GEOLOGIC SUBSURFACE CONDITIONS. October 31, 2017 Project No

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1 October 31, 2017 Project No Ms. Shawna Anderson San Dieguito River Park Joint Powers Authority Sycamore Creek Road Escondido, California Subject: Addendum to Geotechnical Evaluation Proposed Maintenance Facility San Dieguito River Park San Diego, California Dear Ms. Anderson: In accordance with your request, we have prepared this addendum letter to our original project geotechnical report (Ninyo & Moore, 2016) for the proposed Maintenance Facility at the San Dieguito River Park in San Diego, California (Project No ). Specifically, this addendum has been prepared to address the City of San Diego Geology Department s review comment requesting further evaluation of the potential for liquefaction. The City Cycle Issues, dated August 10, 2017, Comment #3 states The project site is located in Geologic Hazard Category 32 as shown on the City s Seismic Safety Study Geologic Hazards Maps and is characterized by a potential for liquefaction. Submit an addendum geotechnical investigation report that addresses liquefaction potential and potential consequences of soil liquefaction on the proposed project as required by San Diego Municipal Code Accordingly, this addendum letter provides supplemental information regarding the potential for liquefaction at the site and our additional recommendations. The recommendations presented below are considered supplemental, and the findings and recommendations provided in the referenced geotechnical report (Ninyo & Moore, 2016) remain valid. GEOLOGIC SUBSURFACE CONDITIONS Based on information provided in our original project geotechnical report (Ninyo & Moore, 2016), the proposed maintenance building location is situated in an area that was previously graded under observation and testing services performed by CTE (CTE, 2009). Furthermore, the compacted fill soil beneath the proposed maintenance building location is underlain by alluvial soils. In addition to the subsurface soil information presented in our original project geotechnical report (Ninyo & Moore, 2016), this supplemental analysis includes soil information from the 5710 Ruffin Road San Diego, California p

2 referenced Geocon (2017) report. As part of the currently proposed Via De La Valle widening project, which bounds the subject project site on the north, a geotechnical evaluation was prepared by Geocon (2017). For the roadway widening project, Geocon (2017) performed exploratory soil boring SM-7 at a location south of Via De La Valle, approximately 250 feet northeast of the proposed maintenance building location. Boring SM-7 extends through the surficial compacted fill soils and the underlying alluvial deposits and extends into materials referred to as the Delmar Formation. The subsurface soil information from this boring has been incorporated into our liquefaction analysis, and a copy of that boring log is presented in Attachment A. Generalized descriptions of the earth units encountered during the subsurface exploration for the original project geotechnical report (Ninyo & Moore, 2016) and the subsurface information encountered within boring SM-7 (Geocon, 2017) are provided in the subsequent sections. Additional descriptions of the subsurface units encountered within boring SM-7 (Geocon, 2017) are presented in Attachment A. Compacted Fill As described in our original project geotechnical report (Ninyo & Moore, 2016), compacted fill was encountered in our borings (B-1 through B-3) at the ground surface and extending to depths of up to approximately 6½ feet below the existing surface grades. The compacted fill materials generally consisted of brown and gray, moist, loose to dense, silty sand with gravel and cobbles, and silty gravel. Scattered concrete/construction debris was also encountered in the fill. As noted, we understand that the observation and testing of the previously placed compacted fill was performed by others (CTE, 2009) during grading of the site. Alluvium Alluvium was encountered in our borings B-1 through B-3 below the compacted fill soils and extended to the depths explored in the borings (Ninyo & Moore, 2016). Additionally, similar alluvial soils were also encountered in boring SM-7 underlying the compacted fill and extending to a depth of approximately 35 feet (Geocon, 2017). The alluvial materials encountered generally consisted of various shades of brown, moist to saturated, very loose to medium dense, silty sand and sand with silt. Ninyo & Moore San Dieguito River Park, San Diego, California October 31,

