Michael J. Pender

Seismic Design and Performance of Surface Foundations

Within the context of shallow foundation design this paper considers the need for more effective interaction between geotechnical and structural design teams so that progress towards the integrated design of structure–foundation systems can be achieved. The paper considers four aspects: (i) the role of the shallow foundation bearing strength surface, (ii) the variability in soil properties relative to variations in structural properties, (iii) the observation that bed-of-spring models cannot represent correctly vertical and rotational stiffness of shallow foundation, and (iv) an example of the numerical prediction of seismic response of a low-rise structure founded on shallow foundations is presented which indicates that moment free connections between the foundations and building columns leads to a more economical design.

Investigation on the Impact of Seasonally Frozen Soil on Seismic Response of Bridge Columns

This paper presents the development of numerical models that investigated the seismic response of a simple two span prototype bridge system during warm and frozen temperatures. Models from both temperature conditions were subjected to a range of seismic intensities to examine the effect of seasonal freezing on the response of the system. Stiffness characteristics were defined using cyclic models of a bridge pier that were previously developed and validated using results from an experimental program on identical full-scale column-foundation units, which were tested during the summer and winter months. Dynamic characteristics of the seismic models were defined using approaches found in the literature. Frozen conditions increased the maximum bending moment and shear force demands for all seismic intensities, with nonlinearity in the column/foundation reducing the difference between the peak responses at higher intensities. At the depth of maximum foundation shear for the frozen model, demand was three times ...

Pile Head Cyclic Lateral Loading of Single Pile

AbstractThis paper presents an elastic continuum model using an extended nonlinear Davies and Budhu equations, which enables the nonlinear behavior of the soil around the long elastic pile to be modeled using a simple expression of pile-head stiffness method. The calculated results were validated with the measured full-scale dynamic field tests data conducted in Auckland residual clay. An idealized soil profile and soil stiffness under small strain (i.e. shear modulus, Gs and shear wave velocity, Vs of the soil) determined from in situ testing was used to model the single pile tests results. The predictions of these extended equations are also confirmed by using the three-dimensional finite-element OpenSeesPL (Lu et al. in OpenSeesPL 3D lateral pile-ground interaction: user manual, University of California, San Diego, 2010). A soil stiffness reduction factor, Gs/Gs,max of 0.36 was introduced to the proposed method and model. It was found to give a reasonable prediction for a single pile subjected to dynamic lateral loading. The reduction in soil stiffness found from the experiment arises from the cumulative effects of pile–soil separation as well as a change in the soil properties subjected to cyclic load. In summary, if the proposed method and model are accurately verified and properly used, then they are capable of producing realistic predictions. Both models provide good modelling tools to replicate the full-scale dynamic test results.

Geotechnical aspects of the Mw6.2 2011 Christchurch New Zealand Earthquake

The 22 February 2011, Mw6.2 Christchurch earthquake is the most costly earthquake to affect New Zealand, causing an estimated 181 fatalities and severely damaging thousands of residential and commercial buildings. This paper presents a summary of some of the observations made by the NSF-sponsored GEER Team regarding the geotechnical/geologic aspects of this earthquake. The Team focused on documenting the occurrence and severity of liquefaction and lateral spreading, performance of building and bridge foundations, buried pipelines and levees, and significant rockfalls and landslides. Liquefaction was pervasive and caused extensive damage to residential properties, water and wastewater networks, high-rise buildings, and bridges. Entire neighborhoods subsided, resulting in flooding that caused further damage. Additionally, liquefaction and lateral spreading resulted in damage to bridges and to stretches of levees along the Waimakariri and Kaiapoi Rivers. Rockfalls and landslides in the Port Hills damaged several homes and caused several fatalities.

Diameter Effects on Pile Head Lateral Stiffness and Site Investigation Requirements for Pile Foundation Design

The effect of pile shaft diameter on unrestrained pile head lateral and rotational stiffnesses of long piles is considered in relation to required site investigation data. Analysis of field test data for lateral load tests on piles of different diameters published in the literature led to the suggestion that the modulus of subgrade reaction appears to increase with increasing pile shaft diameter. This is contrary to the usual understanding that the modulus of subgrade reaction is independent of pile shaft diameter. This puzzle is resolved in the realization that as the pile diameter increases the depth of soil which contributes to the lateral stiffness also increases. If the soil stiffness increases with depth, then the pile head lateral stiffness will be greater than would be predicted on the basis of lateral load testing of smaller diameter piles. Included in the article is discussion of the effect of the variation with pile shaft diameter of the unrestrained pile head lateral and rotational stiffnesses for three distributions of soil modulus with depth, as well as the effect of the pile head moment to shear ratio. A corollary of the article is that accurate estimates of pile head stiffness during foundation design require better than routine site investigation data.

