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Home My WebLink AboutCT 02-14-04; BRESSI RANCH UNIT 4; GEOTECHNICAL RESPONSE RETAINING WALL; 2003-11-07Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY To: November 7, 2003 Lennar Communities 5780 Fleet Street, Suite 320 Carlsbad, California 92009-4700 RECEIVED Project No. 971009-014 NOV i 2 (,003 ENGINEERING DEPARTMENT Attention: Ms. Kristine Zortman Subject: Geotechnical Response to the City of Carlsbad Review Comments Concerning Retaining Wall Design Parameters and Site Drainage Recommendations, Planning Areas PA-6 Through PA-12, Bressi Ranch, Carlsbad, California References: Leighton and Associates, 1997, Preliminary Geotechnical Investigation, Bressi Ranch, Carlsbad, California, Project No. 4971009-002, dated July 29, 1997 Introduction ----, 2001, Supplemental Geotechnical Investigation for Mass Grading, Bressi Ranch, Carlsbad, California, Project No. 971009-005, dated March 14, 2001 In accordance with the request of Mr. David Uson of Project Design Consultants, this letter presents our geotechnical response to the City of Carlsbad review comments concerning retaining wall design parameters and site drainage recommendations for the residential developments (i.e. Planning Areas PA-6 through PA-12) at the Bressi Ranch project located in Carlsbad, California. We understand that the City of Carlsbad review comments included the need to address retaining wall design parameters, site drainage recommendations and soil bearing values. Our responses to the review comments are provided in the following sections. . , Retaining Wall Design Parameters Retaining walls should be designed for the lateral earth pressures exerted on them. The magnitude of these pressures depends on the amount of deformation that the wall can yield under load. If the wall can yield enough to mobilize the full shear strength of the soil, it can be designed for "active" N • 0 Z ~ 0 w :c (.) z cC .... a.. c.-r oz--,4C ~ 3934 Murphy Canyon Road, Suite 8205. San Diego, CA92123-44is' 858.292.8030. Fax 858.292.0771 • www.leightongeo.com , .i 971009-014 pressure. If the wall cannot yield under the applied load, the shear strength of the soil cannot be mobilized and the earth pressure will be higher. Such walls should be designed for "at rest" conditions. If a structure moves toward the soils, the resulting resistance developed by the soil is the "passive" resistance. The noted passive resistance assumes an appropriate setback from the face of slopes. The native backfill soils should be compacted to at least 90 percent relative compaction (based on American Standard of Testing and Materials [ASTM] Test Method D1557). The walls should be constructed and backfilled as soon as possible after backcut excavations. Prolonged exposure of backcut slopes may result in some localized slope instability • F·oundation Setback Recommendations from Slope Faces We recommend a minimum horizontal setback distance from the face of descending slopes or adjacent retaining walls for all structural foundations, footings, and other settlement- sensitive structures as indicated on Table 1. This distance is measured from the outside bottom edge of the footing, horizontally to the descending slope face and is based on the slope height and type of soil. However, the foundation setback distance may be revised by the geotechnical consultant on a case-by-case basis if the geotechnical conditions are different than anticipated. Table 1 Minimum Foundation Setback from Descending Slope Faces Slope Height Minimum Recommended Foundation Setback Less than 5 feet 5 feet 5 to 15 feet 7 feet 15 to 30 feet 10 feet Greater than 30 feet 20 feet Please not~ that the soils within the structural setback area possess poor lateral stability, and improvements (such as retaining wa1ls, sidewalks, fences, pavements, etc.) constructed within this setback area may be subject to latera1 movement andlor differential settlement. Potential distress to such improvements may be mitigated by providing a deepened footing or a pier and grade beam foundation system to support the improvement. The deepened footing should meet the setback as described above. - 2 - Leighton 971009-014 • Lateral Earth Pressures The recommended lateral pressures for the onsite soil or granular~soil and level or sloping backfill are presented on Table 2. ' Table 2 Lateral Earth Pressures Equivalent Fluid Weight (pet) Conditions