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HomeMy WebLinkAboutCT 14-11; CARLSBAD BOAT CLUB & RESORT; GEOTECHNICAL UPDATE; 2015-05-26-.. .. .. .. .. .. .. ... -- .,. -.. "" - -.. .. • --- - GeoTek, Inc • 1384 Poinsettia Avenue, Suite A Vista, CA 92081-8S0S (760) S99-0S09 (760) S99-0S93 www.geotekusa.com VIP Partners Attention: Mr. Jim Courtney 1861 S. View Drive Carlsbad, California 92008 Subject: Response to City Review and Geotechnical Update Carlsbad Boat Club and Resort CT 14-11/CUP 14-10 4509 Adams Street Carlsbad, California Dear Mr. Courtney: C p May 26, 2015 PN 3487-SD ,..., -· .'~--,,,.;'\,,....I\, ·;" --~. ·\~. .: '~· l::. J.I MAY 2 6 2016 In accordance with your request and authorization, GeoTek, Inc. (GeoTek) hereby assumes the geotechnical responsibilities for the project. It is our understanding that Geologic Associates (GLA), has indicated they no longer provided services on this type project. In assuming responsibility, we have reviewed documents provided as indicated on the enclosed reference list and prepared this letter in response to the most recent City of Carlsbad comments (Reference I) for the project. In conjunction with our work, Mr Tim Metcalfe attended a meeting on May 3, 2016 with the City of Carlsbad (City) to discuss the project. We performed a site reconnaissance on April 30, 2016. It should be noted that Mr. Metcalfe is familiar with the site having been on site in non-professional capacities several times over the last 20.± years. Based on the review of the GLA report, general knowledge of the site, review of readily available aerial photographs and recent site observations it does not appear that any changes have occurred affecting geotechnical conditions on site since the 2007 GLA report was prepared. While additional design level studies may be advisable in conjunction with final project structural design, based on our review of the data and recommendations contained in the GLA report are in general keeping with current codes except as modified or updated herein . The City has indicated that the geotechnical update and review may be limited to address concerns expressed in the in Reference I and that prior issues have been resolved. The letter identifies three issues that the geotechnical update should address: Shoring, Storm Water Quality Management Plan (SWQMP) and impacts of possible sea level rise. These are discussed below . GEOTECHNICAL I ENVIRONMENTAL I MATERIAL -... --- - - .. - .. -- .. - .. .. .. .. ---- Response to City Review and Geotechnical Update Carlsbad Boat Club and Resort Carlsbad, California SHORING May 26, 2015 Proj No. 3487-SD Page 2 The City has indicated that the geotechnical report should be revised to include a preliminary shoring design. The City has also indicated that tie backs would not be allowed within the City Right of Way (RoW) for Adams Street. Based on discussion in the meeting with the City it is our understanding the primary concern is related to use of a permanent tie-back system within the RoW and both temporary and permanent tiebacks potentially extending into adjacent private property. The GLA report is rather unclear in separating recommendations for temporary and permanent support conditions. While not specifically indicated as applicable to temporary excavations it our interpretation that the recommendations provided by GLA for shoring were intended for temporary not permanent systems. This interpretation is based on the fact that earth pressures were presented separately for the permanent condition (see Section 6.5 Lateral Earth Presures and Resistance on page 16) and that the shoring recommendations were included in Section 6.6 Slope Excavation and Shoring, along with what are obvious temporary slope recommendations. In fact, use of the shoring design parameters should not be used for design of the permanent conditions. Specific shoring design is not the function of the geotechnical consultant. While, we might be able to offer some rudimentary design it would not be offered to use for construction and as such would be of little use. Shoring system design is structural not geotechnical and should be designed by engineers who are highly familiar with such designs. In our experience this is done in conjunction with final engineering not during project approvals. However, we appreciate the City's concern regarding the feasibility of completing the excavation given that tie-back shoring and temporary slope excavations were the only options presented. A tieback system may or may not be the appropriate method. This is particularly true given the City's stated restriction on tie backs within the Adams RoW and given close proximity of property lines may not be possible along the east and west sides. In our experience, most jurisdictions do not allow pennanent tie-backs for private works, however, they do allow temporary tiebacks to extend into their RoW provided that no portion of the tieback rod is left in place above an elevation which is likely to