Additional material and pictures provided by Ellington Cross LLC.
In the past, the conventional method for handling loose sands, soft clays and poorly compacted fills on a construction project was to undercut and replace those materials or to bypass them with deep foundations. Today, geotechnical and structural engineers design buildings, embankments, bridges and slopes to bear on soils improved in place.
What Is Ground Improvement?
Ground improvement consists of densifying, compacting and/or draining soils in-place to increase bearing capacities and shear strengths, reduce settlements, stabilize slopes and to mitigate liquefaction potential. Improved soils can support structures on shallow foundations or can be used in conjunction with deep foundations to improve lateral support and ultimate pile capacities.
Because ground improvement is a fraction of the cost of undercutting and replacing poor soils or installing deep foundations such as piles or caissons, many sites considered too expensive to develop are now being revisited.
Earthquakes And Soil Liquefaction
During an earthquake, base rock beneath the surface of the earth shakes. As the base rock shakes, all of the soil layers above it shake as well. If the soil is composed of loose sand, the grains naturally want to condense. "It's not unlike marbles in a jar," explains J. Scott Ellington, P.E., president and principal of Ellington Cross LLC. "If the marbles are put in there loosely and you start shaking the jar, they'll settle down as tight as they can go because of gravity."
However, when you have loose sands and a shallow water table in an earthquake zone, the sand cannot condense because the water fills the voids between the grains. "If the water doesn't have a place to go as the sands try to condense," says Ellington, "water pressure builds up. Eventually the water pressure will be equal to the soil strength. Now all of a sudden these grains of sand that were pressing against each other lose contact because the water pressure is pressing them apart. The sand then turns to quicksand and then you have bearing capacity failure, slope failure, and embankment failure."
Analyzing The Site
To determine whether Earthquake Drains should be installed on a site, Ellington Cross sends out a geotechnical engineer to test the site and determine if a liquefaction problem exists. If liquefaction is an issue, Ellington Cross determines the number of drains and the depth of treatment for the site. The drains are installed in a triangular staking pattern across the footprint of the building, and half the treatment depth outside the footprint of the building.
"For example, if you knew you had liquefiable soils up to 30 feet, you would go at least 15 feet beyond the building," explains Ellington. "The structural loads of a building don't go perfectly straight down. They spread out, so you want to be sure that nothing that is carrying any load from the structure is left untreated."
A soil's permeability — or how fast water can move through the soil — the design earthquake and the soil conditions all determine how many Earthquake Drains will be needed. For example, if the soil is very dense, the drains may be installed in an 8-foot triangular pattern. If the soil is very loose, then you might need to space the drains in a 3-foot to 4-foot triangular pattern. The looser the sand, the faster the water pressure builds up during an earthquake.
The Earthquake Drains only need to relieve enough water pressure during an earthquake so that there is only minor settlement. During an earthquake, water will flow to the nearest drain, and the water level in the drain will rise. Often the drain itself has enough space above the water table that it can serve as a reservoir and accommodate the increase in water pressure. When the earthquake is over and the pressure is relieved, the water seeps back out. Sometimes the water table is so shallow that a drainage blanket such as gravel or clean sand must be placed on top of the drains so that water has some place to disperse as the pressure increases.
What Are Earthquake Drains?
The Earthquake Drains consist of a corrugated, perforated pipe wrapped in a special non-woven, polypropylene filter fabric. When the Ellington Cross team shows up on site, they bring enough material to make the specific number and length of drains for that site; for example, 4,000 drains that are each 30 feet long. The drains are cut to the correct length, the filter is wrapped around it, and a sacrificial end plate is attached to the bottom of the pipe.
The team arrives approximately a week early to begin assembling and stockpiling the drains before the installation rigs arrive. The rigs themselves can be either a crane with a vibratory hammer, or a self-contained rig that drives around the site.
A vibro probe is used to drive the drains into the ground. The probe consists of a thin steel pipe with three equally spaced fins that protrude a foot off the pipe. The Earthquake Drain is fed into the steel pipe, which is then driven into the ground with the drain inside of it. The drain is protected by the sacrificial steel plate at the bottom. The drain is driven down to treatment depth and the steel installation pipe is vibrated back out. The end plate stays in the soil so that as the steel pipe comes out the Earthquake Drain stays in place.
