Seismic Design Of Foundations Research Articles

Influence of site conditions on seismic design parameters for foundations as determined via nonlinear site response analysis

Site conditions, including geotechnical properties and the geological setting, influence the near-surface response of strata subjected to seismic excitation. The geotechnical parameters required for the design of foundations include mass density (ρ), damping ratio (βs), shear wave velocity (Vs), and soil shear modulus (Gs). The values of the last three parameters are sensitive to the level of nonlinear strain induced in the strata due to seismic ground motion. In this study, the effect of variations in soil properties, such as plasticity index (PI), effective stress (σ′), over consolidation ratio (OCR), impedance contrast ratio (ICR) between the bedrock and the overlying strata, and depth of soil strata over bedrock (H), on seismic design parameters (βs, Vs, and Gs) was investigated for National Earthquake Hazards Reduction Program (NEHRP) site classes C and D, through 1D nonlinear seismic site response analysis. The Morris one-at-a-time (OAT) sensitivity analysis indicated that βs, Vs, and Gs were significantly influenced by variations in PI, while ICR affected βs more than it affected Vs and Gs. However, the influence of H on these parameters was less significant. It was also found that variations in soil properties influenced seismic design parameters in soil type D more significantly than in soil type C. Predictive relationships for βs, Vs, and Gs were derived based on the 1D seismic site response analysis and sensitivity analysis results. The βs, Vs, and Gs values obtained from the analysis were compared with the corresponding values in NEHRP to determine the similarities and differences between the two sets of values. The need to incorporate PI and ICR in the metrics for determining βs, Vs, and Gs for the seismic design of foundations was highlighted.

Editorial Note - Vol. 14, No. 1 - 2020

We are excited to present this first of two issues in 2020, and hope you enjoy its mix of research and case study papers. 2020 is an exciting year for the DFI Journal as we expanded our editorial board with a set of highly qualified editors with various expertise in deep foundation engineering, ground improvement, slope stabilization, QA/QC of pile elements, load testing, seismic foundation design, and innovative foundation construction technologies. We invited qualified professionals from Academia, Government, Engineering Consulting, and Contracting with the primary objective to provide technical rigor and personal interest and commitment to advancing the foundation engineering practice. Our editorial structure now consists of three teams, including two Editor’s in Chief, a Journal Ombudsman, and a set of highly qualified Journal Editors. Each board member committed to serving a three-year term for the Journal. This structure allows for rotation and/or renewal upon the Editor’s availability and interest after completing the 3-year term on the DFI editorial board. Each editor is devoted to serve as an advocate for the Journal, as a technical resource for our members (in the context of publications) and has declared their willingness to elevate the Journal to a new level. This includes a continued publication of transformative and practice-oriented research as well as state of the art case studies (lessons learned) and novel techniques (engineered and built) that set benchmarks in engineering practice. We are certain that the new structure of the DFI Journal’s editorial board better reflects the broad membership diversity and expertise in the Deep Foundations Institute and are confident that you will find the right editor to work with for your future submissions. A full list of our team can be found on DFI’s Journal website.

Book review

Seismic Design of Foundations: Concepts and Applications

On the variation of mechanical properties of saturated sand during liquefaction observed in shaking table tests

Soil liquefaction induced by earthquakes not only causes settlement and lateral spreading of the ground, but also lowers the stiffness and strength of soil. In this paper, the variation of soil mechanical properties during liquefaction was investigated by revisiting the data of the physical model shaking table tests on saturated sand using a large laminar shear box conducted in National Center for Research on Earthquake Engineering with sinusoidal waves and the record of 1999 Chi-Chi Earthquake as input motions. The excited responses of the ground specimen during liquefaction were firstly characterized through spectrum analysis. One-dimensional shear beam idealization of the ground for wave propagation was further introduced to deduce the stress-strain relationship of the soil body. Thus, the variation of mechanical properties of saturated sand with respect to the development of shear strain and excess pore water pressure can be examined, which is beneficial to estimate appropriate soil parameters for the seismic design of foundations and to reasonably assess the seismic response of liquefied ground.

Field testing of rocking foundations in cohesive soil: cyclic performance and footing mechanical response

This paper presents a field test program of a large-scale soil–footing-structure system designed with a rocking foundation in a cohesive soil to examine the behaviour of the system and to provide case histories for possible performance-based seismic design of foundations. The rocking system was subjected to slow cyclic loadings at various drift ratios up to 7%. Twenty-four tests were conducted for foundations with varying initial factors of safety against the bearing failure, loading directions, rotation amplitudes, and embedment. A geotechnical investigation was carried out to determine soil properties before and after the experiments. The system performance indices, such as damping, stiffness, settlement, and re-centering capability, were quantified and compared with the published literature. Field test results showed that the strength and unit weight of soils at footing edges were increased due to rocking, for the present cohesive soil. The rocking moment capacity increased slightly with the increasing soil strength. An empirical equation for the secant stiffness was developed. The rocking system on the cohesive soil exhibited superior performance in terms of small residual settlement and large re-centering capability. Footing’s mechanical response was quantified using strain gauge readings. The footing remained elastic in tension; the transient soil–footing contact areas were estimated with strain gauges, and they agreed very well with the measured or calculated contact areas.

