This paper presents an examination of ground motion observations from 20 near-source strong motion stations during the most significant 10 events in the 2010-2011 Canterbury earthquake to examine region-specific systematic effects based on relaxing the conventional ergodic assumption. On the basis of similar site-to-site residuals, surfical geology, and geographical proximity, 15 of the 20 stations are grouped into four sub-regions: the Central Business District; and Western, Eastern, and Northern suburbs. Mean site-to-site residuals for these sub-regions then allows for the possibility of non-ergodic ground motion prediction over these sub-regions of Canterbury, rather than only at strong motion station locations. The ratio of the total non-ergodic vs. ergodic standard deviation is found to be, on average, consistent with previous studies, however it is emphasized that on a site-by-site basis the non-ergodic standard deviation can easily vary by ±20%.
The 2010-2011 Canterbury earthquakes were recorded over a dense strong motion network in the near-source region, yielding significant observational evidence of seismic complexities, and a basis for interpretation of multi-disciplinary datasets and induced damage to the natural and built environment. This paper provides an overview of observed strong motions from these events and retrospective comparisons with both empirical and physics-based ground motion models. Both empirical and physics-based methods provide good predictions of observations at short vibration periods in an average sense. However, observed ground motion amplitudes at specific locations, such as Heathcote Valley, are seen to systematically depart from ‘average’ empirical predictions as a result of near surface stratigraphic and topographic features which are well modelled via sitespecific response analyses. Significant insight into the long period bias in empirical predictions is obtained from the use of hybrid broadband ground motion simulation. The comparison of both empirical and physics-based simulations against a set of 10 events in the sequence clearly illustrates the potential for simulations to improve ground motion and site response prediction, both at present, and further in the future.
he strong motion station at Heathcote Valley School (HVSC) recorded unusually high peak ground accelerations (2.21g vertical and 1.41g horizontal) during the February 2011 Christchurch earthquake. Ground motions recorded at HVSC in numerous other events also exhibited consistently higher intensities compared with nearby strong motion stations. We investigated the underlying causes of such high intensity ground motions at HVSC by means of 2D dynamic finite element analyses, using recorded ground motions during the 2010-2011 Canterbury earthquake sequence. The model takes advantage of a LiDAR-based digital elevation model (DEM) to account for the surface topography, while the geometry and dynamic properties of the surficial soils are characterized by seismic cone penetration tests (sCPT) and Multi-Channel Analyses of Surface Waves (MASW). Comparisons of simulated and recorded ground motions suggests that our model performs well for distant events, while for near-field events, ground motions recorded at the adopted reference station at Lyttelton Port are not reasonable input motions for the simulation. The simulations suggest that Rayleigh waves generated at the inclined interface of the surficial colluvium and underlying volcanic rock strongly affect the ground motions recorded at HVSC, in particular, being the dominant contributor to the recorded vertical motions.
The sequence of earthquakes that has affected Christchurch and Canterbury since September 2010 has caused damage to a great number of buildings of all construction types. Following post-event damage surveys performed between April 2011 and June 2011, an inventory of the stone masonry buildings in Christchurch and surrounding areas was carried out in order to assemble a database containing the characteristic features of the building stock, as a basis for studying the vulnerability factors that might have influenced the seismic performance of the stone masonry building stock during the Canterbury earthquake sequence. The damage suffered by unreinforced stone masonry buildings is reported and different types of observed failures are described using a specific survey procedure currently in use in Italy. The observed performance of seismic retrofit interventions applied to stone masonry buildings is also described, as an understanding of the seismic response of these interventions is of fundamental importance for assessing the utility of such strengthening techniques when applied to unreinforced stone masonry structures. AM - Accepted Manuscript
The need for a simple but rigorous seismic assessment procedure to predict damage to reinforced concrete buildings during a seismic event has been highlighted following the Canterbury Earthquake sequence. Such simplified assessment procedure, applied to individual structure or large building inventory, should not only have low requirement in terms of input information and involve straightforward analyses, but also should be capable to provide reliable predictive results within short timeframe. This research provides a general overview and critical comparison of alternative simplified assessment procedures adopted in NZSEE 2006 Guidelines (Assessment and Improvement of the Structural Performance of Buildings in Earthquakes), ASCE 41-13 (Seismic Evaluation and Retrofit of Existing Buildings), and EN: 1998-3: 2005 (Assessment and Retrofitting of Buildings). Particular focus is given to the evaluation of the capability of Simplified Lateral Mechanism Analysis (SLaMa), which is an analytical pushover method adopted in NZSEE 2006 Guidelines. The predictive results from SLaMa are compared to damages observed for a set of reinforced concrete buildings in Christchurch, as well as the results from more detailed assessment procedure based on numerical modelling. This research also suggests improvements to SLaMa, together with validation of the improvements, to include assessment of local mechanism by strength hierarchy evaluation, as well as to develop assessment of global mechanism including post-yield mechanism sequence based on local mechanism.
