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HomeMy WebLinkAboutMS 2018-0006; Breakers View Beach Homes; Preliminary Geotechnical Investigation Update and Response to Third-Party Review; 2020-12-18I COAST GEOTECH, INC. CONSULTING ENGINEERS AND GEOLOGISTS December 18, 2020 Michael Jason Ianni 2337 Catalina Avenue Vista, CA 92084 RE: PRELIMINARY GEOTECHNICAL INVESTIGATION UPDATE AND RESPONSE TO THIRD-PARTY REVIEW Proposed Twin Home Development 3648 Carlsbad Boulevard Carlsbad, California Dear Mr. Ianni: In response to your request and in accordance with our Proposal and Agreement dated December 7, 2020, we have performed an updated preliminary geotechnical investigation on the subject site for the proposed twin home development. This report also includes our response to the referenced third- party review and a geotechnical review of the newest grading plan (separate report). The findings of the investigation, previous laboratory test results, and recommendations for foundation design and remedial grading are presented in this report. From a geologic and soil s engineering point of view, it is our opinion that the site is suitable for the proposed development, provided the recommendations in this report and referenced reports are implemented during the design and construction phases. If you have any questions please do not hesitate to contact us at (858) 755-8622. This opportunity to be of service is appreciated. Respectfully submitted, COAST GEOTECH, INC. ~~ Kevin McFarland Project Geologist ~ Mark Burwell, C.E.G. Engineering Geologist '1~~ Vithaya Singhanet, P.E. Geotechnical Engineer P.O. BOX 230163 • ENCINITAS, CALIFORNIA 92023 (858) 755-8622 PRELIMINARY GEOTECHNICAL INVESTIGATION UPDATE AND RESPONSE TO THIRD-PARTY REVIEW Proposed Twin Home Development 3648 Carlsbad Boulevard Carlsbad, California Prepared For: Michael Jason Ianni 2337 Catalina Avenue Vista, CA 92084 Prepared By: COAST GEOTECH, INC. P.O. Box 230163 Encinitas, CA 92023 December 18, 2020 W.O. U6541 l 15 COAST GEOTECH, INC. TABLE OF CONTENTS Michael Jason Ianni W.O. U6541115 Page 3 of 26 1. INTRODUCTION ........................................................... 5 2. SITE DESCRIPTION AND PROPOSED DEVELOPMENT ......................... 5 2.1 Site Description .......................................................... 5 2.2 Proposed Development .................................................... 5 3. SITE INVESTIGATION AND LABORATORY TESTING .......................... 6 3.1 Site Investigation ......................................................... 6 3.2 Laboratory Testing and Analysis ............................................ 6 4. GEOLOGIC CONDITIONS ................................................... 6 4.1 Regional Geologic Settings ................................................. 6 4.2 Site Geology ............................................................ 7 4.3 Expansive Soil .......................................................... 8 4.4 Groundwater ............................................................ 8 5. GEOLOGIC HAZARDS ..................................................... 8 5.1 Faulting and Seismicity .................................................... 8 5 .2 Landslide Potential ...................................................... 10 5.3 Liquefaction Potential .................................................... 11 5.4 Flood Potential ......................................................... 11 5.5 Tsunami and Flood Potential .............................................. 11 6. CONCLUSIONS ........................................................... 11 7. RECOMMENDATIONS .................................................... 12 7.1 Building Pad-Removals/Recompaction ...................................... 12 7.2 Foundations ............................................................ 13 7 .3 Sulfate and Chloride Tests ................................................ 14 7.4 Slabs on Grade (Interior and Exterior) ....................................... 14 7.5 Lateral Resistance ....................................................... 15 7.6 Retaining Walls ......................................................... 15 7. 7 Dynamic (Seismic) Lateral Earth Pressures ................................... 16 7.8 Settlement Characteristics ................................................. 17 7. 9 Seismic Considerations ................................................... 17 7.10 Site Classification For Seismic Design ...................................... 18 7 .11 Preliminary Pavement Design ............................................. 20 7.12 Permeable Interlocking Concrete Pavers (PICP) .............................. 20 COAST GEOTECH, INC. Michael Jason Ianni W.O. U6541115 Page 4 of 26 7 .13 Utility Trench ......................................................... 20 7.14 Drainage ............................................................. 2 1 7.15 Plan Review .......................................................... 2 1 7.1 6 Geotechnical Observations/Testing ........................................ 21 7 .1 7 Previous Geotechnical Report. ............................................ 22 7.18 Response To Third-Party Review .......................................... 22 8. LIMITATIONS ............................................................ 23 REFERENCES ............................................................... 25 FIGURES: Figure 1: Geotechnical Map Figure 2: Fault Map Figure 3: Flood Hazard Map Figure 4: Typical Isolation Joints and Re-Entrant Corner Reinforcement Figure 5: Typical Pipe/Trench Footing Detail APPENDIX A Seismic Design Parameters APPENDIX B Grading Guidelines COAST GEOTECH, INC. 1. INTRODUCTION Michael Jason Ianni W.O. U6541115 Page 5 of 26 This report presents the results of our updated geotechnical investigation on the subject property. The purpose of this study is to update the Preliminary Geotechnical Investigation Report to current seismic and building code requirements for the proposed twin home development. The current preliminary grading plan was prepared by bHA, Inc. and has been reviewed as part of this study in a separate report. 2. SITE DESCRIPTION AND PROPOSED DEVELOPMENT 2. 1 Site Description The subject property is located north of Juniper Avenue, along the east side of Carlsbad Boulevard, in the city of Carlsbad. The subject property is a relatively level rectangular residential lot. The site includes a single story residence with an attached garage. Access to the rear of the site is via an all ey which enters from Juniper Avenue. The property is bounded along the north, south and east by developed residential lots. Vegetation includes grass, shrubs and a few trees. Drainage is generally by sheet flow to the east. 2. 2 Proposed Development Grading plans for the development of the site were prepared by bHA, Inc. It is our understanding that the project will include the demolition of the existing structure and the construction of a new two (2) unit twin home structure. The structure will be constructed at or near the existing grade and supported on continuous wall footings with a slab on grade floor. Two (2) bio-retention basins are proposed on the southwest and northeast portions of the property, adjacent to the proposed structure.