Potholes: Until the causes are understood just the symptoms will be treated.

The following is a letter to the editor of New Civil Engineer in response to the feature article on potholes appearing in the June 2016 edition. As the letter explains the feature was bemoaning the waste of money by local authorities in the UK who were being encouraged by government funding policies to fill-in the potholes that now bedevil so many roads rather that tackle the root causes of the potholes. Here is the letter:

 

"It was encouraging to read the headline of your feature pointing out that if future spending on pothole repairs is to be effective their causes and not just the symptoms need to be understood (“Potholes? Fix the cause not the symptoms”, NCE, June 2016). It was discouraging, however, to find that the feature was singularly lacking in any identification of the causes of potholes – a subject that appears to be poorly understood. Instead, the feature was a blatant plea for local authorities to stop wasting the considerable sums being allocated by government on “whack-a-mole” direct spending on ad hoc repairs, in favour of diverting these considerable sums of money to the asphalt industry to allow it to lay down “one-off resurfacing projects”. Why these newly laid pavements will not in few years start to develop the self-same potholes as those laid down a few years ago, presumably by this same industry, is curiously not addressed. Surely, without an understanding of their causes and a translation of this understanding into improved pavement design practice the problems will not go away?

It seems clear that the commonly accepted causes of the most prevalent forms of potholes are poorly understood. What does seem uncontroversial is that potholes are the advanced stage of pavement cracking often referred to as “alligator cracking. Some years ago I was encouraged to ponder the similarities between alligator cracking and the development of ice-wedge polygons in periglacial environments – and even nitrogen wedge cracking on the surface of planet Pluto. While at different spatial and temporal scales there do seem to be great similarities – as shown in these photographs. And as suggested in some recent publications (“The role of thermal ratchetting in pavement failures”, Proc. ICE, Transport, August, 2009; “Possible role of thermal ratchetting in alligator cracking of asphalt pavements”, Int. J Pavement Engng., October, 2009; “From asphalt to the Arctic; new insights into thermo-mechanical ratchetting processes”, 3^rd European conference on computational mechanics, solids , structures and coupled problems in engineering, Lisbon, July, 2006) there is a strong possibility that their causes may also share similar features.    

 

Fig 1   Examples of detritus-wedge polygons (alligator cracking) in asphalt pavements.

         

Fig 2   Examples of ice-wedge polygons in Arctic permafrost.

    

Fig 3   Example of nitrogen-ice-wedge polygons on the surface of Pluto.

It is widely recognized that crack-wedge polygons forming within areas of permafrost are the result of seasonal changes in insolation. During winter periods the frozen ground experiences restrained contraction in which the tensile stresses follow the thermal gradients with maximum values at the surface that attenuate with depth. Ice, being weak in tension, cracks, with the width of the cracks as well as the distance between them reflecting the depth of penetration of the seasonal thermal wave - typical polygonal dimensions being measures in 10s of meters. Surface melt water entering the cracks freezes, so that during the expansion occurring during summer warming the cracks fail to close. Over periods of many years the crack widths increase and ramparts form either side as a result of the shoving experienced during the warming, compression phase of the thermal cycle. Relics of these characteristic features developed during the last glacial period can still be found in many parts of the British Isles.

Unlike ice, asphalt when freshly laid is not brittle. However, various processes are known to reduce the ductility of the bituminous binders resulting in a gradual embrittlement of the asphalt: stress fatigue and work hardening, particularly within the wheel tracks; evaporation of surface water resulting in the leaching-out of the volatiles giving the asphalt its ductility, especially in areas of surface depression where puddling forms; and of course low temperatures especially when freezing occurs. Embrittled areas of asphalt then become vulnerable to a form of thermal ratchetting similar to that causing the development of ice-wedge polygons in permafrost. Circadian and even shorter fluctuation in insolation will during the cooling phase in the areas of embrittled asphalt result in polygonal cracking that reflects the depth of penetration of the thermal waves into the asphalt layers or where the asphalt layers are thin the crack separation controlling the polygonal dimensions will be largely determined by the thickness of the asphalt. Unlike the ice-wedge polygons it will be detritus entering into the asphalt cracks that prevents them from closing during the subsequent warming phase. Deprived of tensile strength once alligator cracks have been initiated, any restraint to inward or outward contraction or expansion during cooling and warming will be provided by the shear resistance between the asphalt layer and its subgrade. As this shear resistance begins to break down the polygonal blocks will start to break loose, resulting in the dreaded potholes.

It was a tribute to the editors of the above cited journal papers discussing this mechanism for the development of potholes that, despite very negative and at times hostile review comments from their “expert” advisors, they chose to publish - even though this explanation for their cause goes against accepted wisdom and is clearly a controversial explanation for the development of a great majority of potholes. But despite the importance of this problem and the industry wide challenge offered by this new hypothesis for the causes of potholes, there has been a deafening silence from the asphalt industry that one presumes so avidly read the learned journal in their field.       

 

Until their “causes” are properly understood and design strategies are evolved to prevent them it is unlikely that Government money will be well spent by either the “whack-a-mole” or the “one-off resurfacing” approaches to eradicating potholes.

James Croll FREng, FICE, FIStrucE

Emeritus Professor of Civil Engineering, UCL"

The root causes have been discussed in the various postings of 21 February, 2012. After 6 weeks this letter has not been published. This is regrettable since the repair of potholes is currently costing the UK and other countries a massive proportion of the transport spending. 

Sadly, after such a long hiatus I cannot remember how to paste-in the original Figures. Will try to sort this out over the coming days.