Topographic amplification on steep slopes during seismic events

Abstract

The geomorphology of New Zealand has dictated that many transport corridors and residential areas have been formed on, or adjacent to, steep slopes and undulating terrain underlain by variable geology. Observations of the seismic behaviour of cliff-top properties in the Port Hills during the Canterbury earthquakes suggests the current approach recommended in NZS 1170.5 and the NZTA Bridge Manual may be unconservative at estimating the design peak ground accelerations (PGA’s) for sites influenc... Read more

The geomorphology of New Zealand has dictated that many transport corridors and residential areas have been formed on, or adjacent to, steep slopes and undulating terrain underlain by variable geology. Observations of the seismic behaviour of cliff-top properties in the Port Hills during the Canterbury earthquakes suggests the current approach recommended in NZS 1170.5 and the NZTA Bridge Manual may be unconservative at estimating the design peak ground accelerations (PGA’s) for sites influenced by topographic amplification. A number of sites may pose a higher risk of seismic failure, with potential to cause damage to dwellings and infrastructure, if topographic amplification effects were to be considered under the current codes. It is fundamental to design success that accurate design PGA’s are estimated for assessments of existing slopes, design of slope stabilisation works near steep slopes, and design of structures near the crests of slopes. This is particularly relevant in view of the high probability of an Alpine Fault earthquake of Mw8 and the relative vulnerability of large metropolitan areas like Wellington to seismic events. Eurocode 8 appears to be an exception to existing literature in that it is a code that provides a readily implementable criterion for estimating topographic amplification. A similar, albeit interim, simplified method for estimating topographic amplification factors in New Zealand is proposed, based on back-analysis of observed movement at the Port Hills of Christchurch during the 22 February 2011 earthquake.

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Low temperature recrystallisation of alluvial gold in paleoplacer deposits

Abstract

Detrital gold particles in paleoplacer deposits develop recrystallised rims, with associated expulsion of Ag, leading to the formation of Ag-poor rims which have been recognised in most placer gold particles around the world. Recrystallisation is facilitated by accumulation of strain energy as the gold particles are deformed, particularly on particle margins, during transportation in a fluvial system. The recrystalli- sation process ensues after sedimentary deposition and can occur at low tempe... Read more

Detrital gold particles in paleoplacer deposits develop recrystallised rims, with associated expulsion of Ag, leading to the formation of Ag-poor rims which have been recognised in most placer gold particles around the world. Recrystallisation is facilitated by accumulation of strain energy as the gold particles are deformed, particularly on particle margins, during transportation in a fluvial system. The recrystalli- sation process ensues after sedimentary deposition and can occur at low temperatures (<40 C) over long geological time scales (millions of years). In the Otago placer goldfield of southern New Zealand, paleo- placers of varying ages contain gold with varying transport distances and these display differing degrees of rim formation. Narrow (1–10 mm) recrystallised rims with 0–3 wt% Ag formed on gold particles that had been transported <10 km from their source and preserved in Eocene sediments. Relict, coarse grained ( 100 mm) gold particle cores have 3–10 wt% Ag, which is representative of the source gold in nearby basement rocks. Gold in the Miocene paleoplacers was recycled from the Eocene deposits and transported >20 km from their source. The gold particles now have wider recrystallised rims (up to 100 mm), so that some particles have essentially no relict cores preserved. Gold in Cretaceous paleoplac- ers have wide ( 100 mm) recrystallised low-Ag rims, even in locally-derived particles, partly as a result of diagenetic effects not seen in the younger placers. Gold particles in all the paleoplacers have delicate gold overgrowths that are readily removed during recycling, but are replaced by groundwater dissolution and reprecipitation on a time scale of <1 Ma. The recrystallisation that leads to Ag-poor rim formation is primarily related to the amount of deformation imposed on particles during sedimentary transport, and is therefore broadly linked to transport distance, but is also partly controlled by the age of the paleoplacer on time scales of tens of millions of years. Gold particles that have been derived directly from basement sources can retain their original composition for long distances (tens to hundreds of kilometres) in a river system, with only minor recrystallised rim development. Gold particles that have been recycled through paleoplacer deposits can lose this link to source composition after relatively short transport distances because of extensive recrystallisation.

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Embankment Failures - Local Experiences

Abstract

There have been six embankment dam failures, 40 notices issued for non-compliance with the Building Act (2004), and five abatement notices issued under the Resource Management Act (1991) in respect to small irrigation dams in Otago since 2011. Investigations into three failures in Otago, and one access embankment on the West Coast, are used to illustrate common mechanisms of failure and how these could have been mitigated by good engineering practice. Each case study is summarised, a probable fa... Read more

There have been six embankment dam failures, 40 notices issued for non-compliance with the Building Act (2004), and five abatement notices issued under the Resource Management Act (1991) in respect to small irrigation dams in Otago since 2011. Investigations into three failures in Otago, and one access embankment on the West Coast, are used to illustrate common mechanisms of failure and how these could have been mitigated by good engineering practice. Each case study is summarised, a probable failure mechanism postulated, and remedial measures to avoid recurrence identified. These case studies present common examples from the spectrum of failures associated with these structures, including: foundation/fill contact piping, spillway failure, piping of the conduit, and overtopping. Common design elements to prevent, minimise the likelihood of, or safeguard against failure are introduced, and the challenges of implementing these post-failure emphasized. Because failures tend to develop very rapidly, the importance of getting the right level of design input and QA with any potentially hazardous water retaining structure, irrespective of the size of the structure, is emphasized.

