John Howie

Associate Professor

Relevant Degree Programs

 

Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - May 2019)
Seismic assessment of basement walls in British Columbia (2016)

The current state of practice for seismic design of basement walls in Vancouver is based on the Mononobe-Okabe (M-O) method using a Peak Ground Acceleration (PGA) mandated by the National Building Code of Canada (NBCC, 2010). Because there is a little evidence of any significant damage to basement walls during major earthquakes, the Structural Engineers Association of British Columbia (SEABC) became concerned about designing the walls under the code-mandated PGA and set up a task force to review the current procedure for seismic design of basement walls in British Columbia (BC). The University of British Columbia (UBC) was asked to carry out this investigation. This thesis aims to provide solid base for designing the basement walls using an appropriate fraction of the code-mandated PGA in the M-O analyses. To this end, a series of dynamic nonlinear soil--structure interaction analyses are conducted to examine the seismic resistance of typical basement walls designed according to current practice in BC, for different fractions of the code mandated PGA (100% to 50%). The seismic responses of the walls are evaluated by subjecting them to ensembles of ground motions comprised of shallow crustal, deep subcrustal, interface earthquakes from a Cascadia subduction events and near-fault earthquake motions. Input motions are matched to the intensity of the seismic hazard using both spectral and linear scaling techniques. Representative 4-level and 6-level basement walls are analyzed. The nonlinear hysteretic response of the foundation soil is characterized in order to obtain realistic estimates of an interaction between the basement wall and the surrounding soil. In addition, the effects of the local site conditions in terms of geometrical and geological structure of soil deposits underlying the basement structure on the seismic performance of the basement walls are evaluated. The analyses show that current engineering practice for designing basement walls based on the M-O method and using 100% PGA is too conservative. The analyses suggest that a wall designed using 50% to 60% PGA results in an acceptable performance in terms of drift ratio.

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Investigations into the use of continuous shear wave measurements in geotechnical engineering (2014)

The research presented within this thesis covers the development of a means to continuously monitor shear waves in a laboratory triaxial apparatus and down-hole during seismic cone penetration. This work resulted from an investigation of ageing of Fraser River Sand using a bender element triaxial apparatus. Shear wave propagation times from bender elements were interpreted using published time domain and frequency domain techniques. These techniques provided similar results, but the variability exceeded the effect of ageing. The frequency domain and time domain techniques had different shortcomings. The two techniques could be combined to converge on a single frequency-dependent propagation time that was independent of the trigger signal waveform. This contribution was capable of resolving the small increase in shear wave velocity with age duration.The frequency domain component of the combined bender element technique could run continuously during an experiment. With this further contribution, it was possible to track the change in shear wave propagation time throughout an experiment. The continuous bender element testing was not observed to influence the effect of ageing. It was found that in Fraser River Sand ageing had a small effect on the shear wave velocity, no effect on the ultimate strength, and a significant effect on the shear stiffness over the intermediate small-strain range (observed from 0.01 to 1%). The normalized shear stiffness curve shifts to larger strains and becomes more brittle with ageing.The concepts of the developed continuous bender element method are not restricted to this equipment or even to just bender element testing. The continuous bender element method was adapted to down-hole seismic testing in the field. This contribution resulted in a continuous profile of the shear wave velocity during seismic cone penetration testing that is obtained without stopping the cone penetration. The developments in this thesis provide a continuous measure of the shear wave velocity through a laboratory experiment and a continuous profile with down-hole penetration depth, i.e. the shear wave velocity is measured every time the other parameters are taken.

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Effects of void redistribution on liquefaction-induced ground deformations in earthquakes : a numerical investigation (2009)

Liquefaction-induced ground failure continues to be a major component of earthquake-relateddamages in many parts of the world. Experience from past earthquakes indicates lateral spreads and flow slides have been widespread in saturated granular soils in coastal and river areas.Movements may exceed several meters even in very gentle slopes. More interestingly, failures have occurred not only during, but also after earthquake shaking.The mechanism involved in large lateral displacements is still poorly understood. Sand deposits often comprise of low permeability sub-layers e.g., silt seams. Such layers form a hydraulic barrier to upward flow of water associated with earthquake-induced pore pressures.This impedance of flow path results in an increase of soil skeleton volume (or void ratio) beneath the barrier. The void redistribution mechanism as the focus of this study explains why residual strengths from failed case histories are generally much lower than that of laboratory data basedon undrained condition.A numerical stress-flow coupled procedure based on an effective stress approach has been utilized to investigate void redistribution effects on the seismic behavior of gentle sandy slopes.This study showed that an expansion zone develops at the base of barrier layers in stratified deposits subjected to cyclic loading that can greatly reduce shear strength and results in large deformations. This mechanism can lead to a steady state condition within a thin zone beneath thebarrier causing flow slide when a threshold expansion occurs in that zone. It was found that contraction and expansion, respectively at lower parts and upper parts of a liquefiable slope with a barrier layer is a characteristic feature of seismic behavior of such deposits. A key factor is thepattern of deformations localized at the barrier base, and magnitude that takes place with some delay. In this thesis, a framework for understanding the mechanism of large deformations, and a practical approach for numerical modeling of flow slides are presented.The study was extended to investigate factors affecting the seismic response of slopes, including: layer thickness, barrier depth and thickness, ground inclination, permeability contrast, base motion characteristics and soil consistency.Another finding of this study was that a partial saturation condition results in delay inexcess pore pressure rise, and this factor may be responsible for the controversial behavior of the Wildlife Liquefaction Array, California (USA) during the 1987 Superstition earthquake.It was demonstrated that seismic drains are a promising measure to mitigate the possibledevastating effects of barrier layers.

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Energy transfer and grain size effects during the Standard Penetration Test (SPT) and Large Penetration Test (LPT) (2008)

The Standard Penetration Test (SPT) is the most widely used in-situ soil test in the world. "Large Penetration Test" (LPT) is a term used to describe any scaled up version of the SPT. Several types of LPT have been developed around the world for the purpose of characterizing gravel deposits, as SPT blow counts are less reliable in gravels than in sands. Both tests suffer from the lack of a reliable means of determining transferred energy. Further, the use of LPT blow counts is generally limited to calculation of equivalent SPT blow counts using correlation factors measured in sands. Variation of LPT blow counts with grain size is assumed to be negligible. This research shows that safety hammer energies can be reliably estimated from measurements of hammer impact velocity for both SPT and LPT. This approach to determining transferred energy is relatively simple, and avoids the primary limitation of existing methods, which is the inability to calibrate the instrumentation. Transferred energies and hammer impact velocities are collected from various sources. These data are used to determine the ratio between the hammer kinetic energy and the transferred energy (energy transfer ratio, ETR), which is found to follow a roughly Normal distribution for the various hammers represented. An assessment of uncertainty is used to demonstrate that an ETR based approach could be superior to existing energy measurement methods.SPT grain size effects have primarily been characterized as the variation of an empirical relative density correlation factor, (CD)SPT, with mean grain size. In this thesis, equivalent (CD)LPT data are back-calculated from measured SPT-LPT correlation factors (CS/L). Results of a numerical study suggest that SPT and LPT grain size effects should be similar and related to the ratio of the sample size to the mean grain size. Based on this observation, trend-lines with the same shape as the (CD)SPT trend-line are established for the back-calculated (CD)LPT data. A method for generating the grain size effect trend-line for LPT is then proposed. These trend lines provide a rational approach to direct interpretation of LPT data, or to improved prediction of equivalent SPT blow counts.

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