interfere with existing or future utility excavation. We have participated in projects where tie-backs were provided with couplings to allow removal of the unbounded length above a specific elevation within the public RoW. Attached on Figure S-1 is a typical cross-section of a shoring system where tieback anchors are extended into the public RoW. Sewers are commonly the deepest utility. Given that the sewer already exists in Adams (see Figure S-2) it seems reasonable that temporary tie back construction could be allowed within the RoW with minimal (if any) potential to impact any future public construction. Also as depicted on Figure S-2 if tiebacks are allowed in the RoW the highest elevation they might be within the RoW is on the order of 20 feet below street grade . Assuming the restriction is not modified to allow temporary tiebacks there are several methods to shore the vertical cuts without tiebacks or with modified tiebacks. Some examples of these include: - .. .. .. .... -------- ... Response to City Review and Geotechnical Update Carlsbad Boat Club and Resort Carlsbad, California May 26, 201S Proj No. 3487-SD Page 3 a. b. c . d . Utilizing an internally braced or raker and whaler system (see diagram on Figure S-2). Combining shoring and slot cutting. High strength grouts to increase the bond capacity Cantilevered systems. The east and west sides of the excavation were also noted to be a concern as any tie-back system would necessitate extending the tie-backs off site requiring cooperation and permissions from adjacent owners. Again alternate means particular cantilevered or braced systems could be appropriate. Given the rapid decrease of excavation height along the sides, cross-bracing at the corners with cantilevered piles along the rest of the walls may prove most efficient. STORM WATER QUALITY MANAGEMENT PLAN Due to the updated Regional Water Quality Control Board's Permit, the Project was required to update the Storm Water Management Plan (SWMP), now called Storm Water Quality Management Plan (SWQMP) (Reference 3). Three (3) filtration basins are planned. Basins I and 3 are similar with respect to the geotechnical conditions associated with them, while Basin 2 presents other conditions. Basins I and 3 are at the lower end of the site. The bottoms of the basins are at elevations of 4 to 5 feet. GLA encountered groundwater seepage in Boring B-3 at elevation -2.Sft about 40 feet north and landward of Basin 3. Given this and the basins' proximity to the lagoon a permanent ground water table approximating sea level or slightly above is prudent to assume as GLA did when developing recommendations for the site. Appendix C of the City of Carlsbad BMP Design Manual indicates that: "The depth to seasonally high groundwater tables (normal high depth during the wet season) beneath the base of any infiltration BMP must be greater than IO feet for infiltration BMPs to be allowed." As such infiltration is not recommended for the basins as the groundwater clearance will not meet this standard. Basin 2 is located immediately behind and above the building retaining wall. While introduction of water behind the retaining wall is not desirable even if the water were allowed to infiltrate it would be into the granular wall backfill the base of which would be at about elevation 6ft and again the actual infiltration would not have a minimum of I Oft to the water table. As such infiltration is not recommended for the basin. The SWQMP (Section 7, Page 12) addresses the lack of infiltration by utilizing impermeable liners in the bio-filtration basins to treat the runoff prior to dispersal in the sand. POTENTIAL SEA LEVEL RISE AND FLOODING The potential for sea level rise is truly an unknown with various predictions ranging from about 1.5ft to 6ft by 2100. The California Coastal Commission web site (http://www.coastal.ca.gov/climate/slr/science/) indicates: "According to the 2012 National Response to City Review and Geotechnical Update Carlsbad Boat Club and Resort Carlsbad, California May 26, 2015 Proj No. 3487-SD Page ◄ Research Council (NRC) report, which the Ocean Protection Council established as the best available science on sea level rise for California in March 2013, sea level is projected to increase by 17 to 66 in (42 to 167 cm) along much of the California coast by Year 2100." Figure SL I taken from the NRC report is a graphic representation of future prediction of potential sea level changes. Not surprisingly the predictions are less accurate as time progresses. The reasonable economic life of new construction of this type is often considered to be on the order of 50 years. While the chart is specific to the San Francisco Bay Area it suggests the between 2076 and 2086 (60 to 70 years in the future) the predicted sea level rise may be on the order of 2.3ft (70cm) with peak levels ranging up to approximately 3.28ft ( I 00cm). The basement/garage level of the project would remain almost 5 feet above sea level. Even in the worst case scenario of a 6 foot sea level rise by 2100, while flooding would likely occur it would be periodic and confined to the parking garage. On SL-I, we have denoted the time period from roughly 2076 to 2086 with the red box and the upper limit and mid-range projections with the dashed lines. annual sea level 240 160 cm 80 total time of exceedance hrs above historical 99.99th percentile 1200 hrs 600 0 -t:r''tf-l,,,..Llc:~'lfv'~...