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Evidence of a post-glacial rock avalanche impact on Lake Wanaka, New Zealand

Abstract

The Lake Wanaka Rock Avalanche is a large translational rockslide (est. volume 5–10 × 106 m) near the head of Lake Wanaka in the Southern Alps. Failure occurred in the early post-glacial era, on steep schist slopes weakened by toppling. The rock avalanche impacted the lake and a large tsunami is inferred. The trigger was likely a seismic event or extreme rainfall. There is a high probability of anMw 8 earthquake on the nearby Alpine Fault (Australian-Pacific plate boundary) in the next 50 ye... Read more

The Lake Wanaka Rock Avalanche is a large translational rockslide (est. volume 5–10 × 106 m) near the head of Lake Wanaka in the Southern Alps. Failure occurred in the early post-glacial era, on steep schist slopes weakened by toppling. The rock avalanche impacted the lake and a large tsunami is inferred. The trigger was likely a seismic event or extreme rainfall. There is a high probability of an
Mw 8 earthquake on the nearby Alpine Fault (Australian-Pacific plate boundary) in the next 50 years. The quake is likely to trigger rapid “first-time” rockslides around the steep lakeshore, and the lakeside town of Wanaka is potentially at risk from any earthquake generated tsunami. The public needs to be informed of the risk, and advised to immediately move from shoreline areas to higher ground after a strong earthquake

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Effects of Increased Axle Loadings on Local Roads

Abstract

While heavy vehicle loads will provide benefits once they reach their destination, some axle configurations applied to local roads will contribute disproportionately large adverse effects on the road pavements en-route. Unless the affected pavements have been designed to carry the extra loading, heavily loaded axles will inflict an exceptionally large proportion of additional wear on local pavements. The usual result is premature distress to a terminal condition which necessitates full structura... Read more

While heavy vehicle loads will provide benefits once they reach their destination, some axle configurations applied to local roads will contribute disproportionately large adverse effects on the road pavements en-route. Unless the affected pavements have been designed to carry the extra loading, heavily loaded axles will inflict an exceptionally large proportion of additional wear on local pavements. The usual result is premature distress to a terminal condition which necessitates full structural rehabilitation or reconstruction rather than periodic resurfacing and maintenance.

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Collection and Interpretation of Pavement Structural Parameters using Deflection Testing

Part II: project level

Abstract

Pavement deflection testing is undertaken as the primary means for establishing structural parameters. Part I of this set of two reports discusses “Network Level” deflection testing for asset management purposes. This report, Part II, addresses “Project Level” testing and interpretation for rehabilitation treatment of specific road lengths, or quality control during construction. Typical parameters established from deflection testing on New Zealand roads, including the NZ Transport Agenc... Read more

Pavement deflection testing is undertaken as the primary means for establishing structural parameters. Part I of this set of two reports discusses “Network Level” deflection testing for asset management purposes. This report, Part II, addresses “Project Level” testing and interpretation for rehabilitation treatment of specific road lengths, or quality control during construction. Typical parameters established from deflection testing on New Zealand roads, including the NZ Transport Agency’s (NZTA) Long Term Pavement Performance (LTPP) benchmark sites, are presented.

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Collection and Interpretation of Pavement Structural Parameters using Deflection Testing

Part I: Network Asset Management

Abstract

This document (Part I of a set of two reports) provides a good practice guide for road controlling authorities about “Network Level” pavement deflection testing for Asset Management. Part II of this report series addresses “Project Level” testing and interpretation for specific road lengths’ rehabilitation treatment, or their quality control during construction. Typical structural parameters from past and on-going studies on New Zealand roads, including the NZ Transport Agency’s (NZT... Read more

This document (Part I of a set of two reports) provides a good practice guide for road controlling authorities about “Network Level” pavement deflection testing for Asset Management. Part II of this report series addresses “Project Level” testing and interpretation for specific road lengths’ rehabilitation treatment, or their quality control during construction. Typical structural parameters from past and on-going studies on New Zealand roads, including the NZ Transport Agency’s (NZTA) Long Term Pavement Performance (LTPP) benchmark sites, are presented.This document (Part I of a set of two reports) provides a good practice guide for road controlling authorities about “Network Level” pavement deflection testing for Asset Management. Part II of this report series addresses “Project Level” testing and interpretation for specific road lengths’ rehabilitation treatment, or their quality control during construction. Typical structural parameters from past and on-going studies on New Zealand roads, including the NZ Transport Agency’s (NZTA) Long Term Pavement Performance (LTPP) benchmark sites, are presented.

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Characterisation and use of stabilised basecourse materials

In transportation projects in New Zealand

Abstract

The stabilisation of near-surface granular pavement materials is accepted practice in transportation maintenance and capital development projects in Australasia. Stabilisation in this context involves the mechanical introduction of reactive agents, including cement and foamed bitumen, into existing or manufactured granular materials, with or without existing seal inclusion. Present-day design guides characterise stabilised granular materials as either modified or bound, depending primarily on th... Read more

The stabilisation of near-surface granular pavement materials is accepted practice in transportation maintenance and capital development projects in Australasia. Stabilisation in this context involves the mechanical introduction of reactive agents, including cement and foamed bitumen, into existing or manufactured granular materials, with or without existing seal inclusion. Present-day design guides characterise stabilised granular materials as either modified or bound, depending primarily on the amount and type of reactive agent used in the stabilising process. Modified materials are modelled as unbound granular materials in a pavement. Bound (cemented) materials are modelled as layers with tensile load-carrying capacity within the pavement. Cracking in the bound pavement layer is governed by ‘fatigue relationships’. This research has shown that stabilisation with smaller reactive agent contents (<3% by dry mass) can deliver materials that should be modelled as lightly bound, delivering cost-effective pavement solutions. This research report describes the collection and interrogation of performance data from New Zealand road pavements that utilised stabilised granular materials. The research, carried out from December 2009 to August 2011, compared actual stabilised pavement performance with the expectations in published design guidelines. A conceptual pavement performance model for near-surface ‘lightly bound’ stabilised granular pavement layers that better matches observed pavement behaviour is proposed.