- A minor gravity type retaining wall will line the northwest and southeast margins of the property. COAST GEOTECH, INC. 3. SITE INVESTIGATION AND LABORATORY TESTING 3.1 Site Investigation Michael Jason Ianni W.O. U6541115 Page 6 of26 Site exploration was conducted on January 12, 2016, and included two (2) exploratory borings drilled with a tri-pod hollow-stem drill rig to a maximum depth of 20.0 feet. Investigation details can be found in the original Preliminary Geotechnical Investigation Report. An updated geotechnical map with the pertinent exploratory boring locations is attached to this report as Figure I. 3.2 Laborato,y Testing and Analysis The laboratory tests were performed in accordance with the generally accepted American Society for Testing and Materials (ASTM) test methods or suggested procedures. Lab descriptions and results can be found in the Laboratory Test Results section of the original report. The fo llowing tests were performed: • Classification of Soils (ASTM D-2487) Moisture/Density (ASTM D-2216) Maximum Dry Density and Optimum Moisture Content (ASTM D-1557) Sulfate Ion Content (CTM 643) Shear Test (ASTM D-3080) 4. GEOLOGIC CONDITIONS The geologic conditions at the site are based on our previous field exploration and the review of available geologic and geotechnical literature. 4.1 Regional Geologic Settings The subject property is located in the Coastal Plains Physiographic Province of San Diego. The property is underlain at relatively shallow depths by terrace deposits which have been re-classified as Old Paralic Deposits. These surfaces are relatively flat erosional platforms that were shaped by COAST GEOTECH, INC. Michael Jason Ianni W.O. U6541115 Page 7 of 26 wave action along the former coastlines. The step-like elevation of the marine terraces was caused by changes in sea level throughout the Pleistocene and by seismic activity along the Rose Canyon Fault Zone located west of the coastline. The Rose Canyon Fault Zone is one of many northwest trending, sub-parallel faults and fault zones that traverse the nearby vicinity. Several of these faults, including the Rose Canyon Fault Zone, are considered active faults. Further discussion of faulting in regards to the site is discussed in the Geologic Hazards section of this report. 4. 2 Site Geology Previously published geologic maps conducted by Kennedy and Tan (2008) indicate that the subject property is underlain at shallow depths by Pleistocene Old Paralic Deposits (Qop). The Old Paralic Deposits are underlain at depth by Eocene-age sedimentary rocks, which have commonly been designated as the Santiago Formation on published geologic maps. The Old Paralic Deposits are covered by thin soil deposits and, in part, by minor fill deposits. A brief description of the earth materials encountered on the site follows. • Artificial Fill (at) No evidence of significant fill deposits were observed on the site. However, minor fill deposits, up to approximately 1.0 foot, are present along the perimeter of the residence. The fill is composed of locally derived silty, fine and medium-grained sand. The rear parking area is covered by approximately 4.0 inches of gravel. • Soil (Qs) Approximately 8.0 inches of soil was encountered in the exploratory borings. The soil is composed of brown silty fine and medium-grained sand. The soil is generally slightly moist and loose. The contact with the underlying Old Paralic Deposits is gradational. COAST GEOTECH, INC. • Old Paralic Deposits (Qop) Michael Jason Ianni W.O. U6541115 Page 8 of26 Underlying the surficiaJ materials, poorly consolidated Pleistocene Old Parali c Deposits are present. The Old Paralic deposits are composed of tan to reddish brown, fine and medium- grained sand and are generally moist to very moist and weathered in the upper 2.0 to 3.0 feet. Below the weathered zone, the Old Paralic Deposits are in a medium dense to dense condition. The Old Paralic Deposits generally exhibit little or no cohesion. Regionally, the Pleistocene sands are considered flat-lying and are underlain at depth by Eocene-age sedimentary rock units. Medium-dense to dense Old Paralic Deposits are considered suitable for support of foundations and fills. 4. 3 Expansive Soil Based on our experience in the area and previous laboratory testing of selected samples, the soil and Old Paralic Deposits reflect an expansion potential in the very low range. 4. 4 Groundwater No evidence of perched or high groundwater tables were noted during exploration. However, it should be noted that seepage problems can develop after completion of construction. These seepage problems most often result from drainage alterations, landscaping and over-irrigation. In the event that seepage or saturated ground does occur, it has been our experience that they are most effectively handled on an individual basis. 5. GEOLOGIC HAZARDS 5.1 Faulting and Seismicity The subject site is located within the seismically active Southern California region, which is generally characterized by northwest trending, right-lateral strike-slip faults and fault zones. Several of these fault segments and zones are classified as active by the California Geologic Survey (Alquist-Priolo Earthquake Fault Zoning Act). As a result, ground shaking is a potential hazard throughout the region. COAST GEOTECH, INC. Michael Jason Ianni W.0. U6541115 Page 9 of 26 Based on a review of published geologic maps, no known faults traverse the site (Figure 2). Thus, ground surface rupture is not likely to occur as a result of an earthquake or seismic event. The nearest active fault to the site is the Rose Canyon Fault Zone (offshore), located approximately 4.3 miles west of the site. It should be noted that the Rose Canyon Fault is one of four main fault strands that make up the Newport-Inglewood/Rose Canyon (NIRC) fault system (Treiman, 1984). The four strands form a series of right-stepping en echelon faults situated along the Southern California coastline. A recent study by Sahakian et al. (2017) concluded that the geometry of the NIRC fault system may enable rupture along the entire length of the fault zone. The study also modeled several rupture scenarios in light of the newly defined geometry which suggest earthquake ruptures up to magnitudes (M) of 7 .4 are possible along the NIRC system. While the models are intriguing, the paper recommends further research and modeling on the NIRC fault geometry to improve our understanding of potential hazards and ground shaking along the Southern California coast. Therefore, the modeled rupture magnitude of M = 7.4 on the Rose Canyon Fault was not used for the recommendations for this investigation. Other nearby faults that may affect the site include the Newport-Inglewood fault (offshore), the Coronado Bank fault, and the Julian and Temecula segments of the Elsinore fault. The proximity of major faults to the site, and their estimated maximum earthquake magnitudes and peak site accelerations are enclosed on Table 1 and were determined by EQF AULT version 3.00 software (Blake, 2000). COAST GEOTECH, INC. Table 1: Principal Active Faults (Updated) Fault Name Approximate Distance from site (mi) Rose Canyon (offshore) 4.3 Newport-Inglewood (offshore) 4.8 Coronado Bank 20.5 Elsinore (Temecula) 24.7 Elsinore (Julian) 24.9 Elsinore (Glen Ivy) 34.1 Palos Verdes 35.4 Michael Jason Ianni W.O. U6541115 Page 10 of26 Maximum EQ Peak Site Magnitude (Mmax) Accel. (g) 6.9 0.394 6.9 0.381 7.4 0.226 6.8 0.149 7.1 0.172 6.8 0.113 7.1 0.129 The Rose Canyon Fault is capable of generating a magnitude earthquake which would cause strong ground motions at the subject site. Further analysis on seismicity and the site specific seismic parameters are discussed in the Recommendations section of this report. 