"Pre-Earthquaking" The Site
As Scott Ellington points out, "We're really subjecting the soil to an earthquake as we drive the drains. We have very high vibrations in the vicinity of the pipe as it's driven in.
"If the engineer has determined that the site could get 5 inches to 6 inches of settlement during the design earthquake if the site were left alone, we'll end up getting 2 inches to 3 inches more settlement during our installation process. We may end up getting 8 inches of settlement across the site. So, we're not just installing the drain, we're trying to subject the site to earthquake conditions — or 'pre-earthquake' it. We're vibrating the soil and letting it settle down and get tighter while we put the drains in."
Retrofitting A Site
Earthquake Drains can also be installed in the ground under an existing building. In this case, where low overhead or restricted access can be a challenge such as a crawl space, Ellington Cross uses a hollow stem auger to install the drains. "We'll drill into the ground 5 feet at a time until we get to the depth we need to reach," says Scott Ellington. "The auger will have a plug at the bottom to keep soil from getting inside. When we reach the right depth, we take a rod and knock out the plug at the bottom.
"We put the drain in the hollow stem auger, push it to the bottom, and begin to pull the auger back up. Once you pull the auger up a couple of feet, the soil collapses at the bottom of the drain, holding it in place. You can then pull the auger the rest of the way out."
Earthquake Drains can also be installed under buildings that are slab-on-grade. In this case, a plug is drilled out, the rig is brought in to install the drains and the plug is replaced. If a floor cannot be disturbed, Ellington Cross directionally drills outside the building to install the drains, which go in at an angle to intersect the area to be treated.
When Earthquake Drains Cannot Be Used
In situations where bearing capacity is an issue on a site, Earthquake Drains may not be the best option. In these cases, vibro-replacement columns might be installed to increase the bearing capacity of the soil as well as to mitigate liquefaction.
"If you have a site where you're putting in a one-story warehouse, that's a relatively light-load structure and Earthquake Drains would be your best option to mitigate liquefaction because the soil is already strong enough to hold the structure," Ellington points out.
"However, if you have an 8-story structure and you need 4,000 pounds per square foot of bearing capacity... and you only have 2,000 pounds per square foot on site, Earthquake Drains would keep the soil from liquefying, but you wouldn't have enough bearing capacity. In these cases, stone columns would be the better choice."
Ellington Cross has done combination sites — stone columns and Earthquake Drains. On sites that need more bearing capacity, but don't necessarily need stone columns installed across the entire footprint, this is often a good option. "We did a very large condominium and commercial project where we put stone columns underneath the footings and Earthquake Drains underneath the slab areas," says Ellington.
Ellington points out that if the soil is very silty or has very low permeability, it might be better to densify the site rather than drain the site. "The siltier the site the slower water drains through it. If you have a 40-second earthquake where you want to bleed water off to keep the soil from liquefying, the soil may not be efficient enough to get the water to the drains."
Earthquake Drains are the cheapest, cleanest, most effective way to deal with liquefaction of soils and reduce settlement of a structure during an earthquake. In South Carolina alone, over $20 million of vibro-replacement columns have been eliminated over the last three years using this patented system. For example, on a recent Wal-Mart job, Ellington Cross installed stone columns across the footprint of the store for approximately $3.8 million. "We left that project and went 100 yards down the road to do a Kohl's project with Earthquake Drains," says Ellington. "The Kohl's was half the size of the Wal-Mart, but we did it for $500,000. To compare, our schedule was faster and the cost was less. To do the equivalent stone column job at the Kohl's site would have been $2 million. Wal-Mart had been dead set on stone columns, but they have since changed their minds. We now have two Earthquake Drain jobs lined up with them."
In addition to commercial sites the technique has been accepted by the South Carolina Department of Transportation, the North Carolina State Construction Office and other government agencies. Ellington Cross is now installing Earthquake Drains for critical use buildings such as hospitals, schools and police stations.
Although Scott Ellington's business is growing, Earthquake Drain technology is still only marginally known and understood. However, that certainly appears to be changing.