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HomeMy WebLinkAboutCT 00-06; BRESSI RANCH MASTER; GEOTECH PLAN REVIEW FOR BRESSI RANCH; 2000-03-02^^•™=^ Leighton and Associates A GTG Company GEOTECHNiCAL CONSULTANTS GEOTECHNICAL PLAN REVIEW, OF THE LAND-USE PLAN#6R BRESSI RANCH, CARLSBAD, CALIFORNIA March 2, 2000 ProjectNo. 4971009-005 Prepared For: LENNAR HOMES OF CALIFORNIA 5780 Fleet Street, Suite 320 Carlsbad, Califomia 92008-4700 3934 Murphy Canyon Road, #B205, San Dlego, CA 92123-4425 (858) 292-8030 • FAX (858) 292-0771 • www.leightongeo.com gS^SiSj^ Leighton and Associates A GTG Company GEOTECHNICAL CONSULTANTS March 2,2000 ProjectNo. 4971009-005 To: Lennar Homes of Califomia 5780 Fleet Street, Suite 320 Carlsbad, Califomia 92008-4700 Attention: Mr. Mark Rohrlick Subject: Geotechnical Review of Land-Use Plans for Bressi Ranch, Carlsbad, Califomia Introduction In accordance with your request and authorization, we have performed a geotechnical review of the current land-use plans for the Bressi Ranch project ( PDC, 2000) located in Carlsbad, Califomia. The purpose of our geotechnical plan review was to evaluate the site geotechnical constraints as they relate to the current development plan and to provide additional information to be utilized in the planning process. As part of our plan review we have provided additional geotechnical conclusions and recommendations relative to the proposed site development as indicated on the land-use plans. This report presents a summary of our plan review and additional conclusions and recommendations. The 200-scale land-use plan ( PDC, 2000) was utilized as a base map for our Remedial Grading Map (Plate 1) to present the geotechnical conditions of the site. Land-Use Plan Review. Findinss and Conclusions Based on our review of the preliminary geotechnical investigation report (Leighton, 1997) and the land-use plan for Bressi Ranch ( PDC, 2000), the proposed development appears feasible from a geotechnical standpoint. The significant geotechnical conditions relative to the development of the site as indicated on the current land-use plans are summarized below. In addition, we have identified areas not previously investigated or areas where additional information would be beneficial. The approximate locations of the landslides that will be affected by the proposed land use for the site are shown on the Remedial Grading Map (Plate 1). Adjacent to each landslide we have provided anticipated general remedial grading recommendations. However, once the final grading plans are available these general recommendations should be re-evaluated. The re-evaluation may include stability analysis and/or further investigation. In the meantime, the approximate locations of these landslides and the anticipated remedial measures necessary for each are shown on the Remedial Grading Map (Plate 1). In addition, we have also included the location of anticipated additional borings necessary to further evaluate specific landslides. 3934 Murphy Canyon Road, #B205, San Dlego, CA 92123-4425 (858) 292-8030 • FAX (858) 292-0771 • www.leightongeo.com 4971009-005 • Due to the presence of expansive clayey and silty soil in the Delmar Formation that may be exposed in cut slopes, we recommend that stability/replacement fills be constructed on slopes excavated into the Delmar Formation. The stability fill should have a minimum width at the top of the slopes of 15 feet and be constructed with a 1-1/2:1 back cut. Keys should be constructed a minimum of 15 feet wide but need not be greater than V-i the slope height and at least 5 feet below the toe-of-slope grade, and have a minimum 2 percent into-the-slope inclination. Cut slopes anticipated to require stability/replacementfill slopes have been identified in the main north-south onsite drainage and are shown on the Remedial Grading Map (Plate 1). A typical detail for stability fill constmction is provided in Appendix D of our preliminary investigation report (Leighton, 1997). • Very rough estimates of remedial earthwork quantities have been made based on our review of the referenced project geotechnical report and land-use plan, in-house geotechnical data, and our prior professional experience in the Carlsbad area. It should be noted that the quantities presented below are based on preliminary geotechnical data and a limited evaluation that may be subject to change based on the actual site conditions. Additional subsurface investigation is recommended to better evaluate and further quantify site remedial grading. Preliminary Remedial Grading Estimates The following presents our preliminary estimates of the site remedial grading. Please note that these estimates are based on a series of widely spaced borings and trenches and our experience on similar projects in the site area. Additional evaluation is recommended across the site to better define the necessary remedial grading. Topsoil, Colluvium. and Weathered Bedrock Removals Located in the fill area across the site (assumed average thickness estimated at 4 feet) Estimated Removals 916,500 cy Ouatemary Alluvium Removals Located in the drainages on the site (assumed removal to ground water [15 to 20'] in main north-south canyon and to bedrock in all other drainages). We note that a settlement-monitoring period will be required in the areas where alluvium is left in place. Estimated Removals 893.900 cy Landslide Removals and Stability Fills For landslides located along the margins