Documenting earthquake-induced ground deformation is significant to assess the characteristics of past and contemporary earthquakes and provide insight into seismic hazard. This study uses airborne light detection and ranging (LiDAR) and conducts multi-disciplinary field techniques to document the surface rupture morphology and evaluate the paleoseismicity and seismic hazard parameters of the Hurunui segment of the Hope Fault in the northern South Island of New Zealand. It also documents and evaluates seismically induced features and ground motion characteristics of the 2010 Darfield and 2011 Christchurch earthquakes in the Port Hills, south of Christchurch. These two studies are linked in that they investigate the near-field coseismic features of large (Mw ~7.1) earthquakes in New Zealand and produce data for evaluating seismic hazards of future earthquakes. In the northern South Island of New Zealand, the Australian-Pacific plate boundary is characterised by strike-slip deformation across the Marlborough Fault System (MFS). The ENE-striking Hope Fault (length: ~230 km) is the youngest and southernmost fault in the MFS, and the second fastest slipping fault in New Zealand. The Hope Fault is a major source of seismic hazard in New Zealand and has ruptured (in-part) historically in the Mw 7.1 1888 Amuri earthquake. In the west, the Hurunui segment of the Hope Fault is covered by beech forest. Hence, its seismic hazard parameters and paleoearthquake chronology were poorly constrained and it was unknown whether the 1888 earthquake ruptured this segment or not and if so, to what extent. Utilising LiDAR and field data, a 29 km-long section of the Hurunui segment of the Hope Fault is mapped. LiDAR-mapping clearly reveals the principal slip zone (PSZ) of the fault and a suite of previously unrecognised structures that form the fault deformation zone (FDZ). FDZ width measurements from 415 locations reveal a spatially-variable, active FDZ up to ~500 m wide with an average width of 200 m. Kinematic analysis of the fault structures shows that the Hurunui segment strikes between 070° and 075° and is optimally oriented for dextral strike-slip within the regional stress field. This implies that the wide FDZ observed is unlikely to result from large-scale fault mis-orientation with respect to regional stresses. The analysis of FDZ width indicates that it increases with increased hanging wall topography and increased topographic relief suggesting that along-strike topographic perturbations to fault geometry and stress states increase fault zone complexity and width. FDZ width also increases where the tips of adjacent PSZ strands locally vary in strike, and where the thickness of alluvial deposits overlying bedrock increases. LiDAR- and photogrammetrically-derived topographic mapping indicates that the boundary between the Hurunui and Hope River segments is characterised by a ~850-m-wide right stepover and a 9º-14° fault bend. Paleoseismic trenching at Hope Shelter site reveals that 6 earthquakes occurred at A.D. 1888, 1740-1840, 1479-1623, 819-1092, 439-551, and 373- 419. These rupture events have a mean recurrence interval of ~298 ± 88 yr and inter-event times ranging from 98 to 595 yrs. The variation in the inter-event times is explained by (1) coalescing rupture overlap from the adjacent Hope River segment on to the Hurunui segment at the study site, (2) temporal clustering of large earthquakes on the Hurunui segment, and/or (3) ‘missing’ rupture events. It appears that the first two options are more plausible to explain the earthquake chronologies and rupture behaviour on the Hurunui segment, given the detailed nature of the geologic and chronologic investigations. This study provides first evidence for coseismic multi-segment ruptures on the Hope Fault by identifying a rupture length of 44-70 km for the 1888 earthquake, which was not confined to the Hope River segment (primary source for the 1888 earthquake). LiDAR data is also used to identify and measure dextral displacements and scarp heights from the PSZ and structures within the FDZ along the Hurunui segment. Reconstruction of large dextrally-offset geomorphic features shows that the vertical component of slip accounts for only ~1% of the horizontal displacements and confirms that the fault is predominantly strike-slip. A strong correlation exists between the dextral displacements and elevations of geomorphic features suggesting the possibility of age correlation between the geomorphic features. A mean single event displacement (SED) of 3.6 ± 0.7 m is determined from interpretation of sets of dextral displacements of ≤ 25 m. Using the available surface age data and the cumulative dextral displacements from Matagouri Flat, McKenzie Fan, Macs Knob and Hope River sites, and the mean SED, a mean slip rate of 12.2 ± 2.4 mm/yr, and a mean recurrence interval of ~320 ± 120 yr, and a potential earthquake magnitude of Mw 7.2 are determined for the Hurunui segment. This study suggests that the fault slip rate has been constant over the last ~15000 yr. Strong ground motions from the 2010 Darfield (Canterbury) earthquake displaced boulders and caused ground damage on some ridge crests in the Port Hills. However, the 2011 Christchurch earthquake neither displaced boulders nor caused ground damage at the same ridge crests. Documentation of locations (~400 m a.s.l.), lateral displacements (8-970 cm), displacement direction (250° ± 20°) of displaced