Revision:
| Version | Date | Author | Change Description |
| 1.0 | 27/11/2020 | Initial Document Prepared | |
| 2.0 | 08/03/2022 | P Bellamy | Document Revised |
| 3.0 |
Introduction
Following the principles set out in the Highways Group Lifecycle Plan, this document outlines the data and decision making processes which Plymouth Highways utilise in the maintenance of Highway Structures.
This document details the lifecycle planning for Structures assets Maintained by Plymouth City Council outlining the approach to management and maintenance of the asset group.
Asset Data Management
Inventory Data
Currently all information on Plymouth City Council’s Highway Structures is held on a variety of storage mediums.
As part of Plymouth Highways’ Data management strategy, the importance of holding this information on a singular asset management register which is designed specifically to handle the detailed itemisation, asset management and modelling of highway structure assets has been recognised and implemented. Currently, Bridgestation is used by Plymouth to manage structures assets.
Plymouth City Council’s inventory currently comprises the following Highway Structures, divided in to those on Highway Maintainable at Public Expense (HMPE) and those that are not:
| Asset Type | HMPE | Non – HMPE | TOTAL |
| Bridge | 35 | 62 | 97 |
| Parapet | 7 | 0 | 7 |
| Culvert | 56 | 48 | 104 |
| Footbridge | 26 | 40 | 66 |
| Other | 0 | 6 | 6 |
| Subway | 48 | 10 | 58 |
| Tunnel | 4 | 4 | 8 |
| Viaduct | 1 | 8 | 9 |
| TOTALS | 177 | 178 | 355 |
Retaining walls do not appear in the above inventory as they have historically been managed on a reactive basis because of the sheer numbers involved and the lack of definitive ownership documentation. Our forward plan will be to locate, identify and catalogue retaining walls. Working with Plymouth City Council’s legal department, potential landowners will be identified to bring clarity regarding maintenance responsibilities
Condition Survey
The overall condition of Plymouth City Council’s Highway Structures stock is established through a regime of inspections. An ongoing programme of regular inspections has been established in Bridgestation.
Primary guidance for the inspection of highway structures is taken from the Design Manual for Roads and Bridges (DMRB). The four types of inspection carried out by Plymouth City Council are:
| Type | Used For | Frequency |
| General Inspection | Used to provide information on the physical condition of all visible elements of a highway structure | Carried out at 24-month intervals. When a General Inspection coincides with a due Principal Inspection only the latter is carried out |
| Principal Inspection | Used to provide information on the physical condition of all inspectable parts of a highway structure | Carried out every 6 years. However, this interval can be varied up to a maximum of 12 years subject to a risk assessment. |
| Special Inspection | Used to provide detailed information on a particular part, area or defect that is causing a concern | Special Inspections may comprise a close visual inspection, testing and/or monitoring. The frequency of inspections is risk based and may involve a single one-off inspection or a programmed series inspections |
| Inspection for Assessment | Used to provide information required to undertake a structural assessment | Guidance on Inspections for Assessment is given in BD21 which recommends that they be carried out in conjunction with a Principal Inspection |
Each individual inspection of a highway structure records the severity and extent of defects, if present, on each element that can be inspected. Severity being the degree to which a defect or damage affects the function of the element or other elements on the bridge.
| Severity | |
|---|---|
| 1 | As new condition or has no significant effect on the element (visually or functionally). |
| 2 | Early signs of deterioration, minor defect/damage, no reduction in functionality of element. |
| 3 | Moderate defect/damage, some loss of functionality could be expected. |
| 4 | Severe defect/damage significant loss of functionality and/or element is close to failure/collapse. |
| 5 | The element is non-functional/failed. |
Extent being the area, length or number of a/the bridge element that is affected by the defect or damage.
