The URL can be used to link to this page

Home My WebLink AboutCT 97-13; CARLSBAD OAKS NORTH; PRELIMINARY GEOTECHNICAL INVESTIGATION UPDATE; 2000-01-04GEOCON INCORPORATED Project No. 06442-32-01 December 28, l 999 Revised January 4, 2000 Techbilt Construction Corporation Post Office Box 80036 San Diego, California 92138 Attention: Mr. Ted Tchang - GEOTECHNICAL CONSULTANTS C Subject: CARLSBAD OAKS NORTH BUSINESS PARK; CT 97-13 CARLSBAD, CALIFORNIA GEOTECHNICAL UPDATE Reference: Preliminary Geotechnical Investigation for Proposed Carlsbad Oaks East, prepared by Woodward-Clyde Consultants, dated June 15, 1990. Gentleman: In accordance with your request, this correspondence has been prepared to update the referenced geotechnical report with respect to the latest tentative map for the property. It should be noted that the subject project was referred to as Carlsbad Oaks East in 1990. The present tentative map was prepared by O'Day Consultants, Inc. with a last revised date of October 8, 1999. Although the current development plan for the 414-acre site is similar to that depicted in the referenced geotechnical report, there are some notable differences, which are summarized below: 1. The development area has been reduced from approximately 270 acres to 188 acres (not including Faraday Avenue and El Fuerte Street). 2. The offsite improvement of Faraday Avenue from the project's west boundary to Orion Street has been added. 3. Development of an area underlain by several ancient landslides has been eliminated. 4. Some cut slopes within granitic rock along the northern development margins are planned at 1.5: 1 (horizontal:vertical) with a maximum height of 40 feet. 5. The maximum heights of proposed 2: 1 cut and fill slopes are 60 feet and 70 feet, respectively. Previously a maximum height on the order of 55 feet was contemplated. 6. The number of building lots has been reduced from 55 to 22 (not including the sewer pump station lot). Based on our review of the referenced report and the present Tentative Map for Carlsbad Oaks North Business Park, it is our opinion that the new development plan significantly reduces the impact on areas of potential geologic hazards. The increase in maximum slope height from 55 to 70 still yields a factor of safety for slope stability in excess of 1.5 using the strength parameters presented in the 6960 Flanders Drive ■ San Diego, California 92121-297 4 ■ Telephone (858) 558-6900 ■ Fax ( 858) 558-6159 - geotechnical report. The strength parameters represented a compacted fill soil condition and hence, do not account for the high cohesive strength characteristic of granitic rock in a cut (undisturbed) condition. Using shear strength characteristics more indicative of granitic rock, the proposed 1.5: 1 cut slopes were also found to possess a factor of safety with respect to slope stability well in excess of 1.5 (see Slope Stability Analysis, Figure 1). The slopes proposed for the off site improvement of Faraday Avenue are all planned at 2: 1 and have a maximum height of 30 feet and 50 feet for cut and fill slopes, respectively. No potential geologic hazards, such as ancient landslides are known to exist along the proposed offsite alignment. However, as recommended in the referenced geotechnical report for onsite improvements, more detailed geotechnical studies should be performed prior to finalizing grading and improvement plans. In particular, subsurface exploration should be performed to address the stability of proposed slopes, provide retaining wall design criteria and provide specific recommendations for the grading of the off site roadway, including remedial grading measures, if necessary. The geotechnical report did not address the need for terrace drains on cut or fill slopes and none are proposed for the project as currently planned. In our opinion, the use of terrace drains is not necessary to maintain gross stability of the slopes. It has been our experience that terrace drains that are not continually maintained can actually cause significant slope erosion and possibly slope distress. In summary, it is our opinion that no soil or geologic conditions exist which would preclude the development of the Carlsbad Oaks North Business Park as presently planned. The project has less adverse impact on areas of potential geologic hazards than the plan that was considered in the referenced geotechnical report. Should you have any questions regarding this correspondence or if we may be of further service, please contact the undersigned at your convenience. Very truly yours, GEOCON IN CORPORA TED !±:~ GE 503 David B. Evans CEG 1860 0 !NG . ST / .. -/ ,. , .... - TVL:DBE:kje (3) Addressee ..... _ '. - (3) O'Day Consultants Attention: Mr. Timothy Carroll (3) Ladwig Design Group Attention: Mr. Bob Ladwig Project No. 06442-32-01 -2 -December 28, 1999 Revised January 4, 2000 - PROJECT NO. 06442-32-01 ASSUMED CONDITIONS: 1.5: 1 Cut Slope 40 feet high in granitic rock Slope Height Slope Inclination H = 40 feet 1.5: I (Horizontal : Vertical) Total Unit Weight of Soil Angle of Internal Friction Apparent Cohesion y1 = 140 pounds per cubic foot 4> = 40 degrees C = 300 pounds per square foot No Seepage Forces ANALYSIS: Yc<l> = yH tan!J! Equation (3-3), Reference I C FS = !'.:!.c& Equation (3-2), Reference I yH Yc<l> = 16 Calculated Using Eq. (3-3) Ncf = 36 Determined Using Figure 10, Reference 2 FS = 1.93 Factor of Safety Calculated Using Eq. (3-2) REFERENCES: (1) Janbu, N., Stability Analysis of Slopes with Dimensionless Parameters, Harvard Soil Mechanics, Series No. 46, 1954. (2) Janbu, N., Discussion of J.M. Bell, Dimensionless Parameters for Homogeneous Earth Slopes, Journal of Soil Mechanics and Foundation Design, No. SM6, November 1967. SLOPE ST ABILITY ANALYSIS CARLSBAD OAKS BUSINESS PARK, CT 97-13 CARLSBAD, CALIFORNIA FIGURE 1