicates attitude of bedding, ioint fractures or faults (as indicated) V WATER LEVEL At time of drffling or as Mlcatedy NOTES ON FIELD INVESTIGATION 1. REFUSAL Indlcafos the Inability to extend excavation, practically, wRh equipment being used in the investigation. 2. The log represents measurements and interpretations observed downhole on the cK>rth half of a large diameter boring unless otherwise rrated. •For description of SYf"bolf, »ee Figure DRAWN BY: ch CHECKED BY KEY TO LOGS STAGECOACH PARK PROJECT NO: 56150E-SIO1 DATE: 5-7-86 I FtGURE NO; WOODWARD-CLYDE CONSULTANTS Test Boring 1 Approx imate El. 256.5' DEPTH IN FEET GEOLOGIC LOG TESTS SAMPLE NUMBER DESCRIPTION 5_ 10- 15- 20 25. 30. 35- • f*',\"I'f "-J 40 L-1 C 1-2 [ -3 [ 1-4 1-5 c c 1-6 FILL Damp, light greenish gray, sandy lean clay 3ase of fill flat SLOPEWASH Hard, damp, dark brown, fat clay (CH) DELMAR FORMATION Hard, damp, light olive green, silty claystone (CH) Shear at 13' N60W/30NE Fractured at 18-20' orientations appear random Hard, damp, light brown, clayey siltstone Shear at 23' N45W/25NE ^ Hard, damp, light greenish gray, clayey siltstone Shear at 26' N50W/20SW Remolded clay seam at 28', thickness varies 1/2" to 1/10", undulatory surface, N30E/15SE Interbedded coarse sand at 35' Remolded clay seam at 35', 1/4" to 1/8" thick N/35W Remolded clay seam at 36', 1/2" to 1/4" thick N80E/20SE Fractured at 36-38'; orientations appear random [| Hard, damp, dark green claystone •For detcfiption of ivmbois, tee Figure 3 Continued on next page LOG OF TEST BORING 1 STAGECOACH PARK DRAWN BY: | CHECKED BY: PROJECT NO: 56150E-SIO1 DATE: 5-7-86 FIGURE NO: 4 WOODWARD-CLYDE CONSULTANTS Test Boring 1 (Cont'd) DEPTH IN FEET GEOLOGIC LOG TESTS SAMPLE NUMBER DESCRIPTION 45- 50- 55 60_ 65. 70- 75- 80" bV.d- 1-9 (Continued) Hard, damp, dark green claystone Shear at 40' N75W/20NE Shear at 41' .W/60N Groundwater seepage at 40-41' Hard, damp, greenish gray, clayey to sandy siltstone 1 1-1 1-11 1-12 1-13 1-14 1-15 1-16 1-17 1-18 1-19[^_ l-20[^~ 1-21 Q Become:s sandier Groundwater seepage at 52' Groundwater seepage.at 57' — —Grading to Hard, damp, light green clayey sandstone Becomes moist at 64' Hard, moist, light green clayey siltstone with inter- bedded metavolcanic rock fragments Predominantly angular fragments, brovm, metavolcanic rock with clayey sand matrix 1-23 Hard, moist, light greenish gray siltstone Angular to subrounded metavolcanic rock fragments •For description of symfjols, see Figure 3^ Continued on next page , LOG OF. TEST BORING STAGECOACH 1 (CONT'D) PARK' DRAWN BY: ^h | CHECKED BY:/^^^ PROJECT NO: 56150E-SIOl DATE: 5-7-86 j FIGURE NO: 5 WOODWARD-CLYDE CONSULTANTS Test Boring 1 (Cont'd) DEPTH IN FEET 85. GEOLOGIC LOG TESTS SAMPLE NUMBER 1-25 DESCRIPTION (Continued) angular to subrounded metavolcanic rock fragments Contact N80E/20NW SANTIAGO PEAK VOLCANICS • •• Very dense, moist, angular parting metavolcanic rock, with greenish gray, sandy clay along partings 90- 100- 105- 110- 115_ 120- Bottom of Boring at 125- •For description of symbols, see Figure 3 LOG OF TEST BORING 1 (CONT'' STAGECOACH PARK .// D) DRAWN BY: ch 1 CHECKED BY>^;>^ PROJECT NO: 56150E-SIO1 | DATE: 5-7-86 1 FIGURE NO: 6 WOODWARD-CLYDE CONSULTANTS Test Boring 2 Approximate El. 250' DEPTH IN FEET GEOLOGIC LOG TESTS SAMPLE NUMBER DESCRIPTION 5- 10- 15 - ^ — 20- 25- 30- 35- 40. FILL, • / Moist, brown to greenish gray, silty to sandy clay -Base of fill flat . 