603 2022 FINAL SUPPLEMENTAL ADDENDUM.pdf

PO Box 2540, Camarillo, California 93011 www.Oakridgegeo.com

805-603-4900

A DBE/SWBE/SBE Certified Firm

February 11, 2022 Project No. 012.009

Kennedy/Jenks Consultants, Inc.

32001 32nd Avenue South, Suite 100 Federal Way, Washington 98001-9625

Attention: Mr. Ryan Ray, P.E.

Subject: Geotechnical Addendum Letter, Relocated Networking Center, Administration Building Addition and Crew Building Improvements, Calleguas Municipal Water District, Thousand Oaks, California.

Dear Mr. Ray:

Oakridge Geoscience. (OGI) is pleased to provide this addendum geotechnical letter for two facilities being designed by Kennedy/Jenks Consultants (KJ) at Calleguas Municipal Water District’s (CMWD) Lake Bard Water Filtration Plant at 2100 Olsen Road in Thousand Oaks, California. Based on information provided by KJ, the facilities consist of a relocated Networking Center and an approximately 200-square-foot addition to the existing Administration Building.

The approximate locations of the relocated Networking Center and Administration Building addition are shown on Plate 1. The scope of work for this supplemental study was presented in our proposal dated June 25, 2021 and authorized by a KJ agreement for professional services dated January 27, 2022. As requested by CMWD, the recommendations for this supplemental study are based on review of existing data; no additional subsurface exploration or laboratory testing was performed for the supplemental study. Both structures are anticipated to consist of single-story, concrete masonry block (CMU) walls on shallow foundations with concrete slab-on-grade construction.

The regional geologic setting and seismic design parameters described in the geotechnical report for the Lake Bard Pump Station, Crew Building Improvements, and Conejo Pump Station Rehabilitation (OGI, 2021) are generally considered applicable for the relocated Networking Center and Administration Building addition as modified in this addendum letter. This addendum letter describes the site conditions at the two locations and provides updated and/or additional site-specific foundation recommendations for the new structures based on review of previous geotechnical reports supplied by CMWD. Existing footings encountered during excavations for new improvements can be protected by sloping the excavation away from existing footings at an inclination of 1h:1v or flatter as described in the site preparation portion of this letter for the existing Administration Building.

Kennedy/Jenks Consultants, Inc.

Project No. 012.009

OAKRIDGE GEOSCIENCE, INC.

SITE CONDITIONS

Relocated Networking Center

As shown on Plate 1, the proposed Networking Center site is located northeast of Hardy Lane, about 35 feet west of the Pre-Ozone Contactors. The proposed site has a slight southwestward slope toward Hardy Lane, is covered with gravel, and located about 20 feet south of an approximately 80-foot-high, ascending 2 horizontal to 1 vertical (2h:1v) cut slope. The approximate ground surface elevation (El.) is reportedly about El. +967 feet.

Staal, Gardner, and Dunne, Inc. (SGD, 1992) prepared a geotechnical study for the Lake Bard Water Treatment Plant site. Review of that report indicates the Networking Center site is located in a Sespe Formation bedrock cut area with an original (pre-grading) elevation of about El. +1,000 to +1,010 feet and with about 30 to 40 feet of cut to the existing grade. The bedrock materials in nearby drill holes DH-8, DH-9, and DH-12 and backhoe Trench 3 excavated and logged by SGD (1992) were encountered at an elevation of about El. +970 feet and are generally described as moderately weathered, moderately indurated, moderately hard, fine- to coarse-grained, friable, moist, and thinly to thickly bedded silty sandstone. Lenses of thinly interbedded mudstone and gravel to about 3 inches in diameter were also encountered by SGD’s explorations.

We note the potential exists for fill materials associated with existing utilities and/or previous grading to be located within the project footprint area above the bedrock. Pertinent data consisting of the drill hole logs, trench log, and site map from SGD (1992) are provided in Appendix A.

The existing structural geologic data indicate the silty sandstone bedding in the new Networking Center area generally strikes about N80E and dips about 35 to 45 degrees to the northwest, dipping favorably into the existing cut slope to the north. We note the existing data indicate a zone of south-dipping mudstone was located on the upper portion of the cut slope and that the Report of Rough Grading for the Calleguas (Lake Bard) Water Filtration Plant (SGD, 1993) indicates a buttress fill was constructed on the upper portion of the slope above the proposed Networking Center site. Based on the information presented in the rough grading report (SGD, 1993), the buttress fill constructed in the upper portion of the 2h:1v cut slope suggests that slope stability issues should not impact the proposed Networking Center site.

