Tetsuo Tobita

Soil–pile separation effect on the performance of a pile group under static and dynamic lateral loads

The effect of soil–pile separation is studied with respect to the performance of a laterally loaded pile group. Full-scale tests, which consist of a combination of a single and a 3 × 5 group pile under static and dynamic lateral loads, present a unique opportunity and allow a rigorous study without arbitrary parameter back-fitting. The coupled soil–pile system is idealized through two-dimensional finite elements with soil models idealized by a hyperbolic-type multiple shear mechanism. Nonlinear spring elements are used to idealize the soil–pile interaction through a hysteretic nonlinear load–displacement relationship. Joint elements with a separation–contact mechanism are used to idealize the separation effect at the soil–pile interface. Ignoring soil–pile separation in static tests overestimates the ultimate lateral load–carrying capacity by 43% for a single pile and 73% for the trailing pile in a closely spaced pile group. Moreover, neglecting soil–pile separation in dynamic tests overestimates the tota...

The Effect of Torsional Ground Motion on Structural Response: Code Recommendation for Accidental Eccentricity

In this article, data were collected from the Chiba dense array, which consists of 44 accelerometers with interstation spacing in the range of 5-300 m that are employed to estimate the torsional ground motion. The geodetic method was used to estimate torsional motions from the translational records in the Chiba dense array. The translational and computed torsional motions were then applied to the building models with different structural characteristics to evaluate the effectiveness of the accidental eccentricity levels proposed in various design codes. The results of analy- sis suggest that the 5% accidental eccentricity is on the safe side for most periods of interest in engineering practice. However, in the case of stiff structures (with periods shorter than 0.3 sec), an increase of up to four times in building displacement is ob- served by including the torsional excitation. Furthermore, we found that the acciden- tal eccentricity coefficient increased up to 0.6 at periods shorter than 0.3 sec, which is 12 times larger than what is proposed by the codes.

Vertical loads effect on the lateral pile group resistance in sand

Vertical loads effect on the lateral response of a 3×5 pile group embedded in sand is studied through a two-dimensional finite element analysis. The soil-pile interaction in three-dimensional type is idealized in the two-dimensional analysis using soil-pile interaction springs with a hysteretic nonlinear load displacement relationship. Vertical loads inducing a vertical pile head displacement of 0.1-pile diameter increase the lateral resistance of the single pile at a 60 mm lateral deflection by 8%. Vertical loads inducing the same vertical displacement applied to a pile group spaced at 3.92-pile diameter increase the overall lateral resistance by 9%. The effect on individual piles, however, depends on the pile position. The vertical load decreases the lateral resistance of the leading pile (pile 1) by 10% and increases the lateral resistances of piles 2, 3, 4, and 5 by 9%, 14%, 17%, and 35%, respectively. Vertical loads applied to the pile group increase the confining pressures in the sand deposit confined by the piles but the rate of increase in those outside the group is relatively small, resulting in the difference in a balance of lateral soil pressures acting at the back of and in front of the individual pile.

Seismic intensity estimation through questionnaire survey and collapse rates of various building types in the 2003 Bam, Iran, Earthquake

Seismic intensity in the epicentral area of the 2003 Bam, Iran earthquake is estimated using a questionnaire survey conducted two months after the earthquake. The estimated average seismic intensity on the Japan Meteorological Agency (JMA) scale is 6.1 (VIII to IX in the MMI scale). The peak frequency of the horizontal-to-vertical spectral ratio derived from microtremor measurements conducted during reconnaissance is also compared with the seismic intensity. Collapse rates for various structure types, such as adobe, unreinforced/reinforced masonry, steel-frame, and reinforced concrete, are obtained by counting the number of demolished buildings within an area of about 50-m radius around an observation point. Results show large differences in collapse rates between unreinforced and reinforced masonry, and suggest the upper limit of seismic intensity that unreinforced masonry can sustain. This fact can be utilized for an initial damage assessment within affected areas after large earthquakes.

Estimation of Liquefaction-Induced Manhole Uplift Displacements and Trench-Backfill Settlements

AbstractA simple method to predict the uplift displacement of a manhole and trench-backfill settlement due to liquefaction is proposed. The conventional equilibrium of vertical forces acting on a manhole is solely a function of such forces acting and is incapable of predicting the uplift displacement. In this paper, the proposed method adds variables including the uplift displacement, Δf, and settlements of backfill, Δs, under the condition that the volume of an uplifted portion of a manhole is equal to a settled volume of a trench-backfill. To date, the method is verified through comparison with the results of 1-G and centrifuge model tests. A new safety factor, which takes into account the amount of manhole uplift and backfill settlement, is also derived.