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Extending Pavement Life

Investigation of Premature Distress in Unbound Granular Pavements

Abstract

Premature distress in unbound basecourses has occurred regularly in New Zealand. In 2008, the New Zealand Transport Agency (NZTA) commissioned the assembly of an inventory of problem basecourses and subbases. Study of the inventory found that the long-term degree of saturation of basecourse was highly significant in the case histories of premature distress, ie the pavements failed through shear instability (shoving) in the basecourses. A common feature in basecourses with a high degree of satura... Read more

Premature distress in unbound basecourses has occurred regularly in New Zealand. In 2008, the New Zealand Transport Agency (NZTA) commissioned the assembly of an inventory of problem basecourses and subbases. Study of the inventory found that the long-term degree of saturation of basecourse was highly significant in the case histories of premature distress, ie the pavements failed through shear instability (shoving) in the basecourses. A common feature in basecourses with a high degree of saturation was gap grading in the sand fraction. Existing basecourse specifications limit gap grading through grading shape control requirements but the case histories demonstrate that tighter control is required. The basecourse inventory was used to establish regression equations for predicting the in situ long-term degree of saturation of a basecourse. This approach appears to be very promising. Timely decisions can now be made on acceptance or the need for corrective measures prior to sealing. The above considerations have been used for preparing revised drafts of the NZTA basecourse specification, subbase specification notes as well as a set of recommendations for the compaction specification and the New Zealand supplement to the document, Pavement design – a guide to the structural design of road pavements (Austroads 2004) (Transit NZ 2007a) to implement practical solutions to premature distress in unbound basecourses.

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Rationalisation of the structural capacity definition

and quantification of roads based on falling weight deflectometer tests

Abstract

Pavement performance modelling for New Zealand roading networks, currently relies on an adjusted structural number (SNP) which is a single parameter intended to describe the performance of a multilayered pavement structure in terms of its rate of deterioration with respect to all structural distress modes, as well as non-structural modes. This parameter had its origin in the AASHO road test in the late 1950s, before the advent of analytical methods. Hence refinement to keep abreast of current pr... Read more

Pavement performance modelling for New Zealand roading networks, currently relies on an adjusted structural number (SNP) which is a single parameter intended to describe the performance of a multilayered pavement structure in terms of its rate of deterioration with respect to all structural distress modes, as well as non-structural modes. This parameter had its origin in the AASHO road test in the late 1950s, before the advent of analytical methods. Hence refinement to keep abreast of current practice in pavement engineering is overdue. This research describes the basis for a new set of structural indices and how these can be used to obtain improved prediction of pavement performance: both at network level and for project level rehabilitation of individual roads. The results are (i) effective use of all the data contained in RAMM, (ii) more reliable assignment of network forward work programmes, (iii) reduced cost through targeting only those sections of each road that require treatment and (iv) more efficient design of pavement rehabilitation through informed appreciation of the relevant distress mechanism that will govern the structural life of each individual treatment length.

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Compliance testing using the Falling Weight Deflectometer

For pavement construction, rehabilitation and area-wide treatments

Abstract

The Falling Weight Deflectometer (FWD) which measures pavement deflections was assessed for its ability to predict the life of a newly constructed or rehabilitated pavement. FWD measurements used in the study were from NZ Transport Agency’s test track CAPTIF, roads that have failed and from two performance specified maintenance contracts where the actual life from rutting and roughness measurements could be determined. Three different methods to calculate life from FWD measurements were triall... Read more

The Falling Weight Deflectometer (FWD) which measures pavement deflections was assessed for its ability to predict the life of a newly constructed or rehabilitated pavement. FWD measurements used in the study were from NZ Transport Agency’s test track CAPTIF, roads that have failed and from two performance specified maintenance contracts where the actual life from rutting and roughness measurements could be determined. Three different methods to calculate life from FWD measurements were trialled. The first, a simple Austroads method that uses the central deflection only and was found to either grossly over predict life by a factor of 1000 times more than the actual life or grossly under predict the life. The second two methods trialled were based on Austroads mechanistic pavement design where the life is determined from the vertical compressive strain at the top of the subgrade. For the mechanistic approach the FWD measurements are analysed with specialised software that determines a linear elastic model of the pavement which computes the same surface deflections as those measured by the FWD. From the linear elastic model the subgrade strain is determined and life calculated using the Austroads equation. It was found when using this approach that predictions of life from individual FWD measured points within a project length can range from nearly 0 to over 100 million equivalent standard axles. To cater for this large scatter in results, the 10th percentile value was used as the predicted life of the pavement. In general the Austroads mechanistic approach under-predicted the life, sometimes by a factor of 10 or more. The third approach trialled was adjusting the Austroads mechanistic approach by applying a factor determined from past performance to calibrate the subgrade strain criterion to local conditions. This third approach greatly improved the predictions but it was found that the multiplying factor was not consistent for a geographical area and thus the factor found from one project might not be suitable for another similar project.

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Pavement Performance Prediction II

A Comprehensive New Approach to Defining Structural Capacity (SNP)

Abstract

Pavement performance modelling for New Zealand (NZ) roading networks currently relies on Adjusted Structural Number (SNP); a single parameter intended to describe the performance of a multi-layered pavement structure in terms of its rate of deterioration with respect to all structural distress modes as well as non-structural modes. This parameter had its origin in the AASHO Road Test in the late 1950’s before the advent of analytical methods. Refinement to keep abreast of current practice in p... Read more

Pavement performance modelling for New Zealand (NZ) roading networks currently relies on Adjusted Structural Number (SNP); a single parameter intended to describe the performance of a multi-layered pavement structure in terms of its rate of deterioration with respect to all structural distress modes as well as non-structural modes. This parameter had its origin in the AASHO Road Test in the late 1950’s before the advent of analytical methods. Refinement to keep abreast of current practice in pavement engineering is long over-due.

