IV. ENVIRONMENTAL IMPACT ANALYSIS E. GEOLOGY/SOILS

Transcription

1 IV. ENVIRONMENTAL IMPACT ANALYSIS E. GEOLOGY/SOILS INTRODUCTION The fllwing sectin is a summary f the getechnical reprt cnducted fr the prpsed prject. The Preliminary Getechnical Reprt, Prpsed High Rise Residential Develpment, 6230 Yucca Street, Hllywd, Califrnia, (the Getechnical Reprt ) was prepared by Grup Delta Cnsultants, Inc., dated Nvember 17, A cpy f this reprt can be fund as Appendix F t this Draft EIR. ENVIRONMENTAL SETTING The prject site is lcated at the suthwest crner f Yucca Street and Argyle Avenue in the Hllywd cmmunity f the City f Ls Angeles. The prject site is currently develped with a vacant 18,614 square ft radi statin facility. The prpsed prject wuld invlve the remval f the existing uses and the cnstructin f a mixed-use prject cnsisting f 10 live/wrk units, 85 cndminium units, and apprximately 13,790 square feet f ffice space. Reginal and Lcal Gelgy The prject site is lcated in the nrthwestern area f the Central Blck f the Ls Angeles Basin, brdered n the nrth by the Santa Mnica Muntains, n the nrtheast by the Elysian Hills, n the suth by the Baldwin Hills and Central Plain, and n the west by Beverly Hills. The prject site is underlain by native Quaternary alluvial sils, cnsisting f yung Hlcene fan depsits ver lder Pleistcene fan and terrace depsits. The Hlcene depsits are primarily cmprised f interbedded layers f sand, silt and clay, which generally tend t be lse t medium dense. These yunger alluvial depsits are estimated t be abut 10 t 15 feet thick in the prject area, but culd vary in thickness and culd extend deeper. Belw the yung fan sediments are depsits f Pleistcene lder fan and terrace depsits, which cnsist primarily f denser layers f clay and silt. Seismic Cnditins The entire Suthern Califrnia area is cnsidered a seismically active regin. Based n criteria established by the Califrnia Gelgical Survey (CGS), faults may be categrized as active, ptentially active, r inactive. Active faults are thse that shw evidence f surface displacement within the last 11,000 years (Hlcene age). Ptentially active faults are thse that shw evidence f the last displacement within the last 1.6 millin years (Quaternary age). Faults shwing n evidence f displacement within the last 1.6 millin years may be cnsidered inactive fr mst purpses, except fr sme critical structures. In 1972, the Alquist-Pril Special Studies Zne Act (nw knwn as the Alquist-Pril Earthquake Fault Zning Act) was passed int law. The Act defines active and ptentially active faults utilizing the same age criteria as that used by the CGS. Hwever, the established plicy is t zne active faults and Yucca Street Cnds Draft Envirnmental Impact Reprt IV.E. Gelgy/Sils Page IV.E-1

