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Home My WebLink AboutCT 14-10; POINSETTIA 61; RESPONSE TO GEOTECHNICAL REVIEW COMMENTS (SECOND REVIEW); 2018-04-03• • • AuiauPDELTA April 3, 2018 Lennar Homes 25 Enterprise, Suite 300 Aliso Viejo, California 92656 Attention: Mr. Alan Chik SUBJECT: Mr. Chik: RESPONSE TO GEOTECHNICAL REVIEW COMMENTS (SECOND REVIEW) Poinsettia 61 Development Carlsbad, California We are proving herein our responses to the referenced second set of third party geotechnical review comments for the Poinsettia 61 development in Carlsbad, California (Hetherington, 2018b). Each comment is reiterated below (in italics). followed by our response. Comment No. 1: The Consultant should review the building foundation plans when finalized once the site is rough graded and confirm that the plans have been prepared in accordance with the geotechnical recommendations. We plan to review the residential post-tension slab foundation plans prior to finalization, in order to confirm that they have been prepared per our recommendations, in accordance with the typical standards of practice. Note that the final foundation design should be based on the recommendations provides in the interim as-graded report, once the site is rough graded. Supplemental laboratory testing will be conducted during the grading operations to refine the post-tension slab design parameters recommended for the residential lots. Comment No. 2: The Consultant should review the "Poinsettia Bridges Foundation Plan ... " (Reference 6), provide any additional geatechnical recommendations considered necessary, and confirm that the plans have been prepared in accordance with the geotechnical recommendations. The Poinsettia Bridge Foundation Plan referenced in the second review letter was dated October 4th, 2017. We have reviewed that plan previously, and have incorporated the latest update to the plan dated December 22nd into the Bridge Foundation Plan, Figure 2C, in the referenced Foundation Report (GDC, 2017d). No additional comments are deemed necessary, since we already reviewed this plan prior to submission of the FR. The Bridge Foundation Plan shown in Figure 2C was prepared in accordance with our geotechnical recommendations, and is consistent with the information provided in Tables 2A through 2D of the Foundation Report. • • • Response to Geotechnical Review Comments (Second Review) Poinsettia 61 Development Lennar Homes GDC Project No. SD412B April 3, 2018 Page 2 Comment No. 3: The Consultant should provide a comprehensive, current description of proposed development including grading, structures/improvements, drainage, use, foundation type, estimated structural loads, etc. (Second Request). Site development will include the construction of 122 lightly-loaded wood-framed single-family residential structures. The buildings will be supported by post-tension slab foundations, based on the as-graded design conditions. Various surface improvements will also be constructed, including asphalt concrete residential streets, cul-de-sacs, and alleys, Portland cement concrete sidewalks, and a variety of associated subsurface utilities. Extensive cut and fill grading will be needed to attain finish grades for the planned development. The rough grading plans indicate about 250,000 yd3 of cut/fill grading. In addition, 12 Verdura-style mechanically stabilized earth (MSE) retaining walls are proposed around the perimeter of the site, along with a number of smaller cantilever CMU walls within the interior of the development. Six HPDE lined bio- filtration basins will also be constructed throughout the development to retain storm water. The rough grading plans for the site are shown on the updated Geotechnical Map, Plate 1. Comment No, 4: The Consultant should provide an updated geotechnical map utilizing the latest grading plan for the project to clearly show (at a minimum) a) existing site topography, b} proposed structures/improvements, c) proposed finished grades d} locations of the subsurface exploration, and e) geologic structure. (Second Request). The updated Geotechnical Map (Plate 1) is attached to this response letter. The Geotechnical Map shows the existing and proposed grades, the proposed building pads, retaining walls and streets, the locations of the borings, test pits and cross sections, and the geologic structure. Comment No. 5: The Consultant should provide updated geologic cross-section(s) utilizing the latest grading plan showing (at a minimum) a) existing site topography, b} proposed structures/improvements, c) proposed finished grades, d) geologic structure, contacts, etc., e) slope setbacks for near slope structures/improvements and f) locations