Keep your equipment above water
By David GodkinDemolition LEED
Understanding geological site conditions, before you dig, will save time and money
“Water, water, everywhere and not a drop to…” Well, you know the rest. Trouble is, as necessary as water is to life, it can also be a real pain. Just ask a farmer standing up to his knees in a flooded cornfield. Or better still—ask an equipment operator mired in muck at the bottom of an excavation. Water is a problem for him, and for the building contractor under pressure to bring the project in on time, and budget.
“The other problem is that a lot of the development and construction is happening in areas where you’re very close to environmental features. Even if you’re going into a storm sewer, that runoff of the water from your site now becomes problematic,” says Andy Ingriselli.
As president of AquaTech Dewatering Co. headquartered in Concord, Ont., Ingriselli is well acquainted with water. His job for the past 20-plus years has been to tailor specific technological solutions to water ingress across a range of applications: construction, mining, industrial and environmental development. Dewatering construction sites is an ongoing struggle to control below ground water tables by removing water so excavation can begin.
Ingriselli has a range of tools that allow him to do this—different sized wells that literally suck the water out of the ground to keep the excavation above the water table. Only then, he says, can you excavate, build your structure, backfill to existing grade, shut off the pumps and allow the water to return. “One way or the other we’re going to get rid of this water.” Sometimes the biggest challenge, “is where do I put the water (municipal sewer or natural water body such as a stream) and how do I get it there in a manner that doesn’t have an impact?” adds Ingriselli.
Rule One: Know your stuff
Long before dewatering a site begins Ingriselli needs to know something about the geology containing the aquifer and water table below. Geological reports are “the key to picking the right tool,” explains Ingriselli. It is necessary to identify the presence of sand, gravel, fine grain or other silty materials that hold water and can impede excavation. The most difficult soils, he says, is an interface of water-bearing silt or silty sands over clay or glacial till.
“I have water travelling across the surface of this clay-like material that I still have to dig through. So I have to dewater as close as possible to that interface as I can.”
Dewatering is also complicated by soils that are highly compressible (e.g. organic soils such as peat); these tend to settle as water is removed, destabilizing adjacent structures such as a sewer or building. Dewatering companies go to great lengths to ensure they’re only removing water, and not also removing soil. This is to avoid destabilizing the ground and the structures it supports.
Rule Two: Nature abhors a vacuum
Typically three types of wells are used to fully dewater a site. The first of these is a vacuum well point installation, a series of small diameter tubes inserted around or on either side of a trench from which water is drawn by a vacuum generated by a pump. Well points are installed across a range of soil gradations from fine to coarse, but the key, says Ingriselli, is the distance between those well points. “The close proximity of one well to the next is essential to get as flat a drawdown curve as possible to that interface condition.” The other factor is how the system is driven.
“You may put in sixty to a hundred well points in a row all operating from vacuum drawing water to the surface. Rather than have an electric pump at each well it’s much more efficient to operate well points in close proximity to one another using one common pump.”
Made of plastic or steel, 50-mm well point tubes are jetted under high-pressure water down into drill holes approximately 300 mm in diameter and hooked up to a common header; the vacuum that is created once the system is activated draws the water into the well point’s screen and filter pack. These, in turn, prevent soil particles from mixing with the water drawn to the excavation surface.
Well points are effective to a maximum depth of approximately seven meters. Any deeper and a different dewatering tool is needed. As its name implies, a deep well can go down much further than a well point, and can also be spaced farther apart. Instead of a 50-mm diameter tube, a deep well typically consists of a slotted liner, a PVC screen 150 to 300 mm across and an electric submersible pump, all of which are flushed to the bottom of the well.
“This gets used in ground conditions that are much more porous, [and have] cleaner sands and gravels that produce much more water,” says Ingriselli. “What this allows us to do is create a much larger well, and pump a much larger volume of water from one location.”
Like well points, (the third type) eductor wells are installed relatively close together but are better suited for finer grain, low-permeability soils and require only a single stage to achieve drawdowns of up to 100 feet (30 meters) or more. Two parallel headers, one a high-pressure supply line, the other a low-pressure return line run to a central pump station that feeds water under pressure to the eductors situated at the bottom of the wells. A venturi then draws groundwater into the well screen and up to the surface.
