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HomeMy WebLinkAboutCT 04-10A; Poinsettia Place; Geotechnical; 2010-12-02C. I O I I K GoolMctlnc. l384P0inMniaAvOTiM.SutttA Vista. CA 92081-8505 (760) S99.0509 on, (760) 599-0593 r < www.seotekuta.coni RECC^RB COPY I A December 2, 2010 Imml Me J PN 3368-SD3 Sierra Linda P»v<Bgpiiiirit ' • 219 Meadow Vista Way ^ - '^o , ^ Encinitas, CA 92024 ^<$0^^%^ ^ Subject: Geotechnical Update and Grading Plan Review 1 ^ CT 04-10, Poinsettia Place /1 Poinsettia Lane and Cassia Road [ Carlsbad, California 92009 This report presents an update of the Reference 1 report by GeoTek, Inc. (GeoTek) and is a review of the grading as indicated on the "Grading and Private Storm Drain Plan" for CT 04-10, Poinsettia Place, Carlsbad California prepared by Pangaea Land Consultants, Inc. The undated plan is composed of ten sheets. Sheet 1 is the Title Sheet including the grading notes. The planned site grading is depicted on Sheets 4 through 7, while sheets 8 through 10 show the planned private storm drain. This report does not review the proposed storm drain depicted on plan sheets 8 through 10. A significant portion, 11.55 acres, of the overall property is designated as open space Lot 3. As such, herein indicated references to the "site" or "project" address the developed areas of Lots 1 and 2 which comprise approximately 6.5 acres. Site Background Preliminary geotechnical studies were performed by several companies prior to the site being rough graded in 2000. Geotechnical observation of grading was performed by Pacific Soils Engineering, Inc. Based on available data, GeoTek indicated in Reference 1 that partial alluvial removals were performed prior to fill placement. It is our understanding that post-grading settlements were monitored, but we have not been able to review that data. Proposed Grading O LU The site design is for two entrances into the project from Cassia Road. The main entry ^ road, "Road B", will come to a tee intersection with "Road C" roughly paralleling Cassia along the south side of the project. Road C also turns 90 degrees at the east end of the ^ z GEOTECHNICAL | ENVIRONMENTAL | MATERIALS All material herein © 2010 GeoTek, Inc. All Rights Reserved a. o Sierra Linda Deveiopment December 2, 2010 Geotechnical Update and Grading Plan Review Projea No. 3368-SD3 CT 04-10, Poinsettia Place Page 2 project and then intersects directly with Cassia. Plans indicate there is 2,750 cubic yards (cy) of cut, 43,900 cy of fill and 41,150 cy of import proposed. Proposed grading will create 9 building pads for construction of multi-family residential buildings. With the exception of the 3 westerly most building pads, pad elevations would vary from approximately 3 to 7 feet above current grades. In the southwestern portion of the site below building pads 1 through 3, grading will fill an existing storm drain basin and canyon area. Fill materiais up to approximately 28 feet in depth are proposed in this area so that the pad grades are roughly 5 feet above the future grade of Poinsettia Lane. It is our understanding the grading for Poinsettia Lane will either precede or be concurrent with that for the residential project. With the exception of small areas on the south side of the main road, the road areas are proposed fill areas. Slopes are proposed at gradients of 2:1 (h:v) or flatter. The individual building pads will vary in elevations by approximately 3 to 4 feet from the adjacent pads. The highest fill slope planned is about 20 feet high and adjacent to the proposed bio-retention basin at the south corner of the site. Cut slopes are largely limited to the area adjacent to the open space area at the east and south sides of the project with the highest slope being approximately 10 feet high plus a 4 to 5 feet high retaining wall along the toe. Retaining walls are proposed in several cut areas, primarily along the east and south side of the main road and adjacent to the open space. Walls are up to about 7 feet in height. Supplemental Recommendations The planned development appears feasible from a geotechnical standpoint. GeoTek's Reference 1 report, dated April 13, 2004, presented recommendations and conclusions for site grading and construction. Generally the recommendations contained in GeoTek's 2004 report are considered to be valid. Supplemental and/or modified recommendations for certain aspects of the project are offered below. Site OrwIkMl Site grading is similar to that planned in 2004 so that recommendations offered at that time are generally valid with the following supplements. 1. ASTM standards referenced with dates are subject to the current standards. 