of the subsurface exploration. Slope stability analyses output plots do not provide this data. (Second Request) Geologic Cross Sections are attached to this response letter (see Plate 2). The approximate cross section locations are shown on Plate 1. The Geologic Cross Sections show the existing and proposed grades, the proposed building pads, retaining walls and streets, the locations of the borings and test pits, and the geologic structure . Comment No. 6: The Consultant discusses two sheared claystone beds identified by Geocon at the site, but does not provide the subsurface exploration logs by Geocon. The Consultant should provide the exploration logs by Geocon. We have transmitted a PDF copy of the referenced report to the reviewer (Geocon, 2006). The report contains logs of the previous test pits and exploratory borings conducted at the site. A ~CLP CELT A N,\Prnject,\SD\SD412B ''""" -Poie,ettia 61, T&O of Earthwock Coe,t,octioe\S. Repoct,\18-DD12\18-DD1M.doc • • • Response to Geotechnical Review Comments (Second Review) Poinsettia 61 Development Lennar Homes GDC Project No. SD412B April 3, 2018 Page 3 In Section 3.1 of our 2014 Geotechnical Investigation report, we stated the following: 'Two additional sheared c/aystane beds were previously encauntered by others in large diameter borings at elevations of about 235 and 245 feet MSL (Geocon, 2006). These claystone beds may be continuous across the site, and moy adversely impact cut slope stability. The location ond extent of these claystone beds should be evaluated during grading." The logs for Borings B-1 and B-3 (from 1999) in the referenced report (PDF Pages 46 and 50) show the claystone bed encountered near an elevation of 245 feet MSL in the large diameter borings located near the top of the cut slopes at Station 184+50 on Poinsettia Lane (Geocon, 2006). The approximate locations of Borings B-1 and B-3 are shown on the attached Geotechnical Map (Plate 1), as well as Figure 2 in the referenced report. In Boring B-1, the claystone bed was encountered between elevations of 246 and 250 feet MSL. In Boring B-3, the claystone was encountered between elevations of 244 and 253 feet MSL. In the log for the supplemental Boring B-1 (from 2004 on PDF Page 22) along the northern edge of the site, two 3 to 4-inch thick claystone beds were observed near elevation 245 feet MSL. These two thin beds were described as " ... poorly developed and poorly remolded." We did not encounter sheared claystone in the targeted continuous sampling that we conducted between elevations of 243 and 247 feet MSL in the supplemental explorations in this area (GDC, 2014). Consequently, we did not model the claystone bed in our analyses of the northern cut slopes. The toes of the cut slopes along Poinsettia Lane near the 1999 Borings B-1 and B-3 are currently located at elevations ranging from 240 to 246 feet MSL (these slopes were believed to have been graded in 2010). Note that if a sheared claystone bed with a very low residual strength does exist in the area of Borings B-1 and B-3, we would have expected that the existing 2:1 cut slopes along Poinsettia Lane would show indications of these beds, since the sheared claystone bed would daylight within these cut slopes. We did not observe indications of a sheared claystone bed near the of the existing cut slopes along Poinsettia Lane. These cut slopes appear to have performed well over the last 8 years, and no failures were observed. The second claystone bed noted by Geocon at an elevation of 235 feet was not encountered in their Borings B-1, B-2 or B-3 located within the northern part of the Poinsettia 61 development. This claystone bed at elevation 235 feet MSL was encountered in Boring B-3 from 2004 (see PDF Page 26). We did not encounter this claystone bed in our explorations, and this claystone bed is not expected to adversely impact the cut slope in the southwest portion of the site. Geologic observation will be used during grading to confirm that the claystone bed located near an elevation of 235 feet (if present), and does not adversely impact the stability of the proposed cut slopes in the southwest corner of the site (Lots 34 to 36), near the location where this bed was encountered (Geocon, 2006). ~ ~OLP CELT L\ N,\Pmject;\SD\SD412B Lecoac -Po;e;ett;a 61, T&O of Earthwo,k Coe;trncUoe\S. Report;\18-0012\18-0012,2.doc • • • Response to Geotechnical Review Comments (Second Review) Poinsettia 61 Development Lennar Homes GDC Project No. SD412B April 3, 2018 Page 4 The location and extent of all of the claystone beds within the Santiago Formation should be evaluated during grading using geologic observation. Supplemental recommendations may be provided