Administration Building Addition

As depicted on preliminary site plans by KJ, an about 260-square foot storage area building addition is proposed for the southeastern portion of the Administration Building between the existing building and trellis. The proposed addition footprint will be about 28 feet wide in the east-west direction adjacent to the existing building and will be about nine feet wide in the north-south direction toward the trellis. The proposed building addition location is located on a relatively level graded area covered with landscape chips. The rough grading report (SGD, 1993) indicates the Administration Building site was overexcavated to a depth of about six feet below the foundations (El. +950 feet) and backfilled with compacted fill to the final design elevation. The overexcavation was reported to extend 10 feet laterally beyond the proposed building footprint.

Based on the available geotechnical data, the proposed Administration Building addition should be underlain by a minimum of approximately six feet of fill compacted to a minimum of 90 percent relative compaction.

FOUNDATION RECOMMENDATIONS

Site Earth Materials

As described above, the proposed Networking Center site is located within a previously graded cut area underlain by silty sandstone of the Sespe Formation (and potential fill materials associated with existing utilities and/or previous grading) that is currently covered with gravel and the proposed Administration Building addition is located within a previously graded fill area with about six feet of compacted fill that is currently covered with landscape chips.

Site Preparation

The proposed relocated Networking Center building and the Administration Building addition should be anticipated to utilize shallow conventional foundations bearing entirely into new properly placed compacted fill materials. The intent of the subgrade preparation is to provide a uniform bearing material for the proposed improvement(s) and/or structure(s). If non-uniform bearing materials such as cut/fill transitions, expansive clay beds/materials, disturbed soil, or fill zones, etc. are encountered, they should be addressed on a case-by-case basis during construction based on input from the project geotechnical professional.

The existing unsuitable/surficial gravel, landscape materials, and/or onsite soils within the proposed development areas should be overexcavated and removed to expose the anticipated Sespe Formation silty sandstone bedrock within the relocated Networking Center building area and dense, competent previously compacted fill materials within the Administration Building addition area. The overexcavation for the proposed Administration Building addition should not impact the existing Administration Building foundation.

The recommended overexcavation removals for the Administration Building addition should extend to a minimum depth of three feet below the base of the proposed new foundations and a minimum of three feet laterally beyond the edge of the footing, except where the excavation encounters existing footings. Where the excavation encounters an existing footing, the excavation for the addition should not expose more than half the height of the existing footing and should slope away from the existing Administration Building footing at a 1h:1v slope or flatter based on site conditions encountered during grading. Where the excavation does not encounter an existing footing, the recommended overexcavation removals should extend laterally beyond the planned new foundation limits for a horizontal distance equal to the depth of the grading overexcavation below the proposed new foundations. We suggest the contractor pothole the existing Administration Building footing prior to construction to confirm the footing dimensions shown on the project plans which were developed from the existing Administration Building record drawings.

The exposed overexcavation (”bottom”) surface should be scarified a minimum depth of 12 inches, moisture conditioned, and compacted to a minimum of 90 percent relative compaction as compared to ASTM D1557 (latest edition; Modified Proctor). The excavation should be backfilled to design finished grade with granular earth materials from onsite and/or with import earth materials approved by the project geotechnical professional. The recommended new properly placed, compacted fill materials shall be compacted to a minimum of 90 percent relative compaction, as determined by ASTM D1557. Additionally, the final thickness of the new properly placed compacted fill materials below the proposed improvement(s) and/or structure(s) should not vary by more than 15 percent.

Moisture conditioning of the onsite earth materials, by either drying back or adding water, should be anticipated during the recommended site preparation grading, in order to achieve a new graded fill material mixture with a moisture content near the laboratory-determined optimum moisture content. Also, if expansive soils (EI>50) are encountered within the recommended over-excavation, those materials should not be used as new compacted fill unless they can be mixed with granular materials to create a soil mixture with an EI of less than 50 and with a moisture content at, or within 3 percent above, the optimum moisture content.

