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HomeMy WebLinkAboutRS090125 - SOILSOrange County / Environmental / Corporate ' 3185-A Airway Avenue Cosua Mesa, California 92626 T: 714 549 8921 F: 714 549 1438 e) oq 0 past + present + future it's in our science Engineers. Geologists Environmental Scientists January 16, 2009 J.N. 110-09 MR. AND MRS. JOHN DAHLBERG 9 Pelican Hill Circle Newport Coast, CA 92657 Subject: Geotechnical Recommendations for Proposed Front Entry Renovation and Addition to Single Family Residence, 9 Pelican Hill Circle, Lot 35, Tract 14065, Newport Coast, County of Orange, California. References: See Attached List. Dear Mr. and Mrs. Dahlberg: This letter provides our geotechnical recommendations with respect to site clearing and remedial grading a nd also for design and construction of new foundations and floor slabs for the proposed front entry renovation and addition. LOCATION AND SITE DESCRIPTION The subject property, which is located at 9 Pelican Hill Circle in the Pelican Hills community of Newport Coast, consists of a level building pad that is currently occupied by a two-story single-family residence. The area where the front entry renovation and addition to the existing building is proposed is relatively flat and level and is currently occupied by concrete walkways and also by planters with groundcover, shrubs and small palm trees. PROPOSED CONSTRUCTION AND GRADING Based on the architectural plans and on information provided by Mr. Greg Carlson with Steven L. Ball, Architect, it is proposed to enclose the existing octagonal -shaped entry courtyard. A new parapet wall will be added above the outside perimeter of the courtyard and a new skylight roof will be constructed that will cover the entire courtyard area. Either the existing courtyard walkways and planters will be protected in place, or all or portions of the existing walkways and planters will be removed and replaced with new slabs. Provided that the existing footings can support the new loads, they will be protected in -place. However, if the new loads exceed the bearing capacity of the existing footings, the existing footings will need to be underpinned or new r' Orange County / r. San Diego County r Riverside County r' Los Angeles County r' San Bernardino County r' Desert Region Environmental / Corporate 12225 World Trade Drive, Suite P 38655 Sky Canyon Drive, SuiteA 26639 Valley Center Drive, Suite 109 3535 Inland Empire Blvd., Suite 35 42-240 Green Way, Suite E 3185-A Airway Avenue San Diego, California 92128 Murrieta, California 92563 Santa Clarita, California 91351 Ontario, California 91764 Palm Desert, CA 92211 Costa Mesa, California 92626 858-485-5530 951-600-9271 661-255-5790 909-941-2505 760-340-5303 714-549-8921 MR AND MRS. JOHN DAHLBERG January 16, 2009 J.N. 110-09 Page 2 will be required. The new slabs are expected to be constructed at generally the same elevation proposed o as the footings q existing ground surfaces; therefore, only minor cuts and fills are expected to berequired and for the design and remedial g pad grades within the area of the addition. Recommendations for subsequent sections of this letter. construction of new building foundations, if any, are presented q BACKGROUND INFORMATION the subject site was Based on our review of the previous geotechnical reports for the subject sitel (References), p to approximately 15 feet he site y originally acut/fill transition lot with the southeast portion of Senbedrock of the Monterey Formation of fill and the remainder of the site underlain directly by siltston (Reference No. 1). During the construction of the existing house, the entire building area was overexcavated p bet to a en depth of at least 7 feet and replaced with properly compacted fill in order to eliminate transitions the proposed bedrock and fill beneath the building foundations (Reference No. 2). As a result, the area these addition is currently underlain by approximately 7 feet of compacted fill. Based on the previous reports, fill materi als consist of silty clays and clayey silts that are firm to stiff and moderately expansive with moderate sulfates. CONCLUSIONS AND RECOlVEVIENDATIONS General From a soils engineering and engineering geologic point of view, the subject property is considered suitable owing conclusions and recommendations axe for the construction of the proposed addition provided the specifications. In addition, it is our opinion that the proposed incorporated into the design criteria and project specs properties provided grading and construction will not adversely affect the stability of the site ecomme d tions provided below. that the grading and construction are preformed in accordance with the r Seismic Related Hazards ossible hazards to a particular site include Secondary effects of seismic activity normally considered as p general es of ground failures that Various several types of ground failure as well as induced flooding. subsidence, ound shaking typically include landsliding, ground might occur as a consequence of severe ground ground lurching, shallow probabilityof occurrence of each type of Lure and liquefaction. The ground sup ground failure depends on the severity of the earthquake, distance from faults, topography, subsoils and p y, MR. AND MRS. JOHN DAHLBERG January 16, 2009 J.N. 110-09 Page 3 groundwater conditions, in addition to other factors. Based on our previous subsurface exploration of the subject site and on our observations during previous grading, all of the above secondary effects of seismic