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Forty-year flashback: Why and how does water move in the soil?

July 16, 2013 – Drainage Contractor magazine is celebrating its 40th anniversary in 2013, and to celebrate, we’re taking a look back at what made the headlines 40 years ago with Forty-year flashback, a series of articles from the magazine’s first few editions.

The following article, originally published in the 1975 edition of Drainage Contractor, discusses soil mechanics with Dr. Bob Quigley. 

Water is always trying to go some place. It may be penetrating the soil after a rain or steadily evaporating into the atmosphere after a drought. It may be eroding a gulley or nourishing crops.

Understanding soils and how water moves through them can save a lot of grief. Homes, dams and even towns have slipped into obscurity because the laws of soil mechanics and the effects of water moving through soil were ignored.

Drainage Contractor sought the advice of Dr. R.M. (Bob) Quigley, a graduate of MIT, who now heads the Soil Mechanics Section of Engineering at the University of Western Ontario.

“Permeability is the major influence,” says Dr. Quigley. “Permeability is the measure of velocity of flow of free water through a soil system. The rate of flow in sand is up to 1,000 times faster than through silt, which in turn is 1,000 times faster than in clay, which is without fissures. So if you put a drain into a water bearing sand, then the radius of influence is that much greater than through a silt or clay. We would get very little draw down in clay soils if we didn’t have fissuring – fortunately these fissures, or cracks, which are often forms in clays, give the soil a permeability far higher than would otherwise be the case.”

The spacing and width of fissures governs the effectiveness of a tile system in lowering the water table. Heavy equipment used on damp clay soils can have a major effect in closing these fissures.

In sand, water is classified as “free” water; that is, it is not absorbed by the grains of sand and moves through the soil by gravity flow. Drainage such soils can be a challenge. Says Quigley, “If the sand is at the bottom of a valley there could be water penetrating the sand through an artesian effect. This is not a difficult situation; you have to drain off not only the surplus water from the sand, but also the water that is moving up through that sand. This can happen whenever a geological environment exists, which creates artesian pressure. In Ontario’s Grand Valley there are areas where the artesian head is 60 feet above ground water level . . . and that’s where you can encounter quicksands. If you have to drain the ground water plus the water that is coming up through this artesian effect, you may have to double or even triple the number of tile runs.”

In fine silts, water moves upwards by capillary rise . . . it climbs by its own surface tension. This is why some silt soils will remain wet until evaporation exceeds the rate at which water is rising by capillary action. Says Dr. Quigley, “This is how turf racetracks are kept at the desired moisture level; there is just enough silt in the base to get capillarity.”

After a rain, water may penetrate silts by gravity but most of the water movement is by capillary flow as the water is sucked down into dry soil.

And just how does water penetrate tile? “It flows by gravity into the tile system because the air pressure in the tile is lower than the water pressure in the surrounding area,” says Quigley. “After a series of open pores have been opened up, a parabola develops. It’s a conical effect which spreads further away from the tile – providing it doesn’t rain. When it does rain, the water table rises and there is an equilibrium; then as the water table falls again, capillarity again becomes effective.”

There are also mineralogical complications. For instance, in Southwestern Ontario, the surface soils have been weathered to a point where they absorb more water and remain wetter than the original soils. These are the sorts of soils which compact when worked too wet, and when they’re dry they can become too hard to work.