Shallow Foundation Design Parameters

Shallow conventional foundations for the proposed new Networking Center building and Administration Building addition may be designed using the allowable bearing pressures and lateral earth bearing pressures provided in the referenced geotechnical report for the Lake Bard Pump Station, Crew Building, and Conejo Pump Station Improvements (OGI, 2021). The estimated static and seismic settlement values presented in the referenced report are considered applicable for the proposed relocated Networking Center building and Administration Building addition.

Crew Building Improvements

The preliminary project plans for the Crew Building improvements by KJ (May, 2022) indicate the foundations for the existing Crew Building structure and proposed Crew Building Expansion are less than three feet apart. Based on our review of the preliminary plans, there does not appear to be sufficient space between the existing Crew Building and the proposed Crew Building Expansion to perform the recommended five-foot overexcavation and recompaction to a minimum of 90 percent relative compaction as compared to ASTM D1557 (latest edition; Modified Proctor) for the proposed Crew Building Expansion using a sloped excavation. In our opinion, temporary shoring will be required to support the existing Crew Building foundations to perform the overexcavation and recompaction for the proposed Crew Building Expansion. The temporary shoring should be designed by a registered civil engineer experienced in shoring design. The design should limit the lateral movement of the shoring to ¼-inch or less to reduce the potential to damage the existing Crew Building structure. Excavations setback from existing structures greater than 1.5h:1v may be sloped per OSHA requirements.

After completion of the site grading and new foundation installation, the temporary shoring should be removed to about five feet below the ground surface and the voids left by removal of the shoring should be backfilled with one-sack sand-cement slurry or properly compacted and tested structural fill. Vibratory equipment should not be used to install or remove the shoring to limit potential for damage to existing improvements.

We recommend the shoring be surveyed by a licensed surveyor after installation, during construction, and prior to shoring removal to document that lateral deflection of the shoring was less than ¼-inch. Additionally, we recommend the foundation of the existing Crew Building be surveyed by the licensed surveyor before, during, and after completion of the Crew Building improvements to document that no damage occurred to the existing structure as a result of the construction activities. We recommend the shoring and existing Crew Building foundation surveys be conducted weekly during construction or more frequently if distress/movement is observed or documented by the surveys.

LIMITATIONS

Report Use

This addendum letter-report has been prepared for the exclusive use of Kennedy/Jenks Consultants, Inc. and Calleguas Municipal Water District for the design and construction of the projects as described herein. The limitations described in the referenced geotechnical report are applicable to this addendum letter report. The work performed for the study was based on our review of the conceptual to preliminary project design information provided to us, thus the potential exists for the project designs and/or locations of the various project elements to change from what was evaluated for this study.

The findings, conclusions, and recommendations presented herein were prepared in accordance with generally accepted geotechnical engineering practices of the project region. No other warranty, express or implied, is made.

Although information contained in this report may be of some use for other purposes, it may not contain sufficient information for other parties or uses. If any changes are made to the project as described in this report, the conclusions and recommendations in this letter shall not be considered valid unless the changes are reviewed, and the conclusions and recommendations of this report are modified or validated in writing by OGI.

CLOSURE

We appreciate the opportunity to provide geotechnical services to Kennedy/Jenks Consultants for Calleguas Municipal Water District’s projects. Please contact us if you have any questions regarding information presented herein.

Sincerely, Joseph Barr, III, PE Principal Engineer

Craig D. Prentice, CEG Principal Engineering Geologist

ATTACHMENTS

References Plate 1 – Site Map Appendix A – Previous Geotechnical Data

REFERENCES

Oakridge Geoscience, Inc. (2021), Final Geotechnical Report, Calleguas Municipal Water District’s Lake Bard Pump Station, Crew Building Improvements, and Conejo Pump Station Rehabilitation (Project Nos. 587 and 480), Thousand Oaks, California, OGI Project No. 012.009, dated February 26.

Staal, Gardner, & Dunne, Inc. (1992), Geotechnical Study, Lake Bard Water Treatment Plant, Ventura County, California.

_____ (1993), Report of Rough Grading for the Calleguas (Lake Bard) Water Filtration Plant, Ventura County, California.

Kennedy/Consultants, Inc.

SITE LOCATION MAP

Calleguas Municipal Water District

Thousand Oaks, California

PLATE 1

OAKRIDGE GEOSCIENCE, INC.

Administration Building

Pre-Ozone Contactors

Administration Building Addition Location

Networking Center Location

2h :1v

Cut

Slo pe

APPENDIX A