Attachment C - Supplemental Geotechnical Recommendations Copy.pdf

5710 Ruffin Road | San Diego, California 92123 | p. 858.576.1000 | www.ninyoandmoore.com

July 6, 2022 Project No. 109427001

Mr. Mike Peltz MW Peltz & Associates 143 South Cedros Avenue, Suite B104 Solana Beach, California 92075

Subject: Supplemental Geotechnical Recommendations Los Penasquitos Bridge and Amphitheater Los Penasquitos Canyon County Preserve San Diego County, California

Reference: Ninyo & Moore, 2022, Geotechnical Evaluation, Los Penasquitos Bridge and Amphitheater, Los Penasquitos Canyon County Preserve, San Diego County, California, Project No. 109427001: dated June 17.

Dear Mr. Peltz:

In accordance with your request, we have prepared this letter providing supplemental recommendations for the Los Penasquitos Bridge and Amphitheater project at the Los Penasquitos

Canyon County Preserve in San Diego County, California. This letter specifically provides supplemental recommendations to address the proposed shade structure that is being considered at the amphitheater structure and a potential pile foundation alternative for the bridge structure.

The recommendations provided herein are considered supplementary and the previous findings, conclusions, and recommendations presented in the referenced geotechnical evaluation report

(Ninyo & Moore, 2022) remain applicable and valid.

SHADE STRUCTURE FOUNDATIONS

We understand that a shade structure is being considered for the project as part of the proposed amphitheater improvements. Shade structures typically impose relatively light axial loads on foundations and have more structural demands for lateral and uplift loading. Typically, we recommend that such structures be supported on cast-in-drilled-hole (CIDH) concrete pile foundations. However, we are also providing alternative recommendations to support these structures on shallow spread footings.

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Ninyo & Moore | Los Penasquitos Canyon County Preserve, San Diego County, California | 109427001 | July 6, 2022 2

Shade Structure CIDH Piles As noted earlier, shade structures typically impose relatively light axial loads on foundations.

Although we anticipate that pile dimensions will be generally controlled by the lateral load demand, we recommend that such drilled foundations for the shade structure have a diameter of 18 inches or more. Furthermore, to mitigate for the potential effects of the moderately expansive onsite soils, CIDH pile foundations should extend to depths of 6 feet or more. The pile dimensions

(i.e., diameter and embedment) should be evaluated by the project structural engineer.

The drilled pile construction should be observed by Ninyo & Moore during construction to evaluate if the piles have been extended to the design depths. It is the contractor's responsibility to (a) take appropriate measures for maintaining the integrity of the drilled holes, (b) see that the holes are cleaned and straight, and (c) see that sloughed loose soil is removed from the bottom of the hole prior to the placement of concrete. Drilled piles should be checked for alignment and plumbness during installation. The amount of acceptable misalignment of a pile is approximately 3 inches from the plan location. It is usually acceptable for a pile to be out of plumb by 1 percent of the depth of the pile. The center-to-center spacing of piles should be no less than three times the nominal diameter of the pile. If the CIDH piles extend into groundwater or seepage, the contractor should consider appropriate measures during construction to reduce the potential for caving of the drilled holes, including the use of steel casing and/or drilling mud. In addition, we recommend concrete be placed by tremie method, to see that the aggregate and cement do not segregate during concrete placement, on the same day the CIDH piles are drilled.

Due the variable nature and depth of the existing fill and alluvial materials at the site, we recommend CIDH foundations for the shade structure at the amphitheater be designed using an allowable passive pressure of 600 pounds per square foot (psf) per foot of depth, with an upper bound value of up to 6,000 psf for non-submerged conditions and 280 psf per foot of depth, with an upper bound value of up to 2,800 psf for submerged conditions. These values have been doubled based on the assumption that the posts/poles are designed to tolerate ½ inch of deflection at the surface and that the ground is horizontal for a distance of 10 feet, or three times the height generating the passive pressure, whichever is greater. We recommend that the upper 1 foot of soil not protected by pavement or a concrete slab be neglected when calculating passive resistance.

For frictional resistance to lateral loads, we recommend a coefficient of friction of 0.35 be used between soil and concrete. The allowable lateral resistance values may be increased by one-third during short-term loading conditions, such as wind or seismic loading.

Ninyo & Moore | Los Penasquitos Canyon County Preserve, San Diego County, California | 109427001 | July 6, 2022 3

Shallow Spread Footings – Shade Structure We are also providing alternative recommendations to support shade structures on shallow spread footings. To mitigate for the potential effects of the moderately expansive onsite soils, spread footings for these types of structures should be embedded at depths for 3 feet or more.

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.

Shallow, spread footings for shade structures bearing on competent alluvial soils may be designed using an allowable bearing capacity of 2,000 psf for non-submerged conditions and

1,500 psf for submerged conditions. These allowable bearing capacities may be increased by one-third when considering loads of short duration such as wind or seismic forces. Spread footings should be 36 inches or more in width. The footings should be reinforced in accordance with the recommendations of the project structural engineer.

For resistance of spread footings to lateral loads, we recommend an allowable passive pressure of 300 psf per foot of depth be used with a value of up to 3,000 psf for non-submerged conditions and 140 psf per foot of depth, with an upper bound value of up to

1,400 psf for submerged conditions. This value assumes that the ground is horizontal for a distance of 10 feet, or three times the height generating the passive pressure, whichever is more. We recommend that the upper 1 foot of soil not protected by pavement or a concrete slab be neglected when calculating passive resistance.