Expansion Index Less than Expansion Index Greater 50 than 50 Level 2:1 Slope Level 2:1 Slope Active 35 55 60 70 At-Rest 55 65 70 80 Passive 350 150 350 150 To design an unrestrained wall, such as cantilever wall, the active earth pressure may be used. For a restrained retaining wall, such as a basement wall, the at-rest pressure should be used. Further, for sliding resistance, the friction coefficient of 0.35 may be used at the concrete and soil interface. In combining the total lateral resistance, the passive pressure or the frictional resistance should be reduced by 50 percent. Wall footings may be designed in accordance with structural considerations. The passive resistance value may be increased by one-third when considering loads of short duration including wind or seismic loads. The horizontal distance between foundation elements providing passive resistance should be a minimum of three times the depth of the elements to allow full development of the passive pressure. The total depth of retained earth for design of cantilever walls should be the vertical distance below the ground surface measured at the wall face for stem design or measured at the heel of the footing for overturning .and sliding. All retaining structures should be provided . with a back drain and appropriately waterproofed. Typical wall drainage is presented on Figure 1. Surcharge loading effects from adjacent structures should be evaluated by the geotechnical and structural engineers. • Soil Bearing Value and Minimum Foundation Embedment Depth Foundations for retaining walls in competent formational soils or properly compacted fill should be embedded at least 18 inches below lowest adjacent grade. At this depth, an allowable bearing capacity of 2,000 psfmay be assumed. - 3 -Leighton 971009-014 Control of Surface Water and Drainage Surface drainage should be carefully taken into consideration during precise grading, landscaping, and building construction. Positive drainage (e.g., roof gutters, downspouts, area drain, etc.) should be provided to direct surface water away from structures and towards the street or suitable drainage devices. Ponding of water adjacent to structures should be avoided. The adjacent grade next to structures should have a minimum fall of 2 percent for a distance of at least 5 feet. Roof gutters, downspouts, and area drains should be aligned so as to transport surface water to a minimum distance of 5 feet away from structures. The. performance of structural foundations is dependent upon maintaining adequate surface drainage away from structures. Water should be transported off the site in approved drainage devices or unobstructed swales. We recommend that the minimum flow gradient for the drainage be 1 percent for area drains and drainage swales. In places where the prospect of maintaining the minimum recommended gradient for the drainage swales and the construction of additional area drains is not feasible, provisions for specific recommendations to the homeowners may be necessary, outlining the importance of maintaining positive drainage to streets. • Side-yard Drainage Recommendation Alternative We understand that some of the residential structures of the tract may be located approximately 5 feet from the side-yard property line. Consequently, the recommended surface drainage design (e.g. positive drainage away from the residential structure [at a gradient of at least 2 percent] for a distance of at least 5 feet) cannot be met on some of the lots. As an alternative, we recommend that surface drainage adjacent to the residential structures be accomplished with a gradient of at least 3-1/2 percent away from the structure for a minimum horizontal distance of 3 feet. Drainage should be further maintained by a swale or drainage path at a gradient of at least I-percent to a suitable collection device (Le. area drain, street gutter, etc.). We also recommend that the footings less than 5 feet from the drainage swale be deepened so the bottom of the footing is at least 12 inches below the flow-line of the drainage swale. ".. Where necessary, drainage paths may be shortened by the use of area drains and tightline collector pipes. It is very important that these gradients be maintained throughout the life of the structure. Eave gutters with properly connected downspouts that drain precipitation away from the residential structure and reduce water infiltration into