of the onsite drainages within the limits of the planned grading we have assumed only removal of compressible soils in most cases, on the order of 15 to 25'. In several cases buttress designs will be finalized after additional investigation is performed, however the remedial quantity is not anticipated to change significantly. For cut slopes within the Delmar Formation we have assumed a stability fill as discussed above. Estimated Removals 765,700 cy -2- 4971009-005 Total Estimated Remedial Removals 2,576.100 cv The volume change of excavated on-site materials upon recompaction as fill is expected to vary with materials and location. Typically, the surficial soils and bedrock materials vary significantly in natural and compacted density, and therefore, accurate earthwork shrinkage/bulking estimate cannot be determined. However, the following factors (based on the results of our subsurface investigation, laboratory testing, geotechnical analysis and professional experience on similar sites) are provided on Table 1 as guideline estimates. If possible, we suggest an area where site grades can be adjusted be provided as a balance area. These values also may be refined after additional testing is performed. Table 1 Earthwork Shrinkage and Bulking Estimates Geologic Unit Estimated Shrinkage/Bulking Topsoil/Alluvium/Colluvium 5 to 15 percent shrinkage Landslide Deposits 0 to 10 percent shrinkage Del Mar and Torrey Sandstone Formations 2 to 5 percent bulking Cut/fill transition lots are anticipated within the limits of proposed grading for the Bressi Ranch development. In proposed building areas, the cut portion of these lots should be overexcavated and replaced with compacted fill in accordance with the recommendations presented in the preliminary geotechnical investigation report (Leighton, 1997). The specific lots anticipated to expose cut'fill transition conditions will be identified in our review of the final site grading plans once they are available. The area of undocumented fill in the northeastem portion of the site labeled, "Trash Dump" on the Remedial Grading Map (Plate 1), contains agricultural debris including pesticide containers. Our review of the land-use plans indicates a fill slope is planned in this area. All the undocumented fill is considered potentially compressible and unsuitable for the support of additional fill. Additional borings are recommended in this area to define the anticipated removal limits. The Delmar Formation Claystone is expected to be exposed at finish grade in the main north-south and east-west canyons on the site (along the proposed El Fuerte alignment and the Poinsettia Lane alignment, respectively). A majority (if not all) of this unit is anticipated to be moderately to very highly expansive. As a result, the presence of moderately to very highly expansive materials within 5 vertical feet from finish grade will require special foundation and slab considerations. As an altemative to the special foundation recommendations, you may elect to overexcavate building pads underlain by expansive soils a minimum of 5 feet below finish grade and replace them with properly compacted fill possessing a lower expansion potential (i.e., the sandy soil of the Torrey Sandstone present in the higher elevations of the site). Final foundation design recommendations will be presented in the as-graded geotechnical report upon completion of the rough and fine grading. Specific lots that are anticipated to consist of the Delmar Formational material at finish grade will be identified in our review of the final 4971009-005 site grading plans once they are available. Deep fills are planned in most of the onsite canyons and tributaries. We recommend that the deep fill areas be monitored (by the placement of settlement monuments upon the completion of rough grading and periodic surveying [on a weekly to twice monthly basis]) until the primary settlement is essentially complete. Settlement monitoring periods are estimated at 6-12 months for areas where complete removals are made and 9-18 months in areas where alluvial soils are left in place. Approximately 20 settlement monuments are anticipated, however, a more accurate quantity of monuments and more specific locations will be given after our review of the final site grading plans. Construction of settlement-sensitivestructures in the deep fill areas should be postponed until the anticipated settlement is within tolerable limits based on the analysis of the geotechnical consultant. In areas where construction needs to proceed in a short time frame the settlement period may be reduced by placing fill at 95 percent relative compaction and/or deep overexcavations of transition areas. Canyon subdrains are recommended in the main canyons and the adjacent tributary drainages. The approximate locations of the anticipated subdrains are presented on the Remedial Grading Map (Plate 1). The actual locations of the canyon subdrains should be evaluated in the field during removal of the compressible alluvium, colluvium, topsoil, weathered formational material, etc. Buttress subdrains should be provided in the buttresses and stability fills constmcted on the site. A subdrain should be installed along the heel of the key (or base of the backcut) across the entire length of the buttress or stability fill key. Additional subdrains should be placed along the backcut every 30 vertical feel. The subdrains should have a minimum 1 percent fall toward