Spatial evaluation of liquefaction potential in Christchurch following the 2010/2011 Canterbury earthquakes

Abstract Widespread damage as a result of liquefaction was observed in the Canterbury region following the 2010 Darfield earthquake and the 2011 Christchurch earthquake. To quantify the liquefaction risk in some areas associated with these two events, strong motion records and available boring data were used to produce maps showing distributions of liquefaction potential indices (LPI). It was found that for both events, the distributions of LPI values agree reasonably well with the observed severity of damage. The increased peak accelerations during the February 2011 event along with the elevated water table resulted in more severe damage in eastern Christchurch than during the 2010 earthquake, while the lower shaking intensity in the Waimakariri region led to a severe but more localized liquefaction. In cases where the calculated LPI and observed damage did not agree, the occurrence of lateral spreading and the thickness of the surface crust appear to be the main reasons. Finally, through analysis of boring data, the role of the surface crust in liquefaction manifestation was analyzed.

Macro Element for Pile Head Cyclic Lateral Loading

Interaction between laterally loaded piles and the surrounding soil is a complex phenomenon, particularly when nonlinear soil behaviour is involved; so complex that usually design calculations rely on computer software based on discrete spring formulations using empirically derived nonlinear p-y relationships. This chapter explores a macro element, Davies and Budhu (1986), as an alternative which uses relatively simple formulae that are available for evaluating the lateral stiffness of long elastic piles embedded in elastic soil and an extension to handle nonlinear soil-pile interaction. The predictions of these equations are confirmed using the three dimensional finite element software OpenSeesPL, Lu et al. (2010), as well as data from field lateral load testing on driven piles in a stiff residual soil at a North Auckland site. Furthermore, in this chapter an extension of the macro element to cyclic loading is presented and this is shown to model well the field data and also the predictions of OpenSeesPL. The pile head macro element method is not completely general as it applies only to a homogeneous soil profile, but, since we deal with long piles, the soil homogeneity needs to extend only over the pile shaft active length. Measured lateral load response of the piles at the Auckland site indicates that it is necessary to distinguish the “operational” modulus of the soil from the small strain modulus; the field data indicates a value of about one third to one quarter of the small strain value.

K0 Compression and Stress Relaxation of Pumice Sand

AbstractPumice sand particles have a vesicular nature, making them light and crushable. Previous research showed that the in situ relative density of pumice deposits cannot be estimated from conventional cone penetration testing. Because of this, a need exsits for more study of the geotechnical properties of this material. First, to distinguish compression behavior of loose and dense sand, K0 compression tests were performed on pumice specimens at various strain rates, from 0.33% to 1,000%/min, until a final compression of approximately 33% of the original specimen length was achieved. Second, after compression, the maximum displacement was held constant for a period of time during which the relaxation of the axial stress was monitored. After unloading, the particle-size distribution was measured to confirm particle crushing. From these results, the magnitude of stress relaxation of loose sand was found to be slightly larger than that of dense sand. On the other hand, dense sand particles exhibited more c...

One Dimensional Shallow Foundation Macro-element

Recently a number of macro-element models have been formulated for assessing the performance of shallow foundations during earthquake loading. These provide a computational tool that represents the nonlinear dynamic behavior of the foundation in a manner much simpler than finite element modelling; consequently, they are useful for preliminary design. The basis of this chapter is the shallow foundation moment-rotation pushover curve, which is bracketed by the rotational stiffness at small deformations, determined by the small strain stiffness of the soil, and the moment capacity, which is a function of the soil shear strength and the vertical load carried by the foundation. Between these two limits there is a curved transition. The paper argues that when the vertical load carried by an embedded foundation is a small fraction of the vertical bearing strength, the moment-rotation behavior dominates the response. This means that the structure-foundation system can be reduced to a single degree of freedom (SDOF) model.

Dynamic Soil Stiffness Between WAK, SASW and SCPT Tests

This paper describes an experimental investigation for determining the dynamic soil stiffness by applying the principles of WAK (wave-activated stiffness [K]) test analysis, spectral analysis of surface waves (SASW) method and seismic cone penetration test (SCPT). The WAK and SASW tests were performed by applying an impact load on a circular steel plate of 50 cm diameter in vertical direction. A sledgehammer equipped with a dynamic force transducer was used to produce the impact load. The force time signal from the dynamic loading (input) and acceleration time signals from vertical accelerometers (output) were recorded during the tests. The dynamic stiffness of soil was obtained by considering the soil to be vibrating as a single degree of freedom (SDOF) system. The SCPT was performed by measuring the travel times of body waves propagating between a seismic shear wave source at the ground surface activated at each level and an array of geophones. The dynamic soil stiffness obtained from WAK and SASW tests compared very well with the SCPT test.