the sub grade soils may also be utilized. The impact of heavy irrigation or inadequate runoff gradient can create perched water conditions, resulting in seepage or shallow groundwater conditions where previously none existed. Maintaining adequate surface drainage and controlled irrigation will significantly reduce the potential for nuisance-type moisture problems. To reduce differential earth movements (such as heaving and shrinkage due to the change in moisture content of foundation soils, which may cause -4- Leighton 971009-014 distress to a residential structure and improvements), the moisture content of the soils surrounding the structure should be kept as relatively constant as possible. All area drain inlets should be maintained and kept clear of debris in order to function properly. In addition, yard landscaping should not cause any obstruction to the yard drainage. Rerouting of yard drainage pattern and/or installation of area drains should be performed, if necessary. A qualified civil engineer or a landscape architect should be consulted prior to rerouting of drainage. If you have any questions regarding this letter, please contact this office. We appreciate this opportunity to be of service. Respectfully submitted, LEIGHTON AND ASSOCIATES, INC. lVft-o.CJL William D. Olson, RCE 45283 Senior Project Engineer till1.~~~612 Director of Geology Attachment: Figure 1 -Retaining Wall Backfill and Drainage Detail Distribution: (2) Addressee (1 ) Lennar Communities, Attention: Lisa Galloway (1) Lennar Communities, Attention: Jim Urbina (12) Project Design Consultants, Attention: David Uson -5- - Leighton RETAINING WALL BACKFILL AND DRAINAGE DETAIL OPTION 2: GRAVEL WRAPPED IN FILTER FABRIC For Maximum a-Foot High Walls with On-Site Soils Having Very Low to Low Expansion Potentials (i.e. on Expansion Index of 50 or Less) SLOPE OR LEvEL OPTION 1: PIPE SURROUNDED WITH CLASS 2 PERMEABLE MATERIAL WATERPROOFING --1---1 (SEE GENERAL NOTES) 14 TO 1 ~ INCH SIZE GRAVEL WRAPPED IN FILTER FABRIC SLOPE OR LEVEL , LEVEL OR SLOPE WATERPROOFING (SEE GENERAL NOTES) Class 2 Filter Permeable Water/al Gradation Pef Caltrans Specifications GENERAL NOTES: 12-NINIMUM ClASS 2 PERMEABLE FILTER MATERIAL (SEE GRADATION) -4 INCH DIAMETER PERFORATED PIPE (SEE NOTE 3) Sieve Size 1- 3/4- 3/8- No.4 No.8 No. 30 No. 50 No. 200 '" Waterproofing should be provided where moisture nuisance problem through the wall is und8sirobl8 '" Wat8r proofing of th8 walls is not under purview of the geotechnical engin88r '" All droins should have a gradient of 1 percent minImum Percent Pgsslng 100 90-100 40-100 25-40 18-33 5-15 0-7 0-3 '" Outlet port/on of th8 subdroin should have a 4-inch diamet8r solid pipe discharged into a suitabl8 disposal area d8signed by the project 8ngin88r. The subdrain pipe should be accessible far maintenance (rodding) '" Other subdrain backfill options are subject to the review by the geotechnical, 8ngin88r and modification of design porometers ~: 1) Sand should hav8 a sand equivalent of JO or great8r and may be densified by water jetting 2) 1 Cu. ft. per ft. of 1/4-to 1 112-inch size grovel wropped in filt8r fabric J) Pipe iyptI shauld be ASTM 01527 Acrylanitril8 Butadiene StyrenfJ (ASS) SORJ5 or ASTM 01785 Polyvinyl Chlaridll plastic (P\IC). Schedule 40. Armco A200D ~ or approved equIvalent PipfJ should bll installed with perforotions down. Perfoiatlons should be JI8 inch in diometer placed at thfJ ends of a 120-dBgrBfJ arc in two rows at J-Inch on C8nter (staggfJred) 4) FIlter fabric should be Miran 140NC or approved equivalent 5) W88pholfJ should be J-Inch minimum diameter and provIded at 10-foot maximum intervals. If exposure is permitted. w88pho/es should be located 12 inchfJs above finished grodfJ. If exposure is not permitted such as for a wall adjacfJnt to a sldllwalklcurb. a pipe under thfJ sid8walk to be discharged through thfJ curb face or equivalfJnt should bfJ provided. For a basement-iyptI wall, a proper subdrain outlet system should be provided 6) Retaining wall plans should be reviewed and approved by thfJ geotechnical fJngln88r 7) Walls over six f8fJt in heIght are subject to a special revIew by the geotechnical fJngin88r and modifications to the abovil reqUirements RETAINING WALL BACKFILL AND SUBDRAIN DETAIL (rev. June 2000) PROJECT NO. 971009-014 PROJECT NAME 8,.. .. 1 Ranch Carl.bad. CalifornIa Leighton and Associates, Inc. A LEIGHTON GROUP COMPANY Figure No.1