during grading, if a sheared claystone bed is encountered near the toe of the cut slope along the northern property boundary at elevation 245 feet MSL, or if a sheared claystone bed is encountered near elevation 235 feet in the southwest portion of the site. Prior to commencing with the planned 1:1 temporary cut slopes in these areas, the precise location and extent of these claystone beds should be determined. Exploratory dozer or test pit excavations should be conducted by the grading contractor to search for these claystone beds. Comment No. 7: The Consultant should provide all of the slope stability input/output files included in References 1 and 5. The initial slope stability cross sections developed for our 2014 investigation (Reference 1) did not include the 14 mechanically stabilized earth (MSE) walls that were later added around the perimeter of the site (GDC, 2014). Consequentially, those initial slope stability cross sections are not no longer valid, and are not provided for review. No additional slope stability analyses were needed to prepare References 2, 3 and 4 (GDC, 2015, 2016, 2017a). The currently proposed grading plan is shown on the updated Geotechnical Map, Plate 1. The approximate locations of the eight cross sections we used to revise our slope stability analyses after the MSE walls were added are also shown on Plate 1. The supplemental stability analyses were described in detail in the MSE wall plan review letter, Reference 5 (GDC, 2017b). The input and output files for the SLOPE/W analyses from Reference 5 are attached for review. Comment No. 8: The Consultant should provide the as-graded report for the portion of future Poinsettia Lane that will extend through the Poinsettia 61 development that was reportedly graded in 2010. We have asked the City of Carlsbad to provide the as-graded report for the portion of Poinsettia Lane that was previously graded within the subject site. The City has not provided a copy of the as-graded report. The City indicated that the soil excavated from the large cut slopes along Poinsettia Lane was used to generate fill soil for grading of the SRWB Property located a few hundred feet north of the subject site. The geotechnical conditions at the 5RWB property were described in the referenced reports (GeoTek, 2004, 2010). We discussed the 5RWB project with Timothy Metcalfe at GeoTek, who indicated that remedial excavations were likely conducted prior to placing compacted fill in the area of Poinsettia Lane and elsewhere on the SRWB site. Mr. Metcalf indicated that he would review his records and search for any available as-graded reports for that area. However, no as-graded report has been provided to date for the previous grading operations in the northwest portion of the Poinsettia 61 development. Consequently, this fill is considered to be undocumented, and may potentially be compressible. A ~CUP C>EL T .t\ N,\Pcojects\50\504128 ''""" -Poiasettia 61, T&D of Earthwock Co",tcoctio"\S. Repocts\18-0012\18-0012,2.doc • • ' • • Response to Geotechnical Review Comments (Second Review) Poinsettia 61 Development Lennar Homes GDC Project No. SD412B April 3, 2018 Page 5 As a minimum, we have recommended that 3-foot deep remedial excavations be conducted throughout the area of the existing undocumented fill in the northwest portion of the site, in order to help evaluate the quality of the fill using both a nuclear gauge and geologic observation (GDC, 2016). Note that the Standard Penetration Tests (SPT) that we previously advanced within the existing fill encountered in the northwest portion of the site indicated corrected blow counts (N60) of 25 or more (typically 30 or more), which is indicative of dense conditions consistent with a compacted fill. If pockets of loose, compressible fill or other deleterious materials are observed in the 3-foot deep remedial excavations, deeper excavations may be warranted, based on the conditions observed by our field personnel during grading. In order to help assess the compressibility of the existing undocumented fill in the northwest portion of the property, we recommend that three settlement monuments be installed during the remedial grading operations in that area. The precise locations of the settlement monuments should be determined during grading, in consultation with the grading contractor, to help avoid conflicts with the earthmoving equipment. The plate style monuments should be constructed and monitored in general accordance with California Test Method 112. The monuments should be installed the bottoms of the remedial excavations, prior to placing new fill. The monuments should be surveyed daily while compacted fill is being placed in that