activity including liquefaction are considered unlikely at the site. The types of seismically induced flooding which might be considered as potential hazards to a particular site normally includes flooding due to a tsunami (seismic sea wave), a seiche, or failure of a major reservoir retention structure upstream of the site. Since the site lies approximately 1.3 miles away from the Pacific Ocean at an elevation of approximately 490 feet above mean sea level, and since it does not lie in close proximity to an enclosed body of water or downstream of a major reservoir or other water retention structure, the probability of flooding from a tsunami, seiche or dam break is considered to be non-existent. Earthwork Earthwork Specifications All earthwork and grading should be performed in accordance with all applicable requirements of the Grading and Excavation Code, and the Grading Manual of the County of Orange, California, in addition to the recommendations presented below. Site Clearing Clearing operations should include the removal of all landscape vegetation and existing structural features such as concrete walkways within the areas of proposed construction. Trees and large shrubs, when removed, should be grubbed out to include their stumps and major root systems. Ground Preparation Due to the presence of landscape vegetation, it is expected that the surficial fill materials within the planter areas to depths of 12 to 18 inches will contain variable amounts of roots and organics. In addition, it is expected that the near surface soils will be disturbed during demolition and clearing operations. Therefore, in order to provide proper support of any new slabs, it is recommended that all unsuitable surficial fill materials within planter areas be removed down to competent bearing soils and then replaced as properly compacted fill. This may be accomplished by overexcavating the existing planter areas to a depth of 18 inches and then replacing the excavated materials as properly compacted fill at a minimum relative compaction of 90 percent. • PETRA MR. AND MRS. JOHN DAHLBERG January 16, 2009 J.N. 110-09 Page 4 Within areas where the existing slabs are to be removed and replaced with new slabs, it is expected that the existing underlying soils will have a higher degree of compaction and will not contain roots and organics. Therefore, in these areas, the ground suface should be scarified to a depth of 8 inches, watered as necessary to achieve near optimum moisture conditions, and then recompacted in place to a minimum relative compaction of 90 percent. FM Placement and Testing All fill should be placed in lifts not exceeding 6 inches in thickness, watered or air dried as necessary to achieve near optimum moisture conditions, and then compacted in place to a minimum relative compaction of 90 percent. Each fill lift should be treated in a similar manner. Subsequent lifts should not be placed until the preceding lift has been approved by the project geotechnical consultant. Imported soils, if required, should consist of clean materials exhibiting a very low to low expansion potential (Expansion Index less than 50). Soils to be imported should be approved by the project geotechnical consultant prior to importation. The laboratory maximum dry density and optimum moisture content for each change in soil type should be determined in accordance with Test Method ASTM D 1557-02. Geotechnical Observations Exposed bottom surfaces should be observed and approved by the project geotechnical consultant prior to placing fill. No fills should be placed without prior approval from the geotechnical consultant. The project geotechnical consultant should also be present on site during grading operations to verify proper placement and adequate compaction of fill, as well as to verify compliance with the other recommendations presented herein. Post -Grading Considerations Site Drainage Positive drainage devices such as concrete flatwork, sloped ground surfaces, and area drains should be provided within the areas of new construction to collect and direct all water to a suitable discharge area. Neither rain nor excess irrigation water should be allowed to collect or pond against building foundations. The owner is advised that the drainage system should be properly maintained throughout the life of the proposed MR. AND MRS. JOHN DAHLBERG January 16, 2009 J.N. 110-09 Page 5 development. The purpose of this drainage system will be to reduce water infiltration into the subgrade soils and to direct surface waters away from building foundations, walls and slope areas. Utility Trench Backfill Any new utility trench backfill should be compacted to a minimum relative compaction of 90 percent. Onsite soils cannot be densified adequately by flooding and jetting techniques; therefore, trench backfill materials should be placed in lifts no greater than approximately 12 to 18 inches in thickness, watered or air dried as necessary to achieve a uniform moisture content that is 2 to 3 percentage points over optimum moisture content, and then mechanically compacted in place to a minimum relative compaction of 90 percent. A representative of the project geotechnical consultant should probe and test the backfills to verify adequate compaction. For shallow trenches where pipe may be damaged by mechanical compaction equipment, such as under building floor slab, imported clean sand exhibiting a sand equivalent value (SE) of 30 or greater may be utilized. The sand backfill materials should be watered to achieve near optimum