Auxiliary Bridge Collapse, South America

Continuing with our efforts to publish events from other parts of the world from which Engineering U/W or Risk Managers of infrastructure projects can learn, we bring you a report on an event that happened one month ago in South America. A river swelling carried enormous amounts of wood / logs and debris towards the construction site of a bridge, causing the collapse of an auxiliary bridge that was erected in order to build the definitive bridge.

Several local newspapers reported on January 7th, 2017 about an auxiliary bridge collapse due to a river swelling. The collapsed bridge was a provisional structure for the construction of a 480- meter-long bridge with a total contract value of USD 28 Million approximately.

This temporary construction was built to allow for equipment movement and human resources traffic from one to the other riverside, transporting construction materials and allowing for the erection of pillars of the definitive bridge in the riverbed. In the case we are analyzing, all the pillars of the definitive bridge were already erected and the construction of the deck was about to start. Total work progress of the project was approximately 65%.

The project’s region is subject to yearly precipitations during the rainy season between November and March and every year river swellings are part of the natural environment, especially if the river is an impetuous one.

Not so common, but recurrent, are the “riadas” or “palizadas” as they are known locally: material in form of logs, underwood, silt and other debris is being dragged downstream by the river and accumulates into floating islands with a considerable mass in the case their flow is obstructed. In this case the first signals of a “palizada” were observed on the 31st of December and on the 5th of January the situation was out of control: the enormous mass of dragged logs & woods pushed against the provisional bridge and caused a collapse of 140 meter of the auxiliary bridge that had a total length of 380 meter. Positively, it can be noted that according to the project owner, the definitive bridge’s pillars did not suffer any damage, probably also because the auxiliary bridge was located up-stream and protected them.

To the left the auxiliary bridge. Observe the short distance between their pillars (15 m)

Other view of the work site

Beginning of the accumulation of dragged material  

The floating mass increases

It appears that 50% of the river is covered by logs & timber

Dismantling and removal works of the provisional bridge

It appears that some central pillars of the auxiliary bridge were removed

The provisional structure did not resist the pressure.

The force of nature won

ANALYSIS

An event that develops over 4-5 days and that apparently is recurring in certain South American regions, could give place to following comments:

Knowledge of the local environment

The design of a bridge implies – we take this for granted – knowledge of the region’s hydrology including a list of the river’s tributaries and its flow rate, historical statistics of the maximum water levels, return period of river swellings etc. Taking into account that “riadas” is a known phenomenon in the region – like the “huaicos” in the Andean countries – a professional risk inspection before the rainy season could have pointed out at this special feature.

Design / Subcontractors

Those responsible for the design of the auxiliary bridge – made totally out of steel – did not consider the massive arrival of dragged material / logs while defining the span of the auxiliary bridge’s pillars. Here we have a noticeable planning error. The structure, like it was built, worked as a dam for the massive floating and size-increasing island approaching the bridge. What is surprising is that the project’s supervising entity – and again we take for granted that there was one – did not object the bridges spans.

Definitive Bridge Design

Noticing aforementioned inconsistencies, we should also question the design of the definitive bridge. The spans will be without any doubt much longer – apparently there will be three spans over an length of 480 meter – nevertheless the continuous action of the logs hitting the bridge’s concrete pillars every year during the rainy season, could debilitate the structure on the long term. The maintenance visits should be enforced strictly in this case.

Contingency Plan

Taking into account that this phenomenon is recurrent in the region, it rings the bells that in 5 days – from the first signals of a river swelling about to occur until the consummation of the collapse – the event could not have been avoided. The decision of dismantling a section of the auxiliary bridge came to late and apparently not as efficient as it should have been, if we observe the picture in this respect. We do not know how many people were working on the project and if an emergency task force was available.

Loss Prevention

We believe that loss prevention activities can be taken in order to avoid these types of losses, especially taking into account that this is a recurrent issue. The rivers can be monitored up-stream and also the precipitation intensity can be watched closely. Advanced alerts of the river’s behavior with a two day margin, consulting with local people about the historical behavior of the river at the construction site, the adoption of immediate action with machinery and equipment as well as human resources at the first sign of a “riada” and finally an adequate design of the auxiliary bridge, could without any doubt contribute avoiding or at least diminishing the loss.

Underwriting

We do not have any information about the project’s insurance matters, thus following comments are hypothetical and have more a pedagogical character. In an ideal world the engineering underwriter of the insurance company receives underwriting information and is aware of the local risk environment and will be doing a risk-commensurate underwriting with a fine-tuning of the deductibles or cover limits in case of special circumstances. In case of doubts of the projects details, or in case of incomplete u/w info, the insurance company would send out an inspector for a field visit where the risk can be assessed in a more detailed manner. A very important aspect during the inspection is the information about the project’s chain of responsibilities and the subject of the independent supervision of the project. We touched already this issue in one of our previous publications (News Clip 04 / 2016 Derrumbe de una Ciclovía, available only in Spanish). A critical and independent review of the project’s design is paramount. Unfortunately for financial constrictions, or due to the contractual relationship among the project owner, the main contractor and the project’s designer, the control instances can vanish.

Risk Inspection

Another subject is the risk inspection. The pictures of the case we are examining show that certain aspects of industrial safety are not being taken too seriously at the worksite. Compliance with OSHA is the first step for comprehensive loss prevention in a project and it is an important indicator of the worksite’s quality. We should mention that we did see in the past project contract wordings that force the contractor to follow strictly international safety norms like OSHA.

FINAL WORD

We were so fascinated by this event because during November 2015 in the same region a very similar event happened and with even worse consequences: can’t we learn from our past sins??? Shouldn’t we question working procedure that proofed to be deficient in the past? It is possible that there are no international norms or standards that define the forces that have to be considered in a river carrying massive amounts of wood and logs. The big question that arises is if the construction procedures are adequate: may be it would make more sense using barges for the building of the bridges pillars, instead of provisional structures exposed to the “riadas” for constructions site in those regions.

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