2. Removals of surficial soil were anticipated to be on the 2 to 3 feet. This recommendation is considered applicable to the majority of the site. 3. In the area of building pads 1, 2 and 3, no specific testing was performed and a portion of the area has been used as a desilting basin for several years. Removals are likely to be on the order of 10 to 12 feet. Given the depths of fill to be placed and that only partial alluvial removals will be performed it is Sierra Linda DeveiopnMnt Geotechnical Update and Grading Plan Review CT 04-10, Poinsettia Place December 2, 2010 Project No. 3368-SD3 Page 3 recommended that the area be monitored for settlement using survey monuments. Monuments should be placed and read within 3 days following the fill reaching finish grade. Monitoring should then be performed on a weekly basis for a period of 4 weeks and bi-weekly thereafter for a minimum of 6 months or until the estimated remaining settlements are within the project parameters. 1. Fill slopes should be constructed as outlined in the prior report. 2. The cut slope south of building pad 7 is a shallow cut and may expose mostly topsoil/colluvial soil. Corrective grading may be necessary based on specific conditions exposed. Such corrective grading would likely be in the form of a stabilization fill slope. Fwndatiqn Ptwgn Piranwtgr? Foundation recommendations were based on the then current standards; different standards are currently in effect. As such, supplemental foundation recommendations and seismic design parameters will be needed. The site however is planned to have significant import soil in order to achieve finished grades with most of the building pads on the order of 3 to 7 feet above existing elevations. As such, the imported material will be the dominant foundation soils and updated foundation recommendations cannot be reasonably provided until imported soils are placed. Updated seismic design parameters are offered below. Seismic Desiqn Parameters The site is located at latitude 33.1128 and longitude 117.2779. Site spectral accelerations (Ss and SI), for 0.2 and 1.0 second periods and 2 percent probability of exceedance in 50 years (MCE) was determined from the USGS Website, Earthquake Hazards Program, Interpolated Probabilistic Ground Motion for the Conterminous 48 States. The site is considered a Site Class "D". The results are presented in the following table: SITE SEISMIC PARAMETERS Mapped 0.2 sec Period Spectral Acceleration, Ss (g) 1.177 Mapped 1.0 sec Period Spectral Acceleration, SI (g) 0.444 Site Coefficient, Fa 1.029 Site Coefficient, Fv 1.556 Maximum Considered Earthquake Spectral Response Acceleration 1.211 Sierra Linda Development Geotechnical Update and Grading Plan Review CT 04-10, Poinsettia Place December 2, 2010 Project No. 3368-SD3 Page 4 Parameter at 0.2 Second, SMS (g) Maximum Considered Earthquake Spectral Response Acceleration Parameter at 1 second, SMI (g) 0.698 Design Spectral Response Acceleration Parameter for 0.2 Second, SDS (g) 0.908 Design Spectral Response Acceleration Parameter for 1.0 Second, SDI (g) 0.464 Seismically resistant structural design in accordance with local building ordinances should be followed during the design of the structures. The Caljfornia Building Code (CBC) has been devetoped to reduce the potential for structural damage. However, as the result of ground shaking generated by nearby earthquakes some level of damage and associated economic consequences are considered likely in this general area. Retaining Wall Desiqn Parameters Retaining wall design parameters were presented in Reference 1. It is our understanding that there was some question regarding the applicability of San Diego County Regional Standard Retaining Walls for conditions on the site. Drawing C7 indicates that walls are designed for an equivalent fluid pressure of 36pcf which applies to the level surcharge. It also indicates that: "Walls shown for 172:1 unlimited sloping surcharge are designed in accordance with Rankine's formula for unlimited sloping surcharge with a <t)=33°..." Using Rankine's formula is considered an acceptable method for design of walls with sloping surcharges and should result in earth pressure comparable to those provided in Reference 1. Simon Wong Engineering is designing the larger retaining walls and has requested that we review the foundation parameters presented. The following parameters may be used: For retaining wall foundations an allowable bearing capacity of 2,500 psf may be utilized. Such foundations should have a minimum embedment depth into competent bearing material of 18 inches and be 24 inches wide (minimum). Allowable bearing capacity may be increased by 200 psf for each additional foot of width to a maximum of 3,100 psf. Passive resistance may be computed as an equivalent fluid pressure having a density of 250 psf/ft to a maximum of 2,500 psf. A coefficient of friction between the soil and