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Home My WebLink AboutCT 14-10; POINSETTIA 61; REMEDIAL GRADING SUMMARY AND PLAN REVIEW, POINSETTIA DEVELOPMENT EIR 15-03; 2016-09-21; ~U<CUPDELTA September 21, 2016 Lennar Homes 25 Enterprise, Suite 300 Aliso Viejo, California 92656 Attention: Mr. Andrew Han SUBJECT: Remedial Grading Summary and Plan Review Poinsettia Development (EIR 15-03) Carlsbad, California References: Group Delta Consultants {2014). Report of Geotechnicol Investigation, Poinsettia Development, Carlsbad, California, Document No. 14-0188, November 20. Mr. Han: O'Day Consultants (2016). Vesting Tentative Map for Poinsettia, 10 Sheets, Job No. 14108, revised September 19. Based on our recent conversation, it is our understanding that several changes in the Poinsettia development have occurred since the referenced subsurface investigation was completed (GDC, 2014). Specifically, two five-acre lots have been removed from the southeast portion of the site (APN 215-050-4500 and APN 215-050-4600), resulting in a revised total development area of 50.4 acres. Note that no development was proposed for these lots at the time that the referenced investigation was completed. Consequently, no change to our geotechnical recommendations is needed to reflect the deletion of these two 5-acre lots. In addition, the 17 lot neighborhood once proposed in the southwest portion of the site will no longer be developed. The two 5-acre lots in the southwest portion of the site (APN 215-050-4400 and APN 215-050-4700) will remain as undeveloped open space. Consequently, the development will now have 123 lots, rather than the initial 140 lot development. The revised site layout is shown in Figure 1. The development is shown in more detail in Figures 2A through 2E. In accordance with your request, we have reviewed the recently revised Vesting Tentative Map for the development, and have provided our response to one review comment from the City of Carlsbad. The following report also summarizes our remedial grading recommendations for the project. Each of these items is addressed separately below. • Remedial Grading Summary and Plan Review Poinsettia Development Lennar Homes PLAN REVIEW GDC Project No. SD412 September 21, 2016 Page 2 No grading notes are currently provided on the referenced Tentative Map (O'Day, 2016). However, it has been our experience that the final grading plans for projects within the City of Carlsbad will typically contain a Grading Note stating: "All grading shall be conducted in general accordance with the recommendations provided in the referenced soils report (GDC, 2014)." Provided that this note is shown on the final grading plans, it is our opinion that the preliminary grading shown on the referenced Tentative Map will adequately incorporate the intent of our referenced geotechnical recommendations. RESPONSE TO CITY REVIEW COMMENT The City of Carlsbad has provided the following review comment: "As previously requested, provide a letter from the soils engineer confirming that the proposed preliminary grading (including remedial grading) shown on the preliminary site plan complies with the grading recommendations in the soils report section 6.3.3, requiring all alluvium within the area described by 1:1 plane extended down and out from the toe of the planned fill slopes, along the southern edge of the northern development, should be excavated." The requested letter was provided above. The comment illustrates the potential difficulties that may be encountered during remedial grading along the southern edge of the site. Note that the intent of our recommendations was to remove all alluvium beneath any settlement sensitive improvements located along the southern edge of the property. We anticipate that alluvium depths of 15 feet or more may be encountered along the southern property