For frictional resistance to lateral loads, we recommend a coefficient of friction of 0.35 be used between soil and concrete. The passive resistance values may be increased by one-third when considering loads of short duration such as wind or seismic forces.

PEDESTRIAN BRIDGE – PILE FOUNDATION ALTERNATIVE

Although the referenced geotechnical report provides recommendations for the use of shallow foundations to support the pedestrian bridge, we understand that the project team has also requested recommendations for CIDH pile foundations. Geotechnical recommendations for CIDH pile foundations are provided below. The type, depth, and size of the foundations should be evaluated by the structural engineer based on the loading conditions and these geotechnical recommendations. We recommend that the foundation plans and design submittal be reviewed by this office for general conformance to these recommendations prior to finalizing.

Ninyo & Moore | Los Penasquitos Canyon County Preserve, San Diego County, California | 109427001 | July 6, 2022 4

The key design parameters and considerations for the CIDH concrete piles are presented below.

• The CIDH concrete piles should be 24 inches or more in diameter.

• The spacing of piles should be evaluated by the project structural engineer based on the estimated total service (dead and live) loads. However, from a geotechnical standpoint, we recommend that the CIDH concrete piles be constructed at a center-to-center spacing of three pile diameters or more.

• The CIDH piles may be designed using a submerged allowable downward (axial) frictional resistance value of 100 psf, based on a factor of safety of 2.0, and an uplift frictional resistance value 80 psf, based on a factor of safety of 1.5. The axial and uplift pile capacities presented are based on side friction. We recommend that end bearing not be incorporated into the pile design capacities. The values above may be increased by one-third when considering loading of short duration such as wind or seismic forces (i.e., dynamic loads)

• Groundwater was present during our subsurface exploration. Groundwater will have an impact on construction of the CIDH concrete piles. The contractor should be prepared to case the excavations where caving occurs. Drilling mud may be used in lieu of casing to aid in retaining the soils. Concrete should be tremied into place with an adequate head to displace water or drilling mud. Concrete should not be placed freefall or in such a manner as to hit the sidewalls of the excavation.

The drilled pile construction should be observed by the geotechnical consultant during construction to evaluate if the piles have been extended to the design depths. The drilled holes should be cleaned of loose soil and gravel. It is the contractor's responsibility to (a) take appropriate measures for maintaining the integrity of the drilled holes, (b) see that the holes are cleaned and straight, and (c) see that sloughed loose soil is removed from the bottom of the hole prior to the placement of concrete. Drilled piles should be checked for alignment and plumbness during installation. The amount of acceptable misalignment of a pile is approximately 3 inches from the plan location. It is usually acceptable for a pile to be out of plumb by 1 percent of the depth of the pile. The center-to-center spacing of piles should be no less than three times the nominal diameter of the pile.

Due to the presence of a shallow groundwater table at the proposed pedestrian bridge location at the creek, the contractor should plan to use appropriate measures during construction to reduce the potential for caving of the drilled holes, including the use of steel casing and/or drilling mud. In addition, we recommend concrete be placed by tremie method, to see that the aggregate and cement do not segregate during concrete placement, on the same day the CIDH piles are drilled.

Lateral Resistance – Pedestrian Bridge CIDH Piles For the analysis of the CIDH pile foundations using lateral resistance pressures, we recommend an allowable passive pressure of 280 psf per foot of depth be used with a value of up to 2,800 psf.

For this case, an effective width of two times the diameter of the piles may be considered when

Ninyo & Moore | Los Penasquitos Canyon County Preserve, San Diego County, California | 109427001 | July 6, 2022 5 evaluating lateral resistance. These values have been doubled based on the assumption that the posts/poles are designed to tolerate ½ inch of deflection at the surface. Furthermore, this value assumes that the ground is horizontal for a distance of 10 feet, or three times the height generating the passive pressure, whichever is greater. We recommend that the upper 1 foot of soil not protected by pavement or a concrete slab be neglected when calculating passive resistance.

For frictional resistance to lateral loads, we recommend a coefficient of friction of 0.4 be used between soil and concrete. The lateral resistance values presented above may be increased by one-third when considering loads of short duration such as wind or seismic forces.

Static Settlement – Pedestrian Bridge CIDH Piles We estimate that the proposed structure, designed and constructed as recommended herein, will undergo total settlement on the order of 1 inch. Differential settlement on the order of ½-inch over a horizontal span of 40 feet should be expected.

LIMITATIONS

The geotechnical services described in this report have been conducted in general accordance with current practice and the standard of care exercised by geotechnical consultants performing similar tasks in the project area. No warranty, expressed or implied, is made regarding the conclusions, recommendations, and opinions presented in this report. Our conclusions and recommendations are based on an analysis of the observed site conditions and the references listed. If actual conditions differ from those described in this report, our office should be notified and additional recommendations, if warranted, can be provided upon request.

We appreciate the opportunity to be of continued service on this project.

Respectfully submitted, NINYO & MOORE

Christina Tretinjak, PG, CEG Senior Project Geologist

Jeffrey T. Kent, PE, GE Principal Engineer

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