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Stone age science

Geotechnical engineers in Bahrain have studied the local limestone to better understand its behaviour when used as armourstone for coastal protection. NCE reports.

The trend for coastal development has led to some interesting research by engineers in Bahrain into the rocks used for shoreline protection. Locally sourced rock is often used as armourstone in Bahrain due to the complexities and costs involved in importing heavy rock segments. However this throws up challenges to the engineers trying to predict design performance, as the limestone sourced from the local quarry at Askar is highly variable in terms of quality.

In order to get a more accurate understanding of the rock performance, engineers from URS Scott Wilson studied Bahrain limestone in a self-funded investigation.

Poorer quality rock

“We were aware that the quality of the local rock was getting poorer and poorer. Inspections on revetments built in the past five to 10 years showed that the rocks were subject to degradation due to solution weathering,” explains marine design team leader Andrea Caricato.

These signs of marginal quality had prompted URS Scott Wilson to apply a factor of safety when designing rock structures but this was mainly based on experience and engineering judgments.

“Inspections on revetments built in the past five to 10 years showed that the rocks were subject to degradation due to solution weathering”

Andrea Caricato, URS Scott Wilson

Thanks to the extensive development of Bahrain over the past decade there was plenty of data available on local limestone. Analysis of the test results confirmed that rock quality was indeed highly variable. Of the 970 density tests available, the difference between the average and exceedance densities was 100kg/m3 below the mean density. This figure is at the extreme limit of the normal range suggested by the 2007 Rock Manual by the Construction Industry Research and Information Association.

Mass loss prediction

Two methods were then applied in order to predict the mass loss, the Micro-Deval method and the Armourstone Quality Designation method. Both of these models calculate a parameter that represents the site aggressiveness and a parameter that represents the intrinsic durability of the rock, and use them to estimate mass loss over time.

The study concluded that designers consider around 20% loss of mass for a typical 3t local rock during 50 years in service. Smaller rocks might experience greater losses and for 1.5t segments designers would need to consider losses of around 24%. This was determined by adopting rock proprieties and site conditions which are deemed generally representative of the local rock and for the Bahrain coast. “The marginal durability of local rock needs to be recognised as a potential critical factor for the new developments,” says Caricato, who adds that a range of approaches is available to designers.

Multiplier factor

“One option is to apply a multiplier factor when sizing the rock used as revetment, to take into account the fact that the rock designed and placed today might not be able to withstand the critical environmental conditions towards the end of the structure’s life,” he says.

However this is only a feasible route providing that the oversized rock is still within the limits of the quarry production capability. It is possible that the local quarry might not be able, in the near future, to procure rock over a certain size.

Another option for clients in Bahrain is to consider importing rock from the United Arab Emirates or Oman. However, the cost will play a key role in the final decision and so far clients in Bahrain have been reluctant to take this route.

Of course, the use of precast concrete armour units is an alternative that has been adopted in the region as well as in Bahrain. This becomes a feasible option only for projects where economy of scale applies, as for example in Dyiar Al Muharraq, where URS Scott Wilson proposed precast Accropodes on the most exposed edges for optimised protection.

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