Fluvial Design Guide - Chapter 11

River and canal structures

11.4 Rehabilitation of structures

There are large number of existing structures on our rivers and canals. Each one requires consideration of how best to maintain it, in order to prolong its useful life, while planning for its eventual replacement or retirement.

The importance of this is recognised in work during phase 1 of project FD2318 (Performance and reliability of flood and coastal defences) under the joint Environment Agency/Defra flood and coastal erosion risk management R&D research programme (Environment Agency, 2005; Buijs et al, 2007). For full details of this project and links to the various project outputs, use the project search tool on the programme website (http://sciencesearch.defra.gov.uk/Document.aspx?Document=FD2318_7087_PR.pdf).

The second phase involving a performance-based asset management system (PAMS) to develop and demonstrate the methodologies is underway. This will lead to a third phase of supporting manuals and software. For information about the project see the PAMS website (http://www.pams-project.net/index.htm).

The principal steps in decision-making are discussed in the following sections.
 


11.4.1 Inspection and residual life

To aid decision-making when planning the management of assets on rivers and canals, it is essential to ascertain the residual life of the asset – usually through means of an inspection. Inspection often presents various access and safety issues, and problems associated with assessing the condition of buried structures and foundations.

Inspection for failure – serviceability and ultimate

An ultimate limit state is a limit beyond which a structure no longer serves its primary purpose. Emergency works may be required if the structure is failing, or about to fail in its primary purpose. This is especially true if the structure in question is a flood risk or navigational asset.

A serviceability limit state is one that affects functions of only secondary importance such as finishes or appearance. Such issues may be resolved with less urgency, although it is often more cost-effective to intervene sooner rather than later.

Inspection for residual life

There are many inspection methods available; some of the most important ones are listed in Table 11.1. Before interpreting any results that the inspection may yield, it is essential to understand the limitations of the inspection method.

Assessing residual life

The performance-based approach to assess residual life of structures developed with the concept of fragility curves as part of the joint Defra/Environment Agency flood and coastal erosion risk management R&D programme (Buijs et al, 2007) can be applied to river and canal structures. The fragility curve is a measure of the variation of the probability of failure of an asset with the range of loading conditions to which it is subjected.

Table 11.1 Inspection methods – applications and limitations

Method

Application

Limitations

Walkover/visual, with photographic record

Assessing structural form, condition, materials and access restrictions to site
Identifying potential problems such as accumulations of sediment

Can only see what is visible at the surface

Diver survey

Assessing extent and condition of submerged structural elements
Mapping bed levels
Identifying scour hole locations

Health and safety issues
Limited visibility in certain conditions

Trial pit

Finding structure footprint
Assessing buried services and structures as a precursor to other surveys

Depth limited by excavator arm and stability of excavation

Borehole

Assessing soil conditions and geology

Small sample may not be representative, so may require several boreholes

Drilled core

Assessing concrete condition and extent
Collecting samples for destructive testing

Small sample may not be representative, so may require several cores
Drilling may cause damage to structure

Non-destructive materials testing

Can test different regions within a structure for comparative strengths
Finding weak points
Can be correlated with destructive testing results

Large margin for error

Destructive materials testing

Assessing material qualities, strength, type and chemistry

Small sample may not be representative
Damage to structure

Geophysical methods

Assessing geology and soil structure
Identifying weak points, voids, buried structures and services for further investigation

Limited penetration of hard surfaces
Ambiguity of results

Total station survey

Mapping levels and locations of key features to a given accuracy

Level of accuracy
Line of sight required

Laser survey

Mapping levels and locations of solid surfaces (for example, lock chambers) to a very high accuracy
Identifying instability and movement

Relatively expensive
Line of sight required


11.4.2 Intervention strategy

Choosing the correct strategy for rehabilitation of an asset requires a structured method of comparing a variety of options. Innovative options may present themselves, or be suggested, for specific structures. There are also various generic strategies that should be considered in every case. Strategy options are summarised in Table 11.2.

Table 11.2 Intervention strategy options

Option

Comments

Do nothing

The effects or risks arising from this option must be assessed to form a baseline by which other options are measured in a benefit to cost ratio.

Patch and repair

This approach entails simply fixing issues as and when they arise. Examples include:

  • carrying out a localised repair to a damaged revetment;
  • replacing a broken hinge on a flapgate.

Periodic significant maintenance

A schedule of major works that will replace whole structural elements that are obsolete or at the end of their serviceable life. Examples include:

  • replacement of non-standard trash screens;
  • substitution for a pumping station with newer, more efficient, electric motors.

Demolish and reconstruct

Demolition and replacement of the present structure with a new one designed to the same or similar design criteria.


11.4.3 Whole-life costing

A whole-life costs approach should be taken to appraising rehabilitation options. This involves accounting for design, planning, maintenance and decommissioning costs as well as the cost of the capital construction works to be undertaken.

To ensure carbon dioxide (CO2) emissions are taken into account, carbon costs for options should be calculated and compared using the Environment Agency’s carbon calculator for construction activities (http://www.environment-agency.gov.uk/business/sectors/37543.aspx).

Both cash and carbon costs require net present value discounting.

Details of methods of discounting financial costs can be found in The green book (HM Treasury, 2003).

For discounting carbon costs see the Defra guidance on the shadow price of carbon (http://www.defra.gov.uk/Environment/climatechange/research/carboncost/index.htm).
 


11.4.4 Repair and rehabilitation techniques

Seasonal working

Rehabilitation of river and canal structures is often constrained by:

  • prevailing fluvial conditions;
  • the need to avoid disrupting navigation;
  • the requirements of stakeholders such as anglers.

These considerations may significantly reduce the period in a year when it is acceptable to undertake works (except, of course, in an emergency), as illustrated in Figure 11.5.

Figure 11.5 Penton Hook lock refurbishment

Works were carried out to refurbish the lock at Penton Hook. The work included sprayed concrete to resurface the walls and replacement rubbing strips and edge stones. This required draining the lock and therefore temporary lock closure.

As most of the boat traffic on the River Thames is during the summer, closure of the lock had to take place during winter months. The increased risk of flooding had to be reflected in the contract terms.

Dry working

Some operations are entirely possible without closure of the watercourse. Some require works to be carried out in the dry. This necessitates the construction of temporary works (see Section 8.7) such as:

  • a cofferdam using techniques such as sheetpiling;
  • water-filled dams;
  • portable temporary dam (membrane supported by a steel A-frame and similar units).
     

 

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