area, and weekly thereafter, until the settlement is deemed substantially completed by the project geotechnical consultant. Settlement sensitive buildings and surface improvements including utilities, pavements and sidewalks should be delayed until after the fill settlement is completed. We estimate that this may require a 4-week waiting period after fill placement. Per Caltrans requirements, any existing fill and alluvium located near the proposed bridges at the southern terminus of Poinsettia Lane will need to be completely excavated and replaced as structural approach fill with at least 95 percent relative compaction. The compacted fill for this zone was described in the referenced bridge Foundation Report (GDC, 2017d). We appreciate this opportunity to be of continued professional service. Please feel free to contact the office with any questions or comments, or if you need anything else. ~---- GROUP DELTA CONSULTANTS Matthew A. Fagan, G.E. 2569 Senior Geotechnical Engineer Associate Engineering Geologist Distribution: (1) Addressee, Mr. Alan Chik (Alan.Chik@lennar.com) (1) Hetherington, Mr. Mark Hetherington (mdh@hetheringtonengineering.com) (1) O'Day Consultants, Mr. George O'Day (Georgeo@odayconsultants.com) (1) City of Carlsbad, Ms. Tecla Levy (Tecla.Levy@carlsbadca.gov) A GR.CLP CELT A N,\P,ojects\SD\SD412B ''""" · Po/asettia 61, T&D of Earthwo,k Coastrnct/oa\S. Reports\18·DD12\18·001M.dac • • • REFERENCES Geocon Incorporated (2006). Soil and Geologic Reconnaissance, The Bridges at Poinsettia, Carlsbad, California, Project No. 07381-32-02, June 22. Geotek, Inc. (2004). Geotechnical Evaluation for Proposed Residential Building, SRWB Property, South Side of Cassia Road, East of Poinsettia Lane, Carlsbad, California, Project No. 2555-SD3, April 13. Geotek, Inc. (2010). Geologic Update and Grading Plan Review, CT 14-10, Poinsettia Place, Poinsettia Lane and Cassia Road, Carlsbad, CA, Project No. 3368-SD3, December 2. Group Delta Consultants (2014). Report of Geotechnical Investigation, Poinsettia 61 Development, Carlsbad, California, Document No. 14-0188, November 20. Group Delta Consultants (2015). Slope Stability Considerations, Poinsettia 61 Development, Carlsbad, California, Document No. 15-0029, March 6. Group Delta Consultants (2016). Remedial Grading Summary and Plan Review, Poinsettia Development (EIR 15-03), Carlsbad, California, Document No. 15-0102, September 21. Group Delta Consultants (2017a). Supplemental MSE Retaining Wall Recommendations, Poinsettia 61 Development, Carlsbad, California, Document No. 17-0081, July 12. Group Delta Consultants (2017b). Geotechnical Review of MSE Wall Plans, Poinsettia 61 Development, Carlsbad, California, Document No. 17-0100, August 28. Group Delta Consultants (2017c). Supplemental Infiltration Information, Poinsettia 61 Development, Carlsbad, California, Document No. 17-0145, December 8. Group Delta Consultants (2017d). Foundation Report, Poinsettia Lane Bridges, Carlsbad, California, Document No. 17-0065R, December 22. Hetherington Engineering (2018a). Third-Party Geotechnical Review (First Review), Poinsettia 61 Development, Carlsbad, California, GR2017-0052, 4 Pages, January 2. Hetherington Engineering (2018b). Third-Porty Geotechnical Review (Second Review), Poinsettia 61 Development, Carlsbad, California, GR2017-0052, 3 Pages, March 5. O'Day Consultants (2017). Rough Grading, Storm Drain and Retaining Waif Plans and Details for Poinsettia 61, Sheets 1 through 38, August 21. Moffatt and Nichol (2017). Poinsettia Bridge Foundation Plans, 2 Sheets, December 22. Soil Retention Designs Inc. (2017a). Verdura Retaining Waif Plans for Poinsettia 61, Sheets 21 through 38, August 18 . Soil Retention Designs Inc. (2017b). Verdura Retaining Waif Design Summary and Calculations for Poinsettia 61, SRD Project No. 1704-003, August 21. A c;FICUP DEL.TA N:\Projects\SD\504128 Lennar -Poinsettia 61. T&O of Earthwork Construction\5. Reports\18-0012\18-0012v2.doc ' PLATES • • • .. ~ <c IJ .. -.. .. .. .. .. .. '"It .. g ~ i i i j j j j j '& j~ --j ! j • a. fl Jf Bf Jj Bi 1H, 1f!-.81!1 J:!-J I h BJH ~ ~ z I !t Iii u~ U[ itf nu Hd UH Ud du ih ~ 0 ~j hu a. V .; ~ .:: h ~~ irl I C <( h a . h j z !i l,11 :5 i ~ ~ Q. CL I "ti' g >< ! ~ ' i i ~ ,1 w .. d I !II <( i I • .. >, :l ~ . !1l !1l !1l b ~ .. a U: .,. • • I .... • ~-'· e· l· ,;i'i'~· . • .,. •' ~. • ' I' ,, ' 4' ~ i· l· ... "' • ' ... t , ' ' (' ' • ,.. a • • • • ,_ .. ,,, . • li ~~ I ,,----.... ....... I I I • I I I \ ,,' •, .. .. ... • .. 2 I 1'. '· ... • \ t" ... ~ I \ ii I I I '-t. I i l ~. ) • I ! I -· __) _______ I ... I \ ' • .., j ... ~ "' ~ I ~-; J • • • • • • • • • • • z 0 ~ :5 Q. 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