moisture conditions and then tamped in place. No specific relative compaction will be required; however, observation, probing, and, if deemed necessary, testing should be performed by a representative of the project geotechnical consultant to verify that the sand backfill is adequately compacted and will not be subject to settlement. Where an interior or exterior utility trench is proposed parallel to a building footing, the bottom of the trench should not be located below a 1:1 plane projected downward from the outside bottom edge of the adjacent footing. Where this condition exists, the adjacent footing should be deepened such that the bottom of the utility trench is located above the 1:1 projection. Foundation Design Recommendations Earthquake Loads Structures within the site should be designed and constructed to resist the effects of seismic ground motions as provided in Section 1613 of the 2007 California Building Code (CBC). The method of design is dependent on the seismic zoning, site characteristics, occupancy category, building configuration, type of structural system and on the building height. MR. AND MRS. JOHN DAHLBERG January 16, 2009 J.N. 110-09 Page 6 For structural design in accordance with the 2007 CBC, a computer program, Earthquake Ground Motion Parameters Version 5.07, developed by the United States Geological Survey (USGS, 2007) was utilized to provide ground motion parameters for the subject site. The program includes hazard curves, unifoiiu hazard response spectra and design parameters for sites in the 50 United States, Puerto Rico and the United States Virgin Islands. Based on the latitude, longitude and site classification, seismic design parameters and spectral response for both short periods and 1-second periods are calculated including Mapped Spectral Response Acceleration Parameter, Site Coefficient, Adjusted Maximum Considered Earthquake Spectral Response Acceleration Parameter and Design Spectral Response Acceleration Parameter. The program is based on USGS research and publications in cooperation with the California Geological Survey for evaluation of California faulting and seismicity (USGS, 1996a; 1996b; 2002; 2007). The offshore segment of the Newport -Inglewood Fault (approximately 3 miles or 4.8 kilometers to the southwest of the site) should be considered to be the causative fault for the subject site and is expected to generate the most significant ground motions at the site with an anticipated maximum moment magnitude (Mw) of 6.9 and an anticipated slip rate of 1.5 mm/year (CGS, 2002). The following 2007 CBC seismic design coefficients should be used for the proposed structures. These criteria are based on the site class as determined by existing subsurface geologic conditions, on the proximity of the site to the nearby fault and on the maximum moment magnitude and slip rate of the nearby fault. 2007 GBCySection thquake Loads D Site Class Definition (Table 1613.5.2) Mapped Spectral Response Acceleration Parameter, SS (Figure 1613.5(3) for 0.2 second) 1.688 Mapped Spectral Response Acceleration Parameter, Si (Figure 1613.5(4) for 1.0 second) 0.605 Site Coefficient, Fa (Table 1613.5.3 (1) short period) 1.0 Site Coefficient, F, (Table 1613.5.3 (2) 1-second period) 1.5 Adjusted Maximum Considered Earthquake Spectral Response Acceleration Parameter, SMs (Eq. 16-37) 1.688 Adjusted Maximum Considered Earthquake Spectral Response Acceleration Parameter, SMi (Eq. 16-38) 0.908 Design Spectral Response Acceleration Parameter, SDs (Eq. 16-39) 1.125 Design Spectral Response Acceleration Parameter, Sim (Eq. 16-40) 0.605 MR. AND MRS. JOHN DAHLBERG Existing Footings January 16, 2009 J.N. 110-09 Page 7 Existing interior footings that are at least 12 inches deep and exterior footings that are at least 18 inches deep may be used to support the new additions provided that total loads from the existing and new construction do not exceed 1500 and 1800 pounds per square foot, respectively. If these load limits are exceeded, or if the existing footings do not meet the recommended embedment depths below fmish grade, the existing footings will need to be underpinned or the new loads will need to be transferred to new footings. After the foundation plans have been prepared, the existing footings in all areas to receive additional loads should be randomly exposed and observed by the project structural engineer and geotechnical engineer to verify that they will adequately support the new structure and loads. Allowable Soil Bearing Capacities An allowable bearing value of 1,500 pounds per square foot may be used for 24-inch-wide pad footings and 12-inch-wide continuous footings founded at a minimum depth of 12 inches into compacted fill. This value may be increased by 20 percent for each additional foot of depth, to a maximum value of 2,500 pounds per square foot. Recommended allowable bearing values include both dead and live loads, and maybe increased by one-third for short duration wind and seismic forces. Additional recommendations for design of footings based on the expansiveness of on -site soils are provided in the "Minimum Footing and Floor Slab Recommendations" section of this report. Settlement Under the above -recommended allowable soil bearing values, total settlement of the footings is expected to be V2 inch, and maximum differential settlement is expected to be approximately'/4 of an inch over a span of 40 feet. It is anticipated that the majority of the footing settlements will occur during construction as building loads are applied. To