boundary east of the bridge, as shown in Figure 2C. The Tentative Map shows a 20-foot wide grading easement along the southern property boundary beyond the toe of the fill slope. In some areas, the limits of grading extend up to 40 or SO-feet beyond the toe of the fill slope. In general, for areas with less than 10 feet of alluvium, the 20-foot wide easement should be adequate to remove all alluvium beneath the toe of the slope using 1:1 temporary excavations. Where deeper alluvium is encountered, we recommend starting the 1:1 temporary forecuts at the limits of grading, and extending the excavations northward into the development until all of the alluvium is removed. Group Delta should observe these excavations during grading, and provide remediation alternatives where needed, or evaluate the potential impact to settlement sensitive improvements. Improvements located north of the slope top should not be affected . It should be noted that we previously understood that cast-in-place (CIP) retaining walls would be used on the slopes along the southern edge of the site, as depicted in Cross Section E-E' in Appendix C of the referenced report (GDC, 2014). We understand that the proposed retaining walls shown on the revised Tentative Map will now consist of Mechanically Stabilized Earth (MSE) walls that can experience much more settlement than CIP walls without damage and cracking. Consequently, if some alluvium does remain near the toe of the subject fill slopes, the potential for damage to the proposed slope improvements will be substantially diminished. A GROUP CELT~ N:\Projects\SD\50412 Lennar Poinsettia Geotechnica/ lnvestigation\15-0102\15-0102R.doc • Remedial Grading Summary and Plan Review Poinsettia Development Lennar Homes GDC Project No. SD412 September 21, 2016 Page 3 SUMMARY OF REMEDIAL GRADING Our remedial grading recommendations were provided in Sections 6.3.1 through 6.3.10 of the referenced geotechnical investigation report (GDC, 2014). In summary, we recommended that deleterious materials be removed from the site, including existing structures, improvements, trees, vegetation, trash and any demolition debris. We also recommended that compressible undocumented fill and alluvium be completely excavated throughout the site, cleared of deleterious materials, dried where necessary, and replaced as a uniformly compacted fill. Within the building pad areas, we recommended that cut/fill transitions, and deep fill transition lots be over-excavated. We also recommended that expansive clays be removed from finish pad grade (if encountered). Finally, we recommended that a buttress be constructed along the northern edge of the site, and that various fill slope keyways be constructed. The anticipated remedial excavations are summarized in the attached Figures 2A through 2E. Figure 2A shows the estimated configuration of the recommended buttress keyway excavation for the retaining walls and cut slope along the northern property line. As described in Section 6.3.9, and shown in Appendix C of the referenced report, the buttress is intended to stabilize the fissured clay bed encountered within the formational materials at an elevation of about 264 feet. The Tentative Map indicates that the base of the upper MSE wall on the northern slope is generally located at an elevation of about 262 to 263 feet. For the planned buttress, we recommended that a 1:1 temporary cut be excavated from about elevation 285 feet on the cut slope face, down to a few feet below the base of the MSE wall (and fissured clay bed). As shown in Figure 2A, the base of the cut slope should be located at least 15 feet behind the face of the MSE wall, and should provide sufficient room for the MSE wall geogrid reinforcement. A continuous subdrain is also recommended, as shown in Figure 2A (and detailed in Figure 8 of the referenced