The difference between sand boils and seepage
Information courtesy of the April 2006 issue of Esprit, the St. Louis District's newsletter.
The image to the right is an example of seepage. In this case it is water seeping through a levee. Note that the water is clear.
The U.S. Army Corps of Engineers is the world’s premier builder of levees and other flood damage reduction structures. But Mother Nature is a formidable adversary whose arsenal includes such tools as high water and wave action, underseepage, plus the insidious, invisible-until-almost-too-late sand boil.
We went to geotechnical engineer Patrick Conroy to explain this potentially destructive phenomenon.
Conroy explained that sand boils start with underseepage during floods.
When the floodwaters are higher than the land, the groundwater, under pressure from the river, exerts an upward pressure on the land inside the levee. With time this increased “head pressure,” as it is known to engineers, can drive groundwater through the underside of a levee 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 told. In fact, you may even see hundreds or thousands of ‘pin boils’, small watery bumps that look like a water fountain with insufficient pressure, he added.
When flood waters remain high for a long time though, underseepage can increase in volume and velocity and begin the destructive process of moving sand from the foundation, through the ground, to the surface, forming ‘sand boils’.
"When the seepage begins making sand boils", Conroy said, "geotechnical engineers get nervous! Sand boils indicate more serious underseepage." Left unchecked, the process can eventually lead to levee failure.
So what do you do when you find sand boils during a flood?
In the image to the right, a sand bag levee rings a sandboil. You build the sand bag 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. Click here or on the photo for larger image.
“You fight back – flood fight – by building a sand bag ring around the emerging ‘sand boil,’ which now can be seen as a bubbling, water-sand slurry,” Conroy said. You build the sand bag ring higher and higher, until the weight of the water inside the ring slows or stops the water flow from the boil.
Then you’re done – there at least – until sometime later 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. Again. And again.
The theory behind fighting sand boils is an intriguing exercise in engineering. The practical part of flood fighting sand boils is usually just plain hard, dirty work.
So what do levee designers do? “We have some effective tools to combat underseepage.” Says Conroy. “Most commonly, we use relief wells or landside seepage berms. Less commonly we use of a sheetpile cutoff or slurry trench cutoff.”
Relief wells allow groundwater to flow out of the aquifer, thereby 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 underseeapge travel a longer distance from the levee. The longer seepage path reduces the power and force of the underseepage so that when it does exit landside of the seepage berm, it causes no problems.