5.2 landslide Potential A landslide is the displacement of a mass of rock, debris, or earth down a slope caused by topographic, geological, geotechnical and/or subsurface water conditions. Potential landslide hazards for the site were assessed using the review of published geologic and topographic maps for the area No landslides have been mapped on or in the immediate vicinity of the subject property. The geologic conditions and the sloping terrain indicate a low to moderate potential for failure susceptibility and retreat. According to the Landslide Hazards map, Oceanside Quadrangle (Tan and Giffen, 1995), the site is located within Susceptibility Area 2 where slopes are marginally susceptible. COAST GEOTECH, INC. 5.3 Liquefaction Potential Michael Jason Ianni W.O. U6541115 Page 11 of 26 Liquefaction is a process by which a sand mass loses its shearing strength completely and flows. The temporary transformation of the material into a fluid mass is often associated with ground motion, resulting from an earthquake and high groundwater conditions. Owing to the moderately dense nature of the Old Paralic Deposits and anticipated depth to groundwater, seismically-induced liquefaction and soil instability is considered low. 5. 4 Flood Potential The site is not located in the 100-year flood zone according to the FEMA Flood Map Service Center (Figure 3). 5. 5 Tsunami and Flood Potential Tsunamis are large sea waves generated by earthquakes, volcanic eruptions, or landslides that potentially cause the displacement of substantial volumes of water. The Tsunami Inundation Map fo r Emergency Planning: Oceanside -Del Mar Quadrangle (California Emergency Management Agency, 2009) suggests that the site is not susceptible to flooding from tsunamis. 6. CONCLUSIONS Based on the results from our evaluation of the site, construction of the proposed structure is feasible, provided the recommendations within this report and referenced reports are incorporated in the design and construction of the project. The fo llowing geotechnical considerations for the project site include: 1) The subject property is located in an area that is relatively free of potential geologic hazards such as landsliding, liquefaction, high groundwater conditions and seismically induced subsidence. COAST GEOTECH, INC. Michael Jason Ianni W.O. U6541115 Page 12 of 26 2) The existing fill , soil and weathered Old Paralic Deposits are not suitable for the support of structural footings, concrete flatwork or proposed fills in their present condition. 3) The existing fill, soil and weathered Old Paralic Deposits should be removed and replaced as properly compacted fill deposits in the building footprints, areas of concrete flatwork and exterior improvements. Removals are anticipated to be on the order of 3.5 feet below existing grade. 4) Subsurface cut off walls were recommended in our Revised City of Carlsbad Report, dated July 25, 2016, and should be install ed prior to foundation construction. 7. RECOMMEND A TIO NS 7.1 Building Pad-Removals/Recompaction The existing fill , soil and weathered Old Paralic Deposits should be removed and replaced as properly compacted fill. Removals should include the entire building pad, extending a minimum of 5.0 feet beyond the building footprint. The approximate limits of removals are shown on the enclosed grading plan and Cross Section A-A' of the Revised City of Carlsbad Report, dated July 25, 2016. Removals are anticipated to be on the order of3.5 feet below the existing grade. Deeper removals will be necessary in the area of subsurface piping and tree stumps. Cut/fill transitions should be overexcavated a minimum of 3.5 feet and replaced as properly compacted fill. Most of the existing earth deposits are generally suitable for reuse, provided they are cleared of all vegetation, debris and thoroughly mixed. Prior to placement of fill, the base of the removal should be observed by a representative of this firm. Additional overexcavation and recommendations may be necessary at that time. The exposed bottom should be scarified to a minimum depth of 6.0 inches, moistened as required and compacted to a minimum of 90 percent of the laboratory maximum dry density. Fill should be placed in 6.0 to 8.0 inch lifts, moistened to approximately 1.0-2.0 percent above optimum moisture content and compacted to a minimum of90 percent of the laboratory maximum dry density. Fill, soil and terrace deposits in areas of proposed concrete flatwork and driveways should be COAST GEOTECH, INC. Michael Jason Ianni W.O. U6541115 Page 13 of 26 removed and replaced as properly compacted fill. Removals in these areas are anticipated to be on the order of 2.0 feet. Imported fill, if necessary, should consist of non-expansive granular deposits approved by the geotechnical engineer. 7.2 Foundations The following design parameters are based on footi ngs founded into non-expansive approved compacted fill deposits or competent Old Paralic Deposits. Footings for the proposed structure should be a minimum of 15 inches wide and founded a minimum of 18 inches below the lower most adjacent subgrade at the time of foundation construction for single-story and two-story structures. Isolated spread footings should be at least 24 inches square and founded a minimum of 18 inches below the lowermost adjacent grade. Proposed slurry cut off walls underlying footings should be enlarged to the same width as proposed footings. The cut off walls should be constructed prior to foundation construction. Footings should be reinforced with a minimum of four No. 4 bars, two along the top of the footing and two along the base. Where parallel wall footings occur, the upper footing should be deepened below a 45 degree plane projected up from the base of the lower footing, or the lower wall should be designed for the additional surcharge load from the upper wall. Footing recommendations provided herein are based upon underlying soil conditions and are not intended to be in lieu of the project structural engineer's design. The base of footings should be maintained a minimum horizontal distance of 10 lateral feet to the face of the nearest slope. For design purposes, an allowable bearing value of 2000 pounds per square foot may be used for foundations at the recommended footing depth for single and two-story structures. COAST GEOTECH, INC. Michael Jason Ianni W.O. U6541115 Page 14 of 26 For footings deeper than 18 inches, the bearing value may be increased by 250 pounds per square foot for each additional 6.0 inches of embedment to a maximum of3000 pounds per square foot. The bearing value may be increased by one-third for the sho11 durations of loading, which includes the effects of wind and seismic forces. The bearing value indicated above is for the total dead and frequently applied live loads. This value may be increased by 33 percent for short durations of loading, including the effects of wind and seismic forces. 7.3 Sulfate and Chloride Tests Previous testing on selective samples suggest the surficial soils have a water soluble sulfate content of 0.034 which is considered negligible. 7. 