2 City f Ls Angeles April 2007 nly thse ptentially active faults that have a relatively high ptential fr grund rupture. Therefre, nt all faults identified as ptentially active by the CGS are zned under the Alquist-Pril Act. The clsest active fault is the Hllywd Fault, which is lcated abut 0.3 miles nrth f the Prject site and is capable f generating a magnitude 6.4 earthquake. The Upper Elysian Park and Puente Hills blind thrust faults are lcated belw the prject site at depths f 1.8 and 3.5 miles, respectively. The Newprt- Inglewd Fault Zne and Santa Mnica Fault are lcated at a distance f 5.5 and 5.7 miles frm the prject site and are cnsidered capable f generating magnitude 7.1 and 6.6 earthquakes, respectively. Subsurface Cnditins Subsurface explratin was cnducted fr the Getechnical Reprt, which cnsisted f tw brings, drilled t depths f apprximately 60 feet belw the existing grund surface (bgs). A detailed discussin f the field investigatin, including bring lgs, is included in the appendix t the Getechnical Reprt (included as Appendix F t this Draft EIR). Bth f the brings encuntered generally similar sil cnditins. The existing pavement is generally tw inches thick with tw inches f aggregate base. The brings encuntered pssible fill that may be up t six feet deep. The fill cnsisted f generally lse t medium dense silty sand, with scattered gravel. It shuld als be anticipated that ld fill culd be present anywhere n the site and culd be lcally deeper. There is als the ptential fr the remnants f previus cnstructin t be encuntered anywhere n the site, including fundatins, slabs, basements, pits, buried tanks, cesspls, and utilities. Belw any fill, the brings encuntered native alluvial sils cnsisting f interbedded layers f sand, silt, and clay. The sandy layers were generally medium dense t dense, and the clayey layers were very stiff. Grundwater Brings B-1 and B-2 encuntered grundwater at depths f 24 and 44 feet, respectively, at the time f drilling. Based n the Califrnia Divisin f Mines and Gelgy (CDMG) Open File Reprt , the histric highest grundwater in the site area is reprted at a depth f mre than 80 feet. It is believed that the grundwater fund in the brings is a perched cnditin, where the water is perched n tp f clay layers. The site is lcated alng the te f hills present t the nrth and is underlain with interbedded alluvial sils. With such a gelgic envirnment, it is nt unusual fr perched grundwater levels t vary significantly ver relatively shrt hrizntal distances. Liquefactin Liquefactin invlves the sudden lss f strength in saturated, chesinless sils that are subjected t grund vibratin and which results in temprary transfrmatin f the sil int a fluid mass. If the liquefying layer is near the surface, the effects are much like that f quicksand fr any structures lcated n tp f it. If the layer is deeper in the subsurface, it may prvide a sliding surface fr the material abve it. The effects f liquefactin include the lss f the sil s ability t supprt ftings and fundatins which may cause buildings and fundatins t buckle. Yucca Street Cnds Draft Envirnmental Impact Reprt IV.E. Gelgy/Sils Page IV.E-2

3 City f Ls Angeles April 2007 ENVIRONMENTAL IMPACTS Threshlds f Significance In accrdance with Appendix G t the State CEQA Guidelines, a prject culd have a ptentially significant gelgy and sils impact if it were t cause ne r mre f the fllwing cnditins: a) Expse peple r structures t ptential substantial adverse effects, including the risk f lss, injury, r death invlving: i) Rupture f a knwn earthquake fault, as delineated n the mst recent Alquist-Pril Earthquake Fault Zning Map r based n ther substantial evidence f a knwn fault; ii) Strng seismic grund shaking; iii) Seismic-related grund failure, including liquefactin; iv) Landslides; b) Result in substantial sil ersin r the lss f tpsil; c) Be lcated n a gelgic unit r sil that is unstable, r that wuld becme unstable as a result f the prject, and ptentially result in n- r ff-site landslide, lateral spreading, subsidence, liquefactin, r cllapse; d) Be lcated n expansive sil, as defined in Table 18-1-B f the Unifrm Building Cde (1994), creating substantial risks t life r prperty; r e) Have sils incapable f adequately supprting the use f septic tanks r alternative waste water dispsal systems where sewers are nt available fr the dispsal f waste water. Based n the City f Ls Angeles Draft L.A. CEQA Threshlds Guide, the prpsed prject wuld als result in a significant getechnical impact if it exceeds the fllwing threshld: (a) A prject wuld nrmally have a significant gelgic hazard impact if it wuld cause r accelerate gelgic hazards which wuld result in substantial damage t structures r infrastructure, r expse peple t substantial risk f injury. Yucca Street Cnds Draft Envirnmental Impact Reprt IV.E. Gelgy/Sils Page IV.E-3