boulders, in addition to their hosting socket geometries (< 1 cm to 50 cm depth), the orientation of the ridges (000°-015°) indicate that boulders have been displaced in the direction of instrumentally recorded transient peak ground horizontal displacements nearby and that the seismic waves have been amplified at the study sites. The co-existence of displaced and non-displaced boulders at proximal sites suggests small-scale ground motion variability and/or varying boulder-ground dynamic interactions relating to shallow phenomena such as variability in soil depth, bedrock fracture density and/or microtopography on the bedrock-soil interface. Shorter shaking duration of the 2011 Christchurch event, differing frequency contents and different source characteristics were all factors that may have contributed to generating circumstances less favourable to boulder displacement in this earthquake. Investigating seismically induced features, fault behaviour, site effects on the rupture behaviour, and site response to the seismic waves provides insights into fault rupture hazards.
The recent Canterbury earthquake sequence in 2010-2011 highlighted a uniquely severe level of structural damage to modern buildings, while confirming the high vulnerability and life threatening of unreinforced masonry and inadequately detailed reinforced concrete buildings. Although the level of damage of most buildings met the expected life-safety and collapse prevention criteria, the structural damage to those building was beyond economic repair. The difficulty in the post-event assessment of a concrete or steel structure and the uneconomical repairing costs are the big drivers of the adoption of low damage design. Among several low-damage technologies, post-tensioned rocking systems were developed in the 1990s with applications to precast concrete members and later extended to structural steel members. More recently the technology was extended to timber buildings (Pres-Lam system). This doctoral dissertation focuses on the experimental investigation and analytical and numerical prediction of the lateral load response of dissipative post-tensioned rocking timber wall systems. The first experimental stages of this research consisted of component testing on both external replaceable devices and internal bars. The component testing was aimed to further investigate the response of these devices and to provide significant design parameters. Post-tensioned wall subassembly testing was then carried out. Firstly, quasi-static cyclic testing of two-thirds scale post-tensioned single wall specimens with several reinforcement layouts was carried out. Then, an alternative wall configuration to limit displacement incompatibilities in the diaphragm was developed and tested. The system consisted of a Column-Wall-Column configuration, where the boundary columns can provide the support to the diaphragm with minimal uplifting and also provide dissipation through the coupling to the post-tensioned wall panel with dissipation devices. Both single wall and column-wall-column specimens were subjected to drifts up to 2% showing excellent performance, limiting the damage to the dissipating devices. One of the objectives of the experimental program was to assess the influence of construction detailing, and the dissipater connection in particular proved to have a significant influence on the wall’s response. The experimental programs on dissipaters and wall subassemblies provided exhaustive data for the validation and refinement of current analytical and numerical models. The current moment-rotation iterative procedure was refined accounting for detailed response parameters identified in the initial experimental stage. The refined analytical model proved capable of fitting the experimental result with good accuracy. A further stage in this research was the validation and refinement of numerical modelling approaches, which consisted in rotational spring and multi-spring models. Both the modelling approaches were calibrated versus the experimental results on post-tensioned walls subassemblies. In particular, the multi-spring model was further refined and implemented in OpenSEES to account for the full range of behavioural aspects of the systems. The multi-spring model was used in the final part of the dissertation to validate and refine current lateral force design procedures. Firstly, seismic performance factors in accordance to a Force-Based Design procedure were developed in accordance to the FEMA P-695 procedure through extensive numerical analyses. This procedure aims to determine the seismic reduction factor and over-strength factor accounting for the collapse probability of the building. The outcomes of this numerical analysis were also extended to other significant design codes. Alternatively, Displacement-Based Design can be used for the determination of the lateral load demand on a post-tensioned multi-storey timber building. The current DBD procedure was used for the development of a further numerical analysis which aimed to validate the procedure and identify the necessary refinements. It was concluded that the analytical and numerical models developed throughout this dissertation provided comprehensive and accurate tools for the determination of the lateral load response of post-tensioned wall systems, also allowing the provision of design parameters in accordance to the current standards and lateral force design procedures.
