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Home My WebLink AboutCT 14-10; POINSETTIA 61; SUPPLEMENTAL MSE RETAINING WALL RECOMMENDATIONS; 2017-07-12• .. A u ilCUP DEL TL\ July 12, 2017 Lennar Homes 25 Enterprise, Suite 300 Aliso Viejo, California 92656 Attention: Mr. Andrew Han SUBJECT: SUPPLEMENTAL MSE RETAINING WALL RECOMMENDATIONS Poinsettia 61 Development Carlsbad, California References: Group Delta Consultants (2014). Report of Geotechnical Investigation, Poinsettia Development, Carlsbad, California, Document No. 14-0188, November 20. Mr. Han: O'Day Consultants (2017). Poinsettia 61, Rough Grading Plan, 20 Sheets, Project No. CT14-108, dated June 28. In response to our recent project team meeting, we are providing herein supplemental geotechnical recommendations for use in the design of the Mechanically Stabilized Earth (MSE) retaining walls recently proposed at the Poinsettia 61 residential development in Carlsbad. Geotechnical parameters for the design of standard cantilever retaining walls were provided in Section 6.6 of the referenced geotechnical report {GDC, 2014). Those recommendations remain applicable to design of the cast-in-place and masonry retaining walls proposed at the site. The following design parameters are provided for the Verdura style MSE walls that will be designed by Soil Retention Systems, as shown on the referenced rough grading plans {O'Day, 2017). The proposed MSE retaining walls will retain a variety of different soil conditions. In general, all of the MSE walls may be assumed to retain compacted fill soils that will primarily consist of silty and clayey sand (SM and SC). This assumption will be slightly conservative in the cut areas. The terraced MSE walls proposed along the northern property boundary, as well as the MSE wall proposed in the cut area near the southwest corner of the site, will likely retain dense Santiago Formation, which mostly consists of silty or clayey sandstone (SM or SC). For design of these MSE walls, a higher shear strength than that of the fill may be applicable to the retained zone. However, several one to four-foot thick fat claystone {CH) beds were encountered within the Santiago Formation at the site. A buttress was previously recommended behind the northern MSE wall due to the presence of the fissured claystone. Once the preliminary MSE wall designs are completed by Soil Retention Systems assuming fill conditions, the global stability of the various cut slopes and MSE walls should be evaluated by Group Delta Consultants. • .. • • Supplemental MSE Retaining Wall Recommendations Poinsettia 61 Development Lennar Homes GDC Project No. SD412 July 12, 2017 Page 2 MSE retaining walls should be designed using soil parameters that reflect the strength in the reinforced and retained zones. For design, we recommend that the retained and foundation area materials for all of the proposed MSE walls be assumed to consist of fill soil with a shear strength of 32° and 100 lb/ft3 cohesion, and a moist unit weight of 125 lb/ft3• An allowable bearing pressure of 2,000 lbs/ft' may be assumed. For lateral resistance, a coefficient of friction of 0.35, and an allowable passive pressure of 300 lbs/ft' is recommended. The bottom of the MSE wall should be deepened where needed to provide a minimum slope setback of 8 feet, measured horizontally from the bottom outside edge of the MSE wall units to the slope face. All MSE walls should contain an adequate drainage system to relieve the buildup of hydrostatic pressure. Typical MSE wall drain details are provided in the attached Figure 1. The soil placed within the reinforced zone of the MSE walls should consist of granular soil with an Expansion Index less than 20 based on ASTM D4829. For design purposes, we recommend that the material in the retained zone be assumed to consist of granular fill soil with a minimum friction angle of 32° with zero cohesion, and a moist unit weight of 120 lb/ft3• Retaining wall backfill should be compacted to at least 90 percent relative compaction per ASTM D1557, and should be sampled and tested during construction for remolded shear strength per ASTM D3080 to confirm that the minimum required 32° friction angle is attained. Much of the on-site soil, including all of the poorly graded and silty sand (SP and SM), as well as some of the clayey sand (SC) generated from the proposed excavations, may be suitable for use within the reinforced zone of the MSE walls, provided that these soils have an Expansion Index less than 20 based on ASTM D4829. The Expansion Index testing conducted previously for the