Thursday, 5 May 2016

The Road to Repair

On 27 June 2008, a note appeared on the States website about the long road works which had been taking place on Victoria Avenue, under Deputy Guy de Faye’s term of office as Minister for Transport and Technical Services.

This was the refit of the road which initially did not provide room for emergency services to go Westbound at Bel Royal, and which gave us the “kinks” in the Avenue travelling West. It seemed at the time that aspects of the project had not been properly managed.

“The major road works project on Victoria Avenue between Bel Royal and Rue de Galet which has been causing unavoidable traffic delays, is due to be finished on Wednesday 2 July and the road will be back to normal on the morning of 3 July. As well as the new road surface, the project has also delivered new and improved safety measures which will help to reduce the accident rate in and around Bel Royal and the car park entrance near the hamburger bar, better pedestrian facilities, improved street lighting, drainage refurbishment and provision for fuel tankers unloading at Bel Royal garage.”

“The project was originally scheduled to take only ten weeks, however Transport and Technical Services (TTS) found the condition of the ground below the surface of the road was much poorer than expected. The project had to be lengthened by two and a half weeks to allow an extra thickness of asphalt to be laid to form a new road construction that will ensure a fifteen year life for this road which has 28,000 vehicles a day using it.”

And there was also work done in 2015, when Eddie Noel said: "It's got a life span typically of about 15 years and it's about halfway through that you end up doing patch up work to the surface, just to keep the road surface to a good enough standard and to make it last for the rest of its life."

So I rather wondered what was happening when I read this about road works taking place this May:

“The Department for Infrastructure is carrying out repairs on the road surface on Victoria Avenue. The area of work is from La Rue du Galet to Bel Royal, westbound lanes. The work starts on Tuesday 3 May for two weeks (weather permitting).”

What happened to the fifteen years? I asked Deputy Eddie Noel. He was good enough to check, and apprise me of the reason why.

“Following recent investigations we have determined that some high stressed areas of Victoria Avenue have failed earlier than the predicted 15 year design life. There are a number of reasons for failures of this nature and in this instance it is due to failure of the underlying layers of the road. Our works in 2008 repaired the surface and the immediate layers below the surface but did not include these lower layers. When inspected at the time they were found to be in good condition.”

“The repairs will include the removal of the surface course, the inspection and appropriate repair of the underlying layers and the replacement of the surface course. It is anticipated that the majority of the areas that have failed will be strengthened, using a thicker layer of asphalt.”

“DfI regrets that these areas have failed after being laid 8 years ago; however it is not unusual that localised failures occur. It is imperative that the areas are repaired and the surface strengthened, sealed and made watertight to minimise further damage.”

It is clear then that this is rectifying a problem, and the Minister is taking action sooner rather than later to ensure that the damage does not get worse; it is like getting teeth repaired before decay reaches into the roots.

But why might the road fail in this way?

In general, most damage is done by cumulative vehicle passages: the impact of vehicles going over the road and tearing it up. The force exerted on the road by a given vehicle is related to the vehicle’s weight (load) as transmitted through the vehicle’s axles.

The best scientific study, which has been largely replicated by later studies, was that done in the USA, where it was reported that:

“The AASHTO Road Test established that heavier vehicles reduce the serviceability of a road structure much faster than light vehicles. Results from the test indicated that the damage to the road structure varies approximately according to the fourth power of the axle load, which provides the basis for the so-called ‘fourth power law’”

When we look at heavy vehicles, we find that the following vehicles are not permitted on Jersey roads without a permit:

Vehicles or trailers over 7ft 6.5ins (2.3m) wide
Rigid vehicles over 30ft 6ins (9.3m) long
Articulated vehicles over 36ft 1in (11m) long
Trailers (draw bar type) over 22ft (6.7m) long
Vehicles with a maximum axle weight over 10,500 kg (23,148 lbs)
Vehicles with a gross weight over 32 metric tons (70,548 lbs)

These have a P30 permit and are only allowed to travel on specified routes

Heavy trucks obviously cause more road damage than cars, but how much more? According to the American figures, road damage from one 18-wheeler is equivalent to 9600 cars

The study assumed a fully loaded tractor-trailer at 80,000 pounds, and a typical passenger car at 4,000 pounds. That’s 20 times difference in weight, but the wear and tear caused by the truck is exponentially greater.

As can be seen above, that’s over the gross weight requiring a P30 permit, but not massively so.

The University of Michigan economist Richard C. Porter, writing in “Economics of the Wheel: The Costs of Cars and Drivers” notes that: “Typically, a truck will carry 10 times as much weight per axle as a car, and hence that truck will do 1000 times as much damage to the road (per axle). For practical purposes, road damage is done by trucks, not cars.”