The seismic performance and parameter identification of the base isolated Christchurch Women’s Hospital (CWH) building are investigated using the recorded seismic accelerations during the two large earthquakes in Christchurch. A four degrees of freedom shear model is applied to characterize the dynamic behaviour of the CWH building during these earthquakes. A modified Gauss-Newton method is employed to identify the equivalent stiffness and Rayleigh damping coefficients of the building. The identification method is first validated using a simulated example structure and finally applied to the CWH building using recorded measurements from the Mw 6.0 and Mw 5.8 Christchurch earthquakes on December 23, 2011. The estimated response and recorded response for both earthquakes are compared with the cross correlation coefficients and the mean absolute percentage errors reported. The results indicate that the dynamic behaviour of the superstructure and base isolator was essentially within elastic range and the proposed shear linear model is sufficient for the prediction of the structural response of the CWH Hospital during these events.
The Global Earthquake Model’s (GEM) Earthquake Consequences Database (GEMECD) aims to develop, for the first time, a standardised framework for collecting and collating geocoded consequence data induced by primary and secondary seismic hazards to different types of buildings, critical facilities, infrastructure and population, and relate this data to estimated ground motion intensity via the USGS ShakeMap Atlas. New Zealand is a partner of the GEMECD consortium and to-date has contributed with 7 events to the database, of which 4 are localised in the South Pacific area (Newcastle 1989; Luzon 1990; South of Java 2006 and Samoa Islands 2009) and 3 are NZ-specific events (Edgecumbe 1987; Darfield 2010 and Christchurch 2011). This contribution to GEMECD represented a unique opportunity for collating, comparing and reviewing existing damage datasets and harmonising them into a common, openly accessible and standardised database, from where the seismic performance of New Zealand buildings can be comparatively assessed. This paper firstly provides an overview of the GEMECD database structure, including taxonomies and guidelines to collect and report on earthquake-induced consequence data. Secondly, the paper presents a summary of the studies implemented for the 7 events, with particular focus on the Darfield (2010) and Christchurch (2011) earthquakes. Finally, examples of specific outcomes and potentials for NZ from using and processing GEMECD are presented, including: 1) the rationale for adopting the GEM taxonomy in NZ and any need for introducing NZ-specific attributes; 2) a complete overview of the building typological distribution in the Christchurch CBD prior to the Canterbury earthquakes and 3) some initial correlations between the level and extent of earthquake-induced physical damage to buildings, building safety/accessibility issues and the induced human casualties.
Live monitoring data and simple dynamic reduced-order models of the Christchurch Women’s Hospital (CWH) help explain the performance of the base isolation (BI) system of the hospital during the series of Canterbury earthquakes in 2011-2012. A Park-Wen-Ang hysteresis model is employed to simulate the performance of the BI system and results are compared to measured data recorded above the isolation layer and on the 6th story. Simplified single, two and three degree of freedom models (SDOF, 2DOF and 3DOF) show that the CWH structure did not behave as an isolated but as a fixed-base structure. Comparisons of accelerations and deflections between simulated and monitored data show a good match for isolation stiffness values of approximately two times of the value documented in the design specification and test protocol. Furthermore, an analysis of purely measured data revealed very little to no relative motion across the isolators for large events of moment magnitude scale (Mw) 5.8 and 6.0 that occurred within 3 hours of each other on December 23, 2011. One of the major findings is that the BI system during the seismic events on December 23, 2011 did not yield and that the superstructure performed as a fixed-base building, indicating a need to reevaluate the analysis, design and implementation of these structures.
