The available simplified analytical methods for the seismic design of underground 5 structures either assume yielding or rigid-unyielding conditions. Underground reservoir 6 structures do not fall into either of these categories. In this paper, we present the results of three 7 centrifuge experiments that investigate the seismic response of stiff-unyielding buried structures 8 in medium dense, dry sand and the influence of structure stiffness and earthquake motion 9properties on their performance. The structure to far-field spectral ratios were observed to 10 amplify with increased structural flexibility and decreased soil confining pressure at the 11 predominant frequency of the base motion. Lateral earth pressures and racking displacements for 12 a range of structural stiffnesses were compared with procedures commonly used in design. Pre-13 earthquake measured lateral earth pressures compared well with expected at-rest pressures. 14 However, none of the commonly used procedures adequately captured the structural loading and 15 deformations across the range of stiffness and ground motions for which these reservoirs must be 16 designed. Further, it is unclear if the current methods of analysis provide conservative or 17 unconservative results for engineering design purposes. This identifies a critical need for 18 improved methodologies to analyze and design underground reservoir structures. ManuscriptClick here to download Manuscript Journal paper manuscript_vf.docx 4 of these structures are not fully captured by simplified seismic design procedures. Soil-structure-5 interaction (SSI) for these buried structures is complex and depends on foundation fixity, 6 properties of the surrounding soil, flexibility of the structure relative to soil, and the 7 characteristics of the earthquake motion. There is an increasing need in engineering practice to 8 obtain a better understanding of the seismic performance of these underground structures. For 9 example, the Los Angeles Department of Water and Power (LADWP) is replacing some of its 10 open water reservoirs with buried, reinforced-concrete reservoirs to meet water quality 11 regulations. Understanding the seismic performance of these restrained underground structures 12 will improve the structural and geotechnical seismic design of these type of projects. 13 Traditionally, underground structures are categorized either as yielding or rigid-unyielding, 14 and are designed differently based on the categorization. A yielding wall is one that displaces 15 sufficiently to develop an active earth pressure state. The current state of practice for assessing 16 seismic earth pressures on yielding structures relies heavily on the Mononobe-Okabe (Okabe 17 1926; Mononobe and Matsua 1929) and Seed-Whitman (Seed and Whitman 1970) methods. For 18 rigid-unyielding walls that don't undergo any deformation, the method of choice is often the 19 simplified solution proposed by Wood (1973), which assumes a completely rigid wall (with no 20 flexure). Underground reservoir structures fall in ...

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The seismic response of underground reservoir structures is a complex soilstructure interaction problem that depends on the properties of the earthquake motion, surrounding soil, and structure. More experimental and field data of the response of these structures under different boundary conditions is needed to validate analytical and numerical tools. This paper presents the results of four centrifuge experiments that investigate the seismic performance of reservoir structures, restrained from rotational movement at their roof and floor, buried in dry, medium-dense sand and compacted, partially saturated, silty sand. This study focuses on the influence of backfill soil properties, cover, and slope on accelerations, strains, and lateral earth pressures experienced by the buried structure. The structure to far-field acceleration spectral ratios were observed to approach unity with added soil confinement, density, and stiffness. Both dynamic thrust and accelerations on the structure showed a peak near the effective fundamental frequency of the backfill soil. The addition of a soil cover and stiffness increased seismic earth pressures and moved its centroid upward, hence increasing seismic moments near the base. The added stiffness, density, and apparent cohesion of the compacted site-specific soil did not influence the magnitude of earth thrust noticeably but moved its centroid upward. A sloping backfill reduced the earth pressures and bending moments near the top of the wall. The trends in the results indicates that

This paper summarizes the results from a centrifuge experiment conducted at the University of Colorado at Boulder to evaluate seismic soil-structureinteraction, lateral seismic earth pressures, and dynamic response of equivalent model structures representing buried water reservoirs during a selected suite of input earthquake motions. This paper presents a summary of the design and planning, instrumentation challenges, and test results for the first baseline experiment in a series of centrifuge tests. The preliminary results indicate that underground structures similar in type to those tested in this study can experience seismic lateral earth pressures of engineering importance. The insight from these tests is useful for understanding the performance of underground reservoir structures worldwide and is applicable to an entire class of unyielding underground structures that are restrained at their roof and floor levels.

The seismic performance of underground reservoir structures depends on the properties of the structure, soil, and ground motion as well as the kinematic constraints imposed on the structure. This paper seeks to understand the influences of site response, structural stiffness, base fixity, and excitation frequency on the performance of buried structures through the evaluation of results from four dynamic centrifuge experiments on relatively stiff and unyielding reservoir structures buried in dry, medium-dense clean sand. The magnitude of seismic thrust increased and the distribution of seismic earth pressures changed from approximately triangular to parabolic with increasing structural stiffness. Heavier and stiffer structures also experienced increased rocking and reduced flexural deflection. Fixing the base of the structure amplified the magnitudes of acceleration, seismic earth pressure, and bending strain compared to tests where the structure was free to translate laterally, settle, or rotate atop a soil layer. The frequency contents of transient tilt, acceleration, dynamic thrust, and bending strain measured on the structure was strongly influenced by the frequency content of the base motion and site response, while it was unaffected by the fundamental frequency of the structure (f structure). None of the available simplified procedures could capture the distribution and magnitude of seismic earth pressures experienced by this class of underground structures. The insight from this experimental study is aimed to help validate analytical and numerical methods used in the seismic design of reservoir structures worldwide.