Ahwahnee_-_URS_seismic_report_2002_Part_1_of_3.pdf

Prepared By: Prepared For:

221 Main Street, Suite 600 San Francisco, CA 94105

U.S. Department of the Interior National Park Service Denver Service Center Denver, Colorado

Seismic Rehabilitation Alternatives for the Ahwahnee Hotel

Yosemite National Park, California

FINAL REPORT

February 2002

Indefinite Quantity Contract No. 1443CX2000-97-028 NPS Seismic Safety Program (2001-A250-405)

Mr. Richard Silva, P.E.

November 6,2000

I Scheme & Performance Level Cost Scheme A.. . . Life Safety (Wall Arrangement Alcompaction grout) $17,972,724.00

I Scheme B.. . Life Safety (Wall Arrangement B/compaction grout) $17,886,519.00

Scheme C . . . Limited Damage (Wall Arrangement C/jet grout) $22,295,247.00

The three schemes are also Qscussed in the Historic Architectural Report in Appendix D.

I It has been a pleasure for all of us here at URS to work on this exciting project. Please l feel free to contact us concerning any item in the report.

Sincerely, Joseph Baldelli Project Manager

CAMurdock Highlight

CAMurdock Highlight

CAMurdock Highlight

Ahwahnee Hotel Yosemite National Park, California

FEMA 3 10 Seismic Evaluation October 2000

1.0 INTRODUCTION

As part of the FEMA 310 evaluation for the Ahwahnee Hotel in Yosemite National Park, California, we present in this Appendix a Geotechnical Engineering and Geologic Hazards Report. We also completed a Foundations Checklist in accordance with the evaluation described in the FEMA 3 10 procedure (FEMA, 1998; ASCE, 2000). The following sections provide a description of the site, the building, and the soil subsurface conditions based on field exploratory and soil laboratory testing programs. The potential geologic hazards at the site, including the liquefaction potential, are evaluated. A Foundations Checklist is included in Appendix H. The report also presents a description of preliminary seismic retrofit methods for the foundation system.

2.0 SITE DESCRIPTION

The Ahwahnee Hotel is located in Yosemite National Park, California. The coordinates of the site are N 37.7462" and -1 19.5737". The hotel is in Yosemite Valley in the Ahwahnee Meadow, about 2,200 feet east of the Yosemite Village. The site is bounded by an almost vertical cliff (approximately 1,600 feet to the north and adjacent to the North Dome), the Royal Arch Creek (about 600 feet to the east), the Merced River (on the south and east about 600 feet to the southeast at its closest point), and a more or less flat area (part of the Ahwahnee Meadow at approximate elevation 3737 feet). Approximately 150 feet to the east of the Ahwahnee Hotel is a creek that flows from north to south, more or less parallel to the Royal Arch Creek. A few bungalows, which are part of the hotel complex, are farther to the east. In addition, there is a small pond about 120 feet north of the hotel. The ground surface dips gently to the south toward the Merced River. Grass, landscaping, a parking lot, other minor structures, and access roads cover the site. The totality of the hotel footprint is within the boundaries of an archeological site.

3.0 BUILDING DESCRIPTION

The following building descriptions are based on observations made by URS structural engineers during their visit to the site. The Ahwahnee Hotel, which is a historic site, is a multi-story structure with an irregular footprint shaped approximately as a "Y" and with approximate maximum planar dimensions of 300 feet by 350 feet. The footprint consists of various wings.

The south wing (great lounge, south lounge, and solarium on ground floor) is about 155 feet by 5 1 feet and expands into side areas (mural room and wing club room on the ground floor) near its end. The west wing (dining room on the ground floor) is about 1 10 feet by 5 1 feet and expands into the northwest wing (kitchen), which is approximately 117 feet by 86 feet. The east wing (registration lobby and bar on the ground floor) is about 1 16 feet by 5 1 feet and expands into the gift shop (about 60 feet by 29 feet) and the entry gallery. The wings converge to the core that accommodates the elevator lobby on the ground floor. The height of the structure varies. The core is seven stories high, while most of the south wing and part of the east and west wings are four stories high. The remaining of the south and east wings are two stories high. The rest of the building area, including exterior galleries, loggias, and terraces are one story high. A one-level basement occupies most of the core and the portion of the east wing that is immediately adjacent to the core.

The building is a steel framed, wooden and stone masonry structure built in the late 1920's. The building foundation consists of individual footings that are mostly square and with approximate

URS

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Ahwahnee Hotel FEMA 3 10 Seismic Evaluation Yosernite National Park, California October 2000 maximum planar dimensions of 4.5 feet by 4.5 feet along the footing shaft, and 6.5 feet by 6.5 feet along the pedestal. The embedment depth of the footings has not been clearly established.

Based on existing architectural drawings dated 1927, the bottom of the footings is about 6.5 feet below the ground floor level. The type, amount, and layout of reinforcement in the footings are unknown. At the ground floor level is a continuous 8-inch-thick reinforced concrete slab.

Below this level, the footings are apparently not connected. Under the slab is a crawl space between 3.5 feet to 4 feet high that houses various pipelines and conduits.

