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Excavation is essentially moving dirt from one place to another, whose
environmental impact is in the disturbance at both sites and the energy used to
move the dirt. Like most places in Seattle, our site has little to no
topsoil (which was probably lost soon after the land was logged many years ago),
so although the impact to our lot is a nearly complete destruction of the
ecosystem, we will add good topsoil when the construction is complete (see
the site section in the tutorial). Since all our
excavation will occur in the dry season, and most of it will be below the
surrounding grade there is little chance of runoff impacting surrounding land
(or the storm sewer).
Although the old house had a good size crawl space (about 3.5 to 4 ft deep),
the additional 3 to 3.5 feet we went down combined with the driveway cut and
extending the house seven feet out the back made for a lot of dirt. In
addition, we excavated for a 17w x 11d x 7.5h rainwater tank (not shown in the
picture) .
All the removed dirt is being taken to an old gravel pit in Maltby, about 20
miles away. We have no idea of how they manage their site, but since its
and old gravel pit it was obviously already completely disturbed.
Summary of Excavation (Embodied energy)
Initial excavation - It took about
13 truck loads (24 yards at a time) to remove the dirt for the initial
excavation, and two truck loads to carry away the broken concrete. The process
took about five days.
Slab backfill - this took one day. We used some of the dirt we set
aside, and one truck load of gravel.
Foundation backfill - about one day. We used the remaining saved backfilled
dirt and another truckload of gravel for the footing drain. We didn't
complete the backfill, because the rainwater tank adjoins the backfil area on
the south side.
Rainwater tank - We removed a total of 60 yards of dirt in three
truckload (two doubles and a single). The process took one day, mostly
with the trackhoe.
Driveway - (TBD)
Guess on embodied energy of excavation:
truck loads * 40mi/5mpg + 10gal/day trackhoe + 5 gal/day bobcat
Some thoughts on the process
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View of excavation from future driveway. Footing forms show where the house will be.
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There is no doubt that excavation is not a pretty thing, and the
environmental impact is non-trivial. Beyond that we could probably argue
whether it is a sensible thing to do for a long time. At the moment is
still seems like there was no better choice, and part of that is based on that
fact that the city required us to build an off-street parking space and further
required that any garage we built meet the setback requirements.
On the positive side, basement space is relatively cheap to build and is the
ideal space to store bicycles, outdoor gear, and yard tools. It also makes
an acceptable (although dark and often cold) place for a workshop.
In rural or suburban settings where land is available, people often use garages
or sheds for the same purpose, but on a small city lot, that isn't
possible. We are also using the basement space to build a mechanical room
(H/W, heat, solar tank, ventilation; pressure tank and pump for the rainwater
system, electrical panel and plumbing manifolds). The mechanical equipment
uses about 40 sq ft of floor space, which would otherwise be located
elsewhere. We also put a utility sink in the basement, so if we can use it
as a mud room.
As with many other things, its often easy to find ways to use space.
By rebuilding the foundation, we got an opportunity to evaluate our soil for
its drainage potential, and to build the new basement so that it will stay dry
(see concrete). Looking at the overall view
of the excavation (above) you can see that we've hit a layer of almost pure
clay, which drains very poorly. The picture below is the soil profile of the
west part (back of the house) excavation. The red lines indicate the
approximate boundaries of the soil layers, and their slope corresponds to the
angle of both the street and the neighbors property. We speculate that the
soil in the upper left is fill dirt that was put there when the original house
was built.
We know that there is a constant seep of water coming out of the south west
corner of our lot almost all winter, and it is likely that it is running on top
of the lower clay layer. Since the north side of the house (on the right in the
picture, below), is mostly clay and is also uphill, this foundation wall will be
the most vulnerable to water penetration.
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Soil profile: lower right is nearly pure clay, the middle is a silt/sand mix,
and the upper left is similar to the middle layer.
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