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Home My WebLink AboutCT 16-02; OCEAN CONDOMINIUMS; ADDENDUM TO GEOTECHNICAL DESIGN REPORT OCEAN CONDOMINIUMS; 2021-11-21TAYLOR GROUP, INC. November 21, 2021 TGI Project No. 19.00911 Carlsbad 17 GP, LLC Attn: Mr. Richard Woolsey 234 Venture Street, Suite 100 San Marcos, California 92078 Subject: Addendum to Geotechnical Design Report Recommendations for Slot-Cut Excavations along Northerly Property Line Proposed Ocean Condominium Project 2501 State Street, Carlsbad, California CT 16-02 / GR2019-0028 / DWG 519-5A Reference: Taylor Group, Inc., “Geotechnical Design Report, Proposed Ocean Condominium Project, 2501 State Street, Carlsbad, California, CT 16-02 / GR2019-0028 / DWG 519-5A” , TGI Project No. 19.00911, dated April 24, 2020 Dear Mr. Woolsey: Taylor Group, Inc. (TGI) has prepared this letter as an addendum to the above-referenced geotechnical report for the subject project to provide geotechnical recommendations related to slot- cut excavations that are proposed along the northerly boundary of the project site. BACKGROUND Remedial grading for the project will entail removal of existing undocumented fill and weathered native deposits to a depth of approximately 10 feet below the existing grade and placement of approved site soils and imported soil an engineered fill. Temporary shoring will be constructed along the southerly and easterly site boundaries to facilitate the removals. Temporary cuts inclined at 1.5:1 (horizontal: vertical) were initially proposed along the westerly and northerly boundaries of the site. The temporary cuts would extend onto adjacent properties to allow the remedial grading to extend at least 3 feet beyond the building footprint. Permission to grade onto the property to the west has been granted by the owner NCTD and will be performed as previously proposed. Permission to grade has not been granted by the owner of the property to the north. Temporary shoring along the northerly site boundary has been considered as an alternative to the temporary cut. However, this alternative is complicated by the condition that the shoring would encroach into an existing public sewer easement. It is likely that the City of Carlsbad would require that the shoring would need to be constructed outside of the easement. This requirement would significantly complicate the building construction or require that the width of the structure be reduced by about 18 inches. 301 Mission Avenue Suite 201 Oceanside California 92054 tel: 760.721.9990 fax: 760.721.9991 www.visitTGI.com Recommendations for Slot-Cut Excavations Page | 2 Ocean Condominiums – 2501 State Street, Carlsbad, CA TGI Project No. 19.00911 November 21, 2021 TAYLOR GROUP, INC. Slot cutting has been proposed as an alternative to avoid grading on the adjacent property or construction of temporary shoring within the public easement. Slot-cut excavation methods are commonly used to maintain stable slopes during excavation adjacent to structures or, as is proposed in this case, at the toe of steep slopes. The method generally consists of having alternating open cut areas (slots) and uncut area (buttresses). The slot cuts are excavated and backfilled while the uncut buttress areas stabilize the soil mass behind the cut area. Once engineered fill is compacted in the slots, the areas are reversed with the former slots becoming the buttresses and the previously uncut areas being slot-cut. Slot-cut excavations are generally more stable than continuous cuts due to the contribution of out-of-plane resistance provided by the buttresses adjacent to the slot cuts and the three-dimensional (3D) effect this has on slope stability for the cut. ANALYSIS AND CONCLUSIONS TGI has performed stability calculations for the proposed temporary slot cuts using the soil model developed for the site and the same strength parameters that have been used for shoring design as summarized in Table 1 below. Table 1. Soil Properties Used for Slot Cut Stability Analysis Formational Material In-Situ Unit Weight (lbs/ft3) Dry Unit Weight (lbs/ft3) Phi Angle (Degrees) Cohesion (lbs/ft2) Existing Fill 124 105 27 1,075 Weathered Terrace Deposits 120 107 38 35 Terrace Deposits 130 115 37 105 Based on data from borings in the area of the northerly property boundary, the model assumes that clayey fill soil is present in the uppermost 5 to 9 feet of the subgrade (above about El. 27 to El. 30 feet). The fill is underlain by weathered terrace deposits consisting of medium dense silty sand. The temporary slot cuts were modeled as follows: • Ground surface at the top of the cut at El. 32 feet prior to the slot cut excavation (at the elevation of the top of curb to the north of the property line) • A 4-foot vertical cut in clayey fill soils at the property line • A cut slope inclined at ¾:1 from a depth of 4 feet to the limit of the removal. A 200 psf surcharge was conservatively assumed in the driveway area to the north of the cut slope. Two-dimensional slope stability was calculated using the several limit-equilibrium methods including Bishop, Janbu and ordinary method of slices. Our analysis indicates that the static factor of safety for the temporary cuts is 2.4 or greater. Results of the analysis are summarized in Attachment 1 to the Recommendations for Slot-Cut Excavations Page | 3 Ocean Condominiums – 2501 State Street, Carlsbad, CA TGI Project No. 19.00911 November 21, 2021 TAYLOR GROUP, INC. letter. Based on these results, we determined that it was not necessary to perform a more rigorous 3-dimensional analysis to assess the additional stabilizing effect of shear resistance at the margins of the narrow cuts. We estimate that such an analysis would yield minimum factors of safety of 3.0 or greater. Based on our analysis and experience, it is our professional opinion that slot cutting provides a feasible option to perform remedial grading along the northerly boundary if the earthwork is performed following the recommendations presented in this letter. RECOMMENDATIONS TGI’s recommendations related to the implementation of the slot cutting alternative for the subject project are provided in the following sections. Initial Temporary Cut Slope Prior to any slot cutting, a temporary cut shall be made along the entire northerly property boundary (panel “A” on Figure 1) The top of the cut shall be at the property line at approximately the elevation of the top of curb (El. 32.0 to 32.5 feet) along the south side of the private drive on the adjacent property. The cut shall be inclined no steeper than 1.5:1. This will place the toe of the temporary cut approximately 10 to 13 feet south of the northerly property boundary. The area to the south of the toe of the temporary cut slope should be at the same elevation as the toe (i.e., the base of removal for remedial grading) for about 20’ to facilitate access to slot-cut areas and handling of materials. Slot Cuts Slot cutting shall be performed by what is commonly referred to as the “A-B-C” slot cut method per the sequence described and illustrated on Figure 1. Slot cuts shall be no wider than 8 feet and shall be spaced such that there is a minimum no farther than 16 feet apart as illustrated on Figure 2. Slot cutting shall commence with excavation of the “A” slots. The geometry of the cuts (panel “B” on Figure 1) will be dictated by conditions encountered in the field, but in general the cuts may be vertical for maximum depth of 5 feet where surface soils consisting of clayey fill soils are present. Below the depth of the base of the clay soil layer(s), the slot cuts are expected to encounter weathered native soils consisting predominantly of medium dense to dense silty sands with limited cohesion. Temporary slot cuts in the granular native deposits shall be inclined at not steeper than ¾:1. Recommendations for Slot-Cut Excavations Page | 4 Ocean Condominiums – 2501 State Street, Carlsbad, CA TGI Project No. 19.00911 November 21, 2021 TAYLOR GROUP, INC. Slots shall be backfilled (Panel C on Figure 1) with approved site soils or imported fill following the recommendations provided in Section 6.3 of TGI’s geotechnical report. Backfilling shall commence as soon as practical