3 Delmar Formation As presented on the log for boring SM-7 (Geocon, 2017), materials of the Delmar Formation were encountered underlying the alluvium. As encountered, the Delmar Formation generally consisted of olive and light brown, damp to wet, medium dense to very dense, fine-grained sandstone and sandstone with silt. Groundwater During the subsurface exploration for the original project geotechnical report (Ninyo & Moore, 2016), groundwater at the proposed maintenance building site was encountered in our borings B-1 and B-2 at depths of approximately 17 and 15 ½ feet, respectively. These depths correspond to a groundwater elevation of approximately 7 feet above mean sea level (MSL). Fluctuations in the groundwater level and perched conditions may occur due to variations in ground surface topography, subsurface geologic conditions and structure, rainfall, irrigation, water elevation in the nearby lagoon, and other factors. Liquefaction and Seismically Induced Settlement Liquefaction is the phenomenon in which loosely deposited granular soils (with silt and clay contents of less than approximately 35 percent) and non-plastic silts located below the water table undergo rapid loss of shear strength when subjected to strong earthquake-induced ground shaking. Ground shaking of sufficient duration results in the loss of grain-to-grain contact due to a rapid rise in pore water pressure and causes the soil to behave as a fluid for a short period of time. Liquefaction is known generally to occur in saturated or near-saturated cohesionless soils at depths shallower than 50 feet below the ground surface. Factors known to influence liquefaction potential include composition and thickness of soil layers, grain size, relative density, groundwater level, degree of saturation, and both intensity and duration of ground shaking. Our subsurface exploration (Ninyo & Moore, 2016) and information provided on boring SM-7 (Geocon, 2017) indicates that the site is underlain by soils that are susceptible to liquefaction during a nearby seismic event. Accordingly, the liquefaction potential of subsurface soils was evaluated using the soil sampler blow counts recorded at various depths in our exploratory borings (B-1 through B-3), boring SM-7 (Geocon, 2017), and our laboratory test results. The liquefaction analysis was based on the National Center for Earthquake Engineering Research (NCEER) procedure (Youd, et al., 2001) developed from the methods originally recommended by Seed and Idriss (1982) using the computer program LiquefyPro (CivilTech Software, 2007). For purposes of our evaluation, groundwater was assumed to be at a depth of 7 feet above mean sea level (MSL). Ninyo & Moore San Dieguito River Park, San Diego, California October 31,

4 Our liquefaction analysis indicates that portions of the alluvial deposits encountered below the assumed groundwater level in the borings referenced for this analysis are generally susceptible to liquefaction during the design seismic event. The controlling magnitude for liquefaction analyses was a moment magnitude 6.9 that corresponds to the magnitude of a likely event on the nearby Rose Canyon fault zone (Del Mar Section) with a peak ground acceleration of 0.48g. Due to the very loose to medium dense nature of the alluvial deposits underlying the site and based on our evaluation and previous nearby evaluations, the potential for liquefaction is considered high up to a depth of approximately 35 feet below the ground surface. As a result of liquefaction, the proposed structure may be subject to several hazards, including liquefaction-induced settlement. In order to estimate the amount of post-earthquake settlement, the method proposed by Ishihara and Yoshimine (1992) was chosen for the evaluation of dynamic settlement. The amount of soil settlement during a strong seismic event depends on the thickness of the liquefiable layers and the density and/or consistency of the soils. Under the current conditions, post-earthquake total settlement of up to approximately 3 inches was calculated for the site using the computer project LiquefyPro (CivilTech Software, 2007). Lateral Spreading Lateral spread of the ground surface during an earthquake usually takes place along weak shear zones that have formed within a liquefiable soil layer. Lateral spread has generally been observed to take place in the direction of a free-face (i.e., retaining wall, slope, channel, etc.) but has also been observed to a lesser extent on ground surfaces with very gentle slopes. An empirical model developed by Youd et al. (2002) is typically used to predict the amount of horizontal ground displacement within a site. For sites located in proximity to a free-face, the amount of lateral ground displacement is correlated with the distance of the site from the free-face. Other factors such as earthquake magnitude, distance from the causative fault, thickness of the liquefiable layers, and the fines content and particle sizes of the liquefiable layers also influence the amount of lateral ground displacement. The project site gently slopes to the southeast. Our liquefaction analysis indicates that layers of alluvium and old paralic deposits with corrected standard penetration test (SPT) sampler blow counts of less than 15 (i.e., generally susceptible to seismically induced lateral spread) are present below the site. Based on the empirical model Youd and Bartlett (2002), we estimate that lateral spreading on the order of 1 foot or more may occur at the project location due to the sloping ground surface. It should be noted that due to the empirical nature of the Youd and Bartlett (2002) model, the lateral spread estimate should be considered as preliminary. The Youd and Bartlett (2002) model has a small displacement accuracy of no better than approximately 2 to 3 feet. Ninyo & Moore San Dieguito River Park, San Diego, California October 31,