4 City f Ls Angeles April 2007 As discussed in the Initial Study (included as Appendix A t this Draft EIR), the prpsed prject wuld have n impact with respect t Threshld (e) listed abve, under the Appendix G Guidelines. As such, n further analysis f this tpic is required. Prject Impacts Seismic Hazards Grund Surface Rupture The prject site is lcated in the seismically active regin f Suthern Califrnia. Hwever, the prject site is nt lcated within an Alquist-Pril Earthquake Fault Zne. N knwn active faults are mapped as crssing the prject site r prjecting twards the prject site. The clsest active fault is the Hllywd Fault, which is lcated at a distance f abut 0.3 miles frm the prject site and is capable f generating a magnitude 6.4 earthquake. The Upper Elysian Park and Puente Hills blind thrust faults are lcated belw the prject site at depths f 1.8 and 3.5 miles, respectively. Nne f these faults pse a surface rupture ptential t the prject site. On this basis, grund rupture due t faulting is nt cnsidered a significant hazard at the prject site. Grund Shaking The principal seismic hazard t the prpsed prject is strng grund shaking frm earthquakes prduced by lcal faults. Mdern, well-cnstructed buildings are designed t resist grund shaking thrugh the use f shear walls and reinfrcements. The prpsed cnstructin wuld be cnsistent with all applicable prvisins f the City f Ls Angeles Building Cde, as well as the seismic design criteria cntained within the Unifrm Building Cde. Althugh the prject site is lcated within 0.3 miles f the active Hllywd Fault, and by many ther faults n a reginal level, the ptential seismic hazard t the prject site wuld nt be higher than in mst areas f the City f Ls Angeles r elsewhere in the regin. As the entire Suthern Califrnia area is cnsidered a seismically active regin, every building in the regin is susceptible t grundshaking and earthquakes. The City f Ls Angeles Building Cde includes regulatins and requirements designed t reduce risks t life and prperty t the maximum extent feasible. Therefre, the risks frm seismic grund shaking are cnsidered t be less than significant. Liquefactin Liquefactin invlves the sudden lss in strength f a saturated, chesinless sil (predminantly sand) caused by the build-up f pre water pressure during cyclic lading, such as prduced by an earthquake. This increase in pre water pressure can temprarily transfrm the sil int a fluid mass, resulting in vertical settlement and can als cause lateral grund defrmatins. Typically, liquefactin ccurs in areas where there are lse sands and the depth t grundwater is less than 50 feet frm the surface. Seismic shaking can als cause sil cmpactin and grund settlement withut liquefactin ccurring, including settlement f dry sands abve the water table. Yucca Street Cnds Draft Envirnmental Impact Reprt IV.E. Gelgy/Sils Page IV.E-4

5 City f Ls Angeles April 2007 The site is nt lcated within a State f Califrnia Seismic Hazard Liquefactin Zne. Hwever, the prject site des fall within an area identified in the City f Ls Angeles Safety Element as being susceptible t liquefactin (being underlain with recent alluvial depsits and grundwater at less than 30 feet deep). The subsurface sil prfile at the site cnsists f interbedded layers f sand, silt, and clay. The sandy layers were generally medium dense t dense and the clay layers were very stiff t hard. A preliminary evaluatin f liquefactin ptential at the prject site was perfrmed as part f the getechnical investigatin. The results f the analysis indicate that sme liquefactin is pssible within the varius sandy layers underlying the site. Therefre, with respect t liquefactin a ptentially significant impact wuld ccur. Landslides The tpgraphy f the prject site is relatively flat and the site is nt lcated near any fthills r muntains, meaning that the pssibility f landslides ccurring n the prject site is minimal. Therefre, the ptential impact assciated with landslides wuld be less than significant. Ersin and Tpsil Althugh prject develpment has the ptential t result in the ersin f sil during site preparatin and cnstructin activities, ersin wuld be reduced by implementatin f apprpriate ersin cntrls during grading. Minr amunts f ersin and siltatin culd ccur during prject grading, which wuld be cllected in a cntrlled manner. Additinally, the ptential fr sil ersin during the peratin f the prpsed prject is lw due t the generally level tpgraphy f the area and the fully develped aspects f the prject site at the cmpletin f build-ut. All grading activities require grading permits frm the Department f Building and Safety, which include requirements and standards designed t limit ptential impacts t acceptable levels, and all grading shuld als cnfrm t the requirements f the City f Ls Angeles Grading Divisin. In additin, all nsite grading and site preparatin wuld cmply with applicable prvisins f Chapter IX, Divisin 70 f the Ls Angeles Municipal Cde which addresses grading, excavatins, and fills. With implementatin f the applicable grading and building permit requirements and the applicatin f Best Management Practices, a less than significant impact wuld ccur with respect t ersin r lss f tpsil. Expansive Sils Expansive sils are typically assciated with fine-grained clayey sils that have the ptential t shrink and swell with repeated changes in the misture cntent. A representative sample f the upper 20 feet f sil was tested t identify its expansive characteristics. The test result indicated that the tested sil has a very lw expansin ptential. With cnstructin f the prpsed prject in accrdance with the Ls Angeles Building Cde Chapter IX, a less than significant impact assciated with expansive sils wuld ccur. CUMULATIVE IMPACTS Getechnical impacts related t future develpment in the City wuld invlve hazards related t sitespecific sil cnditins, ersin, and grund shaking during earthquakes. These impacts wuld be site- Yucca Street Cnds Draft Envirnmental Impact Reprt IV.E. Gelgy/Sils Page IV.E-5