An advanced model for pavement structural capacity is under development overseas (NCHRP), but it is not in general use because of its complexity (including requirements for destructive test information). The focus of a recent NZ Transport Agency (NZTA) research study has been to set the framework for obtaining the most practical indices for New Zealand pavements based on parameters which are currently stored in RAMM, while at the same time maintaining flexibility for ongoing upgrades that might utilise future developments in the way of pavement data collection. In many pavements, structural distress can be assigned to one or more of at least four discrete categories: rutting, roughness, crack initiation and shear instability (shoving). In this study, data from all NZ’s Long Term Pavement Performance (LTPP) sites have been analysed to explore the benefits of replacing SNP with four separate “Structural Indices” - each being determined mechanistically (considering stresses and strains induced by an Equivalent Standard Axle) from data commonly available in RAMM.

Each Structural Index has been developed to fall within the same range as the traditional SNP, allowing straightforward implementation with minimal additional calibration needed to implement these in existing NZ Pavement Deterioration models and asset management software such as dTIMS. This paper presents the new developments resulting from the ongoing study, and shows the basis for the new set of Structural Indices and how these can be used to obtain improved prediction of pavement performance, both at network level and for project level rehabilitation of individual roads. The results enable: (i) effective use of all the data contained in RAMM, (ii) more reliable assignment of network Forward Work Programmes, (iii) reduced cost by targeting only those sections of each road that require treatment, and (iv) more efficient design of pavement rehabilitation through informed appreciation of the relevant distress mechanism that will govern the structural life of each individual treatment length.

Pavement performance modelling for New Zealand (NZ) roading networks currently relies on Adjusted Structural Number (SNP); a single parameter intended to describe the performance of a multi-layered pavement structure in terms of its rate of deterioration with respect to all structural distress modes as well as non-structural modes. This parameter had its origin in the AASHO Road Test in the late 1950’s before the advent of analytical methods. Refinement to keep abreast of current practice in pavement engineering is long over-due. An advanced model for pavement structural capacity is under development overseas (NCHRP), but it is not in general use because of its complexity (including requirements for destructive test information). The focus of a recent NZ Transport Agency (NZTA) research study has been to set the framework for obtaining the most practical indices for New Zealand pavements based on parameters which are currently stored in RAMM, while at the same time maintaining flexibility for ongoing upgrades that might utilise future developments in the way of pavement data collection. In many pavements, structural distress can be assigned to one or more of at least four discrete categories: rutting, roughness, crack initiation and shear instability (shoving). In this study, data from all NZ’s Long Term Pavement Performance (LTPP) sites have been analysed to explore the benefits of replacing SNP with four separate “Structural Indices” - each being determined mechanistically (considering stresses and strains induced by an Equivalent Standard Axle) from data commonly available in RAMM. Each Structural Index has been developed to fall within the same range as the traditional SNP, allowing straightforward implementation with minimal additional calibration needed to implement these in existing NZ Pavement Deterioration models and asset management software such as dTIMS. This paper presents the new developments resulting from the ongoing study, and shows the basis for the new set of Structural Indices and how these can be used to obtain improved prediction of pavement performance, both at network level and for project level rehabilitation of individual roads. The results enable: (i) effective use of all the data contained in RAMM, (ii) more reliable assignment of network Forward Work Programmes, (iii) reduced cost by targeting only those sections of each road that require treatment, and (iv) more efficient design of pavement rehabilitation through informed appreciation of the relevant distress mechanism that will govern the structural life of each individual treatment length.

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Mechanistic Approach to Pavement Rehabilitation

Maintenance and Forward Work Programmes Cutting Costs Using Structural Analysis

Abstract

A roading authority that has pavement assets to manage needs well developed strategies for network maintenance based on desired levels of service, pavement deterioration and budget constraints supported by options for cost effective pavement rehabilitation once a terminal condition is reached. A purely reactive approach to maintenance or rehabilitation can result in both inefficiencies and unexpected costs. This is being further highlighted by the need to consider impacts of the newly increased... Read more

A roading authority that has pavement assets to manage needs well developed strategies for network maintenance based on desired levels of service, pavement deterioration and budget constraints supported by options for cost effective pavement rehabilitation once a terminal condition is reached. A purely reactive approach to maintenance or rehabilitation can result in both inefficiencies and unexpected costs. This is being further highlighted by the need to consider impacts of the newly increased VDM limits.

In recent years, a wealth of pavement structural data has been collected and stored in RAMM, but it is only used superficially. The bulk of the data is neglected. The mechanistic approach to pavement management uses Austroads principles to analyse the structural data already collected in RAMM, then calculates layer stiffnesses and stresses and strains induced in the pavement structure by trafficking to rationalise and quantify distress modes. This article addresses the role of structural analysis using a mechanistic approach to reduce the cost of maintenance and rehabilitation by effectively identifying the relevant distress mechanism, and focussing treatment only on subsections where work is essential, rather than using a continuous, and sometimes ineffective, treatment over an assumed treatment length. Often only the carriageway interval or top surface type is used for identification of treatment lengths.

In many cases, the existing raw data already stored in the RAMM database can be reanalysed to minimise any further data collection costs. Significant advancements in the application of pavement structural analysis have taken place in 2008 and 2009 as part of Austroads and NZTA research, and these are summarised in this article.