Simplified formulas for the seismic bearing capacity of shallow strip foundations

The seismic bearing capacity of shallow foundations is affected by inertia forces acting both on the structure and in the supporting soil. Even though the former have been recognised to play often the major role, by increasing the horizontal load and the overturning moment transferred to the foundation, both of them must be taken into account in the seismic design of foundations. Using a pseudostatic approach and based on the upper bound theorem of limit analysis, a comprehensive set of formulas is derived for the computation of the seismic bearing capacity of strip footings resting on cohesive-frictional and purely cohesive soils. Results are given in terms of: (i) reduction coefficients for the Terzaghi's equation of the vertical bearing capacity and (ii) ultimate failure envelopes in the space of normalised loading variables. These formulas extend to more general conditions other literature results, allowing to take into account easily the effects of inertia forces acting both on the superstructure (load inclination and eccentricity) and into the foundation soil. The reliability of the proposed equations, suitable for the design practice, is verified through a thorough comparison with other rigorous and approximate solutions.

A study on the liquefaction risk in seismic design of foundations

A fully coupled non-linear effective stress response finite difference (FD) model is built to survey the counter-intuitive recent findings on the reliance of pore water pressure ratio on foundation contact pressure. Two alternative design scenarios for a benchmark problem are explored and contrasted in the light of construction emission rates using the EFFC-DFI methodology. A strain-hardening effective stress plasticity model is adopted to simulate the dynamic loading. A combination of input motions, contact pressure, initial vertical total pressure and distance to foundation centreline are employed, as model variables, to further investigate the control of permanent and variable actions on the residual pore pressure ratio. The model is verified against the Ghosh and Madabhushi high acceleration field test database. The outputs of this work is aimed to improve the current computer-aided seismic foundation design that relies on ground’s packing state and consistency. The results confirm that on seismic excitation of shallow foundations, the likelihood of effective stress loss is greater in deeper depths and across free field. For the benchmark problem, adopting a shallow foundation system instead of piled foundation benefitted in a 75% less emission rate, a marked proportion of which is owed to reduced materials and haulage carbon cost.

Static and seismic bearing capacity of shallow strip footings

In this study, the evaluation of static and seismic bearing capacity factors for a shallow strip footing was carried out by using the method of characteristics, which was extended to the seismic condition by means of the pseudo-static approach. The results, for both smooth and rough foundations, were checked against those obtained through finite element analyses.Under seismic conditions the three bearing capacity problems for Nc, Nq and Nγ were solved independently and the seismic bearing capacity factors were evaluated accounting separately for the effect of horizontal and vertical inertia forces arising in the soil, in the lateral surcharge and in the superstructure.Empirical formulae approximating the extensive numerical results are proposed to compute the static values of Nγ and the corrective coefficients that can be introduced in the well-known Terzaghi׳s formula of the bearing capacity to extend its applicability to seismic design of foundations.

Characterization of Seasonally Frozen Soils for Seismic Design of Foundations

An experimental investigation was performed on five widespread soil types common in the United States to characterize the effects of freezing temperatures on the unconfined compressive strength (qu), the modulus of elasticity (E), and strain at the unconfined compressive strength (εqu). Soil specimens were subjected to monotonic and cyclic loading with varying strain rates at temperatures ranging from 20 to −23°C (68 to −9.4°F). When compared with test results at 20°C (68°F), testing at −20°C (−4°F) showed an increase in qu by a factor of 100, an average increase in E by a factor of 300, and an average decrease in εqu by 5% strain. Increase in the soil compaction, moisture content, and applied strain rate amplified the cold temperature effects on qu. Additional testing at −20°C (−4°F) resulted in an increase in εqu with no change in E when the applied strain rate was increased. Cyclic experimentation produced data trends comparable to the monotonic experimentation for the mechanical properties but allowed residual deformation as a function of cold temperature to be identified. To assist with current seismic design practice, experimental trends were incorporated into a p-y curve development and the impact of observed soil response as a function of temperature is demonstrated using a series of pushover analyses on a column continued into the subsurface as a drilled shaft foundation.

The role of non-linear dynamic soil-foundation interaction on the seismic response of structures

In this paper we provide an overview of recent research work that contributes to clarify the effects of non-linear dynamic interaction on the seismic response of soil-foundation-superstructure systems. Such work includes experimental results of seismically loaded structures on shallow foundations, theoretical advancements based on improved macro-element modeling of the soil-foundation system, examples of seismic design of bridge piers considering non-linear soil-foundation interaction effects, and numerical results of incremental non-linear dynamic analyses. The objective of this paper is to support the concept of a controlled share of ductility demand between the superstructure and the foundation as a key ingredient for a rational and integrated approach to seismic design of foundations and structures.

MODIFICATION OF SITE RESPONSE IN PRESENCE OF LOCALISED SOFT LAYER

Experimental and numerical simulations are performed to evaluate the modification of ground response resulting from either the presence of soft layers or occurrence of partial liquefaction. Results from two densely instrumented dynamic centrifuge tests are presented to show the ambiguous role played by the presence of a soft layer. It was found that the lateral extent of the soft layer has significant influence on the overall response of the layered strata and any structure founded on it. The experimental observations are supported by simplified numerical analysis. The amplification or deamplification of the input motion is found to be a function of the ratio of the width of soft layer to the wave length. Based on the numerical analysis, a general function describing the site amplification is presented which may be used as a guide in seismic design of foundations in such layered strata.

Geotechnical factors in seismic design of foundations state-of-the-art report

This paper revises the factors that influence the behavior of foundations in seismic environments. It discusses aspects related with seismic load definition, dynamic soil properties, field and laboratory testing equipment, geoseismic instrumentation of prototypes, foundation seismic stability, use of artificial intelligence, among others. It also points out areas where more research is needed to better our knowledge on the physics of the problem and to improve experimental and numerical techniques, with the purpose of making more reliable and less costly foundation systems.