University of Auckland Centre for Earthquake Engineering Research


Liquefaction

Liquefaction results in ground settlements and lateral spreading, causing extensive damage to residential houses, buried pipelines and other infrastructure. To understand sub-surface movements during the liquefaction process, model ground is constructed in a large-scale laminar box. A number of smart sensors are installed to investigate the soil response.

         

 

 

Improved understanding of liquefaction effects on shallow foundations for enhanced aseismic design


N. Chouw, R. Orense, T. Larkin

The research addresses a knowledge gap, ie, an understanding of the intrinsic subsurface behaviour of liquefied sand and its influence on shallow foundations. This will be achieved by performing physical experiments using a large laminar box on a shake table and numerical analyses. The study combines expertise from geotechnical engineering, earthquake engineering, structural engineering, computer and electronic engineering. The research will elucidate the development of failure of coupled soil-shallow foundation systems in liquefied soil. The insights obtained will lead to understanding of the subsurface motion of soil in both the pre-liquefaction and liquefaction states. The relationship between liquefied soil and shallow foundation behaviour will be used in numerical analyses to develop enhanced aseismic design recommendations.

 

Investigation of liquefaction-induced ground displacements using smart particles


R Orense, T Larkin, N Chouw, Z Salcic

Past large-scale earthquakes, such as the recent Canterbury earthquakes, have shown that liquefaction and the associated ground deformations are major geotechnical hazards during earthquakes. Liquefaction has resulted in ground settlements and lateral spreading, causing extensive damage to residential houses, buried pipelines, and other infrastructures. While the manifestations of liquefaction, such as sand boils and lateral movements, can be directly observed on the ground surface, not much has been known on the mechanism of subsurface ground deformation, mainly because they cannot be observed.

To understand subsurface movements during liquefaction process, we are undertaking physical model tests using a shaking table. For this purpose, model grounds are constructed in a large-scale laminar box inside of which different transducers are installed to investigate the response.

To make this model testing different, we embedded “smart particles” whose motions in space within the model ground can be recorded. These pebble-sized smart particles have not been used in liquefaction studies anywhere in the world.

The novel aspects of the project are three-fold.

  1. Real-time detection of particle movements for a better understanding of the relation between subsurface and ground surface movements.
  2. Application of smart particles in model testing, possibly a first in the world.
  3. Combined expertise of the researchers from different fields in addressing current knowledge gaps in soil liquefaction studies, ie, soil and structural dynamics, geotechnical engineering, earthquake engineering and computer science and electronic engineering.
     

Find out more about this project

 
 

Collaborative study on soil liquefaction based on case histories from the 2011 Japan and NZ earthquakes


R Orense, M Pender, T Larkin, N Chouw

The 22 February 2011 Christchurch earthquake in New Zealand and the 11 March 2011 Tohoku earthquake in Japan have caused significant damage over very large extent of urban areas, possibly unprecedented in the world, due to soil liquefaction and associated ground deformations.

The University of Auckland and the University of Tokyo are combining their research expertise and resources, as well as their field experience and well-documented studies from their reconnaissance works, to undertake a collaborative research project to investigate the soil liquefaction occurrence on different types of soils, ranging from the naturally-sedimented soils of Christchurch to artificial fills in Tokyo Bay and to mitigate liquefaction-induced damage to lightweight structures.

This is done by

  1. Sharing laboratory test and shaking table test results, numerical simulations, boring data and other field information.
  2. Exchange visits of staff/postgraduate students for 1-2 week duration per year.
  3. Joint publications of research results through international journals of reputable standing.
  4. Joint sponsorship of a Japan-NZ Workshop on Earthquake Geotechnical Engineering to collate and disseminate research outcomes.

It is expected that the research output will further enhance our current understanding of liquefaction mechanisms and will result in economical ways of mitigating its damaging effects, especially to residential houses.

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Dynamic properties of Auckland residual soil under very small strain range


R Orense, M Pender

Auckland’s landform is typified by Waitemata sedimentary rocks and the geomorphology studies suggested that the Waitemata sedimentary basin was fully formed 20 million years ago (early Miocene). In-situ weathering of Waitemata group sandstones and siltstones produced cohesive residual soils, which are either dominated by silt or clay materials. The nature of these subsurface materials is highly variable, with the depth of the in situ weathering profile ranging from a few metres to a few tens of metres at the very maximum. 

This project focuses on the laboratory investigation of the small strain shear modulus (Gmax) of Auckland residual soil using an integrated small strain triaxial system.

A modified small strain triaxial apparatus was developed and capable of measuring accurately the small strain behaviour of undisturbed Auckland residual soil. Three submersible miniature linear variable differential transducers (LVDTs) mounted on yokes, which clamped onto the soil specimen at three locations (120o apart from each other), were used to measure axial strains over a gauge length of 100 mm. Load cell with 0.4 N resolution and 16 bit AD converter provided good accuracy in the load and axial strain measurements. The device was capable of consistently resolving displacements of less than 1 μm and measuring axial strains less than 0.003%. Furthermore, bender elements system was incorporated in the readily available small strain triaxial apparatus. Piezoelectric bending actuators were installed at the top cap and bottom pedestal of the modified triaxial apparatus.