4.0 SUBSURFACE CONDITIONS

The subsurface conditions in the adjacency of the site were obtained from field investigation and laboratory testing programs conducted in September of 2000. Three exploratory borings were drilled for the field investigation by Kleinfelder of Fresno, California. Because the hotel is within the boundaries of an archeological site, it would have been necessary to request special permits to drill within these boundaries. As the duration of the permitting process (probably a few months) would have had a major impact on the project schedule, it was decided that the borings would be drilled immediately outside of the archeological boundary and as close as practically possible to the hotel in order to expedite the soil investigation. Boring B-l was drilled about 160 feet northeast of the hotel. In the first attempt the drilling encountered refusal at a depth of about 13 feet. In the second and third attempts, each offset approximately 20 feet from the first boring location, refusal occurred at a depth of about 3 feet. Boring B-2 was drilled about 100 feet southeast of the hotel footprint down to a depth of 5 1.5 feet. Boring B-3 was drilled approximately 200 feet northwest of the hotel footprint down to a depth of 5 1.5 feet.

Attachment 1 contains a letter report from Kleinfelder summarizing the field investigation and also presents the logs of borings.

These borings indicate that the soil conditions consist of granular soil composed predominantly of poorly graded and silty loose sand and poorly graded gravel. In some locations, cobbles were encountered. The refusal in boring B-1 may indicate the presence of boulders in the area immediately adjacent to the hotel. At depths of about 20 to 25 feet and 50 feet, higher blow counts were recorded in B-2 and B-3. Bedrock was not encountered down to terminal depth.

Groundwater was observed during drilling at a depth of 18.3 feet in B-2 and 12.6 feet in B-3.

Because it appears that some consistency exists in the soil conditions encountered in borings B-2 and B-3, it is expected that for the purpose of this evaluation, which is preliminary in nature, the soil conditions underneath the hotel footprint are similar to those observed in the exploratory borings.

A soil laboratory testing program consisting of grain size distributions and dry unit weight were conducted on selected samples. Lab test results are also included in Attachment 1.

5.0 GEOLOGIC HAZARDS AND GEOTECHNICAL ISSUES AT THE SITE

5.1 Seismic Hazards

5.1.1 Seismotectonic Setting

The modem tectonic setting of central California is dominated largely by the transform plate boundary contact between the Pacific and North American plates south of the Mendocino triple

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CAMurdock Highlight

CAMurdock Highlight

Ahwahnee Hotel Yosemite National Park, California

FEMA 31 0 Seismic Evaluation October 2000 on a more site-specific basis, "design" events be defined, and their respective return periods be estimated.

For our calculations we obtained response acceleration parameters from the USGS Internet site.

These values are adequate for this preliminary evaluation. However, because local sources may impact the seismic exposure at the site and the building sits on potentially liquefiable soils, we recommend that a site-specific probabilistic seismic hazard analyses be conducted if a seismic retrofit is deemed necessary.

There is no significant evidence indicating that the foundation of the Ahawhnee Hotel has not performed well in the past. However, as stated above, it is our professional opinion that major foundation improvements and retrofit are necessary to reduce and mitigate the potential damages associated with liquefaction under the building during the design seismic event.

9.0 LIMITATIONS

This report was prepared to provide support for the FEMA 3 10 evaluation of the Ahawhnee Hotel owned by the National Park Service (NPS). Because of the time constraint and limited access to the building and its adjacency, the three exploratory were drilled between 100 feet and 200 feet from the building footprint. The borings were drilled by Kleinfelder of Fresno. The recommendations presented in this report are based on the assumption that the soil and geologic conditions under the building do not deviate substantially from those encountered or extrapolated from the exploratory borings. Additional borings, immediately adjacent to the buildings, should be drilled for the design of retrofit measures. The presence of boulders under the buildings should be assessed.

Descriptions of the site and the structure are based on the observations made by URS structural engineers during their visit at the site. No URS geotechnical engineer participated in this visit.

In addition, the scope of the work called for a simplified determination of the response acceleration parameters using a USGS web site. We used this web site using coordinates of the building site measured during the visit by URS engineers. We recommend that a site-specific probabilistic seismic hazard analysis and a site response analysis be conducted in order to provide a more robust estimate of the seismic response at the site.

The elevations and building measurements used in this report are based on site observation made by URS structural engineers and a review of copies of architectural drawings dated 1927.

The environmental impacts of each retrofit scheme should be evaluated according to existing pertinent documents, e.g., National Environmental Policy Act (NEPA) guidelines drafted by the National Park Service (NPS, 1997). Additionally, it is known that there is an intact deposit of a prehistorichistoric Indian village site located near the parking lot and employee dormitory areas.

The assessment of the impact of the proposed retrofit schemes to historic resources must be performed following procedures in accordance with federal and states agencies and after consultation with Indian Tribes. Procedures to minimize impacts on cultural resources should be carefully developed. Environmental and cultural resources impact assessments are beyond the scope of this seismic evaluation. Although the preliminary retrofit schemes presented in this report may be feasible from the technical and economic viewpoints, it is not known at this time the impact environmental and archeological considerations will have on the proposed solutions.

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