Lijun Deng

Centrifuge Modeling of Bridge Systems Designed for Rocking Foundations

In good soil conditions, spread footings for bridges are less expensive than deep foundations. Furthermore, rocking shallow foundations have some performance advantages over conventional fixed-base foundations; they can absorb some of the ductility demand that would typically be absorbed by the columns, and they have better recentering characteristics than conventional reinforced-concrete (RC) columns. Foundations designed for elastic behavior do not have these benefits of nonlinear soil-structure interaction. One potential disad- vantage of rocking systems is that they can produce significant settlement in poor soil conditions. Centrifuge model tests were performed to account for the interaction between soil, footing, column, deck and abutments systems. Bridge systems with rocking foundations on good soil conditions are shown to perform well and settlements are small. An improved method for quantification of settlements is presented. The model tests are described in some detail. One of the important factors limiting the use of rocking foundations is the perception that they might tip over; experiments show that tipping instability is unlikely if the foundations are properly sized. In one experiment, a column for a system with large fixed-base foundation collapsed while the systems with smaller rocking foundations did not collapse. DOI: 10.1061/(ASCE)GT .1943-5606.0000605.

Probabilistic Seismic Performance of Rocking-Foundation and Hinging-Column Bridges

Many engineers are hesitant to specify rocking foundations for ordinary bridges because of the unsubstantiated notion that rocking bridges are more susceptible to instability than conventional fixed-base bridges. A parametric study using a finite element model including large deformation effects compares the performance and stability of stiff, flexible, tall, and short hinging-column and rocking-foundation systems. Eighty different ground motions, scaled using incremental dynamic analysis, were considered. Results show that, in a probabilistic sense, bridges with rocking foundations are more stable than bridges with hinging columns if their fundamental periods are the same and if base shear coefficients to initiate hinging or rocking mechanisms are the same. Maximum drifts are not much affected by changing between rocking and hinging mechanisms except near collapse, but residual drifts are smaller for rocking systems. The results also challenge the notion that rocking systems require a different design approach than hinging column systems.

Seismic Design of Rocking Shallow Foundations: Displacement-Based Methodology

AbstractThis paper proposes a direct displacement-based design (DDBD) methodology for seismic design of rocking shallow foundations for ordinary bridges under earthquake loads. A multilinear model is developed to represent the backbone curve of the nonlinear moment-rotation behavior. In addition, a new empirical relationship is proposed that correlates the initial rotational stiffness to the moment capacity of a rocking foundation; this correlation is proposed as an alternative to calculation of stiffness based upon elasticity theory. In the proposed design procedure, a bridge system consisting of a deck mass, a rocking foundation, and a damped elastic column is integrated into a single element from which the equivalent linear damping and period can be determined. The DDBD methodology uses the equivalent system damping and period along with a design displacement response spectrum to determine the seismic displacement demand. The approach is checked by comparing displacements predicted by this method to th...

Reconnaissance of liquefaction case studies in 2015 Gorkha (Nepal) earthquake and assessment of liquefaction susceptibility

Abstract The Gorkha (Nepal) earthquake of the moment magnitude M w 7.8 occurred on 25 April 2015, with the epicenter 77 km north-west of Kathmandu at a focal depth of 13 km. After the main shock of the earthquake, a field investigation was carried out in the Kathmandu Valley to collect case histories of geotechnical and structural damages. This paper provides the observations of liquefaction case histories and the liquefaction potential assessment based on Standard Penetration Tests for liquefied and non-liquefied sites were performed. Recorded ground motions from the event are presented and the local site effect on the occurrence of liquefaction is described briefly. Observed liquefaction case studies are presented. Ejected soils at liquefied sites were collected and the particle size distributions of ejected soils were analysed. SPT blow counts and the soil profiles at eight liquefied and non-liquefied sites were obtained. The factors of safety against liquefaction with depth and liquefaction potential index of the eight sites were estimated and compared with observed liquefaction areas during Gorkha earthquake. The liquefaction potential indices obtained from the analysis were found to be consistent with the field observation.

Field Testing of Axial Performance of Large-Diameter Helical Piles at Two Soil Sites

AbstractHelical piles are being used extensively in engineering applications in western Canada. There are insufficient studies on the axial load-transfer mechanism (ALTM) for helical piles, which r...

