9 April 2015

The continuing mystery of the Pakyong Airport slope

Posted by Dave Petley

Pakyong Airport

Back in January I posted about apparent ongoing slope stability issues at the remarkable Pakyong Airport site in northern India.  To recap, this is a huge project to construct a large new airport on a greenfield site; to create the bench for the runway and associated infrastructure, a very substantial amounts of earthworks have been undertaken, including the construction of a range reinforced slope.  This image, from an article in NCE, shows the projected works at Pakyong Airport:



The NCE article describes very well the proposed works, and is worth a read.  In summary:-

“Strictly speaking the huge rising structures are not walls as such. They are gabion-basket facings for massive reinforced earth embankments, which help create a flat platform for the runway and the aircraft apron and terminal. Above the gabions, the upper parts of the steep sided slopes are less steep and are faced with a vegetated mesh. Maccaferri is supplying the gabions and mesh and has worked closely on the design details with main consultant Mott MacDonald India.  Such a dramatic use of reinforced earth is not only unusual but probably unprecedented. Most such structures rise not more than perhaps 15m with a few examples in the world reaching 40m for the entire embankment.”

Maccaferri also have a nice PDF article about the project, whilst Geosynthetica has a nice article about how the slope works for Pakyong airport were awarded “International Project of the Year” in the Ground Engineering Geotechnical Awards.  But the focus of these articles tends to be on the works are on the downslope side of the airport – i.e. those that were constructed to widen the bench for terminal and runway.  I have seen no suggestions that these slopes are causing problems.  There seems to be much less information about the slope works on the uphill side of the site – i.e. the locations in which the slope has been cut back and reinforced.  The Maccaferri article states that on this side of the site the plan was:-

“Gabion toe walls (3 m high) at the uphill “cut” side to stabilize the slopes”

But there have been numerous reports in the last year or so of major slope issues at Pakyong.  I described some of these in my earlier post, and there is a nice example in The Telegraph from last August.  And today the Voice of Sikkim reports on the collapse of an abandoned building (possibly a school) on the slopes near to Pakyong.  There are images of the site on their Facebook page:-


Interestingly, this collapse is apparently on the downslope side of the airport site.  There seems to be significant questions about the upslope (“cut”) side of the Pakyong Airport site, and if this is correct it raises questions about what is happening below the site as well.  Given the high profile nature of the celebration of this project over the last few years, the industry needs to know whether the design approach has started to yield problems at Pakyong Airport.  It is interesting that this is a second example of a highly innovative slope design associated with an airport apparently being associated with significant issues.