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Research papers, University of Canterbury Library

Geosynthetic reinforced soil (GRS) walls involve the use of geosynthetic reinforcement (polymer material) within the retained backfill, forming a reinforced soil block where transmission of overturning and sliding forces on the wall to the backfill occurs. Key advantages of GRS systems include the reduced need for large foundations, cost reduction (up to 50%), lower environmental costs, faster construction and significantly improved seismic performance as observed in previous earthquakes. Design methods in New Zealand have not been well established and as a result, GRS structures do not have a uniform level of seismic and static resistance; hence involve different risks of failure. Further research is required to better understand the seismic behaviour of GRS structures to advance design practices. The experimental study of this research involved a series of twelve 1-g shake table tests on reduced-scale (1:5) GRS wall models using the University of Canterbury shake-table. The seismic excitation of the models was unidirectional sinusoidal input motion with a predominant frequency of 5Hz and 10s duration. Seismic excitation of the model commenced at an acceleration amplitude level of 0.1g and was incrementally increased by 0.1g in subsequent excitation levels up to failure (excessive displacement of the wall panel). The wall models were 900mm high with a full-height rigid facing panel and five layers of Microgird reinforcement (reinforcement spacing of 150mm). The wall panel toe was founded on a rigid foundation and was free to slide. The backfill deposit was constructed from dry Albany sand to a backfill relative density, Dr = 85% or 50% through model vibration. The influence of GRS wall parameters such as reinforcement length and layout, backfill density and application of a 3kPa surcharge on the backfill surface was investigated in the testing sequence. Through extensive instrumentation of the wall models, the wall facing displacements, backfill accelerations, earth pressures and reinforcement loads were recorded at the varying levels of model excitation. Additionally, backfill deformation was also measured through high-speed imaging and Geotechnical Particle Image Velocimetry (GeoPIV) analysis. The GeoPIV analysis enabled the identification of the evolution of shear strains and volumetric strains within the backfill at low strain levels before failure of the wall thus allowing interpretations to be made regarding the strain development and shear band progression within the retained backfill. Rotation about the wall toe was the predominant failure mechanism in all excitation level with sliding only significant in the last two excitation levels, resulting in a bi-linear displacement acceleration curve. An increase in acceleration amplification with increasing excitation was observed with amplification factors of up to 1.5 recorded. Maximum seismic and static horizontal earth pressures were recorded at failure and were recorded at the wall toe. The highest reinforcement load was recorded at the lowest (deepest in the backfill) reinforcement layer with a decrease in peak load observed at failure, possibly due to pullout failure of the reinforcement layer. Conversely, peak reinforcement load was recorded at failure for the top reinforcement layer. The staggered reinforcement models exhibited greater wall stability than the uniform reinforcement models of L/H=0.75. However, similar critical accelerations were determined for the two wall models due to the coarseness of excitation level increments of 0.1g. The extended top reinforcements were found to restrict the rotational component of displacement and prevented the development of a preliminary shear band at the middle reinforcement layer, contributing positively to wall stability. Lower acceleration amplification factors were determined for the longer uniform reinforcement length models due to reduced model deformation. A greater distribution of reinforcement load towards the top two extended reinforcement layers was also observed in the staggered wall models. An increase in model backfill density was observed to result in greater wall stability than an increase in uniform reinforcement length. Greater acceleration amplification was observed in looser backfill models due to their lower model stiffness. Due to greater confinement of the reinforcement layers, greater reinforcement loads were developed in higher density wall models with less wall movement required to engage the reinforcement layers and mobilise their resistance. The application of surcharge on the backfill was observed to initially increase the wall stability due to greater normal stresses within the backfill but at greater excitation levels, the surcharge contribution to wall destabilising inertial forces outweighs its contribution to wall stability. As a result, no clear influence of surcharge on the critical acceleration of the wall models was observed. Lower acceleration amplification factors were observed for the surcharged models as the surcharge acts as a damper during excitation. The application of the surcharge also increases the magnitude of reinforcement load developed due to greater confinement and increased wall destabilising forces. The rotation of the wall panel resulted in the progressive development of shears surface with depth that extended from the backfill surface to the ends of the reinforcement (edge of the reinforced soil block). The resultant failure plane would have extended from the backfill surface to the lowest reinforcement layer before developing at the toe of the wall, forming a two-wedge failure mechanism. This is confirmed by development of failure planes at the lowest reinforcement layer (deepest with the backfill) and at the wall toe observed at the critical acceleration level. Key observations of the effect of different wall parameters from the GeoPIV results are found to be in good agreement with conclusions developed from the other forms of instrumentation. Further research is required to achieve the goal of developing seismic guidelines for GRS walls in geotechnical structures in New Zealand. This includes developing and testing wall models with a different facing type (segmental or wrap-around facing), load cell instrumentation of all reinforcement layers, dynamic loading on the wall panel and the use of local soils as the backfill material. Lastly, the limitations of the experimental procedure and wall models should be understood.

