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

Because of the complexity of the soil-structure interaction (SSI) effect on high-rise buildings, contemporary design codes allow the use of results from an advanced numerical analysis in the design of structures without providing further stipulation. The information on SSI effects, however, is only available for low rise buildings with simple analysis procedures. Two hypothetical 20-story buildings and one 30-story real building were subjected to seismic response analyses using SSI3D under the following conditions: rigid base, flexible base with linear foundation springs, flexible base with linear soil, flexible base with nonlinear springs, and the full SSI analysis with flexible base with nonlinear soils for two hypothetical buildings. For the real building, the calculated natural periods, base shears, and top-floor displacements were compared to the values evaluated using the recorded building motions. It was observed that the natural periods increase and the base shears decrease as the base becomes more flexible, but further study is needed to examine the top-floor side sway.

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.

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.

The popular theories currently used to estimate earth pressure^1,2 were developed many years ago³. Although these theories are considered appropriate for many earth pressure problems, they may not be suitable to determine the earth pressures developed from or transferred through reinforced soil walls and abutments. Estimates for earth pressures, from reinforced soils, acting on structural facings are considered especially important. A new large scale testing apparatus is developed to study the behavior of reinforced fill walls and abutments, especially for the development and distribution of earth pressures. The concept, design, construction, preliminary testing, and future plans for this testing system are presented in this paper.