Special Bd. of Aldermen — 10/26/15 Page 9
Ms. Taylor | would say that is more likely if you remove it than if you built the secant wall.
Alderman Dowd
| don’t think some of the people here have heard your explanation of the approach that you are tending to use
and | think if you explain how you are going to contain that area and address the potential of not having leaks
and all of that and how it is built will probably answer a lot of questions that you might get.
Ms. Taylor Sure we can do that. We could have Darrin Santos who is the Environmental Consultant that will
be working on this should this go through, we could have him do it or we can explain it.
Alderman Dowd
| would say whoever knows the most about it.
Mr. Millan-Ramos | would defer to Darrin.
Darrin Santos, Geolnsight, Manchester, New Hampshire Good evening, my name is Darren Santos and I’m
with Geolnsight in Manchester, New Hampshire. | represent the developer Blaylock Holdings, Bernie Plante.
We have been working with EPA, pretty close, hand-in-hand in developing this containment approach. The
secant wall we find the most favorable for a few reasons. It is essentially a slurry wall and you all may know
about the Gilson Road Superfund site. They have a slurry wall there. That is traditionally done with an
excavator, where they trench down and mix cement slurry down while they are doing that. It is an effective
approach but at our site, the secant wall is essentially a controlled slurry wall installed with these shafts that
are drilled down.
So you drill the shafts down, there is a casing and an auger and it displaces the soil and then you pump the
cement grout into it. So when you reach your target depth and start pumping the grout in, you know you are
filling that shaft up, versus a trench system where you are down 25 feet and the wall could collapse on one of
the sides. For our site, if it is the riverside, then we have just breached the natural bank that is there, so again
a controlled manner here. Just quickly the way it is put in, is the skip every shaft and then they come back and
drill the ones they skipped on which then overlap the previous two and actually drills out and creates this
overlap.
Ms. Taylor Kind of like Olympic rings.
Mr. Santos They are, | would say traditionally seen in infrastructure projects, things like the Big Dig or down in
the city in Boston when you need to build a big building and you need a foundation, they do secant, you can do
reinforced concrete walls and parking garage structures, and it will hold back water and soil. The design here
is to go with what is called a cement bentonite slurry. It is called low strength and it has flexibility to it.
Reinforced concrete when it cures will tend to crack. This cement, the bentonite is an expanding clay so that
additive to it will cause these to expand and essentially self-heal to cracks. Kelsey, can we get the cross-
section?
So in pink or magenta you see the secant shafts that go down. These will be very impermeable to liquid. The
secant shafts themselves; they will also be seeded down into the glacial till, which is hard compact soil
compacted by the glaciers years and years ago. It is relatively impermeable itself. So that essentially creates
the bottom liner to the system. And then as Gerardo explained before, we have an actual reinforced concrete
panel or poured retaining wall that would be above grade to receive the Fimbel sludge and that then would be
capped with an impermeable cap that you would see in a landfill.