investigation was summarized in Figure B-2 of the referenced report (GDC, 2014). Note that all of the on-site clay (CL and CH), as well as some of the clayey sand (SC), may have an Expansion Index above 20, and would be considered unsuitable for use as MSE wall backfill. We understand that seismic design is required for all earth retaining structures over 6 feet in height. The Design level peak ground acceleration (PGA) for the site may be taken as 40 percent of the short period spectral ordinate (Sos), as shown in Tables 1 and 2 of the referenced report (GDC, 2014). One-half of the Design level PGA is typically used for pseudo-static seismic wall design, which would equate to 0.14g or 0.15g in this case, depending upon the wall location and underlying Site Class. For seismic design of retaining walls throughout the site, we have provided design parameters based on the higher PGA of 0.15g (corresponding to a Site Class D). We recommend that seismic retaining wall design be conducted using the Mononabe-Okabe solution which incorporates a pseudo-static horizontal load. The Mononabe-Okabe solution is based on active earth pressures, and requires that the retaining walls are free to yield at least Y, percent of the wall height. We recommend that the equivalent seismic pressure increment (ye) of 13 PCF be added to the active earth pressure for seismic wall design. Note that the seismic pressure increment may be idealized as an inverted triangular pressure distribution with the resultant acting at a height of 0.6H above the base of the wall, as shown in Figure 2. A ~CLP CELT .t\ Nc\Projects\SD\SD412 Lenoar-Poinsettia Geotechnical lnvestigation\17-0081 MSE Walls\17-0081.doc • • .. • Supplemental MSE Retaining Wall Recommendations GD( Project No. S0412 Poinsettia 61 Development July 12, 2017 Lennar Homes Page 3 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 1?1~4~ Matthew A. Fagan, G.E. 2569 Senior Geotechnical Engineer Distribution: (1) Addressee, Mr. Andrew Han (andrew.han@lennar.com) (1) Soil Retention Systems, Mr. Nick Jansson (njansson@soilretention.com) (1) O'Day Consultants, Mr. Tim Carroll (timc@odayconsultants.com) )~ ~ ~CL.JFI OEL. TA N:\Projects\SD\SD412 Lennar -Poinsettia Geotechnical lnvestigation\17-0081 MSE Walls\17-0081.doc FIGURES • • • • NOTES • L__ Approximate Excavation line Interceptor Drain Consisting of Geocomposite Panel or Crushed Rock Blanket (SEE DETAIL)\ T Height of Drain Equal to 2/3 Height of Wan . L ~ / G~avel Collector ~ W1thPipe ROCK BLANKET ALTERNATIVE Filter Fabric Surrounding ~ Crushed Rock Minus 314" Crushed Rock Approximate Excavation Line ~ At Least 2" of Crushed Rock Surrounding Pipe 4" Diameter Perforated Pipe, ABS or PVC, Draining to Gravity Outfall GEOCOMPOSITE PANEL ALTERNATIVE Geocomposite Panel Drain ~ Approximate Excavation \ Line ~ At Least 2" of Crushed Rock Surrounding Pipe 1) Perforated pipe should outlet through a solid pipe to a free gravity outfall. Perforated pipe and outlet pipe should have a fall of at least 1 o/o. 4" Diameter Perforated Pipe, ABS or PVC, Draining to Gravity Outfall 2) Geocomposite panel drain should consist of Miradrain 6000, J-DRain 400, Supac DS-15, or approved similar product. d-4 GRCII..P CEL i~ MOU~DEi.TACONSUCT.t.NTS.i~ -e.o.,:c,,.,.,,,- ENGINE~SAJ<O GEc:tOC1STS SD412 J,:~;g"'J:'.~~i'~~~&.i '"''""""'-" -,oo.ec,;..;a, · 17-0081 3) Filter fabric should consist of Mirafi 140N, Supac 5NP, Amoco 4599, or similar approved fabric. Filter fabric should be overlapped at least 6-inches. Poinsettia 61 Development ''""'''"""""'"- Lennar Homes 1 _ MSE WALL DRAINS • • • • INPUT PARAMETERS Unit Weight of Soil [PCF] Backfill Soil Friction Angle(~)[°]: Wall Friction Angle (6) [°): Soil Backfill Angle (a) [0 ]: Wall Batter Angle (13) [0 ]: Horizontal Acceleration (Ki,) [g's]: Vertical Acceleration (K.,) (g's]: Active Pressure Resultant: F8 = 1/2 Ya H2 Earthquake Pressure Resultant: F. = 1/2 y. H2 H Granular Backfill y ~ c=O 120 32 21 0 90 0.15 0.00 • • • CALCULATED PARAMETERS Active Pressure Coefficient (K.): Equivalent Fluid Pressure (y8): Seismic Pressure Coefficient (K..): Equivalent Fluid Pressure (y88): Equivalent Seismic Pressure (y.): • ~ DD ~ ~ I 13 I • • Horizontal Component of Active Pressure Resultant F ah = Fa cos(6+90-f3) Horizontal Component of Seismic Pressure Resultant Feh = F. cos(6+90-f3) --------a_\ __ YeH + Ye ' • Ye-13 PCF 14' ( Fah H/3 l YaH ~ GRCJLF CEl.. TA Gllt()Uj,Dft.TACONSlA.TAJffl,INC, ~.,.---Jrll..lM60" ~~"=~l: _ 50412 5ANDIE00 CAWUl'6lla.Jl.~100D "°""...,...,_.---~· 17--0061 Poulsenio 61 o..v..__, -- Lennor Homes 2 SEISMIC WALL LOADS •