Corps has experience dealing with common flood danger
By Alan Dooley
Sand boils are one of the common problems faced by the U.S. Army Corps of Engineers, but few of the Corps’ employees have heard of them, or know what causes them, or how to defeat them.
The Corps is the world’s premier builder of levees and other flood damage reduction structures. But Nature is a formidable adversary whose arsenal includes high water, wave action, underseepage, and the insidious, "invisible-until-almost-too-late" sand boil.
Patrick Conroy, St. Louis District geotechnical engineer, explained this potentially destructive phenomenon.
According to Conroy, sand boils start with underseepage during floods, especially with levees built on a surficial (of or relating to the surface) clay blanket that covers a sandy or gravelly aquifer. Another requirement is an adjacent river channel deep enough to provide a direct hydraulic connection between the river and the underlying aquifer.
In non-flood times, groundwater usually flows from the aquifer into the stream, following the contour of the land above, running downhill into streams, ponds and, lakes.
But during high water, ground water will actually flow back underground, completely filling and saturating the aquifer. When the floodwaters are higher than the landside ground, the water in the aquifer, under pressure from the river, exerts an upward pressure on the bottom of the clay blanket.
With time this increased "head pressure," as engineers call it, can drive the water through the clay blanket to the surface.
Conroy explained that this is expected. "We call it underseepage. When water stands against an earthen levee for a long time, we expect underseepage to occur. In fact, investigations into the occurrence and control of underseepage are a major consideration of the geotechnical design of a flood protection system."
Underseepage that allows clear water to reach the landside of the levee is not a problem. "It’s not uncommon to see an area of dampness in an otherwise dry field when this happens," Conroy said. "In fact, you may even see hundreds or thousands of ‘pin boils,’ small watery bumps that look like a water fountain with insufficient pressure."
But when flood waters remain high for a long time, underseepage can increase in volume and velocity and begin the destructive process of moving sand from the foundation aquifer, through the clay blanket, to the surface. The water mixed with sand looks like liquid sand boiling up from the earth, so they are called sand boils.
"When the seepage begins making sand boils, geotechnical engineers get nervous," said Conroy. "Sand boils indicate more serious underseepage."
The closer to the levee that a sand boil appears, the more dangerous it is. Left unchecked, a sand boil can eventually lead to levee failure.
So what do you do when you find sand boils during a flood?
"You fight back by building a sandbag ring around the emerging sand boil, which now can be seen as bubbling, water-sand slurry," Conroy said. You build the sandbag ring higher and higher, until the weight of the water inside the ring slows the water flow from the boil to where there is just clear water coming from it.
Then you’re done, until sometime later when another boil appears. The next boil may occur in the adjacent field, or it might occur right next to the sand bag ring you just built. But as long as the flood continues they will appear anew somewhere else along the levee.
The theory behind fighting sand boils is an intriguing exercise in engineering. The practical part of fighting sand boils is usually hard, dirty work. Just ask someone who has filled and toted the sandbags.
So what do levee designers to permanently stop the sand boils, and prevent them in the first place?
"We have some effective tools to combat underseepage," said Conroy. "They include relief wells or landside seepage berms, sheet pile cutoffs or slurry trench cutoffs, "Conroy said.
Relief wells are drilled on the land side of levees to allow groundwater to flow out of the aquifer, reducing the head-pressure in the aquifer. This pressure reduction decreases the upward seepage through the landside clay blanket and preserves the integrity of that blanket.
Seepage berms allow some of the excess seepage pressure to bleed through the surficial clay blanket and into the berm. There, the seepage flows horizontally to the end of the berm. The downward weight of the seepage berm also counteracts the upward seepage forces acting on the surficial blanket.
Finally, the width of the seepage berm makes the underseepage travel a longer distance from the levee. The longer seepage path reduces the power and force of the underseepage so that when it exits on the landside of the seepage berm, it causes no problems.
"Sheet pile cutoffs and slurry trenches are functionally the same," Conroy indicated. They are groundwater resistant barriers that block or impede groundwater from flowing through the underlying aquifer under the levee."
Sheet pile cutoffs are continuous vertical steel sheet pile walls driven from the surface down through the aquifer. Slurry trench cutoff walls start with a continuous trench, excavated from the surface through the aquifer. That trench is backfilled with a clay slurry. During a flood, both devices limit ground water flow under levees, and thus limit the pressure head that otherwise may build in the aquifer inside the levee.
So sand boils are just one weapon in the arsenal that Nature uses to attack the Corps’ flood control structures. But, as we have seen, the Corps has an arsenal of its own to defeat them.