The following aspects are addressed:

  • Part I - Network maintenance and determination of Forward Work Programmes
  • Part II - Pavement structural rehabilitation
  • Part III - Quality assurance of newly constructed pavements and demonstration that the intended design life will be achieved
  • Part IV - Maintenance cost implications of increased vehicle weight limits

The conclusions to each of Parts I , II, III & IV highlight the key benefits that can now be obtained from a mechanistic approach to pavement asset management giving cost-effective design, reduced incidence of premature failure, reduced cost for maintenance, and informed budget estimates for future planning.

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Mechanistic Approach to Pavement Rehabilitation, Maintenance and Forward Work Programmes

Cutting Costs Using Structural Analysis

Abstract

A roading authority that has pavement assets to manage needs well developed strategies for network maintenance based on desired levels of service, pavement deterioration and budget constraints supported by options for cost effective pavement rehabilitation once a terminal condition is reached. A purely reactive approach to maintenance or rehabilitation can result in both inefficiencies and unexpected costs. This is being further highlighted by the need to consider impacts of the newly increased... Read more

A roading authority that has pavement assets to manage needs well developed strategies for network maintenance based on desired levels of service, pavement deterioration and budget constraints supported by options for cost effective pavement rehabilitation once a terminal condition is reached. A purely reactive approach to maintenance or rehabilitation can result in both inefficiencies and unexpected costs. This is being further highlighted by the need to consider impacts of the newly increased VDM limits.

In recent years, a wealth of pavement structural data has been collected and stored in RAMM, but it is only used superficially. The bulk of the data is neglected. The mechanistic approach to pavement management uses Austroads principles to analyse the structural data already collected in RAMM, then calculates layer stiffnesses and stresses and strains induced in the pavement structure by trafficking to rationalise and quantify distress modes. This article addresses the role of structural analysis using a mechanistic approach to reduce the cost of maintenance and rehabilitation by effectively identifying the relevant distress mechanism, and focussing treatment only on subsections where work is essential, rather than using a continuous, and sometimes ineffective, treatment over an assumed treatment length. Often only the carriageway interval or top surface type is used for identification of treatment lengths.

In many cases, the existing raw data already stored in the RAMM database can be reanalysed to minimise any further data collection costs. Significant advancements in the application of pavement structural analysis have taken place in 2008 and 2009 as part of Austroads and NZTA research, and these are summarised in this article. The following aspects are addressed:

    Part I - Network maintenance and determination of Forward Work Programmes
    Part II - Pavement structural rehabilitation
    Part III - Quality assurance of newly constructed pavements and demonstration that the intended design life will be achieved
    Part IV - Maintenance cost implications of increased vehicle weight limits

The conclusions to each of Parts I , II, III & IV highlight the key benefits that can now be obtained from a mechanistic approach to pavement asset management giving cost-effective design, reduced incidence of premature failure, reduced cost for maintenance, and informed budget estimates for future planning.

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A small rock avalanche in toppled schist, Lake Wanaka, New Zealand

Abstract

A rock avalanche of 100,000 m3 occurred in a glacial valley in the Southern Alps of New Zealand in 2002. It originated on a 35◦ slope and released debris over a steep bluff. The resulting rock avalanche travelled 300 m, coming to rest on a gently sloping glacial bench. Individual boulders continued downslope and a number hit the Haast Pass Highway 600 m below. The bedrock in the region is mica schist, dipping at 50◦ into the slope. Large scale toppling is evident in the source area, with... Read more

A rock avalanche of 100,000 m3 occurred in a glacial valley in the Southern Alps of New Zealand in 2002. It originated on a 35◦ slope and released debris over a steep bluff. The resulting rock avalanche travelled 300 m, coming to rest on a gently sloping glacial bench. Individual boulders continued downslope and a number hit the Haast Pass Highway 600 m below. The bedrock in the region is mica schist, dipping at 50◦ into the slope. Large scale toppling is evident in the source area, with dips reduced to 20–35◦ in fractured, dilated rock. Aerial photos taken several years before the rock avalanche show a fresh scarp around the head, indicating significant slope deformation prior to failure. It is inferred that the scarp was the result of incipient sliding that eventually led to a catastrophic failure through loss of strength by strain weakening. The residual friction angle on the sliding surface is believed to be significantly less than the 35◦ slope inclination, providing conditions for rapid sliding.

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Video-analysis of an extremely rapid rockslope failure, Nevis Bluff, New Zealand

Abstract

A rockfall blocked a highway at the Nevis Bluff, South Island, New Zealand in September 2000. The fall was recorded on videotape, providing rare opportunity to study the details of an extremely rapid rockslope failure. Foliation in the steep, 100m high mica schist rock face,  dips into the slope at 45°. Persistent downslope oriented joints act as release surfaces for toppling or sliding. It is inferred that natural toppling processes were accelerated by roading excavations at the toe of the sl... Read more

A rockfall blocked a highway at the Nevis Bluff, South Island, New Zealand in September 2000. The fall was recorded on videotape, providing rare opportunity to study the details of an extremely rapid rockslope failure. Foliation in the steep, 100m high mica schist rock face,  dips into the slope at 45°. Persistent downslope oriented joints act as release surfaces for toppling or sliding. It is inferred that natural toppling processes were accelerated by roading excavations at the toe of the slope in 1972. Continued slow toppling reduced stability until failure eventually occurred. Slow motion analysis of the videotape showed an initial rockfall of less than 100m³, followed by the main failure of 10,000m³. Large schist blocks fell in a cascade from the bluff, followed by small rock avalanches of disintegrated schist. The release mechanism could not be determined from the video, but was possibly a combination of sliding and toppling. The final phase of the failure was a retrogressive translational slide from the head of the slope, that disintegrated and generated a small rock avalanche. A cone of debris accumulated on the highway below the bluff, with some spilling down the riverbank towards the Kawarau River. Understanding of the failure mechanism has facilitated risk assessment and monitoring of the rock face.