Base isolation is arguably the most reliable method for providing enhanced protection of buildings against earthquake-induced actions, by virtue of a physical separation between the structure and the ground through elements/devices with controlled force capacity, significant lateral deformation capacity and (often) enhanced energy dissipation. Such a design solution has shown its effectiveness in protecting both structural and non-structural components, hence preserving their functionality even in the aftermath of a major seismic event. Despite lead rubber bearings being invented in New Zealand almost forty years ago, the Christchurch Women's hospital was the only isolated building in Christchurch when the Canterbury earthquake sequence struck in 2010/11. Furthermore, a reference code for designing base-isolated buildings in New Zealand is still missing. The absence of a design standard or at least of a consensus on design guidelines is a potential source for a lack of uniformity in terms of performance criteria and compliance design approaches. It may also limit more widespread use of the technology in New Zealand. The present paper provides an overview of the major international codes (American, Japanese and European) for the design of base-isolated buildings. The design performance requirements, the analysis procedures, the design review process and approval/quality control of devices outlined in each code are discussed and their respective pros and cons are compared through a design application on a benchmark building in New Zealand. The results gathered from this comparison are intended to set the basis for the development of guidelines specific for the New Zealand environment.
The Canterbury region experienced widespread damage due to liquefaction induced by seismic shaking during the 4 September 2010 earthquake and the large aftershocks that followed, notably those that occurred on 22 February, 13 June and 23 December 2011. Following the 2010 earthquake, the Earthquake Commission directed a thorough investigation of the ground profile in Christchurch, and to date, more than 7500 cone penetration tests (CPT) have been performed in the region. This paper presents the results of analyses which use a subset of the geotechnical database to evaluate the liquefaction process as well as the re-liquefaction that occurred following some of the major events in Christchurch. First, the applicability of existing CPT-based methods for evaluating liquefaction potential of Christchurch soils was investigated using three methods currently available. Next, the results of liquefaction potential evaluation were compared with the severity of observed damage, categorised in terms of the land damage grade developed from Tonkin & Taylor property inspections as well as from observed severity of liquefaction from aerial photography. For this purpose, the Liquefaction Potential Index (LPI) was used to represent the damage potential at each site. In addition, a comparison of the CPT-based strength profiles obtained before each of the major aftershocks was performed. The results suggest that the analysis of spatial and temporal variations of strength profiles gives a clear indication of the resulting liquefaction and re-liquefaction observed in Christchurch. The comparison of a limited number of CPT strength profiles before and after the earthquakes seems to indicate that no noticeable strengthening has occurred in Christchurch, making the area vulnerable to liquefaction induced land damage in future large-scale earthquakes.
When disasters and crises, both man-made and natural, occur, resilient higher education institutions adapt in order to continue teaching and research. This may necessitate the closure of the whole institution, a building and/or other essential infrastructure. In disasters of large scale the impact can be felt for many years. There is an increasing recognition of the need for disaster planning to restructure educational institutions so that they become more resilient to challenges including natural disasters (Seville, Hawker, & Lyttle, 2012).The University of Canterbury (UC) was affected by seismic events that resulted in the closure of the University in September 2010 for 10 days and two weeks at the start of the 2011 academic year This case study research describes ways in which e-learning was deployed and developed by the University to continue and even to improve learning and teaching in the aftermath of a series of earthquakes in 2010 and 2011. A qualitative intrinsic embedded/nested single case study design was chosen for the study. The population was the management, support staff and educators at the University of Canterbury. Participants were recruited with purposive sampling using a snowball strategy where the early key participants were encouraged to recommend further participants. Four sources of data were identified: (1) documents such as policy, reports and guidelines; (2) emails from leaders of the colleges and academics; (3) communications from senior management team posted on the university website during and after the seismic activity of 2010 and 2011; and (4) semi-structured interviews of academics, support staff and members of senior management team. A series of inductive descriptive content analyses identified a number of themes in the data. The Technology Acceptance Model 2 (Venkatesh & Davis, 2000) and the Indicator of Resilience Model (Resilient Organisations, 2012) were used for additional