concrete of 0.35 may be used. When combining passive pressure and frictional resistance the passive pressure component should be reduced by one-third. These values assume that the import is reasonable consistent with on site materials, if Sierra Linda Development December 2, 2010 Geotechnical Update and Grading Plan Review Project No. 3368-SD3 CT 04-10, Poinsettia Place Page 5 expansive or otherwise fine grained, lower phi angle soils are brought in and placed in the foundation zone these may not be valid. Recommendations regarding wall backfill and drainage remain valid. PaYwniMit Ptiifln In addition to the prior pavement and construction recommendations the following should be considered: 1. Pavement design and construction should conform to the minimum requirements of the City of Carlsbad. 2. Final pavement design should be based on R-Value testing of actual subgrade soils. 3. Permeable pavements are proposed at the driveway approaches and "Road C". These areas are to be underlain with a minimum thickness of 24 inches of granular material which we understand the intent of is to act as filtration layer. Both percolation and filtration are desired. These areas should be graded to slope at a minimum of 2% toward the collector pipes at their low ends which tie into the storm drain system. Excessive water should be prevented from percolating into the ground by placing a very low permeability membrane in the bottom and wrapping up the sides. Following placement of the membrane and subject to City approval, we would suggest a 50-50 mix of washed concrete sand and Vz to % inch crushed rock, or a similar blend of material be placed. The material should be properly compacted to at least 90 % of the maximum dry density in accordance ASTM 1557. The top of the permeable mix should be covered with a layer of Mirafi 140N or comparable fabric prior to placing the standard road base section. It should be noted that introduction of excessive amounts of water into base materials and subgrade soils negatively impacts their support capability. The potential introduction of water into the underlying base and subgrade soils may significantly reduce useable pavement lifespan. As such, increased maintenance may be required. Rln-Pfttantinn Ba«n Sheet 3 of the Plans indicates that a minimum 24 inches of aggregate permeable material per the Soils Engineer specification is required across the bottom of the Bio- Retention basin. It is our understanding the purpose of this basin is to act as a filtration device for run-off prior to it leaving the site. Subject to City approval, we would suggest a 50-50 mix of washed concrete sand and Yz to % inch crushed rock, or a similar blend of material. Testing could be performed to determine whether this blend provides desired permeability and filtration. Sierra Linda Development Geotechnical Update and Grading Plan Review CT 04-10, Poinsettia Place December 2, 2010 Project No. 3368-SD3 Page 6 Closure The materials observed on the project site appear to be representative of the area; however, soil and bedrock materials vary in character between excavations and natural outcrops or conditions exposed during site construction. Site conditions may vary due to seasonal changes or other factors. GeoTek, Inc. assumes no responsibility or liability for work, testing or recommendations performed or provided by others. Since our recommendations are based on the site conditions observed and encountered, and laboratory testing, our conclusion and recommendations are professional opinions that are limited to the extent of the available data. Observations during construction are important to allow for any change in recommendations found to be warranted. These opinions have been derived in accordance with current standards of practice and no warranty is expressed or implied. Standards of practice are subject to change with time. We appreciate the opportunity to be of service on this project, undersigned if you have any questions. Please contact the Respectfully submitted, GeoTek, Inc. Timothy E. Met^lfe, CEG 1142 Expires 04/30/12 Principal Geologist Enclosures: List of cited references Ronald A. Reed, GE 2524 Expires 06/30/2011 Senior Project Engineer Sierra Linda Development December 2, 2010 Geotechnical Update and Grading Plan Review Projett No. 3368-SD3 CT 04-10, Poinsettia Place References Cited References: 1) Geotechnical Evaluation, Proposed Residential Buildings, SRWB Property, South Side of Cassia Road, East of Poinsettia Lane, Carlsbad, California by GeoTek, Inc. dated April 13, 2004 2) Grading and Private Storm Drain Plan" for CT 04-10, Poinsettia Place, Carlsbad California prepared by Pangaea Land Consultants, Inc. 3) Stock Pile Grading Plan: CT 04-10, Poinsettia Place", prepared by Pangaea Land Consultants Inc. undated