Images, UC QuakeStudies

A photograph of Siobhan Murphy's house at 436 Oxford Terrace. The front walls has been covered with plastic sheeting. A bow made out of curtains has been pinned to the plastic where the fireplace juts out from the wall. The photographer comments, "The bow is a memorial to Murphy's living room and her life in the house".

Images, UC QuakeStudies

A photograph of Siobhan Murphy's house at 436 Oxford Terrace. The front walls has been covered with plastic sheeting. A bow made out of curtains has been pinned to the plastic where the fireplace juts out from the wall. The photographer comments, "The bow is a memorial to Murphy's living room and her life in the house".

Images, UC QuakeStudies

A photograph of Siobhan Murphy outside her house at 436 Oxford Terrace. The front walls of the house have been covered with plastic sheeting. A bow made out of curtains has been pinned to the plastic where the fireplace juts out of the closest wall. The photographer comments, "The bow is a memorial to Murphy's living room and her life in the house".

Images, UC QuakeStudies

The Octagon Live Restaurant, formerly the Trinity Congregational Church, on the corner of Worcester and Manchester Streets. There has been considerable damage to the front wall, and wire fencing has been placed around the building.

Images, Canterbury Museum

One model of the Temple for Christchurch with a rectangular base of Jarrah and solid silver conical shapes and wave like walls representing the movement of the 22 February 2011 earthquake. Based on the Temple for Christchurch sculpture that was designed by Hippathy Valentine.

Images, UC QuakeStudies

A photograph of a house at 7 Rees Street. The side of the house has been covered in plastic sheeting. Plywood has been used to board up the door. The number of the house has been spray-painted on the wall next to the door. The letterbox of the house next door also has its house number spray-painted on it.

Images, eqnz.chch.2010

There are occasional sewerage spills into the Avon River while all the sewer and road repairs are carried out. This rock wall was level and well above high tide level prior to the eathquakes. All the houses that can been seen here (except for those on the distant Port Hills) are in the suburban "red zone" and are still to be demolished.

Images, UC QuakeStudies

A photograph of graffiti on one of the walls of the kitchen in Donna Allfrey's house at 406 Oxford Terrace. Parts of the graffiti read, "Quakes, a national disaster", "Recovery, a national disgrace", "Ring fn EQC, ring fn insurance", "Useless fn council", "Don't let the bastards get you down", "Avon Loop - park or developers fodder?", and "Never trust a Carter". There are also shopping and to-do lists scrawled amongst these messages.

Images, UC QuakeStudies

A photograph of the earthquake damage to a section of Robin Duff's house at 386 Oxford Terrace. The chimney has fallen off the roof and is now resting in the patio. Behind the chimney, one of the walls has a noticeable lean and the glass of one of the French doors is broken. The photographer comments, "The glass was broken by looters who entered the house and took bits that interested them".

Images, UC QuakeStudies

A photograph of graffiti on one of the walls of the kitchen in Donna Allfrey's house at 406 Oxford Terrace. Parts of the graffiti read, "Quakes, a national disaster", "Recovery, a national disgrace", "'It's been a lot of fun' - John Key, yeah right!", "Brownlee sucks, "Ring fn EQC, ring fn insurance", "Useless fn council", "Sideshow Bob is an idiot", "Don't let the bastards get you down", "Avon Loop - park or developers fodder?", and "Never trust a Carter". There are also shopping and to-do lists scrawled amongst these messages.

Images, UC QuakeStudies

A photograph of graffiti on one of the walls of the kitchen in Donna Allfrey's house at 406 Oxford Terrace. Parts of the graffiti read, "Quakes, a national disaster", "Recovery, a national disgrace", "'It's been a lot of fun' - John Key, yeah right!", "Brownlee sucks, "Ring fn EQC, ring fn insurance", "Useless fn council", "Sideshow Bob is an idiot", "Don't let the bastards get you down", "Avon Loop - park or developers fodder?", and "Never trust a Carter". There are also shopping and to-do lists scrawled amongst these messages.