analyses of each of the three cases. Within the University case, the cases of two contrasting Colleges were embedded to produce a total of three case studies describing e-learning from 2000 - 2014. One contrast was the extent of e-learning deployment at the colleges: The College of Education was a leader in the field, while the College of Business and Law had relatively little e-learning at the time of the first earthquake in September 2010. The following six themes emerged from the analyses: Communication about crises, IT infrastructure, Availability of e-learning technologies, Support in the use of e-learning technologies, Timing of crises in academic year and Strategic planning for e-learning. One of the findings confirmed earlier research that communication to members of an organisation and the general public about crises and the recovery from crises is important. The use of communication channels, which students were familiar with and already using, aided the dissemination of the information that UC would be using e-learning as one of the options to complete the academic year. It was also found that e-learning tools were invaluable during the crises and facilitated teaching and learning whilst freeing limited campus space for essential activities and that IT infrastructure was essential to e-learning. The range of e-learning tools and their deployment evolved over the years influenced by repeated crises and facilitated by the availability of centrally located support from the e-Learning support team for a limited set of tools, as well as more localised support and collaboration with colleagues. Furthermore, the reasons and/or rate of e-learning adoption in an educational institution during crises varied with the time of the academic year and the needs of the institution at the time. The duration of the crises also affected the adoption of e-learning. Finally, UC’s lack of an explicit e-learning strategy influenced the two colleges to develop college-specific e-learning plans and those College plans complemented the incorporation of e-learning for the first time in the University’s teaching and learning strategy in 2013. Twelve out of the 13 indicators of the Indicators of Resilience Model were found in the data collected for the study and could be explained using the model; it revealed that UC has become more resilient with e-learning in the aftermath of the seismic activities in 2010 and 2011. The interpretation of the results using TAM2 demonstrated that the adoption of technologies during crises aided in overcoming barriers to learning at the time of the crisis. The recommendations from this study are that in times of crises, educational institutions take advantage of Cloud computing to communicate with members of the institution and stakeholders. Also, that the architecture of a university’s IT infrastructure be made more resilient by increasing redundancy, backup and security, centralisation and Cloud computing. In addition, when under stress it is recommended that new tools are only introduced when they are essential.
The Canterbury Earthquake Sequence (CES) of 2010-2011 produced large seismic moments up to Mw 7.1. These large, near-to-surface (<15 km) ruptures triggered >6,000 rockfall boulders on the Port Hills of Christchurch, many of which impacted houses and affected the livelihoods of people within the impacted area. From these disastrous and unpredicted natural events a need arose to be able to assess the areas affected by rockfall events in the future, where it is known that a rockfall is possible from a specific source outcrop but the potential boulder runout and dynamics are not understood. The distribution of rockfall deposits is largely constrained by the physical properties and processes of the boulder and its motion such as block density, shape and size, block velocity, bounce height, impact and rebound angle, as well as the properties of the substrate. Numerical rockfall models go some way to accounting for all the complex factors in an algorithm, commonly parameterised in a user interface where site-specific effects can be calibrated. Calibration of these algorithms requires thorough field checks and often experimental practises. The purpose of this project, which began immediately following the most destructive rupture of the CES (February 22, 2011), is to collate data to characterise boulder falls, and to use this information, supplemented by a set of anthropogenic boulder fall data, to perform an in-depth calibration of the three-dimensional numerical rockfall model RAMMS::Rockfall. The thesis covers the following topics: • Use of field data to calibrate RAMMS. Boulder impact trails in the loess-colluvium soils at Rapaki Bay have been used to estimate ranges of boulder velocities and bounce heights. RAMMS results replicate field data closely; it is concluded that the model is appropriate for analysing the earthquake-triggered boulder trails at Rapaki Bay, and that it can be usefully applied to rockfall trajectory and hazard assessment at this and similar sites elsewhere. • Detailed analysis of dynamic rockfall processes, interpreted from recorded boulder rolling experiments, and compared to RAMMS simulated results at the same site. Recorded rotational and translational velocities of a particular boulder show that the boulder behaves logically and dynamically on impact with different substrate types. Simulations show that seasonal changes in soil moisture alter rockfall dynamics and