2-10[^ 2-11 2-1 2-12 2-13 DELMAR FORMATION . . Hard, damp, greenish gray siltstone : . ;_ Grading to Hard,' damp, greenish gray with purplish mottling, Silty claystone Shear discontinuous N55E/20NW At 8-10', intebedded yellowish brown, .clayey sand At 11-13', fractured orientations appear random Yellowish brown, clayey sand lenses at 15' Remolded clay seam at 15", upper and lower surfaces striated, 1/8" to 1/10" thick, base flat Remolded clay seam at 16', 1/8" to 1/4" thick, N70W/15NE Hard,' damp, dark green claystone/siltstone Gradational to brownish gray, clayey siltstone 2-3 [ Shears, at 26' and 28', N30W/25SW Hard, damp, light brown, clayey sandstone .-4 E Hard, damp to moist, dark green with purplish mottling sandy claystone Interbedded metavolcanic rock fragments Groundwater seepage at 35' 2-5 [ •For description of symbols, see Figure 3 Continued on next page LOG OF TEST BORING 2 \ , \ rf}^ STAGECOACH' PARK DRAWN BY: ch [ CHECKED ^y/i'/^ PROJECT NO: 56150E-SIO1 DATE: 5-7-86 j FIGURE NO: 7 WOODWARD-CLYDE CONSULTANTS Test Boring 2 (Cont'd) DEPTH IN FEET GEOLOGIC LOG TESTS SAMPLE NUMBER DESCRIPTION 45_ 50. 55- 60- 65_ (Continued) hard, damp to moist, dark green with purplish mottling, sandy claystone Groundwater seepage 2-7 •Groundwater seepage Hard, moist, greenish gray claystone/siltstone' 2-9 ^ Increasing percentage of interbedded rock fragments SANTIAGO PEAK VOLCANICS Very dense, moist, dark brown, angular metavolcanic rock fragments 70- 75- Refusal at' 66' 80-" 'For description of symbols, see Figure 3 LOG OF.TEST BORING 2 (CONT'D) STAGECOACH PARK DRAWN BY: ch | CHECKED BYj^^^ | PROJECT NO: 56150E-SIO1 [oATE: 5-7-86 | FIGURE NO:""?" WOODWARD-CLYDE CONSULTANTS Test Boring 3 Approximate El. 2 26' DEPTH IN FEET GEOLOGIC LOG TESTS SAMPLE NUMBER DESCRIPTION 10. 15- 20- 25. 30- 35- 40- .go 1 FILL Damp, gray., silty clay/clayey silt 3-1 3.-2 3-3 3-4 3-5 [ c -6 [ 3-7 DELMAR FORMATION ... Hard, damp, light greenish gray with purplish mottling silty claystone Shear at 5','discontinuous N40W/40NE Fractured at 6'-8',.orientations random Remolded clay seam at 10', 1/10" thick, N65E/25NW Remolded clay seam at .13 ' , 1/4" to 1/10" thick, N35E/25SE. ' . Hard, damp to moist, dark greenish gray with purplish mottling clayey siltstone Groundwater seepage at 18' Groundwater seepage at 25'-28' (estimated flow rate several g.p.m.) Hard, moist to wet, bluish green to dark brown, clayey fine sandstone with interbedded angular fragments metavolcanic rock •For description of symbols, see Figure Continued on next page LOG OF TEST BORING 3 STAGECOACH PAR:K DRAWN BY: I CHECKED BY, PROJECT NO: 56150E-SIO1 DATE: 5-7-86 FIGURE NO: 9 WOODWARD-CLYDE CONSULTANTS Test Boring 3 (Cont'd) DEPTH IN FEET GEOLOGIC LOG TESTS SAMPLE NUMBER DESCRIPTION -(Continued) increasing percentage metavolcanic rock 45- SANTIAGO PEAK VOLCANICS Very dense, moist to wet, dark brown, angular metavol- canic rock fragments Bottom of Boring at 46' 50- 55-; 60- 65- 70- 75- - 80 •For description of symbols, see Figure 3 LOG OF TEST BORING 3 (CONT'D) STAGECOACH PARK' .// DRAWN BY: CHECKED BY//^. PROJECT NO: 56150E-SIO1 1 DATE: 5-7_86 1 FIGURE NO: 10 WOODWARD-CLYDE CONSULTANTS