4 Slabs on Grade (Interior and Exterior) Slabs on grade should be a minimum of 5 .0 inches thick and reinforced in both directions with No. 3 bars placed 18 inches on center in both directions. Exterior slabs on grade should be a minimum of 4.5 inches thick and reinforced with No. 3 placed 18 inches on center in both directions. The slab should be underlain by a minimum 2.0-inch coarse sand blanket (S.E. greater than 30). Where moisture sensitive floors are used, a minimum 10.0-mil Visqueen, Stego, or equivalent moisture barrier should be placed over the sand blanket and covered by an additional two inches of sand (S.E. greater than 30). Utility trenches underlying the slab may be backfilled with on-site materials, compacted to a minimum of 90 percent of the laboratory maximum dry density. Slabs should be reinforced as indicated above the provided with saw cuts/expansion joints, as recommended by the project structural engineer. All slabs should be cast over dense compacted subgrades. At a minimum, interior slabs should be provided with softcut contraction/control joints consisting of sawcuts spaced 10 feet on center maximum each way. C ut as soon as the slab will support the weight of the saw, and operate without disturbing the final finish, which is normally within 2 hours after final finish at each control joint location or 150 psi to 800 psi. The softcuts should be a COAST GEOTECH, INC. Michael Jason Ianni W.O. U6541115 Page 15 of 26 minimum of3/4 inch in depth, but should not exceed I inch deep maximum. Anti-ravel skid plates should be used and replaced with each blade to avoid spalling and raveling. Avoid wheeled equipment across cuts for at least 24 how-s. Provide re-entrant comer (270 degrees comers) reinforced for all interior slabs consisting of minimum two, I 0-feet long No. 3 bars at 12 inches on center with the first bat placed 3 inches from re-entrant corner (see Figure 4 ). Re-entrant corners will depend on slab geometry and/or interior column locations. Exterior slabs should be provided with weakened plane joints at frequent intervals in accordance with the American Concrete Institute (ACI) guidelines. Our experience indicates that the use of reinforcement in slabs and fow1dations can reduce the potential for drying and shrinkage cracking. However, some minor cracking is considered normal and should be expected as the concrete cures. Moisture barriers can retard, but not eliminate moisture vapor movement from the underlying soil s up through the slab. 7.5 Lateral Resistance Resistance to lateral load may be provided by friction acting at the base foundations and by passive earth pressure. A coefficient of friction of 0.35 may be used with dead-load fo rces. Design passive earth resistance may be calculated from a lateral pressure corresponding to an equivalent fluid density of 300 pounds per cubic foot with a maximum of 3000 pounds per square foot. 7. 6 Retaining Walls Cantilever walls (yielding) retaining nonexpans1ve gran ular soils may be designed for an active-equivalent fluid pressure of37 pounds per cubic foot for a level surcharge and 47 pounds per cubic foot for a sloping backfill. Restrained walls (nonyielding) should be designed for an "at-rest" equivalent fluid pressure of 60 pounds per cubic foot. Wall footings should be designed in accordance with the foundation design recommendations. All retaining walls should be provided with adequate backdrainage system. A geocomposite blanket drain such as Miradrain 6000 or equivalent is recommended behind walls. The soil parameters assume a nonexpansive select granular backfill compacted to a minimum of 90 percent of the laboratory maximum dry density. COAST GEOTECH, INC. 7. 7 Dynamic (Seismic) Lateral Earth Pressures Michael Jason Ianni W.0. U6541115 Page 16 of 26 For proposed restrained walls (non-yielding), potential seismic loading should be considered. For smooth rigid walls, Wood (1973) expressed the dynamic thrust in the following fom1: ~Pe = kh YH2 (nonyielding) where kh is ½ peak ground acceleration equal to 50 percent of the design spectral response acceleration coefficient (Sds) divided by 2.5 per C.B.C. (2007), Y is equal to the unit weight of backfill, and H is equal to the height of the wall. The pressure diagram for this dynamic component can be approximated as an inverted trapezoid with stress decreasing with depth. The point of applicati on of the dynamic thrust is at a height of 0.6 above the base of the wall. The magnitude of the resultant is: ~Pe= 19.5 H2 (nonyielding) This dynamic component should be added to the at-rest static pressure for seismi c loading conditions. For cantilever walls (yielding), Seed and Whitman (1970) developed the dynamic thrust as: ~Pe = 3/8 kh YH2 (yielding) The pressure diagram for this dynamic component can be approximated as an inverted trapezoid with stress decreasing with depth and the resultant at a height of 0.6 above the base of the wall. The magnitude of the resultant is: ~Pe= 7.3 H2 (yielding) COAST GEOTECH, INC. Michael Jason Ianni W.O. U6541115 Page 17 of 26 This dynamic component should be added to the static pressure for seismic loading conditions. 7.8 Settlement Characteristics Estimated total and differential settlement over a horizontal distance of30 feet is expected to be on the order of 1.0 inch and ¾ inch, respectively. It should also be noted that long term secondary settlement due to irrigation and loads imposed by structures is anticipated to be ¼ inch. 7. 9 Seismic Considerations Although the likelihood of ground rupture on the site is remote, the prope1ty wi ll be exposed to moderate to high levels of ground motion resulting from the release of energy should an earthquake occur along the numerous known and unknown faults in the region. The Rose Canyon Fault Zone located approximately 4.3 miles west of the property is the nearest known active fault and is considered the design fault for the site. In addition to the Rose Canyon fault, several other active faults may affect the subject site. Seismic design parameters were determined as part of this investigation in accordance with Chapter 16, Section 1613 of the 2019 California Building Code (CBC) and ASCE 7-10 Standard using the web-based United States Geological Survey (USGS) Seismic Design Tool. The generated results for the parameters are presented on Table 2. COAST GEOTECH, INC. Table 2: Seismic Design Parameters Factors Site Class Seismic Design Category Site Coefficient, Fa Mapped Short Period Spectral Acceleration, S5 Mapped One-Period Spectral Acceleration, S1 Short Period Spectral Acceleration Adjusted for Site Class, SMs Design Short Period Spectral Acceleration, S05 7.10 Site Classification For Seismic Design Michael Jason Ianni W.O. U6541115 Page 18 of 26 Values D II 1.06 1.101 0.397 1.166 0.778 Site soils are classified based on the upper 100 feet maximum of site subsoil profile. In the absence of sufficient or specific site data, appropriate soil properties are permitted to be estimated by the project geotechnical consultant based on known geotechnical conditions and Site Class Dis typically used as a "default", unless otherwise noted. Site Classes A and B shall not be assigned to a site, if there is more than IO feet of soil ( or fill) between the top of the underlying rock surface and bottom of the foundation. Site Classes A and Bare most commonly supported by shear wave velocity determination (us, ft/s). Site Class