reduce the potential for distress due to differential settlement between the existing building footings and any new building footings, either the new footings should be doweled into the existing footings, or an architectural joint should be constructed through the walls, slabs and footings in order to accommodate differential movement that may occur along the interface between the old and new building sections. MR. AND MRS. JOHN DAHLBERG January 16, 2009 J.N. 110-09 Page 8 Lateral Resistance A passive earth pressure of 250 pounds per square foot per foot of depth to a maximum value of 2,500 pounds per square foot, may be used to resist lateral loads. In addition, a coefficient of friction of 0.30 times the dead load forces may be used between concrete and the supporting soils to determine lateral sliding resistance. The above values may be combined without reduction provided the lateral sliding resistance does not exceed one- half the dead load. An increase of one-third of the above values may also be used when designing for short duration wind or seismic forces. Minimum Footing and Floor Slab Recommendations Results of previous laboratory tests (References) indicate onsite soils exhibit a moderate expansion potential as determined in accordance with ASTM D 4829 and should be considered to be an expansive soil per Section 1 802.3.2 of the 2007 CBC. Section 1805.8.2 of the 2007 CBC specifies that slab -on -ground foundations (floor slabs) resting on expansive soils should be designed in accordance with Wire Reinforcement Institute (WRI) publication "Design of Slab -on Ground Foundation," which was last updated in 1996. The design procedures outlined in the WRI publication are based on the weighted plasticity index of the different soil layers existing within the upper 15 feet of the building site. Previous testing (References) indicates that a plasticity index of 31 should be used for the in accordance with the WRI publication; however, the WRI publication also states that the weighted plasticity index of the building site must be modified (multiplied) by correction factors that compensate for the effects of sloping ground and the unconfined compressive strength of the soil materials. The addition will be constructed on a level pad; therefore, the weighted plasticity index value does proposed not need to be corrected for the effects of sloping ground. In order to approximate the unconfined penetration tests with a pocket penetrometer were previously compressive strength of the onsite fill materials, p performed on several undisturbed samples of fill that were obtained during our original subsurface exploration of the site. The unconfined compressive strength of the stiff fill materials ranged from approximately 4 to greater thanto greater tsf 5 (8 than 10 ksf). Based on these unconfined compressive strengths, it is / recommended that the weighted plasticity index (31) be multiplied by a factor of 1.2 in order to determine the I/ MR. AND MRS. JOHN DAHLBERG January 16, 2009 J.N. 110-09 Page 9 value of the effective plasticity index (per Figure 9 of the WRI 1996 update). In summary, an effective plasticity index of 37 should be used for the addition in accordance with the WRI publication. The design and construction recommendations that follow are based on the above soil conditions and may be considered for reducing the effects of moderately expansive soils and long-term differential settlement. These recommendations have been developed on the basis of previous experience of this firm on projects with similar soil conditions. Although construction performed in accordance with these recommendations has been found to reduce post -construction movement and/or cracking, they generally do not positively mitigate all potential effects of expansive soils and future settlement. The effective plasticity index provided above should be utilized by the project structural engineer to design slab -on -ground foundations with an interior grade beam grid system in accordance with the WRI publication. Based on this design, thicker floor slabs, larger footing sizes and/or additional reinforcement may be required and should govern the design if more restrictive than the minimum recommendations provided below. 1. New Footings a. New exterior continuous footings, if any, should be founded at a minimum depth of 18 inches below the lowest adjacent final grade. New interior continuous footings, if any, may be founded at a minimum depth of 12 inches below proposed pad grade. In addition, all new continuous footings should have a minimum width of 12 end 15, for one-story and two-story construction, respectively. All new continuous footings should be reinforced with a minimum of four No. 4 bars, two top and two bottom. b. New interior isolated pad footings, if any, should be a minimum of 24 inches square and founded at a minimum depth of 12 inches below the bottoms of the adjacent floor slabs. ew pad footings should be reinforced with No. 4 bars spaced a maximum of 18 inches on centers, both ways, near the bottoms of the footings. c. New exterior isolated pad footings intended for support of roof overhangs such as second -story decks, patio covers and similar construction should be a minimum of 24 inches square, and founded at a minimum depth of 18 inches below the lowest adjacent final grade. The new pad footings should be reinforced with No. 4 bars spaced a maximum of 18 inches on centers, both ways, near the bottoms of the footings. d. If the floor of the enclosed courtyard is completely