-, ....... ~..,.__------+--+----t-0 1960 2000 2040 2080 yr FIGURE SL-I Projected number of hours (blue bars) of extremely high sea level off San Francisco under an assumed sea-level rise and climate change scenario. In this exercise, a sea-level event registers as an exceedance when San Francisco's projected sea level exceeds its recent ( 1970-2000) 99.99th percentile level, I .'I m above historical mean sea level. In the recent historical period, sea level has exceeded this threshold about one time (I hour) every 14 months. Sea-level rise (black line) during 1960- 1999 was arbitrarily set to zero, then increased to the committee's projected level for the San Francisco area over the 21st century. (Explanation from source.) CODE REVISIONS AND UPDATES There are triennial building code revisions as such multiple code revisions have been adopted since the 2007 GLA report was prepared. The 2016 CBC is due to be published in July with adoption January I, 2017. It is our understanding that final engineering is unlikely to be performed prior to 2017. Based on our current understanding the code changes, the coming adoption should ----- ----------- ---- --.. -.. -- -... - Response to City Review and Geotechnical Update Carlsbad Boat Club and Resort Carlsbad, California May 26, 2015 Proj No. 3487-SD Page 5 not have a significant impact on geotechnical recommendations. However, specific updated recommendations should be requested from this office prior to start of final design project design. LIMITATIONS This report is based on review of data presented by others. Based on our general knowledge of the area, the conditions discussed appear reasonable. 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, data by others, our conclusions 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. If you have any questions, please do not hesitate to contact the undersigned Respectfully submitted, GeoTek, Inc. Enclosure: Selected References Figure S-1 -Shoring within Public Right of Way Figure S-2 -Shoring Options ---- --- -.. • ---- --- ... .. - - --- ... .. Response to City Review and Geotechnical Update Carlsbad Boat Club and Resort Carlsbad, California Selected References May 26, 2015 Proj No. 3487-SD References 1) "3rd Review for CT 14-11/CUP 14-10-Carlsbad Boat Club and Resort" by City of Carlsbad dated April 13, 2016 2) "Geotechnical Report, Proposed Condominium Complex, 4509 Adams Street Carlsbad, California" by Geologic Associates dated March 6, 2007 3) "Preliminary Storm Water Quality Management Plan for CT 14-11 Cup 14-10 Carlsbad Boat Club Resort, 4509 Adams Street Carlsbad, California" by O'Day Consultants, Inc date revised January 25,2016 4) "Carlsbad Boat Club & Resort -City of Carlsbad Tentative Map -Tentative Map & Site Plan" by Excel Engineering, dated 4/28/16 5) Architectural Plans, Carlsbad Boat Club & Resort by MAB Architecture & Planning revision date 4/18/18 GEOTECHNICAL I ENVIRONMENTAL I MATERIAL Carlsbad Boat Club and Resort Shoring within Public Right of Way SAFETY C LE (T'l'PICAL AAOU E Sl-1 I PR COi.i IANCE STAND,.RDS, 0 TOM Cf EXCAVATIOI GEOTEK "too E)(lS NG GRA.OE RY (A.BOVE lOE) :"-''" . ~ . UKBONIJED . l!'NGJH I I I I I 6 . \ SOLDIER AM (SIT SCHEOOLE) FAILURE PLANE / PVC Ovt EN11RE EMOVA E SECTION ~ TIEBACJ( 0 PRfl/OlT B C1i THR£ADBAR \,lnllN Cl Y RIGHT-Of-WAY / rPORTION OF TIEBACK 1 TO BE REMO'¥£0 L __________ J VlllH SET SCREWS AT EIAOV"81.E ~D A BELOW GRADE OST ~OUT LIN[, ( 11" SCH40 PVC PIPE 'M TH U ER VALVE'S A !','-if 0.C) 20'-0" 'O.C. (S(E SCHEDULE FCR SIZE) I 1 /2 SAO( SL RY TOE, 0 !JRI\JEN [To ACIUTATE REMOVAL-NO POST GROUT 7 IVALVES SHALL BE PLACED Yl11HIN 5' F'EEl I 1 OF BAR COUPLER 1 ~---------------------J SHAF OIAMffiR -.-.-(SEE SCH.EllULE FOR SIZE) NOlES: 1. W STRUCTURE 'NALL NOT SHOl FOR CLARITY 2, LAOOlt4G NOT SHOWN AT TIEBACK f'OR CLAAITY 3. fO DIM SICtlS .,._., ·0·1 • & •. SrE SCt!XER BEAM & llEBACI< SCH OUl.E O SHEET [S5.0. SOLDIER BEAM SECTION (ONE TIEBACK) EI) TIEBACK WITHIN~ RIGHT-OF-WAY Figure S-1 ! I ~ I ,,PAM§ &JBEEI ~ I Raker and Whaler Shoring System with Temporary 1 :1 Construction Slope 29' Emm E/evatio ofTieBac, PL. Typical Temporary Soldier Beam and Tie-back Shoring System Approx. Scale l.._20' at I htl7 l.._32' at l½xl I Bulldins Section from Sheet AS -2 "Bulldlns Sec. & Boat Launch Plan Carlsbad Boat Club & Resort'' prepared by MAB~ .. and Plannln1 date 04/11/2016 ~ GEOTEK l ·-, ~ U)pl,RJ,ffl, 6}'-e>' Fl'.• ,&O'-#)' FF.~ ""1-#J' F~.• 4'·0' LOJ"AIW'!t • 6> -o' ,~. ,&o-f,' F.f •. • ..CO.fl' Fl',• t2'f4' -UE OF-C&llER LI Shoring Options Carlsbad Boat Club MAY 2016 PN: 3487-SD Fi2ure 5-2