5 FOUNDATIONS To mitigate the effects of potential liquefaction (including seismic settlement, lateral spreading, sand boils, etc.), we recommend that the proposed Maintenance Facility building be supported on a mat foundation system bearing on compacted fill materials prepared in accordance with the site preparation recommendations presented in our geotechnical evaluation (Ninyo & Moore, 2016). It is anticipated that liquefaction and resulting ground deformations will still occur. Ancillary structures such as site walls may be supported on conventional shallow continuous or spread footings bearing on at least 18 inches of recompacted fill materials. Foundations should be designed in accordance with structural considerations and the following recommendations. In addition, requirements of the appropriate governing jurisdictions and applicable building codes should be considered in the design of the structures. Other alternative measures to mitigate the effects of liquefaction of the existing fill soils may also be implemented. These alternative measures may include supporting the building on deep foundations that bear within the Delmar Formational materials and soil improvement that involves in-place densification of the fill soils at depth to reduce the soils susceptibility to liquefaction. If these or other alternative foundations systems/measures are desired, we can provide recommendations upon request. Be advised that as part of the foundation design election process, there is always a cost/benefit evaluation. Although we are providing recommendations for a specific building foundation system, we have not conducted the cost/benefit evaluation. Mat Foundations - Buildings An allowable bearing pressure of 2,000 pounds per square foot (psf) may be assumed for mat foundations constructed on the engineered fill recommended as part of the remedial grading. This allowable bearing capacity may be increased by one-third when considering loads of a short duration such as wind or seismic forces. Thickness and reinforcement of the mat foundation should be in accordance with the recommendations of the project structural engineer. Mat foundations typically experience some deflection due to loads placed on the mat and the reaction of the soils underlying the mat. A design coefficient of subgrade reaction, K v1, of 150 pounds per cubic inch may be used for evaluating such deflections at the subject site. This value is based on a unit square-foot area and should be adjusted for the planned mat size. The coefficient of subgrade reaction K b for a mat of specific width may be evaluated using the following equation: K b = K v1 [(b+1)/2b] 2 where b is the width of the foundation in feet. Ninyo & Moore San Dieguito River Park, San Diego, California October 31,

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7 REFERENCES CivilTech Software, 2007, Liquefy Pro (Version 5.5j), A Computer Program for Liquefaction and Settlement Analysis. City of San Diego, 2017, Cycle Issues, Project , August 10. Construction Testing & Engineering, Inc. (CTE), 2009, Final Summary for Testing of Compacted Fill, San Dieguito Wetlands Restoration Project, San Diego and Del Mar, California: dated February 25. Geocon Incorporated, 2017, Geotechnical Investigation, Via de la Valle Street Widening, San Andreas Drive to El Camino Real, San Diego, California, August 30. Google, Inc., 2017, accessed in August. Ishihara, K. and Yoshimine, M., 1992, Evaluation of Settlements in Sand Deposits following Liquefaction during Earthquakes, Soils and Foundations, JSSMFE, Vol. 32, No. 1: dated March. Ninyo & Moore, In-House Proprietary Data. Ninyo & Moore, 2016, Geotechnical Evaluation, Proposed Maintenance Facility, San Dieguito River Park, San Diego, California, August 31. Seed, H.B., and Idriss, I.M., 1982, Ground Motions and Soil Liquefaction During Earthquakes, Earthquake Engineering Research Institute Monograph, Oakland, California, Tan, S.S. and Kennedy, M.P., 1996, Geologic Maps of the Northwestern Part of San Diego County, California; California Geological Survey, Open File Report Tokimatsu, K., and Seed, H.B., 1987, Evaluation of Settlements in Sands Due to Earthquake Shaking, Journal of Geotechnical Engineering, American Society of Civil Engineers, 113(8), Treiman, J.A., 1993, The Rose Canyon Fault Zone, Southern California: California Geological Survey, Open File Report Youd, T.L., Idriss, I.M., Andrus, R.D., Arango, I., Castro, G., Christian, J.T., Dobry, R., Finn, W.D., Harder, L.F., Hynes, M.E., Ishihara, K., Koester, J.P., Liao, S.S.C., Marcuson, W.F., Martin, G.R., Mitchell, J.K., Moriwaki, Y., Power, M.S., Robertson, P.K., Seed, R.B., and Stokoe, K.H., II., 2001, Liquefaction Resistance of Soils: Summary Report from the 1996 NCEER and 1998 NCEER/NSF Workshops on Evaluation of Liquefaction Resistance of Soils, Journal of Geotechnical and Geoenvironmental Engineering: American Society of Civil Engineering 124(10), pp Youd, T.L., Hansen, C.M., and Bartlett, S.F., 2002, Revised Multilinear Regression Equations for Prediction of Lateral Spread Displacement, Journal of Geotechnical and Geoenvironmental Engineer, Vol. 128, No. 12, ASCE: dated December 1. Ninyo & Moore San Dieguito River Park, San Diego, California October 31, 2017