F, which may require a site response analysis, consists of liquefiable or collapsible soils and highly sensitive clayey soil profile. Site Classes C, D, and E soils may be classified using an average field Standard Penetration Resistance (N) method for soil layers based on Section 20.4.2 of ASCE 7-16. Where refusal is met for a rock layer (blow counts of 50 or greater for 6 inches or less penetration), Ni is taken as I 00 blows per foot. Site Classification is then established based on Table 20.3-1 ofASCE7-16. COAST GEOTECH, INC. Michael Jason Ianni W.O. U6541115 Page 19 of 26 Requirements provided below are also applicable and should be incorporated in the project designs where appropriate: I. Site specific hazard analysis is required (see Section 11.4.8) in accordance with Chapter 21.2 of ASCE 7-1 6 for structures on Site Class E sites with values of Ss greater than or equal to 1.0g and structures on Site Class D and E sites with values of S 1 greater than or equal to 0.2g. However, the following 3 exceptions are permitted for Equivalent Lateral Force design (ELF) using conservative values of seismic design parameters in lieu of performing a site specific ground motion analysis: • Structures on Site Class E sites with Ss greater than or equal to 1.0, provided the site coefficient Fa is taken as equal to that of Site Class C. • For structures on Site Class D sites with S 1 greater than or equal to 0.2, a long period coefficient (Fv) of 1.7 may be utilized for calculation of Ts, provided that the value of Seismic Response Coefficient (Cs) is determined by Equation (12.8-2) for values of the fundamental period of the building (T) less than or equal to l .5Ts and taken as 1.5 times the value computed in accordance with either Equation 12.8-3 for T greater than 1.5 Ts and less than or equal to TL or Equation 12.8-4 for T greater than TL. Structures on Site Class E sites with S 1 greater than or equal to 0.2, provided that Tis less than or equal to Ts and the equivalent static force procedure is used for the design. 2. Where Site Class B is recommended and a site specific measurement is not provided, the site coefficients Fa, Fv, and FPGA shall be taken as unity (1.0) in accordance to Section 11.4.3 of ASCE 7-16. COAST GEOTECH, INC. Michael Jason Ianni W.O. U6541115 Page 20 of 26 3. Where Site Class Dis selected as the "default" site class per Section 11.4.3 of ASCE 7-16, the value of Fa shall not be less than 1.2. Where the simplified procedure of Section 12.4 is used, the value of Fa shall be determined in accordance with Section 12.14.8.1 and the values of Fv, SMS and SM I need not to be determined. At the project site, dense metavolcanic bedrock occurs beneath the site at shallow depths on the order of 2.0 to 6.0 feet below existing ground surfaces (BOS), and based on our past experience with similar deposits, Site Class D (Stiff Soil), can be conservatively considered for the project site subsoil profile, unless otherwise noted. 7.11 Preliminary Pavement Design The following pavement section is recommended for proposed driveways: 4.0 inches of asphaltic concrete on 6.0 inches of select base (Class 2) on 12 inches of compacted subgrade soils or 5.5 inches of concrete on 12 inches of compacted subgrade soils 7 .12 Permeable Interlocking Concrete Pavers (PICP) Permeable Interlocking Concrete Pavers (PICP), recommendations were provided in the Revised City of Carlsbad Response Report, dated July 25, 2016, and remain applicable. Impervious interlocking concrete pavers may be underlain by a minimum of 2.0 inches of clean sand (S.E. greater than 30) or per the manufacture's specifications. 7.13 Utility Trench We recommend that all utilities be bedded in clean sand to at least one foot above the top of the conduit. The bedding should be flooded in place to fill all the voids around the conduit. Imp011ed or on-site granular material compacted to at least 90 percent relative compaction may be utilized for backfill above the bedding. Where utilities penetrate footings, please see Figure 5. COAST GEOTECH, INC. Michael Jason Ianni W.O. U6541115 Page 21 of 26 The invert of subsurface utility excavations paralleling footings should be located above the zone of influence of these adjacent footings. This zone of influence is defined as the area below a 45 degree plane projected down from the nearest bottom edge of an adjacent footing. This can be accomplished by either deepening the footing, raising the invert elevation of the utility, or moving the utility or the footing away from one another. 7.14 Drainage Specific drainage patterns should be designated by the project architect or engineer. However, in general, pad water should be directed away from foundations and around the structure to the street. Roof water should be collected and conducted to a suitable discharge location via non-erodible devices. Pad water should not be allowed to pond. Vegetation adjacent to foundations should be avoided. If vegetation in these areas is desired, sealed planter boxes or drought resistant plants should be considered. Other alternative may be available, however, the intent is to reduce moisture from migrating into foundation subsoils. Irrigation should be limited to that amount necessary to sustain plant life. All drainage systems should be inspected and cleaned annually, prior to winter rams. Lateral mitigation of infiltrated water could adversely affect the adjacent residence. Deepening the retaining wall footing or providing a subsurface cutoff wall should be considered in this regard. It should be noted that the adjacent residence is located 3.0 lateral feet from the bio- retention basin on the southwest comer of the site. 7.15 Plan Review Foundation plans were not available for review at the time of this study. A copy of the final foundation plans should be submitted to this office for review prior to the initiation of constructions. Additional recommendations may be necessary at that time. 7.16 Geotechnical Observations/Testing 1) Observation of bottom excavations prior to placement of fill. 2) Density testing and observation during placement of fill. COAST GEOTECH, INC. Michael Jason Ianni W.O. U6541115 Page 22 of26 3) Laboratory testing for Maximum Dry Density and Optimum Moisture Content for each soil type used as fi 11. 4) Observation of subsurface slurry cut off wall excavation. 5) Observation of footing excavations prior to placement of steel. 6) Utility trench observation and testing. 7) Pavement subgrade observation and testing. 7. I 7 Previous Geotechnical Report All of the recommendations of the previous reports which are not superceded by this report remain applicable and should be implemented. 7. I 8 Response To Third-Party Review 1) All reports are signed and sealed by appropriate individuals. 2) Plans, grading, foundations and other pertinent data are addressed in this Updated Geotechnical Report and are consistent with the 2019 C.B.C. 3) The current grading plans have been reviewed and are in general conformance with the recommendations of the Preliminary Geotechnical Investigation Report, Supplemental Reports and this Updated Geotechnical Report. Foundation plans were not available for review at the time of this update report. 4) Geotechnical conditions, exploratory boring locations and other pe11inent data are plotted on the enclosed Geotechnical Map which is based on the current grading plan. 5) Detailed descriptions are provided in the Updated Geotechnical Report. 