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Performance based specifications for unbound granular pavements

Procedures for Demonstrating Achievement of Design Life

Abstract

Pavement maintenance specifications are increasingly calling for “Operational Performance Measures” for rehabilitation treatments. One of these requires that the treatment is constructed so that post-construction materials testing and FWD testing confirm achievement of the design life. Often only post-construction testing is available and the procedures which maintenance contractors should follow to demonstrate achievement, are less well defined. This article is intended as a discussion docu... Read more

Pavement maintenance specifications are increasingly calling for “Operational Performance Measures” for rehabilitation treatments. One of these requires that the treatment is constructed so that post-construction materials testing and FWD testing confirm achievement of the design life. Often only post-construction testing is available and the procedures which maintenance contractors should follow to demonstrate achievement, are less well defined. This article is intended as a discussion document, and gives some alternative methods, depending on test data available, with suitable rationale for application. Rehabilitation treatments are addressed primarily. In some cases, new pavement construction can be assessed similarly but with limitations.

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Frost Resistant Design and Construction of Pavements in Central Otago

Abstract

Frost damage is a significant factor in the premature demise of unbound granular pavements in Central Otago. During the 1970s and 1980s, extensive lengths of new and rehabilitated pavements were constructed in the Cromwell Gorge and Lindis Pass (where frost penetration to 600 mm was noted). Experience obtained in these projects was documented at the time under research grants addressing both construction in schist and the frost resistance of pavement materials. The work was carried out for the N... Read more

Frost damage is a significant factor in the premature demise of unbound granular pavements in Central Otago. During the 1970s and 1980s, extensive lengths of new and rehabilitated pavements were constructed in the Cromwell Gorge and Lindis Pass (where frost penetration to 600 mm was noted). Experience obtained in these projects was documented at the time under research grants addressing both construction in schist and the frost resistance of pavement materials. The work was carried out for the National Roads Board to provide guidelines for the frost resistant design and construction of unbound pavements in similar terrain where schist and glacial deposits are the principal soil types. This update is intended for local practitioners involved with design and/or construction supervision and quality assurance of pavements in frost prone regions.

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Production Level FWD Back-Analysis Using the Full Time History

Abstract

When pavement structural evaluation is carried out using the Falling Weight Deflectometer, the full displacement-time history for each geophone is measured in the field, but for normal production only the peak deflection data are stored for subsequent processing. Velocity and acceleration information can however be readily captured. A preliminary study has been made to examine whether useful time-dependent parameters could be extracted from the full time history and used by practitioners. Softwa... Read more

When pavement structural evaluation is carried out using the Falling Weight Deflectometer, the full displacement-time history for each geophone is measured in the field, but for normal production only the peak deflection data are stored for subsequent processing. Velocity and acceleration information can however be readily captured. A preliminary study has been made to examine whether useful time-dependent parameters could be extracted from the full time history and used by practitioners. Software has been developed for capturing wave speeds and correlating with approximate moduli of the surface layer, to complement conventional back-analyses. Surface wave speeds appear promising indicators to allow rapid distinction of near surface material types and structural condition, at no additional cost or time.

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Performance-Based Specifications Using Deflection Measurements

Modular Ratios for Unbound Granular Pavement Layers

Abstract

Unbound granular pavement layers show a characteristic increase in moduli from the subgrade to the surface. The ratio of moduli between successive layers provides a direct and effective measure of pavement quality that makes due allowance for site conditions and seasonal effects beyond a contractor’s control. The modular ratio is therefore an effective indicator for performance-based specifications. Modular ratios are readily determined from back-analyses of deflection monitoring. Their applic... Read more

Unbound granular pavement layers show a characteristic increase in moduli from the subgrade to the surface. The ratio of moduli between successive layers provides a direct and effective measure of pavement quality that makes due allowance for site conditions and seasonal effects beyond a contractor’s control. The modular ratio is therefore an effective indicator for performance-based specifications. Modular ratios are readily determined from back-analyses of deflection monitoring. Their applications to new construction quality assurance and performance-based network maintenance contracts are described.

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Pavement Performance Prediction (I)

Determination and Calibration of Structural Capacity (SNP)

Abstract

Realistic prediction of pavement performance is a critical component of asset management. Performance in terms of structural capacity is generally measured by the Adjusted Structural Number (SNP), hence a review has been carried out testing alternative methods for deriving this parameter for unbound granular pavements. Simplified methods may work reasonably well when calibrated to typical local conditions, but they are less likely to provide the same reliability in different regions or with diff... Read more

Realistic prediction of pavement performance is a critical component of asset management. Performance in terms of structural capacity is generally measured by the Adjusted Structural Number (SNP), hence a review has been carried out testing alternative methods for deriving this parameter for unbound granular pavements. Simplified methods may work reasonably well when calibrated to typical local conditions, but they are less likely to provide the same reliability in different regions or with different pavement structures. The rigorous methods (which can be readily applied) are recommended wherever deflection bowl information has been recorded. However, it is important to note that the standard equations are based on isotropic moduli for each layer, rather than anisotropic moduli, which the Austroads Pavement Design Guide has adopted for mechanistic analysis. Typical field measured moduli for unbound granular pavements are presented and appropriate calculation procedures suggested.