| Extent | ||
|---|---|---|
| A | None | No significant defect |
| B | Slight | not more than 5% of surface area/length/number |
| C | Moderate | 5%- 20% of surface area/length/number |
| D | Wide | 20% - 50% of surface area/length/number |
| E | Extensive | more than 50% of surface area/length/number |
Permissible combinations are as follows:
| Extent | Severity | Severity | Severity | Severity | Severity |
|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | 5 | |
| A | 1A | ||||
| B | 2B | 3B | 4B | 5B | |
| C | 2C | 3C | 4C | 5C | |
| D | 2D | 3D | 4D | 5D | |
| E | 2E | 3E | 4E | 5E |
These individual element severity and extent values, combined with a complex interaction of variables, contribute towards the calculation of two condition factors for the overall structure. These factors are known as the Bridge Condition Indicator (BCI) scores and are defined as:
BCIav: The average BCI score for a structure taking into account the condition of all structural elements on the structure. This score provides an overview of the structure condition.
BCIcrit: The BCI score for the critical load bearing element of a structure that is in the worst condition. This score provides an indication of the criticality of the structure with regard to its load carrying capacity.
An average value for Plymouth City Council’s whole bridge stock, known as the Bridge Stock
Condition Index (BSCIav), can also calculated based on the BCIav scores for each Highway Structure.
Individual defects, identified through the regime of inspections, are currently added to a ‘Work Bank’ which is a list of maintenance work required to bring the structures up to standard. The BCI scores for each individual element and each highway structure are then used to establish appropriate intervention timescales and to prioritise maintenance actions.
Data Management
Information stored on Bridgestation will be regularly updated in a format fit for the challenges of the current asset management approach. It is a complete asset management tool for Highway Structures within which all important files can be uploaded and stored against each highway structure.
The management tools within the software enable the user to search, interrogate and report on the data in order to:
- Prioritise the programme of inspections
- Review Bridge Condition Indicator (BCI) scores that are automatically calculated and reported for each highway structure.
- Produce prioritised lists of maintenance actions.
- View GIS mapping to aid the preparation of the inspection and works management programmes.
- Produce the required asset valuations; Whole Government Account (WGA), Gross Replacement Cost (GRC) and Depreciated Replacement Cost (DRC), which will support senior management and council members in future bids to central Government.
- Provide senior management and council members with Lifecycle planning deterioration models based upon inspected elements and predicted and/or desired levels of investment.
- A Gap analysis of the data stored in order to track progress and identify missing or erroneous data.
- Build financial and condition reports made up of the fields specified and/or data filters applied.
This approach enables stakeholders to make effective and informed decisions and to understand the impact of those decisions on the asset and the subsequent level of service and performance required to maintain the assets in a fit for purpose state.
Performance and Programme Planning
Performance Management
The performance for structures is measured by BCI and BSCI scores. Each individual structure has its own BCI score which is based on its condition at the time of the inspection. As a guide, the BCI scores represent the following:
- 100 – 95 Asset or Bridge stock in Very Good condition;
- 94 – 85 Asset or Bridge stock in Good condition;
- 84 – 65 Asset or Bridge stock in Fair condition;
- 64 – 40 Asset or Bridge stock in Poor condition;
- 39 – 0 Asset or Bridge stock in Very Poor condition.
If maintenance work is carried out following the general course of inspections it would be expected that the BCI score for the structure would increase accordingly. Likewise, the BSCI score as this is directly dependant on the BCI scores. The same bracketed scores and definitions apply to the whole bridge stock (BSCI) as they do to the individual asset (BCI).
Targets against these performance measures are reviewed annually to consider changes to best practice, legislation, the corporate objectives and to consider any changes to funding levels.
Current performance measures for structure are:
| Performance Monitor | Definition | 2020/21 Target | 2020/21 | Reporting Frequency |
| SE-ST-001 | % of bridge stock in poor condition | <20% | 14% | Annually |
| SE-ST-002 | % of council road bridge stock with height, width or weight restrictions | <10% | 10% | Annually |
| SE-ST-003 | Bridge stock condition indicator average BSCIav | TBC | 80.4 | Annually |
| SE-ST-004 | Bridge stock condition indicator average BSCIcrit | >63 | 65 | Annually |
| SE-ST-005 | % of general inspections (GI's) completed on time | 100% | 100% | Annually |
| SE-ST-006 | % of principal inspections (PI's) completed on time | 100% | 100% | Annually |
Reactive Maintenance and Defect Repairs
The public can report defects and damage through the Firmstep system, by email or by telephone. Defects are inspected by Plymouth City Council staff, and if appropriate, programmed for repair. Repairs may be carried out by Plymouth City Council’s contractor, but some work may require specialist contractors or sub-contractors. In such instances, repairs cannot be carried out within short notice.