8 FIGURE Ninyo & Moore San Dieguito River Park, San Diego, California October 31, 2017

9 VIA DE LA VALLE SM A' 30 Qcf Qal B-3 TD= B-2 TD= A B-1 TD= _ _BL_93.mxd AOB LEGEND A B-1 TD=16.5 PROPOSED BUILDING NOTE: DIRECTIONS, DIMENSIONS AND LOCATIONS ARE APPROXIMATE PROJECT NO. BORING TD=TOTAL DEPTH IN FEET SM-7 BORING (GEOCON, 2017) A' CROSS SECTION DATE /17 Qcf Qal 32 COMPACTED FILL OVER ALLUVIUM TOPOGRAPHY CONTOUR (RINEHART HERBST, 2015) FLOOD ZONE BORING LOCATIONS PROPOSED MAINTENANCE FACILITY SAN DIEGUITO RIVER PARK SAN DIEGO, CALIFORNIA SCALE IN FEET SOURCE: GOOGLE EARTH, RINEHART HERBST, FEMA, FIGURE 1

10 ATTACHMENT A Boring Logs (Ninyo & Moore, 2016 and Geocon, 2017) Ninyo & Moore San Dieguito River Park, San Diego, California October 31, 2017

11 DEPTH (feet) Bulk SAMPLES Driven BLOWS/FOOT MOISTURE (%) DRY DENSITY (PCF) SYMBOL CLASSIFICATION U.S.C.S. BORING LOG EXPLANATION SHEET 0 Bulk sample. Modified split-barrel drive sampler. 2-inch inner diameter split-barrel drive sampler. No recovery with modified split-barrel drive sampler, or 2-inch inner diameter split-barrel drive sampler. Sample retained by others. 5 Standard Penetration Test (SPT). No recovery with a SPT. XX/XX Shelby tube sample. Distance pushed in inches/length of sample recovered in inches. No recovery with Shelby tube sampler. 10 SM CL Continuous Push Sample. Seepage. Groundwater encountered during drilling. Groundwater measured after drilling. MAJOR MATERIAL TYPE (SOIL): Solid line denotes unit change. Dashed line denotes material change. 15 Attitudes: Strike/Dip b: Bedding c: Contact j: Joint f: Fracture F: Fault cs: Clay Seam s: Shear bss: Basal Slide Surface sf: Shear Fracture sz: Shear Zone sbs: Shear Bedding Surface The total depth line is a solid line that is drawn at the bottom of the boring. 20 BORING LOG Explanation of Boring Log Symbols PROJECT NO. DATE FIGURE