6) The geotechnical feasibility of the proposed development is addressed. COAST GEOTECH, INC. Michael Jason Ianni w.o. 0 6541115 Page 23 of 26 7) ASTM standards for laboratory testing are addressed in the Updated Geotechnical Report. 8) Hardscape recommendations are provided in the Updated Geotechnical Report. 9) A li st of recommended observations and testing is provided in the Updated Geo technical Report. I 0) Current maps, reports and codes have been referenced in the Updated Geotechnical Report. 8. LIMITATIONS This report is presented with the provision that it is the responsibility of the owner or the owner's representative to bring the information and recommendations given herein to the attention of the project's architects and/or engineers so that they may be incorporated into the plans. If condition encountered during construction appear to differ from those described in this report, our office should be notified so that we may consider whether modification are needed. No responsibility for construction compliance with design concepts, specifications, or recommendations given in this report is assumed unless on-site review is performed during the course of construction. The subsurface conditions, excavation characteristics, and geologic structure described herein are based on individual exploratory excavations made on the subject property. The subsurface conditions, excavation characteristics, and geologic structures discussed should in no way be construed to reflect any variations which may occur among the exploratory excavations. Please note that fluctuations in the level of groundwater may occur due to variations in rainfall, temperature, and other factors not evident at the time measurements were made and reported herein. Coast Geotech, Inc. assumes no responsibility for variations which may occur across the site. COAST GEOTECH, INC. Michael Jason Ianni W.O. U6541115 Page 24 of 26 The conclusions and recommendations of this report apply as of the current date. ln time, however, changes can occur on a property whether caused by acts of man or nature on this or adjoining properties. Additionally, chan ges in professional standards may be brought about by legislation or the expansion of knowledge. Consequently, the conclusions and recommendations of this report may be rendered wholly or partially invalid by event beyond our control. This report is therefore subject to review and should not be relied upon after the passage of two years. The professional judgements presented herein are founded pa11ly on om assessment of the technical data gathered, partly on our understanding of the proposed construction, and partly on our general experience in the geotechnical field. However, in no respect so we guarantee the outcome of the project. This study has been provided solely fo r the benefit of the client, and is in no way intended to benefit or extend any right or interest to any third party. Tru s party is not to be used on other projects or extensions to this project except by agreement in writing with Coast Geotech, Inc. COAST GEOTECH, INC. REFERENCES Michael Jason Ianni W.O. U6541115 Page 25 of 26 bHA, Inc. (2020). Grading Plan. Breakers View Beach Homes. 3648 Carlsbad Boulevard, Carlsbad, California. Blake, T. F. (2000). EQFAULT: A Computer Program for the Dete1ministic Estimation of Peak Acceleration using Three-Dimensional California Faults as Earthquake Sources, Version 3.0 , Thomas F. Blake Computer Services and Software, Thousand Oaks, CA. California Building Standards Commission. (January 1, 2019). 2019 California Building Code, Cali fornia Code of Regulations. California Geologic Survey, (2010), Fault Activity Map of California, Map Scale l "=750,000'. Coast Geotechnical, (February 11, 20 I 6). Preliminary Geotechnical Investigation: Proposed Twin Home Development, 3648 Carlsbad Boulevard, Carl sbad, California. Coast Geotechnical, (July 25, 20 16). Revised City of Carlsbad Response, Proposed Twin Home Development, 3648 Carlsbad Boulevard, Carlsbad, California. Coast Geotechnical, (May 16, 2018). Geotechnical Update Report, 3648 Carlsbad Boulevard, Carlsbad, California. Coast GeotechnicaJ, (August 20, 2018). Site Plan Review, Tentative Parcel Map, PUD 2018-0006 Breakers View Beach Homes, 3648 Carlsbad Boulevard, Carlsbad, California. COAST GEOTECH, INC. Michael Jason Ianni W.O. U6541115 Page 26 of 26 Hetherington Engineering, Inc. (October 26, 2020). Third-Party Geotechnical Review (First), Proposed Twin Home Development, 3648 Carlsbad Boulevard , Carlsbad, California, Project I.D. MS 2018-0006. Kennedy, M. P., and Tan, S.S. (2008). California Geological Survey, Regional Geologic Map No. 3, 1: 100,000 scale. OSHPD, U.S. Seismic Design Maps. https://seismicmaps.org Sahakian, V ., et al.(2017). Seismic Constraints on the Architecture of the Newport-Inglewood/Rose Canyon Fault: Implications for the Length and Magnitude of Future Earthquake Ruptures. American Geophysical Union (In progress). DOI: l0.1002/2016JB013467. Seed, H.B., and Whitman, R.V. (1970). Design of earth retaining structures for dynamic loads. In Proceedings of the ASCE Special Conference on Lateral Stresses, Ground Displacement and Earth Retaining Structure, Ithaca, N.Y., pp. 103-147. Tan, S.S., and Giffen, D.G., (1995), Landslide Hazards in the Northern Part of San Diego Metropolitan Area, San Diego County, Plate 35A, OFR 95-04, Map Scale 1 :24000. Treiman, J.A., (1984), The Rose Canyon Fault Zone: A Review and Analysis, CA Division of Mines and Geology. Wood, J.H. (1973). Earthquake-induced soil pressures on structures. Ph.D. thesis, the Californ ia Institute of Technology, Pasadena, Calif. United States Geological Survey, U. S. Seismic Design Maps, Scale = Variable. https://earthquake.usgs.gov/designmaps/us/application.php GEOTECHNICAL MAP 3648 CARLSB --N ARD, CARLSBAD, CA t ADBOULEV SCALE: I"= 20' I I -- af/Qs Qop o· A.pproximatc Lnnits ofG . 1radmg Geologic Co A ntact pproximatcd Proposed S1ruc1urc !oring Location pproxinrntcd Fill/Soil Old Paralic Deposits 20' \ Project Number-U654 Fi •u · 1115 g re Number: I REGIONAL FAULT MAP SAN DIEGO COUNTY REGION \ ' ' Holocen& '"°It d1~1 (dunno Poll 11.100 yea,sJ wrthout t1t,toncre«lf'd la,_ Ou.tanary laul ~ (durr41 P,HI 700,000 yean.) Ou&te~ry tati\ (age uncldferentialed} Pr&-Oulitemtuy faull (older thai 1 6 millon years) or f:wtt Wllhou1 recognized Ouatema,y displacement AOOITIONAL FAULT SYMBOLS Bar and btlD on oownuwown l40e (rela!Jve or ~,tnl) -:::::- Atrowi aJong foufl Indicate ,e-1a1rve or opp.!M'en1 c•ecaon ot Jate16' """""'""'· . , Low angle fault (barbs on uppet plate). COAST GEOTECH, INC. 5931 Sea Lion Place, Sui le I 09 Carlsbad, CA 920 I 0 Me xic.:i l1 Map is reproduced from Cali fornia Geological Survey, "Fault Acti vity Map of California (201 O)". Project Number: U6541l15 Figure Number: 2 FLOOD HAZARD MAP 3648 CARLSBAD BOULEVARD, CARLSBAD, CA PIN MAP PANELS ' Appr®mate location based on user in pin and doo not repres1cnt an a11tlloritati11e propertylocallion a Scl!N:t;sl Floodf,lap SOundary Di~ Data A\'a!al>le r:o Olg'rtal Data A\'aJable unmappi;t! ~ Area of Minimal Flood Hazard z~,, x 111---• • Effe.."liloet or,11,s Area of Undetermined Flood Hazard Z>o, o I,'' , ':' '. 