Page 360 - 360.revista de Alta Velocidad - Nº 5

Campos Zaldiernas, Javier. Díaz García, Sarai. Galán Alguacil, Álvaro. González Pérez, Javier.

diagnosis, the key pathologies or risk factors that affect the culvert are identified and based on
this information, a series of standard actions are proposed to address the deficiencies found.
Each action has a specific cost and an economic repercussion on the assessed risk level defined
as the reduction of the annual risk of failure caused by such remedial works. Based on these
parameters, it is possible to carry out a cost-benefit study of each of the considered actions and
lastly, selecting the actions in each case according to profitability criteria in the short, medium
and long term.
The repairs or remedial works selected must fit within the budget availability, so that the most
cost-effective actions are carried out in the first place

2.1 Failure probability

The annual failure probability of a culvert is determined as the sum of the failure probabilities
of each of the failure mechanisms operating in the influence area of the culvert, causing partial
or total traffic disruption in the railway. These events are grouped into four main categories
called failure modes (Roads and Traffic Authority, 2010):

1. Failure mode 1: Structural collapse. There is only one potential failure mechanism:

1.1 Structural collapse of the culvert barrel.
2. Failure mode 2: Slope instability. There are four potential failure mechanisms:

2.1. Slope instability caused by afflux.
2.2. Slope instability caused by leakage out of barrel.
2.3. Slope instability caused by headwall collapse.

2.4. Slope instability caused by undermining at inlet or outlet.
3. Failure mode 3: Piping. There are three potential failure mechanisms:
3.1. Piping into culvert.

3.2. Piping on outside of culvert due to afflux
3.3. Piping on outside of culvert due to leakage out of culvert.
4. Failure mode 4: Hydraulic flow. There are two potential failure mechanisms:
4.1. Erosion by overtopping flows.

4.2. Cross catchment flooding.

Thus the previous mechanisms are considered independent phenomena and therefore the
annual probability of failure of the culvert (global) is assessed as the sum of the probabilities of
each failure mechanism. Each mechanism is constituted by a series of events that must occur
sequentially so that the failure mechanism is able to take place. These events are divided into
two different categories according to its nature:

• Precedent events: these events caused the failure situation, corresponding to an initial
situation linked in most cases to a rainfall event or the current condition of the culvert
barrel, headwall and wingwalls (deterioration). Adverse circumstances are essentially
weather-related precedent events – either exceptional rainfall or extreme groundwater
levels. In such cases the probability of failure may be governed by the return period of
the necessary precedent event, which would be reflected in the assessment of an annual
probability of failure.

358 360.revista de alta velocidad