12 SOIL CLASSIFICATION CHART PER ASTM D 2488 SECONDARY DIVISIONS PRIMARY DIVISIONS GROUP SYMBOL GROUP NAME DESCRIPTION GRAIN SIZE SIEVE SIZE GRAIN SIZE APPROXIMATE SIZE CLEAN GRAVEL less than 5% fines GW GP well-graded GRAVEL poorly graded GRAVEL Boulders > 12 > 12 Larger than basketball-sized COARSE- GRAINED SOILS more than 50% retained on No. 200 sieve GRAVEL more than 50% of coarse fraction retained on No. 4 sieve SAND 50% or more of coarse fraction passes No. 4 sieve GRAVEL with DUAL CLASSIFICATIONS 5% to 12% fines GRAVEL with FINES more than 12% fines CLEAN SAND less than 5% fines SAND with DUAL CLASSIFICATIONS 5% to 12% fines SAND with FINES more than 12% fines GW-GM GP-GM GW-GC GP-GC GM GC GC-GM SW SP SW-SM SP-SM SW-SC SP-SC SM SC SC-SM well-graded GRAVEL with silt poorly graded GRAVEL with silt well-graded GRAVEL with clay poorly graded GRAVEL with clay silty GRAVEL clayey GRAVEL silty, clayey GRAVEL well-graded SAND poorly graded SAND well-graded SAND with silt poorly graded SAND with silt well-graded SAND with clay poorly graded SAND with clay silty SAND clayey SAND silty, clayey SAND Gravel Sand Cobbles Coarse 3/4-3 3/4-3 Fine #4-3/ Coarse #10 - # Medium #40 - # Fine #200 - # Fines Passing #200 < PLASTICITY CHART Fist-sized to basketball-sized Thumb-sized to fist-sized Pea-sized to thumb-sized Rock-salt-sized to pea-sized Sugar-sized to rock-salt-sized Flour-sized to sugar-sized Flour-sized and smaller FINE- GRAINED SOILS 50% or more passes No. 200 sieve SILT and CLAY liquid limit less than 50% SILT and CLAY liquid limit 50% or more INORGANIC ORGANIC INORGANIC ORGANIC CL ML CL-ML OL (PI > 4) OL (PI < 4) CH MH OH (plots on or above A -line) OH (plots below A -line) lean CLAY SILT silty CLAY organic CLAY organic SILT fat CLAY elastic SILT organic CLAY organic SILT Highly Organic Soils PT Peat PLASTICITY INDEX (PI), % CH or OH CL or OL MH or OH CL - ML ML or OL LIQUID LIMIT (LL), % APPARENT DENSITY APPARENT DENSITY - COARSE-GRAINED SOIL SPOOLING CABLE OR CATHEAD SPT (blows/foot) MODIFIED SPLIT BARREL (blows/foot) AUTOMATIC TRIP HAMMER SPT (blows/foot) MODIFIED SPLIT BARREL (blows/foot) Very Loose < 4 < 8 < 3 < 5 Loose Medium Dense Dense Very Dense > 50 > 105 > 33 > 70 CONSIS- TENCY CONSISTENCY - FINE-GRAINED SOIL SPOOLING CABLE OR CATHEAD SPT (blows/foot) MODIFIED SPLIT BARREL (blows/foot) AUTOMATIC TRIP HAMMER SPT (blows/foot) MODIFIED SPLIT BARREL (blows/foot) Very Soft < 2 < 3 < 1 < 2 Soft Firm Stiff Very Stiff Hard > 30 > 39 > 20 > 26 USCS METHOD OF SOIL CLASSIFICATION Explanation of USCS Method of Soil Classification PROJECT NO. DATE FIGURE