a suitable outlet. The basal subdrain (along the back of the key) will likely only have one or two outlets (due to the depth of the key below the existing ground surface). The backcut subdrains should be outletted every 200 linear feet. All outlets should be constmcted with solid PVC pipe. The approximate locations of the buttress or stability fill keys anticipated are provided on the Remedial Grading Map (Plate 1). Prior to the backfllling of the subdrains, the subdrain locations should be surveyed by the project civil engineer (especially the outlet locations). Upon completion of the buttresses and fine grading of the slopes, all of the subdrain outlets (which were previously surveyed) should be located and the outlet pipe extended past the fill slope face. The final lot developer, homeowner's association, maintenance district, etc. should be informed of the subdrain outlets and told of the importance of maintaining the integrity of the subdrains to reduce potential slope instability or seepage problems. Cut slopes exposing the contact between the permeable Torrey Sandstone Formation and the underlying dense and relatively impermeable claystone of the Delmar Formation are likely to have groundwater seepage conditions (once the site is developed and landscape irrigation is started) along the contact on the slope face. As a result, we recommend that the slopes exposing the contact between the Torrey and Delmar Formations and/or contacts between sandstone and claystone beds within the Delmar Formation he constructed with a toe-of-slope subdrain. Fill slopes to be constructed in deep alluvial areas where complete removals are not practical may require extra stabilization measures. These may include such things as geofabric layers, stmctural mats (rock aggregate sandwiched between layers of geofabric) or rock columns constructed along the toe-of- slope in areas where a structural fill key can not be constructed due to high ground water levels. Locations where these structural measures are anticipated to be necessary include proposed fill slopes in the main north-south and east-west canyons are shown on the Remedial Grading Map (Plate 1). The need for additional stabilization measures will be determined after review of the site tentative map. -4- 4971009-005 • As part of our review for the Bressi Ranch development we have also reviewed the grading plans for the adjacent site (O'day, 1998) along the southwest property line and have identified areas where grading of Bressi Ranch and/or grading of the adjacent site to the southwest may adversely affect one or both sites. The locations and brief descriptions of these concems are shown on the Remedial Grading Map (Plate 1). Cooperation between developers and sharing of information during site planning and grading for the two sites is suggested in order to mitigate these concems. We have contacted the geotechnical consultant for the adjacent property ( Geocon Inc) and they will provide us with a copy of their site investigation when it is complete. This report is anticipated to be issued in the very near future. • Development plans were not available at the time of our previous investigation for the site, therefore the geotechnical data for the site is incomplete with regard to the proposed development. Accordingly, we have provided the locations for proposed borings in those areas not previously investigated or in areas where additional information would be beneficial. The location of these borings is shown on the Remedial Grading Map (Plate 1), as is a description of the data and/or use they would be expected to provide. General Recommendations Geotechnical recommendations conceming the grading of Bressi Ranch development (including the constmction phases of site development) that have been presented in the Preliminary Geotechnical Investigation Report (Leighton, 1997) are still considered pertinent and applicable to the proposed project. This report is intended as a supplement to the previous project geotechnical report (Leighton, 1997). Therefore, recommendations herein that supersede previous recommendations should be incorporated into the final design, grading and constmction of the site. However, development plans for the site are in the early stages of planning, we anticipate fiirther subsurface investigation will be necessary as more precise grading plans are developed and uhimately a supplemental geotechnical report for the site will be provided. 4971009-005 If you have any questions regarding our report, please contact this office. We appreciate this opportunity to be of service. Respectfully submitted, LEIGHTON AND ASSOCIATES, INC. Kevin B. Colson Sr. Staff Geologist KBC/MRS Michael R. Stewart, CEG 1349 (Exp. 03/31/01) Vice President/Directorof Geology Attachments: Appendix A - References Plate 1 Remedial Grading Map Distribution: (4) Addressee (4) Project Design Consultants, Attn: Ms Marina Wurst (2) Mr. Dale Greenhaigh 4971009-005 APPENDIXA REFERENCES Project Design Consultants, 2000, "Land-Use Plan" for Bressi Ranch, Carlsbad, Califomia, Scale: 1" = 200'undated, received Febmary 2000. Leighton and Associates, Inc., 1997, Preliminary Geotechnical Investigation, Bressi Ranch, Carlsbad, Califomia, ProjectNo. 4971009-002, dated July 29,1997. , In- house Unpublished and Published Data O'day Consultants, 1998, Master Tentative Map for La Costa Greens, Carlsbad, Califomia, ProjectNo. 97- 1057,18 Sheets, Scale: 1"= 100', dated October 1998