Rule three: monitor, monitor, monitor
Aquatech also uses a single-pipe eductor system, which is a large diameter service pipe encasing a smaller pipe that acts as the return line. Water is pumped under high pressure along the annulus between the two pipes and is forced through the nozzle and venturi. The groundwater then passes through the well and into the return pipe, creating a vacuum that lowers the water table, stabilizing the fine-grained soils. Depending on the size of the project as many as 300 or more eductor wells can be activated by a single pumping station. In addition, a single pumping station for the eductor and well point systems is less costly to maintain than a pump for each individual well.
The downside to eductor wells, says Jose Somera, project manager for Griffin Dewatering Corporation in Ontario, Calif.; are the volumes of water that can be removed (less than 200 gallons per minutes). Another challenge—and all dewatering systems face this—is that “it’s hard to read soils underground.” Russell Thomas agrees. While “taking his hat off” to engineers who tell us much about soil conditions underground “during underground construction,” Aquatech’s senior project manager says, “you don’t have a complete 100 per cent picture of what’s beneath the ground surface.”
“So you’ll run into situations where the size of the well may have to change, where you may have to pump more water than was anticipated.”
Both Thomas and Somera rely heavily on piezometers or observation wells to address those challenges, quickly determining water level underground during pumping and making adjustments when dewatering is not proceeding as expected. “They may have observation wells in the centre of the excavation or they may have them between two eductor wells,” says Somera. “These measure the water levels even before they start the system.”
A case study
It is one of Greater Toronto’s most ambitious building projects in decades, an 8.6 km extension of the city’s subway system that will cross under buildings, railway tracks, arterial roads, major utilities, conservation areas and water courses. And while water is less of a concern in the Spadina Subway Extension’s 6.7 km of bored twin tunnels, underground aquifers pose potential problems for e
xcavation at six brand new subway stations.
Long before Aquatech could tackle water problems there, however, the six excavation sites had to be stabilized, some with water-tight shoring system, others such as the new Finch West station using a soldier pile and lagging system. Once steel H piles were installed down to depth, Aquatech dewatering crews followed in behind using jet poles to flush the eductor wells—each fitted with a PVC screen and casing – down the 300-mm drill holes to depths of between 18 and 24 meters.
Once this was completed and dewatering is underway the construction crew was able to install wooden lagging between the H piles, confident heavy water bearing soils wouldn’t suddenly start pouring in on them. Because the excavation box at the Finch West station is a bit larger than the other stations, it employs up to 300 eductor wells and pumps approximately 200,000 litres of water per day.
Throughout the process crews closely monitor water levels at surrounding observational wells to ensure the water table is still below the excavation. “If it rises, that would indicate a problem,” says Thomas. Monitoring gives his crew time to figure out what if anything, in the system is not functioning properly. Sometimes it can be as simple as another contractor on site accidentally loosening or breaking a part of the surface equipment.
“Or sometimes there’s a buildup of iron in the water which can plug up the nozzle on the eductor assembly and it’s not going to be producing as much water as it should.”
Aquatech’s “workhorse” is a custom-built, track-mounted drill rig specifically engineered for dewatering construction projects. “We’ve taken the bucket and stick off a tracked 324 Cat excavator and installed a mast and drill assembly,” says Thomas. The result: a machine equipped to work in extremely wet conditions. Adds Ingriselli, “We’re bringing water with us every time we install, so the ground conditions become saturated and more difficult for a rubber-tired machine. So tracks become essential.”
The pride of Aquatech’s equipment list, however, is its drill. “It’s simply the biggest, most robust and fastest drill rig out there,” says Thomas. “It can bore more wells in a single shift than any of our competition can.” Ingriselli adds that when digging particularly deep wells Aquatech will sometimes employ a dual rotary drill that actually drills the casing into the ground, drilling out the material inside the casing “so that you can install eductor wells or deep wells to whatever depth you want. We’ve installed 2-inch (50 mm) eductor wells up to 24-inch (609 mm) diameter deep wells with that rig.”
The dewatering technology is rounded out by a wide ranging support fleet of boom trucks, service trucks, rubber-tired loaders and material handlers such as large fork lifts. All of these support elements “right down to the loaders and trucks that make deliveries, are critical to a turnkey operation like this,” says Ingriselli. “We’re a service business. It’s all about the end result. So you have to have control over all the critical factors…”
David Godkin is a B.C.-based freelance writer and regular contributor to On-Site. Send comments to firstname.lastname@example.org.