Assessment of Financial Aspects of Risks Associated with Structures. S. M. Hockey and S. K. Liew, WS Atkins Safety and Reliability, Woodcote Grove, Ashley Road, Epsom, Surrey KT 18 5BW
INTRODUCTION
Construction activities, transfer of ownership of structures
or assessment of maintenance requirements at a plant or structure
often require an preliminary assessment of risks in financial, as
well as technical, terms. Cost-effective mitigating measures can
also readily be identified. This enables levels of compensation
or contingency to be determined. In addition, assessment of the
most apposite time to undertake maintenance, and the increase in
cost associated with delay in maintenance, provides a useful
guide to budgetary planning.
This paper demonstrates that simple variations in standard analysis techniques can enable financial aspects of risk to be evaluated. Such analyses rely on historical data on the actual structure and similar sites as well as professional judgement and experience, both in safety and civil or geotechnical disciplines. The methodology is illustrated by examples using geotechnical structures and renovation or extension of existing structures. These include the following:
APPROACH
This paper uses the following definitions in describing the
hazard assessment:
| • Hazard: | An object or event that could cause undesirable consequences |
| • Frequency: | The probability of an incident occurring per unit time |
| • Consequence: | The likely outcome of an incident, in technical (damage to people and property), economical (cost of |
| loss of trade, cleanup) and/or remedial (work required to restore structure and /or environs) terms | |
| • Risk: | The combination of frequency and consequence |
Although advanced hazard and risk analysis techniques can be
used to assess technical risks in systems where there are few
unforeseen hazards in the behaviour of the system, considerable
problems arise when such techniques are applied to man-made or
geotechnical structures or to projects in their early stages.
Hence, any methodology adopted must seek to address all
significant mechanisms but at the same time be flexible enough to
address all deficiencies in information. Such deficiencies
include not only an imprecise understanding of present day
conditions but also a lack of knowledge of the long term
evolution of any particular disposal site and its surrounding
region The methodology is demonstrated using three examples using
geotechnical structures.
Assessment of essential maintenance of a reinforced earth
structure
A reinforced earth structure supporting a road had been built
across geologically and geotechnically difficult terrain. Shortly
after completion effects of settlement were observed and concern
was expressed that these settlements could adversely affect the
structural stability of the structure. As ownership of the road
section was due to change, a preliminary hazard assessment was
required to assess the maintenance requirements of the structure
prior to handover of the structure. The site was surveyed to
assess its current condition and the data from this study was
used as the basis of the assessment of the stability of the
structure and its maintenance requirements. The large number of
hazards and uniqueness of the structure meant that little
historical data was applicable. Linguistic categories were
therefore adopted for both probabilities and consequences, as
shown in the table below.
The reinforced earth structure was divided into a number of locations and hazards were identified and assessed for each. A criticality matrix, analogous to the simplified example shown in Figure 1 below, was drawn up to assess the most critical risks.
The most critical risks to the long-term stability of the
structure were obviously those which, where possible, had to be
reduced. However, some mitigation measures could be implemented
prior to handover, whereas others would have to be tackled on a
periodic basis and may depend on the changes in condition of the
structure. Once measures were identified which could be
implemented immediately, the cost of other possible mitigation
measures could be evaluated and compared with the associated
risk. Useful maintenance measures included:
| Before handover: | After handover: |
| • Drill seepage holes, | • Bi-annual condition survey |
| • Repair areas of bad spalling |
Assessment of construction risks (during modifications to
a bridge) for compensation and evacuation purposes.
The assessment of risks from construction activity to the locality of a bridge was required to enable the most hazardous areas to be identified and hence enable decisions to be made regarding compensation and temporary resettlement of people for the duration of the works for those living and working in the vicinity. The first step in the assessment was to divide the locality into a number of zones. All hazards were then identified and the magnitude of the probabilities and the consequences was assessed for each zone, where appropriate, according to the methodology described above. The judgement on the magnitudes of probabilities and consequences was based on the following:
The total risk rankings were then evaluated for each zone for
both personal and material risks and this enabled the area to be
divided into regions of priority for resettlement and
compensation purposes. It also assisted development of the site
safety plan by highlighting critical risks in particular areas,
and by highlighting areas in which particular risks would have
high personal or material consequences so that mitigation
measures could be implemented.
Contingency for historic dam
The dam in this example was once an impounding reservoir for a blast furnace and dates from the beginning of the industrial revolution. Although the dam is still intact, its stability and present condition are not well known. In addition, the area around it has changed considerably since the date of its construction and there are many external hazards which could have an impact on the dam. Its ownership was also due to be transferred and, in a manner analogous to that demonstrated in the first example, the hazards were identified and evaluated in financial terms. Since the furnace building had been turned into a museum, there were hazards which involved the following:
The sum of the risks was used to evaluate the contingency
(around £200,000).
CONCLUSIONS
Simple variations in standard risk analysis techniques can be
used to assess contingency for structures or maintenance
requirements for a variety of constructions where the nature and
magnitude of the risks is not readily determinable and detailed
risk calculations are not appropriate. The cost effectiveness of
maintenance measures can be determined and the most apposite time
to undertake maintenance, from a economical viewpoint, can be
estimated. Whilst being conservative, the methodology allows for
data ranges in frequencies and consequences and is readily
adaptable to a variety of structures.