Reconnaissance Report on Geotechnical Engineering Aspect of the 2015 Gorkha, Nepal, Earthquake

ABSTRACT An earthquake of moment magnitude Mw 7.8 struck Nepal at 06:11 UTC on April 25, 2015. Field reconnaissance focused on the geotechnical engineering aspect of the earthquake was carried out in the Kathmandu Valley and regions near the epicenter. This paper presents briefly the geology of Nepal, the accelerogram, and results of standard penetration tests at selected sites in the valley. The study shows the failure case histories including landslides, road embankment settlement, bridge foundations and abutment damage, and liquefaction. The paper also highlights the impact of local soil properties and basin and ridge effect on the severity of damage.

Analyses of embedded piles reinforced landslides using strength reduction finite element method

Embedded stabilising piles for landslide reinforcement were proposed as a viable alternative to conventional full-length piles. Embedded piles are shorter than full-length piles, while still providing a sufficient factor of safety (FS) to landslides; however, the cost of embedded piles may be significantly less than full-length piles due to the smaller bending moment demand. The concept of effective length was introduced. A strength reduction finite-element model was developed for the analyses of embedded anti-slide pile-reinforced landslides. A method for searching for the optimum effective length and estimating lateral loads on piles was developed. A conceptual landslide reinforced with a single embedded pile was adopted to exercise the proposed method. Results showed that the FS increased with the pile length at the plastic limit failure state, the maximum bending moment of the pile increased significantly with the pile length. However, for a given design safety factor (target FS), the lateral load acting on the embedded pile was similar to that of the full-length pile. Finally, the proposed method was applied to stabilise a massive landslide using 18 embedded piles; field monitoring data implied that the performance of the reinforced landslide is acceptable.

Development of mechanical properties of Edmonton stiff clay treated with cement and fly ash

Abstract Cohesive soils are often mixed in situ with cementitious binders to serve as a deep foundation. However, there is limited research on cemented stiff clay in applications where high soil strength is required. The present research is aimed to determine the mechanical properties of soilcrete produced with Edmonton stiff clay. The equivalent cement content is high, between 18 and 30%. Two cementitious binders were used: 100% ordinary Portland cement and a mix of 90% cement and 10% fly ash. Unconfined compressive strength tests were carried out at different curing ages. Scanning electron microscopy images were taken to inspect soilcrete texture and examine the effects of fly ash. Results showed that soilcrete behaves similar to an overconsolidated clay; the specimens reach peak strength at strain lower than 1% at mature age (>56 days). The peak strength decreases with increasing water to cement ratio. The measured moduli range widely from 30 to 270 MPa; the initial and secant moduli have a linear relation. The residual strength is nearly linearly related to the peak strength. SEM images show that addition of 10% fly ash helps disperse the cement and reduce cement clusters; the damage on soilcrete occurs along the failure plane due to crushing of cement clusters.

Influence of Physical Modeling on Adoption of Rocking Foundations in Practice

Several centrifuge model tests have been conducted using the NEES facility at UC Davis to study the dynamic behavior of rocking shallow foundations and the interaction with highway bridges or building systems. The results suggest that rocking should be encouraged as one mechanism for absorbing energy and thereby reducing ductility demand on the structure. During these tests, the interactions between researchers and practictioners has been valuable (a) as a technology transfer mechanism, (b) to figure out the direction of research to convince practitioners that results are definitive, and (c) for academics and practitioners to collaborate in appropriate changes in national standards such as ASCE/SEI-41 (ASCE standard for Seismic Rehabilitation of Existing Buildings). This paper provides examples to illustrate the roles and influences of physical modeling in the evolution of practice related to rocking foundations for bridges and buildings.

Effects of Ground Motion Characteristics on Seismic Response of Rocking- Foundation Bridges

Past studies have revealed that the different characteristics of near- fault pulse-like and far-field broadband motions have varying effects on the response of structures. In this paper, a suite of pulse-like and broadband motions are utilized to investigate the response of rocking-foundation and hinging column bridges including large deformation effects. Incremental Dynamic Analyses were carried out on simplified models of which the following parameters were varied: the rocking strength, the column height, and the fundamental period. Results show that, in a probabilistic sense, rocking systems are more susceptible to tip-over in pulse-like motions than in broadband motions. The re-centering effect of rocking systems makes them relatively immune to damage in motions that consist of many small inelastic pulses. Elastic-perfectly plastic hinging systems have less re-centering capability and hence they are more susceptible to accumulated deformation and eventual collapse in many small cycles that might occur in long duration broadband motions.