Calibration or correction of SNP may also be required in a range of circumstances when refining the structural capacity for use in long-term pavement performance prediction. Where representative benchmark sites are set up and actual performance is monitored, appropriate calibration factors, based on specific modes of distress can be developed. These result in effective SNP values for a given network, giving due regard to the local range of materials and construction techniques.

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Applications of Mechanistic Pavement Design in New Zealand (II)

Case Histories Comparing Design Prediction with Post Construction Measurement and Analysis

Abstract

The AUSTROADS Pavement Design Guide has been used for the mechanistic design of pavement rehabilitation in New Zealand for several years. A number of projects have now been completed where pavement testing has been carried out before and after construction, giving the opportunity to compare design prediction with the in situ performance achieved by the rehabilitation. Case histories involving different forms of construction in various parts of New Zealand are presented, quantifying the improveme... Read more

The AUSTROADS Pavement Design Guide has been used for the mechanistic design of pavement rehabilitation in New Zealand for several years. A number of projects have now been completed where pavement testing has been carried out before and after construction, giving the opportunity to compare design prediction with the in situ performance achieved by the rehabilitation. Case histories involving different forms of construction in various parts of New Zealand are presented, quantifying the improvement in performance resulting from the use of AUSTROADS procedures. Data is also presented for typical in situ material properties of unbound granular overlays and cement based stabilised basecourses.

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Applications of Mechanistic Pavement Design in New Zealand

Case histories comparing overlay design methods including the TNZ Supplement to the Austroads pavement design guide

Abstract

Overlay design for pavement rehabilitation in New Zealand is based on mechanistic procedures given in the AUSTROADS Pavement Design Guide. In July 1997, Transit New Zealand issued a Supplement to the AUSTROADS Guide, promoting some variation to the existing methods to give due regard to the past performance of any road section programmed for rehabilitation.The New Zealand Supplement overlay design methods have now been applied to many projects on state highways and local authority roads. This ar... Read more

Overlay design for pavement rehabilitation in New Zealand is based on mechanistic procedures given in the AUSTROADS Pavement Design Guide. In July 1997, Transit New Zealand issued a Supplement to the AUSTROADS Guide, promoting some variation to the existing methods to give due regard to the past performance of any road section programmed for rehabilitation.

The New Zealand Supplement overlay design methods have now been applied to many projects on state highways and local authority roads. This article presents the results from a number of case histories for unbound pavements where overlay requirements have been calculated using both the original AUSTROADS design methods and the procedures set out in the New Zealand Supplement. Comparisons show the strengths and limitations of the various methods, providing a guide to the most useful conditions under which each method should be applied to five both cost effective design and assurance of long term performance.

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Pavement Deflection Measurement & Interpretation for the Design of Rehabilitation Treatments

Abstract

A mechanistic design procedure has been adopted by Transit New Zealand for designing rehabilitation treatments for New Zealand roads. A computer program such as CIRCLY (Wardle 1980) is sued to analyse the reaction of various pavement rehabilitation designs (modelled as multiple layers of linear elastic materials) under a standard wheel load. Other programs such as ELMOD include allowance for nonlinear elastic material. Strains within various critical layers are computed for each rehabilitation d... Read more

A mechanistic design procedure has been adopted by Transit New Zealand for designing rehabilitation treatments for New Zealand roads. A computer program such as CIRCLY (Wardle 1980) is sued to analyse the reaction of various pavement rehabilitation designs (modelled as multiple layers of linear elastic materials) under a standard wheel load. Other programs such as ELMOD include allowance for nonlinear elastic material. Strains within various critical layers are computed for each rehabilitation design being considered. The designs which are acceptable are those which meet or exceed specific performance criteria for asphalt, cemented bases and subgrade layers. Mechanistic design has the advantage of allowing the design of a range of rehabilitation treatments including: strengthening the existing pavement layers (stabilisation or other means); granular overlay; asphalt overlay; or any combination of these.

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Pavement Structural Condition Evaluation - Residual Life Assessment and Rehabilitation Design using the Falling Weight Deflectometer (FWD)

Abstract

Knowledge of the structural condition of a pavement is required to make effective decisions on the type of maintenance or rehabilitation to be carried out on a pavement section. Reliable quantification of the extent of necessary remedial work will ensure the design life is achieved with maximum benefit/cost ratio.The Falling Weight Deflectometer determines the full dynamic deflection bowl beneath a standard wheel load. Using the associated analysis software, it is possible to quickly and accurat... Read more

Knowledge of the structural condition of a pavement is required to make effective decisions on the type of maintenance or rehabilitation to be carried out on a pavement section. Reliable quantification of the extent of necessary remedial work will ensure the design life is achieved with maximum benefit/cost ratio.

The Falling Weight Deflectometer determines the full dynamic deflection bowl beneath a standard wheel load. Using the associated analysis software, it is possible to quickly and accurately determine the structural condition of the pavement system. The remaining life and appropriate rehabilitation requirements of a pavement section are determined from calculated stresses and strains in each layer. Thus, by using non-linear layered elastic theory, a pavement structure is analysed in the same way as other civil engineering structures. From non-destructive FWD testing, the required overlay or other rehabilitation alternatives are quantified from soundly based principles, producing maximum pavement life at minimum cost.

FWD data are combined with environmental data, layer thicknesses, material response functions and traffic load information to determine remaining life, critical layer, failure mode and required treatment (if any), at each FWD test point. Determination of structurally uniform subsections is based on a statistical evaluation of remaining life for all test points along the design section of pavement.

