Editor: Yail J. Kim and Nien-Yin Chang

Soil-structure interaction has been of interest over several decades; however, many challenging issues remain. Because all structural systems are founded on soil strata, transient and long-term foundation displacements, particularly differential settlement, can severely influence the behavior of structural members in buildings and bridges. This is particularly important when a structure is constructed in earthquake-prone areas or unstable soil regions. Adequate subsurface investigation, design, and construction methods are required to avoid various damage types from structural and architectural perspectives. Typical research approaches include laboratory testing and numerical modeling. The results of on-site examinations are often reported. Recent advances in the-state-of-the-art of soil-structure interaction contribute to accomplishing the safe, reliable, and affordable performance of concrete structures. This Special Publication (SP) encompasses nine papers selected from two technical sessions held in the ACI Fall convention at Denver, CO, in Nov. 2015. All manuscripts submitted are reviewed by at least two experts in accordance with the ACI publication policy. The Editors wish to thank all contributing authors and anonymous reviewers for their rigorous efforts. The Editors also gratefully acknowledge Ms. Barbara Coleman at ACI for her knowledgeable guidance.

Note: The individual papers are also available. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP-316

Increasingly contemporary bridge abutments are supported on mechanically stabilized backfill (MSB) or geotextile reinforced soil (GRS) mass to enhance smooth ridership when vehicles transitioned from embankment to bridge deck with facing concrete blocks, rigid concrete panels or steel sheet piles to retain backfill. The bridge dead load and live load flow from bridge deck and girders to abutment sill, MSB (or GRS) mass, geo-textile, earth retaining structures and subgrade underlying MSB through interface interaction. Evaluation of the MSB abutment performance requires clear understanding of load transfer through interface interaction among neighboring materials. The replacement of the I-70 Twin Bridge in Aurora, Colorado made possible the implementation of a comprehensive instrumentation program to monitor the performance of abutment. Clean crushed rocks with minus 2-inch (50-mm) grain size, angular grains and less than 10% fines were used as backfill. Instrument monitor results were used in the calibration of two selected computer codes. Finite element analysis results using both LS-DYNA and SSI2D were found to be in good agreement with the field instrument monitoring results. Earth pressures behind steel sheet pile façade, lateral deformation at top of sheet piles and geo-fabric tensile loads were found small at the time when this article was written; abutment settlements and lateral movement were less than one inch (25 mm).

Two concrete masonry buildings, at adjacent sites in Glenwood Springs, Colorado, are located atop existing collapsible debris fan soils. Both buildings were constructed on concrete foundations with spread footings, and both suffered serious and damaging differential settlements. Compaction grouting was utilized for underpinning and lifting both buildings. Compaction grout columns are comprised of a low slump and low strength grout made from a combination of sand, soil, pea gravel, cement, and water. When installed under pressure, the grout densifies the surrounding soils supporting the building foundation, and when carried to the underside of footings, the grout can offer direct support. The grout was also used to lift and partially level the buildings. But here the similarity ends; each had unique circumstances and the repair designs were custom tailored. One was underpinned with deep (100’) compaction grout columns while the other received a much shallower underpinning treatment. Each had unique drainage problems. Both projects were challenging and required cooperation among the Owners, Structural, Geotechnical and Civil Engineers, and the Contractors. The geotechnical studies, the structural design for repair, the drainage provisions for each, and the construction are described, with a focus on structural damage, design of the underpinning to be compatible with the structural capacities, and control systems utilized during construction.

Performance-based seismic design of structures calls for many analyses and therefore becomes computationally expensive in large-scale soil-structure interaction (SSI) problems where the superstructure and its surrounding semi-infinite soil must be considered together. The substructure method is an attempt to reduce this computational cost through substituting the far-field elastic soil with a set of impedance functions, which are generally nonlinear functions of frequency (depending on soil heterogeneity and foundation geometry). This in turn results in integro-differential equations if one wants to take nonlinear frequency dependence of the impedances into account exactly in the time domain. In practice, representative linear functions—i.e., constant stiffness and damping coefficients, are used to avoid this complexity. However, the accuracy of this simplifying approach in predicting the responses of structures, especially when considering inelastic behavior, is not well understood. To address this knowledge gap, herein the inelastic response of superstructures subject to SSI effects is studied. Rational functions are used to approximate nonlinear impedance functions. They are represented efficiently as digital filters in the time domain and are solved along with the superstructure’s equations of motion to obtain response histories when subjected to select ground motions. These results are compared to those obtained both assuming a fixed base (neglecting SSI effects) and using linear impedance functions. The effects of inertial SSI are observed as the difference between the fixed base and substructure responses. Additionally, potential inaccuracies induced through the use of representative linear functions are identified and discussed. The work concludes by offering ductility maps, which graphically depict the effects of inertial SSI on inelastic structural systems, as an example of a practical application of the filter method.

Prediction of the seismic response of civil structures without considering the flexibility and damping provided by their supporting soil-foundation systems can be unrealistic, especially for stiff structures. In engineering practice, the substructure method is generally preferred for considering Soil-Structure Interaction (SSI) due to its computationally efficiency. In this method, soil is modeled using discrete spring elements that are attached to the superstructure; and the Foundation Input Motions (FIMs)—which are usually calculated through analytical transfer functions from recorded/anticipated free-field motions—are applied at the ends of these springs. Whereas the application of the substructure method itself is simple, the determination of FIMs and the soil-foundation systems’ dynamic stiffnesses are challenging. In the present study, we propose two new approaches to identify the dynamic stiffness of soil-foundation systems from response signals recorded during earthquakes. In these approaches, the superstructure is represented either by a numerical (finite element) or by an analytical (Timoshenko beam) model, and the soil is represented by discrete frequency-dependent springs. In both approaches, the superstructure and soil-foundation stiffnesses are all identified through model updating. We present various forms for the second approach (involving the Timoshenko beam) and verify these through comparisons with the results from the first approach (involving the finite element model) obtained using earthquake data recorded at the Robert A. Millikan Library at the Caltech campus in Pasadena, CA.