runout predictions within RAMMS, and adjustments are made to the calibration to reflect this; suggesting that in hazard analysis a rockfall model should be calibrated to dry rather than wet soil conditions to anticipate the most serious outcome. • Verifying the model calibration for a separate site on the Port Hills. The results of the RAMMS simulations show the effectiveness of calibration against a real data set, as well as the effectiveness of vegetation as a rockfall barrier/retardant. The results of simulations are compared using hazard maps, where the maximum runouts match well the mapped CES fallen boulder maximum runouts. The results of the simulations in terms of frequency distribution of deposit locations on the slope are also compared with those of the CES data, using the shadow angle tool to apportion slope zones. These results also replicate real field data well. Results show that a maximum runout envelope can be mapped, as well as frequency distribution of deposited boulders for hazard (and thus risk) analysis purposes. The accuracy of the rockfall runout envelope and frequency distribution can be improved by comprehensive vegetation and substrate mapping. The topics above define the scope of the project, limiting the focus to rockfall processes on the Port Hills, and implications for model calibration for the wider scientific community. The results provide a useful rockfall analysis methodology with a defensible and replicable calibration process, that has the potential to be applied to other lithologies and substrates. Its applications include a method of analysis for the selection and positioning of rockfall countermeasure design; site safety assessment for scaling and demolition works; and risk analysis and land planning for future construction in Christchurch.
Following the Mw 6.2 Christchurch Earthquake on 22 February 2011, extensive ground cracking in loessial soils was reported in some areas of the Port Hills, southeast of central Christchurch. This study was undertaken to investigate the mechanisms of earthquake-induced ground damage on the eastern side of the Hillsborough Valley. A zone of extensional cracking up to 40m wide and 600m long was identified along the eastern foot-slope, accompanied by compression features and spring formation at the toe of the slope. An engineering geological and geomorphological model was developed for the eastern Hillsborough Valley that incorporates geotechnical investigation data sourced from the Canterbury Geotechnical Database (CGD), the findings of trenching and seismic refraction surveying carried out for this research, and interpretation of historical aerial photographs. The thickness and extent of a buried peat swamp at the base of the slope was mapped, and found to coincide with significant compression features. Ground cracking was found to have occurred entirely within loess-colluvium and to follow the apices of pre-1920s tunnel-gully fan debris at the southern end of the valley. The ground-cracking on the eastern side of the Hillsborough Valley is interpreted to have formed through tensile failure of the loess-colluvium. Testing was carried out to determine the tensile strength of Port Hills loess colluvium as a function of water content and density, in order to better understand the occurrence and distribution of the observed ground cracking. A comprehensive review of the soil tensile strength testing literature was undertaken, from which a test methodology was developed. Results show remoulded loess-colluvium to possess tensile strength of 7 - 28 kPa across the range of tested moisture contents (10-15%) and dry densities (1650-1900kg/m3). A positive linear relationship was observed between tensile strength and dry density, and a negative linear relationship between moisture content and tensile strength. The observed ground damage and available geotechnical information (inclinometer and piezometer records provided by the Earthquake Commission) were together used to interpret the mechanism(s) of slope movement that occurred in the eastern Hillsborough Valley. The observed ground damage is characteristic of translational movement, but without the development of lateral release scarps, or a basal sliding surface - which was not located during drilling. It is hypothesised that shear displacement has been accommodated by multiple slip surfaces of limited extent within the upper 10m of the slope. Movement has likely occurred within near-saturated colluvial units that have lost strength during earthquake shaking. The eastern Hillsborough Valley is considered to be an ‘incipient translational slide’, as both the patterns of damage and shearing are consistent with the early stages of such slide development. Sliding block analysis was utilised to understand how the eastern Hillsborough Valley may perform in a future large magnitude earthquake. Known cumulative displacements of ~0.3m for eastern Hillsborough Valley during the 2010-2011 Canterbury Earthquake Sequence were compared with modelled slope displacements to back-analyse a lower-bound yield acceleration of 0.2 - 0.25g. Synthetic broadband modelling for future Alpine and Hope Fault earthquakes indicates PGAs of approximately 0.08g for soil sites in the Christchurch area, as such, slope movement is unlikely to be reactivated by an Alpine Fault or Hope Fault earthquake. This does not take into account the possible role of strength loss due to excess pore pressure that may occur during these future events.