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Alarm Criteria and Monitoring for Hazardous Landslides

Abstract

Where hazardous landslides have been identified, case histories show that prediction of the onset of any rapid movement has not always been successful. Studies have been made of the precursory movements for a number of notable landslides and relevant geotechnical characteristics examined. Guidelines are provided for effective monitoring and improved evaluation of the potential for rapid landsliding of a hazardous slope. Read more

Where hazardous landslides have been identified, case histories show that prediction of the onset of any rapid movement has not always been successful. Studies have been made of the precursory movements for a number of notable landslides and relevant geotechnical characteristics examined. Guidelines are provided for effective monitoring and improved evaluation of the potential for rapid landsliding of a hazardous slope.

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Prediction of Effective Strength Parameters from Residual Strength Tests

Abstract

The concept of partial safety factors in geotechnical design is reviewed. Sets of comparative data from both simple and more time consuming shear tests are presented and the relevance of partial safety factors to slope stability and bearing capacity discussed. Methods for the prediction of effective strength parameters for silts and clays are suggested. Read more

The concept of partial safety factors in geotechnical design is reviewed. Sets of comparative data from both simple and more time consuming shear tests are presented and the relevance of partial safety factors to slope stability and bearing capacity discussed. Methods for the prediction of effective strength parameters for silts and clays are suggested.

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The Performance of Unbound Basecourse Under Simulated Traffic

Abstract

The performance of unbound granular basecourse surfaced with chip seal has been studied in a field situation using simulated traffic loads. Test variables included subgrade deflection, aggregate type, grading and shape.Pavement distress due to basecourse instability may be attributed fundamentally to thickness change due to densification and/or lateral shear movement of material beneath a wheel path. Contrary to reports involving laboratory models, the field trials found that lateral movement pr... Read more

The performance of unbound granular basecourse surfaced with chip seal has been studied in a field situation using simulated traffic loads. Test variables included subgrade deflection, aggregate type, grading and shape.

Pavement distress due to basecourse instability may be attributed fundamentally to thickness change due to densification and/or lateral shear movement of material beneath a wheel path. Contrary to reports involving laboratory models, the field trials found that lateral movement provided no measurable contribution to rut depth unless either high saturation or excessive pavement deflection had occurred. The most significant factor affecting the stability of basecourses was densification at constant moisture content resulting in a high degree of saturation (over 80%). This produced progressive rutting and lateral shear. The character of highly saturated basecourses was deceptive: excavated specimens were strongly cohesive and appeared quite “dry”. (Testing showed that moisture contents as low as 3.5% give excessive saturation in some basecourses after less than 10% of design traffic loading). The most adequate means of predicting susceptibility of stockpile material to excessive densification is examination of the shape of the particle graduation curve. The sand equivalent test, which reputedly determines the presence of undesirable fine material in aggregates, was found to correlate poorly with basecourse performance. Guidelines are offered for the practitioner wishing to utilise local materials, which do not comply with current NRB requirements for basecourse or subbase.

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Large-scale toppling of schist in North-West Otago, Southern Alps, NZ; a precursor to rockslides and rock avalanches

Abstract

In north-west Otago, New Zealand, large-scale toppling is widespread on scarp-slopes in schist, where foliation dips inwards at greater than 45°. It also occurs extensively in other areas of the Southern Alps with steeply dipping foliation. Toppling commonly extends along the sides of major glacial valleys, penetrating to depths of 50–150 m. Flexural toppling occurs in weak, mica-rich schist, and block-flexural toppling is seen in stronger quartz-rich lithologies. It is likely that toppl... Read more

In north-west Otago, New Zealand, large-scale toppling is widespread on scarp-slopes in schist, where foliation dips inwards at greater than 45°. It also occurs extensively in other areas of the Southern Alps with steeply dipping foliation. Toppling commonly extends along the sides of major glacial valleys, penetrating to depths of 50–150 m. Flexural toppling occurs in weak, mica-rich schist, and block-flexural toppling is seen in stronger quartz-rich lithologies. It is likely that toppling began in post-glacial times with the withdrawal of glacial ice, which debuttressed the slopes. Additional defects probably developed due to the relief of the high tectonic stresses, with rock-mass dilation and toppling being promoted by both major earthquakes on the Alpine Fault (plate boundary) and ingress of water from extreme rainfalls. The closely jointed and toppled rock is highly susceptible to rockslides, debris flows and gully erosion. Rockslides originating in toppled schist are typically slowly creeping, with downslope movement producing schist debris landslides similar to others in the Otago region. However in some cases rockslides can be extremely rapid, generating rockfalls, rock avalanches and debris flows. These forms of instability in toppled schist have caused ongoing engineering problems for a 25 km section of the Haast Pass Highway (State Highway 6), near the head of Lake Wanaka. A major earthquake on the nearby Alpine Fault has a high probability in the next 50 years. This is likely to cause extensive landsliding in the Southern Alps and result in damage and blockages to the highway, particularly in regions where toppled schist occurs on the slopes above.

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Modular ratios for unbound granular pavement layers

Abstract

Unbound granular pavement layers show a characteristic increase in moduli from the subgrade to the surface. The ratio of moduli between successive layers provides a direct and effective measure of pavement quality that makes due allowance for site conditions and seasonal effects beyond a contractor’s control. The modular ratio is therefore an effective indicator for performance-based specifications. Modular ratios are readily determined from backanalysis of Falling Weight Deflectometer bowls r... Read more

Unbound granular pavement layers show a characteristic increase in moduli from the subgrade to the surface. The ratio of moduli between successive layers provides a direct and effective measure of pavement quality that makes due allowance for site conditions and seasonal effects beyond a contractor’s control. The modular ratio is therefore an effective indicator for performance-based specifications. Modular ratios are readily determined from backanalysis of Falling Weight Deflectometer bowls recorded during construction monitoring or long-term structural condition studies. Their applications to new construction quality assurance and performance-based network maintenance contracts are described.

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