6 City f Ls Angeles April 2007 specific and wuld nt be cmmn t nr shared with the impacts n ther sites. Furthermre, develpment f each f the related prjects and the prpsed prject wuld be subject t unifrm site develpment and cnstructin standards that are designed t prtect public safety. Therefre, cumulative getechnical impacts wuld be less than significant. MITIGATION MEASURES The fllwing mitigatin measures are recmmended fr the prpsed prject: E-1 Further evaluatin f the ptential fr liquefactin t ccur at the prject site during strng grund shaking shall be perfrmed as part f the design-level getechnical investigatin, using data develped frm Cne Penetratin Test (CPT) prbes, which shuld be perfrmed in additin t additinal brings. E-2 The prject shall cmply with the recmmendatins listed n pages 6 thrugh 13 in the Preliminary Getechnical Reprt, Prpsed High Rise Residential Develpment, 6230 Yucca Street, Hllywd, Califrnia, (which is incrprated herein by reference), prepared by Grup Delta Cnsultants, Inc., dated Nvember 17, Nn-design related structural methds are presented belw. The reprt cntains additinal specific design requirements that the cntractr must implement: Earthwrk All grading shuld als cnfrm t the requirements f the City f Ls Angeles Grading Divisin and the fllwing general grading recmmendatins: The grading cntractr is respnsible fr ntifying the prject getechnical engineer f a pre-grading meeting prir t the start f grading peratins and anytime the peratins are resumed after an interruptin. Prir t the start f earthwrk the existing imprvements will require demlitin, as discussed in Sectin 5.2. Existing utilities shuld be remved, relcated r prtected, as apprpriate. As discussed in Sectin 5.3, any existing fill is uncertified, but will be remved during the planned basement excavatin. The sides f the basement excavatin will require shring with ne t tw rws f tie-back anchrs, as discussed in Sectin 5.5. The bttm f the cmpleted excavatin shuld be bserved by the prject getechnical engineer, while it is prfrlled with laded equipment. Any lse r yielding sils shuld be verexcavated and recmpacted t the limits determined by the prject getechnical engineer. The bttm f the excavatin shuld then be scarified t a depth f 6 inches, misture cnditined between 0 t 2 percent wet f the ptimum misture Yucca Street Cnds Draft Envirnmental Impact Reprt IV.E. Gelgy/Sils Page IV.E-6

7 City f Ls Angeles April 2007 cntent, and cmpacted t at least 95 percent relative cmpactin as determined by ASTM Test D1557. Any fill placed under structures r pavement and any backfill placed adjacent t buried walls is defined as structural fill. All structural fill shuld cnsist f predminantly sandy sils and shuld be free f expansive clay, rck greater than 3 inches in maximum size, debris and ther deleterius materials. All structural fill shuld be cmpacted t at least 95 percent f the maximum dry density determined bye ASTM D1557. Fill placed in nnstructural and landscape areas shuld be cmpacted t at least 90 percent. In general, the sandy layers encuntered in ur brings may be used as structural fill. Hwever, the clayey and silty sils encuntered in ur explratins will nt be acceptable fr reuse as fill r backfill. All fill sls shall be apprved by the prject getechnical engineer. All earthwrk and grading shuld be perfrmed under the bservatin f the prject getechnical engineer. Cmpactin testing f the fill sils shall be perfrmed at the discretin f the prject getechnical engineer. Testing shuld be perfrmed fr apprximately every 2 feet in fill thickness r 500 cubic yards f fill placed, whichever ccurs first. If specified cmpactin is nt achieved, additinal cmpactive effrt, misture cnditining, and/r remval and recmpactin f the fills sils will be required. All materials used fr asphalt cncrete and base shall cnfrm t the 2006 Green Bk r the equivalent, and shall be cmpacted t at least 95 percent relative cmpactin. If, in the pinin f the getechincal engineer, cntractr, r wner, an unsafe cnditin is created r encuntered during grading, all wrk in the area shall be stpped until measures can be taken t mitigate the unsafe cnditin. An unsafe cnditin shall be cnsidered any cnditin that creates a danger t wrkers, n-site structures, n-site cnstructin, r any ff-site prperties r persns. Temprary Excavatins - General All excavatin slpes shuld meet the minimum requirements f the Occupatinal Safety and Health Assciatin (OSHA) Standards. Maintaining safe and stable slpes n excavatins is the respnsibility f the cntractr and will depend n the nature f the sils and grundwater cnditins encuntered and his methd f excavatin. Excavatins during cnstructin shuld be carried ut in such a manner that failure r grund mvement will nt ccur. Temprary Excavatins Dewatering Yucca Street Cnds Draft Envirnmental Impact Reprt IV.E. Gelgy/Sils Page IV.E-7