4 Ottl.:mise Protected Area OTHER AREAS ~ coastal Barri;;r l<'esouroe System Area Exerpt from the FEMA Flod Map Service Center COAST GEOTECH, lNC. 5931 Sea Lion Place, Suire I 09 Carlsbad, CA 920 I 0 SPECIAL FLOOD HAZARD AREAS OTHER AREAS OF FLOOD HAZARD Without Base FloOd Elevation (BFE) Zor,4 ~ V.All!I With BFE or Deptn Regulatory FloOdway Z••· AE.AO. AH.~ AR 0.2% Annual Chance Flood Hazard. Areas of 1'K. annual chance flood with average depth less than one foot or with drainage areas of less than one square mjle =• ~ Fu1ure Conditions 1% Annual cnanre FloOd Hazard z ... x Area wJth Reduced Flood Risk due to Levee. See Notes. Zo11<1 x Area Wllh Flood Rtsk due to Lewe :ono o Project Number: U654 II l 5 Figure Number: 3 (a) (b) / ISOLATION JOINTS ~ CONTRACTION JOINTS -------- RE-ENTRANT CORNER ~ REINFORCEMENT NO. 3 BARS PLACED MID-HEIGHT IN SLAB ( c) I NO SCALE I ------RE-ENTRANT CORNER RACK 3" 1. Isolation Joints around the columns should be either circular as shown in (a) or diamond shaped as shown in (b). If no isolation joints are used around columns, or if the corners of the isolation j oints do not meet the contraction joints, radial cracking as shown in (c) may occur (reference AC!). 2. In order to control cracking at the re-entrant corners(+/-270 degree corners), provide reinforcement as shown in (c). 3. Re-entrant corner reinforcement shown herein is herein is provided as a general guideline only and is subject to verification and changes by the project architect and / or structural engineer based upon slab geometry, location, and other engineering and construction factors. COAST GEOTECH, INC. 593 1 Sea Lion Place. Suite I 09 Carlsbad, CA 92010 TYPICAL ISOLATION JOINTS AND RE-ENTRANT CORNER REINFORCEMENT Project Number: U6541 It 5 Figure Number: 4 d " " 4 " " 4 d LOCATE PIPE TRENCH SO THAT FOOTING WILL NOT BE UNDERMINED NO PIPE SHALL PASS THROUGH FOOTING PAD BACKFILL PER SPECIFJCATIONS "4 " " 4 " " d 4 d d " <1 d 4 " 4 " 4 e " k--18" MIN 4 2" MIN 4 " " d <1 8" MIN PLACE CONCRETE FILL AROUND SLEEVES BEFORE PLACING FOOTING PADS. FILL SHALL BE SAME AS FOOTING AND FULL PLACE CONCRETE FILL. I NO SCALE I PROVIDE METAL SLEEVES WlTH I.D. 2" GREATER THAN THE O.D. OF THE PIPE TYPICAL PIPE I TRENCH FOOTING DETAIL A SPECIFIC DESIGN SHOULD BE PREPARED BY TI-IE PROJECT STRUCTURAL ENG INEER COAST GEOTECH, INC. 5931 Sea Lion Place, Suite I 09 Carlsbad, CA 92010 Project Number: U65411 I 5 Figure Number: 5 APPENDIX A OS HPD 3648 Carlsbad Blvd, Carlsbad, CA 92008, USA Latitude, Longitude: 33.1500489, -117.3469344 Go gle Date Design Code Reference Document Risk Category Site Class Type Value Ss 1.101 S1 0.397 SMs 1.166 SM1 null -5 ee Section 11.4.8 Sos 0.778 501 null-See Section 11.4.8 Type Value soc null-See Section 11.4.8 Fa 1.06 Fv null -See Section 11.4.8 PGA 0.487 FPGA 1.113 PGAM 0.542 h 8 SsRT 1.101 SsUH 1.234 SsD 1.5 SlRT 0.397 SlUH 0.439 5 1D 0.6 PG Ad 0.606 CRs 0.892 CR1 0.904 \ TamarackD ' '\ t ' ~- \ ""' Surf Beach T\ \ "o,. \ ~-ie ,, \ (j~ ·" ' 'o~'o ft \ -<._'o~ T Carlsbad Vacation ' \ El an Tamarack ft \ Shores Apartments T ' \ ' ' \ \ ' \ Map data ©2020 Description 12/15/2020, 12:39:39 PM ASCE7-16 D · Stiff Soil MCE R ground motion. (for 0.2 second period) Description MCE R ground motion. (for l .Os period) Site-modified spectral acceleration value Site-modified spectral acceleration value Numeric seismic design value at 0.2 second SA Numeric seismic design value at 1.0 second SA Seismic design category S Ite amplification factor at 0.2 second Site amplification factor at 1.0 second MCE G peak ground acceleration S ~e amplification factor at PGA Site modified peak ground acceleration Long-period transition period in seconds Probabilistic risk-targeted ground motion. (0.2 second) Factored uniform-hazard (2% probability of exceedance in 50 years) spectral acceleration Factored deterministic acceleration value. (0.2 second) Probabilistic risk-targeted ground motion. (1.0 second) Factored uniform-hazard (2% probability of exceedance in 50 years) spectral acceleration. Factored deterministic acceleration value. (1.0 second) Factored deterministic acceleration value. (Peak Ground Acceleration) Mapped value of the risk coefficient at short periods Mapped value of the risk coefficient at a period of 1 s APPENDIXB GRADING GUIDELINES 1. General Grading should be performed to at least the minimum requirements of the local governin g agencies, the California Building Code, 2019, the geotechnical report and the guidelines presented below. All of the guidelines may not apply to a specific site and additional recommendations may be necessary during the grading phase. It is the responsibility of the Contractor to adhere to these specifications, grading plans, and the provision of equipment to carry our the grading methods. 2. Site Clearing and Preparatio11 Trees, dense vegetation, and other deleterious materials should be removed from the site. Removal shall consist of the complete removal of roots, stumps and other buried vegetation in the areas to be excavated. Non-organic debris or concrete may be placed in deeper fill areas under direction of the Soi ls engineer. 3. Treatment of Existi11g Ground All heavy vegetation, rubbish and other deleterious materials should be disposed of off site. • Materials used for compacted fill shall first be determined as suitable by the project engineer or geologist. All material with organics, rubbish or of other unsuitable composition might be considered unsuitable and shall not be used in fills. Any pavement material in the excavation area shall be properly cleared and di sposed of in an approved off-site faci lity or if feasible in an area on-site approved by the Project Engineer. All surficial deposits including alluvium and colluvium should be removed unless otherwise indicated in the text of this report. Groundwater existing in the alluvial areas may make excavation difficult. Deeper removals than indicated in the text of the report may be necessary due to saturation during winter months. Subsequent to removals, the natural ground should be scarified and processed to a depth of six inches, moistened to near optimum moisture conditions and compacted to fill standards. 4. Fill Placeme11t Fi 11 placement sha 11 be accomp I ished by compaction equipment such as sheepsfoot ro I lers, vibratory rollers and other types of equipment. The equipment should be able to properly and uniformly compact fill material to the specified relative compaction. Most site soil and bedrock may be reused for compacted fill; however, some special processing or handling may be required (see report). Highly organic or contaminated soil should not be used for compacted fill. Material used in the compacting process should be evenly spread, moisture conditioned, processed, and compacted in thin lifts not to exceed six inches in thickness to obtain a uniformly dense layer. The fill should be placed and compacted on a horizontal plane, unless otherwise found acceptable by the Soils Engineer. If the moisture content or relative density varies from that acceptable to the Soils engineer, the Contractor should rework the fill until it is in accordance with the following: I. Moisture content of the fill should be at or above optimum moisture as determined by ASTM D 1557. Moisture should be evenly distributed without wet and dry pockets. Pre-watering of cut or removal areas should be considered in addition to watering during ft 11 placement, particularly in clay or dry surficial soils. If the moisture content of the soil is below that of optimum, water shall be added by the Contractor until it reaches the specified range. 