following completion of slot-cut excavations and shall be completed during the same shift that the slot-cut excavation is made. After the “A” slots are completed, the slot cutting can proceed to the “B” and then “C’ cuts. The sequence may be modified so long as there is always a 16-foot earthen buttress between adjacent active slot cuts. Panels “F’ and “G” on Figure 1 illustrate construction sequences that fill follow completion of the slot cutting operations. Geotechnical Observation and Testing All work associated with slot cuts shall be observed on a continuous basis by the Geotechnical Engineer. This shall include: observation of operations in general; observation of the temporary slot cuts to assess their overall stability; observation of fill moisture conditioning, placement and compaction operations, and; density testing of compacted fill. If the recommendations presented herein do not address any element of the planned slot-cut construction or if you require additional information, please contact us for any clarification the additions that you might require. Sincerely, TAYLOR GROUP, INC. Larry R. Taylor Principal Engineer G.E. 2602 2530354020PLExist ground surface8' sewer esm't10' water esm't1.51Exist. 10" VCP sewer mainINVERT EL. 22.6 TO 25.0Exist. 8" ACP water main2530354020PLApprox. limit of removal forremedial grading8' sewer esm't10' water esm'tExist. 10" VCP sewer mainINVERT EL. 22.6 TO 25.0Exist. 8" ACP water main2530354020PL4.0'FF 32.66Approx. limit of removal forremedial grading118' sewer esm't10' water esm't21FG 34.00Exist. 10" VCP sewer main to bereplaced w/ 12" PVC sewer main perDWG 519-5INVERT EL. 22.6 TO 25.0EL. 27.00Exist. 8" ACP water main2530354020PLSlab subgrade (El. 32.2)8' sewer esm't10' water esm'tExist. 10" VCP sewer mainINVERT EL. 22.6 TO 25.0Exist. 8" ACP water main2530354020PL8' sewer esm't10' water esm'tExist. 10" VCP sewer mainINVERT EL. 22.6 TO 25.0Exist. 8" ACP water mainApprox. limit of removal forremedial gradingApprox. limit of removal forremedial gradingA. Construct initial temporary cut slope at property line.B. Excavate "A" Slot Cuts to approved depth of removalC. Place and Compact Fill in "A" SlotsG. Place fill to finished surface after sewer main replacementE. Place and compact fill to slab subgrade elevationMax. 8' wide slot cutsspaced @ 8'3/415' max. vertical in clay soils2333 State Street2333 State Street2333 State Street2333 State Street2333 State StreetPlace fill after construction of newsewer mainTrench excavation for sewer mainreplacementGround surface adjacent to slotGround surface adjacent to slotD. Repeat B through C for "B" Slots and then for "C" Slots2530354020PL4.0'FF 32.66Approx. limit of removal forremedial grading118' sewer esm't10' water esm'tEL. 27.00Exist. 8" ACP water mainF. Construct deepened footing, slab and stemwall.2333 State StreetPlace fill after construction of newsewer mainExist. 10" VCP sewer mainINVERT EL. 22.6 TO 25.0FIGURE 1. SCHEMATIC ILLUSTRATION OF A-B-C SLOT CUTTING Project Name:Ocean Condominiums - 2501 State Street Project Number:14.00711 Client Name:Neil & Nicole Billock Drawn by:LRT Date:11.19.2021 tgi 301 Mission Avenue Suite 201 Oceanside California 92054 tel. 760.721.9990 fax. 760.721.9991 www.visitTGI.com Geotechnical Engineering Civil Engineering Checked by:LRT Date:11.19.2021 FMFMFMFMFMFMFMFMFMFMFMFMFMFMFMFMFMFMFMFMFMSSEEEEEEWWWWWWWWWWWWWWWWWWWTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTWWGGGGGGSSSSSSSSSSSSSSSSSSSSSSSSSS S SS S S GGGGGGGGGGWWWWWWWWWWWWWWWSSSSSSSSSSSB4B4D4C4C4A4A4G6G6D4E6E6STATE STREET212ENGINEER'S EARTHWORK NOTESOCEAN CONDOMINIUMSDEMOLITION, SHORING & INTERIM GRADING PLANtgi301 Mission Avenue Suite 201 Oceanside CA 92054tel. 760.721.9990 fax. 760.721.9991 www.visitTGI.comGeotechnical Engineering Civil EngineeringREGISTERE D P ROFESSIONAL ENGINEERCIVILLAR RY RUSSEL TAYLORSTATE OF CALI FORN IA GENERAL DEMOLITION NOTESCONSTRUCTION WASTE MANAGEMENT PLANEASEMENT NOTESNo. 58274Exp. 06.30.2022SLOT-CUTTING EARTHWORK NOTESFIGURE 2 TAYLOR GROUP, INC. ATTACHMENT 1 OCEAN CONDOMINIUMS STABILITY OF SLOT-CUT EXCAVATIONS SLOPE-W ANALYSIS (2D ONLY)