Understanding the Reasons for Needing to Improve Drainage

Do you know the answer to the old question, "What are the three most important aspects of highway design?" That's right: drainage, drainage, and drainage!

The answer usually brings a smile to the face of laymen and highway officials alike. But, is it really true that the only important thing is drainage? I may be drifting outside the range of political correctness, but throwing caution to the winds I will risk a no!

Please let me explain.

Much of what we know about the importance of drainage is founded on the work done in England from 1780-1830 by a Scot named John L. MacAdam. Others in Europe came to the same conclusions at about the same time, but MacAdam is so widely recognized for his efforts that many laymen think the word "macadam" is synonymous with "pavement." For well over 100 years the MacAdam concept provided the highest quality of highway surface. It is still in use today on some major roads such as the New Jersey Turnpike.

MacAdam's concept involved placing large stone, 1 « to 3 inches in size, close together in a single course on the earth. The holes between the large stones were "chinked" (driven into place using a maul) with medium sized stones. Then the valleys between the medium stones were filled with fine sizes. To make a thicker surface the process was repeated, adding layer on layer, until the road was typically 9 to 12 inches thick.

The road was particularly resistant to the forces of steel-tired, horse drawn vehicles, which moved along slowly. But the stone-surfaced MacAdam roads did not hold up very well when faster and heavier automobiles and trucks came along. They caused the surface stones to loosen.

To reduce this problem, toward the end of the 19th century, asphalt and tar were often poured over the medium sized stones, before the finer size was put down.

This surface was called asphalt-bound macadam, penetration macadam, or "penny mac" for short. It was better suited for use with the faster and heavier traffic of the early 20th century, and it soon became the standard for major roads. By 1914 it was in place on 95 percent of the main highways in Massachusetts. In the middle 1970's it was reported that many miles of these early pavements were still in service.

Needless to say, the process was extremely labor-intensive, which is why it is not used anymore. But there is a lot that we can learn by considering why the MacAdam road was so durable.

One of the "secrets" of the MacAdam road was the shaping of the subgrade. Before the first layer of stones was placed, the earth was given a substantial crown.

The stone surface was very permeable to water, and the crown underneath assured that the water would drain off to the side of the road. If this had not been done, flooded areas, or "birdbaths," would have been created under the road, thereby softening the subgrade. The support for the stone surface would have been weakened, and areas of the road would have failed.

A second aspect of MacAdam's success was due to the fact that the edge[s] of his roads were free to drain. He built his roads on top of the earth. Water could not get trapped in the pores of the base course. And, the water did not have time to percolate into the subgrade beneath the road.

The third major factor was the choice of aggregate gradation. By using coarse quarried stones, good inter-particle interlock was assured, as was good drainage. It would have been necessary then, as today, that the thickness of the road be adequate to spread the pressure of the wheel loads over the subgrade.

If the contact pressure on the subgrade is too high, the subgrade soil will be squeezed into the pores between the base course stones, thereby rendering the drainage in the base ineffective. This process takes place over a period of years, and it has the net effect of gradually making the effective thickness of the base thinner and thinner.

MacAdam showed that three aspects of drainage are important:

• Drainage through the road surface (by using permeable stone)
• Drainage across the surface of the subgrade (by using a crown slope)
• Drainage out of the edge of the road (by placing the road on top of the earth)

All three of these are elements of subsurface drainage. In addition, surface drainage would have to be attended to, by providing ditches and culverts, as a means of channeling the drained water away from the road and into the creeks and streams that existed before the advent of the road.

If we were still building roads in the manner of MacAdam, drainage would be the most important aspect of road building. But, with the onset of mechanized highway construction, we began to do things differently. Not all of the changes have been for the better.

Today we seldom expect to get 75 or 100 years of use out of a road surface. To understand why adding sub-surface drainage now may not be a panacea for our problems, we have to examine what we have done differently over the past 75 years, or so. While the total mileage of roads and streets in this country has not changed very much since 1920, the nature of the construction has changed entirely.

In 1916 we began a nationwide effort to "get the farmer out of the mud." By then both the automobile and farm truck were in common use. Springtime mudbaths, called roads, were a common sight. Automobiles would get totally stuck, and had to be pulled out by horses. The need for all-weather roads was abundantly evident.

Much of the construction under this program, on secondary roads and even some main roads, was with gravel surfaces. To withstand the tire forces and the vacuum under the higher speed vehicles, it was necessary that the surface materials have some silt and clay in them. This provided the required cohesion. But, it also greatly changed the permeability characteristics of the material, as contrasted with MacAdam's open graded stones.

The drainage emphasis moved toward providing good surface drainage by means of putting a steep crown on top of the gravel road. When a good crown was present, less water percolated into the dense-graded gravel surface than was the case with the MacAdam road. What little did seep in would evaporate away during fair weather.

As heavy construction equipment became available, around 1920, we made a second change from MacAdam's method. We began removing the topsoil from where the road was built. This made sense, we thought, because the organic layer in the upper subgrade becomes very weak when it gets wet.