geotechnical report). The subdrains should be graded to flow at one percent or more to solid PVC collectors. The collector pipes should outlet into the storm drain system. Figures 2B, 2C and 2D show the remedial excavations proposed in the alluvial drainage canyon along the eastern edge of the northern portion of the property. Estimated remedial excavation depths of 3, 5, 10 and 15 or more feet are shown on these figures. Note that a canyon subdrain is also recommended for this area, at the approximate location shown in Figures 2C and 2C. The canyon subdrain should outlet into the storm drain, or a natural gravity outlet. Note that in the areas of the deepest alluvium removals (shown in green). saturated soil and groundwater is anticipated. The saturated soils will need to be dried back to a moisture content suitable for placement as compacted fill. Dewatering may be needed to accomplish these remedial excavations. Note also that the remedial grading excavations will generally need to extend 30 to 40 feet beyond the toe of the proposed fill slopes, in order to remove all alluvium located within a 1:1 plane extended down and out beyond the toe of the fill slopes. Figure 2E shows the remedial grading proposed in the northwest portion of the site. In general, we have recommended that a minimum 15-foot wide keyway be constructed along the toe of the new fill slope proposed in that area. In addition, this portion of the site contains a large !j\ t'.~ GROUP DEL TL\ N:\Projects\SD\50412 Lennar Poinsettia Geotechnical lnvestigation\15-0102\15-0102R.doc • Remedial Grading Summary and Plan Review Poinsettia Development Lennar Homes GDC Project No. SD412 September 21, 2016 Page 4 area of previously placed fill. The SPT data from our borings indicates that the existing fill is generally medium dense to dense, and may be suitable for support of new fill and foundation loads. However, as a minimum, we have recommended three foot deep remedial excavations (below existing grades) throughout these existing fill areas. If pockets of loose, compressible fill or other deleterious materials are observed by Group Delta during these remedial excavations, deeper excavations may be warranted, as determined in the field by our personnel. Our remedial grading recommendations for cut/fill transitions were provided in Section 6.3.4 and Figure 5 of the referenced report (GDC, 2014). In general, cut/fill and deep fill transition lots should be over-excavated to a depth equal to half the maximum fill depth, with a minimum over-excavation depth of three feet, and a maximum depth of 10 feet. Several cut/fill transition lots are shown in Figures 2B through 2E. The potential cut/fill lots include Lots 19, 24, 28, 38, 39,40,41,50,52, 79,80,82,83,85,86,96, 97,98,99, 108,113,114,119,126,128,129,134, 135 and 136. Additional cut/fill lots may need to be over-excavated during grading, based on the actual extent and depths of the remedial excavations needed to remove compressible soils. In addition to the remedial grading areas shown in Figure 2A through 2E of this report, some lots may expose expansive clays at or near finish grade. Our recommendations for expansive soils were provided in Section 6.3.3 of the referenced report. It is difficult to determine which lots may be affected by the presence of expansive clays near finish grade at this time. However, for such lots, we have recommended a maximum 4-foot deep over-excavation (where needed), based on the conditions observed by Group Delta during grading. Note that the Expansion Potential of the finish pad soils will be determined during grading. For remedial grading quantity estimation purposes, we recommend assuming that roughly 20 cut lots may need to be over-excavated at the site due to the presence of expansive clays. An average lot area, and an average remedial excavation depth of 3 