13 DEPTH (feet) 0 SAMPLES Bulk Driven BLOWS/FOOT MOISTURE (%) DRY DENSITY (PCF) SYMBOL CLASSIFICATION U.S.C.S. GM DATE DRILLED 7/21/16 BORING NO. B-1 GROUND ELEVATION 24' (MSL) SHEET 1 OF METHOD OF DRILLING 8" Diameter Hollow Stem Auger (Baja Exploration) (CME-75) DRIVE WEIGHT 140 lbs. (Auto-Trip) DROP 30" SAMPLED BY NMM LOGGED BY NMM REVIEWED BY TCS DESCRIPTION/INTERPRETATION FILL: Brown, moist, loose to medium dense, silty GRAVEL with sand SM ALLUVIUM: Grayish brown, moist, medium dense, silty fine SAND; trace gravel. Light brown to light reddish brown. Trace clay Greater moisture content Total Depth = 20 feet. Groundwater encountered at approximately 17.0 feet during drilling. Backfilled with approximately 6 cubic feet of bentonite shortly after drilling on 7/21/16. Note: Groundwater may rise to a level higher than that measured in borehole due to seasonal variations in precipitation and several other factors as discussed in the report. The ground elevation shown above is an estimation only. It is based on our interpretations of published maps and other documents reviewed for the purposes of this evaluation. It is not sufficiently accurate for preparing construction bids and design documents PROJECT NO BORING LOG PROPOSED MAINTENANCE FACILITY SAN DIEGUITO RIVER PARK, SAN DIEGO, CALIFORNIA DATE 8/16 FIGURE A-1

14 DEPTH (feet) 0 SAMPLES Bulk Driven BLOWS/FOOT MOISTURE (%) DRY DENSITY (PCF) SYMBOL CLASSIFICATION U.S.C.S. SM DATE DRILLED 7/21/16 BORING NO. B-2 GROUND ELEVATION 24' (MSL) SHEET 1 OF METHOD OF DRILLING 8" Diameter Hollow Stem Auger (Baja Exploration) (CME-75) DRIVE WEIGHT 140 lbs. (Auto-Trip) DROP 30" SAMPLED BY NMM LOGGED BY NMM REVIEWED BY TCS DESCRIPTION/INTERPRETATION FILL: Brown to grayish brown, moist, loose to medium dense, silty fine SAND; little gravel. 1 SM ALLUVIUM: Brown, moist, loose to medium dense, silty fine to medium SAND Medium dense Loose; trace clay. 9 Brown; medium dense; wet. Total Depth = 16.5 feet. Groundwater encountered at approximately 15.5 feet during drilling. Backfilled with approximately 5 cubic feet of bentonite shortly after drilling on 7/21/ Note: Groundwater may rise to a level higher than that measured in borehole due to seasonal variations in precipitation and several other factors as discussed in the report. The ground elevation shown above is an estimation only. It is based on our interpretations of published maps and other documents reviewed for the purposes of this evaluation. It is not sufficiently accurate for preparing construction bids and design documents BORING LOG PROPOSED MAINTENANCE FACILITY SAN DIEGUITO RIVER PARK, SAN DIEGO, CALIFORNIA PROJECT NO DATE 8/16 FIGURE A-2

15 DEPTH (feet) 0 SAMPLES Bulk Driven BLOWS/FOOT 50/2" MOISTURE (%) DRY DENSITY (PCF) SYMBOL CLASSIFICATION U.S.C.S. SM DATE DRILLED 7/20/16 BORING NO. B-3 GROUND ELEVATION 24' (MSL) SHEET 1 OF METHOD OF DRILLING 8" Diameter Hollow Stem Auger (Baja Exploration) (CME-75) DRIVE WEIGHT 140 lbs. (Auto-Trip) DROP 30" SAMPLED BY NMM LOGGED BY NMM REVIEWED BY TCS DESCRIPTION/INTERPRETATION FILL: Brown to grayish brown, moist, loose to medium dense, silty GRAVEL; some sand; trace cobbles up to approximately 7 inches in 2.5': Concrete piece approximately 6.5 inches in 3.5': Gray; trace travel and cobbles up to approximately 5 inches in 4.5': Brown SM ALLUVIUM: Reddish brown, moist, dense, silty fine to medium SAND. Total Depth = 7.2 feet. (Refusal) Groundwater encountered during drilling. Backfilled shortly after drilling on 7/20/16. Note: Groundwater may rise to a level higher than that measured in borehole due to seasonal variations in precipitation and several other factors as discussed in the report. The ground elevation shown above is an estimation only. It is based on our interpretations of published maps and other documents reviewed for the purposes of this evaluation. It is not sufficiently accurate for preparing construction bids and design documents BORING LOG PROPOSED MAINTENANCE FACILITY SAN DIEGUITO RIVER PARK, SAN DIEGO, CALIFORNIA PROJECT NO DATE 8/16 FIGURE A-3

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