Centrifuge Modeling and Mitigation of Manhole Uplift due to Liquefaction

AbstractBecause low-compacted trench backfill around a manhole is normally liquefiable, the manhole could suffer uplift damage associated with soil liquefaction during a strong earthquake. In this study, 22 dynamic centrifuge models were tested to investigate the response of a buried manhole subjected to a dynamic load. The models were tested under 20g, and a substitute pore fluid was used to avoid the scaling law conflict between the dynamic and diffusion processes. It was found that excess pore water pressure is one of the contributing factors to the magnitude of the manhole uplift. Using this result, new mitigation methods against the uplift in liquefied ground were developed. Their effectiveness was also examined through the tests. A model manhole mitigated with the proposed methods was tested alongside regular model manhole. From the test results, the magnitude of manhole uplifts with the mitigation methods decreased as buildup of the excess pore water pressure was restrained in high-compacted backfi...

Seismic Performance and Design of Port Structures

This paper describes an emerging methodology for seismic evaluation and design of port structures. The methodology is based on minimum life-cycle cost principle through probabilistic evaluation of performance. While conventional seismic design of port structures are based on a particular return period specified for design, the methodology based on life-cycle cost allows for consideration of ground motions with all (or varying) return periods and uncertainty in geotechnical conditions. The methodology based on life-cycle cost also allows the probability of failure being evaluated rather than prescribed by an authority. Since ordinary port structures are for commercial use, the method based on the life-cycle cost has potential advantages over conventional design. An example is presented. In particular, the simplified design charts that are based on a series of parametric studies of effective stress analyses have great benefit for implementing the proposed methodology in practice. Apart from the ordinary port structures, higher priority should be assigned for safety as a performance objective. This issue is especially relevant if port structures are essential parts of post-earthquake emergency strategies for recovery and restoration of urban areas. The paper also discusses the importance of this issue and the future direction of study.

Seismic Performance Evaluation of Geotechnical Structures

The paper describes an emerging methodology for evaluating seismic performance of geotechnical structures that extend over tens of kilometers along a coastal protection line. The challenging aspect in establishing this type of methodology for performance evaluation is in the fact that site-by-site detailed study is not practical. The paper proposed a performance-based approach utilizing simplified design charts that are newly developed based on a parameter study of effective stress analyses of soil-structure systems. The coastal protection lines over a distance of 70km along Osaka Bay Area, Japan, are used as an example to demonstrate the advantages of the proposed approach.

Soil-Pile Interaction in Horizontal Plane

Two dimensional model tests are performed on a horizontal cross section of a soil-pile system in a pile foundation. The objective of the model tests is to evaluate local soil displacement field in the vicinity of the piles associated with a global displacement of soil around the pile foundation. Two dimensional effective stress analyses in horizontal plane are also performed to generalize the findings from the model tests. An effective stress model based on multiple shear mechanism is used through a computer code FLIP. Primary findings from this study are as follows: (1) In dry condition, displacement vectors are directed away from pile front, and displacement at pile side rapidly decreases with an increasing distance from soil-pile interface. In undrained condition, displacement field shows vortexes at pile side associated with push-out/pull-in pattern of displacements in front of and behind the pile. (2) Distribution of local soil displacement between piles deployed perpendicular to direction of global displacement of soil shows high strain concentration (i.e. discontinuity in displacement) at soil-pile interface.

Analysis of Group Pile Behavior under Lateral Spreading

Results of a series of centrifuge experiments to study the dynamic response of pile foundations under lateral spreading were compared to the results of numerical analysis. Experiments were carried out under the centrifugal acceleration of 40 G. Piles in the model foundation were lined up 3 by 3 pattern with a spacing of three pile diameter, and both pile head and bottom were rotation fixed. Piles were placed in the inclined ground of saturated sands and applied lateral loads due to the ground deformation. The effective stress finite element analysis was conducted to simulate these experiments. Computed time histories of pile head acceleration and displacement were consistent with those obtained from experiments. However, much smaller surface ground deformation in simulation might cause small amplitude of bending moments. Some calibration in the numerical modeling may be required to have more consistent results on bending moments.