“In determining the structural life of a road pavement, the effect of cars and similar vehicles is negligible…. It is therefore the heaviest axles in the stream of commercial vehicles that cause a disproportionately large amount of structural damage to a flexible pavement.”

An Ohio study looked at the effects in detail, and noted that:

"Clearly, different types of vehicles (and different weights) will do different amounts of damage. So we need some way of standardizing these impacts. "

This they did by calculating an engineering standard: ESAL = damage done by 18,000 lbs on a single axle. (18,000 lbs = 18 kips, kilo-pounds)

Representative ESAL numbers are:

Passenger car 0.0008
Urban Transit Bus 0.6806
SU2 Truck 0.1890
SU3 Truck 0.1303
CS3 Truck 0.8646
CS4 Truck 0.6560
DS5 Truck 2.3187

Key: SU = single unit; CS = conventional semi-trailer ; DS = Double Trailer. n = number of axles.

The first policy implications of their study were this: When discussing road wear cars don’t matter - road damage is effectively caused by trucks.

The study also asked: How many times (passages) can such a truck pass over a durability road before it deteriorates from new to needing re-surfacing?

The statistics showed that the number of passages rises steeply with the number of axles. Going from 2 to 4 single axles results in a factor-of-10 increase in road life. The effect is even stronger with tandem axles. Going from 2 single axles to 2 tandem axles quadruples life. Going from 2 single axles to 4 tandem axles is a factor-of-33 improvement .

It also noted that an earlier than anticipated failure of a road surface was directly related to the volume of heavy traffic, either increased number of vehicles or greater loads on existing regular vehicles.

So the second policy implication they came up with was this: Public policy should provide incentives for truckers to distribute their loads over as many axles as possible, and to use tandem axles. This minimizes forces on the road, and hence road wear.

In conclusion...

Without adequate support, a pavement or gravel surface deflects too much under trucks carrying heavy loads, developing cracks or ruts. These then let more moisture penetrate, worsening the cycle until the road fails completely. The amount of damage a road sustains is directly related to the weight of the load and how often it is applied, according to tests by AASHTO (the American Association of State Highway and Transportation Officials). Passenger autos and light duty vehicles are not a problem. It is trucks carrying legal weight loads of up to 80,000 GVW over weakened surfaces which do the damage.

A UK study on costs noted that “In general terms, cars pay much more tax and charges relative to their costs than heavy vehicles. For HGVs it is clear from most studies that they do not cover the costs for which there are established values, especially road construction and maintenance, carbon and congestion”

Of course, any extra charges levied on HGVs would probably impact on other areas, as for instance, where supermarket supply is concerned, and part of the cost would certainly go to the consumer. But nevertheless, if any mechanism needs to be found to recover costs of damage by heavy vehicles, no burden should fall on ordinary cars, which as the figures show, cause negligible damage to road surfaces such as that being done.

But heavy vehicles, the numbers licenced in Jersey, figures on their weight, tyres and axles, should be available and collected under the P30 permit system, and should be part of any future strategy for the longevity of Jersey's road systems.


As James has pointed out, Double Decker buses now use the Avenue frequently. They provide an approximately 50% increase in the number of passengers, but impose higher axle loadings (and road wear).

A New Zealand study noted that a two axle double decker would have the rear axle loaded above the standard 8.2 tonne limit. While this does vary by vehicle model, a typical loading would be 10 tonnes on the rear twin-tyred drive axle. 10 tonnes represents 120% above the standard weight allowance, and is calculated to produce increased road wear (at 4th power) of 207%.


James said...

To my knowledge there were no regular bus services using the avenue in 2008: there are now double-decker buses running down there regularly.

wayne le brocq said...

A very interesting article. Though there is some truth to the answer given by the Infustucture Minister there also has to be noted that road reinstatement doesn't always mean a full reinstatement. In recent years it is mainly the surface course that get renewed. Though there is usually strengthening of the base course, this is mainly done in areas that look weak after the surface course has been removed. One would presume that most of the roads in the island are "past their use by date" and could probably do with a full restructure. But due to cuts and the presumption from previous Ministers that infrastructure isn't at the top of the list of important items. The roads in the island have been neglected and have had numerous amounts of patch work done to them. As you mentioned about car numbers and lorry sizes increasing this will cause stress to the roads. But we must wonder what the civil engineers at TTS/Infustucture are contracted for if they are not picking up the roads weakness when these reinstatements are being undertaken. Wouldn't it be better to pay more now rather then later?