2. Each six inch layer should be compacted to at least 90 percent of the maximum density in compliance with the testing method specified by the controlling governmental agency. In this case, the testing method is ASTM Test Designation D-1557-91. • Side-hill fills should have a minimum equipment-w idth key at their toe excavated through all surficial soil and into competent material (see report) and ti lted back into the hill. As the fill is elevated, it should be benched through surficial deposits and into competent bedrock or other material deemed suitable by the Soils Engineer. Rock fragments less than six inches in diameter may be utilized in the fill, provided: I. They are not placed in concentrated pockets; 2. There is a sufficient percentage of fine-grained material to surround the rocks; 3. The distribution of the rocks is supervised by the Soi Is Engineer. Rocks greater than six inches in diameter should be taken off site, or placed in accordance with the recommendations of the Soils Engineer in areas designated as suitable for rock disposal. In clay soil large chunks or blocks are common; if in excess of six (6) inches minimum dimension then they are considered as oversized. Sheepsfoot compactors o r other suitable methods should be used to break the up blocks. The Contractor should be required to obtain a minimum relative compaction of90 percent out to the finished slope face of fill slopes. This may be achieved by either overbuilding the slope and cutting back to the compacted core, or by direct compaction of the slope face with suitable equipment. If fill slopes are built "at grade" using direct compaction methods then the slope construction should be performed so that a constant gradient is maintained throughout construction. Soil should not be "spilled" over the slope face nor should slopes be "pushed out" to obtain grades. Compaction equipment should compact each lift along the immediate top of slope. Slopes should be back rolled approximately every 4 feet vertically as the slope is built. Density tests should be taken periodically during grading on the flat surface of the fill three to five feet horizonta lly from the face of the slope. In addition, if a method other than over building and cutting back to the compacted core is to be employed, slope compaction testing during construction should include testing the outer six inches to three feet in the slope face to determine if the required compaction is being achieved. Finish grade testing of the slope should be performed after construction is complete. Each day the Contractor should receive a copy of the "Daily Field Report" which would indicate the results of field density tests that day or as verbally commun icated to the contractor. • Fill over cut slopes should be constructed in the fo llowing manner: I. All surficial soils and weathered rock materials should be removed at the cut-fill interface. 2. A key at least I equipment width wide (see report) and tipped at least I foot into slope should be excavated into competent materials and observed by the Soils Engineer or his representative. 3. The cut portion of the slope should be constructed prior to fil l placement to evaluate if stabilization is necessary, the contractor should be responsible for any additional earthwork created by placing fill prior to cut excavation. • Transition lots ( cut and ft 11) and lots above stabi I ization ft lls shou Id be capped with a fo ur foot thick compacted fil l blanket (or as indicated in the report). • Cut pads should be observed by the Geologist to evaluate the need for overexcavation and replacement with fil l. This may be necessary to reduce water infiltration into highly fractured bedrock or other permeable zones, and/or due to differing expansive potential of materials beneath a structure. The overexcavation should be at least three feet. Deeper overexcavation may be recommended in some cases. Exploratory backhoe or dozer trenches still remaining a~er site removal should be excavated and filled with compacted fill if they can be located. 5. Subdrainage During grading, the Geologist and Soils Engineer should evaluate the necessity of pl ac ing additional drains. Actual locations will be evaluated in the fie ld. Depending on the soil and geologic cond itions, add itional subdrains may be necessary upon the discretion of the Geologist and Soils Engineer and per the local requ irements. Al I subdrainage systems should be observed by the Geologist and Soils Engineer during construction and prior to covering with compacted fill. Consideration should be given to having subdrains located by the project surveyors. Outlets should be located and protected. The final grading plans should denote the locations of the subd ra ins. It is the contractor's responsibility that th e subdrains are installed properly and perform as expected. 6. Utility Trench Backfill Uti lity trench backfill should be placed to the fol lowing standards: • Ninety percent of the laboratory standard if native material is used as backfill. As an alternative, clean sand may be utilized and flooded into place. No specific relative compaction would be required; however, observation, probing, and if deemed necessary, testing may be required. Exterior trenches, paralleling a footing and extending below a I: I plane projected from the outside bottom edge of the footing, should be compacted to 90 percent of the laboratory standard. Sand backfill, un less it is similar to the in place fi ll, should not be allowed in these trench-backfill areas. • Density testing along with probing should be accomplished to verify the desired results. 7. Grading Observation and Testing Observation of the fi ll placement should be provided by the Soils Engineer during the progress of grading. In general, density tests would be made at intervals not exceeding two feet of fi ll height or every 1,000 cubic yards of fi ll placed. This criteria will vary depending on soil conditions and the size of the fill. In any event, an adequate number of field density tests should be made to evaluate if the required compaction and mo isture content is generally being obtained. • Density tests may be made on the surface material to receive fi ll, as required by the Soils Engineer. All field testing should conform to the following ASTM Standards: I. ASTM D 1556, Density of Soil In-Place By the Sand-Cone Method 2. ASTM 06938, Density of Soil and Soil Aggregate In-Place by Nuclear Methods (Sha llow Depth) • All lab testing shall conform to the following ASTM Standards: I. ASTM D 1557, Moisture Density Relations of Soils and Soil-Aggregate Mixtures Using I 0- Pound Hammer and 18-lnch Drop 2. ASTM 04829, Expansion Index Test Cleanouts, processed ground to receive fill, key excavations, subdrains and rock disposal should be observed by the Soils Engineer prior to placing any fi ll. It will be the Contractor's responsibili ty to notify the Soil s Engineer when such areas are ready for observation. A Geologist shoul d observe subdrain construction. A Geologist should observe benching prior to and during placement of fill. All original existing slopes that have a gradient greater than 5: I (Horizontal:Vertical), the slope should be properly benched in accordance with the report.