feet may be assumed for estimation. We appreciate this opportunity to be of continued professional service. Feel free to contact the office with any questions or comments, or if you need anything else. GROUP DELTA CONSULTANTS 111~4~ Matthew A. Fagan, G.E. 2569 Senior Geotechnical Engineer Distribution: ';\ t~ GRCLF CEL T.l\ N:\Projects\SD\50412 Lennar Poinsettia Geotechnical lnvestigation\15-0102\15-0102R.doc • FIGURES N A NO SCALE GROUP DELTA CONSULTANTS, INC. ENGINEERS ANO GEOLOGISTS 9245 ACTIVITY ROAD, SUITE 103 SAN DIEGO. CA 92126 (658) 536-1000 PROJECT NAME Poinsettia 61 Development Lennar Homes PRC>..ECT NUMBER SD412 DOCUMENT NUMBER 15-0102 FfGURE NUMBER 1 SITE VICINITY PLAN \ DESCRIPTION: I l ······> ······:> lil,,. ,,,.... ~d>. \ .,-, ~ \ \ I SEClldN A-A I T 2 ' ~6 I -·-L -r ~-, --<==--<=-~,,i'c:---7'.,.----~. ---_.__-1.(}61_L;:i;-~-\\' ;r -::V: ---Jo{~ -s:. ..=r-~ -::;e.. r-, -= _=>-,... ---~ ( J0~.7 .. ...._ -... J I U.L Approximate location of buttress keyway (keyway elevation below clay and finish surface grades for MSE wall). Top of 1: 1 cut at -285' MSL. Approximate location of perforated buttress subdrain and solid outlet pipes. Grade keyway and subdrains to outlets at 1 % gradient or more. Reference: O'Day Consultants (2014). Vesting Tentative Map for Poinsettia 61 , Sheets 1 to 10, September. N A ,r GROUP DELTA CONSULTANTS, INC. ENGINEERS ANO GEOLOGISTS 9245 ACTIVITY ROAD, SUITE 103 SAN DIEGO. CA 92126 (858) 536-1000 PROJECT NAME Poinsettia Development Lennar Homes PRD.ECT NUMBER SD412 DOCUMENT NIJMBER 15-0102 FtGURE MJMBE.R 2A NO SCALE I REMEDIAL GRADING PLAN I DESCRIPTION: N 1 3· 1 L 5· 1 10· 1t 15·+ Approximate remedial excavation depth (3', 5', 1 O' or 15' + ). A .•••. ·>-••. ···> ..... -~ Approximate location of perforated canyon subdrain . Reference: O'Day Consultants (2014). Vesting Tentative Map for Poinsettia 61 , Sheets 1 to 10, September. NO SCALE GROUP DELTA CONSULTANTS, INC. ENGINEERS AND GEOLOGISTS 9245ACTIVITY ROAD. SUITE 103 SAN DIEGO. CA 92126 (858) 536-1000 PROJECT NAME Poinsettia Development Lennar Homes PRO.ECT NUMBER S0412 DOCUMENT NUMBER 15-0102 FIGURE NUMBER 28 REMEDIAL GRADING PLAN DESCRIPTION: N Approximate remedial excavation depth (3', 5', 1 O' or 15' + ). A I 3· 1 L 5· I 10· 1[ 15·+ ••••• -~ ••••• -~ ••••• ·"> Approximate location of perforated canyon subdrain. Reference: O'Day Consultants (2014 ). Vesting Tentative Map for Poinsettia 61 , Sheets 1 to 10, September. NO SCALE -..... ' ,··t· ' I '\ ' i ' ~ GRClUP CELT .L\ GROUP DELTA CONSULTANTS, INC. ENGINEERS AND GEOLOGISTS 9245 ACTIVITY ROAD, SUITE 103 SAN DIEGO, CA92126 (858) 536-1000 PROJECT NAME Poinsettia Development Lennar Homes PRo.E.CT NUMBER SD41 2 DOCUMENT NUMBER 15-0102 FK;URE NUMBER 2C REMEDIAL GRADING PLAN ,fP l ,, \ \ ;---~-) i \. I / DES_CRIPTION: \ \ \ \ l \ \ I 5' Approximate remedial excavation depth (3', 5', 10' or 15'+). 1 1 o· =i __ __. I 3· 1 __ _ 15'+ \ \ Reference: O'Day Consultants (2014 ). Vesting Tentative Map for Poinsettia 61 , Sheets 1 to 10, September. \ '· !Ol!5£ llll.l!H OH '* "· .._ - N A NO SCALE GROUP DELTA CONSULTANTS, INC. ENGINEERS AND GEOLOGISTS 9245ACTIVITY ROAD, SUITE 103 SAN DIEGO. CA92126 (858) 536-1000 PROJECT NAME Poinsettia Development Lennar Homes PRD.ECT NUMBER $0412 DOCUMENT NUMBER 15-0102 FtGURE NUMBER 20 REMEDIAL GRADING PLAN • -... --- ·,-:: --___ NO'.JJ~S.g;:----C -,, • • • • • • • • DESCRIPTION: 5' 10' 15'+ Approximate remedial excavation depth (3', 5', 1 O' or 15' + ). • Reference: O'Day Consultants (2014). Vesting Tentative Map for Poinsettia 61 , Sheets 1 to 10, September. ---s~ >z NO SCALE ~ GFfDLF 0EL TA GROUP DELTA CONSULTANTS, INC. ENGINEERS AND GEOLOGISTS 92<15ACTMTY ROAD, SUITE 103 SAN DIEGO. CA92126(858) 536-1000 ~o.ECTNAME Poinsettia Development Lennar Homes PROJECT NUMBER SD412 DOCUMENT NlN8ER 15-0102 FIGURE NUMBER 2E REMEDIAL GRADING PLAN