Published: Friday, May 17, 2013

The Tale of Two Ponds

Rana Plaza and Spectrum Sweater Factory

Aerial view of Rana Plaza in 2010, note    the remaining portion of pond in green. (Source: Google Earth)

Aerial view of Rana Plaza in 2010, note the remaining portion of pond in green. (Source: Google Earth)

As I write this, the death toll at the collapsed Rana Plaza at Savar, Bangladesh has crossed  1000. Unfortunately the final death toll will possibly go even higher. Hundreds have been seriously injured and thousands have been rescued and are lucky to have escaped death.

The collapse of Rana Plaza reminded me of the April 12, 2005 collapse of Spectrum Sweater factory near Savar. Both these tragedies seem to be connected with an invisible thread. The eerie connection is the fact that both these structures were built partly on land created from filling up ponds.

The aerial view of Rana Plaza in 2010 from Google Earth is shown in Figure 1 and it clearly shows the remaining portion of the pond behind the massive eight-story building (reportedly 100′ x 250′ foot-print). Figure 2 shows Rana Plaza after it collapsed on April 24, 2013 and the partially filled pond at the top right.

Collapsed Rana Plaza in 2013, note the remaining portion of pond at top right corner. (Source: Internet)

Collapsed Rana Plaza in 2013, note the remaining portion of pond at top right corner. (Source: Internet)

The nine-story Spectrum Sweater factory was located less than 10 km north of Rana Plaza.  Figure 3 shows a photograph of the rubble after the collapse. According to a media report (Prothom-Alo, April 12, 2005), part of the factory building was built by filling a “jheel”. Figure 4 shows a photograph of the rubble of Spectrum building and the jheel/pond that was partially filled to build the structure.

In a land-deficient country like Bangladesh, ponds are regularly filled to construct buildings. The vanishing ponds and wet lands have given rise to serious problems of storm-water drainage in the urban areas after heavy monsoon down pours, causing flooding of streets and houses. But more serious and relevant to this article is the fact that these rampant pond fillings have created death traps.

Though readers might be aware of the term “settlement” of buildings, we need also to discuss the technical term “differential settlement”. Simply put, it is the difference in the amount of settlement between two parts of the building. Normally, buildings are considered to withstand differential settlement of up to 1 inch (25 mm) without much distress.

Collapsed Spectrum Sweater Factory in 2005. (Source: The Daily Star)

Collapsed Spectrum Sweater Factory in 2005. (Source: The Daily Star)

Now let us bring the partially filled pond in the picture. As reported in the media, about half of the 250-foot long Rana Plaza near the Dhaka-Aricha Highway was built on natural undisturbed ground and the eastern half was resting on filled up pond. The filling was not what is called ‘engineered fill” that is, filled with proper compaction of every 1-foot thick layer. So the filled ground ended up being loose compared to the undisturbed soil. Not only was the proper compaction missing, but added to it was the highly compressible organic soil (muck) at the bottom of the pond. A proper “engineered fill” would require that the organic soil at the pond bottom be removed first by what is generally called a “de-mucking” operation.

The organic soil at the bottom of the pond may be several feet thick and due to its porous, fibrous nature is almost like sponge and is highly compressible. The mucky soil compresses (settles) initially under the load of the fill placed on top of it, and then continues to settle with time as the organic fibers decay over many years in a process called “secondary consolidation” in geotechnical literature.

In the case of Spectrum Sweater building, according to published post-failure investigation report by the IEB (Institute of Engineers Bangladesh) committee (The Daily Star, October 5 and 12, 2007) the nine-story structure was provided with pile foundation.

This writer on May 20, 2005 in a letter to the editor of The Daily Star had suggested that “down drag” (negative skin friction) of the settling soil of the reported 30-foot deep filled soil at the Spectrum site had acted on the piles. Thus, in addition to the applied load from the structure, the top 30 feet of the fill soil around the piles was applying a dragging load on the piles. This, in lay-man’s term, is a double whammy! Not only was the pile not getting any friction resistance to carry the building load from the top 30 feet of filled soil, rather it was applying additional downward load to the piles.

Collapsed Spectrum Sweater Factory in 2005, note the partially filled up pond in the foreground. (Source: Internet)

Collapsed Spectrum Sweater Factory in 2005, note the partially
filled up pond in the foreground. (Source: Internet)

The IEB investigation concluded that poor quality concrete in the north-east corner column triggered the catastrophic Spectrum building collapse. The analyses by the IEB committee found the capacity of the piles to be adequate but the published report does not mention whether the possible development of “downdrag” (negative skin friction) due to the settlement of the 30 feet thick fill was considered in their analyses. The IEB investigation committee observed the presence of mud within the column indicating concreting under water and that brings back the pesky question of ponds. Thus, the pond filling and the ponding of water in one way or other seem to be related to the fateful Spectrum Sweater building collapse.

The tragic story of the pond at Rana Plaza followed a similar, and even more sinister plot. Half of the 250 – long building was resting on undisturbed natural soil and the other half on recently placed fill soil that filled a pond with possibly several feet of organic soil at the pond bottom. The possibility of detrimental “differential settlement” did not get any attention at all. According to media reports, Rana Plaza was originally constructed as a four-story shopping/office building resting on shallow foundation (footings under the columns). As if the news of the tragic collapse of Spectrum Sweater building had not reached 10 km south, the four-story Rana Plaza was raised to eight-stories. The foundation designed for a four-storied shopping/office building cannot be expected to carry the load of an eight-story factory building with all the machinery load, not to speak of the fact that half of the building was resting on pond filled ground and the other half on natural undisturbed ground. This was a case of gross disregard for safety that ended up costing the lives of more than a thousand.

Before I finish my tale of the two ponds, I would like to emphasize the importance of paying special attention to multi-story factory buildings constructed on filled ground. According to media reports the honorable prime minister has requested engineers to stop future constructions on pond filled ground. This definitely is a laudable call indeed. But what about the possibly of hundreds of factory buildings already constructed on pond filled ground where thousands of workers are presently working day and night? The bereaved nation expects the honorable prime minister to use her good offices to call for thorough inspection of the safety and integrity of all existing factory buildings. All multi-story factory buildings built on pond filled ground especially those with part of the building resting on natural soil and part on filled ground should be considered as ticking time-bombs that can cause death and injury of thousands.

As a national priority, serious efforts should be made to identify all existing factory buildings built on filled ground. After the buildings are identified and the depth of fill soil thoroughly investigated, the foundation loads of these buildings need to be transferred to stable soil beneath the settling pond filled soil. There are well established techniques of under pinning such as mini piles, jacked down piles or helical piles that can be employed to retrofit the existing multi-story factory buildings on pond filled soil.

To avoid repetition of the terrible tragedy of Rana Plaza, we all (eng ineers/ developers/owners/financiers) must be aware of the scourge of the ponds. We must keep the possibility of “differential settlement” and “down drag loads” in our minds whenever dealing with pond filled soil. All future multi-story factory buildings on pond filled soil should be built on deep foundation (pile foundation). Moreover, pile design involving newly filled ground should always consider the possibility of “down drag” (negative skin friction) load acting on the piles.

Finally, as a nation, we must be aware that if tragedies like Rana Plaza are allowed to continue, the day is not far off when consumers of our garment products in the west will start to avoid our garments as “blood garments”. It is high time we start to value human lives and sincerely follow all aspects of the building codes to make our buildings safe.

The writer is a geotechnical engineer and lives in Orlando, Florida.