Defects
The main causes of asset defects and/or deterioration are itemised in the table below.
| Causes | Descriptions | Typical defects |
|---|---|---|
| Design | In appropriate design decisions made, no design for maintenance consideration | Structural failure, unachievable maintenance tasks, inherent maintenance defects, immediate maintenance costs following completion of construction |
| Durability | Failure to consider environmental loads or to account for local micro-climate conditions | Wear and tear, ageing, weathering, corrosion and/or chemical attack, cracking |
| Materials | Inadequate material choices | Increased rate of deterioration due to chemical attack, reaction to aggressive environments, cracking |
| Construction | Poor workmanship/site control during construction, poorly compacted concrete, areas of low cover etc. | Delamination, spalling, corrosion |
| Loading | Increased vehicle loading, unsuitable traffic, presence of excess water | Excessive deflections, greater earth pressures, displacement of abutments or wingwalls, structural failure, increased rate of deterioration/reduced life expectancy |
| Maintenance | Insufficient inspection and/or monitoring, inadequate repair strategies/repair methods | Increased rate of deterioration |
| Liquefaction | Saturation and/or settlement of supporting and/or supported material | Structural failure and/or failure of adjacent highway |
| Scour | Flooding, blockages, abrasive action of water and/or debris | Increased rate of deterioration, flooding of adjacent landscape |
| Climate | Additional freeze/thaw action, flooding, heat, unplanned water table changes | Unexpected severe damages, increased rate of deterioration of structural elements |
| Impact | Vehicular collisions | Structural failure, reduction in structural capacity, road closure |
| Vandalism | Fire, structural damage, theft | Structural failure, reduction in structural capacity, road closure |
Structural Maintenance Strategy and Investment
A programme of routine maintenance has also been developed from the Condition Inspection results. The attributed BCI scores will be used to prioritise work according to need and these can then be repaired in line with the availability of funding.
Over goal is to:
- Improve the overall condition of highway structures.
- Prevent backlog of maintenance work on highway structures.
- Reduce the risk to highway users from substandard structures
The drive is to assure we
- Improve the average condition of the bridges assets
- Improve the condition of bridges assets so that all bridges have an individual BCI Critical condition of Good or better
- To ensure that all sub standard bridges (prior to strengthening / replacement) are managed in accordance with BD 79.
Such works include patching of specialist surfacing to footbridges, brickwork re-pointing, and vegetation clearance and re-painting of masonry and/or parapet guardrails.
Lifecycle Planning
The analysis parameters, maintenance strategies, activities, rates, schedules and the deterioration profiles detailed below are all modelled on the ‘Structures Toolkit’ which published by CIPFA and used industry wide for WGA returns.
Condition Data
Each inspection carried out, whether that be General, Principal or otherwise, will record condition values, defect severity and extents, within a bespoke asset management software package for highway Structures; BridgeStation (BS). Condition values are based on a matrix of defect severity (A to E) and extent of defect (1 to 5) …. A1 being no defects present to 5E for a widespread major defect. The combination of severity and extent values for the basis of the condition trigger levels described below.