removed and replaced with a new slab, it should be supported by an interior concrete grade beam system due to the underlying expansive soils. The spacing and layout of the interior concrete grade beam system should be determined by the project architect or structural engineer in accordance with the WRI publication. MR. AND MRS. JOHN DAHLBERG 2. New Building Floor Slabs January 16, 2009 J.N. 110-09 Page 10 a. The project architect or structural engineer should evaluate minimum thickness and reinforcement of new floor slabs in accordance with WRI publication based on the effective plasticity index provided previously. Unless a more stringent design is recommended by the architect or structural engineer, we recommend a minimum slab thickness of 4 inches for new floor slabs, and reinforcement consisting of No. 3 bars spaced a maximum of 18 inches on centers, both ways. All slab reinforcement should be supported on concrete chairs or brick to ensure the desired placement near mid -depth. b. New floor slabs should be underlain with a moisture vapor retarder consisting of a polyethylene or polyolefin membrane such as 10-mil Visqueen, or equivalent. All laps within the membrane should be sealed, and at least 2 inches of clean sand should be placed over the membrane to promote uniform curing of the concrete. To reduce the potential for punctures, the membrane should be placed on a pad surface that has been graded smooth without any sharp protrusions. If a smooth surface cannot be achieved by grading, consideration should be given to removing and additional inch from the pad and then placing a 1-inch-thick leveling coarse of sand across the pad surface prior to the placement of the membrane. c. Prior to placing concrete, the subgrade soils below new floor slabs should be prewatered to achieve a moisture content that is at least 1.2 times the optimum moisture content. This moisture should penetrate to a depth of approximately 12 inches into the subgrade. Footing Observations Foundation excavations should be observed by a representative of the project geotechnical consultant to verify that they have been excavated into competent fill materials and to the minimum depths recommended herein. These observations should be performed prior to the placement of forms or reinforcement. The excavations should be trimmed neat, level and square. All loose, sloughed or moisture -softened materials and/or any construction debris should be removed prior to the placement of concrete. Excavated soils derived from footing and utility trenches should not be placed in slab -on -grade areas unless they are compacted to at least 90 percent of maximum dry density. Soluble Sulfates Results of previous laboratory tests performed in accordance with California Test Method No. 417 (References) indicate on -site soils contain water soluble sulfate contents of less than 0.1 o percent. Based on Section 1904.3 of the 2007 CBC, concrete that will be exposed to sulfate -containing soils shall comply with the provisions of ACI 318-05, Section 4.3. According to Table 4.3.1 of the ACI 318-05, a Negligible MR. AND MRS. JOHN DAHLBERG January 16, 2009 J.N. 110-09 Page 11 exposure to sulfate can be expected for concrete placed in contact with the onsite soil materials; therefore, no special cement will be required. REPORT LIMITATIONS This report i s b ased o n the proposed project and geotechnical data as described herein. The materials encountered on the project site, described in other literature, and utilized in our previous laboratory investigations are believed representative of the total project area, and the conclusions and recommendations contained in this report are presented on that basis. Observation and testing by a geotechnical consultant during the construction phase of the project are essential to confirming the basis of this report. To provide the greatest degree of continuity between the design and construction phases, consideration should be given to retaining Petra Geotechnical, Inc., for construction services. This report has been prepared consistent with that level of care being provided by other professionals providing similar services at the same locale and in the same time period. The contents of this report are professional opinions and as such, are not to be considered a guaranty or warranty. This report should be reviewed and updated after a period of one year or if the site ownership or project concept changes from that described herein. This report has not been prepared for use by parties or projects other than those named or described herein and may not contain sufficient information for other parties or other purposes. Respectfully submitted, PETRA GEOTECHNICAL, INC. David Hansen Senior Associate Engineer RCE 56591 DH/kg W:\2009\100\110-09\100\Addition Recommendations.doc • PETRA MR. AND MRS. JOHN DAHLBERG January 16, 2009 J.N. 110-09 Page 12 REFERENCES 1) Notification of Soils Engineer and Engineering Geologist of Record, Site Reconnaissance, Lot Recertification, and Grading Plan Review, Lot 35, Tract No. 14065, Newport Coast, County of Orange, California; report by Petra Geotechnical, Inc., dated June 30, 1997 (J.N. 542-96). 2) Geotechnical Report of Rough Grading, Lot 35, Tract No. 14065, Newport Coast, County of Orange, California; report by Petra Geotechnical, Inc., dated June 3, 1998 (J.N. 542-96). 3) Final Soils Report, Utility Trench Backfill, Lot 35, Tract No. 14065, Newport Coast, County of Orange, Califomia; report by Petra Geotechnical, Inc., dated May 13, 1999 (J.N. 542-96).