A function generator created a change of voltage in the transmitter to induce bending and transmission of shear wave through the specimen. The arrival of the shear wave at the other end of the specimen was recorded by an oscilloscope. The shear wave velocity, Vs, was calculated from the travel time of shear wave, taken as "start-to-start" between two instants at generation and at reception of shear wave, and the “tip-to-tip” distance between the elements. Gmax was then calculated from the known soil density and Vs. Input pulse using various kinds of wave form over a wide range of frequency was used. In the tests, the shear modulus was obtained at different levels of consolidation pressure.

 
 

Liquefaction characteristics of pumice


R Orense

Pumice materials are frequently encountered in many engineering projects in the North Island of New Zealand. Because of their lightweight, highly crushable and compressible nature, they are problematic from engineering and construction viewpoints.

Most existing engineering correlations originally developed for ordinary sands are not applicable to this material. In terms of evaluating liquefaction potential, empirical procedures currently available for sands were derived primarily from hard-grained (quartz) sands. No information is available whether these procedures are applicable to pumice deposits because there has been very little research done to examine the liquefaction characteristics of pumice. Thus, a research programme was undertaken to understand the cyclic/dynamic properties of pumice.

Several series of undrained cyclic triaxial tests were performed on two sets of pumice samples. One set was undisturbed samples taken from pumiceous deposits and the corresponding liquefaction resistances were determined. The other set was reconstituted specimens of commercially-available pumice sands where the effects of various parameters, such as relative density, confining pressure, and particle gradation, on the undrained behaviour and liquefaction resistance were investigated.

A comparison of the results was made with those of hard-grained sands. Particle crushing, both at the end and at various stages of the tests, was examined. The results of the cyclic undrained triaxial tests were supplemented by monotonic undrained tests on reconstituted pumice sand specimens. The influence of relative density, effective confining pressure and gradation characteristics on the undrained response was also investigated.

Finally, geotechnical investigations, consisting of cone penetration testing and seismic dilatometer tests were performed at two sites where the undisturbed samples were obtained. The cyclic resistance ratio (CRR) obtained for the undisturbed samples were then compared with the estimated CRR from conventional methods which are based on field testing.

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Timber liquefaction piles


R Orense

Timber piles applied as a liquefaction countermeasure have previously been designed through the use of empirical equations. These methods are based on the assumption that the soil and pile have equal strain during dynamic loading, essentially that the pile deforms purely in shear and there is no slip or gapping at the soil-pile interface. Empirical design methods also assume that the pile does not structurally fail during dynamic loading, and in-situ soil compaction occurring due to the pile driving process is ignored.

This research aims to assess if existing empirical design methods accurately predict timber pile effectiveness as a liquefaction countermeasure. The research objectives are addressed through numerical modelling employing the three-dimensional finite element program OpenSees. Numerical modelling is conducted to verify the OpenSees output accuracy, followed by an assessment of the effect that the grid of timber piles has on excess pore water pressure generation. Each of the assumptions employed by existing empirical design methods is examined in detail. Comparison is also made between empirically and numerically estimated excess pore water pressures for three New Zealand design examples.

Results indicate that the grid of timber piles alone provides very little restraint to excess pore water pressure generation. Significant magnitudes of soil-pile interface slip and gapping are recorded, indicating that the equal strain assumption may be inappropriate. The assumption that the pile does not structurally fail during dynamic loading does appear to be appropriate for the ground conditions and earthquake excitations investigated. Incorporating in-situ soil compaction due to pile driving significantly increases the pile effectiveness as a liquefaction countermeasure. Overall, existing empirical design methods may significantly overestimate timber pile effectiveness as a liquefaction countermeasure.

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Effectiveness of stone columns as a countermeasure for liquefaction-induced lateral spreading


R Orense

The risk of liquefaction and associated ground deformation can be reduced by various ground improvement methods including densification, solidification, and stone columns. The use of stone columns is a rather recent development compared with the more traditional soil densification approaches. There is a great need for better understanding of stone columns liquefaction hazard mitigation mechanisms.

The aim of this research is to assess the performance of stone columns as a countermeasure against liquefaction-induced lateral spreading. In particular, how the effects of densification of surrounding soils, drainage through the stone columns can be analysed using numerical modelling and how they affect the effectiveness of stone columns. Also, how 3D effects can be incorporated in a 2D model. For this purpose, the finite element computational platforms FLAC 2D and FLAC 3D is employed .

The performance of stone columns as a countermeasure for liquefaction is evaluated by varying stone column parameters such as column spacing, column diameter, material stiffness, drainage and insitu geology. The mechanism involved on how stone columns mitigate lateral spreading is investigated by examining the response of soils at different locations within the grid of columns with respect to untreated ground and how drainage between and within the stone columns affect the overall performance.

Through this study, it is anticipated that the design process of stone columns can be improved via numerical modelling and how 3D effects can be incorporated into the design of stone columns.

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Impact of liquefiable soil on the behaviour of coupled soil-foundation-structure systems in strong earthquakes

N Chouw, T Larkin, R Orense, M Pender

This research focuses on the dynamics of the soil-foundation-structure system in order to develop recommendations for the seismic design of multiple interacting structure-foundation footing systems. The research will address recommendations of the Canterbury Earthquake Royal Commission and ultimately improve the earthquake resilience of NZ cities.