8 City f Ls Angeles April 2007 During cnstructin, it is anticipated that grund water can be cntrlled using shallw trenches, sumps and pumps. T prvide further definitin f the grund water cnditins at the site, ne r mre mnitring wells are recmmended t be installed during the design-level getechnical investigatin. If it is necessary t dispse f water during cnstructin, a discharge permit will be required frm the Reginal Water Quality Review Bard. This will require testing f the grund water fr cntaminants, and shuld be planned fr the prject schedule. Temprary Excavatins Shring A sldier pile and tied-back shring system will be required t prtect adjacent prperty and streets. The design f the shring system will be the respnsibility f the shring designer. Since the tied-back anchrs will extend ffsite, apprval will be required frm the City the adjacent prperty wners. This shuld be planned fr in the prject schedule. The design f the sldier piles and anchrs are specified in the Preliminary Getechnical Reprt. Temprary Excavatins Mnitring f Shring A survey-mnitring prgram shuld be implemented t mnitr shring displacements during cnstructin. In additin, nearby imprvements shuld als be surveyed and phtgraphs and/r vide taken t dcument baseline cnditins. The deflectin at the tp f the shring shuld be limited t 1 inch. If the shring exceeds 1 inch r if distress r settlement is nted adjacent t the tp f shring, an evaluatin shuld be perfrmed and crrective measures taken. E-3 All structural elements shall be designed and built t resist strng grund mtins in accrdance with the requirements f the City f Ls Angeles Building Cde and the Califrnia Building Standards Cde. These measures shall be reviewed and apprved by the City f Ls Angeles Department f Building and Safety prir t issuance f building permits. Grading E-4 All grading activities require grading permits frm the Department f Building and Safety which include requirements and standards designed t limit ptential impacts t acceptable levels. Chapter IX, Divisin 70 f the Ls Angeles Municipal Cde addresses grading, excavatins, and fills. Additinal prvisins are required fr grading activities within Yucca Street Cnds Draft Envirnmental Impact Reprt IV.E. Gelgy/Sils Page IV.E-8

9 City f Ls Angeles April 2007 Hillside areas. The applicatin f BMPs includes but is nt limited t the fllwing mitigatin measures: Excavatin and grading activities shall be scheduled during dry weather perids. If grading ccurs during the rainy seasn (Octber 15 thrugh April 1), diversin dikes shall be cnstructed t channel runff arund the site. Channels shall be lined with grass r rughened pavement t reduce runff velcity. Apprpriate ersin cntrl and drainage devices shall be prvided t the satisfactin f the Building and Safety Department. These measures include interceptr terraces, berms, vee-channels, and inlet and utlet structures, as specified by Sectin f the Building Cde, including planting fast-grwing annual and perennial grasses in areas where cnstructin is nt immediately planned. Stckpiles and excavated sil shall be cvered with secured tarps r plastic sheeting. General Cnstructin E-5 Where truck traffic is frequent, gravel appraches shall be used t reduce sil cmpactin and limit the tracking f sediment int streets. LEVEL OF SIGNIFICANCE AFTER MITIGATION A ptentially significant impact with respect t liquefactin wuld ccur. With implementatin f Mitigatin Measure E-1, impacts with respect t liquefactin wuld be less than significant. With implementatin f recmmended Mitigatin Measures E-2 thrugh E-5, the prpsed prject wuld have a less than significant impact with respect t getechnical issues. Yucca Street Cnds Draft Envirnmental Impact Reprt IV.E. Gelgy/Sils Page IV.E-9