In many instances, the net result was to build the road in a trench. Sometimes this is called "box construction." The edges of the base were boxed in, and they were no longer free draining. Whatever water did get in had to rely on wicking action to get out by evaporation through the gravel surface.

As traffic volumes increased, gravel surfaced roads began to pose maintenance problems. Dust was objectionable, gravel replacement was required often, and blading and reshaping was a constant problem.

Coming out of World War II we began a nationwide program to pave the gravel roads. Generally we put a light-duty surface on top, maybe cold-mix, maybe a chip seal, sometimes hot-mix. This did a fine job of blocking the surface evaporation and trapping the water under the road, especially where box construction was used.

Today we are the victims of previous abuses. The silt and clay in the base, necessary once upon a time to make the road trafficable, now softens the base and makes it frost- susceptible (frost heaves in the winter, frost boils in the spring).

The water retained due to box construction softens the subgrade. Even if we construct good ditches, the water stays under the road and does its damage.

We get 3 to 5 years of life from a chip seal, rather than the 15 to 20 years that is entirely possible. Some highway agencies are turning their formerly paved roads back to gravel surfaces in an effort to make them more affordable.

If you take a core out of the road, or dig a test pit to investigate what is underneath the surface, it is not uncommon for the hole to rapidly fill up with water during the spring. The obvious solution is to put in some underdrains. But I say, "hold on a minute," and think about the problem.

The dirty bases are like a sponge. The term "dirty" refers to the presence of excessive amounts of silt and clay, material passing a no. 200 sieve.

Think about what happens when you soak a sponge in water until it is saturated. When you lift it out of the water it will drain a little. But i[t] still holds a lot of water. If you squeeze the sponge you can remove more water than will drain out of it due to gravity alone. The sponge dries due to evaporation.

The same is true of the road with a dirty base. It makes no difference whether the drain is placed along the edge of the road or on the centerline. Underdrains are only able to remove a small portion of the water from a dirty base by means of gravity flow. And so the base and the subgrade remain weak all year round, except when they are frozen.

What can you do i{f} you are faced with a poor sub-surface drainage situation?

Invest in underdrains

Underdrains are best suited for use when you encounter box construction and clean, free draining base course materials. If you have a dirty base, perhaps a gravel surface that was paved over, underdrains may help a little, but they will not be a smashing success.

Do some in-place recycling

Break up the old asphalt surface and mix it into the base. You may need to add some clean (no silt or clay) crushed gravel or stone to reduce the amount of silt-clay in the mixture to less than 10 percent. A testing lab can assist you in figuring out how much new material to add.

If you accomplish this, and you have box construction, it would be worthwhile to consider putting in underdrains. The best approach is to put a drain on both sides of the road, near the edge of the pavement. This keeps the area under the road as dry as possible. Don't make the water run under the road to get to the drain.

Backfill around the drain with concrete sand, not open-graded crushed stone. This stops the migration of fine material into the drain. Fine soil can be washed along with the water through the crushed stone, eventually plugging the drain. Water velocity through the sand is slow enough that the fine soil is not carried along.

Stabilize the existing base course

This usually begins with in-place recycling of the old surface into the base. If it will require too much clean material to get the fines down to less than 10 percent, then try mixing about 5 percent by weight portland cement into the old base. This binds the silt and clay together, reducing the moisture softening, and adding strength. It also reduces the permeability of the base. A new chip seal or thin hot-mix should be added to provide a wearing course.

Underdrains along both edges of the road, placed deeply to block access of water to the subgrade, may be worthwhile. The choice depends on the availability of water and the type of subgrade soil. If the drainage direction is away from the road, or if the subgrade is an impermeable clay, the underdrains will probably not help very much.

Remove and replace the old base

This is what should have been done when the road was first paved. Today it is usually the most expensive alternative. Of course the new base must be clean, angular, and free draining.

Before you do this, consider each of the previous alternatives, and check to see if you have box construction. If the material under the shoulders blocks the lateral drainage of water, you have box construction. When you are finished reconstructing the road you want the new base to be able to drain. To do so, either replace the material under the shoulders, or construct underdrains on both sides of the road. Often the shoulder replacement option costs less. Either approach will greatly enhance pavement surface life.

If you have gravel surfaced roads that are being considered for an asphalt surface, please put down an adequate thickness of clean material to serve as the base. Don't keep making the mistakes of our forefathers.

So, why do I say that drainage is not the most important this, and if it isn't, what is? I think that understanding the reason for the problem is most important.

There are times when putting in underdrains will not do much good. Like trying to drain a sponge with a pipe.

MacAdam got it right, and then we began doing things differently. Today we need to use our brains first, and our money second.

If we do not correct the problems now that have been built into our roads, we will have to keep paying, and paying, and paying for those problems. We never really saved anything by doing it wrong when we got the farmer out of the mud.

Just imagine, going out to a 15 year old chip seal, deciding whether to put on a new seal this year or next, and knowing that if you do, it will be 15 or 20 years from now before another one has to be built. Impossible? No! I have seen it with my own two eyes. I know that it can be done! Start now, and you will be doing the taxpayers a real service.