Maintenance Strategies
Within BS there are five pre-set Maintenance Strategies that comprise its Life Cycle Plan Toolkit model. These are:
- Planned Preventative
- Planned Targeted
- Planned Do Minimum
- Unplanned Reactive
- Do-nothing
Condition trigger levels
Within BS there are default ‘condition trigger levels’ for each element type, for each material type and for each of the above five maintenance strategies. However, if the end user desires, these condition trigger levels can be adapted to suit an individual’s ethos towards asset management. The default condition triggers are as follows:
| Strategy | Condition Trigger level |
|---|---|
| For a planned preventative maintenance approach | All elements of any importance are treated once they reach the condition triggers of either 3C, 2C or 4B. |
| For a planned targeted maintenance approach | Very High, High and Medium importance elements are treated once they reach the condition trigger of either 3C or 4B. |
| For a planned do-minimum maintenance approach | Very High and High importance elements are treated once they reach the condition trigger of 4D. |
| For an unplanned reactive maintenance approach | (Demonstrating the consequences of a zero-budget). It is anticipated that, if no funding is available, the stock condition and value would decline over the evaluation period. However, all elements irrespective of their importance are treated when they reach condition 5B with the appropriate maintenance activity. |
| For a do-nothing maintenance approach | This is a ‘Custom strategy’ that reviews deterioration over time. No maintenance defects are added to the workbank and no intervention levels are set. |
That being said; where insufficient budget is available to intervene at the trigger condition for a specific strategy, or where elements of other importance reach condition 5B, an appropriate intervention will be applied when they reach that level.
Analysis
To take life cycle planning to its conclusion, BS also contains functionality that compares ‘asset life expectancy/deterioration’ for chosen ‘levels of investment’ i.e. the adoption by PCC of one or other of the five maintenance strategies.
Programme
The analysis functionality described above has yet to be used by PCC due to inconsistencies in historic severity and extent values. Raw data is still being collated, inconsistencies being rectified and new severity and extent values registered following scheduled inspections. It is expected that from mid-2022 this should be available for use and meaningful data extracted.
However, in order to eliminate the historic back-log of defects, and as a pre-emptive measure, the following maintenance programmes are in place.
Yearly Programme
| Asset | Frequency | Activity |
|---|---|---|
| Culverts | Monthly | Known flooding Hot-spots. Clear debris and de-silt to maintain uninterrupted flow through the culvert. |
| Quarterly | Culverts where flooding would affect residences. Clear debris and de-silt if required so as to maintain free flow. | |
| Bi-annually | Culverts where flooding would affect Businesses and/or public spaces. Prune overhanging vegetation, clear debris. | |
| All, during the above | Minor repairs to head walls, protective guardrails, inverts and embankments 10m before and after inlet/outlet. |
5-Year Programme
| Asset | Activity |
|---|---|
| Footbridges | Clear encroaching vegetation, jet wash, weld repairs, deck drainage repairs, joint replacement, repainting, resurfacing. |
| Subways | Clear encroaching vegetation, jet wash structure, weld, masonry and/or concrete repairs, drainage jetting, mapping and repairs, joint replacement, repainting, resurfacing. |
| Tunnels | Confined space Principal inspections. Programme repairs if required. |
| Bridges | Jet wash, service drainage, repaint. If required replace waterproofing, surfacing and bearings |
The extent of the council’s responsibility for the maintenance of retaining walls, hedgebanks and cliff faces is unknown. Over the coming years assets will be identified and ownership/maintenance liability established. Assets will be added to BS, survey/inspection work carried out and a maintenance programme developed.
It is proposed to change the way culverts are managed within PCC. Rather than individual owners managing individual assets along a watercourse it is proposed to create a ‘water management’ team to manage watercourses as an entity itself. The PCC team will comprise members from all necessary disciplines and will establish working relationships with all affected external third parties.
Network Resilience
Failure of a structures asset, although relatively infrequent, can have catastrophic consequences including loss of life. Any failure will also have major implications for network resilience since repair or replacement will be a long drawn out process. Failure of an asset can be associated with many factors; however, by far the most common is as a result of an extreme weather event. These events are becoming more frequent as a result of climatic uncertainty and it is possible that the effects of a future extreme event could have implications for highway structures assets. Of particular concern are those assets that span watercourses that may be susceptible to scour. A detailed assessment of risk is ongoing. This will identify high risk sites and allow suitable mitigation measures to be implemented to manage any potential network implications.
An embedded diversion arrangements is already in place should partial or catastrophic failure occur.